Core-Shell Structured NiCo@NiCoP Nanorod on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

The development of a clean and renewable energy carrier is necessary owing to the tremendous devastation of the environment and exhaustion of fossil fuel resources. Due to the high energy density and renewability of hydrogen gas, it is regarded as one of the energy alternatives to traditional fossil fuels. Electrochemical water splitting, which consists of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is considered as sustainable technology for high-purity hydrogen gas production. However, the thermodynamically sluggish kinetics of electrochemical reaction at each electrode have hampered the practical applications in the industrial fields. To overcome this issue, ruthenium- and platinum-based materials are generally used for electrocatalysts for improving OER and HER performances, respectively. However, their high cost, scarcity, and insufficient durability have limited their large-scale applications. Recently, transition metal-based oxide, sulfide, phosphide have been considered as promising electrocatalysts for water splitting owing to their remarkable advantages such as low cost, abundance, rich redox properties, and outstanding chemical stability. Among them, transition metal phosphides (TMPs) are emerging as a promising bifunctional electrocatalyst for both of OER and HER. However, their intrinsic low electrical conductivity still remained as a challenge<br/>Herein, core-shell structured nickel cobalt alloy-nickel cobalt phosphide (NiCo@NiCoP) nanorod directly grown on nickel foam (NF) was synthesized by hydrothermal, thermal reduction, electrochemical oxidation, and phosphidation process. First, NiCo carbonate hydroxide (NiCoCH) was synthesized by hydrothermal method, and then it was calcined under the H₂ flow to form NiCo alloy core. The core-shell structured NiCo@NiCo hydroxide (NiCo@NiCoOH) was obtained by electrochemical oxidation at an anodic current density of 300 mA cm^-2 for 60 min in 1 M KOH solution. Finally, NiCo@NiCoP was prepared by the annealing method which was placed NaH₂PO₂ and NiCo@NiCoOH in a tube furnace under Ar atmosphere. The metallic core encourages the charge transport to the surface phosphide, and the phosphide shell provides abundant active sites for HER and OER. Also, such a homogeneous 1D structure can easily absorb a liquid-phase solution onto the electrode surface owing to the strong capillary force, thus reducing the gas-solid interface friction and enhancing the release of generated oxygen gas bubbles. As a result, the NiCo@NiCoP nanorods on NF require the low overpotentials of 146 mV (vs. RHE) and 340 mV (vs. RHE) for HER and OER, respectively, at a current density of 100 mA cm^-2 in alkaline condition. Moreover, the NiCo@NiCoP nanorods show superior long-term stability for 50 h, compared to commercial Pt/C and RuO₂ on NF.