Abstract
Spinel NiCo2X4 (X = O or S), comprising two geometrical cobalt ions, Co2+ in the tetrahedral site (Co-Td(2+)) and Co3+ in the octahedral site (Co-Oh(3+)), has been widely evaluated as a promising pseudocapacitor electrode material. Previous literature mainly demonstrated that much higher specific capacitance of NiCo2S4 than that of NiCo2O4 was ascribed to the higher electronic conductivity. However, we argue that only a small amount of capacitance can be induced by the electronic conductivity, while the significance of electrochemical active species in these system has long been ignored. Here, we propose that geometrical-site-dependent pseudocapacitive activity will generate enhanced specific capacitance through the interface structural design. It reveals that specific capacitance of NiCo2S4 (1862 F g(-1) at 4 A g(-1)) is 50% higher than that of NiCo2O4 (1230 F g(-1) at 4 A g(-1)), which is derived from the designed increase of Co-Td(2+) ions (cobalt ions in the tetrahedral site) in NiCo2S4. These results have significant implications for the design and optimization of the electrochemical properties of transition-metal-based pseudocapacitors.