Urea-assisted synthesis of nickel sulfide arrays with enriching high-valence W species to boost Ni3+active sites for water oxidation

Introduction of high-valence transition metal in nickel sulfides is an effective strategy to boost Ni3+ active sites, resulting in excellent OER performance. However, it is a challenge to enhance the amount of high-valence transition metal for ensuring Ni3+-rich specie in nickel sulfides. In this work, one-pot urea-assisted synthesis method was applied to fabricate Ni3+-rich W,V co-modified nickel sulfide nanohorn arrays grown on nickel foam (W, V-NiS@NF-UR1) as OER electrocatalyst in alkaline media. Urea can not only improve the crystallinity of electrocatalysts but also regulate their composition, which is favorable for the formation of the high content of W6+ species in the electrocatalyst, consequently increasing the Ni3+ active sites and enhancing the electrocatalytic OER performance. As a result, a small overpotential of 214 mV is required for a current density of 10 mA cm-2 by the as-prepared W, V-NiS@NF-UR1 electrocatalyst and a small Tafel slope of 69 mV dec � 1 is observed in 1 M KOH. Meanwhile, the introduction of V species is also beneficial to keeping the high-valence W6+ in the electrocatalyst during the sulfurization process. For comparison, the counterpart electrocatalysts were also investigated systematically without urea.