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Developing bifunctional efficient and durable non-noble electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable and challenging for overall water splitting. Herein, Co-Mn carbonate hydroxide (CoMnCH) nanosheet arrays with controllable morphology and composition were developed on nickel foam (NF) as such a bifunctional electrocatalyst. It is discovered that Mn doping in CoCH can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and tune the electronic structure of Co center to effectively boost its intrinsic activity. As a result, the optimized CoMnCH/NF electrode exhibits unprecedented OER activity with an ultralow overpotential of 294 mV at 30 mA cm, compared with all reported metal carbonate hydroxides. Benefited from 3D open nanosheet array topographic structure with tight contact between nanosheets and NF, it is able to deliver a high and stable current density of 1000 mA cm at only an overpotential of 462 mV with no interference from high-flux oxygen evolution. Despite no reports about effective HER on metal carbonate hydroxides yet, the small overpotential of 180 mV at 10 mA cm for HER can be also achieved on CoMnCH/NF by the dual modulation of Mn doping. This offers a two-electrode electrolyzer using bifunctional CoMnCH/NF as both anode and cathode to perform stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm. These findings may open up opportunities to explore other multimetal carbonate hydroxides as practical bifunctional electrocatalysts for scale-up water electrolysis.

Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such a Se-doped FeOOH electrode via an FeSe precatalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm–2 is secured at a low overpotential of 348 mV. The application of such an Fe-rich OER electrode in a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.

A binder-free efficient MoNi /MoO nanorod array electrode with 3D open structure is developed by using Ni foam as both scaffold and Ni source to form NiMoO precursor, followed by subsequent annealing in a reduction atmosphere. It is discovered that the self-templated conversion of NiMoO into MoNi nanocrystals and MoO as dual active components dramatically boosts the hydrogen evolution reaction (HER) performance. Benefiting from high intrinsic activity, high electrochemical surface area, 3D open network, and improved electron transport, the resulting MoNi /MoO electrode exhibits a remarkable HER activity with extremely low overpotentials of 17 mV at 10 mA cm and 114 mV at 500 mA cm , as well as a superior durability in alkaline medium. The water-alkali electrolyzer using MoNi /MoO as cathode achieves stable overall water splitting with a small cell voltage of 1.6 V at 30 mA cm . These findings may inspire the exploration of cost-effective and efficient electrodes by in situ integrating multiple highly active components on 3D platform with open conductive network for practical hydrogen production.

The exploration of new efficient OER electrocatalysts based on nonprecious metals and the understanding of the relationship between activity and structure of electrocatalysts are important to advance electrochemical water oxidation. Herein, we developed an efficient OER electrocatalyst with nickel boride (Ni B) nanoparticles as cores and nickel(II) borate (Ni-B ) as shells (Ni-B @NB) via a very simple and facile aqueous reaction. This electrocatalyst exhibited a small overpotential of 302 mV at 10 mA cm and Tafel slope of 52 mV dec . More interestingly, it was found that the OER activity of Ni-B @NB was closely dependent on the crystallinity of the Ni-B shells. The partially crystalline Ni-B catalyst exhibited much higher activity than the amorphous or crystalline analogues; this higher activity originated from the enhanced intrinsic activity of the catalytic sites. These findings open up opportunities to explore nickel(II) borates as a new class of efficient nonprecious metal OER electrocatalysts, and to improve the electrocatalyst performance by modulating their crystallinity.

Creating high-density durable bifunctional active sites in an air electrode is essential but still challenging for a long-life rechargeable zinc–air battery with appealing power density. Herein, we discover a general strategy mediated by metastable rock salt oxides for achieving high-density well-defined transition-metal nanocrystals encapsulated in N-doped carbon shells (M@NC) which are anchored on a substrate by a porous carbon network as highly active and durable bifunctional catalytic sites. Small-size (15 ± 5 nm) well-dispersed Co2Fe1@NC in a high density (metal loading up to 54.0 wt %) offers the zinc–air battery a record power density of 423.7 mW cm–2. The dual protection from the complete graphitic carbon shells and the anchoring of the outer carbon network make Co2Fe1@NC chemically and mechanically durable, giving the battery a long cycling life. Systematic in-situ temperature-dependent characterizations as well as DFT modeling rationalize the rock salt oxide-mediated process and its indispensable role in achieving high-density nanosized M@NC. These findings open up opportunities for designing efficient electrocatalysts for high-performance Zn–air batteries and diverse energy devices.

Practical electrochemical water splitting requires cost-effective electrodes capable of steadily working at high output, leading to the challenges for efficient and stable electrodes for the oxygen evolution reaction (OER). Herein, by simply using conductive FeS microsheet arrays vertically pre-grown on iron foam (FeS/IF) as both substrate and source to in situ form vertically aligned NiFe(OH) x nanosheets arrays, a hierarchical electrode with a nano/micro sheet-on-sheet structure (NiFe(OH) x /FeS/IF) can be readily achieved to meet the requirements. Such hierarchical electrode architecture with a superhydrophilic surface also allows for prompt gas release even at high output. As a result, NiFe(OH) x /FeS/IF exhibits superior OER activity with an overpotential of 245 mV at 50 mA cm −2 and can steadily output 1000 mA cm −2 at a low overpotential of 332 mV. The water-alkali electrolyzer using NiFe(OH) x /FeS/IF as the anode can deliver 10 mA cm −2 at 1.50 V and steadily operate at 300 mA cm −2 with a small cell voltage for 70 h. Furthermore, a solar-driven electrolyzer using the developed electrode demonstrates an exceptionally high solarto-hydrogen efficiency of 18.6%. Such performance together with low-cost Fe-based materials and facile mass production suggest the present strategy may open up opportunities for rationally designing hierarchical electrocatalysts for practical water splitting or diverse applications.

Green and scalable syntheses of highly dispersed supported metal nanocatalysts (SMNCs) are of significant importance for heterogeneous catalysis in industry. In order to achieve nanosized SMNCs and prevent metal nanoparticles (NPs) from aggregation, the traditional liquid syntheses commonly require organic capping agents and low metal loading, which are unfavorable for practical production of SMNCs. Herein, a green and facile solid‐state approach is reported for a general synthesis of Rh, Ru, and Ir NPs highly dispersed on different carbon supports via a room‐temperature mortar grinding. The synthesis is easy to scale up and no organic solvent is needed. Metal NPs are free of capping agents and in a couple of nanometers with a uniform size distribution. Benefiting from the above features and high intrinsic activity, Rh NP/C shows the superior activity for hydrogen evolution reaction (HER) in terms of an ultralow overpotential of 7 mV at 10 mA cm−2, outperforming the state‐of‐the‐art HER electrocatalysts. The cell voltage to output a stable current density of 10 mA cm−2 is only 1.53 V for the electrolyzer with Rh NP/C cathode. These results indicate that the present scalable solid‐state synthetic strategy paves a new avenue for mass production of highly efficient SMNCs for diverse applications.