Morphology and degradation of high temperature polymer electrolyte fuel cell electrodes

The polybenzimidazole-based high-temperature polymer electrolyte fuel (HT-PEFC) has become a major topic in the clean energy field in recent years due to the appealing advantages, e.g., improved electrochemical kinetics and enhanced tolerance to carbon monoxide. However, the introduction of phosphoric acid (PA) in the membrane electrode assembly (MEA) leads to several issues, e.g., phosphate contaminations, accelerated Pt corrosion, and PA leaching. What is more, compared to classic PEFCs, the required higher Pt loading is another factor that impedes the commercialization of HT-PEFCs. The dissertation focuses on the electrochemical processes in the MEA, especially on the cathode side, to investigate the relations among the parameters of the MEA as well as study the degradation mechanism under different operating conditions. Various types of gas diffusion electrodes are fabricated by using different methods and recipes. Electron characterizations and electrochemical measurements demonstrate that the electrodes with more regular cracks and less PTFE agglomerations perform better. The PTFE binder plays a role in adjusting the PA distribution in the MEA and 10-25 wt.% PTFE is the advised content in the cathodecatalyst layer (CCL). In addition, the interactions of another three parameters in MEAs (i.e., Pt loading, thickness of CCL, and PA doping level) are investigated by using the design of experiment (DoE) method. The results propose a direction for the preparation of MEAs: a relatively high PA doping level and a low ratio of Pt/C catalyst are beneficial to improve the Pt utilization efficiency inHT-PEFCs. The degradation in HT-PEFCs is another emphasis of this dissertation. Five stressors that include high temperature, open circuit voltage, thermal cycling, low-load cycling and high-load cycling areapplied to the above optimized MEAs. A Pt band is unexpectedly visualized in the membrane of an MEA only run a standardized break-in procedure by using the focused ion beam milling technique. This concretely proves the corrosive environment in HT-PEFCs. The Pt band formation is explicitly influenced by the operating conditions. The OCV and low-load cycling stressors cause the largest amount of Pt loss into the membrane, indicating that the high potential is the most pronounced factor in Pt corrosion. During the aging tests, the membrane degradation and PA leaching only have minor effects on the performance loss, while the main cause is the degradation of the Pt/C catalyst