Any attractive system for light-driven fuel production comprises water oxidation.¹ Desirable catalysts are based on abundant elements and can operate under benign conditions (room temperature, moderate pH). The biological catalyst of photosynthesis, the protein-bound Mn₄Ca(µ-O)₅ cluster, fulfills these requirements.¹ In recent years, catalysts based on amorphous oxides of first-row transition metals came into focus. Aside from the redox-active metal (Mn, Fe, Co, Ni), these oxides often contain redox-inert cations (K, Na, Ca, ..), anions that could act as a proton acceptor (phosphate, borate, ..), and intercalated water. They may exhibit catalytic activity of the bulk material and rate-determining proton transfer to buffer molecules.² Catalytic oxides based on Co, Ni, and Mn, which were directly electrodeposited or screen-printed on electrodes,³ are compared and common properties are discussed. These oxides share common structural motifs and can be bi-functional.⁴ They are redox-active meaning that the metal ions undergo oxidation-state and structural changes when exposed to various (electrode) potentials in the vicinity of the equilibrium potential for water-oxidation. Oxidation state changes can be described in terms of proton-coupled electron transfer reactions involving short-range and long-distance proton transfer. Structure-function relations are observable.⁵ A conceptual approach is outlined for the mode of water oxidation in these heterogeneous catalysts, which feature certain molecular properties. Additionally, the question is addressed whether (surface) amorphisation^6,7 is essential for catalysis by initially crystalline materials. Recent results on amorphous H₂-formation catalysts, namely molybdenum sulfides ‘doped’ with cobalt and nickel, may be mentioned as well.

¹Dau, H., Limberg, C., Reier, T., Risch, M., Roggan, S., and Strasser, P. (2010) ChemCatChem 2, 724-7612.

²Klingan, K., Ringleb, F., Zaharieva, I., J. Heidkamp, P. Chernev, D. Gonzalez-Flores, M. Risch, A. Fischer and H. Dau (2014) ChemSusChem 7, 1301–1310.

³Lee, S.Y., González-Flores, D., Ohms, J, Trost, T., Dau, H., Zaharieva, I. and Kurz, P. (2014) ChemSusChem DOI: 10.1002/c ssc.201402533.

⁴Cobo, S., Heidkamp, J., Jacques, P.-A., Fize, J., Fourmond, V., Guetaz, L., Jousselme, B., Ivanova, V., Dau, H., Palacin, S., Fontecave, M., and Artero, V. (2012) Nature Materials 11, 802-807.  

⁵Bergmann, A., Zaharieva, I., Dau, H. and Strasser, P. (2013) Energy Environ. Sci. 2013, 6, 2745-2755.

⁶Indra, A., P. W. Menezes, I. Zaharieva, E. Baktash, J. Pfrommer, M. Schwarze, H. Dau and M. Driess (2013). Angew. Chem. Int. Ed. 52, 13206–13210.

⁷González-Flores, D., Sánchez, I., Zaharieva, I., Klingan, K., Heidkamp, J., Chernev, P., Menezes, P.W., Driess, M, Dau, H., Montero, M.L. Angew. Chem. Int. Ed., in press.