Optimization of Gas Diffusion Electrode for High-Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

Recently, significant attention on high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) has been focused due to their tolerance to CO and simple water management in the system level. In HT-PEMFCs, a membrane electrode assembly (MEA), which is composed of a polymer electrolyte membrane, gas diffusion layer (GDL) and catalyst layer (CL), plays a pivotal role in determining a whole fuel cell performance. Since the electrochemical reactions occur on the three phase boundary (TPB) in the CL, where electrocatalyst, electrolyte, and reactant all contact, the properties of CL are critical to the fuel cell performance. In this work, several factors to affect properties of CL were investigated ranging from CL fabrication methods, binders in catalyst slurry to hydrophobicity of CL to optimize a gas diffusion electrode (GDE) which is composite of GDL and CL. As for CL fabrication techniques, cathode GDEs prepared with various CL deposition methods such as spraying, bar coating, and screen printing were examined. The result showed that all three GDEs with different CL deposit methods had similar performance at activation and ohm regions of polarization curve. However, in the case of GDE manufactured by spraying displays the highest power density (383mW/cm2) mainly due to a better mass transport from presence of the pores smaller than 1 m. Secondly, the performance of MEA with different binders in the CL was explored. Especially, new polymers (poly(phenylene oxide), PPO) with phosphate groups in the side chain, showing good proton conductivities are applied as binders for the GDE. Furthermore, the influence of the wettability of cathode CL on performance by adding phosphonic acid (PA) and vinylphosphoric acid (VPA) was also studied, which revealed that the change in hydrophobicity of CL had an impact on ohmic resistance. It can be explained by the fact that hydrophobicity affects the distribution of PA on the GDE. Finally, The influence on performance of the Pt loading of the anode was also conducted in a range from 0.2 to 1 mgptcm-2 to decrease total Pt loading in the MEA. It was suggested that the Pt loading of anode does not affect the performance within the studied range by tuning the anode CL. To prevent acid flooding at the anode, the GDE was prepared spraying method with hydrophobic binder.