Characterizing the Pt-Electrolyte Interface in PEM Fuel Cells

Abstract

Much progress in reducing the platinum loading in proton exchange membrane (PEM) fuel cell cathodes to a target of 0.1 mg-Pt/cm2 has been made. However, large performance losses are observed at high current densities at low cathode platinum loadings. Recent studies have shown that the large losses are caused due to resistances at or near the catalyst-ionomer interface.[1] This resistance can be countered through the use of ionomers designed to interact with Pt in a way that does not limit the oxygen reduction reaction. However, the development of tools to quantify and characterize this resistance is needed to enhance our understanding of this resistance and correlate it with performance loss. In the early 1990’s Feliu et al. developed a method for measuring adsorbed ion charges on Pt single crystals in an aqueous electrolyte.[1] We have validated this CO displacement method on polycrystalline Pt and carbon-supported Pt nanoparticles in an aqueous electrolyte and present here results from implementing this methodology in a fuel cell. Figure 1 illustrates the current transient measured during chronoamperometry as CO gas is introduced to the cathode of a fuel cell. The measured displacement charge at potentials less than 0.2 V represent a displacement of adsorbed protons, corresponding to a positive displacement current. At potentials greater than 0.3 V, a negative current is measured corresponding to the displacement of negatively charged adsorbed species. In the case of a fuel cell cathode, these negative species are the sulfonate group present in the ionomer. The adsorption charge can then be integrated to determine the amount of species adsorbed on the surface at each potential. Using this tool, ionomer adsorption for different ionomer types at different operating conditions can be examined. [1] Kongkanand A, Mathias MF, J. Phys. Chem. Let. 7 (2016) 1127. [2] Feliu JM, Orts JM, Gomez R, Aldaz A, Clavilier J. J. Electroanal. Chem. 372 (1994) 265. Figure 1. Chronoamperometry at different potentials during CO Displacement. Introduction of CO to the cathode occurs at t=120s. Figure 1