Size-dependent capacitive behavior of homogeneous MnO nanoparticles on carbon cloth as electrodes for symmetric solid-state supercapacitors with high performance

As promising electrode materials for supercapacitors, manganese oxides still have big challenges such as the low material utilization and poor ionic/electronic conductivity. Reducing particle sizes through nanotechnology has been used to improve material conductivity and electrochemical active sites at materials/electrolyte interfaces. Nevertheless, the extremely small particle size may result in physical and/or chemical instability, mass loss and subsequent capacitance attenuation. Understanding this trade-off effect of electrode materials size with their electrochemical properties is critical to fabricate high-performance supercapacitors. In this work, we prepare homogenous and size-tunable MnO particles (with mean diameters of 80, 41, 20, 15 and 9 nm) on carbon cloth via a facile gel-like film assisted method. It is found that the medium-size nanoparticle (20 nm) displays the best performance instead of the smallest one. These observations are different from the traditional view about material size-property relationship. Instead this work provides a new insight referring to both the size-dependent solubility and ionic/electronic transport. Beneficial from the good flexibility and high conductivity/stability of carbon cloth, the optimized MnO/carbon cloth electrode demonstrates extraordinary performance in symmetric solid-state supercapacitors with energy densities of 86 and 70 Wh kg −1 at the power densities of 450 W kg −1 and 9 kW kg −1 , respectively.