Optimally Generated Active Sites on Nanostructured Nickel Sulfide Electrocatalysts for Designing Economical Electrolyzers

Development of electrocatalysts is an obstacle in the way of designing durable and low-cost water splitting systems. Thus, particular attention should be paid to researching the catalyst design phase, functionality, and active sites to reduce the activation of potential barriers. Herein, we report a straightforward one-pot hydrothermal method for the fabrication of highly efficient electrocatalysts, leveraging control over sulfur source concentrations. The resulting porous nickel sulfide (Ni x S y ) superstructures exhibit distinctive flower- and petal-like morphologies synthesized using thiourea (TU) and thiosulfate (TS), respectively. Notably, Ni x S y (TS) demonstrates exceptional electrocatalytic activity for the hydrogen evolution reaction, achieving a low overpotential of 118 mV in 1.0 M KOH at 10 mA/cm 2 . Conversely, Ni x S y (TU) exhibits high electrocatalytic activity for the oxygen evolution reaction, with a low overpotential of 188 mV. The TS route facilitates the formation of an oxidative Ni 3 S 2 heterostructure, while TU leads to the development of mixed nickel sulfide nanocrystals including nickel ions with higher oxidation states. The resulting electrocatalysts, characterized by an open porous network, rich heterogeneous interfaces, abundant exposed active sites, and electronic interactions, demonstrate outstanding performance in overall water splitting. Integration of optimized NF/TUS5 and NF/TSS2 (NF, nickel foam) achieves remarkable current densities of 10 mA/cm 2 at 1.546 V and 50 mA/cm 2 at 1.86 V, sustaining for 22 h without significant degradation. This study offers promising insights for the development of cost-effective electrocatalysts based on metal chalcogenides, advancing the prospects for overall water-splitting applications.