WO2019025785A1

WO2019025785A1 - Polymer-based energy storage device

Info

Publication number: WO2019025785A1
Application number: PCT/GB2018/052184
Authority: WO
Inventors: Kara Danielle FONG; Tiesheng WANG; Hyun-Kyung Kim; Stoyan Kostadinov SMOUKOV; Ramachandran Vasant Kumar; Rumen TOMOV; Paulina R LIBRIZZI
Original assignee: Cambridge Enterprise Limited
Priority date: 2017-07-31
Filing date: 2018-07-31
Publication date: 2019-02-07
Also published as: GB201712274D0

Abstract

An energy storage device comprising: two electrodes; and an electrolyte, wherein each of said two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, and wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, and said two electrodes are each in the form of a freestanding film.

Description

Polymer-Based Energy Storage Device

INTRODUCTION

The present invention relates to an energy storage device such as a supercapacitor, processes for making said devices, and uses of said energy storage device.

BACKGROUND

Supercapacitors are a promising energy storage technology which can bridge the gap between the high energy densities of batteries and the high power densities of conventional capacitors. While most commercially-available supercapacitors are made from carbon or metal oxides, devices made from conducting polymers have recently received much attention due to their high theoretical specific capacitance, relatively low cost, and unique mechanical properties such as flexibility, toughness and stretchability. For example, WO 99/66572 discloses electrochemical energy storage devices comprising at least one polymer electrode.

These lightweight, flexible supercapacitors could potentially enable novel technologies such as wearable electronics, roll-up displays or bio-implantable devices. However, significant improvements in conducting polymer-based materials will be necessary in order to make these supercapacitors commercially competitive.

One of the main limitations of conducting polymer-based supercapacitors is poor ion mobility within the electrode material. This is because the electrolyte ions necessary for charge storage reactions cannot diffuse past the first few nanometers of the electrode surface, rendering much of the electrode material wasted and thus decreasing specific capacitance (see, for example, Horng, Y.-Y. et al, Flexible Supercapacitor Based on Polyaniline Nanowires/carbon Cloth with Both High Gravimetric and Area-Normalized Capacitance, J. Power Sources 2010, 195, 4418- 4422).

SUMMARY OF INVENTION

There is a need to develop improved charge storage devices such as batteries and capacitors, especially supercapacitors in which the problems of poor ion mobility that is encountered in conducting polymer-based supercapacitors is addressed. It has surprisingly been discovered that improved charged storage devices can be obtained by the preparation of charge storage devices such as supercapacitors comprising two electrodes wherein each of said two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer. Use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer decreases ion diffusion length scales and makes virtually all of the active material accessible for charge storage. Furthermore, the use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer also enables the electrodes to be formed as freestanding films, eliminating the need for additional binders, substrates or supports which can add inactive weight to the electrode and decrease specific capacitance.

Thus, in a first aspect of the present invention there is provided an energy storage device comprising: two electrodes; and an electrolyte, wherein each of said two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, and wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, and said two electrodes are each in the form of a freestanding film.

In a second aspect of the present invention there is provided a process for preparing an energy storage device according to the first aspect of the present invention comprising:

(i) preparing two electrodes each comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer; and

(ii) providing an electrolyte between said two electrodes.

In a third aspect of the present invention there is provided a device comprising an energy storage device according to the first aspect of the present invention, wherein said device is selected from the group consisting of a regenerative braking system, a memory backup system, preferably for portable electronics, a power buffer, a load- leveling device, preferably for an industrial scale system, a wearable electronic device, a roll-up display and a bio-implantable device.

In a fourth aspect of the present invention, there is provided a process for preparing an electrode comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, said process comprising:

(i) mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors and a solvent (and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials);

(ii) printing the mixture onto a substrate; and

(iii) polymerising and/or cross-linking (preferably polymerising) said polymer precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

In a fifth aspect of the invention, there is provided a process for preparing an energy storage device, said process comprising:

(i) preparing an electrode according to a process of the fourth aspect of the present invention;

(ii) printing one or more ionically conducting polymer precursors onto said electrode;

(iii) polymerising and/or cross-linking (preferably polymerising) said polymer precursors to produce an ionically conducting polymer layer;

(iv) mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors and a solvent (and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials);

(v) printing the mixture onto said ionically conducting polymer layer; and (vi) polymerising and/or cross-linking (preferably polymerising) said polymer precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

BRIEF DESCRIPTION OF FIGURES

Figure 1a shows a poly(3-4,ethylenedioxythiophene) (PEDOT) polymer and a poly(ethylene oxide) (PEO) based network;

Figures 1 b and 1c show semi-interpenetrating network films (sIPNs) comprising an interpenetrating network of PEDOT and a PEO-based network in flat (b) and bent (c) configurations;

Figure 1d is a schematic diagram showing a process for preparing the sIPNs of the invention;

Figures 1e to 1g show the initial mixture of PEO-based oligomers and EDOT in the starting mixture (Figure 1 e), the conversion of the PEO-based oligomers to a PEO- based network by exposure to UV light (Figure 1f) and finally conversion to an interpenetrating PEDOT-PEO-based network by oxidative polymerisation using FeCI₃;

Figure 2 is a plot showing thermogravimetric analysis (TGA) of a PEO-based film impregnated with EDOT.

Figure 3a is a Raman spectrum (Intensity (a.u.) v wavelength (cm)) for a PEO-based matrix;

Figure 3b shows a Raman spectrum (Intensity (a.u. v wavelength (cm)) for a sIPN film compared with neat PEDOT powder;

Figure 3c is an energy-dispersive X-ray spectrum of mapping of a cross-section of a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT to determine sulfur content thereof;

Figure 3d and 3e depict energy-dispersive X-ray spectra of mapping of a cross-section of a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT to determine carbon and oxygen content thereof respectively; Figure 3f shows scanning electron microscope (SEM) images of the surface of a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 4a is a plot of Galvanostatic charge-discharge data at rates of 1-10 A/g for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 4b shows cyclic voltammograms at scan rates ranging from 5 mV/s to 100 mV/s for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 4c is a Nyquist plot, with the inset showing greater magnification of the high frequency region for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 4d is a Bode plot showing impedance phase angle as a function of frequency for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 5 is a nitrogen adsorption isotherm for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 6a shows charge-discharge data at 1 A/g for a range of sIPNs prepared according to Example 1 comprising 14 wt%, 34 wt%, 49 wt% and 61 wt% PEDOT;

Figure 6b is a plot of specific capacitance (F/g) against scan rate (mV/s) for a range of sIPNs prepared according to Example 1 comprising 4 wt%, 14 wt%, 34 wt%, 49 wt% and 61 wt% PEDOT;

Figure 7a shows Nyquist plots of sIPNs prepared according to Example 1 with 61 wt%, 49 wt%, and 34 wt% PEDOT;

Figure 7b is a plot of four-point probe measurements of electrical conductivity versus PEDOT concentration for slPNs prepared according to Example 1 ;

Figure 8 is a plot of specific capacitance (measured by cyclic voltammetry at 5 mV/s) versus film thickness for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 9 shows Nyquist plots of a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT and neat PEDOT electrodes, with an inset magnifying the high frequency region; Figure 10a shows cyclic voltammetry curves at 5 mV/s for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT and neat PEDOT electrodes;

Figure 10b shows charge-discharge curves at 1 A/g for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT and neat PEDOT electrodes;

Figure 10c shows rate capability for charging rates up to 20 A/g for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT and neat PEDOT electrodes;

Figures 10d is a representation of a sIPN according to Example 1 demonstrating the morphological origin of the specific capacitance and cycling stability trends of the sIPN, where the crosslinked PEO matrix provides a reservoir of electrolyte ions and locally constrains swelling of the PEDOT during cycling;

Figure 10e is a representation of a PEDOT only bulk polymer film, where ion accessibility is limited to the electrode surface and swelling-induced strain can yield cracking and failure of the material;

Figure 10f is a plot of cycling stability, as measured by retention of capacitance after repeated cycling at 10 A/g for a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT and neat PEDOT electrodes;

Figure 11 is a plot of Coulombic efficiency (discharge time/charge time) over long- term cycling of a sIPN according to Example 1 comprising PEO and 61 wt% PEDOT;

Figure 12a shows digital images of an electrode prepared according to Example 1 bent around a wire with radius of 180 μηι; inset image shows a top-down view of the wrapped electrode;

Figure 12b shows cyclic voltammograms (20 mV/s) of sIPN electrodes prepared according to Example 1 comprising PEO and 61 wt% PEDOT before and after 1 ,000 cycles of bending and unbending;

Figure 13 shows plots of compressive stress-strain curves of a sIPN comprising PEO and 61 wt% PEDOT prepared according to Example 1 and PEO network films;

Figure 14a is a schematic illustration of a symmetric solid-state supercapacitor formed using sIPN electrodes prepared according to Example 1 ; Figure 14b shows CV scans of the device of Figure 14a, which exhibit the nearly symmetric shape characteristic of an ideal capacitor;

Figure 14c is a digital image of two supercapacitors as illustrated in Figure 14a in series lighting a 1.7 V LED;

Figure 15 is a plot of cyclic voltammetry data for a PEDOT/PEO sIPN electrode prepared according to Example 1 ;

Figure 16 is a plot of cyclic voltammetry data for a PANI/PEO sIPN electrode prepared according to Example 2a;

Figure 17 is a plot of cyclic voltammetry data for a trilayer PEDOT/PEO supercapacitor device according to Example 3 in which PEO acts as the electrolyte and two PEDOT/PEO sIPNs are formed as distinct electrode layers either side of this electrolyte.

Figure 18 is a plot showing the shear stress of prepolymer printing ink samples prepared according to Example 4 over a range of shear rates.

Figure 19 is a plot showing the viscosity of prepolymer printing ink samples prepared according to Example 4 over the same range of shear rates tested in Figure 18.

DEFINITIONS

An electrode in accordance with the present invention is a conductor through which electrons enter or leave a non-metallic medium, namely the electrolyte. The electrodes of the present invention comprise an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer form a continuous phase.

An electrolyte in accordance with the present invention is a phase which conducts ions. In preferred devices of the present invention, the electrolyte is a solid electrolyte consisting of a matrix polymer and an electrolyte salt from which ions can be dissociated. The matrix polymer has ion dissociation power and has two functions: keeping the ion conductive solid in the solid state and behaving as a solvent for the electrolyte salt. Armand et al. of the Grenoble University (France) made a report on an example of such solid electrolytes in 1978, in which they achieved an ion conductivity of the order of 1x10^"7 S/cm in a system where lithium perchlorate was dissolved in polyethylene oxide matrix. Since then, a variety of polymer materials have been examined, including polymers having a polyether structure such as a polyoxyalkylene structure.

As used herein, the term interpenetrating network (IPN) refers to a combination of two or more polymers in network form, at least one of which is synthesised and/or cross-linked in the immediate presence of the other (or of the precursor(s) of the other). Preferably at least two of the two or more polymers are synthesised and/or cross-linked in the immediate presence of the other polymer (or of the precursor(s) of the other). The two or more polymers in an IPN are at least partially interlaced but are not covalently bonded to one another. IPNs are distinct from other multipolymer combinations, such as polymer blends (wherein two or more polymers are simply mixed together), and block and graft copolymers (wherein two or more polymers are chemically bonded to one another). IPNs may be semi-interpenetrating networks or fully interpenetrating networks. Unless otherwise specified the term interpenetrating network as used herein is intended to cover semi-interpenetrating networks and fully interpenetrating networks. An interpenetrating network in accordance with the present invention comprises two or more different polymer compounds which form an interpenetrating network, wherein at least one of the polymer compounds is an electrically conducting polymer and at least one of the polymer compounds is an ionically conducting polymer, wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer form a continuous phase.

An electrically conducting polymer in accordance with the present invention is an organic polymer that is capable of conducting electricity. Such compounds can have metallic conductivity or can be semiconductors. Electrically conducting polymers are able to conduct electricity as a result of the transfer of charges across the polymer chains. Preferred electrically conducting polymers are those comprising an extended linear backbone allowing charge transfer across the extended pi-cloud formed along the chain. Examples of such compounds include polyacetylenes, polypyrroles, polythiophenes and polyanilines.

An ionically conducting polymer in accordance with the present invention is a polymer which is capable of allowing the conduction of ionic species, preferably cationic species. This ion conductivity is a consequence of the presence of one or more bonds in the polymer that are able to co-ordinate with the ionic species, e.g. carbonyl groups. Examples of ionically conducting polymers include polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof.

A freestanding film in accordance with the present invention is a film which is not supported by a backing material to provide rigidity, e.g. a substrate.

As used herein, the term fully interpenetrating network refers to an IPN in which each (i.e. every) polymer is cross-linked. A fully interpenetrating network in accordance with the present invention preferably comprises an electrically conducting polymer in an ionically conducting polymer matrix wherein both polymers are cross-linked or partially cross-linked.

As used herein, the term semi-interpenetrating network (semi-IPN) refers to an IPN in which one or more polymers are cross-linked and one or more polymers are linear or branched (i.e. are not cross-linked). A semi-interpenetrating network in accordance with the present invention preferably comprises an electrically conducting polymer in an ionically conducting polymer matrix wherein at least one of the polymers is cross- linked or partially cross-linked.

A binder in accordance with the present invention is a material or substance that holds or draws other materials together.

A biocompatible material in accordance with the present invention is a material that is able to show an appropriate host response in a specific situation. It can also be defined as the ability of a material to perform its desired function in a biological system (e.g. as an implant), without eliciting any undesirable local or systemic effects in the recipient of that material, but generating the most appropriate beneficial cellular or tissue response in that specific situation, and optimising the clinically relevant performance of the receipt of that material, e.g. as a therapy. A cross-linked polymer in accordance with the present invention is one in which the chains of the polymer are bonded to each other. The cross-linking can be partial or full.

A swellable gel in accordance with the present invention is a polymeric material, the hydrophobic or hydrophilic structure of which renders them capable of holding large amounts of solvent (aqueous or organic) in their three-dimensional structures.

A battery in accordance with the present invention is a device consisting of one or more electrochemical cells with external connections provided to power electrical devices, wherein each electrochemical cell comprises a positive and negative electrode (a cathode and anode) in accordance with the present invention and an electrolyte.

A capacitor in accordance with the present invention is an electrical component that stores electrical energy in an electric field, wherein said capacitor comprises two electrodes in accordance with the present invention and an electrolyte.

A supercapacitor in accordance with the present invention is a high-capacity capacitor with capacitance values much higher than other capacitors (but lower voltage limits) that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries. The supercapacitor of the present invention comprises two electrodes in accordance with the present invention and an electrolyte, wherein the nature of the electrodes and the electrolyte are suitably adjusted compared to the capacitors to give the high-capacity required. This is achievable with the electrodes of the present invention.

DESCRIPTION OF INVENTION

The present invention provides an energy storage device comprising: two electrodes; and an electrolyte, wherein each of the two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, and wherein in the interpenetrating network each of the electrically conducting polymer and the ionically conducting polymer forms a continuous phase, and the two electrodes are each in the form of a freestanding film.

Advantageously, use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer enables the formation of electrodes and energy storage devices with unique mechanical properties such as flexibility and stretchability whilst maintaining high specific capacitance and cycling stability. Without wishing to be bound by theory it is thought that poor ion mobility in electrically conducting polymers per se can render material more than a few tens of nanometers from the surface of the polymer inaccessible for charge storage, limiting performance. Use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer decreases ion diffusion length scales and makes virtually all of the active material accessible for charge storage. The use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer also enables the electrodes to be formed as freestanding films, eliminating the need for additional binders, substrates or supports which can add inactive weight to the electrode and decrease specific capacitance.

In the energy storage device of the present invention, in the interpenetrating network the electrically conducting polymer and the ionically conducting polymer each form a continuous phase. Preferably, in the interpenetrating network there is micrometer to nanometer separation between phases and more preferably nanometer separation between phases.

In a preferred energy storage device of the present invention, the interpenetrating network is a fully interpenetrating network or a semi-interpenetrating network. In certain preferred energy storage devices, the interpenetrating network is a fully interpenetrating network. More preferably the interpenetrating network is a semi- interpenetrating network.

In the energy storage device of the present invention, each of the two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer. The electrically conducting and ionically conducting polymers may be present in the interpenetrating network in any suitable amounts, and may be selected so as to form electrodes and devices with desired electrochemical and mechanical properties. Preferably, however, the interpenetrating network comprises 10 to 90 wt%, more preferably 20 to 80 wt%, still more preferably 30 to 75 wt% and yet more preferably 40 wt% to 70 wt% of the electrically conducting polymer, based on the total weight of the interpenetrating network. Most preferably, the interpenetrating network comprises 50 to 65 wt% (e.g. 55 to 65 wt%) of the electrically conducting polymer, based on the total weight of the interpenetrating network. At lower concentrations of electrically conducting polymer the capacitance of the electrode may be limited by poor continuity of the electrically conducting polymer phase, whilst at higher concentrations ionic conductivity and mechanical properties may suffer.

In the energy storage device of the present invention, the interpenetrating network preferably comprises 10 to 90 wt%, more preferably 20 to 80 wt%, still more preferably 25 to 70 wt% and yet more preferably 30 wt% to 60 wt% of the ionically conducting polymer, based on the total weight of the interpenetrating network. Most preferably, the interpenetrating network comprises 35 to 50 wt% (e.g. 35 to 45 wt%) of the ionically conducting polymer, based on the total weight of the interpenetrating network. The quantity of electrically conducting polymer and ionically conducting polymer in the interpenetrating network film may be calculated as set out in the examples section below.

In a preferred energy storage device of the present invention, the interpenetrating network extends throughout the whole thickness of each electrode. Preferably, the electrically conducting polymer is present throughout the whole thickness of the interpenetrating network. Preferably the electrically conducting polymer is present throughout the whole thickness of each electrode. This may be assessed using elemental mapping techniques (e.g. energy-dispersive X-ray spectroscopy). Advantageously this allows the whole thickness of the electrode to be utilised for efficient charge storage.

Preferably, the ionically conducting polymer is present throughout the whole thickness of the interpenetrating network. Preferably the ionically conducting polymer is present throughout the whole thickness of each electrode. Advantageously this decreases the ion diffusion length scales throughout the bulk of the electrode, enabling the whole electrode to be utilised for charge storage, even in thicker electrodes. The energy storage device of the present invention comprises two electrodes which are each in the form of a freestanding film. Each film preferably has a thickness of 10 μηι to 3 mm, preferably 20 μηι to 2 mm, more preferably 30 μηι to 1 mm, still more preferably 40 μηι to 500 μηι, yet more preferably 50 μηι to 250 μηι (e.g. 100 μηι to 150 μηι). Preferably each electrode has a thickness of 10 μηι to 3 mm, preferably 20 μηι to 2 mm, more preferably 30 μηι to 1 mm, still more preferably 40 μηι to 500 μηι, yet more preferably 50 μηι to 250 μηι (e.g. 100 μηι to 150 μηι). In a preferred energy storage device of the present invention each electrode has a thickness of 100 μηι to 1000 μηι. Advantageously the present invention allows the use of relatively thick electrodes due to good ion mobility throughout the bulk of the film. The electrodes in the energy storage device of the present invention may preferably have a thickness of about 130 μηι. The thickness of the electrode films may be measured using any conventional technique, e.g. using a stylus profilometer or viewing the film cross- section under a scanning electron microscope.

The energy storage device of the present invention has desirable mechanical properties. In the energy storage device of the present invention, each film preferably has a Young's modulus from 10 kPa to 2 GPa, more preferably from 30 to 500 MPa, still more preferably from 40 to 250 MPa and yet more preferably 50 to 100 MPa.

In the energy storage device of the present invention, the two electrodes are each in the form of a freestanding film. This advantageously eliminates the need for additional binders, substrates or supports which can decrease specific capacitance by adding inactive weight to the electrode. Thus in a preferred energy storage device of the present invention, the interpenetrating network does not comprise a binder. In a preferred energy storage device of the present invention, the interpenetrating network does not comprise a substrate. In a preferred energy storage device of the present invention, the interpenetrating network does not comprise a support.

In a preferred energy storage device of the present invention, the interpenetrating network consists of the electrically conducting polymer and the ionically conducting polymer.

In another preferred energy storage device of the present invention, the interpenetrating network further comprises one or more further polymers. The further polymer is preferably a functional polymer. Preferred functional polymers are selected from rubber, shape-memory polymer, epoxy resins, self-healing polymers and mixtures thereof. The incorporation of further polymers into the interpenetrating network allows the properties of the device to be modified or improved by e.g. increasing the stability of the electrodes. Preferred rubbers are nitrile rubbers, styrene-butadiene rubbers, isoprene rubbers, fluoroprene rubbers and chloroprene rubbers. Preferred shape-memory polymers are polyurethane-based, polyethyleneoxide (PEO)-based, polyethylene terephthalate (PET)-based, polycaprolactone (PCL)-based, polystyrene-based, polyester-based, Nafion, and their copolymers. Preferred epoxy resins are bisphenol A epoxy resin, bisphenol F epoxy resin, novolac epoxy resin and glycidylamine epoxy resin. Preferred self- healing polymers are polyurethane-based, thiol-based, and polymers which can undergo healing process such as Diels-Alder and retro-Diels-Alder reactions.

In a preferred energy storage device of the present invention the interpenetrating network may further comprise additional active materials. The interpenetrating network may comprise one or more additional active materials. The additional active materials may act to enhance the properties (e.g. the energy storage properties) of the device. Preferably the additional active materials are selected from metal oxides (e.g. Mn0₂, Ru0₂, NiO/Ni(OH)₂ and Co₃0₄/Co(OH)2, preferably Mn0₂ and Ru0₂) and carbon. In energy storage devices wherein the interpenetrating network further comprises additional active materials, the interpenetrating network may serve as a matrix for the additives.

In a preferred energy storage device of the present invention, the interpenetrating network consists of the electrically conducting polymer, the ionically conducting polymer and the one or more further polymers and/or additional active materials. In one preferred energy storage device of the present invention, the interpenetrating network consists of the electrically conducting polymer, the ionically conducting polymer and the one or more further polymers. In another preferred energy storage device of the present invention, the interpenetrating network consists of the electrically conducting polymer, the ionically conducting polymer and the one or more additional active materials. In another preferred energy storage device of the present invention, the interpenetrating network consists of the electrically conducting polymer, the ionically conducting polymer, the one or more further polymers and the one or more additional active materials. The energy storage device of the present invention comprises two electrodes that comprise an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer. Any suitable electrically conducting polymer may be used in the interpenetrating network. Electrically conducting polymers are beneficial materials for use in electrodes of energy storage devices due to their high energy and power densities, low cost, high conductivity, and robust mechanical properties such as flexibility and stretchability. Preferably, the electrically conducting polymer exhibits pseudocapacitance (i.e. is pseudocapacitative). Preferably, the electrically conducting polymer exhibits double-layer capacitance.

In a preferred energy storage device of the present invention, the electrically conducting polymer comprises repeat units comprising substituted or unsubstituted alkene (e.g. Ci_₈ alkene), arylene (e.g. C₅.₂o aryl), heteroarylene (e.g. C₅.₂o heteroaryl) groups, or mixtures thereof. Particularly preferably the electrically conducting polymer comprises repeat units comprising substituted or unsubstituted thiophene or derivatives thereof, substituted or unsubstituted aniline or derivatives thereof, substituted or unsubstituted pyrrole or derivatives thereof, substituted or unsubstituted vinylene or derivatives thereof, substituted or unsubstituted phenylene or derivatives thereof, substituted or unsubstituted polycyclic aromatic hydrocarbons (e.g. fluoranthene) or derivatives thereof, or mixtures thereof. Especially preferably the electrically conducting polymer comprises repeat units comprising substituted or unsubstituted thiophene or derivatives thereof, substituted or unsubstituted aniline or derivatives thereof, substituted or unsubstituted pyrrole or derivatives thereof, substituted or unsubstituted vinylene or derivatives thereof, or mixtures thereof.

In energy storage devices of the present invention wherein the electrically conducting polymer comprises repeat units comprising substituted alkene, arylene or heteroarylene groups, suitable substituents include groups such as OR', =0, SR', SOR', S0₂R', N0₂, NHR\ NR'R', =N-R\ NHCOR', N(COR')₂, NHS0₂R', NR'C(=NR')NR'R\ CN, Si(OH)R'R\ Si(OR')R'R', Si(OR')(OR')(OR'), OSiR'R'R', OSi(OR')(OR')(OR'), P(OR')R\ OPR'R', OP(OR')R\ OP(OR')(OR'), (P=0)R'R\ (P=0)(OR')R\ (P=0)(OR')(OR'), 0(P=0)R'R\ 0(P=0)(OR')R\ 0(P=0)(OR')(OR'), halogen, COR', COOR', OCOR', OCONHR', OCONR'R', CONHR', CONR'R', protected OH, substituted or unsubstituted C Ci₂ alkyl, substituted or unsubstituted C₂-Ci₂ alkenyl, substituted or unsubstituted C₂-Ci₂ alkynyl, substituted or unsubstituted aryl, and substituted or unsubstituted heterocyclic group, wherein each of the R' groups is independently selected from the group consisting of hydrogen, OH, N0_2, NH₂, SH, CN, halogen, COH, COalkyl, C0₂H, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C2-C12 alkenyl, substituted or unsubstituted C₂-Ci₂ alkynyl, substituted or unsubstituted aryl, and substituted or unsubstituted heterocyclic group. Where such groups are themselves substituted, the substituents may be chosen from the foregoing list.

Preferably, the electrically conducting polymer is selected from poly(3,4- ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), polyaniline (PANI), polythiophene (PT), poly(p-phenylene), poly(p-phenylene vinylene), poly(3- vinylperylene), polyaniline-polypyrrole (PANI-PPy), polyacetylene and mixtures thereof. More preferably, the electrically conducting polymer is selected from poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), polyaniline (PANI) and polyacetylene and yet more preferably from poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy) and polyaniline (PANI). Especially preferred electrically conducting polymers are poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) (e.g. poly(3,4-ethylenedioxythiophene) (PEDOT)). Preferred electrically conducting polymers advantageously exhibit good electrical conductivity, chemical and thermal stability, biocompatibility and high theoretical specific capacitance. PEDOT, for example, has a competitive theoretical specific capacitance of 210 F/g, possesses a high conductivity (up to 4500 S/cm) and can be operated under a larger voltage window than most other conducting polymers.

Preferably, the electrically conducting polymer is biocompatible.

In a preferred energy storage device of the present invention, the electrically conducting polymer has a theoretical specific capacitance of 100 to 2000 F/g, more preferably 150 to 1000 F/g and still more preferably 200 to 800 F/g (e.g. 210 to 750 F/g). Preferably, the electrically conducting polymer has a conductivity of 10^"7 to 10⁴ S/cm, more preferably of 1 to 10⁴ S/cm and even more preferably of 100 to 10⁴ S/cm.

In a preferred energy storage device of the present invention, the interpenetrating network comprises at least one polymer that is a cross-linked polymer. The electrically conducting polymer may a cross-linked polymer. The ionically conducting polymer may be a cross-linked polymer. Both the electrically conducting and the ionically conducting polymers may be cross-linked polymers. When the interpenetrating network comprises one or more further polymers the one or more further polymers may be cross-linked polymers. Preferably, the ionically conducting polymer is a cross-linked polymer. Use of a cross-linked polymer (e.g. a cross-linked ionically conducting polymer) advantageously leads to a more durable electrode with improved stability (e.g. cycling stability) and flexibility. Without wishing to be bound by theory, it is thought that these improved properties are observed because non-cross- linked polymers are more readily dissolvable and undergo structural disintegration more easily.

The energy storage device of the present invention comprises two electrodes which each comprise an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer. Wthout wishing to be bound by theory it is thought that the ionically conducting polymer generates an ion reservoir throughout the electrode, enabling the electrically conducting polymer even within the bulk of the material to be accessible for charge storage reactions. This strategy increases the material utilization efficiency of the electrically conducting polymer without employing complex synthesis methods or sacrificing mechanical stability. Moreover, the flexible framework of the ionically conducting polymer can accommodate the volumetric changes associated with ion intercalation/de-intercalation in the electrically conducting polymer, minimizing mechanical stress in the electrode (and reducing the propensity of the electrode to crack or break) and yielding excellent cycling stability. In a preferred energy storage device of the present invention, the ionically conducting polymer is a gel, preferably a solvent (e.g. water) swellable gel. Preferably, the ionically conducting polymer is a cross-linked polymer.

A preferred ionically conducting polymer comprises polyalkylene oxide (e.g. polyethylene oxide (PEO) or polypropylene oxide, preferably polyethylene oxide), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof. More preferably the ionically conducting polymer comprises polyethylene oxide (PEO).

The energy storage device of the present invention comprises an electrolyte. Preferably, the electrolyte comprises an ionically conducting polymer. The electrolyte preferably comprises polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof and more preferably polyethylene oxide (PEO).

The electrolyte may further comprise a salt. Preferably, the salt is selected from lithium chloride (LiCI), lithium perchlorate (LiCI0₄), lithium sulfate (Li₂S0₄), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF₆), sodium chloride (NaCI), sodium perchlorate (NaCI0₄), sodium sulfate (Na₂S0₄), sodium nitrate (NaN0₃), sodium hexafluorophosphate (NaPF₆), potassium chloride (KCI), potassium iodide (Kl), potassium perchlorate (KCI0₄), potassium sulfate (K₂S0₄), magnesium chloride (MgCI₂), magnesium perchlorate (Mg(CI0₄)₂), magnesium sulfate (MgS0₄), tetraethylammonium tetrafluoroborate (TEABF₄) and 1- ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄). The electrolyte may further comprise an acid (e.g. H₂S0₄).

The energy storage device of the present invention is preferably selected from a battery, a capacitor and a supercapacitor. More preferably, the energy storage device is a supercapacitor and still more preferably a solid-state supercapacitor.

The energy storage device (e.g. supercapacitor) of the present invention preferably has a capacitance of 10 to 2500 F/g, preferably 100 to 2500 F/g (e.g. 1000 to 2000 F/g) at a scan rate of 5 mV/s. The energy storage device (e.g. supercapacitor) of the present invention preferably has an energy density of 0.1 to 1000 Wh/kg, preferably 10 to 1000 Wh/kg. The energy storage device (e.g. supercapacitor) of the present invention preferably has a power density of 1 to 10000 W/kg, preferably 100 to 10000 W/kg. The energy storage device (e.g. supercapacitor) of the present invention preferably has a capacitance retention of 85 to 100%, preferably 90 to 100% and more preferably 95 to 100% after 3000 cycles at 10 A/g.

Preferably, in the energy storage device of the present invention the electrodes each have a specific capacitance of up to (e.g. less than) 2500 F/g, up to 800 F/g or up to 600 F/g, at a charging rate of 1 A/g. The specific capacitance may be measured using a three-electrode configuration as described in the examples section below. The electrodes preferably have a specific capacitance of greater than 50 F/g, more preferably of greater than 100 F/g and yet more preferably of greater than 150 F/g (e.g. greater than 200 F/g, or greater than 400 F/g), at a charging rate of 1 A/g. The electrodes may preferably each have a specific capacitance of 10 to 2500 F/g, more preferably 150 to 2500 F/g at a charging rate of 1 A/g. In one preferred energy storage device of the present invention, the electrodes each have a specific capacitance of 50 to 300 F/g and more preferably 100 to 300 F/g at a charging rate of 1 A/g.

Preferably, in the energy storage device of the present invention the electrodes each have a capacitance retention of 85 to 100%, preferably 90 to 100% and more preferably 95 to 100% (e.g. 98 to 100%) after 3000 cycles at 10 A/g. The electrodes preferably have 95 to 100%, preferably 98 to 100% and still more preferably about 100% coulombic efficiency over 3000 cycles at 10 A/g. Advantageously, high coulombic efficiency is characteristic of highly reversible (pseudo)capacitive processes and a lack of parasitic side reactions.

In the energy storage device of the present invention each electrode is in the form of a freestanding film. Each electrode is preferably in the form of a flexible freestanding film. Preferably, the electrodes can be bent, folded or rolled up without breaking and without significantly affecting the performance (e.g. the capacitance) of the electrodes or of the energy storage device. Preferably, the electrodes can be bent (e.g. repeatedly bent) to a 2 mm radius of curvature without breaking, more preferably to a 1.5 mm radius of curvature without breaking, and yet more preferably to a 1 mm radius of curvature without breaking. Still more preferably the electrodes can be bent to a 500 μηι radius of curvature without breaking and especially preferably to a 180 μηι radius of curvature without breaking. Preferably, the electrodes can be bent (e.g. repeatedly bent) to at least a 2 mm radius of curvature without breaking, more preferably to at least a 1.5 mm radius of curvature without breaking, and yet more preferably to at least a 1 mm radius of curvature without breaking. Still more preferably the electrodes can be bent to at least a 500 μηι radius of curvature without breaking and especially preferably to at least a 180 μηι radius of curvature without breaking. Preferably, the electrodes can be bent (e.g. repeatedly bent) to a 150 μηι to 2 mm radius of curvature without breaking, more preferably to a 150 μηι to 1.5 mm radius of curvature without breaking, and yet more preferably to a 150 μηι to 1 mm radius of curvature without breaking. Still more preferably the electrodes can be bent to a 150 μηι to 500 μηι radius of curvature without breaking and especially preferably to a 150 μηι to 200 μηι radius of curvature without breaking (e.g. to about a 180 μηι radius of curvature without breaking).

Preferably, the electrodes in the form of a freestanding film substantially retain their initial capacitance following multiple cycles of bending and unbending. A preferred energy storage device of the present invention therefore comprises electrodes that retain 90 to 100 % of their initial capacitance, more preferably 95 to 100 % of their initial capacitance and still more preferably 97 to 100 % (e.g. about 99 %) of their initial capacitance after 1000 cycles of bending/unbending, e.g. to a 1.5 mm radius of curvature.

In a preferred energy storage device of the present invention, the electrodes have a surface electrical conductivity of 10^"7 to 10⁴ S/cm, preferably 1 to 10⁴ S/cm, even more preferably 100 to 10⁴ S/cm. Preferably the electrodes have a surface electrical conductivity of 10 to 1000 S/cm (e.g. 10 to 500 S/cm), even more preferably 100 to 1000 S/cm (e.g. 150 to 500 S/cm).

In the energy storage device of the present invention, the electrodes may further comprise an additional active material. The additional active materials may act to enhance the properties (e.g. the energy storage properties) of the device. Preferably the additional active materials are selected from metal oxides (e.g. Mn0₂, Ru0₂, NiO/Ni(OH)₂ and Co₃0₄/Co(OH)2, preferably Mn0₂ and Ru0₂) and carbon. Preferably the electrodes do not comprise a binder. Preferably the electrodes do not comprise a substrate. Preferably the electrodes do not comprise a support. Substrates or supports can undesirably add inactive weight to the electrode and device.

In an alternative preferred energy storage device of the present invention, the electrodes do not further comprise an additional active material. In a particularly preferred energy storage device of the present invention, the electrodes consist of the interpenetrating network.

In the energy storage device of the present invention the electrodes are preferably prepared by a process comprising:

(i) mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials; and (ii) polymerising and/or cross-linking (preferably polymerising) the precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

Further preferred features are as described below in relation to the process of the present invention.

The energy storage device of the present invention may comprise further components in addition to the two electrodes and the electrolyte. The energy storage device may further comprise a current collector and preferably two current collectors.

More preferably, however, the electrodes themselves advantageously also serve as current collectors and so no separate current collectors are required. Thus in a preferred energy storage device of the present invention the energy storage device does not further comprise a current collector. In a particularly preferred energy storage device of the present invention, the energy storage device consists of the two electrodes and the electrolyte.

In a preferred energy storage device of the present invention, the two electrodes and the electrolyte are each separate bodies. Preferably the two electrodes and the electrolyte are each formed as a separate body. Thus the two electrodes are provided as two separate freestanding films and an electrolyte is also separately provided and positioned between the two electrodes.

In another preferred energy storage device of the present invention, the two electrodes and the electrolyte are formed as a single body. Preferably the two electrodes and the electrolyte are a single body (i.e. constitute a single body). Preferably the energy storage device of the present invention consists of a single body. In such a device, an interpenetrating network is formed on two opposing sides of an ionically conducting polymer, with bulk ionically conducting polymer or ionically conducting materials (not electrically conducting) in the middle serving as the electrolyte and the two interpenetrating networks serving as the two electrodes.

The present invention also provides a process for preparing an energy storage device as hereinbefore defined, comprising:

(i) preparing two electrodes each comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer; and (ii) providing an electrolyte between the two electrodes.

Preferred features of the energy storage device, electrodes, interpenetrating network, electrically conducting polymer, ionically conducting polymer and electrolyte in the process of the present invention are as hereinbefore defined in relation to the energy storage device of the invention.

The interpenetrating network can be fabricated using a simple, two-step synthesis to give freestanding film electrodes without the need for complex or costly synthesis methods such as compositing or nanostructuring.

In the process of the present invention the electrodes are preferably prepared by a process comprising:

(i) mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors (and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials); and

(ii) polymerising and/or cross-linking (preferably polymerising) the precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

The polymer precursors may be selected from monomers, oligomers and polymers, preferably from monomers and oligomers. Liquid-monomers and oligomers typically have low viscosity and so their mixing advantageously requires a relatively low amount of energy, force and time, making the process compatible with large-scale fabrication processes. Furthermore, advantageously no solvents are required to dissolve liquid monomer/oligomer precursors. In situ polymerisation, rather than simple mixing of ionically conducting and electrically conducting polymers, ensures the formation of a truly interpenetrating network wherein each of the electrically conducting polymer and the ionically conducting polymer form a continuous phase. Advantageously the synthesis method is generally applicable to the synthesis of interpenetrating network electrodes of many types of different electrically conducting and ionically conducting polymers without the use of surfactants, and without the need for the two polymers (the ionically conducting polymer and the electrically conducting polymer) to be soluble in the same solvent. The one or more ionically conducting polymer precursors may be polymerised and/or cross-linked to form the ionically conducting polymer before, at the same time, or after the one or more electrically conducting polymer precursors are polymerised and/or crosslinked to form the electrically conducting polymer. Preferably the one or more ionically conducting polymer precursors are polymerised before the one or more electrically conducting polymer precursors are polymerised to form the electrically conducting polymer. Preferably the one or more ionically conducting polymer precursors are cross-linked before the one or more electrically conducting polymer precursors are polymerised to form the electrically conducting polymer.

The precursors may be polymerised by any conventional polymerisation method. Preferred methods are free radical polymerisation (e.g. free radical co- polymerisation), photochemical polymerisation and chemical polymerisation (e.g. oxidative chemical polymerisation). Preferably polymerisation of the ionically conducting polymer precursors is carried out using free radical polymerisation, in which case an initiator (e.g. benzoin methyl ether or azobisisobutyronitrile) may preferably be incorporated into the polymer precursor mixture. Preferably polymerisation of the electrically conducting polymer precursors is carried out using oxidative chemical polymerisation (e.g. using aqueous FeCI₃ solution).

Preferred ionically conducting polymer precursors are selected from poly(ethylene glycol) methyl ether methacrylate (PEGM), poly(ethylene glycol) dimethacrylate (PEGDM), ethylene oxide, vinyl alcohol, acrylonitrile, acrylic acid, methyl methacrylate, vinyl carbonate, vinylidene fluoride, tetrafluoroethylene (TFE), a derivative of a perfluoro (alkyl vinyl ether) with sulfonyl acid fluoride, and acrylamide, and more preferably poly(ethylene glycol) methyl ether methacrylate (PEGM) and poly(ethylene glycol) dimethacrylate (PEGDM).

Preferably at least one ionically conducting polymer precursor (e.g. PEGM) which provides dangling chains within the interpenetrating network is used. Dangling chains provide free volume to facilitate swelling of the network with electrolyte. Preferably at least one cross-linkable ionically conducting polymer precursor (e.g. PEGDM) is used. The use of a cross-linkable ionically conducting polymer precursor provides mechanical integrity to the interpenetrating network. Thus the process of preparing the electrodes preferably further comprises cross-linking at least one of the polymers, preferably the ionically conducting polymer. The cross linking step may be carried out before, at the same time, or after the polymerisation step but is preferably carried out at the same time as the polymerisation step (e.g. at the same time as the polymerisation of the ionically conducting polymer precursors).

Preferred electrically conducting polymer precursors are selected from 3,4- ethylenedioxythiophene (EDOT), aniline, pyrrole, thiophene, acetylene and other polymerisable polycyclic aromatic hydrocarbons such as fluoranthene, and more preferably from 3,4-ethylenedioxythiophene (EDOT) and aniline.

Preferred further polymer precursors are selected from polymerisable acrylic monomers, alcohols, allyl monomers, amine monomers, anhydride monomers, bisphenol and sulfonyldiphenol monomers, carboxylic acid monomers, epoxide monomers, isocyanate monomers, norbornene monomers, silicone monomers, styrene and functionalized styrene monomers, vinyl esters, vinyl halides, and other vinyl monomers with desired function(s). Particularly preferred precursors are selected from carbamate esters (such as urethanes), terephthalates (such as bis(2- hydroxyethyl) terephthalate), lactone-based monomers (such as ε-caprolactone and lactic acid monomers), thiol-based monomers, butadiene, chloroprene, fluoroprene, dicarboxylic acid monomers, diamines and silicon-based organic monomers.

Preferred additional active materials are as hereinbefore defined.

The present invention also provides an energy storage device obtainable by a process as hereinbefore defined.

Thus, preferably the energy storage device is obtainable by a process comprising:

(i) preparing two electrodes each comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer by mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials;

(ii) polymerising and/or cross-linking (preferably polymerising) the precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer; and

(iii) providing an electrolyte between the two electrodes. The present invention also provides an energy storage device obtained by a process as hereinbefore defined.

Thus, preferably the energy storage device is obtained by a process comprising:

(iii) providing an electrolyte between the two electrodes.

Preferred features of the energy storage device, electrodes, interpenetrating network, electrically conducting polymer, ionically conducting polymer, ionically conducting polymer precursors, electrically conducting polymer precursors, further polymers, further polymer precursors, additional active materials and electrolyte in the process of the present invention are as hereinbefore described.

An interpenetrating network film electrode of desired thickness may be produced by flattening the mixture of one or more ionically conducting polymer precursors and one or more electrically conducting polymer precursors to a desired thickness prior to polymerising the precursors. For example, the mixture may be placed between two plates (e.g. two glass plates) and spacers placed between the edges of the plates to control the final thickness of the films. The mixture may then be polymerised and subsequently removed from between the plates to provide a freestanding film of desired thickness.

The present invention also provides a process for preparing an electrode comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, said process comprising: (i) mixing one or more ionically conducting polymer precursors with one or more electrically conducting polymer precursors and a solvent (and optionally one or more further polymers, one or more further polymer precursors and/or one or more additional active materials);

(ii) printing the mixture onto a substrate; and

Preferred features of the electrodes, interpenetrating network, electrically conducting polymer and ionically conducting polymer in the process of the present invention are as hereinbefore described.

The polymer precursors are preferably selected from monomers, oligomers and polymers, preferably from monomers and oligomers. Liquid-monomers and oligomers typically have low viscosity and so their mixing advantageously requires a relatively low amount of energy, force and time, making the process compatible with large- scale fabrication processes. In situ polymerisation, rather than simple mixing of ionically conducting and electrically conducting polymers, ensures the formation of a truly interpenetrating network wherein each of the electrically conducting polymer and the ionically conducting polymer form a continuous phase. Advantageously the synthesis method is generally applicable to the synthesis of interpenetrating network electrodes of many types of different electrically conducting and ionically conducting polymers without the use of surfactants.

The one or more ionically conducting polymer precursors is preferably polymerised and/or cross-linked to form the ionically conducting polymer before, at the same time, or after the one or more electrically conducting polymer precursors are polymerised and/or crosslinked to form the electrically conducting polymer. Preferably the one or more ionically conducting polymer precursors are polymerised before the one or more electrically conducting polymer precursors are polymerised to form the electrically conducting polymer. Preferably the one or more ionically conducting polymer precursors are cross-linked before the one or more electrically conducting polymer precursors are polymerised to form the electrically conducting polymer. The precursors may be polymerised by any conventional polymerisation method. Preferred methods are free radical polymerisation (e.g. free radical co- polymerisation), photochemical polymerisation and chemical polymerisation (e.g. oxidative chemical polymerisation). Preferably polymerisation of the ionically conducting polymer precursors is carried out using free radical polymerisation, in which case an initiator (e.g. benzoin methyl ether or azobisisobutyronitrile) may preferably be incorporated into the polymer precursor mixture. Preferably, the mixture formed in step (i) is exposed to polymerisation conditions as soon as it has been printed in step (ii) (e.g. via exposure to UV light from a UV lamp mounted on the printhead). Preferably, polymerisation of the electrically conducting polymer precursors is carried out using oxidative chemical polymerisation (e.g. using aqueous FeCI₃ solution). Preferably, polymerisation of the electrically conducting polymer is carried out whilst the mixture is still attached to, or in contact with, the substrate.

Preferred solvents have a low viscosity and good miscibility with the one or more ionically conducting polymer precursors and the one or more electrically conducting polymer precursors. Preferred solvents are selected from ethanol, methanol, isopropanol, toluene, hexane, chloroform and acetone. A particularly preferred solvent is ethanol. In preferred processes of the present invention, the mixture produced in step (i) comprises 1 to 40% solvent by volume, preferably 5 to 35%, more preferably 10 to 30% (e.g. 10%, 20% or 30% solvent by volume).

Preferably, the substrate is sheet material, a membrane material or a film-like material. More preferably, the substrate has a large surface area (e.g. for laboratory-based inkjet printing techniques, the surface area of the substrate is preferably 0.0001 m² to 1 m², more preferably 0.01 m² to 0.1 m²; for production-based roll-to-roll printing techniques, the substrate is preferably 0.1 m to 5 m wide, more preferably 0.5 m to 1.5 m wide, and has a continuous length). Preferred substrates are selected from glass (e.g. a glass slide), textiles, clothing, leather, paper, cellulose films, ionically conducting polymer films or membranes (e.g. polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof) and flexible plastic films (e.g. polyethylene terephthalate (PET) films, nylon films). A particularly preferred substrate is a glass slide.

Once prepared, the electrode is preferably used in conjunction with the substrate (i.e. it is not removed from the substrate before use). For example, the electrode is preferably printed directly onto clothing and used in situ. Alternatively, the electrode is preferably used independently of the substrate (i.e. it is removed from the substrate before use). In this case, the substrate should be pretreated to prevent strong adhesion of the electrode to the substrate (e.g. in the case of a glass slide substrate, the glass slide can be pre-treated with Rain X).

Preferred additional active materials are as hereinbefore defined.

The mixture prepared in step (i) may be printed onto the substrate in step (ii) using any suitable printing technique. Preferred techniques include inkjet printing, aerogel printing and roll-to-roll printing. In order to be suitable for printing, the mixture produced in step (i) preferably has a viscosity of 1 cP to 20 cP, more preferably a viscosity of 1 cP to 10 cP (e.g. 5 cP).

Where the substrate is an ionically conducting polymer film or membrane (e.g. polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof), an electrode can be printed onto each side of the substrate according to the process of the present invention in order to produce an energy storage device.

The present invention also provides a process for preparing an energy storage device, said process comprising:

(i) preparing an electrode according to the process hereinbefore described;

(v) printing the mixture onto said ionically conducting polymer layer; and

(vi) polymerising and/or cross-linking (preferably polymerising) said polymer precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

The present invention also provides the use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer to form an energy storage device as hereinbefore defined. The present invention also provides the use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer as hereinbefore defined, wherein the electrically conducting polymer is as hereinbefore defined. The present invention also provides the use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer as hereinbefore defined, wherein the ionically conducting polymer is as hereinbefore defined.

The energy storage device of the present application is suitable for use in a wide range of applications, devices and systems. Its superior energy storage capability, long-term stability, and low maintenance requirements make it suitable for a variety of applications, such as regenerative braking and power supply in hybrid vehicles, memory backup for portable electronics, and power buffering or load-leveling for industrial scale systems. The energy storage device is also advantageously lightweight and flexible, enabling its use in novel technologies such as wearable electronics, roll-up displays, or bio-implantable devices.

The present invention thus also provides a regenerative braking system comprising an energy storage device as hereinbefore defined. The present invention also provides a memory backup system, preferably for portable electronics, comprising an energy storage device as hereinbefore defined. The present invention also provides a power buffer comprising an energy storage device as hereinbefore defined. The present invention also provides a load-leveling device, preferably for an industrial scale system, comprising an energy storage device as hereinbefore defined.

The present invention also provides a wearable electronic device comprising an energy storage device as hereinbefore defined. The present invention also provides a roll-up display comprising an energy storage device as hereinbefore defined. The present invention also provides a bio-implantable device comprising an energy storage device as hereinbefore defined.

EXAMPLES

Materials

Poly(ethylene glycol) methyl ether methacrylate (PEGM, average Mn 500, Sigma- Aldrich)

Poly(ethylene glycol) dimethacrylate (PEGDM, average Mn 750, Sigma-Aldrich) Benzoin methyl ether (BME, 99%, Sigma-Aldrich) 3,4-ethylenedioxythiophene (EDOT, 99%, Acros Organics) FeCI3 (anhydrous, 98%, Alfa Aesar)

Polyvinylidene fluoride (PVDF, MW = 534,000 g/mol, Sigma-Aldrich) N-methylpyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%) Carbon black (CB, Korea Carbon Black Co, Korea) Aniline (99.8%, Acros Organics) Pyrrole (99+%, Sigma-Aldrich) Ethanol (99%, Fisher)

Tetraethylammonium tetrafluoroborate (TEABF₄, 99%, Sigma-Aldrich) Analysis methods Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on a FEI Nova NanoSEM™. SEM images were acquired using an accelerating voltage of 5.0 kV and a 2.5 nm spot size, while EDX data was collected with an accelerating voltage of 15 kV and a spot size of 4 nm. Samples for cross-sectional imaging were prepared by breaking the films after freezing in liquid nitrogen. All samples were imaged directly, without any additional conductive metal coating. Raman spectra were collected using a silicon-calibrated Renishaw Ramascope-1000 using a 633 nm laser excitation source. Thermogravimetric analysis (TGA, TA instruments Q500) was performed under nitrogen gas. Samples were heated from approximately 20 °C to 900 °C at a rate of 20 °C/min. Nitrogen adsorption isotherms were undertaken at 77 K using a MicroMeritics TriStar 3000 Porosimeter. Prior to the N2 adsorption test, all samples were evacuated for 1 hour at 100 °C under nitrogen flow. The electrical conductivity σ (S/cm) of the films was determined using measurements from a custom-built four-point probe based on the following equation:

where V is the measured voltage (V), I is the applied current (A), t is the film thickness (cm), and k is a correction factor based on the sample dimensions to account for edge effects (k had a value of 1.49 for the samples in this work). Film thickness was measured using a DEKTAK 6M profilometer. Compressive stress- strain curves were obtained using a thermomechanical analyzer (TMA, TA instruments Q400) at room temperature. Compression tests were conducted with a preload force of 0.1 N and force ramp rate of 0.1 N/m. Young's modulus values were determined using a linear regression over the elastic region of the stress-strain curve.

Cyclic voltammetry (CV), charge-discharge tests, and electrochemical impedance spectroscopy (EIS) were performed with a potentiostat/galvanostat (Ivium Stat XRi). Unless otherwise specified, all of these tests were carried out in a three-electrode cell in 1 M lithium perchlorate (LJCI04, Alfa Aesar, 98%) using a platinum foil counter electrode and Ag/AgCI reference electrode. The sIPN films were cut into pieces of approximately 5x3 mm² and connected to the external circuit using Ti foil clamps then submerged in solution.

PEDOT quantification in sIPN films The quantity of PEDOT within the sIPN films (M_PEDOT,finai) was evaluated by performing a simple mass balance on the reaction:

M PEDOT, final EDOT .initial - M PEDOT ,precip (2)

The value of M_EDOT,initiai is the mass of EDOT impregnated in the PEO matrix (Figure 3f), dictated by the initial EDOT content of the reagent mixture (Figure 3e). TGA was used to determine the precise weight percent of EDOT at this stage, as exemplified in Figure 2.

While the majority of this impregnated EDOT remained in the PEO matrix as it polymerised, some diffused into the FeCI₃(aq) solution over the course of the 24-hour polymerisation, forming PEDOT precipitates in solution. These precipitates were isolated using centrifugation, repeatedly rinsed with methanol to wash away all FeCI₃, and dried overnight in air; their final weight gave the value of M_PEDOT,precip to be used in the above equation. Note that any loss of precipitate in this centrifugation process would increase the calculated PEDOT concentration in the film; this quantification method should thus give conservative estimates of the specific capacitance of these materials.

Calculations for Electrochemical Characterization

In the three-electrode configuration, specific capacitance (C, F/g) was calculated from cyclic voltammetry data based on the following equation:

Q

C = (3)

2(E₂ - E m

where Q is the total voltammetric charge passed in the CV scan, Ei and E₂ are the lower and upper potential bounds (V), and m is the mass of the PEDOT in the electrode (g). The total charge Q is — , where /^' is the instantaneous current and v is the scan rate (V/s). The factor of twoin the denominator takes into account the charging that occurs in both the cathodic and anodic sweeps of the CV.

Charge-discharge test data was used to calculate specific capacitance according to:

I x At

C = (4)

AV x m I (A) is the constant charge/discharge current, At (s) is the discharge time, and AV = E₂ - E₁ is the electrochemical potential window. Both At and AV are analyzed after the initial voltage drop. The capacitance of the full symmetric device, C_D, was determined from CV tests based on:

where M is the combined mass of the PEDOT in both electrodes. The energy density (E, Wh/g) and power density (P, W/g) of the devices are calculated using:

C_DAV²

2 x 3600

3600 x E

Example 1 : Synthesis of PEDOT-PEO semi-Interpenetrating Network

Experimental:

To demonstrate the interpenetrating network concept, a poly(3- 4,ethylenedioxythiophene) (PEDOT) and a poly(ethylene oxide) (PEO) based network (Figure 1a) was used in this Example, a pair of polymers which has been studied extensively in the context of sIPNs for actuators, electrochromic devices and tactile sensors but not yet explored for supercapacitor applications. PEDOT, the electrically conducting pseudocapacitive component of the sIPN, has the advantages of chemical and thermal stability, biocompatibility, and a competitive theoretical specific capacitance of 210 F/g. PEDOT also possesses high conductivity (up to 4500 S/cm) and can be operated under a larger voltage window than most other conducting polymers. The other portion of the sIPN, a cross-linked PEO-based network, achieves high ionic conductivity from its ethylene oxide groups, which can coordinate metal cations from the electrolyte and enhance the polarity of the material, improving its ability to swell with aqueous electrolyte. This swelling is also facilitated by dangling chains within the matrix, which create free space to further improve ion mobility. The sIPN films (see Figures 1 b, c) were fabricated using a simple, two-step synthesis as follows and demonstrated dramatically improved specific capacitance, cycling stability, and mechanical properties relative to electrodes made from conventional neat PEDOT. The sIPNs were synthesized according to the process as illustrated in Figure 1d-g and discussed in detail below. In Figures 1f and 1g, black areas marked with white dots show the PEO-based network.

To synthesize the sIPN films, poly(ethylene glycol) methyl ether methacrylate was combined with poly(ethylene glycol) dimethacrylate in a 3:1 ratio. An appropriate quantity of 3,4-ethylenedioxythiophene (EDOT), was then added to the mixture; while a range of initial EDOT concentrations were used to produce different films, 50 wt. % EDOT was used to produce the main samples described in this work. Finally, benzoin methyl ether was added (approximately 2 wt. % with respect to the methacrylate oligomers).

To form flat films, a 100 μΙ_ droplet of this mixture was flattened between two glass plates (treated with a hydrophobic coating to facilitate the removal of the film later); plastic spacers were placed on the edges of the glass plates to control the final thickness of the films. The sample was then exposed to ultraviolet light (365 nm) for ten minutes to polymerise the methacrylate oligomers and obtain a poly(ethylene oxide) (PEO) based matrix. The two glass slides were then pulled apart and the film, which was at this point a freestanding clear gel, was submerged in a 1.5 M aqueous solution of FeCI₃ for 24 hours to polymerise the EDOT. The film was then removed and repeatedly rinsed with methanol until the FeCI₃ had been removed (when no more yellow color could be observed in the rinse). This rinsing step also removed unpolymerised EDOT, PEGM, and PEGDM. The samples were then dried at 80°C under vacuum. The final samples contained 61 wt. % PEDOT; detailed methods for this quantification process are described above.

The neat PEDOT used to benchmark the performance of the sIPNs was synthesized via chemical oxidative polymerisation (using a method as described in Li et al, Application of Ultrasonic Irradiation in Preparing Conducting Polymer as Active Materials for Supercapacitor, Mater. Lett. 2004, 59, 800-803). 75 μΙ_ EDOT was combined with 5 mL of 1.5 M FeCI₃ (aq) and stirred for 24 hours. The resulting dark blue powder was isolated using centrifugation, repeatedly rinsed free of FeCI₃ with methanol, and dried. Electrodes were created from this powder using a standard procedure (see Kim et al. Scalable Fabrication of Micron-Scale Graphene Nanomeshes for High-Performance Supercapacitor Applications, Energy Environ. Sci. 2016, 9, 1270-1281 ; and Zhang, J et al, Preparation of Vertically Aligned Carbon Nanotube/polyaniline Composite Membranes and the Flash Welding Effect on Their Supercapacitor Properties, RSC Adv. 2016, 6, 98598-98605) in which the PEDOT powder was combined with polyvinylidene fluoride dissolved in N-methylpyrrolidone and carbon black as a binder and conducting agent in an 8: 1 : 1 ratio. Approximately 1 mg of this slurry was uniformly casted on an etched titanium foil using a doctor blade then dried at 100 °C overnight.

Results

The PEGM:PEGDM ratio of 3:1 used here has been demonstrated in previous work to provide optimal ionic mobility (Festin, Net al, Robust Solid Polymer Electrolyte for Conducting IPN Actuators, Smart Mater. Struct. 2013, 22, 104005) as it allows for the maximum number of dangling chains within the matrix (provided by the PEGM) while maintaining the mechanical integrity enabled by cross-linking of the PEGDM. These dangling chains provide free volume to facilitate swelling of the gel with electrolyte; indeed, the area of these films increases by approximately 80% when wet. Raman spectroscopy of the films (Figure 3a) confirms the synthesis of a PEO-based material.

To interpenetrate PEDOT within the films, EDOT was incorporated into the initial PEGM/PEGDM reagent mixture. Crosslinking of the PEO network yielded EDOT- impregnated PEO gels. This EDOT was subsequently polymerised via chemical oxidative polymerisation by immersing the films in FeCI₃ solution, with FeCI₃ serving as both oxidant and dopant. Given that EDOT is insoluble in aqueous solution, water was chosen as the solvent for the FeCI₃ to minimize the migration of EDOT from the PEO gel.

The Raman spectrum of the sIPN film is overlaid with that of neat PEDOT in Figure 3b to demonstrate the successful synthesis of PEDOT. In particular, the peak at 1425 cm^"1 from symmetric Οα=Οβ(-0) stretching indicates the presence of a high degree of conjugation in the PEDOT. The formation of PEDOT was further characterised by quantifying the electrical conductivity of the film surface, which was measured to be 156 S/cm using a four-point probe. This conductivity is of the same order as many other PEDOT-based supercapacitor materials from the literature, despite the addition of the electrically insulating PEO matrix.

It should be cautioned, however, that these Raman spectroscopy and four-point probe measurements have only characterized the PEDOT at the surface of the sIPN. In order to ensure successful supercapacitor performance it is necessary to confirm the continuity of the PEDOT phase throughout the entire depth of the PEO matrix. In order to do so, the distribution of PEDOT throughout 130 μηι depth of the sIPN was therefore probed using energy-dispersive X-ray spectroscopy (EDX) mapping of the film cross-section: sulfur, which is present in PEDOT but not PEO, can be used to detect the distribution of PEDOT in the film. This analysis (Figure 3c) confirmed the presence of PEDOT throughout the entire film, although a decrease in the PEDOT content was observed in the center of the film. Correspondingly increased carbon and oxygen signals were observed in the film center (see Figure 3d, e). The accumulation of PEDOT on the film surface is also apparent in scanning electron microscope (SEM) images (Figure 3f), in which a relatively flat morphology with submicron PEDOT clusters were observed on the surface.

This process thus provides a two-step method to fabricate the sIPN electrodes, successfully achieving interconnectivity between the PEDOT and PEO-based matrix. In addition to its simplicity, the sIPN synthesis process is also highly tunable. The final PEDOT concentration in the films can be easily controlled, enabling a systematic optimization of the material's electrochemical and mechanical properties.

Data from galvanostatic charge-discharge and cyclic voltammetry (CV) tests (Figure 4a and Figure 4b, respectively) demonstrate that the sIPNs exhibit a high specific capacitance of 182 F/g at a charging rate of 1 A/g (158 F/g at 5 mV/s). This is among the highest reported capacitance values for a PEDOT-based pure-polymer supercapacitor electrode (see Table 1 below). The capacitance of the sIPN is especially impressive given its low surface area, inferred from its nitrogen adsorption isotherm, which indicates that the surface is nonporous (see Figure 5). Figure 5 shows a nitrogen adsorption isotherm for a representative sIPN sample. The sIPN films exhibit a Type II nitrogen adsorption isotherm, indicating that the surface is nonporous. Note that while the overall shape of this isotherm gives insight into the sIPNs' lack of porosity, the film surface area could not be accurately calculated due to the very low overall quantity of gas adsorbed.

Performance of PEDOT-based supercapacitors in the literature compared to the PEDOT-PEO film of the present invention

In the above table, the references materials are disclosed in the following papers:

(1) Bai, X et al, In Situ Polymerisation and Characterization of Grafted Poly (3,4- Ethylenedioxythiophene)/multiwalled Carbon Nanotubes Composite with High Electrochemical Performances, Electrochim. Acta 2013, 87, 394-400;

(2) Laforgue, A, All-Textile Flexible Supercapacitors Using Electrospun poly(3,4-Ethylenedioxythiophene) Nanofibers, J. Power Sources 201 1 , 196, 559- 564;

(3) Li, W. et al, Application of Ultrasonic Irradiation in Preparing Conducting Polymer as Active Materials for Supercapacitor, Mater. Lett. 2004, 59, 800-803; (4) Yang, Y. et al, Electrochemical Performance of Conducting Polymer and Its Nanocomposites Prepared by Chemical Vapor Phase Polymerisation Method, J. Mater. Sci. Mater. Electron. 2013, 24, 22452253;

(5) Pandey, G. P. et al, Solid-State Supercapacitors Based on Pulse Polymerised Poly(3,4-Ethylenedioxythiophene) Electrodes and Ionic Liquid Gel Polymer Electrolyte, J. Electrochem. Soc. 2012, 159, A1664-A1671 ;

(6) Zhao, Q et al, The Structure and Properties of PEDOT Synthesized by Template- Free Solution Method, Nanoscale Res. Lett. 2014, 9, 557;

(7) Tong, L et al, Vapor-Phase Polymerisation of poly(3,4-Ethylenedioxythiophene) (PEDOT) on Commercial Carbon Coated Aluminum Foil as Enhanced Electrodes for Supercapacitors, J. Power Sources 2015, 297, 195-201 ;

(8) Zhao, Q. et al, J. Binder-Free Porous PEDOT Electrodes for Flexible Supercapacitors, J. Appl. Polym. Sci. 2015, 132, 42549;

(9) D'Arcy et al, Vapor-Phase Polymerisation of Nanofibrillar Poly(3,4- Ethylenedioxythiophene) for Supercapacitors, ACS Nano 2014, 8, 1500-1510;

(10) Lee, S et al, A Facile Synthetic Route for Well Defined Multilayer Films of Graphene and PEDOT via an Electrochemical Method, J. Mater. Chem. 2012, 22, 1899- 1903; and

(1 1) Bai, X et al, 3D Flowerlike poly(3,4-Ethylenedioxythiophene) for High Electrochemical Capacitive Energy Storage, Electrochim. Acta 2013, 106, 219-225.

The ionic/electronic resistances, charge transfer properties, and capacitive behavior of the sIPNs can be probed further using electrochemical impedance spectroscopy (EIS). The Nyquist plot in Figure 4c shows that the sIPN electrodes exhibit an equivalent series resistance (ESR) value of 14.6 Ω, obtained by extrapolating the vertical portion of the plot to the x-axis. This ESR is only slightly higher than that of neat PEDOT samples despite the presence of approximately 40 wt. % PEO (see Figure 9), indicating good continuity of the PEDOT phase throughout the electrode. In the low frequency region, the vertical slope of the plot indicates nearly ideal capacitive behavior in the electrode. This is represented more quantitatively by the y-intercept of the Bode plot (Figure 4d); the phase angle approaches 72° at low frequencies, which is close to the 90° angle of an ideal capacitor. The Bode plot can also provide insight into the rate capability of the electrodes. The frequency value at a phase angle of -45° (f₀) gives the dielectric relaxation time constant (τ₀) of the system, the minimum charge/discharge time at which the electrode can be operated with at least 50% efficiency. For the sIPN electrode, f₀is 0.095 Hz, corresponding to a TO of 10.6 s. This value of ¾ which is comparable to the time constants for many other polymer-based supercapacitor materials, is most likely limited by the kinetics of ion diffusion within the PEO matrix as well as the charge transfer resistance of the pseudocapacitive charge storage processes.

To demonstrate the tunability of the electrode fabrication process, sIPNs were fabricated with a variety of PEDOT concentrations ranging from 4 wt. % to 61 wt. %. As illustrated by the data in Figures 6a and 6b, the specific capacitance of the sIPNs increases as PEDOT loading increases.

At lower concentrations, the electrode capacitance was most likely limited by poor continuity of the PEDOT phase. These samples consequently experienced increased electrical resistance, which introduced kinetic limitations that decreased specific capacitance. Indeed, the Nyquist plots in Figure 7a indicate that samples with lower PEDOT concentrations have both higher bulk resistance (the x-intercept of the plot, which reflects resistance from the electrolyte and internal resistance of the electrode) and higher charge-transfer resistance (indicated by the increased diameter of the RC semicircle). Furthermore, this trend is supported by the electrical conductivity of the film surfaces as measured by four-point probe (Figure 7b).

Based on these trends, it might in theory be expected that sIPN electrodes with even greater PEDOT content (above the 61 wt. % fabricated here) would have even greater specific capacitance; a PEDOT loading which yields maximum performance might be predicted, above which negligible improvements in ionic conductivity would be observed due to the low PEO content. However, samples with greater PEDOT concentration could not be fabricated due to the mechanical instability of the PEO matrix, which is required to produce the freestanding films. This issue appears in the first step of the synthesis, forming an EDOT-impregnated PEO matrix (Figure 1f). If the EDOT content in the original reagent mixture is too high, the PEO-based oligomers are too dilute to create a stable gel film. The resulting product is too mechanically weak to be transferred to the iron chloride solution used to polymerise the EDOT.

To explore the material utilization efficiency of the PEDOT in our sIPN films, the effect of film thickness on specific capacitance was studied. Ideally, if all PEDOT is fully utilized, constant specific capacitance should be expected regardless of film thickness - if this is the case, even PEDOT in the bulk of a thick film would behave with the favorable ion transport properties of a thin film. As can be seen from Figure 8, the sIPN electrodes prepared according to this Example follow this trend up to a thickness of approximately 130 μηι, at which point specific capacitance cannot be significantly increased by decreasing film thickness.

Inevitably, after some threshold thickness, decreases in specific capacitance were observed due to the slow kinetics of ion diffusion in the PEO gel relative to liquid electrolyte. However, the fact that these diffusion limitations were not observed in films of the present invention up to 130 m is impressive given that many electrode films reported in the literature are one to two orders of magnitude thinner. The formation of supercapacitor electrodes which can maintain both their specific capacitance and flexibility even when very thick is a crucial challenge in developing devices for practical applications.

In order to benchmark the sIPN performance against conventional materials, a neat PEDOT powder was synthesized via chemical oxidative polymerisation, choosing reaction conditions which aligned as closely as possible with those of the PEDOT in the sIPNs, including consistent reaction time and oxidant concentration. This neat PEDOT provides a useful performance benchmark, although one has to take into account differences in surface area as well as the presence of binder and conductive additive required to fabricate the powder-based electrodes. As illustrated by the data in Figure 10a and Figure 10b, the sIPN electrodes exhibited up to 73% increased specific capacitance relative to the neat PEDOT value of 105 F/g at 1 A/g (91 F/g at 5 mV/s). Moreover, the sIPNs maintained higher specific capacitance values than the neat PEDOT even at high charging/discharging rates (see Figure 10c).

These improvements in specific capacitance emerge from the morphology of the sIPN material, illustrated schematically in Figure 10d. The ionically conductive PEO matrix (shown as black areas marked with white dots; electrolyte ions are shown as grey dots) acts as an ion reservoir surrounding the PEDOT, greatly reducing ion diffusion distances throughout the electrode relative to conventional structures. This enables PEDOT even within the bulk of the electrodes to participate in charge storage, as demonstrated by the fact that the specific capacitance of our sIPNs remains relatively constant when increasing the film thickness from 50 μηι to 130 μηι (Figure 8). Fabrication of electrodes which can maintain their specific capacitance at these relatively high thicknesses is much more difficult for conventional polymer films (Figure 10e), where poor ionic conductivity limits the access of electrolyte ions (shown as grey dots) to a few tens of nanometers from the electrode surface (see Horng, Y.-Yet al, Flexible Supercapacitor Based on Polyaniline Nanowires/carbon Cloth with Both High Gravimetric and Area-Normalized Capacitance, J. Power Sources 2010, 795, 4418- 4422; and Simon, P et al, Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845854). In fact, the high-performance 130 μηι films that were prepared are one to two orders of magnitude thicker than many polymer-based electrodes reported in the literature.

The material morphology of the films of the invention that were prepared also greatly enhanced electrode cycling stability, another crucial performance metric which presents particular challenges for polymer-based materials. Many polymer-based supercapacitor electrodes suffer from poor long-term capacitance retention due to the volumetric changes caused by repeated ion intercalation/de-intercalation. The resulting mechanical stress on the material can lead to issues such as delamination, collapse of ion flow channels, or disordering/breakage of polymer chains. This process is schematically illustrated for conventional bulk polymer morphologies in Figure 10e. Indeed, the neat PEDOT control sample showed only 82% capacitance retention after 1 ,200 cycles; other PEDOT-based materials in the literature have shown even poorer cycling stability. In contrast, the sIPN structure of this Example lends itself to excellent stability, as it retained 97.5% of its initial specific capacitance after 3,000 cycles (Figure 10f). It is hypothesized that the flexible, crosslinked PEO network in the sIPN acts as a mechanical buffer to accommodate volumetric changes upon cycling and thus suppress mechanical stress damage within the electrode (see Figure 10d). Improved stability through similar mechanical buffering effects has been observed for a variety of composite polymer electrodes, with structures such as carbon nanotube networks, hydrogels, or graphene oxide sheets providing flexible or open structures to minimize the negative effects of repeated swelling/shrinking. The sIPN electrode also maintained effectively 100% Coulombic efficiency over the 3,000 cycles (Figure 11), characteristic of highly reversible (pseudo)capacitive processes and a lack of parasitic side reactions.

In addition to improved specific capacitance and cycling stability, the sIPN electrodes surpassed the neat PEDOT samples in their mechanical properties as well. The sIPNs were formed as freestanding films, eliminating the need for additional binders or substrates which can decrease specific capacitance by adding inactive weight to the electrode. Furthermore, the sIPN films are flexible, as pictured in Figure 1 c and Figure 12a. The 130 μηι thick films can be bent to radii of curvature below 200 μηι without breaking. Even after 1 ,000 cycles of bending/unbending, the CV profile of the electrode remains essentially unchanged (Figure 12b), retaining 99% of its initial capacitance.

Further characterization of the sIPN mechanical properties can be found in Figure 14. Based on the data of Figure 13, the Young's modulus of the PEO-based network used as a framework for the sIPN is 13.2 MPa; this is highly consistent with the value of 1 1 MPa reported for similar PEO networks reported in the literature. In contrast, the sIPN exhibits more robust mechanical properties, with a Young's modulus of 60.1 MPa. Thus, as expected, we observe that the interpenetrated PEDOT/PEO sIPN yields intermediate mechanical properties between those of neat PEO and neat PEDOT (the latter of which typically has a Young's modulus near 2 GPa).

The PEDOT/PEO sIPN electrodes of the present invention have also been tested in organic electrolytes and found to have a large working potential window. Figure 15 shows full-cell cyclic voltammetry tests for the PEDOT/PEO sIPN electrodes of the present invention in 1 M TEABF₄ in acetonitrile. In these tests, which were carried out in a coin cell, the working electrode and the counter electrode are both PEDOT/PEO sIPN electrodes of the present invention. The results show that the sIPN material can work in the potential window of at least 0-2.7 V in the organic electrolyte, indicating the sIPNs have a potential application in supercapacitors or as a binding material in a commercial supercapacitor.

Development of Full Supercapacitor Devices from sIPN Electrodes

As a preliminary investigation of the potential applicability of these electrodes, a full solid-state supercapacitor device was fabricated using two identical pieces of the sIPN and a PEO-based gel as the electrolyte (see the illustration of this electrode at Figure 14a). Based on cyclic voltammetry data (Figure 14b), the device capacitance reached 28.8 F/g at a scan rate of 5 mV/s, corresponding to an energy density of 3.2 Wh/kg at a power density of 64.8 W/kg. Two devices in series proved sufficiently powerful to light a LED (Figure 14c). This proof-of-concept demonstration suggests the potential of sIPN-based electrodes to be utilized for novel applications such as wearable or implantable electronics.

These experiments taken together demonstrate that interpenetrating ionically and electrically conducting polymers is a successful strategy to improve the performance of supercapacitor electrodes. The semi-interpenetrating polymer network (sIPN) structure optimises the accessibility of the entire pseudocapacitive polymer to electrolyte ions, resulting in specific capacitance over 70% greater than that of neat PEDOT powder. Furthermore, the robust mechanical structure of the PEO phase confines volumetric changes in the PEDOT upon cycling, minimizing mechanical stress on the electrode to yield drastically improved cycling stability. Finally, the resulting tough and flexible materials (bendable to < 200 μηι radius of curvature) are promising for wearable electronics and other flexible technologies. This sIPN approach has the potential to be generalized as a framework to improve the performance of a myriad of polymer-based energy storage materials.

Example 2a: Development of PANI/PEO sIPN Supercapacitor Electrodes

In this example PEDOT, the electrically conducting polymer of the sIPN of Example 1 was substituted with polyaniline (PANI). PANI has been widely-studied as a pseudocapacitive material and is attractive for its high theoretical specific capacitance (over 3.5 times greater than that of PEDOT).

PANI/PEO sIPN electrodes were prepared in a method analogous to the PEDOT/PEO electrodes. As before, poly(ethylene glycol) methyl ether methacrylate (PEGM) and poly(ethylene glycol) dimethacrylate (PEGDM) were combined in a 3:1 ratio. 50 wt% aniline was then added to the mixture. Benzoin methyl ether (BME) was then added (approximately 2 wt. % with respect to the methacrylate oligomers).

To polymerise the PEO network into a flat, freestanding film, a 200 μΙ_ droplet of the reagent mixture was flattened between two glass plates, which were separated by plastic spacers to control the final thickness of the film. The sample was then exposed to ultraviolet light (365 nm) for ten minutes to polymerise the methacrylate oligomers and obtain a PEO based matrix. The two glass slides were then pulled apart to yield a freestanding clear gel.

Aniline polymerisation was carried out by submerging the film in a solution of ammonium persulfate (APS) in 1 M hydrochloric acid; the ratio of aniline to APS in the solution was fixed at 1 :1. The reaction was carried out at approximately 2°C in a refrigerator. After 24 hours, the film was removed and repeatedly rinsed in water then dried in air. In summary, this synthesis differs from that of the PEDOT/PEO electrodes of Example 1 as follows:

• Aniline was impregnated into the PEO network (instead of EDOT)

• The polymerisation of aniline was carried out under different conditions (different solvent, dopant, etc.)

The electrochemical performance of the resulting PANI/PEO sIPN electrodes was carried out using cyclic voltammetry, with typical results shown in Figure 16. The half- cell tests shown in Figure 16 were carried out in a three-electrode cell in 1 M H₂S0₄ (Sigma-Aldrich, 95%+) aqueous solution using a platinum foil counter electrode and a Ag/AgCI reference electrode. The working electrode was a PANI/PEO sIPN electrode of the present invention.

Example 2b: Development of PPy/PEO sIPN Supercapacitor Electrodes

In this example PEDOT, the electrically conducting polymer of the sIPN of Example 1 was substituted with polypyrrole (PPy).

PPy/PEO sIPN electrodes were prepared in a method analogous to the PEDOT/PEO electrodes. As before, poly(ethylene glycol) methyl ether methacrylate (PEGM) and poly(ethylene glycol) di methacrylate (PEGDM) were combined in a 3: 1 ratio. Approx. 50 wt% pyrrole was then added to the mixture. Benzoin methyl ether (BME) was then added (approximately 2 wt. % with respect to the methacrylate oligomers).

To polymerise the PEO network into a flat, freestanding film, a 100 μΙ_ droplet of the reagent mixture was flattened between two glass plates (themselves treated with a hydrophobic coating to facilitate the later removal of the film), which were separated by plastic spacers to control the final thickness of the film. The sample was then exposed to ultraviolet light (365 nm) for ten minutes to polymerise the methacrylate oligomers and obtain a PEO based matrix. The two glass slides were then pulled apart to yield a freestanding clear gel.

Pyrrole polymerisation was carried out by submerging the film in a 1.5 M aqueous solution of FeCI₃ for 24 hours. After 24 hours, the film was removed and repeatedly rinsed in methanol until all FeCI₃ had been removed (i.e. when the rinse liquid no longer appeared yellow). The rinsing step also removed unpolymerized pyrrole, PEGM and PEGDM. The samples were then dried at 80 °C under vacuum. In summary, this synthesis differs from that of the PEDOT/PEO electrodes of Example 1 in that pyrrole was impregnated into the PEO network (instead of EDOT).

Example 3: Development of Full Supercapacitor Devices from Modified sIPN Structure

This section describes a novel approach for the fabrication of solid-state supercapacitor devices based on our PEDOT/PEO sIPN. In many solid-state devices, lack of effective contact between the gel electrolyte and electrodes can limit performance; not only does poor contact introduce ion transport resistance at the electrode-electrolyte interface, but it can also result in mechanical issues such as delamination. These issues could potentially be avoided by forming the electrodes and electrolyte not as three distinct layers but as a single body. To develop such a device, the sIPN electrode structure described above was modified such that the PEDOT concentration in the film is highly non-homogenous. If PEDOT levels are very high on the film surfaces but nearly zero in the middle (such that there is no electrical contact between the upper and lower surfaces of the film), the top and bottom of the film can serve as the two supercapacitor electrodes while the middle layer containing only PEO is the separator and gel electrolyte. This would effectively eliminate any electrode-electrolyte interfacial resistance, as the two components would be part of the same continuous PEO-based matrix.

To synthesize these devices, first a PEO network was fabricated. PEGM and PEDGM were combined in a 3: 1 ratio with approximately 2 wt. % BME. This mixture was polymerised between two glass slides (as described above) for ten minutes under exposure to 365 nm ultraviolet light, producing freestanding gel films. Note that these PEO films were made approximately 2-3 times thicker than those produced for the original PEDOT/PEO electrodes of Example 1 (which were 130 μηι thick). This PEO film was then submerged in pure liquid EDOT for 24 hours. The EDOT in the film was then polymerised according to the procedure as described in Example 1 , namely submersion in 1.5 M FeCI₃ for 24 hours, followed by rinsing in methanol and drying.

In summary, this synthesis differs from that of the PEDOT/PEO electrodes of Example 1 as follows: • Rather than impregnating EDOT into the PEO by incorporating it directly into the PEGM/PEGDM precursor mixture, EDOT is incorporated after polymerisation of the PEO network via swelling. This resulted in much higher concentrations of PEDOT accumulating at the film surfaces than in the centre after polymerisation of the EDOT.

• The films are 2-3 times thicker than the previous samples; this helps to ensure that there is no electrical contact between the top and bottom surfaces of the film.

Prior to electrochemical testing, the devices were submerged in 1 M LiCI0₄ to introduce the electrolytic salt to the gel electrolyte. The devices were then tested using a standard two-electrode setup.

We have confirmed that this synthesis yields a trilayer device, in which the two electrodes are not electrically connected, was manufactured by using a multimeter to test the electrical resistance between the top and bottom of the film. Cyclic voltammetry data for the PEDOT/PEO full devices is shown in Figure 17.

It is further envisaged that the full supercapacitor devices described above could be prepared using printing techniques.

Example 4: Synthesis of PEDOT-PEO semi-Interpenetrating Network using printing techniques

This section describes the synthesis of a PEDOT-PEO sIPN electrode using an alternative method to that described in Example 1. It has been found that the mixture of polymer precursors can be adapted to printing techniques (e.g. inkjet printing, aerogel printing or roll-to-roll printing etc.) by adding a solvent. It is thought that the solvent works to reduce the viscosity of the mixture of polymer precursors thereby enabling it to be used as an ink that can be adapted to printing technologies. Furthermore, the addition of a solvent has little influence on the polymerisation and/or cross-linking process involved in the preparation of the electrodes.

Optimisation of ink composition

Poly(ethylene glycol) methyl ether methacrylate (PEGM) and poly(ethylene glycol) dimethacrylate (PEGDM) were combined in a 3:1 ratio. 50 wt% 3,4- ethylenedioxythiophene (EDOT), was then added to the mixture. Benzoin methyl ether (BME) was then added (approximately 2 wt. % with respect to the methacrylate oligomers). The prepared precursor was then diluted with ethanol to produce separate samples containing 90%, 80% and 70% precursor by volume. A comparative sample containing 100% precursor by volume (i.e. where no ethanol was present) was also prepared.

Viscosity is a measure of the internal friction of a fluid in motion. Two mathematical representations of viscosity exist: dynamic and kinematic. The dynamic viscosity is the ratio of the measured shear stress of a fluid at a given shear rate. The kinematic viscosity normalizes the dynamic viscosity to the density of the fluid. Equations (8) and (9) are the traditional mathematical expressions of dynamic (μ) and kinematic viscosity (v), respectively. μ = - ^τ [Pa s] (8)

ν = - ^μ [m² s ¹] (9)

Dynamic viscosity measurements were taken using a Brookfield DV-II + Pro EXTRA Viscometer outfitted for small samples. The spindle used was a link-hanging SC4-21. The rheometer was programed to measure the shear stress at shear rates between 20 rotations per minute and 160 rotations per minute, in 20 rotations per minute intervals. At each shear rate, the shear stress was allowed to stabilize over a period of 10 seconds before measurement, then a viscosity was calculated using a computer program.

In order to optimize the prepolymer for inkjet printing, dynamic viscosity measurements were taken. The viscosity of each prepared sample of prepolymer was measured over the same range of shear rates. The results are shown in Figures 18 and 19 where, for example, the label "70% prepolymer" describes a sample containing 30% ethanol by volume and 70% prepolymer by volume. It has been found that the samples having a lower prepolymer concentration (e.g. 70% or 80% prepolymer) and a viscosity of < 10 cP should be suitable for printing. Whilst samples having an even lower prepolymer concentration could also potentially be used in the printing method, it is important to ensure that the samples are not over-diluted as this may cause deterioration in sIPN properties and/or have a negative impact on the coverage/structural integrity of the film after evaporation of ethanol. Inkjet printing general protocol

To prepare the polymer electrodes, a Domino drop on demand nozzle was used in conjunction with a pen plotter. For this type of printer, droplet parameters are controlled by the opening time of the printing nozzle and the pressure applied to the liquid in the nozzle by a stream of compressed air. The diluted prepolymer (at, for example, a volumetric concentration of 70% or 80% prepolymer) was placed in a well prior to the printhead. One pass (e.g. an area of 20 mm x 20 mm) was printed on glass slides pretreated with Rain X. While printing the diluted mixture onto the glass slides, the printed part was exposed to UV light (365 nm) to polymerise the methacrylate oligomers (e.g. using a UV lamp mounted to the printhead). EDOT polymerisation was then carried out by soaking the film in a 1 .5 M aqueous solution of aqueous FeCI₃ for 24 hours. After 24 hours, the film was removed and repeatedly rinsed in methanol until all FeCI₃ had been removed (i.e. when the rinse liquid no longer appeared yellow). The rinsing step also removed unpolymerized EDOT, PEGM and PEGDM. The samples were then dried at 80 °C under vacuum. After the polymerisation and cross-linking processes, the electrical conductivity of the polymerised materials/ink was found to be around 1 S/cm.

The preparation of the sIPN electrodes using the printing technique is both easy and fast, and also allows for the possibility of making patterned polymer films (e.g. in the form of a grid etc.). Furthermore, when the method involves roll-to-roll printing, it is envisaged that the sIPN-based electrode and/or device could be deposited evenly on top of various materials, e.g. textiles, clothing, leathers, papers, ionically conducting polymer films or membranes (e.g. polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof) flexible plastic films etc. (provided the sIPN is able to be connected to the power sources and devices to be powered).

Claims

CLAIMS:

1. An energy storage device comprising: two electrodes; and an electrolyte, wherein each of said two electrodes comprises an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, and wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, and said two electrodes are each in the form of a freestanding film.

2. An energy storage device as claimed in claim 1 , wherein said interpenetrating network is a fully interpenetrating network or a semi-interpenetrating network, preferably a semi-interpenetrating network.

3. An energy storage device as claimed in claim 1 or claim 2, wherein said interpenetrating network comprises 10 to 90 wt%, preferably 50 to 65 wt%, of said electrically conducting polymer, based on the total weight of the interpenetrating network.

4. An energy storage device as claimed in any one of claims 1 to 3, wherein said interpenetrating network comprises 10 to 90 wt%, preferably 35 to 50 wt%, of said ionically conducting polymer, based on the total weight of the interpenetrating network.

5. An energy storage device as claimed in any one of claims 1 to 4, wherein said interpenetrating network extends throughout the whole thickness of each electrode.

6. An energy storage device as claimed in any one of claims 1 to 5, wherein said interpenetrating network further comprises one or more further polymers, preferably one or more functional polymers selected from rubber, shape-memory polymers, epoxy resins, self-healing polymers and mixtures thereof.

7. An energy storage device as claimed in any one of claims 1 to 6, wherein said electrically conducting polymer comprises repeat units comprising substituted or unsubstituted thiophene or derivatives thereof, substituted or unsubstituted aniline or derivatives thereof, substituted or unsubstituted pyrrole or derivatives thereof, substituted or unsubstituted vinylene or derivatives thereof, substituted or unsubstituted phenylene or derivatives thereof, substituted or unsubstituted polycyclic aromatic hydrocarbons (e.g. fluoranthene) or derivatives thereof, or mixtures thereof.

8. An energy storage device as claimed in any one of claims 1 to 7, wherein said electrically conducting polymer is selected from poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), polyaniline (PANI), polythiophene (PT), poly(p- phenylene), poly(p-phenylene vinylene), poly(3-vinylperylene), polyaniline-polypyrrole (PANI-PPy), polyacetylene and mixtures thereof, preferably from poly(3,4- ethylenedioxythiophene) (PEDOT), polypyrrole (PPy) and polyaniline (PANI).

9. An energy storage device as claimed in any one of claims 1 to 8, wherein said electrically conducting polymer has a theoretical specific capacitance of 100 to 2000 F/g, preferably 200 to 800 F/g.

10. An energy storage device as claimed in any one of claims 1 to 9, wherein said ionically conducting polymer is a gel, preferably a solvent swellable gel.

1 1. An energy storage device as claimed in any one of claims 1 to 10, wherein said ionically conducting polymer is a cross-linked polymer.

12. An energy storage device as claimed in any one of claims 1 to 1 1 , wherein said ionically conducting polymer comprises polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof, preferably polyethylene oxide (PEO).

13. An energy storage device as claimed in any one of claims 1 to 12, wherein said electrolyte comprises an ionically conducting polymer, and preferably comprises polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC)), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof.

14. An energy storage device as claimed in any one of claims 1 to 13, wherein said electrolyte further comprises a salt, preferably selected from lithium chloride (LiCI), lithium perchlorate (LiCI0₄), lithium sulfate (Li₂S0₄), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium hexafluorophosphate (LiPF₆), sodium chloride (NaCI), sodium perchlorate (NaCI0₄), sodium sulfate (Na₂S0₄), sodium nitrate (NaN0₃), sodium hexafluorophosphate (NaPF₆), potassium chloride (KCI), potassium iodide (Kl), potassium perchlorate (KCI0₄), potassium sulfate (K₂S0₄), magnesium chloride (MgCI₂), magnesium perchlorate (Mg(CI0₄)₂), magnesium sulfate (MgS0₄), tetraethylammonium tetrafluoroborate (TEABF₄) and 1- ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄).

15. An energy storage device as claimed in any one of claims 1 to 14, wherein said electrolyte further comprises an acid (e.g. H₂S0₄).

16. An energy storage device as claimed in any one of claims 1 to 15, wherein said energy storage device is selected from a battery, a capacitor and a supercapacitor, preferably a supercapacitor.

17. An energy storage device as claimed in any one of claims 1 to 16, wherein said energy storage device has a capacitance of 10 to 2500 F/g, preferably 100 to 2500 F/g at a scan rate of 5 mV/s.

18. An energy storage device as claimed in any one of claims 1 to 17, wherein said electrodes each have a thickness of 10 μηι to 3 mm, preferably of 40 μηι to 500 μηι.

19. An energy storage device as claimed in any one of claims 1 to 18, wherein said electrodes can be bent to a 150 μηι to 500 μηι radius of curvature without breaking and especially preferably to a 150 μηι to 200 μηι (e.g. about 180 μηι) radius of curvature without breaking.

20. An energy storage device as claimed in any one of claims 1 to 19, wherein said electrodes retain 99% of their initial capacitance after 1000 cycles of bending/unbending to a 1.5 mm radius of curvature.

21. An energy storage device as claimed in any one of claims 1 to 20, wherein said two electrodes and said electrolyte are each separate bodies.

22. An energy storage device as claimed in any one of claims 1 to 21 , wherein said two electrodes and said electrolyte are formed as a single body.

23. A process for preparing an energy storage device according to any one of claims 1 to 22 comprising:

(ii) providing an electrolyte between said two electrodes.

24. A process as claimed in claim 23, wherein said electrodes are prepared by a process comprising:

(ii) polymerising and/or cross-linking (preferably polymerising) said polymer precursors to produce an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer.

25. A process as claimed in claim 24, wherein said polymer precursors are selected from monomers, oligomers and polymers, preferably from monomers and oligomers.

26. An energy storage device obtainable by a process as claimed in any one of claims 23 to 25.

27. An energy storage device obtained by a process as claimed in any one of claims 23 to 25.

28. Use of an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer to form an energy storage device according to any one of claims 1 to 22, 26 and 27.

29. A regenerative braking system comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

30. A memory backup system, preferably for portable electronics, comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

31. A power buffer comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

32. A load-leveling device, preferably for an industrial scale system, comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

33. A wearable electronic device comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

34. A roll-up display comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

35. A bio-implantable device comprising an energy storage device as claimed in any one of claims 1 to 22, 26 and 27.

36. A process for preparing an electrode comprising an interpenetrating network of an electrically conducting polymer and an ionically conducting polymer, wherein in said interpenetrating network each of said electrically conducting polymer and said ionically conducting polymer forms a continuous phase, said process comprising:

(ii) printing the mixture onto a substrate; and

37. A process as claimed in claim 36, wherein said polymer precursors are selected from monomers, oligomers and polymers, preferably from monomers and oligomers.

38. A process as claimed in claim 36 or claim 37, wherein the solvent is selected from ethanol, methanol, isopropanol, toluene, hexane chloroform and acetone, and preferably ethanol.

39. A process as claimed in any one of claims 36 to 38, wherein the mixture produced in step (i) has a viscosity of 1 cP to 20 cP, more preferably a viscosity of 1 cP to 10 cP.

40. A process as claimed in any one of claims 36 to 39, wherein the mixture produced in step (i) comprises 1 to 40% solvent by volume.

41. A process as claimed in any one of claims 36 to 40, wherein the substrate is selected from glass (e.g. a glass slide), textiles, clothing, leather, paper, cellulose films, ionically conducting films or membranes (e.g. polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyacrylic acid (PAA), poly(methyl methacrylate) (PMMA), polyvinyl carbonate (PVC), polyvinylidene fluoride (PVDF), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) and chemically modified derivatives thereof, polyacrylamide (PAM), or mixtures thereof) and flexible plastic films (e.g. polyethylene terephthalate (PET) films, nylon films).

42. A process as claimed in any one of claims 36 to 41 , wherein the mixture is printed in step (ii) using an inkjet printing technique, an aerogel printing technique or a roll-to-roll printing technique.

43. A process as claimed in any one of claims 36 to 42, wherein the mixture is printed in step (ii) in the form of a pattern (e.g. a grid).

44. A process as claimed in any one of claims 36 to 43, wherein the mixture formed in step (i) is exposed to polymerisation conditions as soon as it has been printed in step (ii).

45. A process as claimed in any one of claims 36 to 44, wherein polymerisation of the ionically conducting polymer precursors is carried out using free radical polymerisation (e.g. using UV light).

46. A process as claimed in any one of claims 36 to 45, wherein polymerisation of the electrically conducting polymer precursors is carried out using oxidative chemical polymerisation (e.g. using FeCI₃).

47. A process as claimed in any one of claims 36 to 46, wherein the one or more ionically conducting polymer precursors are polymerised and/or cross-linked to form the ionically conducting polymer before the one or more electrically conducting poymers are polymerised and/or cross-linked to form the electrically conducting polymer.

48. A process for preparing an energy storage device, said process comprising:

(i) preparing an electrode according to the process of any one of claims 36 to 47;

(v) printing the mixture onto said ionically conducting polymer layer; and