Study on Catalyst and Catalyst Layer for High Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

For the commercialization of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), it is most important to reduce the amount of platinum in membrane electrode assembly (MEA). Accordingly, a number of studies have been attempted to increase the platinum utilization in MEA for the HT-PEMFCs. Platinum utilization in HT-PEMFCs can be determined not only by the uniform Pt deposition on the carbon support surface, but also by the high accessibility to platinum nanoparticles O2 and phosphoric acid. This is because the actual electrochemical reaction takes place on the triple-phase boundaries where platinum, phosphoric acid, and oxygen all contact at the same time. Since platinum has significantly low Pt utilization due to low tolerance to phosphate and its anions poisoning, various studies have been conducted to improve Pt utilization by preventing the adsorption of phosphate ions onto Pt nanoparticles by using the 'third body effect', which involves pre-adsorbing organic molecules on the platinum surface. In this study, 4-Aminobenzenethiol(4-ABT) was used to mitigate phosphate poisoning. In 4-ABT, the thiol (-SH) group and the amino (NH2) group are attached to a benzene ring in a para position. when a sulfur accepts electrons from the surface of platinum nanoparticles by chemisorbing, NH2 group of 4-ABT in the phosphate solution is protonated to NH3+(positively charged), which plays a role in capturing phosphate anions (PO3-). As a result of X-ray photoelectron spectroscopy (XPS) and Energy Disperse X-Ray Spectrometer (EDX) analysis, it was confirmed that sulfur of 4-ABT was selectively chemisorbed on the platinum surface not on the carbon support. Electrochemical surface area (ECSA) gradually decreased as the 4-ABT coverage on the Pt surface increased in the cyclovoltammetry (CV) experiment using typical three electrode system, signifying that as the concentration of 4-ABT solution increases, the extent of adsorption also increases. As a result of the activity evaluation for the oxygen reduction reaction in the phosphoric acid solution, it was found that the maximum ORR activity in H3PO4 occurs at a 4-ABT coverage of around 30 % with the 20 μM of 4-ABT solution used. In addition, the catalyst layer (CL) structure was modified to increase the number of reaction sites (triple phase boundaries). The first approach to optimize the structure of the CL was to use polybenzamidazole (PBI) polymer coated carbon support. There are two expected effects that could be obtained by selecting PBI as a coating polymer: (i) Pt nanoparticles are mainly deposited on the outer surface of the catalyst support, leading to the improved Pt utilization. (ii) Proton conductivity is expected to be enhanced via the acid-base reaction between the imidazole group of PBI and liquid phosphoric acid in the CL. The N1s signal of XPS profile exhibits that PBI was still present after deposition of Pt, it can be observed that PBI coating did not affect the crystallinity and size of the platinum nanoparticles. When the CV was carried out using a three-electrode system and a potentiostat, the ECSA was improved by about 13% compared to the catalyst without coating. This behavior was highly consistent with the increased oxygen reduction reaction activity. In addition, the synthesized catalysts were fabricated as a cathode CL and applied to MEA for performance evaluation. As a result, when hydrogen / air was used as a fuel, it was shown that the use of PBI-coated catalyst improved about 30mv at 0.2 A /cm2. Compared with hydrogen / oxygen as a fuel, the MEA with PBI coated catalyst exhibited a lower oxygen diffusion resistance in the high current density region than that of the catalyst without coating. As a second approach to optimize the CL structure, the CLs were fabricated in various ways: spray, bar coating, and screen printing, and the effects on the morphologic structure and electrochemical properties were observed. There are no significant differences in the kinetic region for all electrode. However, in the case of gas diffusion electrode (GDE) manufactured by spraying displays the highest power density (383mW/cm2). The GDE manufactured by spraying is more attractive than the other two deposit techniques mainly due to a better mass transport from porous structure of CL. The GDEs fabricated with bar coating and screen printing method is suffered from acid flooding which is ascribed to the large cracks in CL requiring low capillary pressure.