Alcohol-Soluble, Sulfonated Poly(arylene ether)s: Investigation of Hydrocarbon Ionomers for Proton Exchange Membrane Fuel Cell Catalyst Layers

Abstract

Novel sulfonated poly(arylene ether)s, characterized as being highly sterically encumbered, were synthesized for investigation as the ionomer in proton exchange membrane fuel cell (PEMFC) catalyst layers. Catalyst-coated membranes were prepared via their incorporation into alcohol-based catalyst inks, devoid of the high-boiling, polar aprotic solvents typically required for hydrocarbonbased ionomer inks. Catalyst layers thicknesses increased from 8.5 to 9.1 μm when the hydrocarbon ionomer loading was increased from 20 to 40 wt%, but resulted in a 77% loss in pore volume for fully hydrated electrodes. The catalyst layers possessed similar electrochemical surface areas and net ionic conductivity, yet catalyst layers containing 20 wt% ionomer yielded the highest overall fuel cell performance and considerably outperformed catalyst layers prepared from inks that contained high-boiling solvents. Perfluorosulfonic acid (PFSA) ionomers offer exceptional physical and chemical stability for proton exchange membrane fuel cells (PEMFCs) as well as high proton conductivity, 1 but are relatively expensive, possess high gas permeability, and are of limited use at high temperature and low humidity.2 Hence, attention has focused on the investigation of hydrocarbon (HC)-based solid polymer electrolytes for use as PEMs.3 Compared to PFSA ionomers, there is a large breadth of potentially low cost monomers available for synthesis of HC-based analogues. However, while significant progress has been made studying HC solid polymer electrolytes as membranes, 4 there is very little understanding of their incorporation as ionomer in catalyst layers.5 Studies on PFSA ionomer-based catalyst layers have revealed the complex nature of the interactions between ionomer, Pt, and the carbon support.6-8 The agglomeration of PFSA ionomer in catalyst inks and during the catalyst layer deposition process are believed to dramatically affect proton, gas and water transport through the catalyst layer. A similar impact is expected for catalyst layers containing HC-based ionomers. A previous report by J. Peron et al., for example, demonstrated the influence of sulfonated poly(ether ether ketone) (sPEEK) loading in the catalyst layer (CL). 9 In each case, sPEEKbased CLs were found to contain smaller aggregated catalyst particles and smaller pore sizes than their PFSA-based counterparts. Inferior in-situ performance was found for all sPEEK-containing electrodes, 10

The Control and Effect of Pore Size Distribution in AEMFC Catalyst Layers

“…Accordingly, CCMs 1 & 2 showed expected higher absolute and volume-normalized ionomer conductivities. 31 However, the volume-normalized ionomer conductivity of CCM 5 was unexpectedly lower than that for CCMs 3 and 4, despite near-equal cathode thicknesses (Fig. S3).…”

“…31 However, it is postulated that ionomer in the CL operates at an effectively reduced hydration state under these circumstances, which is problematic for obtaining stable, operating anion-exchange ionomers. Therefore, it is possible that the poor mesoporosity observed contributes to instability of the AEMFC, which highlights the need to understand each porosity regime in electrode studies.…”

Sulfophenylated Terphenylene Copolymer Membranes and Ionomers

“…Measurements were performed using two detector positions, which provided a q-range of 0.06 to 6nm À1 .S amples were measured after equilibration with atmospheres of controlled humidity and temperature, or with D 2 Oa t controlled temperature. [61] After disassembly,M EAs were freeze-fractured with liquid N 2 and the membrane and electrode thicknesses were measured by scanning electron microscopy (FEI/Aspex Explorer). These samples were loaded into sealable sample cells with two drops of excess D 2 O.…”

Highly Stable, Low Gas Crossover, Proton‐Conducting Phenylated Polyphenylenes

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Tw oc lasses of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic-based proton exchange membranes.B oth types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures.T hey exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity,and are markedly resilient to free radical attack. [5,6] Many different ion-containing polymers have been investigated with significant focus on those incorporating aromatic groups as part of the polymer main chain, such as sulfonated derivatives of poly(arylene ether)s, [7] poly(arylene ether ketone)s, [8][9][10] poly(arylene sulfone)s, [9] poly(imide)s, [11] poly(benzimidazole)s, [2,12] and poly(para-phenylene)s. [8,13,14] However,i ti st he general consensus that hydrocarbonbased ionomers to date are inhibited by ag reater sensitivity to oxidative degradation either ex situ (e.g., FentonsReagent test) and/or in situ (e.g.,i nP EM fuel cells). [5,6] Many different ion-containing polymers have been investigated with significant focus on those incorporating aromatic groups as part of the polymer main chain, such as sulfonated derivatives of poly(arylene ether)s, [7] poly(arylene ether ketone)s, [8][9][10] poly(arylene sulfone)s, [9] poly(imide)s, [11] poly(benzimidazole)s, [2,12] and poly(para-phenylene)s. [8,13,14] However,i ti st he general consensus that hydrocarbonbased ionomers to date are inhibited by ag reater sensitivity to oxidative degradation either ex situ (e.g., FentonsReagent test) and/or in situ (e.g.,i nP EM fuel cells).

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