Raman Spectroscopy for Diagnostic Analysis of Fuel Cell Catalyst Coated Membranes A critical assessment

Raman spectroscopy is a useful analytical tool for characterising the carbon chemistry of proton exchange membrane fuel cell (PEMFC) catalyst coated membranes (CCMs) and understanding changes in the carbon matrix due to corrosion and degradation processes. However, interpretation of the data is highly sensitive to the sampling and spectral analysis methods employed. Here we critically assess the use of Raman spectroscopy for diagnostic analysis of uncycled PEMFC CCMs and equivalent CCMs subjected to dynamic load cycling (DLC). We first consider different approaches to quantitative analysis of Raman spectra and show that a two peak spectral fitting model which only considers the characteristic D1 and G peaks in the Raman spectrum provides an inferior fit compared to a four peak fitting model that includes the minority D3 and D4 peaks associated with amorphous carbon and disordered graphitic domains. We furthermore demonstrate that in specific cases these two models can generate opposing trends. We then compare quantitative Raman metrics generated from spectral maps at different locations of CCMs subjected to different durations of cycling. A large degree of scatter in the data precluded conclusive correlation between Raman data and duration of cycling, highlighting the importance of sufficiently large sample sizes when performing quantitative analysis. However, a difference in behaviour between cathode and anode was observed, characterised most prominently by a higher degree of scatter in the Raman metrics associated with disordered and amorphous carbon, potentially pointing to contrasting ageing phenomena resulting from the different conditions at the cathode and anode. We also demonstrate that spectral differences across the cycled anode appear to be highly spatially heterogeneous, indicating that the associated chemical changes are localised on the <100 <mu>m scale.