Publication date: 15th December 2014
The efficient catalysis of the four-electron oxidation process of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyser systems. Inspired by the composition of the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, we have developed synthetic routes for calcium manganese oxides and successfully used these materials as heterogeneous catalysts for water oxidation.
In screenings where Ce4+ was used as chemical oxidant, calcium-containing, layered manganese oxides from the birnessite mineral family clearly emerged as most active oxide phase. These oxides contain Mn in average oxidation states of 3.5-3.9 and have rather complex compositions (e.g. a typical stoichiometry is K0.20Ca0.21MnO2.21·1.4H2O). The synthetic parameters during their preparation proved to be of great importance for catalytic rates and we found that especially the Ca/Mn ratio and the temperature used during the post-synthetic treatment in air have to be optimized to enhance the catalytic performance of the materials.[1]
As a next step towards an application of these oxide catalysts in the context of solar fuels, screen-printing methods were investigated to coat conductive surfaces like FTO or nickel with Ca-birnessite layers. After a number of optimization steps, we developed a method yielding birnessite electrodes which can be operated for extended time periods at water-oxidation current densities of >1mA∙cm-2 in an aqueous electrolyte at pH 7.[2]
The presentation will start from a description of the “static” properties of both the as-prepared birnessites and the screen-printed electrodes and then present some of the observable changes occuring when oxides are used as electrocatalysts for water oxidation. As birnessites are highly-disordered, porous materials of variable composition and redox state, such investigations are very complex. On the other hand, it seems likely that exactly these features of chemical and structural flexibility are central to explain the observed good electrocatalytic performances of birnessite anodes.
[1] C. E. Frey et al., Dalton Trans. 43, 4370 (2014). [2] S. Y. Lee et al., ChemSusChem doi: 10.1002/cssc.201402533.
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