Development of Bifunctional Electrocatalyst for Overall Water Electrolysis: Vanadium and Phosphorus Dual-Doping Strategy on Ni3S2 Nanoneedle
Authors: Min Kyeongseok , Kim Junseong , Ko Dasol , Baeck Sung-Hyeon 
Published: August 2024

Along with the rapid consumption of fossil fuels and serious environmental pollution, the development of renewable and clean energy has been receiving a lot of attention recently. Among the various renewable energy sources, hydrogen (H2) is considered as most promising substance in future energy society owing to its high gravimetric energy density and environmental friendliness. Electrochemical water electrolysis system is powerful strategy for high-purity hydrogen production without emission of any pollutants. It consists of two electrocatalytic reactions: oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, the practical efficiency of water electrolysis is limited due to the intrinsically sluggish reaction kinetics of OER and HER. To date, the precious metal-based electrocatalysts such as RuO2 and Pt/C for OER and HER, respectively, were employed as state-of-the-art electrocatalyst to enhance the performance of water electrolysis. However, due to the high expense, mono-functionality, and unsatisfactory stability of the precious metal-based materials, research on bifunctional electrocatalysts with low cost and high activity has become an urgent topic.

To date, numerous scientific endeavors have been dedicated to the advancement of highly efficient non-noble metal-based bifunctional electrocatalysts such as transition metal oxides, hydroxides, sulfides, carbides, nitrides, and phosphides. Among these, Ni3S2 has emerged as a particularly promising electrocatalyst for water splitting, garnering substantial attention owing to its high electric conductivity, rich redox property, abundance in the Earth's crust, and environmentally friendly characteristics. Nevertheless, the electrocatalytic activity of Ni3S2 lags behind that of noble metal counterparts. Therefore, elaborate modification of Ni3S2 catalyst should be conducted to further improve its intrinsic activity toward OER and HER. Among the various strategies, heteroatom doping into Ni3S2 structure could be adopted as powerful technique for enhancement of electrocatalytic performance. The homogeneously incorporated dopants not only generate lattice disorders but elaborately modulate the surface electronic property. Specifically, high-valent cation dopants such as vanadium and molybdenum can effectively mediate the change of chemical states of Ni active sites during the step-wise electrocatalytic procedure. Meanwhile, additionally introduced anion dopants, which have lower electronegativity compared with sulfur, can reduce the electron trapping phenomenon induced by excessive electron transfer from nickel to sulfur.

Inspired by above features, we synthesized V and P co-doped Ni3S2 nanoneedles directly grown on nickel foam (V-Ni3S2-P/NF) through facile two-step preparation method. First, V doped Ni3S2 nanoneedles were homogeneously grown on NF (V-Ni3S2/NF) by hydrothermal process without use of nickel precursor. The vanadium ions promote the nucleation of Ni3S2 species and lead the preferential growth toward 1D direction during the hydrothermal reaction. After that, the V-Ni3S2/NF was annealed in the tube furnace with the NaH2PO2 as a phosphorus precursor under the Ar flow. Under the mild annealing condition, the PH3 gases generated from thermal decomposition of NaH2PO2 reacted with the V-Ni3S2 species. Consequently, the V-Ni3S2 was partially phosphidated into V-Ni3S2-P throughout anion exchange process. As a result, the V-Ni3S2-P/NF electrocatalyst exhibited excellent bifunctional activity toward both OER and HER compared with mono-doped and un-doped counterparts and precious metal-based electrocatalysts. Furthermore, the outstanding electrocatalytic performance of V-Ni3S2-P/NF was well-maintained over the 100 h of continuous operation under alkaline condition. This study will contribute to the development of efficient and cost-effective electrocatalysts for future energy conversion and storage technologies.