For efficient electrolysis of water, catalysts are used to enhance the kinetics of the individual electrochemical reactions and thereby minimizing the electrical voltages required to reach adequate current densities. The aim of the work done at the “Solar Fuels” institute is to promote the basic understanding of electron transfer at electrolyte-catalyst interfaces and to develop efficient catalysts without the use of precious metals for solar driven water splitting. Initially, the catalysts for the partial processes at the cathode (hydrogen evolution) and the anode (i.e. water oxidation to oxygen) are separately investigated and optimized. Then they are combined into a single integrated system for self-sufficient hydrogen production utilizing semiconducting absorber materials (for production of the required photo-voltage). Highly developed photovoltaic cells are also used in combination with catalysts to form monolithic “artificial leafs”, which can under sunlight split water into hydrogen and oxygen without any cabling or additional bias voltage. In order to achieve high efficiencies the nano-structuring of the catalytic materials, their optical transparency, and their stability against the electrolyte pose a considerable challenge.
Fig. 1: Amongst other materials manganese oxides are developed as catalysts for water oxidation by characterizing their electronical properties and the activity. The TEM picture shows a porous Ca-manganese oxide layer which was deposited on coducting glass (FTO) to create an anode. The figure on the right shows the crystal lattice in the material. Unterneath, the activity of the oxygen generation in correlation with the current density is shown.
Fig. 2: In cooperation with the Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin (PVcomB) photovoltaic cells (in this case a-Si:H/ a-Si:H/ µc-Si:H ) were combined with catalysts to form a monolithic “Artificial Leaf” which splits water to hydrogen and oxygen under solar illumination in an acid electrolyte.