Experimental Study on Ionic Conductivity of Carbon Support within Polymer Electrolyte Fuel Cell Catalyst Layers

Abstract

Conventional polymer electrolyte fuel cells (PEFCs) use carbon supported Platinum (Pt/C) as the catalyst. With decades of efforts, researchers have brought down the loading of Pt below 0.1 mg/cm2 on the anode and 0.4 mg/cm2 on the cathode. However, the high cost of Pt based catalyst still serves as a big barrier for PEFCs’ fully commercialization. Within the catalyst layer, polymer electrolyte ionomer (e.g. Nafion) is usually used as the proton conducting media. During the catalyst layer fabrication process, Pt/C particles are present as aggregates. Due to size exclusion, the polymer electrolyte mainly coats the large mesopores of the Pt/C aggregates, i.e. ionomer covers the secondary pores. In high surface area carbon supports, a significant amount of Pt exists within the smaller mesopores and micropores of the support, which is inaccessible to the ionomer. Although it is known that protons are able to reach Pt within the aggregates when those pores are filled with liquid water, the magnitude and mechanism of proton transport is largely unknown. In past studies, the proton conductivity of continuous Pt surfaces have been studied (1), with hypothesis proposing that proton can transport through Pt surface by surface diffusion or through electric double layer (EDL) at the metal/water interface. It has also been reported that proton conductivity is severely hindered over extended glassy carbon surfaces (2). However, the widely used Pt/C catalyst doesn’t have continuous metal surface, and unlike glassy carbon, the carbon support used in the catalyst layer usually has a mixture of functional groups on the surface (e.g., carboxyl and sulfur species), which will alter the surface ionic conductivity through deprotonation. In this work, we measure the conductivity in ionomer free carbon (Vulcan XC72R) and ionomer free Pt/C (10 wt.% HiSPEC 2000) electrodes to better understand the conduction properties of ionomer-free carbon support surfaces. Our experiments evaluate the effects of both electrode potential and the relative humidity (RH) on the conductivity. Electrochemical impedance spectroscopy (EIS) and transmission line modeling show the proton conductivity of the ionomer-free electrode decreases as electrodes’ potential increase (see Figure 1). The potential dependence of the ionic conductivity is supported by a cyclic voltammetry (CV) analysis. Herein, we present several aspects related with the proton conductivity within the ionomer-free carbon domains of the catalyst layer and their effects on fuel cell performances. Acknowledgement This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. References E. L. Thompson and D. Baker, ECS Trans., 41, 709 (2011).U. Paulus, Z. Veziridis, B. Schnyder, M. Kuhnke, G. Scherer and A. Wokaun, J. Electroanalytical Chemistry, 541, 77 (2003). Figure 1