Investigation of Ion Conductivity and ORR on Metal/Water Interfaces in Proton Exchange Membrane Fuel Cells

Abstract

The high cost of platinum (Pt) is a challenging barrier to broad commercialization of proton exchange membrane fuel cells (PEMFCs). Over recent decades efforts have been made to decrease Pt loading by increasing its mass and specific activities through nanostructuring and alloying. One such approach is the nanostructured thin film (NSTF) electrode1, which has Pt or Pt alloy catalyst deposited onto high surface area organic whiskers and lacks the addition of ionomer binder. NSTF cathodes typically have a lower Pt loading than conventional carbon supported Pt (Pt/C) catalyst cathodes, but tend to lose performance in low relative humidity and cold, flooded conditions. Despite the lack of ionomer, the ion conductivity for NSTF and ionomer-free Pt black electrodes has been shown to be good under high relative humidity conditions.2,3 However, the apparent proton transport mechanism is not well understood. The high mobility of adsorbed proton or hydroxyl species and the diffuse charge in the electric double layer could account for high ion conductivity on the metal surface away from the membrane.4 Such mechanisms are complicated by their sensitivities to water activity and interfacial electric potentials. The present work investigates the role of water on the ion conductivity and oxygen reduction reaction (ORR) activity of metal/water interfaces by measuring the dependence of accessible metal surface area on humidity and potential for two ionomer/binder-free metal surfaces: Pt and gold (Au). Pt is studied because of its relevance to PEMFCs and Au is selected as a model surface because of its close ideal behavior and oxide-free surface within the envelope of PEMFC cathode potentials. However, Au catalyst provides low ORR activity relative to Pt. A membrane electrode assembly (MEA) was fabricated by painting a layer of Pt black or Au micro powder (0.5 m2/g) ink on a gas diffusion layer (GDL). A commercial Pt gas diffusion electrode (GDE) was used as the anode. Cyclic voltammetry (CV) was conducted for different relative humidity conditions as shown for Au in Figure 1a. As the inlet gas relative humidity increases, the double layer and Au-OH redox currents increase, indicating increased electrochemical active surface area on the wetted Au. As Figure 1b shows, the increased active area also yields increased current density when operated the cell as an air/H2 fuel cell and polarization curves are measured. This indicates that water films on the Au surface play an essential role in ion conduction and that the active area made accessible by the water also facilitates the ORR. References (1) An, S. J., and Litster, S. (2013) In Situ, Ionic Conductivity Measurement of Ionomer/Binder-Free Pt Catalyst under Fuel Cell Operating Condition. ECS Trans. 58, 831–839. (2) McBreen, J. (1985) Voltammetric Studies of Electrodes in Contact with Ionomeric Membranes. J. Electrochem. Soc. 132, 1112. (3) Sinha, P. K., Gu, W., Kongkanand, A., and Thompson, E. (2011) Performance of Nano Structured Thin Film (NSTF) Electrodes under Partially-Humidified Conditions. J. Electrochem. Soc. 158, B831. (4) Zenyuk, I., and Litster, S. (2014) Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta. Figure 1