Assemblies of carbon monoxide dehydrogenase molecules with CdS nanocrystals show fast CO2 reduction driven by visible light. Activity is strongly influenced by size and shape of nanocrystals, and by the nature of the electron donor.

The development of robust systems for the conversion of solar energy into chemical fuels is an important subject in renewable energy research. Key aspects are efficient and rapid catalysis of both fuel production (reduction of H 2 O or CO 2 ), and water oxidation. Enzymes often have extraordinary and unique capabilities as electrocatalysts, and in this Perspective we consider the role that these molecules can play through their incorporation into model systems for solar fuel production, or as inspiration for synthetic catalysts.

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H+/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H+ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction.