Metalloenzymes as reversible electrocatalysts for artificial photosynthesis - from mechanism to model devices

Artificial photosynthesis, an important route towards future supply of renewable energy, seeks to convert sunlight into storable chemical energy such as fuels. Building upon the principles of biological solar energy conversion, artificial photosynthesis can be broken down into four essential processes: harvesting of visible light, charge (electron-hole) separation, oxidation of water to dioxygen, and fuel formation. Importantly, unlike natural photosynthesis, artificial photosynthesis is solely dedicated to efficient formation of fuels and is not restricted by the availability of arable land. Both water oxidation and fuel formation require efficient and selective catalysts. This work utilises certain metalloenzymes, which have evolved to catalyse fuel-generating reactions such as the formation of H2 or the reductive activation of CO2 to carbon-based fuels with unmatched efficiencies. In contrast to most artificial catalysts, these enzymes are composed solely of abundant elements and operate efficiently at neutral pH. Thus, although not suitable for scale-up, they can be used to mimic conditions under which future devices will have to operate and provide design criteria for the components of applied technologies. In this thesis, physico-chemical techniques are used to study the mechanism of [FeFe]-hydrogenases, the most proficient H2 evolving catalysts that rival platinum in activity, by investigating how reversible inhibitors intercept transient enzyme states. The interaction of fuel-forming enzymes with light-absorbing semiconductor electrodes is also explored, leading to the construction of a photoelectrochemical cell for the selective, light-driven reduction of CO2. Furthermore, this thesis demonstrates that metalloenzymes can be used to establish new directions in artificial photosynthesis research, driving endergonic organic reactions such as specific C=C hydrogenation.