Semi-Artificial Photoelectrochemistry through Perovskite Integration and Elucidation of the Local Environment

Semi-artificial photoelectrochemistry combines state-of-the-art semiconductors developed in the photovoltaic industry with highly selective and active biological catalysts evolved for specific fuel-forming reactions. The biohybrids combine the benefits of both photovoltaic and bioelectrochemical fields to achieve light-driven selective catalysis, with greater efficiency than natural or artificial systems alone. However, at the interface of fields the fundamental governing principles of each can become lost. In this thesis, molecular-level understanding of the bioelectrochemical local environment is combined with the interfacial engineering approach of photoelectrode design to significantly advance semi-artificial photoelectrochemistry. Lead halide perovskite solar cells are the fastest developing technology in the field of photovoltaics and whilst notoriously moisture-sensitive, recent encapsulation strategies have enabled their application as photoelectrodes in aqueous solution using precious metal Pt co-catalysts. However, prior to this research, photoelectrochemical H₂ production with perovskites using Earth-abundant co-catalysts or biological catalysts was unknown. Here, [NiFeSe] hydrogenase from Desulfovibrio vulgaris Hildenborough, was interfaced with a state-of-the-art triple cation lead mixed halide perovskite to produce a highly active photocathode. The recombinant DvH [NiFeSe]r hydrogenase was selected due to its relative ease of purification, high activity, relative O₂ tolerance, resistance to H₂, and intrinsic bias for proton reduction. The biocatalyst was immobilised on a hierarchical inverse opal mesoporous TiO₂ scaffold, and combined with the perovskite by a Ti foil platform, which provided structural support and enhanced moisture protection of the photoabsorber. The positive onset potential of the resultant photocathode enabled combination with a BiVO₄ water oxidation photoanode to give a self-sustaining, bias-free photoelectrochemical tandem system for overall water splitting (solar-to-hydrogen efficiency of 1.1%). This work demonstrates the compatibility of perovskite elements with carefully selected biological catalysts to produce hybrid photoelectrodes with benchmark performance, which establishes their utility in semi-artificial photosynthesis. The local chemical environment has been shown in heterogeneous and heterogenised molecular electrochemical systems to differ significantly from the bulk solution. Previously these considerations have not been applied to bioelectrochemistry due to the use of flat electrode architectures, low enzyme loadings and consequently lower current densities, providing minimal local concentration gradients. However, high-surface area porous electrodes, such as inverse opal metal oxides, have been increasingly adopted for applications in bioelectrocatalytic fuel and chemical synthesis, enzymatic fuel cells and higher resolution enzymology studies. Here, electrochemistry and computational techniques were applied to explore the local environment of fuel-producing oxidoreductases, H₂ase and formate dehydrogenase, within porous electrodes. DvH W/Sec-FDHr was selected due to its high activity for the reduction of CO₂ to formate and relative O₂ tolerance. The changes in local pH and substrate concentration were determined by comparing the catalytic current density in different electrolyte solutions. The bulk electrolyte was then adjusted, with consideration of the effect of buffer concentration on the enzyme activity, to provide the optimum local conditions for bioelectrocatalysis. Finally, in macroporous inverse opal electrodes, even greater advances were achieved through the combination of an optimised electrolyte with increased mass transport, higher loading and more reductive potentials. Thus, an 18-fold increase in current density for CO₂ reduction by FDH was realised compared to the highest reported analogous system, representing the leap in performance achieved when fundamental understanding of the confined enzymatic chemical environment is applied to bioelectrochemistry. These considerations can be directly applied to both enzymatic photoelectrochemical fuel synthesis, and enzymatic fuel cells, to significantly improve device performance. The local environment of bioelectrocatalysis was further probed by examining the effect of different salts and salt concentration on the enzyme activity. The local pH was influenced by the salt concentration due to the electrical double layer and, in the resultant cation enriched space, CsCl was found to be the least inhibitory salt to enzyme activity. Finally, control of the local environment via the electrolyte to facilitate catalysis was applied to the exemplary perovskite-integrated photoelectrochemical system established above to achieve benchmark enzymatic photoelectrochemistry for CO₂-to-formate reduction. Thus solar fuel production by enzymatic photoelectrochemical devices is advanced by elucidation of the fundamental processes governing the system, from the semiconductor to the enzyme-electrode interface.