Semi‐artificial photoelectrochemistry can combine state‐of‐the‐art photovoltaic light‐absorbers with enzymes evolved for selective fuel‐forming reactions such as CO2 reduction, but the overall performance of such hybrid systems has been limited to date. Here, the electrolyte constituents were first tuned to establish an optimal local environment for a W‐formate dehydrogenase to perform electrocatalysis. The CO2 reductase was then interfaced with a triple cation lead mixed‐halide perovskite through a hierarchically structured porous TiO2 scaffold to produce an integrated photocathode achieving a photocurrent density of −5 mA cm−2 at 0.4 V vs. the reversible hydrogen electrode during simulated solar light irradiation. Finally, the combination with a water‐oxidizing BiVO4 photoanode produced a bias‐free integrated biophotoelectrochemical tandem device (semi‐artificial leaf) with a solar CO2‐to‐formate energy conversion efficiency of 0.8 %.

Bioelectrochemistry employs an array of high-surface-area meso- and macroporous electrode architectures to increase protein loading and the electrochemical current response. While the local chemical environment has been studied in small-molecule and heterogenous electrocatalysis, conditions in enzyme electrochemistry are still commonly established based on bulk solution assays, without appropriate consideration of the nonequilibrium conditions of the confined electrode space. Here, we apply electrochemical and computational techniques to explore the local environment of fuel-producing oxidoreductases within porous electrode architectures. This improved understanding of the local environment enabled simple manipulation of the electrolyte solution by adjusting the bulk pH and buffer pKa to achieve an optimum local pH for maximal activity of the immobilized enzyme. When applied to macroporous inverse opal electrodes, the benefits of higher loading and increased mass transport were employed, and, consequently, the electrolyte adjusted to reach −8.0 mA ⋅ cm−2 for the H2 evolution reaction and −3.6 mA ⋅ cm−2 for the CO2 reduction reaction (CO2RR), demonstrating an 18-fold improvement on previously reported enzymatic CO2RR systems. This research emphasizes the critical importance of understanding the confined enzymatic chemical environment, thus expanding the known capabilities of enzyme bioelectrocatalysis. These considerations and insights can be directly applied to both bio(photo)electrochemical fuel and chemical synthesis, as well as enzymatic fuel cells, to significantly improve the fundamental understanding of the enzyme–electrode interface as well as device performance.

Bioelectrochemistry employs an array of high-surface area meso and macroporous electrode architectures to increase protein loading and the electrochemical current response. Whilst the local chemical environment has been studied in small molecule and heterogenous electrocatalysis, conditions in enzyme electrochemistry are still commonly established based on bulk solution assays, without appropriate consideration of the non-equilibrium conditions of the confined electrode space. Here, we apply electrochemical and computational techniques to explore the local environment of fuel-producing oxidoreductases within porous electrode architectures. This improved understanding of the local environment enabled simple manipulation of the electrolyte solution, by adjusting the bulk pH and buffer pKa, to achieve an optimum local pH for maximal activity of the immobilised enzyme. When applied to macroporous inverse opal electrodes, the benefits of higher loading and increased mass transport were employed and, consequently, the electrolyte adjusted to reach −8.0 mA cm −2 for the H2 evolution reaction (HER) and −3.6 mA cm −2 for the CO2 reduction reaction (CO2RR), demonstrating an 18-fold improvement on previously reported enzymatic CO2RR systems. This research emphasises the critical importance of understanding the confined enzymatic chemical environment, thus expanding the known capabilities of enzyme bioelectrocatalysis. These considerations and insights can be directly applied to both bio(photo)electrochemical fuel and chemical synthesis as well as enzymatic fuel cells to significantly improve the fundamental understanding of the enzyme-electrode interface as well as device performance.

Semi‐artificial photoelectrochemistry can combine state‐of‐the‐art photovoltaic light‐absorbers with enzymes evolved for selective fuel‐forming reactions such as CO2 reduction, but the overall performance of such hybrid systems has been limited to date. Here, the electrolyte constituents were first tuned to establish an optimal local environment for a W‐formate dehydrogenase to perform electrocatalysis. The CO2 reductase was then interfaced with a triple cation lead mixed‐halide perovskite through a hierarchically structured porous TiO2 scaffold to produce an integrated photocathode achieving a photocurrent density of −5 mA cm−2 at 0.4 V vs. the reversible hydrogen electrode during simulated solar light irradiation. Finally, the combination with a water‐oxidizing BiVO4 photoanode produced a bias‐free integrated biophotoelectrochemical tandem device (semi‐artificial leaf) with a solar CO2‐to‐formate energy conversion efficiency of 0.8 %.

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