Facile Synthesis of Co, N Dual-Doped Ni₂P Grown on Nickel Foam as Active Electrocatalyst for Oxygen Evolution Reaction

Due to the increasingly severe energy depletion and environmental pollution, a lot of studies on efficient and renewable energy have been actively conducted. Recently, Hydrogen is regarded as the most prospective energy source to replace the traditional fossil fuels. Overall water electrolysis, which consists of hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode, is a noticeable technology for producing high-purity hydrogen. The OER has complicated reaction mechanism involving four electron transfer, thereby resulting in high overpotential. To overcome this issue, noble metal-based materials, such as IrO₂ and RuO₂, are generally employed as electrocatalysts owing to their high activity. However, the high cost and poor stability of precious metal-based electrocatalyst have limited widespread application of water electrolysis. Therefore, non-noble metal-based electrocatalysts with high OER performance have been explored.<br/>Specifically, Ni-, Co-, Fe-, Mn-based electrocatalysts, including their phosphides, sulfides, oxides, chalcogenides, have studied due to their adjustable electronic structure and earth abundance. Among them, transition metal phosphides (TMPs), which have high electrical conductivity, high electrochemical stability, and rich active sites, have been reported as efficient OER electrocatalysts. Furthermore, in order to increase intrinsic OER performance, heteroatom doping strategy has been carried out to modify their electronic configuration structure. The introduction of dual dopants, which have different polarity and electronegativity, can not only modify the electronic state of host species but change the adsorption free energy of reaction intermediates during the OER process.<br/>On the basis of above explanation, Co, N dual-doped Ni₂P grown on nickel foam was synthesized by only two step hydrothermal reaction and subsequent heat treatment. First, a piece of nickel foam (1 cm x 3 cm) (NF) was sonicated with 1.0 M HCl for 15 min to remove the oxide layers and rinsed with DI water and ethanol. Afterward, HCl, K₃[Co(CN)₆], and Na₃C₆H₅O₇●2H₂O were put into a Teflon-lined autoclave (130 mL) with NF, and kept at 95 °C for 24 h to synthesize nickel cobalt Prussian blue analogue on NF (NiCo-PBA/NF). Owing to the surface etching of NF by HCl solution, the released Ni²⁺ species immediately react with the K₃[Co(CN)₆], resulting in formation of bimetallic PBA. Finally, the NiCo-PBA/NF was annealed with NaH₂PO₂ at 430 °C for 2 h under NH₃/Ar atmosphere. During the process, the NiCo-PBA was phosphidized owing to PH₃ gases generated by thermal decomposition of NaH₂PO₂. Simultaneously, the nitrogen dopants can be introduced into Co-doped Ni₂P structure under the NH₃ flows and elevated temperature. As a results, the Co, N dual-doped Ni₂P exhibited a notable enhancement of catalytic activity toward the OER compared with Co-Ni₂P, N-Ni₂P, and pure Ni₂P counterparts. Furthermore, the Co, N-Ni₂P/NF showed lower overpotential and superior long-term durability than state-of-the-art RuO₂ electrocatalyst. Undoubtedly, this work will offer a new synthetic insight on the rational design of defect-rich transition metal-based materials with improved catalytic performance for energy storage and conversion system.