Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors. CONTENTS1. Introduction 16051 2. Photochemistry, Photoelectrochemistry, Photocatalysis, Photosynthesis, Photoelectrosynthesis, and Efficiencies 16052 3. Natural Photosynthesis 16053 4. From Enzymes to Human-Engineered Catalysts 16056 5. Artificial Photosynthesis and Photoelectrosynthetic Cells 16056 6. Molecular-Catalyst-Modified Semiconductors 16059 7. Examples Involving Solid-State Photocathodes Modified with Molecular Catalysts 16060 7.1. Photoelectrochemical H 2 Production 16060 7.2. Photoelectrochemical CO 2 Reduction 16075 8. Examples Involving Light-Absorbing Nanoparticles and Nanorods Modified with Molecular Catalysts 16080 8.1. Molecular-Catalyst-Modified Semiconductor Nanoparticles and Nanorods for H 2 Production 16080 8.2. Molecular-Catalyst-Modified Semiconductor Nanoparticles and Nanorods for CO 2 Reduction 16082 9.

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Understanding and controlling factors that restrict the rates of fuel-forming reactions are essential to designing effective catalyst-modified semiconductors for applications in solar-to-fuel technologies. Herein, we describe GaAs semiconductors featuring a polymeric coating that contains cobaloxime-type catalysts for photoelectrochemically powering hydrogen production. The activities of these electrodes (limiting current densities >20 mA cm–2 under 1-sun illumination) enable identification of fundamental performance-limiting bottlenecks encountered at relatively high rates of fuel formation. Experiments conducted under varying bias potential, pH, illumination intensity, and scan rate reveal two distinct mechanisms of photoelectrochemical hydrogen production. At relatively low polarization and pH, the limiting photoactivity is independent of illumination conditions and is attributed to a mechanism involving reduction of substrate protons. At relatively high polarization or pH, the limiting photoactivity shows a linear response to increasing photon flux and is attributed to a mechanism involving reduction of substrate water. This work illustrates the complex interplay between transport of photons, electrons, and chemical substrates in photoelectrosynthetic reactions and highlights diagnostic tools for better understanding these processes.

The direct integration of electrocatalysts with photovoltaic materials provides a strategy to photoelectrochemically power chemical transformations and store intermittent solar energy as fuels. However, many electrocatalytic components used or proposed for use in such assemblies also absorb visible light. This prompts the questions: to what extent does a selected electrocatalytic coating screen photons from reaching the underlying photovoltaic, do excited-state species associated with coating layers contribute to photocurrent production via mechanisms involving dye-sensitization processes, and are relatively high or low loadings of catalytic sites advantageous. Herein, we highlight how optical and electrochemical characterization techniques can be coupled with structural information to address these questions. The experiments described in this work make use of a p-type gallium phosphide semiconductor that is interfaced with cobalt porphyrin hydrogen evolution reaction catalysts. However, the experimental techniques and discussions presented in this work can likely be applied to other materials and chemical transformations, providing a general yet useful strategy for better understanding the origin of photocurrents and fuel production activities in catalyst-modified semiconductor electrodes.