Oxygen Transport Effects of Cobalt Cation Contamination of Ionomer Thin Films in Proton Exchange Membrane Fuel Cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) are a promising automotive energy conversion technology for reducing the dependence on petroleum and reducing greenhouse gas emissions. However, the high cost of the platinum (Pt) electrocatalyst responsible for increasing the rate of the oxygen (O2) reduction reaction (ORR) in the cathode is one of the primary inhibitors to this technology’s mass market penetration. In order for PEMFCs to become cost competitive with internal combustion engine technologies, the electrocatalyst cost must be significantly reduced. To address this issue, the use of Pt alloys such as Pt3Co has been explored. Alloys such as Pt3Co have been shown to have higher activities than pure Pt electrocatalysts,(1) reducing the mass of the expensive Pt. However, the utilization of Pt3Co poses contamination risks to the ionomers used in PEMFCs due to cobalt’s (Co) propensity to dissolve out of the catalyst, migrate to the ionomer, and replace protons on the sulfonic acid sites(1). Cai et al.(2) demonstrated the effects of this risk through the testing of membrane electrode assemblies (MEAs) containing Co doped ionomers, which showed a decrease in proton conductivity through the ionomer. In addition, Kongkanand et al.(3) have shown that voltage losses at high current densities for low-Pt loaded electrodes can be attributed to localized O2 transport resistances in the thin films of ionomer covering the electrocatalyst surface. Thus, the effects of dissolved Co on the local O2 transport through these thin films of ionomer are of particular interest. The purpose of this work is to experimentally characterize the effects of Co contamination on the localized O2 transport properties of ionomer thin films. To study these effects, we modified an existing fuel cell to allow for the testing of gas (O2) transport through ionomer thin films in a well-controlled environment, similar to the setup used by Liu et al.(4) The setup allows for the films’ transport resistance to be measured under non-polarized (uniform Co distribution) and interface free conditions, in order to isolate the effect of the Co contamination. In addition, the setup allows for the control of the ionomer sample’s temperature and relative humidity, to evaluate the effects of each on the O2 transport resistance of the doped thin films. To test the effects of differing levels of Co cation contamination during PEMFC operation, we take O2 transport resistance measurements for various amounts of controlled Co ion doping. Our preliminary results have indicated significant increases in O2 transport resistances for high Co contamination levels, consistent with our hypotheses derived from the results mentioned above(2, 3). This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. Figure 1