Experimental and Theoretical Investigation of Prussian Blue-Type Catalysts for Artificial Photosynthesis

Artificial photosynthesis, in which high energy compounds such as hydrogen are produced from water by using sunlight, seems to be a very promising solution to the global energy crisis, regarding its huge advantages of sustainability and abundance. Large progress has been made in the development of photoelectrochemical water splitting using Earth-abundant semiconductor materials. Nonetheless, industrial implementation is still hampered by the lack of a stable, efficient, and cost-competitive photoanode for water oxidation catalysis. To overcome these drawbacks and to enhance reaction rates, a co-catalyst can be deposited on such a metal oxide photoanode. In this context, redox catalysts based on Prussian blue (iron hexacyanoferrate), which have shown high catalytic activities and exceeding long-term stabilities, show great promises.

In this Thesis, the Earth-abundant metal oxides, Fe₂O₃  and BiVO₄, which are well-known photoanode materials, were modified with CoFe-PB, the cobalt iron analogue of Prussian blue.

Deposition of CoFe-PB yielded large performance enhancements on BiVO₄, while its beneficial influence on Fe₂O₃  remained rather small. In order to understand this different effect of CoFe-PB, a wide range of photoelectrochemical methods, included impedance spectroscopy, and time-resolved spectroscopy, including transient and photo-induced absorption spectroscopy, were used. In the case of BiVO₄, it was shown to efficiently transfer charges, whereas in the case of Fe₂O₃ , it merely shifted the capacitance of surface states.

Various theoretical methods in the framework of DFT were applied and their applicability to describe the complex catalytic systems was assessed.

A modified hybrid functional with optimised amount of exact exchange was found appropriate, and was therefore used to compute the electronic structures of all employed compounds.

Their alignment demonstrated the thermodynamic feasibility of charge-transfer from BiVO₄ to CoFe-PB and further to H₂O, in line with the experimental

predictions. In CoFe-PB/Fe₂O₃  such a favourable alignment of electronic structures is not given, which may explain its inferior performance compared to CoFe-PB/BiVO₄ .

Taken together, the role of CoFe-PB was concluded to be different on both modified photoanodes:

it acts as a true hole transfer catalyst on BiVO₄, whereas it only affects the surface electronic structure on Fe₂O₃.