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 selective reduction of CO2 with inexpensive solar-driven photoelectrochemical devices is a contemporary challenge in the quest for renewable fuel production. Here we report a molecular catalyst-based photocathode assembled from precious-metal-free components that is active towards solar-driven, aqueous CO2 reduction. The reported photocathode is based on a phosphonated cobalt bis(terpyridine) catalyst that is interfaced via a mesoporous TiO2 scaffold with a light-harvesting p-type silicon electrode. The hybrid photoelectrode reduces CO2 to CO in both organic-water and purely aqueous conditions, achieving a turnover number of ~330 and maintaining stable activity for more than one day. Critically, indepth electrochemical and in situ resonance Raman and infrared spectroelectrochemical investigations alluded to a catalytic mechanism that differs to that reported for the soluble metal bis(terpyridine) catalyst as the consequence of the immobilisation. In addition, it further unlocks an earlier catalytic onset and better electrocatalytic performance while enabling aqueous CO2 reduction with the reported photocathode.

Harvesting solar energy to convert CO 2 into chemical fuels is a promising technology to curtail the growing atmospheric CO 2 levels and alleviate the global dependence on fossil fuels. However, the assembly of efficient and robust systems for the selective photoconversion of CO 2 without sacrificial reagents and external bias remains a challenge. Here, we present a photocatalyst sheet that converts CO 2 and H 2 O into formate and O 2 as a potentially scalable technology for CO 2 utilisation. This technology integrates La and Rh-doped SrTiO 3 (SrTiO 3 :La,Rh) and Mo-doped BiVO 4 (BiVO 4 :Mo) light absorbers modified by phosphonated Co(II) bis(terpyridine) and RuO 2 catalysts onto a gold layer. The monolithic device provides a solar-to-formate conversion efficiency of 0.08±0.01% with a selectivity for formate of 97±3%. As the device operates wirelessly and uses water as an electron donor, it offers a versatile strategy toward scalable and sustainable CO 2 reduction using molecular-based hybrid photocatalysts.

A freestanding H2‐evolution electrode consisting of a copolymer‐embedded cobaloxime integrated into a multiwall carbon nanotube matrix by π–π interactions is reported. This electrode is straightforward to assemble and displays high activity towards hydrogen evolution in near‐neutral pH solution under inert and aerobic conditions, with a cobalt‐based turnover number (TONCo) of up to 420. An analogous electrode with a monomeric cobaloxime showed less activity with a TONCo of only 80. These results suggest that, in addition to the high surface area of the porous network of the buckypaper, the polymeric scaffold provides a stabilizing environment to the catalyst, leading to further enhancement in catalytic performance. We have therefore established that the use of a multifunctional copolymeric architecture is a viable strategy to enhance the performance of molecular electrocatalysts.

Electrolytic or solar-driven reduction of CO2 to CO using heterogenized molecular catalysts is a promising approach towards production of a key chemical feedstock, as well as mitigating CO2 emissions. Here, we report a molecular cobalt-phthalocyanine catalyst bearing four phosphonic acid anchoring groups (CoPcP) that can be immobilized on metal oxide electrodes. A hybrid electrode with CoPcP on mesoporous TiO2 (mesoTiO2) converts CO2 to CO in aqueous electrolyte solution at a near-neutral pH (7.3) with high selectivity and a turnover number for CO (TONCO) of 1949 ± 5 after 2 h controlled potential electrolysis at -1.09 V vs. SHE (~550 mV overpotential). In situ UV-visible spectroelectrochemical investigations alluded to a catalytic mechanism that involves non-rate-limiting CO2 binding to the doubly-reduced catalyst. Finally, the integration of the mesoTiO2|CoPcP assembly with a p-type silicon (Si) photoelectrode allowed the construction of a benchmark precious-metal-free molecular photocathode that achieves a TONCO of 939 ± 132 with 66% selectivity for CO (CO/H2 = 2) under fully aqueous condition. The electrocatalytic and photoelectrochemical (PEC) activity of CoPcP was compared to state-of-the-art synthetic and enzymatic CO2 reduction catalysts, demonstrating the excellent performance of CoPcP and its suitability for the integration in tandem PEC devices.