Photosystem II (PSII) is a multi-subunit enzyme responsible for solar-driven water oxidation to release O₂ and highly reducing electrons during photosynthesis. The study of PSII in protein film photoelectrochemistry sheds light into its biological function and provides a blueprint for artificial water-splitting systems. However, the integration of macromolecules, such as PSII, into hybrid bio-electrodes is often plagued by poor electrical wiring between the protein guest and the material host. Here, we report a new benchmark PSII–electrode system that combines the efficient wiring afforded by redox-active polymers with the high loading provided by hierarchically-structured inverse opal indium tin oxide (IO-ITO) electrodes. Compared to flat electrodes, the hierarchical IO-ITO electrodes enabled up to an approximately 50-fold increase in the immobilisation of an Os complex-modified and a phenothiazine-modified polymer. When the Os complex-modified polymer is co-adsorbed with PSII on the hierarchical electrodes, photocurrent densities of up to ∼410 μA cm⁻² at 0.5 V vs. SHE were observed in the absence of diffusional mediators, demonstrating a substantially improved wiring of PSII to the IO-ITO electrode with the redox polymer. The high photocurrent density allowed for the quantification of O₂ evolution, and a Faradaic efficiency of 85 ± 9% was measured. As such, we have demonstrated a high performing and fully integrated host–guest system with excellent electronic wiring and loading capacity. This assembly strategy may form the basis of all-integrated electrode designs for a wide range of biological and synthetic catalysts.

Bioelectrocatalysis provides access to sustainable and highly efficient technological applications. However, several limitations related either to the intrinsic properties of the biocatalyst or to technical difficulties still hamper or even prevent the integration of such devices into technologically relevant large-scale processes. In this Review, we challenge the common viewpoint suggesting biology-based catalytic systems as a promising approach for the provision of sustainable stored energy and discuss the status of bioelectrocatalytic devices developed for energy conversion. In particular, we focus on two major research areas in the field, that is, H₂-powered hydrogenase-based biofuel cells and biophotoelectrodes for solar energy harvesting. We identify the main limitations that have to be addressed to gain access to applied large-scale bio-based and bio-inspired advanced energy conversion systems. Moreover, we show recent examples and milestones that are paving the way towards potential realization of these technologies by overcoming existing limiting factors.