WO2021198974A1

WO2021198974A1 - Carbon electrodes having improved electrocatalytic activity

Info

Publication number: WO2021198974A1
Application number: PCT/IB2021/052733
Authority: WO
Inventors: Sebastiano BELLANI; Francesco Bonaccorso; Leyla Najafi; Vittorio Pellegrini; Mirko PRATO
Original assignee: Fondazione Istituto Italiano Di Tecnologia
Priority date: 2020-04-03
Filing date: 2021-04-01
Publication date: 2021-10-07
Also published as: EP4128394A1; IT202000007102A1

Abstract

The present invention relates to new hierarchical carbon electrodes having improved electrocatalytic activity, useful as electrodes for electrocatalytic devices, such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems and in particular redox flow batteries; to the process for the production thereof and to the electrocatalytic devices that comprise them.

Description

CARBON ELECTRODES

HAVING IMPROVED ELECTROCATALYTIC ACTIVITY

Field of the invention

The present invention relates to the field of batteries and, in particular, it relates to hierarchical carbon electrodes having improved catalytic activity, useful as electrodes for electrocatalytic devices such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems (electrolyzers) and, in particular, for redox flow batteries ( RFB ). The invention further relates to the process for the production of the aforesaid electrodes, and to the electrocatalytic devices that comprise them. Background of the invention

Redox flow batteries are now one of the most promising technologies for large scale energy storage. Unlike batteries contained in a single container, RFBs store their energy in the electrolyte with redox-active materials that fill external tanks. The electrolyte flows from these tanks towards the active surfaces of the electrodes, where oxidation-reduction reactions take place, possibly more quickly compared to metal-ion batteries (e.g. lithium Li, sodium Na and potassium K batteries, etc.). Accordingly, the total capacity of RFBs can be adapted to the industrial scale application by simply expanding the volume of the external tanks regardless of the power characteristics, which are defined by the dimensions and the number of cells in a module unit. Simultaneously, the power of the RFBs is defined by the sizing of their electrodes.

In order to encourage the entry onto the market of RFB technology, it is however necessary to tackle the need for operating capacity with high charge/discharge current density with high energy efficiency (EE) (e.g. a current density of ~ 100 mA cm^-2 and EE> 80%) [1] In fact, efficient high power density operation can reduce the size of the stacking of cells, thus reducing the cost of an RFB system [2] Therefore, one of the hottest research topics in the sector is that of developing materials for electrodes having: 1) high electrical conductibility, which limits ohmic polarization [3]; 2) electrocatalytic activity towards redox reactions in RFB and 3) hydrophobicity, which provides optimal electrochemical accessibility for redox-active materials. Graphitic materials, in particular graphite felts (GF), are commonly used as electrodes both in commercial RFBs based on vanadium (i.e. VRFB, “Vanadium Redox-Flow Batteries”) and in RFBs based on zinc, thanks to the low production costs and the excellent electrical conductibility, electrochemical stability and porosity. However, unsatisfactory electrochemical activity towards redox reactions of RFBs [4], and the small surface area (<1 m² g^_1) [5] severely limit the voltage efficiency, and therefore the total energy efficiency of RFBs. Furthermore, the hydrophobic nature of graphite-based materials hinders the electrolytic access to the surface of the electrode in aqueous RFBs including VRFBs and RFBs based on zinc, such as so-called “Zinc- Iodine Redox-Flow Batteries” (ZIRFB).

To solve these technical problems, different treatments of graphite electrodes have been proposed to date. One of these is plasma treatment, which enables the activation and functionalization of the surface of the materials that constitute the electrodes, and also its change from hydrophilic to hydrophobic nature. Plasma treatments can be performed in general with various gases, e.g. O2, water vapour, (compressed) air, reducing gases such as H2, or N2; gaseous noble metals such as Ar, He, Xe, Ne, and Kr; NH₃; fluorinated gases such as CF₄ and SFe; and other gases such as CO2 and ethylene.

It is well known that plasma treatment with O2 creates reactive species, e.g. O₃, O radicals, and ionic species such as 0⁺, which can react with carbon surfaces. For example, on graphitized surfaces these species create O based functionalities (e.g., phenol (C-OH), carboxyl (C=0), carbonyl (C-O-C) groups and aliphatic hydrocarbons) [7] These functional groups have been shown to be catalytically active for redox reactions in VRFB batteries. Furthermore, surface changes to carbon materials can also take place during plasma treatments with O2, as a consequence of losses of C caused by the development of CO and/or CO2 [8] Plasma treatments with O2 are also effective for cleaning the materials of the carbon electrodes from organic contaminations.

Likewise, plasma treatments with N2 create N atoms and radicals that form nitrogenous functionalities on the carbon surfaces [9], e.g. by introducing C-N bonds on graphitic surfaces [10], thus introducing pyridinic-N, pyrrolic-N, quaternary N, N-oxides of pyridinic-N and aminic N (more rarely graphitic N) [11] These functionalities have been shown to be catalytically active for the redox reactions involved in VRFB batteries [12] The valency of the N atoms with its 5 electrons contributes with a further charge to the bond of the graphene layers, improving the conductivity of carbon materials. Furthermore, it can create structural defects, e.g. unsaturated C atoms, which react with the O2 present in the material of the electrode or with the O2 in the environment

[1 U

The article by Huang et al. “N, O Co-doped carbon felt for high-performance all vanadium redox flow battery”, International Journal of Hydrogen Energy (2017) describes the successful preparation of a carbon felt co-doped with N and O through plasma treatment of the carbon felt with mixed N2 and O2 plasma, useful for VRFB electrodes. This co-doped carbon felt has improved the performance of the battery thanks to the modified electronic properties, better affinity with the electrolyte and therefore improved electrocatalytic activity due to the presence of the heteroatoms. Plasma treatment (with an RF source of 13.56 MHz) was carried out by modifying the carbon filter various times at room temperature using N2 and O2 as precursors of N and O, respectively. The power of the plasma was 200 W and the pressure was 65 Pa. With this technique, through plasma treatment, O-doped carbon felt (O-CF), N-doped carbon felt (N-CF) and N,0 co-doped carbon felt (N, O-CF) were prepared. The last electrode was made by initially treating CF with plasma treatment with O2 for 9 minutes, followed by plasma treatment with N2 for 1 minute. The results indicate that the electrocatalysis kinetics of the redox process on the electrodes are in the order O- N-CF > O-CF >N-CF > untreated CF. In particular, the N,O-CF show a much better electrochemical performance than CFs doped with a single atom, because of the synergistic effect of the co-doping. The EE of the VRFB battery with N,O-CF was improved passing from 65% of the VRFB with untreated CF with plasma treatment, to 78% at a current density of 50 mA cm^-2, with excellent cyclic stability.

Dixon et al. in “Tuning the performance of vanadium redox flow batteries by modifying the structural defects of the carbon felt electrode”, Beilstein Journal of Nanotechnology (2019) describe a carbon felt electrode with minimum oxygen functional groups and a higher number of defects in the form of doping sites with N, prepared using the N2 plasma technique. The sample treated with N2 plasma demonstrated improved electrochemical performance in a VRFB with respect to an untreated sample with fewer defects. According to the authors, when commercial GFD felts based on polyacrylonitrile (PAN), obtained from SGL Carbon, are subject to plasma treatments with N2, defects form on the carbon felt. Besides the increase in quantity of reactive sites, with N2 plasma treatment, heteroatom defects are also created through the doping of N. The surface of samples treated with plasma was characterized by the presence of pyrrolic and pyridinic N. The felt treated with N2 plasma demonstrated better electrochemical performance with respect to an untreated felt. As both the untreated sample and the one treated with N2 plasma exhibited almost the same amount of functional groups with oxygen (2% and 3% of C=0 and C-O) the best performance observed for the sample treated with N2 plasma is attributed to the additional defects formed during plasma treatment. However, the cell functioning with the sample treated with plasma with N2 demonstrated a loss of energy capacity, or capacity fade, that can most probably be attributed to the development of hydrogen at the negative electrode.

European patent No. EP2626936B1 describes the use of carbon material as an electrode in redox flow cells. More in particular, this patent describes how to prepare graphite and carbon materials intended for use in efficient redox flow batteries through activation with plasma treatments in an atmosphere containing oxygen. In fact, it is known that functional groups containing oxygen act as active centres for a number of electrochemical reactions, and they increase the hydrophilicity of these surfaces too. In this patent, the activation of the carbon material can comprise a modification to the surface, in particular a hydrophilization of the carbon material. The number of functional groups containing oxygen on the surface of the carbon material is increased by a factor of at least 2, at least 5 or at least 10 compared to material not treated with plasma treatment. The functional groups containing oxygen preferably comprise at least one functional group selected from hydroxyl, carbonyl and carboxyl groups. As the working gas for the plasma treatment, air, nitrogen, argon, carbon monoxide, carbon dioxide and/or helium and mixtures thereof can be used. When inert gases such as nitrogen and/or noble gases are used, the working gas is generally mixed with a specific proportion of oxygen, e.g. in the range between 1 and 40% by volume and in particular in the range from 20 to 30% by volume. The plasma treatment is carried out in a pressure range of the working gas between 1 and 500 kPa. Typical exposure times in the plasma treatment of carbon materials are in the range between 1 and 600 s and in particular in the range between 10 and 90 s, e.g. 30 s.

Likewise, the publication of international patent application No. W02003/070998A1 describes the possibility of combining more than one precursor gas in a plasma source for the activation of surfaces with precision molecular coatings. More in particular, this publication describes a method for the deposition of ionized molecules on the surface of an object in a vacuum system. Such method comprises a surface plasma treatment of the object in the vacuum system and a step of deposition of ionized molecules on the surface of the object in a vacuum system. In one example, the plasma treatment described produces dangling bonds on the surface. In another example, the plasma treatment comprises the substitution of chemical groups on the surface. In another example, the plasma treatment comprises the addition of chemical groups on the surface. Such plasma treatment can be conducted with at least one of the following as working gases: O₂, N₂, N₂O, He, Ar, NH₃, CO₂, CF₄ and air. Plasma treatments with gas have also been proposed as a means of functionalization for electrodes already previously functionalized. US patent application 20180108915A1 relates to an electrode for batteries with functionalized flow with conductive nanoparticles and then further treated with plasma treatment. In more detail, this application relates to a porous electrode for a liquid flow battery comprising 1) particulate fibres of a non-electrically conductive polymer in the form of a first porous substrate, wherein the first porous substrate is at least one from among paper, felt, mat and woven or non-woven fabrics and 2) electrically conductive carbon particulate incorporated into the pores of the first porous substrate adhering directly to the surface of the non-electrically conductive polymer particulate fibres of the first porous substrate. The electrically conductive carbon particulate of the porous electrode may be at least one from among carbon particles, carbon flakes, carbon fibres, carbon dendrites, carbon nanotubes and branched carbon nanotubes. The electrically conductive carbon particulate that includes particles, flakes, fibres, dendrites and the like may be graphene. Flakes of particulate include particulate with a length and width each of which is significantly greater than the thickness of the flakes. A flake includes particulate with a length/width ratio and width/thickness ratio each greater than 5, without a particular upper limit. The width and length of the flake may each be from about 0.001 micro to about 50 micron. In some examples, the electrically conductive carbon particulate can be treated on the surface. The surface treatment can increase the wettability of the porous electrode to provide an anolyte or a catholyte or to provide or improve the electrochemical activity of the electrode with respect to the oxidation-reduction reactions associated with the chemical composition of a given anolyte or catholyte. Surface treatments include at least one from among chemical treatments, thermal treatments and plasma treatments. In some examples, the electrically conductive carbon particulate has improved the electrochemical activity, produced by at least one chemical treatment, a thermal treatment and a plasma treatment. The term “improved” means that the electrochemical activity of the electrically conductive carbon particulate is selectively increased after treatment with respect to the electrochemical activity of the electrically conductive carbon particulate before treatment. The improved electrochemical activity can include at least one from among increased current density, reduced oxygen development and reduced hydrogen development at a determined potential. By expanding to surface treatments but still considered functionalized based on

O and N, Jiyeon Kim et al. in “High electrocatalytic performance of N and O atomic co functionalized carbon electrodes for vanadium redox flow battery”, Carbon (2017) describe the effects of co-funzionalizing with O and N a graphite felt (GF) through ammoxidation reactions for the positive and negative electrodes of a vanadium redox flow battery. The ammoxidation treatment was carried out by placing a GF based on commercial PAN in a tubular quartz glass reactor purifying with N2 for 1 hour. The quartz glass tubular reactor was placed in an electric horizontal tubular furnace at a constant reaction temperature with proportional-integral-derivative (PID) control. The temperature of the reactor was then raised to 773 K (ramp = 10 K/min) under a N2 flow and a reactant mixture of NH3 and air was supplied to the reactor for 1 hour. The ammoxidative surface reactions of the pure GF with NH₃/O₂ result in an effective co-doping of N and O mainly with functional groups of N and O significant for speeding up the kinetics of the redox reactions. The final content of the atomic N was evaluated in the range 3.6-3.8%, with the following respective compositions: Pyridinic N = 31-33%, pyrrolic N = 46-49%, quaternary N = 20-22%. The ammoxidation treatment led to a significant increase in the atomic O contents, in the range 8-17% in the treated graphite felt, with a preferential formation of hydroxyl (64%) and carbonyl (36%) groups. With respect to a GF electrode functionalized only with oxygen-containing groups (O-GF), the GF electrode co-functionalized with O and N (O-N-GF) displayed constant intrinsic redox reaction speeds 2-3 times higher for both the redox reactions of the semi-cells, demonstrating the synergistic effects of co-functionalization with O and N for speeding up the kinetics of redox reactions involving vanadium species. In a single symmetrical cell of a VRFB battery, the O-N-GF displayed an initial voltage and energy efficiencies about 4-6% higher with respect to the O-GF electrode during operation with high current density (80-110 mA/cm²). Such effect caused an improvement of about 1.4 times in the discharge energy capacity of the VRFB battery.

Despite all the different methods described above for functionalizing the electrodes, with gas plasma treatments only based on N, only on O, with O-N, as well as with surface treatments of O-N co-functionalization, the electrodes available today for redox flow batteries have unsatisfactory performance levels to compete advantageously with other energy storage technologies. Therefore, it is necessary to improve further the electrodes of the redox flow batteries.

Summary of the invention

The inventors have now found an electrode that helps in overcoming the technical limits highlighted above for the known electrodes and, in particular, it provides an unexpected improvement in the performance levels of the electrode thanks to a plasma treatment in a single step, with a combination of precursor gases of O2 and N2. The functionalization of the electrode obtained with such treatment shows a synergistic effect in terms of performance of the electrode when compared with what is obtained by a treatment with a precursor of the individual gas O2 or N2, or by consecutive treatment with a precursor of the aforesaid gases in sequence. Plasma treatment with the combination of the two gases according to the present invention can be performed on commercial electrodes or on electrodes functionalized with carbon particles, as described in detail in the following.

A subject of the present invention is therefore an electrode made of activated carbon material as claimed in claim 1, which solves the technical problems highlighted above for the known electrodes, providing in particular a carbon electrode having an improved electrocatalytic activity.

A further subject of the present invention is a process for producing the aforesaid electrode as claimed in claim 6, the use thereof, an electrochemical cell and an electrocatalytic device that comprises it as respectively claimed in claims 13, 14 and 15.

Further important characteristics of the subjects of the invention are defined in the dependent claims.

Brief description of the figures The characteristics and advantages of the electrode, of the use thereof and of the process for its production according to the present invention, shall be clearly illustrated in the following description provided by way of an exemplary, non-limiting description of embodiments thereof, also with reference to the appended figures wherein: - Figure 1 shows the polarization curves iR corrected, measured for the VRFB batteries using graphite felt (GF) not treated with plasma treatment (pristine) and graphite felts treated with different gas plasma treatments; in all the cells Nafion^® 155 was used as a proton exchange membrane;

- Figure 2 is an SEM (Scanning Electron Microscopy) image of a fibre representative of a graphite felt (GF) not treated with plasma treatment;

- Figure 3 is an SEM image of a fibre representative of a GF treated with O2 plasma at the pressure of 40 Pa;

- Figure 4 is an SEM image of a fibre representative of a GF treated with N2 plasma at the pressure of 40 Pa; - Figure 5 is an SEM image of a fibre representative of a GF treated with 0₂:N₂

1 : 1 plasma at the pressure of 40 Pa; - Figure 6 shows two SEM images of a fibre representative of a GF treated with C>2:N2 1:1 plasma at the pressure of 16 Pa at two different enlargements: at 10020x (Fig. 6a) and at 40009x (Fig. 6b);

- Figure 7 shows an SEM image of a fibre representative of a GF treated with 0₂:N₂ 1 :1 plasma at the pressure of 4 Pa at three different enlargements: at 4003x (Fig.

7a), at 17482x (Fig. 7b), and at 50006x (Fig. 7c);

- Figure 8 shows cyclic voltammetry measurements for GF not treated (pristine) with plasma and GF treated with different gaseous plasmas for the anode region related to the h/l redox reaction (Fig. 8a) and for the cathode region related to the Zn²7Zn redox reaction. The CV curves were acquired with a potential measurement rate of 2 mV s ¹ ;

- Figure 9 shows in the form of histograms the chemical composition of GFs not treated (pristine) and treated with the different gaseous plasmas indicated, as the elementary composition (Fig. 9a), as distribution of the functionalities of oxygen on the surface of the electrodes (Fig. 9b), and as distribution of the functionalities of nitrogen on the surface of the electrodes (Fig. 9c), taken from the analysis of the XPS spectra C 1s, N 1s and O 1s of the different electrodes.

Detailed description of the invention

The present invention relates to an electrode made of carbon material activated by a plasma treatment characterized in that said carbon material was activated by exposure to an electrical discharge in the atmosphere of a combination of precursors of gaseous N2 and O2.

As demonstrated by the tests performed and described in detail in the following experimental part, the use of a combination of precursors of N2 and O2 instead of the precursors of the same gases individually in a “stand-alone" mode, but also of both gases in sequential mode instead of in combination, enables an activated carbon material to be obtained which has improved electrocatalytic performance.

An even greater improvement in the electrocatalytic properties is further observed with respect to carbon material not treated by a plasma treatment with the combination of gases of the invention. The present electrodes can be applied as electrodes for electrocatalytic devices, such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems and, preferably, as electrodes in redox flow batteries (RFBs).

The carbon material of the present electrodes can, for example, be any graphite material, preferably a graphite felt (GF) based on rayon or polyacrylonitrile (PAN) as precursors. The starting material for making the present electrodes may be, for example, a commercial graphite felt electrode, or another suitable electrode typically used in RFBs.

The combination of the gaseous precursors of O2 and N2 can be for example used in a weight ratio in the range comprised between about 0.05:0.95 and 0.95:0.05, and preferably such weight ratio is about 1:1.

According to a preferred embodiment of the invention, before activation through plasma treatment, the carbon material of the electrode was functionalized, on at least a part of its surface, with electrically conducting carbon particles, in particular two- dimensional (2D) carbon particles, such as, for example, graphene shavings or flakes (single or multiple layers of graphene) or graphene derivatives, and preferably graphene flakes.

By graphene derivatives is meant, for example, reduced graphene oxide. Such graphene flakes are preferably obtained through “wet-jet milling” exfoliation as described in Italian patent application No. IT102015000077259 in the name of the Applicant, the description of which is incorporated herein by reference.

The invention also relates to the process for producing the aforesaid electrode, comprising the steps of providing a piece of carbon material, and subjecting it to activation through plasma treatment by exposure to an electric discharge in the atmosphere of a combination of precursors of gaseous N2 and O2, preferably at a pressure of the combination of gases ranging between about 4 and about 100 Pa, more preferably at a pressure between about 4 and about 40 Pa, and even more preferably at a pressure of about 4, or about 16, or about 40 Pa.

The exposure to electrical discharge for the generation of plasma can be performed according to any known method, and preferably with an inductively coupled radiofrequency reactor, wherein said electrical discharge can be for example of power comprised between about 20 and about 500 W, more preferably between about 50 and about 200 W, and more preferably at a power of about 100 W. Because of its nature, such treatment is very quick, typically it has a duration that varies between about 10 seconds and 60 minutes, and on average it has a duration of about 10 minutes. According to a preferred embodiment of the process of the invention, the process further comprises, prior to the activation of the carbon material with plasma treatment, a functionalization step of the material itself, on at least part of the surface thereof, with conductive carbon particles, e.g. by wet impregnation of said material with a dispersion of said conductive particles and a binding agent, followed by a drying step. As the binding agent a polymer agent is preferably used selected for example from the group consisting of polyvinyldenfluoride (PVDF), copolymers of fluoropolymers based on tetrafluoreoethylene sulfonate (e.g. Nafion), polyketones, poly(ether ether ketone) sulfonate (SPEEK), polyolefins, acrylate polymers, vinyl polymers, polyethers, polyimides, ethylene vinyl acetate (EVA), and polybenzimidazole; preferably the polymer binding agent is polyvinyldenfluoride.

For non-aqueous electrolytes, besides the polymers listed above, the binding agent can also be selected from the group consisting of styrene butadiene rubbers (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyamide imide (PAI), sodium carboxymethyl cellulose, multipolymer acrylonitrile (LA133), polytetrafluoroethylene (PTFE). Further binding agents commonly used in batteries can also be applied in the present invention and can be easily selected according to the conditions of use by any technical operator with ordinary knowledge of the sector.

Excellent results have been obtained for the impregnation of graphite felt as carbon material with a dispersion of graphene shavings or flakes or graphene derivatives as conductive particles in a binder consisting of PVDF.

The binding agent can be added for example in an amount comprised between 1 and 50% by weight with respect to the total weight of the dispersion of conductive particles, preferably in an amount comprised between 1 and 30% by weight. In a preferred aspect of the invention, the binding agent is PVDF and it is added in an amount of about 10% by weight with respect to the total weight of the dispersion of conductive particles. The subsequent drying of the electrode functionalized with the conductive particles before the plasma treatment can for example be performed by heating to a temperature comprised between 50 and 200°C, preferably about 150°C, in vacuum conditions for a certain period of time, typically between 1 and 24 hours, preferably for about 12 hours.

The electrodes of the invention have different functionalizations based on nitrogen and oxygen on their surface. In relation to functionalizations of nitrogen, they comprise pyridinic nitrogen, pyrrolic nitrogen, quaternary nitrogen, N-oxides of pyridinic nitrogen and of aminic nitrogen. In relation to functionalizations of oxygen, they comprise phenol, carboxyl and carbonyl groups and aliphatic hydrocarbons. Through X-ray Photoeletron Spectroscopy (XPS), the electrodes of the invention have displayed certain atomic contents in the range of 75-95% for carbon, 0.1-5% for nitrogen and 5- 20% for oxygen, with the sum of said atomic content for C, O and N being equal to 100%; preferably, the present electrodes have an atomic content of 80-90% carbon, 12-17% oxygen and 0.3-2% nitrogen.

For each of the three elements the atomic content may be attributed to different functional groups. In particular, from the XPS spectrum for 0 1s, the atomic content of O is in the range 1-10% for the carboxyl group (COOH) (bond energy between 533.0 and 534.0 eV), 4-12% for the hydroxyl group (COH) (bond energy between 532.0 and 533.0 eV), 1-8% for the carbonyl group (C=0) (bond energy between 531.5 and 532 eV). Preferably, the atomic content of O is in the range 5-7% for the carboxyl group (COOH), 6-9% for the hydroxyl group (COH), 3-5% for the carbonyl group (C=0). It is to be understood that, as mentioned above, the total atomic content for oxygen is comprised between 5 and 20%, to which total content can contribute in a varying way, to the extent indicated above, the percentages of atomic content for the various functional groups containing oxygen.

From the XPS spectrum for N 1s, the atomic content of N can be attributed to pyridinic N (bond energy between 397.8 and 398.2 eV) for less than 0.1%, to pyrrolic N (bond energy between 400.0 and 400.4 eV) for a range of 0.05-1.00 %, quaternary N (graphitic N) (bond energy between 401.0 and 401.6 eV) for a range of 0.05-1.00%, N oxides (bond energy between 402.2 and 402.5 eV) for a range of 0.01-1.00%. Preferably, the atomic content of nitrogen can be attributed to pyridinic N for less than 0.01%, pyrrolic N for a range of 0.15-0.40% (more preferably in a range of 0.20- 0.35%), quaternary N for a content in the range of 0.05-0.30%, nitrogen oxides for a content in the range of 0.01-0.70%. From the XPS spectrum of C 1s, the electrodes of the invention have displayed for carbon an atomic content in the range 75-95%.

In fact, thanks to the functional groups introduced with the plasma treatment with combined O2 and N2 gases, the present electrodes, as described above, display high catalytic activity towards the redox reactions that take place in electrochemical devices, in particular in RFBs.

Furthermore, with respect to carbon electrodes based on untreated graphite felt (GF) typically used in redox flow batteries and in other electrochemical devices such as fuel cells, the electrodes of this invention have a larger electrochemically accessible surface area, as well as high wettability with the electrolyte, i.e. high hydrophilicity for aqueous and polar electrolytes.

Being carbon based electrodes, preferably graphitic, the electrodes of the invention further have high electric conductivity (i.e. low electrical resistivity).

The composition of the combined gases of the plasma treatment used for the production of the electrodes of this invention can be appropriately varied to select the functional groups most desired to be incorporated into the material of the electrode for the purpose of modulating the electrochemical performance thereof.

The same is valid for the other parameters of the production process that can be varied to create a modulation of the chemical and physical effects such as the type of functional groups, morphological modifications of the surface of the electrodes, phase conversions, etc.

Further advantages of the process for the production of the electrodes of the invention are represented by the plasma treatment speed and by its sustainability both from the economic and the environmental point of view being able to use non-polluting, low cost, commercial gases. The present invention will now be described in detail by means of the following non-limiting examples. EXAMPLE 1

Plasma activation, with multiple and combined gases, of electrodes for aqueous redox flow batteries (RFBs), including vanadium redox flow batteries (VRFBs) and zinc-iodine redox flow batteries (ZIRFBs) The efficacy of the plasma treatment with multiple and combined gases for the activation of the carbon electrodes for redox flow batteries (RFBs), in particular vanadium redox flow batteries (VRFBs), was validated on graphite felts (GFs) (4.6 mm GFD, Sigracell^®). The GFs were treated with multiple combined gas plasma (O2 and N2 plasma, with a composition of 1:1 by weight) and, as a comparison, also with single gas plasma (O2 or N2) and sequential gas plasma (O2 plasma followed by N2 plasma or N2 plasma followed by O2 plasma). Experimentally, the gaseous plasma treatments of the GFs were performed in a radiofrequency (RF) reactor inductively coupled (13.56 MHz) to a 100 W power and a working gas pressure (multiple or single) comprised between 4 and 40 Pa, i.e. 4, 16 and 40 Pa, (background gas pressure fixed at 0.2 Pa) for a time of 10 min.

As is already known in the state of the art, plasma treatments with O2 are effective for introducing O-based functionalities onto the surface of the materials of the electrodes. The O-based functionalities include carboxyl groups (C=0) and carbonyl groups (C-O-C), which are catalytically active towards the redox reactions used in VRFBs and increase the wettability of the electrode by polar electrolytes (e.g. aqueous electrolytes, including those used in VRFB batteries). Plasma treatment with N2 introduces N-based functionalities onto the surface of the materials of the electrodes. The N-based functionalities are N-pyridinic, N-pyrrolic, N-quaternary, N-oxides of N- pyridinic and N-aminic functionalities (more rarely N graphitic), and are catalytically active for the redox reactions that take place in the VRFBs. The five valence electrons of the N atoms contribute to the additional charge of the bond of the graphene layers, improving the electrical conductivity of the carbon materials. Furthermore, the plasma treatment with N2 creates structural defects, e.g. unsaturated C atoms, which can later react with O2 present in the material of the electrode or with environmental O2. Obtaining the subsequent gaseous plasma with different plasma, the effects of the different gases can be combined and possibly controlled to obtain an excellent functionalization ratio based on O and N, as well as an attachment of the morphology that cannot be obtained with O₂ plasma only.

However, the combination of O2 and N2 in a plasma treatment with gases combined according to the invention enables the effects of the two gaseous plasmas to interact with one another, resulting in a new synergistic effect. It is worthy of note that, by varying the pressure of the plasma, it is possible to control the effects of gas plasma treatments. Measurements of the contact angle with water (obtained using DATAPHYSICS, OCA-15 and water drops of MilliO^® (2 mI_) as water references), have confirmed that the electrodes became hydrophilic and displayed a zero contact angle with water after the different plasma treatments, regardless of the pressure and composition of the gas, while for the pristine electrode before the plasma treatment, the graphene surface was hydrophobic (water contact angle = 127.3° ± 4.2°).

The efficacy of the different plasma treatments was evaluated, and therefore validated in VRFB. For the experiments, a single cell VRFB configuration was used with a no-gap coil architecture (XLScribner RFB Single Cell Hardware). In more detail, this hardware system consisted of a pair of bipolar plates based on the “graphite flow-field layout” by Poco Graphite (Poco^®), 2 Teflon^® flow frames, 4 Viton^® gaskets (2 for each electrode) and a pair of gold plated aluminium plates as current collectors with input/output ports for the electrolyte (Swagelok^® equipment). A Naflon^® 115 membrane (thickness 127 mGp) was used as a proton exchange membrane.

Peristaltic pumps (Masterflex L/S^® series) were used to pump the electrolyte into the cell hardware in a one-directional way. The electrochemical measurements of the VRFB batteries were performed with a potentiostat/galvanostat (VMP3, Biologic). The electrolytes were previously prepared electrochemically from a 1 M solution of VOSO4 + 3 M of H2SO4. The initial positive and negative electrolytes (respectively catholyte and anolyte) in the tanks were sized with a volume equal to 30 mL of the following solutions 1 M V0²⁺ + 3 M of H₂S0₄ and 1 M di V³⁺ + 3 M of H₂S0₄, respectively, corresponding to a specific theoretical capacity of 13.4 Ah / L (calculated on the total volume of electrolyte, including both the catholyte and the anolyte). The electrolytes were dispensed into the cell from the peristaltic pumps at a flow rate of 40 ml rnirr¹. Purging with nitrogen into the anolyte tank was performed to prevent the oxidation of the charge from V²⁺ to V³⁺ in the presence of O2 when the battery is in a charged state.

The polarization curve was analysed to evaluate the kinetic activation polarization (kinetic losses) and the ohmic polarization (iR losses) inside the cells. The polarization curves of the VRFBs were performed on completely charged cells (with 1 M V0²⁺ + 3 M H₂SO₄ as the catholyte and 1 M V²⁺ + 3M H₂SO₄ as the anolyte). A constant current of 10 mA cm^-2 was applied until the voltage of 1.7 V (charged state). The cells were then discharged for 30 s at every applied current density (comprised between 1 and 200 mA cm^-2). The voltage measurements of the cells were mediated on 30 s of each current passage to provide a point on the polarization curve. Prior to the acquisition of the polarization curves, the high frequency resistance (at 15-30 kHz) of the VRFB was measured through electrochemical impedance spectroscopy (EIS), according to the previously reported protocols. The magnitude of the voltage disturbance in AC alternating mode was set to 10 mV. The iR losses were calculated from the product of the applied current (i) and the resistance measured through EIS (R). The polarization curves corrected by iR (i = measured current) were obtained by subtracting the iR losses from the measured polarization curves. Galvanostatic charge/discharge measurements (CD) were performed to evaluate the main Figure of Merit (FoM) of the VRFB - where in the present invention “figures of merit” refer to the parameters used to define the performance of the battery, i.e. : the coulombic efficiency (CE), which is the ratio between the electric charge passed from the cell during the discharge (Qdischarge) and that during charging (Qcharge); the voltage efficiency (VE), which is the ratio between the average voltages of the cells during charging and during discharging; EE, which is the product of CE and VE. The galvanostatic measurements CD of the individual cells of the VRFB battery were performed at different current densities, comprised between 25 and 200 mA cm ².

Figure 1 shows the polarization curves (after IR correction to isolate the kinetic activation polarizations, the proton exchange membrane and the electrolyte being the same for all the cells) obtained for VRFBs using GF before and after the different plasma treatments as electrodes (electrodes mentioned in the key of the figures as not treated with plasma and with the details of the gaseous plasma treatment, including the composition of the gas and the pressure of plasma). The VRFB based on the GFs treated with combined gas plasma with 40 and 16 Pa as plasma pressure (i.e. plasma of O2: N2 (1: 1) - 40 Pa, plasma of O2: N2 (1: 1) - 16 Pa) shows the lowest kinetic polarizations of activation. By reducing the pressure of the gas during plasma to 4 Pa, the kinetic activation polarizations increase because of the excessive attachment of the GF fibres by the reactive plasma species (by lowering the plasma pressure, the speed of the particles present in the plasma increases before impacting against the target sample). Furthermore, the VRFBs based on GFs treated with N2 plasma at 40 Pa display polarizations of kinetic activation higher than those displayed by the VRFBs treated with O2 plasma at the same plasma pressure. Therefore, the plasma treatment with N2 is less effective for improving the catalytic activity of GFs towards the redox reactions of VRFBs with respect to O2 plasma treatment. However, the application of N2 plasma treatment after O2 plasma treatment further reduces the activation polarization losses, indicating a greater electrocatalytic activity towards redox reactions in the VRFB battery after sequential plasma treatments. In the same way, treatment with O2 plasma reduces the kinetic polarization of activation of the GF previously treated with N2 plasma. However, neither of the sequential plasma treatments is effective for reducing the kinetic polarization as happens in the case of plasma treatment with multiple and combined gases at the same plasma pressure. This indicates that new effective and synergistic effects can be obtained which increase the catalytic activity of the GF towards VRFBs through the use of multiple gases during the plasma treatments.

The main figure of merit extrapolated with CD galvanostatic measurements, taken at current densities that vary in the range between 25 and 200 mA cm^-2, for the different VRFBs using GFs not treated with plasma, are illustrated in the following Table 1. The best VE and EE were obtained for VRFBs that use GFs treated with a plasma of the mixture 0₂:N₂ 1:1 at the pressure of 16 Pa. As a matter of fact, these VRFBs have obtained the highest VE (e.g. 96.3% and 86.5% at current density between 25 and 100 mA cm^-2, respectively), confirming the lowest polarization of kinetic activation between all the VRFBs subjected to investigation and reported in Figure 1. In particular, at a current density of 25 mA crrr², the VRFBs based on GFs treated with plasma of mixtures C>2:N2 1:1 at 16 Pa display a VE of 96.3%, which is higher than those reported in literature on the subject. Table 1 - Efficiencies (CE, VE, EE) recorded for various current densities for VRFB using GFs not treated with plasma and GFs treated with different gaseous plasma obtained by varying the carrier gas and the plasma pressure. The efficiency values were calculated at the third galvanostatic charge/discharge cycle at the same current density.

The plasma with both gases O2 and N2 led to both the functionalization of the carbon observed for each plasma with only one gas, although their content depends strongly on mutual synergistic effects of the different plasma species, and to concomitant morphological changes on the electrodes. In fact, some gaseous plasmas (e.g. Ar, He, Xe, Ne and N2 plasma) can create electrically charged species that have a strong impact on carbon materials, modifying their morphology (texturization process). In our case, plasmas with combined gases (i.e. , C>2:N2 plasma) were shown to be highly effective in texturizing the graphitic surfaces of the GF fibres, as demonstrated through SEM analysis of GFs not treated with plasma and of GFs treated instead with different gaseous plasmas. In more detail, the carbon fibres of the untreated GFs show smooth surfaces (see Figure 2).

After the plasma treatment with O2 at the pressure of 40 Pa, the surface of the GF fibres still shows a smooth morphology (see Figure 3), which is similar to that observed for the native GF fibres. Therefore, the effects that originated from the O2 plasma are mainly chemical surface modifications, as already described in the state of the art. However, by increasing the power that generates the plasma, the energy of the species in the plasma could have an effect on the graphitic surfaces because of a concomitant evolution of CO and/or CO2.

Unlike O2 plasma, N2 plasma increases the roughness of the surface of the fibres with respect to that of a GF not plasma treated (see Figure 4). This change to the surface morphology is caused by an incision process of the fibres through the formation of structural defects (unsaturated C atoms).

Such effects are significantly prominent on intrinsic defects and functionalities to the oxygen of the GFs. Therefore, in the plasma treatment with 0₂:N₂ 1:1, O2 and N2 the roughness of the surfaces improves synergistically (see Figure 5). Without thereby wishing to be bound to a theory, the inventors believe that the oxygen species in the plasma oxidise the surfaces of the GF, whereas the nitrogen species in the plasma gradually cut into the surface; the incisions caused by the nitrogen species in the plasma would be promoted by the contextual oxidization of the surface, thus producing “deep” incision effects, in the sense that the incised surface can be re-oxidized and subsequently incised again. The incision effects increase significantly as the pressure of the plasma decreases. In fact, the average free paths between species in the plasma are reduced as the pressure reduces. Therefore, the lower the pressure, the longer the acceleration times for the species in the plasma, which then have an impact on the surface of the sample treated at high speed, i.e. with high energy, thus supporting the incision processes. The Figures 6 and 7 show a GF treated with C>2:N2 1:1 mixtures at the pressure of 16 Pa and with C>2:N2 1:1 at the pressure of 4 Pa, respectively, therefore electrodes with lower pressures than that used to treat the sample shown in Figure 5 (i.e. 40 Pa). Both the samples display significant modifications to the surface morphology, including the formation of cavities similar to craters having a diameter of hundreds of nanometres, and texturization of the surfaces on a lower scale than the dimensions of the craters (i.e. surface nano-texturization). Such effects are more clearly pronounced in GFs treated with 0₂:N₂ 1:1 at the pressure of 4 Pa with respect to GFs treated with 0₂:N₂ 1:1 at the pressure of 16 Pa, in accordance with the expectations on the basis of what has been observed above. An excessive incision of the GF reduces the electrochemical performance of the resulting VRFBs with respect to the optimal case based on GFs treated with 0₂:N₂ 1:1 at the pressure of 16 Pa.

To validate the efficacy and versatility of plasma treatments with gases in RFBs, the GFs treated with plasma with gases were also studied for zinc-iodine redox flow batteries (ZIRFB). These aqueous RFBs have received a lot of attention recently due to the excellent electrochemical properties of the redox pair l³VI and the super high solubility of the salts based on I- (e.g. the solubility of Kl is > 8 M) [13] Furthermore, it was demonstrated that the bromide ions (Br), provided by simply dissolving Br based salts in the electrolyte, can act as a complexing agent for stabilizing the free iodine I2 forming I2BG [14], especially in a high state of charge (SOC) and high operating current density, where the low availability of I may not support the formation of I3 (thus causing the precipitation of I2). Cyclic voltammetry measurements (CV) were performed in a configuration of the cell with three electrodes for evaluating the electrocatalytic activity of the GF not treated with plasma and the GFs treated with different plasma treatments. In fact, the electrochemical activity of the electrodes can be evaluated from the analysis of the current density peaks of the redox reactions [15,16,17], the separation of the potentials of the current density peaks (DE) of the redox reactions [15,16,17], and from the corresponding ratios of the anode(cathode) and cathode(anode) current density peak ratios —I _PA/I _PC(I _PC/I pa)— of the redox reactions in the region of the anode(cathode) current [15,16,17] The CV measurements were performed with the same potenziostat/galvanostat used for the electrochemical characterization of the VRFBs (i.e. , VMP3, Biologic), with a potential scanning rate of 2 mV s ¹. An Ag/AgCI electrode saturated with KCI (Biologic) and a carbon bar (Sigma Aldrich) were used as the reference electrode and the counter electrode, respectively. A 0.1 M solution of Kl and 0.5 M of ZnBr (Alfa Aesar) was used as the electrolyte. Figure 8 shows the CV curves obtained for the different electrodes, i.e. for GFs not treated with plasma and for GFs treated with different gaseous plasmas. For the anode region related to the redox reaction l³ (or Br )/h (see Figure 8a) the current density peaks increase after all the plasma treatments. However, the N2 plasma is significantly more effective than the O2 plasma for activating the catalytic activity of the electrode not treated for the redox reaction l³ (o hBr)/! . The plasma treatment with multiple and combined gases at the plasma pressure of 40 Pa (i.e. 0₂:N₂ 1:1 plasma at the pressure of 40 Pa) further increases the catalytic activity of the GFs. The difference DE of the redox reaction l^3_(o Br )/l^_ for GFs treated with plasma shows a slight increase with respect to the untreated GF. However, the I _PA/I _PC values measured for GFs treated with N2 plasma at 40 Pa (0.90) and with 0₂:N₂ 1:1 plasma at the pressure of 40 Pa (0.82) are significantly higher than those measured for GFs not treated with plasma (0.73). This means that gas plasma treatments increase the reversibility of the redox reaction l^3_(o bBr)/! on the untreated GF. The reversibility is not total (condition expressed by I_PA/I_PC=1) because of the possibility of precipitation of I2 close to the electrode in the absence of any flow of electrolyte, which promotes the complexation of I2 in the ZIRFBs. For the cathode region related to the redox reaction Zn2+/Zn (Figure 8b), the current density peaks increase after plasma treatments with N2 at 40 Pa and with C>2:N2 1:1 at 40 Pa. Plasma treatment with O2 at the pressure of 40 Pa does not significantly change the catalytic activities of the GFs. Therefore, it is not suitable as treatment of the electrodes for ZIRFBs. Also in this case, plasma treatment with C>2:N2 1:1 at 40 Pa is more effective than the plasma treatment with N2 at 40 Pa. The difference DE of the redox reaction Zn²7Zn is slightly reduced after plasma treatment with N2 and with mixed gases with respect to untreated GFs. The ratio Ipc/lp_A is less than 0.5 for all the electrodes. This indicates the presence of irreversible effects, possibly caused by the formation of Zn dendrites, which can break and then come detached from the electrode, losing their electrical contact with the electrodes in the absence of any flow of electrolyte. In general, these results represent a significant test of the applicability of plasma treatment with combined gases according to the invention in order to increase the catalytic activity of the carbon electrodes for redox reactions in ZIRFBs. The GFs treated with the plasma treatment according to the invention, in different conditions, were finally subjected to XPS spectroscopy in comparison with GFs not treated and with GFs treated with plasma of only N plasma or of only O. On the basis of the data collected with such XPS spectra C 1s, N 1s and O 1s on the above electrodes, their respective chemical composition was estimated, as elementary composition (Fig. 9a), as distribution of the functionalities of oxygen on the surface of the electrodes (Fig. 9B),i and as distribution of the functionalities of nitrogen on the surface of the electrodes (Fig. 9c). From Figure 9a it can be observed in particular how plasma treatments significantly increase the functionalities of O and N. The electrode treated according to the invention with O2: N2-4 Pa displays the maximum atomic percentage O in % of 15.93%, followed by the electrode treated with O2 plasma: N2-4 Pa 40 Pa and with O2 plasma: N2-I6 Pa (15.21% and 14.16%, respectively). The atomic content values % detected for O are significantly higher than those reported in literature for GFs treated thermally for VRFBs (typically less than <8%) [18,19] The maximum of N as atomic content in % was detected for N₂-40 Pa (1.67%), followed by O2: N2-I6 Pa and O2: N2-4 Pa (respectively 0.68% and 0.87%). As shown in Figure 9b, the plasma treatments increase the percentage of carbonyl groups (C=0), hydroxyl groups (C-OH) and carboxyl groups (COOH), the latter not actually present in untreated GFs. Without thereby wishing to be bound to a theory, the present inventors maintain that all these O functionalities introduced onto the surface of the material of the electrodes thanks to the present plasma treatment with combined gases, can be configured as catalytic sites for the redox reactions that take place in electrochemical devices where such electrodes are used.

The plasma treatments according to the invention have introduced onto the surface of the electrodes also functional groups of N, i.e. pyrrolic N, N oxides and graphitic N, as illustrated in Figure 9c. Unlike the electrodes doped with N obtained through assisted chemical functionalization at a high temperature, in our case pyridinic groups were not observed. Also, these N functionalities can act as catalytic sites for redox reactions in electrochemical devices, as well as the O functionalities, with which synergistic catalytic effects can be caused. EXAMPLE 2

Combined gas plasma activation of electrodes functionalized with graphene flakes for aqueous redox flow batteries (RFB)

A hierarchical carbon electrode was prepared with a coating of graphene flakes on graphite fibres of a graphite felt (GF) available on the market (4.6 mm GFD, Sigracell^®). The graphene flakes were produced in the form of a dispersion in N- methyl-2-pyrrolidone (NMP) through the exfoliation of graphite with the jet-milling method described in Italian patent application No. IT102015000077259 in the name of the Applicant, incorporated herein by reference, as described below.

A mixture of 20 L of N-Methyl-2-pyrrolidone (NMP) (> 97%, Sigma Aldrich) and 200 g of graphite flakes (+100 mesh, Sigma Aldrich) was prepared in a suitable container and mixed with a mechanical stirrer (Eurostar digital Ika-Werke). Subsequently, the mixture was transferred into a processor consisting of five series of different perforated discs interconnected with one another by applying a pressure of 250 Mpa through a hydraulic piston. Two jet flows were created at the second disc, which is provided with two holes of 1 mm diameter. Therefore, the two jets collide between the second and the third disc, which consists of a nozzle with diameter 0.3 mm. During the passage of the sample into the nozzle, the turbulence of the solvent generates a shear force that causes the exfoliation of the graphite. The dispersion as produced is cooled by a cooler, then collected in another container. To obtain the final dispersion SLG/FLG, the sample is processed again various times (e.g. twice) in a WJM machine, passing consecutively through nozzles with a reduced diameter (e.g. 0.15 and 0.1 mm). The SLG/FLG dispersion thus produced is concentrated or even dried in the form of powder with a rotary evaporator (Heidolph HEIVAP INDUSTRIAL, FKV Sri, Italy), setting the temperature of the bath to 80°C reducing the pressure to 5 mbar. When a SLG/FLG dispersion in a different solvent from NMP is required, the SLG/FLG flakes are added to 1.5 L of dimethyl sulfoxide (DMSO) (Merck KGaA, Germany). The mixture thus obtained is poured into five 300 mL Petri dishes made of aluminium and kept in a fridge for 1 hour at -15°C. Then, the Petri dishes are transferred into a freeze drying machine. Here the sublimation process is performed at a temperature of -10°C and a pressure of 0.1 mbar. After 50 minutes, the final freeze dried SLG/FLG powder is obtained.

The graphene flakes thus obtained are a mixture of graphene flakes in a single layer or in few layers (SLG/FLG). The dispersion of such SLG/FLG graphene flakes was purified through ultracentrifugation at 1000 rpm for 30 minutes, and subsequently collecting the supernatant. The dispersion of purified graphene flakes SLG/FLG was concentrated at 15 g L ¹ through evaporation of NMP with a rotavapor (Heidolph, HEIVAP INDUSTRIAL, FKV Sri, Italy) at 60°C. Polyvinyldenfluoride (PVDF) with an average molecular weight of about 534000 Da available on the market (Sigma Aldrich) was used as a polymer binding agent between the SLG/FLG flakes and the fibres of the GF. In practice, the PVDF was added to the dispersion of SLG/FLG flakes in the amount of 10% by weight with respect to the total weight. The dispersion thus obtained is therefore indicated as a dispersion of SLG/FLG flakes and PVDF.

GFs coated with SLG/FLG flakes were therefore prepared with a direct wet impregnation method. In more detail, the SLG/FLG flakes were impregnated in the GFs by filtering such GFs with the dispersion of SLG/FLG flakes as prepared. In practice, 3 mL of the dispersion of SLG/FLG flakes and PVDF as prepared were infiltrated in a piece of GF with an area of 5 cm². Such electrodes were then vacuum dried at 150°C for 12 hours. The hierarchical electrodes obtained display a larger surface area than that of the native GFs. In particular, measurements were performed on the surface area BET (Brunauer-Emmett-Teller), through Kr physisorption at 77 K in Autosorb-iQ (Qantachrome) and using the multipoint BET model (9 points equally spaced out in a relative pressure interval) (P/Po, wherein Po is the pressure of the Kr vapour at 77 K, corresponding to 2.63 Torr) between 0.10 and 0.30). Such measurements have indicated that the surface area of the GFs coated with SLG/FLG flakes (about 3.4 m² g ¹) increases by 750% with respect to that of a native GF (about 0.4 m² g^-1). After having assembled the hierarchical electrodes, the materials of the electrodes were functionalized through plasma treatment with combined gases using N2 and O2 as the carrier gas, as described in Example 1.

To demonstrate the efficacy of such treatment in improving the performance of the graphite electrodes, extra electrochemical tests were performed on VRFB batteries with graphene electrodes and GFs plasma treated with 0₂:N₂ 1:1 at two different pressures, i.e. at 16 Pa and at 4 Pa. In particular, the pressure of 4 Pa was also tested as the coating of SLG/FLG flakes on the fibres of the GFs could partially protect the latter from excessive incision. Furthermore, the plasma on the SLG/FLG flakes could cause different effects from those observed on GFs.

The main figure of merit FoM extrapolated with CD galvanostatic measurement, performed with current density in the range of 25 and 100 mA cm^-2, for the different VRFBs using GF/graphene electrodes plasma treated with 0₂:N₂ 1:1, is illustrated in the following Table 2.

The VRFB that uses the GF/graphene electrodes plasma treated with 0₂:N₂ 1:1 at the pressure of 4 Pa shows the highest VE (e.g. 97.1% and 87.0% at 25 and at 100 mA cm^-2 respectively) and EE (e.g. 93.1% and 84.9% at 25 and at 100 mA cm^-2 respectively). In particular, at a current density of 25 mA cm^-2, the VRFBs that use GF/graphene treated with 0₂:N₂ 1:1 plasma at the pressure of 4 Pa show a VE of 97.1%. By increasing the current density to 200 mA cm^-2, the VRFB that uses GF/graphene treated with 0₂:N₂ 1:1 plasma at the pressure of 4 Pa shows a significantly higher VE (77.5%) and EE (76.9%) than those obtained with the reference VRFB that uses GF treated with plasma without graphene (VE = 74.3% and EE = 73.7%).

It is significant to note that coating the GF with SLG/FLG flakes can also protect the GF from excessive degradation induced by plasma treatments at low pressure, as already discussed for the pressure of 4 Pa.

Table 2: Efficiencies recorded at various current densities, of the VRFBs that use untreated GF/graphene or GF/graphene treated with q2:N2 1 :1 plasma by varying the plasma pressure. The efficiency values were calculated at the third galvanostatic charge/discharge (CD) cycle at the same current density.

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Claims

I. An electrode of carbon material activated by plasma treatment characterised in that said carbon material is activated by exposure to an electric discharge in an atmosphere of combined precursors of gaseous N2 and O2. 2. The electrode of claim 1 , wherein said carbon material is a graphite felt.

3. The electrode of claim 1 or claim 2, wherein said carbon material before being activated is functionalised, on at least part of its surface, with conductive carbon particles.

4. The electrode of any one of the preceding claims, wherein said particles are graphene flakes.

5. The electrode of any one of the preceding claims, wherein said activated carbon material has an atomic content determined by X-ray Photoeletron Spectroscopy (XPS) in a range 75-95% for C, in a range 0.1-5% for N, and in a range 5-20% for O, with the sum of said atomic content for C, O and N being equal to 100%. 6. A process for the manufacture of an electrode as defined in the claims 1-5, comprising the steps of providing a piece of carbon material, and subjecting it to activation by plasma treatment by exposure to an electric discharge in an atmosphere of combined precursors of gaseous N2 and O2.

7. The process of claim 6, wherein said electric discharge is obtained in an inductively coupled radiofrequency reactor.

8. The process of claim 6 or claim 7, wherein said electric discharge has a power ranging between 20 and 500 W.

9. The process of any one of the claims 6-8, wherein said combination of precursors of gaseous N2 and O2 has a pressure ranging between 4 and 40 Pa. 10. The process of any one of the claims 6-9, further comprising, before said activation by plasma treatment, a step of functionalisation of said carbon material, on at least part of its surface, with conductive carbon particles.

II. The process of claim 10, wherein said functionalisation is carried out by wet impregnation of said material with a dispersion of said conductive particles and a bonding agent, followed by a drying step.

12. The process of claim 11, wherein said material is graphite felt, said particles are graphene flakes and said bonding agent is polyvinylidene fluoride (PVDF).

13. Use of the electrode as defined in claims 1-5, in electrocatalytic devices.

14. An electrochemical cell comprising as electrode an electrode as defined in claims 1-5.

15. An electrocatalytic device comprising an electrode as defined in claims 1-5.

16. The electrocatalytic device of claim 15, which is a redox flow battery, a fuel cell, a metal ion battery, a supercapacitor, a water splitting system.

17. The electrocatalytic device of claim 15 or claim 16, which is a redox flow battery.