Supercapacitor is one of the most promising energy storage devices to meet surging demands of high-power supply and long-cycle life. Aqueous electrolytes are advantageous over organic electrolytes due to its low cost, safety and ease of assembly but have a major drawback of low energy density due to their small potential windows (~1.2 V). While neutral water is preferred over its acidic or alkaline counterpart to mitigate the drawback (i.e. small potential window), its wettability on carbon, the usual supercapacitor electrode material, has significant room to improve. To address this issue, we considered the incorporation of a metal oxide, MnO_x, to the carbon-based structure. MnO_x is chosen due to its superior pseudocapacitive capability, wider electrochemical potential window, abundance, and low cost.

In this study, we functionalized active carbon-based structures with a metal oxide (MnO_x) by electro-pulse deposition to improve both the electrode wettability and pseudocapacitance. Pulse deposition is chosen to afford a precise control over coating thickness and surface chemistry by controlling the pulse amplitude (0.5 – 3 V), duration (1 – 500 ms) and number of pulsing process. Results showed that there is an optimum in the resulting capacitance; electrodepositing of MnO_x at 1 V for 50 ms showed the best performance by improving the overall specific capacitance by 30%. However, the higher pulsing amplitude/duration generally improved wettability. A series of observations indicates that the overall capacitance is co-limited by multiple factors including wettability, surface area and area-specific pseudocapacitance.