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.

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.

One of the challenges to realize large-scale water splitting is the lackofactive and low-cost electrocatalysts for its two half reactions:H 2 and O 2 evolution reactions (HER and OER). Herein, we report that cobalt-phosphorous-derived films (Co-P) can act as bifunctional catalysts for overall water splitting.T he as-prepared Co-P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with ac urrent density of 10 mA cm À2 at overpotentials of À94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively.T hey can be employed as catalysts on both anode and cathode for overall water splitting with 100 %F aradaic efficiency,rivalling the integrated performance of Pt and IrO 2 .T he major composition of the asprepared and post-HER films are metallic cobalt and cobalt phosphide,w hichp artially evolved to cobalt oxide during OER.

The development of high-performance nonprecious electrocatalysts with both H2 and O2 evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize three-dimensional hierarchically porous urchin-like Ni2P microsphere superstructures anchored on nickel foam (Ni2P/Ni/NF) as bifunctional electrocatalysts for overall water splitting. The Ni2P/Ni/NF catalysts were prepared by template-free electrodeposition of porous nickel microspheres on nickel foam followed by phosphidation. The hierarchically macroporous superstructures with 3D configuration can reduce ion transport resistance and facilitate the diffusion of gaseous products (H2 and O2). The optimal Ni2P/Ni/NF exhibited remarkable catalytic performance and outstanding stability for both the HER and OER in alkaline electrolyte (1.0 M KOH). For the HER, Ni2P/Ni/NF afforded a current density of 10 mA cm–2 at a low overpotential of only −98 mV. When it served as an OER electrocatalyst, Ni2P/Ni/NF was partially oxidized to nickel oxides/hydroxides/oxyhydroxides (mainly NiO) on the catalyst surface and exhibited excellent OER activity with small overpotentials of 200 and 268 mV to reach 10 and 100 mA cm–2, respectively. Furthermore, when Ni2P/Ni/NF was employed as the electrocatalyst for both the cathode and anode, a water splitting electrolyzer was able to reach 10 and 100 mA cm–2 in 1.0 M KOH at cell voltages of 1.49 and 1.68 V, respectively, together with robust durability. Various characterization techniques and controlled experiments indicated that the superior activity and strong stability of Ni2P/Ni/NF for overall water splitting originated from its electrochemically active constituents, 3D interconnected porosity, and high conductivity.

As an environmentally friendly approach to generate H2 , electrocatalytic water splitting has attracted worldwide interest. However, its broad employment has been inhibited by costly catalysts and low energy conversion efficiency, mainly due to the sluggish anodic half reaction, the O2 evolution reaction (OER), whose product O2 is not of significant value. Herein, we report an efficient strategy to replace OER with a thermodynamically more favorable reaction, the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), catalyzed by 3D Ni2 P nanoparticle arrays on nickel foam (Ni2 P NPA/NF). HMF is one of the primary dehydration intermediates of raw biomass and FDCA is of many industrial applications. As a bifunctional electrocatalyst, Ni2 P NPA/NF is not only active for HMF oxidation but also competent for H2 evolution. In fact, a two-electrode electrolyzer employing Ni2 P NPA/NF for simultaneous H2 and FDCA production required a voltage at least 200 mV smaller compared with pure water splitting to achieve the same current density, as well as exhibiting robust stability and nearly unity Faradaic efficiencies.