PEMFC Performance Recovery from Air Pollutants Contamination

Abstract

Air pollution is a challenge that impedes proton exchange membrane fuel cell (PEMFC) commercialization. Many potential contaminants were studied with single cells and accelerated or long-term tests. These airborne contaminants cover a wide range of organic functional groups and inorganic compounds, including unsaturated hydrocarbons (alkenes, alkynes, and aromatics) and O (CO, alcohols, ketones, aldehydes, ethers, and esters), N (nitriles, ammonia, NOx), S (SO2, H2S, and COS), and halogen (halocarbons and HCl) species. In-situ and ex-situ studies indicated that contaminants adsorb on a Pt surface, compete with oxygen for sites, and may inhibit the oxygen reduction reaction (ORR). N compounds also showed an ionic conductivity effect, which was associated with an ammonium intermediate that replaces the proton in the catalyst layer ionomer and membrane. Cell performances showed that most catalyst and membrane contamination effects are mitigated by neat air operation with the exception of S and halogen species. These latter contaminants in the gas phase permeate through the electrolyte film to the catalyst surface, and are converted into anions, such as SO4 2 - and X-. Anions remain at the interface between the electrolyte film and the catalyst surface, and their subsequent removal is difficult due to repulsive effects associated with the ionomer (Donnan exclusion). In the present report, several strategies were used to restore the cell performance after exposure to anion intermediates originating from acidic gases (SO2, HCl) or halocarbons (bromomethane). Strategies encompassed elements such as oxidant gases (N2O, O3) and variations in operating conditions (current, relative humidity, temperature) in part to promote the presence of liquid water for rinsing and entraining anions. Oxidant gases and high cathode potentials were expected to oxidize Cl- and Br- ions into Cl2 and Br2, and facilitate desorption from the catalyst surface and release into gas streams. All experiments were conducted at 80 °C with a 50 cm2 single cell and Gore catalyst coated membranes (CCMs, 0.1 mg Pt cm- 2 on each side). Anode and cathode compartments were respectively fed with H2 and air at 100/50 % relative humidity, 2/2 stoichiometry and 48.3/48.3 kPag. The cell was operated at 0.4 A cm- 2. Results indicate that the application of N2O or high cathode potentials are insufficient to oxidize the adsorbed Cl- and Br- ions. In contrast, O3 is too potent as an oxidant, which leads to permanent damage to the catalyst layer and causes additional cell performance losses. Variations in cell current, relative humidity and temperature partially mitigate the presence of Cl- and Br- ions but they cannot fully restore the cell performance. A water rinse can completely restores the cell performance for all cases. As an example, Figure 1 illustrates the cell performance degradation and recovery resulting from a temporary bromomethane exposure. Acknowledgments The authors are indebted to the Office of Naval Research (award N00014-15-1-0028) for financial support of this project. The authors are also grateful to the Hawaiian Electric Company for their ongoing support of the operations of the Hawaii Sustainable Energy Research Facility. Figure 1: Cell voltage response to the 50 ppm BrCH3 exposure and the subsequent water rinse induced recovery. Figure 1