Bulk-Phase and Surface Dual-Defect Engineering-Enabled P-NiCo ₂ S _{4– <i>x</i>} Bifunctional Electrodes for Efficient Supercapacitors and Overall Water Splitting

Developing active catalytic energy-storage electrodes holds significant scientific value and research relevance for energy conversion and storage. In this work, P-NiCo2S4–x electrodes with bulk P-doping and surface S-vacancy were synthesized for improved supercapacitor performance and water-splitting activity. Microstructural characterization combined with density functional theory (DFT) calculations manifested that P-doping regulated the d-band center, while S-vacancy lowered reaction energy barriers and boosted charge transfer efficiency. Defect states induced by synergistic effects promoted electron excitation and transfer near the Fermi level, generating active sites that enhanced the electrocatalytic reaction kinetics. The optimized P-NiCo2S4–x delivered a specific capacitance of 2612.50 F g–1 at 1 A g–1, showed 2.4 times of pristine NiCo2S4 (1085.75 F g–1). The assembled ASC (P-NiCo2S4–x//AC) demonstrated exceptional energy density (56.2 Wh kg–1 at 800 W kg–1) and reasonable cycling stability (81.42% over 10000 cycles). Meanwhile, the dual-defect P-NiCo2S4–x electrocatalyst demonstrated low overpotentials (HER, 240 mV at 10 mA cm–2; OER, 416 mV at 50 mA cm–2). This work developed a viable strategy for the controlled construction of dual defects in multifunctional electrodes, thereby establishing a scalable strategy for advancing high-performance energy storage and conversion.