WO2010063888A1

WO2010063888A1 - A catalyst layer for electrochemical applications

Info

Publication number: WO2010063888A1
Application number: PCT/FI2009/050973
Authority: WO
Inventors: Pertti Kauranen; Ali Harlin; Pirjo HEIKKILÄ; Lisa WICKSTRÖM; Eino Sivonen; Jari Keskinen; Antti Pasanen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2008-12-02
Filing date: 2009-12-02
Publication date: 2010-06-10
Also published as: FI20086154A0

Abstract

A catalyst structure for an electrochemical cell is produced by electrospinning of a polyacrylonitrile (PAN) and carbon nanotube (CNT) or nanofibre (CNF) composite nonwoven followed by carbonization of the structure and deposition of a catalyst on it. Electrospinning method enables implementation substances such as CNTs into the fibre, and electrospun fibres having characteristically high specific area can be used as carrier for subsequent fixation of substances such as catalyst to fibre surfaces. The structure thus produced is thin and shows improved electrical conductivity and mechanical strength over carbonized electrospun PAN nonwovens without CNT or CNF addition.

Description

A catalyst layer for electrochemical applications

Field of the Invention

The present invention relates to a catalyst layer for electrochemical applications, especially to a composite catalyst layer structure, a membrane electrode assembly, and methods for preparing those.

Background of the Invention

The catalyst layer of proton exchange membrane (PEM) fuel cell should be quite thin (typically 5 - 20 μm) in order to be effective. The catalyst layer is traditionally prepared by making a printing ink of carbon powder supported noble metal catalyst and dissolved PEM ionomer. The catalyst layer is either printed directly on the electrolyte membrane or the gas diffusion layer carbon paper or indirectly on a polymer film from which it is transferred onto the membrane. The catalyst coated membrane is then laminated with carbon papers or catalyst coated carbon papers are laminated with the electrolyte membrane for a five layer structure. The carbon supported catalyst powders are prepared either by chemical reduction of precursor salts in a batch process, (as disclosed by the patent US-5,068,161 ), or by continuous aerosol synthesis (as disclosed by the patents US-6,338,809, US-6, 103,393). One limitation of such catalyst layer is that the electrolyte may form electric insulation between the catalyst particles which may limit the electronic conductivity of the layer. This limitation could be overcome by using a continuous carbon web structure as the supporting structure for the noble metal catalyst.

Such thin nonwovens can be produced by an electrospinning process. Electrospinning is a method that can be used in the production of polymeric nanofibres. Polymer solution is drawn into nanosized fibres utilizing electric field arranged between the solution and a collector. Electrospinning equipment is typically consisting of nozzles through which the solution is fed into electric field, but electrospinning can also be obtained from free solution surface. Electrical forces accelerate the solution towards the collector forming polymer jets, which undergo instabilities such as whipping. Instabilities are largely responsible for the high stretching of the jet and the nanosized diameters of the obtained fibres. In addition to neat polymers, solutions containing additives and fillers can also be used to functionalize fibres for use in specific applications. The nanosized structure of electrospun fibre web has intrinsic properties, such as small fibre diameter, small pore size, and high surface area, which are advantageous for many applications.

Electrospun webs can be used as a carrier for subsequent fixation of various substances onto the fibre surfaces as well as for their direct implementation into the fibre. Polymeric nanofibres can be used as precursor fibres, which are carbonized into carbon nanofibres. This kind of approach can be used in order to prepare support carbon nanostructure for the deposition of noble metal catalyst. Electrospun fibres can be functionalized using additives and fillers of many kinds in order to improve the electrospinnability and properties of electrospun fibres.

U.S. patent US7229944 describes a method for preparation of catalyst structure utilizing electrospinning method. This method includes preparation of electrospun neat polyacrylonithle (PAN) or PAN/platinum structure which is carbonized. Neat carbon web is subsequently coated with platinum (Pt).

Korean patent KR 715155 (KR application KR10-2005-0037241 ) describes utilization of electrospinning in preparation of catalyst layer. This patent publication includes preparation of carbon nanotube (CNT) containing electrospun composite web, which is carbonized, but instead of the direct use of the carbonized web as the catalyst support, the carbon web is ground up into stable fibres, which are coated with the catalyst and then used in preparation of catalyst structure thus involving more processing steps.

Summary of the Invention

It is an object of this invention to provide improved structure and properties of a catalyst layer and a membrane electrode assembly, and to provide methods of making those structures. A further object of the present invention is to construct more economic and effective processing lines for catalyst layer and membrane electrode assembly production, for example, for the electrochemical applications such as membrane fuel cells. These objects are achieved by a composite support structure of the catalyst layer, which catalyst layer structure may also be used as an electrode for membrane electrode assembly, and by a continuous roll-to-roll process for the manufacture of the catalyst layer and membrane electrode assembly. According to a first aspect of the present invention, there is provided a catalyst layer for electrochemical applications, which include a composite catalyst support structure, which comprises polymer precursor and carbon nanotubes and/or carbon nanofibres. The composite catalyst support structure may be formed by electrospinning of a carbonizable polymer precursor and carbon nanotubes and/or carbon nanofibres. The composite catalyst support structure may further be converted to a carbonized composite web by subsequent heat treatment (carbonization). After the heat treatment a catalyst material may be deposited onto the carbonized composite web by physical or chemical methods.

According to an embodiment of the invention, a carbonizable polymer precursor may be polyacrylonithle (PAN), pitch, polyvinyl alcohol (PVA), polybenzimidatzole, polypyrrole or suchlike polymer having high carbon yield.

According to another embodiment of the invention, a method of heat treatment forming carbonized composite web is provided. Polymer/CNT/CNF composite web is heat treated at 200-350⁰C in oxidizing atmosphere. Heat treatment in inert atmosphere is carried out at 300-3000⁰C, preferably at 300- 1800⁰C. The heat treatment steps are carried out continuously as roll-to-roll processes.

According to an embodiment of the invention, the thickness of the carbonized composite web is from 1 to 50 microns, preferably from 2 to 20 microns.

According to an embodiment of the invention, the catalyst material may be platinum or platinum alloy, e.g. Pt-Ru, Pt-Pd, Pt-Co, Pt-Ni, Pt-Co-Cr, Pt-Co- Mn, an organometallic compound like Co-Tetraphenylporphyhne, Fe- Tetraphenylporphyhne, Co-Tetramethoxyphenylporphyhne or Co- Phthalocyanine, silver or nickel, perovskite or spinel, e.g. Lao,iCao,9MnO3 or MnCθ2θ₄. Catalyst material may also be deposited as nanoparticles the size of which is from 1 to 20 nm, preferably from 2 to 10 nm. Catalyst material may be deposited using physical methods like physical vapour deposition (PVD), chemical vapour deposition (CVD), arc discharge, atomic layer deposition (ALD) or liquid flame spray. Catalyst material may also be deposited using chemical methods like reduction of precursor salts by methods known as microemulsion, impregnation, colloidal method or electrodeposition.

According to an embodiment of the invention, a membrane electrode assembly (MEA) for a membrane fuel cell, e.g. polymer electrolyte membrane, direct methanol, direct ethanol or anion exchange membrane fuel cell, comprises electrodes which are produced according to a first aspect of the invention and that the electrodes are laminated with the electrolyte membrane.

According to an embodiment of the invention, one electrode layer may be laminated on each side of the membrane or alternatively more than one electrode layer may be laminated on one or both sides of the membrane.

According to an embodiment of the invention, same catalyst material is used on both sides of the membrane or alternatively different catalyst materials may be used on different sides of the membrane.

According to an embodiment of the invention, one or more electrode layers are pretreated with the electrolyte solution before a lamination step. Electrode layers may be also pretreated with a hydrophobic agent, e.g. polytetrafluoroethylene (PTFE), before the lamination step.

According to an embodiment of the invention, a lamination may be performed by hot pressing.

According to an embodiment of the invention, additional pieces of carbon paper or cloth may be laminated with the electrolyte membrane and the catalyst layers.

According to an embodiment of the invention, a membrane fuel cell, e.g. polymer electrolyte membrane, direct methanol, direct ethanol or anion exchange membrane fuel cell, which use membrane electrode assemblies comprising electrodes which are produced according to first aspect of the invention, and that the electrodes are laminated with the electrolyte membrane.

Electrospinning is one processing method for polymeric nanosized fibres, thin nonwovens and fibre webs, which enables implementation substances such as carbon nanotubes (CNT) or carbon nanofibres (CNF) or both into the polymeric fibres CNT and CNF fillers can act as reinforcement components, but they may also modify the electrical and surface properties of the electrospun fibres, nonwovens and fibre webs. Composite fibres, containing CNTs or CNFs, are more stable in the carbonization process since CNTs nor CNFs do not shrink during heat treatment.

Due to the composite fibres also the shrinkage of precursor composite web (composite support structure) during carbonization is reduced. The tensile strength and modulus of the precursor composite web, oxidized composite web and carbonized composite web are increased and thus their processability is improved allowing the roll-to-roll processes. Also conductivity of the carbonized composite web is higher in comparison to carbonized web obtained from neat PAN or alternatively the same conductivity can be obtained in lower carbonization temperature, which is a clear cost factor.

Electrospun nanosized fibres have characteristically high specific surface area and thus electrospun fibres and fibre webs may be used as effective carrier for subsequent fixation of substances such as catalyst onto fibre surfaces. The specific surface area can further be increased by CNT or CNF addition.

The carbonized electrospun composite structure (carbonized composite web) thus produced is thin and shows improved electrical conductivity and mechanical strength over carbonized electrospun PAN nonwovens without CNT or CNF addition. Description of the Drawings

In the following, the invention will be discussed with reference to accompanying figures, where

Fig.1 shows a schematic view of a catalyst layer and membrane electrode assembly,

Fig. 2 shows a schematic view of roll-to-roll process of heat treatment,

Fig. 3 shows a schematic view of producing membrane electrode assembly by roll-to-roll process,

Fig. 4 scanning electron microscopy image of an electrospun polyacrylonithle fibre structure,

Fig. 5 scanning electron microscopy image of an electrospun composite support structure

The schematic figures 1-3 are not intended to be drawn to scale.

Detailed Description of the Invention

The invention provides a composite catalyst layer structure, which comprises carbonized composite web and catalytic material. These composite catalyst layer structures may further be used as electrodes of a membrane electrode assembly.

Composite catalyst structure, for example for an electrochemical cell, is produced by electrospinning e.g. co-electrospinning of a carbonizable polymer, such as polyacrylonithle (PAN) or alike, and carbon nanotube (CNT) or nanofibre (CNF) composite nonwoven (composite support structure) followed by carbonization of the structure (carbonized composite web) and direct deposition of a catalytic material on it.

In order to produce a composite support structure, an essential part of the present invention is that it includes increment of CNTs or CNFs or both into electrospun structure. CNTs or CNFs can be used as fillers in electrospun fibres, by adding them into spinning dope. The length of carbon nanotubes of nanofibres may be preferably less than 20 microns. Diameter preferably from 2 to 200 nm. The amount of CNT and/or CNF filler is preferably at most 10 weight-%.

Fig. 1 shows a schematic view of a membrane electrode assembly 1 according to one embodiment of the invention. The structure is formed of an electrolyte membrane 2, a catalyst layer structure 4, 4^* and a carbon paper or cloth 6, 6\ The catalyst layer 4, 4^' comprises a fibrous carbonized nonwoven web structure 8 where the carbonized fibres comprise carbon nanotubes and/or carbon nanofibres 10 and have catalytic material particles 12 supported on the fibres.

Fig 2. shows a schematic view of a continuous roll-to-roll heat treatment process. Electrospun composite support structure is reeled off from the roll 3 and conveyed on a conveyor through a heat treatment at 200-350⁰C in oxidizing atmosphere in an oven 5 and reeled up on a roll 7. Another heat treatment to produce carbonized composite structure, may be carried out using the same procedure but in the difference heat treated in inert atmosphere at 300-3000⁰C, preferably at 300-1800⁰C . Oxidizing and carbonizing heat treatment steps of the composite support structure may be carried out as individual steps or as a continuous process.

Fig 3. shows a schematic view of producing membrane electrode assembly by continuous roll-to-roll process. A electrolyte membrane 2 is reeled off from the roll 3 and the catalyst layer structure 4 and 4^' are applied and laminated 5 onto it. Further carbon paper or cloth 6 and 6^'are laminated and after that thus formed membrane electrode assembly 1 is reeled up on a roll 7. Instead of continuous process, membrane electrode assembly may also be produced through a multi-stage process, wherein the different layers are laminated together individually.

Heat treatment process of composite support structure, deposition of catalyst layer on it, and further process to produce a membrane electrode assembly may also be carried out continuously. The following nonlimiting examples are provided to further illustrate the present invention.

Example 1 According to one embodiment of the invention the catalyst support layer and the membrane electrolyte assembly may be produced as follows.

Electrospinning solution containing PAN and CNTs in dimethylformamide (DMF) is prepared by first dissolving small amount of PAN into DMF with the aid of heat and stirring. After that CNTs are added and dispersed with ultrasonic homogenizer. Small amount of polymer acts as a dispersing agent without increasing the viscosity, which would slow down the dispersing of the CNTs. When homogenous dispersion is obtained rest of the polymer is dissolved. The final polymer concentration should be high enough in order to ensure the sufficient viscoelastic forces to hold the jet coherent during the electrospinning process so that fibrous structures are formed. The sufficient amount depends on the molecular weight of the polymer and the structure of the polymer chain.

The solution is fed into nozzle system and electric field applied. PAN is very responsive to process parameters especially when conductive CNTs increase the conductivity of the solution. CNTs in composite solution increase the conductivity of the solution and thus increase the electrical forces of the process. Too high voltage and too high flow rate of the solution may cause the formation of 3-dimensional fibre structures instead of thin fibrous sheet. Electrospun composite fibres are collected onto suitable substrate. Precursor fibre web is removed from the substrate and, if necessary, calandered in order to compress the precursor web without affecting the fibrous structure. Preparation of precursor web is then followed by well known heat treatment steps to convert the PAN precursor to carbon structure at temperatures of 200 - 3000 ⁰C. CNT or CNF addition to the electrospinning process will lead to smaller shrinkage of fibres during the carbonization process and improved mechanical and electrical properties of the carbonized web.

Platinum or platinum alloy nanoparticles are then deposited onto the carbonized web using well known physical or chemical deposition methods, e.g. PVD, CVD, ALD, liquid flame, reduction of precursor salts or electrodeposition. As an alternative, different spinel or perovskite type oxides, organic macrocycles, silver or nickel can be used as catalyst materials in alkaline environment.

Finally, one or more layers of the catalysed webs are laminated on both sides of the electrolyte membrane by hot pressing to form a so called membrane- electrolyte-assembly (MEA). The catalyzed webs can be treated with an electrolyte (e.g. Nafion) solution before the lamination to improve the protonic conductivity of the catalyst layer.

Example 2

Electrospun web was prepared from solution containing 13.3 % PAN in dimethylformamide. Viscosity of the solution was 1290 centipoise (cP). Solution was electrospun to web having basis weight of 10 g/m². Electrospinning was conducted with continuous electrospinning equipment with following parameters: nozzle size 0.4 mm, number of nozzles 40, spinning distance 150 mm, spinning voltage 28 kV. Electrospun fibres have diameter of 480 ± 160 nm and their surface was smooth. Appearance of fibres is presented in Fig. 4.

90 mm wide web of the electrospun PAN fibres was continuously stabilized (i.e. oxidized) in a tube furnace in air atmosphere with the following parameters: conveyor speed 0,5 m/h, effective heating rate 100 °C/h, maximum temperature 265 ⁰C, time at maximum temperature 70 min, inner diameter of the furnace tube 100 mm, length of the furnace tube 1 ,8 m, number of heating zones 6. The oxidized web was further carbonized in the same furnace under nitrogen atmosphere with the following parameters: conveyor speed 0,2 m/h, effective heating rate 200 °C/h, maximum temperature 1100 ⁰C, time at maximum temperature 45 min.

Example 3

Electrospun composite web was prepared from solution containing 13 % PAN and 0.25 % of CNT (Nanocyl™, Nanocyl s.a Belgium, NC7000 - MWCNT 90% C purity for industrial applications, av. diameter 9.5 nm, av. length 1.5 μm) in dimethylformamide. Viscosity of the solution was 2320 cP. Sol ution was electrospun to web having basis weig ht of 1 0 g/m². Electrospinning was conducted with continuous electrospinning equipment with following parameters: nozzle size 0.4 mm, number of nozzles 40, spinning distance 150 mm, spinning voltage 28 kV. Electrospun fibres contained approximately 1 ,9 wt% of CNT. Fibres have diameter of 430 ± 110 nm and the tips of CNTs could be seen on their surface. Appearance of fibres is presented in Fig. 5. where the nanotubes are seen as surface roughness. For example, in Fig. 5 the nanotubes on the surface of the fibre are seen inside the white circles.

The PAN/CNT composite web was stabilized and carbonized using the same parameters as in Example 2. Table 1 shows some properties of nonwoven PAN and PAN/CNT webs.

The shrinkage was measured as the reduction of the web width.

The embodiments described above are only exemplary embodiments of the invention and a person skilled in the art recognizes readily that there may be further embodiments without deviating from the basic underlying invention.

Claims

Claims:

1. A catalyst layer for electrochemical applications, wherein

a catalyst support structure is formed by electrospinning of a carbonizable polymer precursor and carbon nanotubes or carbon nanofibres

and

the composite catalyst support structure is converted to a carbonized composite web by subsequent heat treatment steps

and

a catalyst material is deposited onto the carbonized composite web by physical or chemical methods.

2. A catalyst layer according to claim 1 , wherein the carbonizable polymer precursor is polyacrylonithle (PAN), pitch, polyvinyl alcohol (PVA), polybenzimidatzole, polypyrrole or other polymer having high carbon yield.

3. A catalyst layer according to claim 1 , wherein the polymer/CNT/CNF composite web is heat treated at 200 - 350 ⁰C in oxidizing atmosphere.

4. A catalyst layer according to claim 1 , wherein the composite web is heat treated in inert atmosphere at 300 - 3000 ⁰C, preferably at 300 to 1800 ⁰C.

5. A catalyst layer according to claims 3 and 4, wherein the heat treatment is carried out continuously as one or more roll to roll processes.

6. A catalyst layer according to claim 1 , wherein the thickness of the carbonized composite web is 1 to 50 microns, preferably, 2 to 20 microns.

7. A catalyst layer according to claim 1 , wherein the catalyst material is platinum or platinum alloy, e.g. Pt-Ru, Pt-Pd, Pt-Co, Pt-Ni, Pt-Co-Cr, Pt-Co- Mn.

8. A catalyst layer according to claims 1 or 7, wherein the catalyst material is deposited as nanoparticles the size of which is from 1 to 20 nm, preferably from 2 to 10 nm.

9. A catalyst layer according to claims 1 or 7, wherein the catalyst material is deposited using at least one of the following methods:

- physical vapour deposition,

- chemical vapour deposition,

- arc discharge,

- atomic layer deposition or - liquid flame spray.

10. A catalyst layer according to claims 1 or 7, wherein the catalyst material is deposited using reduction of precursor salts by microemulsion, impregnation, colloidal method or electrodeposition.

11. A catalyst layer according to claim 1 , wherein the catalyst material is an organometallic compound like Co-Tetraphenylporphyhne, Fe- Tetraphenylporphyhne, Co-Tetramethoxyphenylporphyhne or Co- Phthalocyanine.

12. A catalyst layer according to claim 1 , wherein the catalyst material is silver or nickel.

13. A catalyst layer according to claim 1 , wherein the catalyst material is perovskite or spinel, e.g. Lao,iCao,9MnO3 or MnCθ2θ₄.

14. A membrane electrode assembly for a membrane fuel cell, wherein the fuel cell electrodes are produced according to claim 1

and

that the electrodes are laminated with the electrolyte membrane.

15. A membrane electrode assembly according to claim 14, wherein one electrode layer is laminated on each side of the membrane.

16. A membrane electrode assembly according to claim 14, wherein more than one electrode layer is laminated on one or both sides of the membrane.

17. A membrane electrode assembly according to claim 14, wherein same catalyst material is used on both sides of the membrane.

18. A membrane electrode assembly according to claim 14, wherein different catalyst materials are used on different sides of the membrane.

19. A membrane electrode assembly according to claim 14, wherein one or more electrode layers are pretreated with the electrolyte solution before the lamination step.

20. A membrane electrode assembly according to claim 14, wherein one or more electrode layers are pretreated with a hydrophobic agent, e.g. polytetrafluoroethylene (PTFE), before the lamination step.

21. A membrane electrode assembly according to claim 14, wherein the lamination is performed by hot pressing.

22. A membrane electrode assembly according to claim 14, wherein additional pieces of carbon paper or cloth are laminated with the electrolyte membrane and the catalyst layers.

23. A membrane electrode assembly according to any of the claims 14 to 22, wherein the membrane fuel cell is one of the following:

- a polymer electrolyte membrane,

- a direct methanol,

- a direct ethanol or

- an anion exchange membrane fuel cell.

24. A membrane fuel cell using membrane electrode assemblies according to claim 14.