Improving PEMFC Performance Using Short-Side-Chain Low-Equivalent-Weight PFSA Ionomer in the Cathode Catalyst Layer

Abstract

Incorporation of short-side chain (SSC) ionomers in the catalyst layers (CL) of proton exchange membrane fuel cells (PEMFCs) can improve performance, particularly at low relative humidities. We attempt to understand this effect by comparing PEMFCs with cathode CLs containing Pt on carbon-black (CB) and either SSC Aquivion ionomer or a standard long-side-chain (LSC) Nafion ionomer at 50% and 100% RH. The CL microstructures are characterized for their micro- and mesoporosity. The CLs are formed into PEMFCs and probed with polarization curves, cyclic voltammetry, O2 gain, limiting current measurements, and electrochemical impedance spectroscopy. PEMFCs containing the SSC ionomer in the cathode CL have superior polarization curves compared to those containing the LSC ionomer in the mass transport region under all conditions. We find that the SSC ionomer imparts lower proton transport resistances, lower charge transfer resistance to the cathode near 0.60 V, and lower mass transport resistance at 0.40 V. We attribute some of the performance improvements to the superior proton conductivity of the SSC ionomer, and the remainder to the higher micropore volume in the SSC-containing CLs which can more effectively evaporate water to the gas phase, improving both the availability of catalyst sites for charge transfer and mesopores for gas transport.