The success of fuel cells relies on the performance of electrocatalysts and their effective electrochemical conversion of fuel (e.g., hydrogen, borohydride) and oxygen in air to electricity. For hydrogen fuel cells, the focus is on the performance of the cathode electrocatalyst, as the oxygen reduction reaction is kinetically limited. In direct fuel systems, such as direct borohydride fuel cells, the kinetic limitations are usually for the fuel oxidation reaction at the anode. The typical measure of catalyst effectiveness is its kinetic activity, which is measured analytically at low overpotentials (0.9 V for a hydrogen fuel cell), and often in a half cell by rotating disk electrode voltammetry. However fuel cells are operated at high current densities (~0.6 V) where mass transport of the reactants and buildup of the reaction byproducts become reaction limiting. We show how the mass transport of the electrodes is affected by the physical attributes of the catalyst layer, including its catalyst loading, thickness, hydrophobicity and the type if ionomer used for proton conduction in the electrode. The thickness, hydrophobicity and porosity of the gas diffusion media (GDM) also plays a dominant role at high current densities. Our presentation will review our publications on catalyst characterization, and compare results measured in half cells by rotating disk electrode voltammetry (RDE) to that in hydrogen fuel cells and direct borohydride fuel cells. The effect of gas diffusion media on apparent electrocatalyst kinetics will be discussed for hydrogen fuel cells.

Critical evaluation of the performance of fuel cell electrocatalysts

ACS Spring Meeting 2021

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