Article
Expanding application of XPS to fuel cells - seeing beyond the surface
Surface Analysis Spotlight: XPS
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by Kateryna Artyushkova President |
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There is not a single peer-reviewed paper on electrocatalysts for fuel cells that do not have XPS among a set of analytical techniques used in characterizing the surface properties of catalysts and electrodes. Our customers have been using PHI XPS instruments because XPS provides critical information on the overall composition, chemical state of the active phase, i.e. metal such as Pt or transition metals; carbon chemistry, such as graphitic versus aliphatic; surface oxides, etc. 1-3 |
With the development of analytical instrumentation, there are many novel types of experiments in the portfolio of analysts beyond simple elemental and chemical analysis.
In the upcoming invited talk "The Role of Surface Chemistry in Electrocatalytic Activity: Studying Interfaces Using Novel Surface Analytical Methods" at the GRC Fuel cell conference, I will be discussing some of the new applications such as:
- Depth-resolved analysis using high energy sources in HAXPES
- Novel ion guns for damage-free depth profiling
- Microprobe technology – in-situ micro-SEM
- Mitigating damage for X-ray-beam sensitive materials.
In transition metal−nitrogen−carbon (MNC) – based electrodes, the distribution of active sites that perform oxygen reduction reaction, ionomer morphology, and structure of ionomer-catalyst interface within the cathode catalyst layers is critical. It is important to probe the structure of the electrode as a function of depth. Gas Cluster Ion Beam (GCIB) is optimal for XPS depth profiling through organic structures such as polymers. We have used a GCIB ion gun to remove the top layer of the ionomer to probe the chemistry beneath it. The sputter rate for this setting of GCIB is 54 nm per minute as determined for PMMA. After one minute of cleaning, most F and S from the ionomer is being removed, which is confirmed by a significant decrease of CF2 peak from Nafion in the C1s spectrum. Important changes in N chemistry are detected as well. The surface-facing groups such as hydrogenated nitrogen are being removed to expose deeper active sites such as pyridinic nitrogen and nitrogen coordinated to iron.
