GB2558058A

GB2558058A - Immersion-cast catalyst layers for fuel cells and a method for their production

Info

Publication number: GB2558058A
Application number: GB1717589.4A
Authority: GB
Inventors: Kas Onur; Sylvia Ryan; Moore Ashley; Tkacik Gabriel
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2016-10-26
Filing date: 2017-10-26
Publication date: 2018-07-04
Also published as: GB201717589D0

Abstract

A process for producing a porous catalyst layer for use in a PEM fuel cell comprises providing a casting solution comprising a metal supported on a carrier, an ionomer, a pore forming polymer and a solvent. The composition is cast onto a substrate, for example by doctor blading, and a phase inversion is effected to obtain a porous layer. The phase inversion may be effected by immersion precipitation in which the catalyst layer is immersed into a bath of non-solvent. The non-solvent being selected such that it is a non-solvent with respect to the pore forming polymer but is miscible with the solvent of the composition. The solvent may for example be N-methylpyrrolidone (NMP) and the non-solvent may be water. The pore-forming polymer may be polyethersulfone (PES). Platinum on carbon may be used as the catalyst in the composition and an example of a suitable ionomer is Nafion (RTM).

Description

(56) Documents Cited:

WO 2001/071835 A2 JP 2000173625 A US 6214891 B1

JP 2009146772 A US 6602630 B1 (71) Applicant(s):

Merck Patent GmbH (Incorporated in the Federal Republic of Germany) 250 Frankfurter Strasse, Darmstadt 64293, Germany (72) Inventor(s):

Onur Kas Ryan Sylvia Ashley Moore Gabriel Tkacik (74) Agent and/or Address for Service:

Venner Shipley LLP

200 Aldersgate, LONDON, EC1A 4HD,

United Kingdom (58) Field of Search:

INT CL H01M

Other: WPI, EPODOC, Patent Fulltext (54) Title of the Invention: Immersion-cast catalyst layers for fuel cells and a method for their production Abstract Title: Porous catalyst layer prepared by phase inversion (57) A process for producing a porous catalyst layer for use in a PEM fuel cell comprises providing a casting solution comprising a metal supported on a carrier, an ionomer, a pore forming polymer and a solvent. The composition is cast onto a substrate, for example by doctor blading, and a phase inversion is effected to obtain a porous layer.

The phase inversion may be effected by immersion precipitation in which the catalyst layer is immersed into a bath of non-solvent. The non-solvent being selected such that it is a non-solvent with respect to the pore forming polymer but is miscible with the solvent of the composition. The solvent may for example be Nmethylpyrrolidone (NMP) and the non-solvent may be water. The pore-forming polymer may be polyethersulfone (PES). Platinum on carbon may be used as the catalyst in the composition and an example of a suitable ionomer is Nation (RTM).

Figure 1

630b

1/2

2/2

Immers

I Cells and a Method for Their Production

Technical Field

The present invention relates to immersion-cast catalyst layers for fuel cells and a method for their preparation from a casting composition comprising a fuel ceil catalyst, a proton conducting polymer, a pore forming polymer and a solvent.

Background and description of the prior art

Proton exchange membrane fuel cells show great potential as future source of electrical energy. However, commercialization has been hindered by cost. The cost of the membrane electrode assembly (MEA) constitutes a significant portion of the cost of the fuel cell stack and is mostly due to the cost of platinum, which Is used as a catalyst.

While recently progress has been made in bringing down costs of the fuel cell stack by reducing the loading of platinum catalyst, further progress needs to be made also in respect to manufacturing costs per se.

Current catalyst layer manufacturing processes Involve slot die coating of casting compositions of inks onto moving substrates, such as for example a gas diffusion layer (GDI) or a membrane. The resulting structures are generally weak in mechanical properties and do not fulfill the durability requirements as demanded by Department of Energy (DOE) targets. Because in these structures porosity is created by the catalyst particles it Is believed that it will be challenging to produce such structures with consistent properties In commercial large-scale production,

Alternatively the catalyst may be applied to a carrier substrate or a membrane by a physical vapor deposition method, such as disclosed for example in US 5,039,561. However, due to the need for operating under vacuum conditions the process disclosed therein is considered difficult to perform and also requires substantial capital investment for commercial large-scale operation.

WO 2012/001061 AX discloses « process for producing a gas diffusion electrode comprising the step of casting a porous electrically conductive web with a suspension of particiss of an electrically conductive materiel in a solution of a first binder to provide: Ifeefectrically conductive layer for a biological fuel celt

However, none of these documents discloses a process for producing an immersion-cast catalyst Layer that Is suited for being used In a proton exchange membrane foci cell said immersion-cast catalyst layer preferably allowing for reduced catalyst loading while essentially maintaining performance.

1.0

The present, application therefore alms at providing a method of producing such ipmwsioneast catalyst layer in a relatively easy and simple manner, preferably also/allowlngfer large-scale commerclai production..

IS

Summary of the invention

The present inventors have now surprisingly found that the above objects may be attained either individually or in any combination fey the process and the porous catalyst layer of the present application,

The present application therefore provides for s process for producing a porous catalyst layer for a proton exchange membrane fuel cell, said process comprising the steps of

a) providing a : casting composition comprising^ metal supported on a carrier, an ionomer, a pore forming polymer and a solvent;

b| casting said casting composition onto a substrate to yield a catalyst layer; and c> effecting a phase Inversion in said catalyst layer to obtain a porous 30 catalyst layer,

The-present application therefore also provides for an Immersion-cast porous catalyst' layer comprising a metal supported on a carrier, an ionomer and a pore forming polymer, obtained fey such process,

Brief description of the figures

Figure 1 shows a schematic: representation of an exemplary membrane electrode assembly, '5

Figure 2 shows the performance curve for the membrane electrode assembly of

Detailed deseripf ion of the invent ion

For the purposes of the present application the term ionomer” is used to denote, a polymer composed of nsacromoleeules In which a small but significant proportion of the constitutional units has Ionic or ionhable groups or both (see also Pure and

Applied Chemistry, Vol, 78, No. 11, pp. 2667^2074, particularly page 2072),

For the purposes of the present application the term ’’monomer unit is used to denote the largest constitutional unit contributed to the structure of the polymer by a single monomer molecule,

For the purposes of the present application the terms monomer” or monomer moiecule” are used synonymously and denote a molecule 'which can undergo polymerization, thereby contributing a constitutional unit (monomer unit 1 to the polymer. The term hafagenated monomer is used to denote a monomer

2.5 comprising halogen.

For the purposes of:the present application an asterisk {*) denotes a linkage to an adjacent unit or group, and in ease of an oligomer or polymer if may denote a link to an adjacent repeating unit or to any other group comprised in the oligomer or polymer,

I n gen e ra! terms, the present app!Ication relates to a process of prod uci n g a poresus catalyst layer by immersion-casting,

The process of the present application comprises a first step of

-4 a) providing a casting composition comprising a metal supported on a carrier, an Ionomer, a pore forming polymer and a solvent.

While any metal that can be supported on a carrier may be used in the present process, it is nevertheless preferred that the metal is selected from: the group consisting of Sc, Y, Ti, 2r, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, lanthanides, actinides and any biend thereof, More preferably said metal may be selected from the group consisting of Co, Rh, Ir, Ns, Pd, Pt,, Cu, Ag and Au, Even more preferably said metal may be selected from the group consisting of NI, Pd, Pt, Cu, Ag and Au, Still even more preferably said metal may be selected from the group consisting of Ni, Pd and Pt, Most preferably said metal is Pt.

The carrier on which the metal Is supported may be any Inert material onto which metals can be supported. Suitable examples of such carriers may be selected from the group consisting of carbon, oxides, halides and blends thereof. Suitable oxides may for example be selected from the group consisting of alumina, magnesia, silica, and any blends of these, A suitable halide Is for example magnesium chloride, The most preferred carrier on which the metal is supported is carbon.

The ionomer used herein may be described by formula (I)

wherein

Ma Is an electrically neutral repeating unit;

Ma is an ionic or ionteable repeating unit; a is at least 1 and at most 50; b is 1; and c Is at least 5 and at most 10,000,

Preferred electrically neutral repeating units Ma may be represented by general formula -CRⁱR²-CR^sR⁴~(X¹}₍r wherein R\ R², R³, R \ X¹ and d are as defined herein,

R¹, R², R³ and R⁴ are independently of each other selected from the group consisting of H, F, Ci, 8r, I, alkyl having from 1 to 20 carbon atoms, aryl having from 6 to 10

-5 :carbon atoms; and nryl having from f> to 10 carbon atoms substituted with an Wry; having from 1 to 10 carbon atoms as Well as analogous alkyl and aryl wherein one: or more hydrogens are replaced by fluorine. More preferably R~, Ffy R³and R⁴ are independently of each other selected from the group consisting of R, F> alkyl having from 1 to 10 carbon atoms, aryl having from 6 to 10 carbon atoms and aryl having from 6 to 10 carbon atoms substituted with en alkyl having from 1 to 10 carbon atoms as well as analogous alkyl and aryl wherein one or more hydrogens are replaced by fluorine. Even more preferably R³. R\ R³ and R'· are independently of each other selected from the group consisting of H, n. methyl, ethyl and phenyl as well as the respective analogues of methyl ethyl and phenyl wherein one or more hydrogens are replaced by fluorine. Moat preferably R-\ R< R³ and R⁴ are R

Examples of alkyl having from 1 to 10 carbon atoms are methyl, ethyl, mpropyl, k propyl, n-butyi, bbetyl, s-butyl, t-butyl, Zmtetbylbtriyl, n-peotyi., s-pentyl, cyclopentyl, neo-pentyl., p-hexyl, eyclohexyl, neo-hexyl, n~heptyb cycloheptyl, noctyl, cyciooctyl., 2-ethylhexyl, nonyl ahd decyl as well as the respective f’iuorinated analogues wherein one: or more hydrogen Is replaced by fluorine. Preferred; examples of alkyl having from 1 to 10 carbon atoms are methyl, ethyl, n-propyl, ipropyl, s-butyl, n-pentyl and n-bcxyl as well as the respective fiuorinaled analogues

20: wherein one or more hydrogen is replaced by fluorine,

X·³ may be selected from the group consisting Of 0, 5, NR⁵ and PR⁵, with R⁵ being selected from the group consisting of H, alkyl having from 1 to 20 carbon atoms, aryl having from 6 to 3.0 carbon atoms and aryl having: from 6 to 10 carbon atoms

25: substituted with an alkyl haying from 1 to 10 carbon atoms as well as analogous alkyl and aryl wherein one or more hydrogens are replaced by fluorine, Most preferably X¹ Is O, d may be 0 or 1, Preferably d is 0.

Preferred ionic or ionisable repeating units Ma may be represented by general formula -(CR^eR^?)_e-{CR^sR^iti'- wherein R^s. Ry M, R³ and e are as defined below,

Ffr, R’ and R^s are Independently of each other selected as defined above for Ry Rh

R³ and Rh el may be 0 or 1, .Preferably e 4s 1 may be represented by general formula -iR^hi-X² wherein P²⁰, t and X^z are as defined below.

f may be G or :1, Preferably f Is 1

X? may be -SCnH or-POsHs· Preferably X^:? is -SCsH

R^ie Is represented by general formula h, ΐ add J as defined below.

gj h, Land j may Independently of each other be at feast 0 and at most 10, preferably at least 1 and at most S,

to IQ carbon atoms as well as analogous alkyl and aryl wherein one or more hydrogens; are replaced by fluorine,

Pxarnples of allranediyi having from 1 to 10 carbon atoms are methylene i-Crh-j, ethanedlyl, n-propanediyl,, i-propanedlyl etc. as well as the respective analogues wherein one or more hydrogen is replaced by a fluorine.

Particularly preferred are ionorners of general formula (II)

wherein k Is at least 6 and at most IS, I is 1,. m is Q or 1 and n is at least 2 and at: most 4,. Ah example of such an ionomer Is Nation™,, Which Is commercially available from DuPont,

The choice of pore forming polymer is not particularly limited- Suitable examples of pore forming polymers may be selected from the group consisting of polyfaerylic acid), polymethacrylate, polyamide, polyimide, polyurethane,, poiybenzimidaeote, polycarbonate, polyacrylonitrile, poly(vinyl alcohol)., polyOactic acid), polyethylene oxide, polystyrene, polyaniline, polyethylene terephthsiate, polybutylene terephtbaiate, poly(aoryl amide), polycaprolactone, poiyfvmyhdene fluoride), polyethylene cowinyl alcohol), poiysulfone (P$U), polyethersuffone (RES), and any blends of these, A preferred example of a pore forming polymer Is polyethersuifone (RES),

IS

Examples of suitable solvents may be selected from the. group consisting of ethers of general formula R¹³-O-Rⁱ⁴, alcohols of general formula Rⁱ⁵-OH, ketones of general formula R^:6“C{“G)-R'^:?, amides of general formula (Rⁱⁱ⁵hN-C{~O)~R^i? and any blends thereof, wherein h® R*⁴, R^3S and R^1S are independently of each other selected from alkyl having from i t© ID carbon atoms arm fiuorinafed alkyl having from 1 to 10 carbon atoms, and Rⁱ⁷ is selected from the group consisting of H, alkyl having from 1 to ® carbon atoms and fiuorinafed alkyl having from 1 to ID carbon atoms, or R⁷³and R^w may together be selected from alkanedlyl having from 4 to 6 carbon atoms and fiuorinafed alkanedlyl having from 4 to 6 carbon atoms, or Rⁱ⁵ and R^i? may together be selected from alkanedlyl haying from 4 to 6 carbon atoms and fiuorinafed alkanediyi having from 4 fo_: δ carbon atoms. With respect to R¹³, R^u, R^is, R^x6 and R⁵⁷ the term ‘'fiuorinafed” means that at least one hydrogen is replaced by fluorine,

Examples of particularly suited ethers are dimethylefher, ethylmethyiether, diethylether, bufylethylefher, difeoproyiether, fetrahydrofurane, the: respective fluorinated analogues arid any blend of these,

Examples of particularly suited alcohols are methanol, ethanol, n-propanol. I35 propanol, I-butanol, 2-butanol, 2-metbyi-l-propanol, i-psntanoi, 2-pentanoi, idpentanol, z-methyhl-buianol, 2-methyl-2-butanol, 3-methyl-l-butanol, 3-methyiS2-bufanol, the rsspecdve fiuorirrafed analogues and any blends of these. Preferred examples are methanol, ethanol, n-propawb i-propenol, the respective fluorinated analogues and any blends of these.

5: Examples of particularly suited ketones are acetone, 2-tatanone fethylmethyiRetone), 2~pertanone, 3-pentanone, 2-hexanOn.e, S-hexanone, 4~ hexanone, Yoeatanone, 3-ocfanone, AoctanoPs, acetophenone, the respective floorfeefed' analogues and any blend of these. The most preferred ketone is acetone,

Examples· of particularly suited amides are Ν,Ν-dimefhylformamide f pMFT and M,N -dimethylacetamide (“OMAe).

Further examples of solvents Include N-methyl pyrredidone, tetrametbylurea, acetone, dlmethylsuifoxide and trlethyl phosphate.

Preferred examples of solvents include M,N~dimethylfermamlde (”OMF^W) and N..N· dimethylacstamide fDMAc'h, N--methyl pyrrolldone. tetrgmethylurea, acetone, dimethyisulfoxlde and trlethyl phosphate.

The present casting compositions,· yihfcb: in the following are cast onto a substrate toyield the catalyst layer, may be obtained by mixing of the respective components. Such mixing may for example be performed by ball milling, stirring, such as magnetic or mechanical stirring, shaking, sonication, homogenization or any method making use of more than one of these, The choice in mixing method may depend upon the nature of the easting compositions to be mixed. For .example It might be useful to mix byball milling if the metal supported on the. carrier is present: in bigger particles, which need to be reduced in site. Sonication and magnetic stirring and a combination of both, possibly several repeat intermittent cycles Of sordcation and magnetic stirring have proven to be particularly useful.

The duration of mixing is not particularly limited and may also depend upon, the nature of the respective casting composition. For example the duration of mixing may be chosen in the range from st few seconds to one week or even longer, it may for example be at least 1 s, 5 s .10 s, 30 s, 1 min, 5 min, 10 min, 30 min, 1 h, 2 h, 3 h, 4 b, 5 h, 6 h, S h, 12 h. 15 h. 18 h, 21 b, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, days or oven longer. Preferably said mixing time may be at least 12 b, 15 b/18/h, 21 h, 1 day,-.2 days, 3 daysj 4 days,. S days, δ days, 7 days or even longer. More preferably said mixing time Is at least 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days or even longer, Even more preferably said mixing time Is at least 2 days, 3 days,

S 4 days, 5 days, 6 days., 7 days or even longer,

As an alternative to room temperature the mixing may also be performed at higher or lower temperature. The choice in temperature may also depend on the nature Of tbe_: respective casting composition to be mixed end may for example be chosen

IO based on the·: mtsoibSity and/or solubility and/or thermal stability of the components ofthe casting composition to be mixed.

Preferably the present casting composition comprises at least (for example at least 1 wt% or 2 wt% or 3 wt% or 4 wtM or 5 :wt%) of the combined: fractions of metal with carrier, ionomer abd pore forming polymer,,with wt% being relative to the total weight of said casting composition.

Preferably the present casting composition comprises at. most 30 wt% (for example at most 25 wt.% or 20 wt% or 19 wt% wt% or IS wtM or 17 wlM or IB ^.-or 15 wt%l of the eomblned amounts of metal with carrier, ionomer and pore forming polymer, with wt% being relative to the total weight of said casting composition, preferably said casting composition comprises metal with carrier,, ionomer and pore forming polymer in a ratio of A; B : C, wherein

A i.e. metal togefherlwith carrier, is at least 10 parts and at most 80 parte, for example at least 12 or 14 or 16 or 13 or 20 or 2.2 or 24 or 26 or 28 or 30 or 32 or 34 or 36 or 38 or 40 or 42 or 44 or 46 or 48 or 50 parts, and for example at most 80 or 78 or 76 or 74 or 72 or 70 parts;

is at least 1 part and at most 40 parts, for example at least 2 or 3 or 4 or 5 or

6 or 7 or 8 or 9 or 10 or 12 or 12 or 13 or 14 or 15 parts,, andfor example, at most38 or 36 or 34 or 32 or 30 parts; and C Is at most SO parts, for example at most 48 or 46 or 44 or 42 or 40 or 38 or 36 or 34 or 32 or 30 Or 28 or 26 or 24 or 22 or 20 or IS or 16 or 14 or 12 or 10 or 8 or 6 or 4 or 2 or 1,5 or 1 or 0,5 or 0,1 or 0.01 or 0,001 parts, '· xo provided that the sum of A, B and C is 100 parts, with parts being given in weight percent relative to the total weight of the catalyst (metal with carrier^ Ipnomer and pore forming polymer.

Further, the process of the present application comprises a second step of bj casting said casting composition onto a substrate to yield a catalyst layer.

Said casting may be performed by any suitable method, such as for example slot die coating, knife coating, doctor blade coating or lubrication flow coating. Such

XQ methods are generally known to the skilled person and therefore need hot be described in detail.

Said substrate is preferably a gas diffusion layer, An example of such a gas diffusion layer is a hydrophohlzed carbon paper gas diffusion layer with a mlcroporons coating. Such gas diffusion layers may bo oommerclully obtained from a variety of sources,

Optionally,· step b is followed by a drying step. Preferably In said drying step a part of the solvent is evaporated,

Subsequently the process of the present application then comprises the step of c) effecting a phase Inversion in said catalyst layer to obtain a porous catalyst layer.

Preferably the phase Inversion is effected by immersioh precipitafish, wherein/the previously obtained catalyst layer is immersed in a bath of a non -solvent. Said solvent is preferably selected such that It is a non-solvent, or at best a very poor solvent for the pore forming polymer but Is miscible with the previously defined •Solvent Without wishing to be bound by theory it Is believed that munerslon in the non-solvent will lead to the absorption of water and removal of solvent from the catalyst layer,, thus leading to precroitation and formation of the porous catalyst layer.

Preferably, said non-soivent is selected from the group consisting of water, ethylene glycol compounds, propylene glycol compounds and any blend of any of ·· ί 1 these. More preferably,, said non- solvent is selected from the group consisting of water, triethylerm glycol and a blend of these.

The temperature of the non-solvent bath is not particularly limited. It is, however , preferably at least IOT, more preferably at least IS’C and most preferably at. least

20®C Preferably it-is at most 80”C, more preferably at most ?0°C or SO’C, even more preferably at most 5CAC or 40’C and most preferably at most. 30’C,

The porous catalyst layer obtained by the present process may be used In a membrane eteewde assembly of a proton exchange fuel celt Hence> the present: process preferably further comprises the following step of id) using said porous catalyst layer in a membrane electrode assembly of a proton exchange membrane fuel celi,

A schematic representation of a membrane electrode assembly (600), as Is for example used in a fuel cell., such as for example a proton exchange membrane fuel cell, is shown in Figure LA typical, membrane electrode assembly for a fuel cell comprises two: gas diffusion layers (GDI.) (630a, 630b), with adjacent catalyst electrode layers (620a, 620b), cathode and anode. At the anode, a first catalyst layer helps in: separating hydrogen into protons and, electrons, The resulting protons then travel through proton exchange membrane (6X0) to the cathode, at which a second catalyst layer helps in combining, the protons with oxygen and electrons to form water and heat, 0:n either side,, such membrane electrode assembly may further comprise a device, such as for example a: flow-field iafate, directing hydrogen to the anode and oxygen to the cathode.

Catalyst layers 620a. and 620b may be the same or different, In an aspect of the present invention such catalyst layers comprise the porous: catalyst layer in accordance with the present invention, As fuel cel) catalyst frequently platinum is used.

Examples

The following non-limiting examples further illustrate the advantages of the present invention.

-12 Example: la?

Nation™ ion exchange resin (02021, 20 % lonomer in alcoboi/water, obtained from 5 DuPont.. Wilmington, Delaware, USA) was dried and re-suspended in Nmethylpyrroltdone (IMMP) by hybridizing at 40*0 for 48 hours- Then polyethersulf'one (FES, 7600P obtained from Sumitomo Chemicals, Japan) was added and the mixture hybridised at 4CTC for 24 hours. Both polymers, i.e, the

Nation Ion exchange resin and the polyethersuiforse, could he: dispersed / id suspended in the N· rnethy Ipyrrolidone, in this, way a first and a second solution was prepared with the characteristics as indicated in Table 1,

Table 1

Solution	e^HHHMeooooooooewoooooooo«NeeeM<MMoooeoooMoooMo«oowoooooooMeeeeNeewSMMM Ratio PES,- Nation™ T<	atai polymer content (wt%l
A	15 :1	16
8	:10:1 11 ieMeeeeeeeeeMMMeeeeeeeeeMMKeNeNe^roKMMeeeeeeeeeeeeewMMMOMNeeeeftn&MMMMMMOoooooMewQQQQQQMMeraenmeSMMMMMOOQOooo

Solutions A and S were oast by doctor blading onto the surface of a glass-plate.and then submerged in -water at 25X

Solutions A and 8 were diluted with trielhylenegiyeul (TEG) until the ratio of NMP to TEF was 2 ; .1, Resulting solutions C and D had a total polymer content of 20,7 wt% and 7,3 wt%, respectively, relative io the total weight of the solution. Formulations C and D were cast by doctor blading onto the surface of a glass plate., submerged in 'water at 25“C and then dried.

Example 2a

A catalyst solution was prepared by mixing 40% Pt on carbon black (HiSpec^ 4000 Ft/C catalyst powder obtained from Johnson MaUhey Pic, London) with triethylenegiycoi (TECft via homogenization so as to have a solution comprising 10 wt% Pt/C, relative to On total weight of the catalyst solution.

A polymer solution with a 1 : 1 ratio of Nation™ and PES and 17 wt% polymer content, relative to the total weight of the polymer solution, was prepared as described in Example la,

A casting composition was prepared by mixing catalyst solution and polymer solution in a 44 ; 56 weight ratio in a 40 ml vessel with ceramic milling beads In a hybridizer, The casting composition was cast onto glass by doctor blading and then immersion-east hi a water bath for 5 minutes at a temperature of 21’C so as to allow for complete pore formation and solvent removal, yielding an asymmetric mlcroporous structure with a measure thickness of 94 pm. Catalyst loading was calculated at 0,53 mg Pt/cm²,

Example 2b

A casting composition was prepared by adding 40% Pt on carbon black (HiSpec™ 4000 Pt/C catalyst powder obtained from Johnson Matthey Pic, London) to solution C of Example lb, which had a total polymer content of 10,7 wt%, relative to its total weight, until a ratio of 41; 18:41 of Pt/C : Nation™ : PES was reached. The casting composition was mechanically mixed with an overhead mixer for six hours.

The casting composition was cast onto a glass by doctor blading and then immersion-cast in a wafer bath for 5 minutes at a temperature of 21*C so as to allow for complete pore formation and solvent removal, yielding an asymmetric mlcroporous structure.

Example 2c

A solution having a total polymer content of 12 wt%, relative to the total weight of the solution, a 2:1 ratio of Nation™: PES and a 1; 1 ratio of NMP: TEG was prepared by the method described for Example lb.

A casting composition was prepared by adding 40% Pt on carbon black (HISpec™ 4000 Pt/C catalyst powder obtained from Johnson Matthey Pic, London) to the previously prepared solution until a ratio of 25 ; SO : 25 of Pt/C: Nation™ : PES was

-14reached. The:casting composition was mechanically mixed with an overhead mixer for ssx hours.

The casting composition was cast onto a glass fey doctor blading and then immersion-cast In a water hath to? 5 minutes at a temperature of 21°€ so as to allow for complete pore formation and solvent removal yielding sn asymmetric, mlcroporous structure,

Example 3

The casting composition ol Example 2b was hand-painted onto a gas diffusion layer .fSigmceP* 25BC with an average thickness of 235 pm and an average area weight Of 86 g/m\ SGI. Group., Wiesbaden, Germany), then immersion-cast into a water bath at a temperature of 21T and allowed to soak for 5 minutes so as to allow foils complete pore formation and solvent extraction, An Immersion-cast gas diffusion electrode (iCGDEI with a mieroporous catalyst layer having a catalyst content of

0.134 mg Pt / cm² (determined by back calculation of weight addition) was obtained,

The so-obtained Immersion-cast gas diffusion electrode was used as cathode in a membrane electrode assembly (MEA), A standard hon-structured (I.e, not immersion cast), hand-painted gas diffusion electrode with a calculated catalyst content q#:D,25 g Pt/cm³ wasused as anode. The MEA was then prepared by hotpressing the electrodes io either side of a membrane (hlafion.™ 211, DuPont,

Wilmington, Delaware. USA) at e temperature of^:140^?C using a Carver bench top hydraulic press (No. 3S12> Carver in e., Wabash, Indiana, USA),

Performance of the resulting N1EA was measured In a hydrogen/air fuel, cell at 80*€ and a relative humidity of 100 % without any back pressure. Results are shown in figure 2. The MEA was found to deliver ca, 205 mA / cm^? at. 0.65 V, hi general terms,, the present examples show the versatility of the present approach. It is shown that ionically conductive polymers, such as for example

Nafion™, can in combination with a pore forming polymer, such as for example PE'S, form a porous microstructure that can, for example,, advantageously find .10

-is application in a fuel ceil, It has also bean surprisingly found that the present, method is applicable to a wide range of polymer contents as well as varying ratios of the components of the castingoumpositions,. This versatility advantageopslyailows for easy adaptation of the process to the desired characteristics of the final product30

Claims

Claims

X, A process for producing a catalyst layer for a proton exchange 'membranefuel celt said process comprising the steps of

a) providing a casting composition: comprising metal supported oh a carrier, an ionomer, a pore forming polymer and a solvent;

b) casting said casting composition onto a substrate to yield a catalyst layer; and

c) effecting a phase inversion in said catalyst layer to obtain a porous catalyst layer.
2,. The process of claim i> wherein the metal is selected from the group: consist log of Sc.Y, Ti.. Zr> Hi, V.. Mb., Ta, Cr; Mo, W._t Fe., Ru, Os, Co, Rhjr, Hi, Pd., Pt, Cep Ag, Au, To, Cd, H<, lanthanides, actinidesahdanyblend thereof,,

TS
3, The process according to any one or more of the preceding claims,, wherein the earner is selected from the group consisting of carbon, silica., metal oxides,, metal halides and any blend thereof.

20 4. The process: according to any one or more of the preceding claims, wherein the ionomer comprises electrically neutral repeating units and ionized or lonhabie repeating units,

S, The process according to any one or more of the preceding claims, wherein

35 the solvent is selected from the group consisting of water, alcohols, ketones, ethers., amides and any blend thereof.

8:, The process according to any one or more of the preceding claims, wherein the process further comprises the stop of

30 Id) using said porous catalyst layer in a membrane electrode assembly of a proton exchange membrane fuel cell.
7. An Immersion-past catalyst layer comprising a metal supported on a carrier,, an ionomer and a pore forming polymer, obtained by the process of one or

35 more of claims 1 to 5,

Intellectual

Property

Office

Application No: GB1717589.4 Examiner: Dr Fiona Rogers