Hierarchical Hollow Microspheres Assembled from Sulfide-incorporated NiFe-Layered Double Hydroxides for Efficient Electrocatalytic Water Splitting with Low Overpotentials

This study investigates the enhancement of electrocatalysis for overall water splitting through sulfide incorporation into layered double hydroxides (LDHs). Using a one-pot hydrothermal synthesis, hierarchical hollow microspheres of optimized sulfide-doped NiFe–OH (hNiFe-S2) were prepared with thioacetamide as the sulfur precursor. Among the four synthesized LDHs (hCoFe–OH, hNiFe–OH, hCoMg–OH, and hNiMg–OH), hNiFe–OH exhibited the most promising catalytic performance. Accordingly, the sulfide-incorporated derivative hNiFe-S2 demonstrated significantly enhanced bifunctional electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). For OER, hNiFe-S2 achieved a low overpotential (η) of 235 mV at 10 mA cm–2, significantly lower than hNiFe–OH (338 mV) and even outperforming commercial Ir/C (336 mV). Notably, hNiFe-S2 also showed improved HER activity, exhibiting η = 175 mV at 50 mA cm–2, which is significantly lower than that of hNiFe–OH (308 mV). Additionally, hNiFe-S2 demonstrated a Tafel slope of 23 mV dec–1 and excellent stability over 50 h of continuous operation. In a symmetric two-electrode electrolyzer, hNiFe-S2 enabled efficient overall water splitting, achieving a cell voltage of 1.527 V at 10 mA cm–2 with outstanding long-term durability. Structural analysis revealed that sulfide incorporation improved conductivity, reduced charge-transfer resistance to 1.4 Ω, and increased the electrochemical surface area (Cdl = 28.54 mF cm–2). These findings demonstrate that hNiFe-S2 is a highly efficient and cost-effective bifunctional catalyst for water splitting, underscoring the importance of structural engineering and compositional tuning in catalyst design for sustainable energy applications.