Mn-Doped Nickel-Cobalt Sulfides as Bifunctional Electrodes for High-Performance Supercapacitors and Electrocatalysts for Hydrogen Evolution Reactions

The development of active electrode materials with dual functions of energy storage and electrocatalyst is important for future energy storage and energy conversion devices. In this work, manganese-doped nickel cobalt sulfide (Mn-NCS) mesoporous microspheres were prepared by a simple two-step solvent-thermal method and the effects of different Mn2+ doping levels on the electrochemical properties and hydrogen evolution reactions (HER) of the prepared samples were investigated. As a supercapacitor electrode material, optimized Mn0.5-NCS had a maximum specific capacitance of 1175 C g-1 at a current density of 1 A g-1 and remained an outstanding capacitance retention rate of 81% at a current density of 20 A g-1. The asymmetric supercapacitor was assembled with the optimal electrode material as the positive electrode and activated carbon as the negative electrode, and a high energy density of 55.4 Wh kg-1 was achieved at a power density of 797 W kg-1. The Mn0.5-NCS exhibited the best HER performance in alkaline medium, shown as the lowest overpotential of 95 mV and the smallest Tafel slope of 71 mV dec-1 at a current density of 10 mA cm-2. Experimental tests and density functional theory (DFT) calculations reveal that manganese preferentially occupies nickel sites and acts as an electron donor in the electrochemical process to realize the electron redistribution, increasing electron density and accelerating charge transfer. The results indicate that manganese-doped nickel-cobalt sulfides have great potential as bifunctional electrode materials for high-performance supercapacitors and HER.