Accessing fundamental characteristics of fuel cells by in operando high-energy XRD

V. Petkov
Central Michigan University,
United States

Keywords: fuel cells, in operando studies, catalysts

Summary:

Devices for efficient and clean energy conversion such as fuel cells are an attractive alternative to mankind’s dependence on fossil fuels. However, for the fuel cell technology to become commercially viable a key challenge is to be resolved. That is, affordable and efficient in terms of activity and stability catalysts for speeding up the chemical reactions driving cells’ operation have to be developed. Recently, thanks to the sustained effort of an army of researchers, a number of excellent catalysts for fuel cells, including catalysts for the sluggish Oxygen Reduction Reaction (ORR) taking place at the cathode of virtually all fuel cells, were developed. Unfortunately, as practice has shown, catalysts proven excellent when tested in a Lab do not necessarily perform excellently inside operating fuel cells. The reality check underlines the fact that transferring excellent catalytic properties into real devices without losing the former is not a trivial task by itself. We will show that the task can be facilitated by in operando high-energy XRD coupled to atomic pair distribution functions (PDFs) analysis. In particular, we will show results from recent in operando high-energy (110 keV) XRD and atomic PDFs studies on binary and ternary NM-TM nanometer-sized catalysts (NM=Pd, Pt and TM=Ni, Co) as tested for ORR activity and stability at the cathode of a fully functional Proton Exchange Membrane Fuel Cell (PEMFC) operated (cycled between 0.6 V and 1.2 V as suggested by DOE) for12 hours. Structural characteristics of prime importance to the ORR activity and stability of NM-TM NPs such as the NP structure type, size, phase state, metal-to-metal bond lengths and coordination numbers at the NP surface and roughness of NP surface are derived from the in operando diffraction data. The PEMFC current output is also recorded during the in operando XRD experiments and used as a measure of the ORR activity and stability of NM-TM NPs. Experimental structure and catalytic data will be compared side by side and hardly appreciated so far relationships between the concurrently evolving atomic ordering and efficiency (activity & stability) of NM-TM NPs as ORR catalysts will discussed. Particular attention will be paid to the decay of ligand, strain and atomic ensemble effects in NM-TM NPs due to leaching of TM species from the NPs during ORR. The relationship between that decay on one side and the NP coalescing and Ostwald ripening, also taking place during ORR, on the other will be looked over as well. Routes for improving the ORR activity and stability of NM-TM NPs and, hence, the efficiency of fuel cells will be suggested.