The synthesis of renewable fuels from abundant water or the greenhouse gas CO 2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO 2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule–material hybrid systems are organized as “dark” cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond “classical” H 2 evolution and CO 2 reduction to C 1 products, by summarizing cases for higher-value products from N 2 reduction, C x >1 products from CO 2 utilization, and other reductive organic transformations.

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

The photoelectrochemical (PEC) production of syngas from water and CO 2 represents an attractive technology towards a circular carbon economy. However, the overpotential (i.e. energy loss), low selectivity and cost of commonly employed catalysts make this sustainable alternative challenging. Here, we demonstrate highly tunable PEC syngas production by integrating an earth abundant, cobalt porphyrin catalyst immobilized on carbon nanotubes with triple-cation mixed halide perovskite and BiVO 4 photoabsorbers. Empirical data analysis is used to clarify the optimal electrode selectivity at low catalyst loadings. The perovskite photocathodes maintain selective aqueous CO 2 reduction for one day at light intensities as low as 0.1 Sun. Perovskite-BiVO 4 PEC tandems sustain an unprecedented bias-free syngas production for three days with solar-to-H 2 and solar-to-CO conversion efficiencies of 0.06% and 0.02%, respectively, operating as standalone artificial leaves in neutral pH solution. These systems provide potential pathways for maximizing daylight utilisation, by sustaining CO 2 conversion even under low solar irradiance. Syngas, a mixture of CO and H 2 , is a crucial intermediate in the industrial production of methanol, higher alcohols, longchain hydrocarbons, lubricants, waxes and fuels via the Fischer-Tropsch process. 1-3 Applications extend to pharmaceuticals, bulk chemical synthesis and fertilisers. The conventional reforming of methane to syngas relies on fossil fuels to operate at high temperatures and pressures 3 and biomass gasification can introduce contaminants. 4 The solar-driven production of syngas from aqueous CO 2 is an ambient conditions and clean alternative process. 5 Although silicon, 6 dye, 7 metal oxide, 8-10 or perovskite 11,12 photoabsorbers provide enough driving force in tandem devices for bias-free water splitting, very few examples of bias-free PEC CO 2 reduction are known. 13-16 Due to the large overpotentials which need to be overcome for simultaneous CO 2 reduction and water oxidation, 17 most of those systems employ up to six photovoltaic (PV) solar cells connected in series, 13,18,19 or complex noble metal based nanostructures. 14,16,20 Accordingly, a vast library of molecular catalysts employing earth abundant-metals for CO 2 reduction remains underexplored. 21-24 Such catalysts are known to achieve improved selectivities towards CO production at lower overpotentials, with Co porphyrin and phthalocyanin recently demonstrating selective aqueous CO 2 reduction to CO when immobilized onto carbon nanotube (CNT) based electrodes. 25-27 Here, we tap into that library of molecular catalysts by using the commercially available cobalt(II) meso-tetrakis(4methoxyphenyl)porphyrin (abbreviated CoMTPP), which can be readily immobilized via π-π stacking interactions 26 onto CNT sheets, also known as buckypaper (Fig. 1, right side). The composite is employed in electrodes, state-of-the-art perovskitebased photocathodes and perovskite-BiVO 4 PEC tandem devices, which couple tunable syngas to O 2 pro...

Light-driven enzymatic catalysis is enabled by the productive coupling of a protein to a photosensitizer. Photosensitizers used in such hybrid systems are typically costly, toxic, and/or fragile, with limited chemical versatility. Carbon dots (CDs) are low-cost, nanosized light-harvesters that are attractive photosensitizers for biological systems as they are water-soluble, photostable, nontoxic, and their surface chemistry can be easily modified. We demonstrate here that CDs act as excellent light-absorbers in two semibiological photosynthetic systems utilizing either a fumarate reductase (FccA) for the solar-driven hydrogenation of fumarate to succinate or a hydrogenase (Hase) for reduction of protons to H. The tunable surface chemistry of the CDs was exploited to synthesize positively charged ammonium-terminated CDs (CD-NHMe), which were capable of transferring photoexcited electrons directly to the negatively charged enzymes with high efficiency and stability. Enzyme-based turnover numbers of 6000 mol succinate (mol FccA) and 43,000 mol H (mol Hase) were reached after 24 h. Negatively charged carboxylate-terminated CDs (CD-CO) displayed little or no activity, and the electrostatic interactions at the CD-enzyme interface were determined to be essential to the high photocatalytic activity observed with CD-NHMe. The modular surface chemistry of CDs together with their photostability and aqueous solubility make CDs versatile photosensitizers for redox enzymes with great scope for their utilization in photobiocatalysis.

We report the design of a novel glucose/O2 biofuel cell (GBFC) integrating carbon nanotube-based 3D bioelectrodes and using naphthoquinone-mediated oxidation of glucose by glucose oxidase and direct oxygen reduction by laccase. The GBFCs exhibit high open circuit voltages of 0.76 V, high current densities of 4.47 mA cm(-2), and maximum power output of 1.54 mW cm(-2), 1.92 mW mL(-1) and 2.67 mW g(-1). The GBFC is able to constantly deliver 0.56 mW h cm(-2) under discharge at 0.5 V, showing among the best in vitro performances for a GBFC. Using a charge pump, the GBFC finally powered a Light Emitting Diode (LED), demonstrating its ability to amplify micro watts to power mW-demanding electronic devices.