Master thesis presentation Astrid Hjern, MPMCN

Abstract:
The proton exchange membrane fuel cell is a potential key player in reducing greenhouse gas emissions. There is, however, a need for further improvements in Pt/C catalyst durability and costs. The purpose of this thesis is to improve catalyst durability by investigating what upper potential limit (UPL) voltage clipping should be implemented during load cycling. It is further investigated how scan rate affects catalyst durability. Based on previous research, possible causes and pathways of Pt degradation are discussed. Five accelerated stress tests (ASTs) are conducted on three Pt/C catalysts. In two tests, the scan rate is 50 mV/s or 250 mV/s. The potential window is set between a lower potential limit (LPL) of 0.6 V and various UPL between 1.0 and 0.7 V. The electrochemical surface area (ECSA) is calculated to track catalyst degradation at specific intervals during the ASTs.
The degradation of the catalyst increases with a higher scan rate from 23 to 26% for scan rates of 50 mV/s and 250 mV/s, respectively. All three catalysts show a decrease in degradation when the UPL is lowered from 1.0 to 0.8 V. With decreased UPL a thinner oxide layer is formed. The degradation increased for two catalysts at UPL 0.7. The Pt/C catalyst with the highest support area of 750 m2/g displayed the lowest degradation, possibly due to large inter-particle distance. The primary degradation mechanisms for platinum in the potential window of 0.6 and 1.0 V are dissolution and agglomeration. Based on these results, it can be concluded that a decreasing UPL and increasing support area decrease the catalyst degradation. However, initial ECSA variations, low coating quality, and ink age questions the accuracy of the results. Therefore, it would be beneficial to repeat the current test with a more controllable method.

Password: 950544

Examiner: Björn Wickman
Supervisor: Marika Männikkö
Opponent: Nora Malmquist