Controlled synthesis of NiCo₂S₄ nanostructured arrays on carbon fiber paper for high-performance pseudocapacitors

A facile hydrothermal method is utilized to produce nanostructured NiCo₂S₄ arrays on carbon fiber paper with controlled morphologies to study the effect of morphology on their electrochemical performance in supercapacitors. Specifically, NiCo₂S₄ solid nanofiber, nanotube, and hollow nanoneedle of the same crystalline structure are synthesized by controlling the conditions of the hydrothermal synthesis. Among the three different morphologies studied, the hollow nanoneedle of NiCo₂S₄ shows the highest capacity and the longest cycling life, demonstrating a specific capacitance of ~1154Fg^-1 at a charge-discharge current density of 1Ag^-1 and negligible capacity loss after 8000 cycles (at a rate of 10Ag^-1). This high performance is attributed to the unique nanostructure of the hollow nanoneedle, suggesting that the morphology of NiCo₂S₄ plays a vital role in determining the electrochemical performance. Further, an asymmetric capacitor consisting of NiCo₂S₄ hollow nanoneedle electrode and a tape-cast activated carbon film electrode achieves an energy density of ~17.3Whkg^-1 at 1Ag^-1 and a power density of ~0.2kWkg^-1 at 20Ag^-1 in a voltage range of 0-1.5V, implying that it has a great potential for a wide variety of practical applications.