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Substantial progress has recently been made in the general understanding of the interfacial CO 2 reduction reaction (CO 2 RR) in electro- and photocatalysis, but the influence of the local chemical environment and its effects on the catalytic interface at the molecular level remains largely elusive. Here, we introduce a classification scheme to group different aspects influencing the interfacial CO 2 RR thermodynamics and kinetics. This categorization allows a systematic survey of the literature focusing on the local chemical environment encompassing surface effects (adsorbates, support), solution interactions (electrolyte constituents) and three-dimensional chemical surroundings (polymers, metal organic frameworks (MOFs), covalent organic frameworks (COFs)). The review concludes with an outlook discussing possible future concepts for next-generation electrocatalytic and photocatalytic CO 2 RR. The electro- and photo-catalytic reduction of carbon dioxide are important processes in the context of developing a sustainable carbon-neutral economy. In this Review Article, the authors discuss how the local chemical environment in the proximity of the catalytic active site can influence the reactivity and selectivity of the processes and detail different approaches to achieve their modulation.

Solar reforming (SR) is a promising green‐energy technology that can use sunlight to mitigate biomass and plastic waste while producing hydrogen gas at ambient pressure and temperature. However, practical challenges, including photocatalyst lifetime, recyclability, and low production rates in turbid waste suspensions, limit SR's industrial potential. By immobilizing SR catalyst materials (carbon nitride/platinum; CNx|Pt and carbon nitride/nickel phosphide; CNx|Ni2P) on hollow glass microspheres (HGM), which act as floating supports enabling practical composite recycling, such limitations can be overcome. Substrates derived from plastic and biomass, including poly(ethylene terephthalate) (PET) and cellulose, are reformed by floating SR composites, which are reused for up to ten consecutive cycles under realistic, vertical simulated solar irradiation (AM1.5G), reaching activities of 1333 ± 240 µmolH2 m−2 h−1 on pre‐treated PET. Floating SR composites are also advantageous in realistic waste where turbidity prevents light absorption by non‐floating catalyst powders, achieving 338.1 ± 1.1 µmolH2 m−2 h−1 using floating CNx versus non‐detectable H2 production with non‐floating CNx and a pre‐treated PET bottle as substrate. Low Pt loadings (0.033 ± 0.0013% m/m) demonstrate consistent performance and recyclability, allowing efficient use of precious metals for SR hydrogen production from waste substrates at large areal scale (217 cm2), taking an important step toward practical SR implementation. Reusable floating composites of carbon nitride containing hollow glass microspheres separate to the air‐water interface where they catalyze solar reforming (SR) reactions, transforming waste streams to hydrogen gas. Driven by sunlight, the floating composite produces hydrogen gas from plastic and biomass substrates over consecutive cycles and enables SR in turbid waste suspensions, addressing some key concerns for potential application.

The clean conversion of carbon dioxide and water to a single multicarbon product and O 2 using sunlight via photocatalysis without the assistance of organic additives or electricity remains an unresolved challenge. Here we report a bio-abiotic hybrid system with the non-photosynthetic, CO 2 -fixing acetogenic bacterium Sporomusa ovata grown on a scalable and cost-effective photocatalyst sheet consisting of a pair of particulate semiconductors (La and Rh co-doped SrTiO 3 (SrTiO 3 :La,Rh) and Mo-doped BiVO 4 (BiVO 4 :Mo)). The biohybrid effectively produces acetate (CH 3 COO – ) and oxygen (O 2 ) using only sunlight, CO 2 and H 2 O, achieving a solar-to-acetate conversion efficiency of 0.7% at ambient conditions (298 K, 1 atm). The photocatalyst sheet oxidizes water to O 2 and provides electrons and hydrogen (H 2 ) to S. ovata for the selective synthesis of CH 3 COO – from CO 2 . To demonstrate utility in a closed carbon cycle, the solar-generated acetate was used directly as feedstock in a bioelectrochemical system for electricity generation. These semi-biological approaches thus offer a promising strategy for sustainably and cleanly fixing CO 2 and closing the carbon cycle. Conversion of CO 2 to fuels or chemicals via artificial photosynthesis usually requires the assistance of organic additives or electricity. Now, a biohybrid system is reported consisting of a photocatalyst sheet and bacteria producing acetate and O 2 from CO 2 and H 2 O using sunlight as the sole energy input.

Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal–oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)–O–M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O–O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.

The performance of heterogeneous catalysts for electrocatalytic CO 2 reduction suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilized CO 2 reduction enzymes—such as formate dehydrogenase—can operate with high turnover and selectivity at a minimal overpotential and are therefore ‘ideal’ model catalysts. Here, through the co-immobilization of carbonic anhydrase, we study the effect of CO 2 hydration on the local environment and performance of a range of disparate CO 2 reduction systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilization of carbonic anhydrase increases the kinetics of CO 2 hydration at the electrode. This benefits enzymatic CO 2 reduction—despite the decrease in CO 2 concentration—due to a reduction in local pH change, whereas it is detrimental to heterogeneous catalysis (on Au) because the system is unable to suppress the H 2 evolution side reaction. Understanding the role of CO 2 hydration kinetics within the local environment on the performance of electrocatalyst systems provides important insights for the development of next-generation synthetic CO 2 reduction catalysts. Carbonic anhydrase enzymatically catalyses CO 2 hydration, and its effect on enzymatic and heterogeneous CO 2 reduction has now been studied. Through the co-immobilization of carbonic anhydrase, it has been shown that faster CO 2 hydration kinetics are beneficial for enzymatic catalysis (using formate dehydrogenase) but detrimental for heterogeneous catalysts, such as gold.

Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of low amounts of moisture makes them, a priori , unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. Here, we present a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Field’s metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst. A record photocurrent density of −9.8 mA cm −2 at 0 V versus RHE with an onset potential as positive as 0.95±0.03 V versus RHE is obtained. The photoelectrodes show remarkable stability retaining more than 80% of their initial photocurrent for ∼1 h under continuous illumination. Lead-halide perovskites are sensitive to humidity, which limits their use in water splitting applications. Here, the authors protect the perovskite layer with Field’s metal, driving photoelectrocatalytic hydrogen evolution in an aqueous solution for approximately one hour under constant illumination.

Pollution from microplastics and anthropogenic fibres threatens lakes, but we know little about what factors predict its accumulation. Lakes may be especially contaminated because of long water retention times and proximity to pollution sources. Here, we surveyed anthropogenic microparticles, i.e., microplastics and anthropogenic fibres, in surface waters of 67 European lakes spanning 30° of latitude and large environmental gradients. By collating data from >2,100 published net tows, we found that microparticle concentrations in our field survey were higher than previously reported in lakes and comparable to rivers and oceans. We then related microparticle concentrations in our field survey to surrounding land use, water chemistry, and plastic emissions to sites estimated from local hydrology, population density, and waste production. Microparticle concentrations in European lakes quadrupled as both estimated mismanaged waste inputs and wastewater treatment loads increased in catchments. Concentrations decreased by 2 and 5 times over the range of surrounding forest cover and potential in-lake biodegradation, respectively. As anthropogenic debris continues to pollute the environment, our data will help contextualise future work, and our models can inform control and remediation efforts.

The reduction of CO2 to renewable fuels must be coupled to a sustainable oxidation process to devise a viable device that produces solar fuels. In photoelectrochemical cells, water oxidation to O2 is the predominant oxidation reaction and typically requires a pair of light absorbers or an applied bias voltage when coupled to CO2 reduction. Here, we report a bias-free photoelectrochemical device for simultaneous CO2 reduction to formate and alcohol oxidation to aldehyde in aqueous conditions. The photoanode is constructed by co-immobilization of a diketopyrrolopyrrole-based chromophore and a nitroxyl-based alcohol oxidation catalyst on a mesoporous TiO2 scaffold, which provides a precious-metal-free dye-sensitized photoanode. The photoanode is wired to a biohybrid cathode that consists of the CO2 reduction enzyme formate dehydrogenase integrated into a mesoporous indium tin oxide electrode. The bias-free cell delivers sustained photocurrents of up to 30 µA cm−2 under visible-light irradiation, which results in simultaneous aldehyde and formate production. Our results show that in the absence of an external bias, single light absorber photoelectrochemical cells can be used for parallel fuel production and chemical synthesis from CO2 and alcohol substrates.In photoelectrochemical (PEC) cells, water oxidation to O2, when coupled to CO2 reduction, typically requires a pair of light absorbers or an applied bias voltage. Now, a bias-free PEC cell with a single sunlight absorber drives simultaneous CO2 reduction to give formate, and the oxidation of an organic substrate in aqueous conditions.