Devices which split water into oxygen and hydrogen using either sunlight directly or solar-electricity to power water electrolysis are attractive renewable energy technologies. In such devices two fundamental electrochemical processes are typically combined: 1) water oxidation, 2H2O ? O2 + 4H+ + 4e-; and (2) proton reduction, 2H+ + 2e- ? H2. Our specific interest is in using earth-abundant transition metals to develop robust and active catalysts capable of promoting the complex water oxidation reaction.We developed robust photoanodes, by doping manganese clusters into Nafion coated electrodes, which catalyze water oxidation for extended periods on application of a bias. Catalytic activity is significantly enhanced by visible light irradiation.Examination of the films during catalysis via XAS, TEM and EPR spectroscopy subsequently revealed that the clusters dissociate in Nafion forming Mn2+ species which on biasing are oxidized to MnOx nanoparticles. A birnessite-like layered oxide (see Figure) forms which undergoes reduction and releases oxygen on illumination. The fact that birnessite, a common mineral formed in aquatic systems by bacterial oxidation of Mn2+, is responsible for catalysis suggests that the water oxidation cycle in Nafion has similarities to the broader Mn-biogeochemistry, which is dominated by oxidative formation of nanoparticulate MnIII/IV oxides followed by photoreduction to Mn2+. Building on these findings, we have shown that a variety of metal complexes as well as ionic liquids and screen-printing methods can be used to process highly active water oxidation catalysts.

Metal oxides nanoparticles as water oxidation electro-catalysts

247th National Meeting (2014)

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