Abstract and subjects
The focus of this presentation will be the relatively new class of non-precious metal (or platinum group metal-free, PGM-free) electrocatalysts for oxygen reduction reaction (ORR). This particular class of materials has been the subject of a growing interest worldwide since the late 1990s, as one of two main approaches to address the catalyst-cost challenge of polymer electrolyte fuel cells (PEFCs) and some other electrochemical systems that heavily depend on precious metals, typically platinum and its alloys, for oxygen reduction. (With the other approach relying on various ways of Pt thrifting, for example, alloying, core-shell nanoparticle catalyst development, and thin-layer catalyst designs.) The most successful methods of synthesizing PGM-free ORR catalysts typically involve the heat treatment of precursors of transition metals, nitrogen and carbon. These methods have become more sophisticated over the years, yielding catalysts with oxygen reduction activity approaching that of precious metal-based formulations already implemented in the first commercially available fuel cell cars. The most impressive progress in the field appears to have been achieved predominantly via the improvements to the ORR active-site density and catalyst-layer porosity, especially microporosity, using pore-forming agents, sacrificial templates and by other approaches.
All the recent progress in PGM-free electrocatalysis notwithstanding, the growing conviction in the electrocatalysis community in the last few years has been that further development of such catalysts hinges on the understanding of the source of the ORR activity in materials without any precious metals and, possibly, without obvious two adjacent active sites for promoting the dissociative mechanism of oxygen reduction. Such understanding is also needed to reveal the reasons for the relatively fast performance loss in fuel cells and also in the electrochemical cells, as well as for the development of remediation strategies, which, together with the activity enhancements, are required to overcome the performance gap between non-precious materials and the incumbent Pt-based cathode catalysts.
In this presentation, we will concentrate on both the unquestionable progress in the PGM-free electrocatalysis, accomplished over a relatively short period of time, and significant challenges still facing the field. In the context of challenges, we will summarize efforts specifically targeting better understanding of the sources of PGM-free catalysts activity, identification and quantification of the ORR active sites, and the reaction mechanism, as important steps towards a rational design of catalysts in the future. The results from both theoretical and experimental studies will be presented. Of a number of both in situ and ex situ techniques used for gaining a better insight into the origins of ORR activity in PGM-free catalysts, we will concentrate on the microscopic and X-ray absorption spectroscopic methods, as well as on the implementation of molecular dioxygen analogues as probes for the ORR active sites on the PGM-free catalyst surface, which promises to make otherwise bulk techniques, surface-specific. The characterization part of the presentation will include research performed in close collaboration with partners in the DOE Electrocatalysis Consortium (ElectroCat).