Unraveling Surface-Controlled versus Diffusion-Controlled Mechanisms in Coir-Derived Carbon and NiCo-LDH for High Energy Density Asymmetric Supercapacitors

The present work focuses on fabricating asymmetric supercapacitors (ASCs) using AC from coir fiber as the anode and nickel–cobalt double-layered hydroxide (NC-LDH) as the cathode material. The kinetic study of AC and NC-LDH was examined using a power law relationship and the Dunn model. The AC electrode showed a dominant surface-controlled charge storage mechanism of about 99.76%, with a very minimal diffusion-controlled process. The NC-LDH electrode exhibited the predominance of a pseudocapacitive process, with diffusion and surface-controlled charge storage mechanisms of about 93.50% and 6.49%, respectively. The surface areas of AC and NC-LDH were found to be 1330 and 52 m2/g, with total pore volumes of 0.81 and 0.38 cm3/g, respectively. The AC and NC-LDH demonstrated specific capacitances of 294 F/g and 586 F/g at current densities of 1 and 0.5 A/g, respectively, showing good cyclic performance and Coulombic efficiencies of about 99.61% and 98.96% over 5000 cycles in a three-electrode system. An asymmetric supercapacitor (ASC) device was fabricated, which displayed both diffusion- and surface-controlled processes of about 81.63 and 18.36% at a scan rate of 50 mV/s, and delivered a specific capacitance of 125.4 F/g at 0.5 A/g, offering an energy density of about 34 Wh/kg at a power density of 1300 W/kg. The device exhibited superior long-term cyclic stability, showing 92.38% capacitive retention and 99.85% Coulombic efficiency after 20,000 prolonged charge–discharge cycles. Thus, the kinetic study of the ASC device underscores the synergistic combination of AC and NC-LDH, thereby improving electrochemical performance through dual mechanisms (nonfaradaic and faradaic), which aid in increasing the overall capacitance of the ASC device.