Electrocatalytic water splitting driven by renewable energy input to produce clean H has been widely viewed as a promising strategy of the future energy portfolio. Currently, the state-of-the-art electrocatalysts for water splitting in acidic solutions are IrO or RuO for the O evolution reaction (OER) and Pt for the H evolution reaction (HER). Realization of large-scale H production from water splitting requires competent nonprecious electrocatalysts. Despite the advances of decades in this field, several challenges still exist and need to be overcome: (1) Most efforts in the design of nonprecious electrocatalysts have focused on developing HER catalysts for acidic conditions but OER catalysts for alkaline conditions owing to their thermodynamic convenience, potentially resulting in incompatible integration of the two types of catalysts and thus inferior overall performance. (2) In conventional water electrolysis, HER and OER are strictly coupled and therefore H and O are produced simultaneously, which may lead to explosive H/O mixing due to gas crossover. Meanwhile, the coexistence of H, O, and electrocatalysts could produce reactive oxygen species that might shorten the lifetime of an electrolyzer. (3) The HER rate is often limited by that of OER due to the more sluggish kinetics of the latter, which lowers the overall energy conversion efficiency. Moreover, the product of OER, O, is not highly valuable. (4) It remains challenging to develop efficient and low-cost H storage and transport systems for the future H economy. In this Account, we describe recent progress in innovative strategies to address the aforementioned four challenges in conventional water electrolysis. These novel strategies include (1) overall water electrolysis based on bifunctional nonprecious electrocatalysts (or precursors) to drive both HER and OER under the same conditions, (2) decoupled water electrolysis achieved by redox mediators for temporally and spatially separating HER from OER, (3) hybrid water electrolysis by integrating thermodynamically more favorable organic upgrading reactions to replace OER, and (4) tandem water electrolysis by utilizing biocatalysts for converting the in situ produced H with foreign compounds (e.g., CO and N) to more valuable products. Finally, the remaining challenges and future perspectives are also presented. We hope this Account will function as a momentum call for more endeavors into the development of advanced electrocatalytic systems and novel strategies for practicable H production from water as well as the electrocatalytic upgrading of diverse organic compounds.

Inorganic solids are an important class of catalysts that often derive their activity from sparse active sites that are structurally distinct from the inactive bulk. Rationally optimizing activity is therefore beholden to the challenges in studying these active sites in molecular detail. Here, we report a molecule that mimics the structure of the proposed triangular active edge site fragments of molybdenum disulfide (MoS(2)), a widely used industrial catalyst that has shown promise as a low-cost alternative to platinum for electrocatalytic hydrogen production. By leveraging the robust coordination environment of a pentapyridyl ligand, we synthesized and structurally characterized a well-defined Mo(IV)-disulfide complex that, upon electrochemical reduction, can catalytically generate hydrogen from acidic organic media as well as from acidic water.

Conventional water electrolyzers produce H and O simultaneously, such that additional gas separation steps are needed to prevent H/O mixing. The sluggish anodic O evolution reaction (OER) always results in low overall energy conversion efficiency and the product of OER, O, is not of significant value. In addition, the potential formation of reactive oxygen species (ROS) may lead to degradation of cell membranes and thus premature device failure. Herein we report a general concept of integrating oxidative biomass upgrading reactions with decoupled H generation from water splitting. Five representative biomass substrates, ethanol, benzyl alcohol, furfural, furfuryl alcohol, and 5-hydroxymethylfurfural (HMF), were selected for oxidative upgrading catalyzed by a hierarchically porous NiS/Ni foam bifunctional electrocatalyst (NiS/NF). All the five organics can be oxidized to value-added liquid products at much lower overpotentials than that of OER. In particular, the electrocatalytic oxidation of HMF to the value-added 2,5-furandicarboxylic acid (FDCA) was further studied in detail. Benefiting from the more favorable thermodynamics of HMF oxidation than that of OER, the cell voltage for integrated H production and HMF oxidation was significantly reduced by ∼200 mV relative to pure water splitting to achieve 100 mA cm, while the oxidation product (FDCA) at the anode was much more valuable than O. When utilized as electrocatalysts for both cathode and anode, NiS/NF demonstrated outstanding durability and nearly unity Faradaic efficiencies for both H and FDCA production. Overall, such an integration of oxidative biomass valorization and HER via earth-abundant electrocatalysts not only avoids the generation of explosive H/O mixture and ROS, but also yields products of high value at both electrodes with lower voltage input, maximizing the energy conversion efficiency.

Growing global energy demands and climate change motivate the development of new renewable energy technologies. In this context, water splitting using sustainable energy sources has emerged as an attractive process for carbon-neutral fuel cycles. A key scientific challenge to achieving this overall goal is the invention of new catalysts for the reductive and oxidative conversions of water to hydrogen and oxygen, respectively. This review article will highlight progress in molecular electrochemical approaches for catalytic reduction of protons to hydrogen, focusing on complexes of earth-abundant metals that can function in pure aqueous or mixed aqueous-organic media. The use of water as a reaction medium has dual benefits of maintaining high substrate concentration as well as minimizing the environmental impact from organic additives and by-products.