The development of high-performance and cost-effective catalysts for the hydrogen and oxygen evolution reactions is key to efficient electrocatalysis of water, which offers a promising solution to convert and store those green but unsteady energies. Herein, we report a hierarchical nickel–carbon composite, fabricated by directly growing sheet-like Ni–MOFs on commercial nickel foam prior to high-temperature annealing, as a highly efficient bifunctional catalyst. This composite shows remarkable catalytic activities for both the hydrogen and oxygen evolution reactions in an alkaline electrolyte, affording a current density of 10 mA cm⁻² at an overpotential of 37 mV for the HER and 265 mV for the OER. Furthermore, an electrolyzer employing the composite as a bifunctional catalyst in both the cathode and the anode delivers a current density of 35.9 mA cm⁻² at a cell voltage of 1.60 V with extended stability, which is even superior to the integrated Pt/C and RuO₂ counterparts. This excellent performance is believed to be a result of a concerted synergy due to its hierarchical structure, enabling excellent reaction kinetics. Further ex situ XRD and XPS analyses reveal that while metallic nickel is responsible for the HER, Ni nanoparticles with an oxide shell encapsulated in graphitic carbon are the OER catalytically-active sites formed in situ.

Water electrolysis for hydrogen production has long been regarded as an ideal tactic for renewable energy conversion and storage, but is impeded by the sluggish kinetics of both the hydrogen and oxygen evolution reactions, which are therefore in urgent need for high-performance but low-cost electrocatalysts. Herein, nanoframes of transition metal phosphides (TMPs) with the 3D framework carved open have been demonstrated as highly potent bifunctional catalysts for overall water splitting, reaching the benchmark performance of the Pt/C‖RuO₂ couple, and are much superior to their nanocubic counterparts. This excellent water splitting behavior can be attributed to the enlarged active surface area, less obstructed electrolyte infiltration, promoted charge transfer, and facilitated gas release. Further through in-depth activity analysis and post-electrocatalysis characterization, special attention has been paid to the fate and role of phosphorus in the electrocatalytic process, suggesting that despite the chemical instability of the TMPs (especially under OER conditions), excellent electrocatalytic stability can still be achieved through the amorphous bimetallic hydroxides/oxides formed in situ.