With the energy crisis and global warming, exploration of novel energy sources to replace fossil fuels is a hot topic. H₂ is considered as an eco-friendly energy carrier due to its high energy density per unit mass, and when consumed in engines or fuel cells, it produces only water as the byproduct. Conventional approach to H₂ synthesis strongly depends on fossil fuels by steam reforming process, leading to serious CO₂ emission. Nowadays, an eco-friendly alternative to produce H₂ on a large scale is via electrochemical water splitting [1-7]. The overall water splitting reaction consists of two half-reactions: the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), both of which require catalysts to improve their efficiency and reduce their overpotentials [8]. Although the state-of-the-art catalysts of Pt/C and IrO₂/RuO₂ exhibit high performance for HER and OER, their widespread uses are hindered by their scarcity and high cost. Moreover, using a bifunctional HER and OER electrocatalyst has advantages of simplifying the system and lowering the cost. As such, it is highly attractive to design and develop earth-abundant bifunctional water-splitting electrocatalysts.

Transition-metal phosphides are an important class of compounds formed by the alloying of metals and phosphorus, and their good electrical conductivity is of great benefit to enhanced electrochemical performances [9-12]. Recently, Fu's group from Heilongjiang University have prepared NiCoP nanosheets grown on N-doped carbon coated Ni foam (NiCoP/NF@NC) [13]. The linear sweeping voltammetry (LSV) curves show that the NiCoP/NF@NC electrode requires overpotentials of 31.8 mV for the HER and 308.2 mV for the OER to achieve 10 mA/cm² in 1.0 M KOH. The HER overpotential is much lower than those for NiP/NF@NC (126.6 mV) and CoP/NF@NC (112.1 mV), demonstrating NiCoP/NF@NC is superior in HER performance to CoP/NF@NC and NiP/NF@NC due to the synergistic effect. Moreover, for the OER catalysis, the overpotential of NiCoP/NF@NC electrode is also lower than those for NiP/NF@NC (349.1 mV) and CoP/NF@NC (383.3 mV). Notably, NiCoP/NF@NC has excellent long-term stability without obvious attenuation of performance after 10000 time cycles. Such 3D catalyst electrode has the following advantages for water splitting: (1) it acts as a bifunctional HER and OER electrocatalyst; (2) Ni foam accelerates the diffusion of ions and improve the electrocatalytic performance; (3) introduction of N-doped carbon nanostructures enhances the electrical conductivity and facilitates the transfer rate of electron.

In summary, Ni foam-supported 3D nanostructured NiCoP nanosheets are proven as high-active and durable electrocatalyst for overall water splitting. The whole fabrication process is cost-effective and easy to scale-up. All these remarkable features promise its practical use as an advanced catalyst electrode in water-splitting technological devices.