Metal oxides are gaining increasing interest in electrocatalysis research as alternative to commonly used high surface area carbon supports for electrocatalyst nanoparticles. On the one hand, metal oxide supports can offer improved corrosion resistance in electrocatalytic processes occurring at high electrode potentials where carbon supports suffer from severe oxidation. On the other hand, intrinsic catalytic properties of the supported catalyst nanoparticles can be influenced by interactions between catalyst and metal oxide support. Thus, metal oxides open a wide field for optimization of supported electrocatalysts similar to the situation in heterogeneous catalysis.

However, in electrocatalysis applications, electronic conductivity of the support material is necessary, a requirement which is difficult to be met by metal oxides. Whereas few metal oxides possess intrinsic metallic conductivity, a wider selection of semiconducting materials is achieved by generating defects, such as dopant atoms or oxygen vacancies, inside the lattice of an intrinsically isolating metal oxide. Our research on semiconducting Sb-doped SnO₂ (ATO) support for Pt nanoparticles in the context of oxygen reduction reaction (ORR) electrocatalysis revealed a fundamental descriptor for the choice of suitable metal oxide support materials [1]: Well-known from the theory of the semiconductor–electrolyte interface, the metal oxide flat-band potential separates the potential range where the oxide surface is enriched with mobile charge carriers from the potential range of charge carrier depletion leading to a drastically reduced support conductivity at certain electrode potentials. This potential-dependent in situ conductivity switching can be seen as direct electrochemical analog of a field-effect transistor.

On the one hand, this electrochemical transistor effect can limit metal oxide support conductivity and, therefore, electrocatalyst performance if the conductivity switching potential, i.e. the metal oxide flat-band potential, overlaps with the potential range of application. On the other hand, as shown for Pt/ATO ORR catalyst, the electrochemical transistor switching of ATO conductivity has a highly beneficial effect on the stability of supported Pt nanoparticles by limiting electrochemical dissolution and Ostwald ripening at high electrode potentials. Therefore, the electrochemical transistor effect of metal oxide supports offers a way for the development of ORR catalysts combining high performance with strongly improved stability.

Acknowledgement

This work was supported by CCEM Switzerland and Umicore AG & Co KG within the project DuraCat. The authors thank the BESSY II synchrotron at Helmholtz-Zentrum Berlin, Germany, for providing beamtime at the 7T MPW SAXS beamline.

References

[1] T. Binninger, R. Mohamed, A. Patru, K. Waltar, E. Gericke, X. Tuaev, E. Fabbri, P. Levecque, A. Hoell, T.J. Schmidt, Chem. Mater. 29 (2017), 2831–2843.