The Power of in-Situ Studies: How X-ray Absorption Spectroscopy Can Help to Develop PEMFC Catalysts with Increased Activity and Stability

Low and high temperature polymer electrolyte membrane fuel cells (LT- and HT-PEMFC) are promising systems in a future sustainable energy scenario, where renewable energies can be implemented by a power-to-gas strategy. The supply of hydrogen or methanol produced by intermittently available wind and solar energy will then be converted into electricity on demand using a combination of water electrolysis and fuel cells. However, in order to make this dream a viable reality, costs need to be dramatically reduced and the material’s stability significantly improved. In our opinion, this is only possible after detailed insights into the reaction and degradation mechanisms in these systems have been obtained, so that a knowledge-based design of specifically tailored materials becomes possible.

State-of-the-art catalysts in fuel cells are Pt(-M) with M=Ru, Co nanoparticles with particle sizes ranging between 2 nm and 5 nm and metal loadings between 20 and 80 wt.% supported on a high surface area carbon material. Recent research focuses also on functionalized carbons, ordered mesoporous and templated carbons, as well as carbon nanotubes (CNT) and even graphene. However, carbonaceous materials are very prone to corrosion, when subjected to the harsh operation conditions, in particular at the cathode side of the fuel cell. That is why current efforts in our group and others focus on the search for alternative support materials, such as electron-conducting oxides and polymers. Oxide-supported catalysts are well-known in heterogeneous catalysis, so that a closer collaboration between the two communities appears desirable in the future.

X-ray absorption spectroscopy (XAS) is a technique optimally suited to studies under fuel cell operation conditions, as neither UHV conditions in the experiment, nor long-range order in the sample are required. It can be applied to monitor the geometric structure of the catalytically active nanoparticles directly following particle oxidation and growth during operation. With the delta µ XANES method and complementary theoretical FEFF8 calculations, it is furthermore possible to identify adsorbed species on small Pt nanoparticles and follow their change in coverage and their competition for free adsorption sites elucidating the reaction mechanism. Since diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) has been applied manifold in heterogeneous catalysis for the study of oxidic support materials and – in contrast to XAS – is applicable in the lab, we decided to design a combined DRIFTS-XAS in-situ cell to investigate nanoparticle-support interactions in different gas atmospheres.

This talk will highlight how sophisticated in-situ techniques, such as X-ray absorption spectroscopy, but also others, will help with rational materials design. When the reaction mechanisms can be unraveled, the catalyst can be tailored and hopefully lesser fractions of the costly noble metals will be required. Getting more insight into degradation phenomena should help to optimize the relevant operation conditions in order to avoid those stressing the catalyst unnecessarily. We will give examples of the anode and cathode side of a HT-PEM fuel cell and also show first results measured in our DRIFTS-XAS in-situ set-up.