Effect of Fuel Cell Operating Potential Window on Pt/C Catalyst Durability
Examensarbete för masterexamen
Materials chemistry (MPMCN), MSc
The proton exchange membrane fuel cell is a potential key player in reducing green house gas emissions. There is, however, a need for further improvements in Pt on carbon support catalyst (Pt/C) 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 investi gated how scan rate affects catalyst durability. Based on previous research, possible causes and pathways of Pt degradation are discussed. Three catalysts from different producers with varying support surface area are examined with accelerated stress tests (ASTs) using triangular wave . One AST was performed with a scan rate of 250 mV/s in the potential window of 0.6 to 1.0 V. Four ASTs were performed with a scan rate of 50 mV/s with a set lower potential limit (LPL) of 0.6 and varying UPL between 0.7-1.0 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 increases for two catalysts at UPL 0.7, possibly due to error sources in the method. The Pt/C catalyst with a support area of 750 m2/g displayed the lowest degradation possibly due to a large inter-particle distance. The primary degradation mechanisms for platinum in the potential window of 0.6 and 1.0 V are commonly described as Pt dissolution and agglomeration. Based on the results, it can be concluded that a decreasing UPL in the region 1.0 to 0.8 V and increasing support area results in lower catalyst degradation. However, the influence of initial ECSA variations, low coating quality, and ink age questions the accuracy of the results. Therefore, it would be beneficial to repeat the current tests with a more controllable method.
Proton exchange membrane fuel cell, accelerated stress test, triangular wave, electrochemical surface area, rotating disk electrode, upper potential limit, scan rate, Pt/C.