Solar energy has a tremendous potential as an energy source but only a small fraction is currently used in photovoltaics and solar thermal heating, or in the broader sense to grow biomass for energy production. To take full advantage of the enormous potential of the sun, solutions to the capture of solar energy and its conversion into readily utilizable and storable forms are urgently required.

Artificial photosynthesis is a promising concept to convert solar energy into useful and storable forms of energy by the splitting of water into H2 and O2. Hydrogen evolved from H2O has the potential to be used as clean, carbon free energy carrier whose consumption would only produce H2O as a waste product.

However, to evolve hydrogen from overall light-driven water splitting in an artificial photosynthetic system, it is important to find ways to combine different reaction steps. Suitable redox potentials have to be provided by light-harvesting molecular dyes or solid state absorbers. The photochemistry of these dyes or absorbers has to be coupled to the much slower multi-electron redox chemistry of water oxidation and proton reduction. To meet the global energy demand in a sustainable way, the materials used in the overall process have to be based on abundant and non-toxic elements.

Catalysis of the water oxidation half-reaction represents one of the major challenges to achieve an efficient water-splitting process. Layered Co-, Mn-, Ni-and Zn-oxides, as materials based on abundant and cheap elements, have each been found to exhibit catalytic activity towards this reaction. The performance of these oxides is strongly associated with their composition, synthesis conditions, the phase of the material formed and the conditions of testing. Thus, one part of our research project is focused on the preparation and characterization of electrodes for water oxidation based on various Co-, Mn-, Ni- and Zn-oxides.

Another major challenge is the integration of the catalytic oxide films into photo-electrochemical devices. In these devices, dye-sensitized solar cells and multi-junction photovoltaic cells can be used to convert solar energy into an electrochemical potential to drive the water splitting reaction. In this part of our research project, methods have to be developed to prepare catalysts exhibiting both, a high catalytic activity towards the water oxidation reaction and a good electrical contact to the device. The conditions under which these materials are prepared also have to be suitable to the stability of the dye-sensitized solar cells or to the multi-junction photovoltaic cells.

If such integrated devices for light-driven water-splitting could in the end be set up with high solar to fuel conversion efficiencies, this would be a major breakthrough for alternative energy production to meet the global demand.

Scheme for a photoelectrochemical cell for light-driven water splitting. In this scheme, a dye-sensitized solar cell for light harvesting is decorated with a co-catalyst (Catox) to drive the water oxidation half reaction. The catalyst for proton reduction (Catred) is connected via an ohmic contact.