Metal oxides are of great interest for supercapacitor application, however, they suffer from capacity fading during cycling and limited cycle life. In this work, a one-pot bottom-up approach is proposed to design cobalt oxide (Co3O4) nanoparticles confined in a mesoporous carbon. This involved the coassembly of a phenolic resin, a surfactant, and a cobalt salt followed by a high temperature pyrolysis (600−800 °C) and a subsequent low temperature oxidation (190−240 °C) step. Very small Co3O4 particle size (2.3−7.4 nm) could be achieved for high loadings of Co3O4 (up to 59%) in the carbon network. Both the pyrolysis and oxidation temperature increase led to an increase of nanoparticle size, porosity and electronic conductivity. At low temperatures, i.e., 600 and 650 °C, and despite the low particle size, the performances are poor and limited by the carbon low electronic conductivity. At high temperature (800 °C), the conductivity is improved translating in a higher capacitance, but the larger and more aggregated nanoparticles induced low rate capability. The best compromise to maintain high capacitance and rate capability was observed at 700 and 750 °C and thus for composite materials combining simultaneously dispersed nanoparticles, high porosity, and good electronic conductivity. In particular, the material treated at 750 °C presents, in a 2 electrode system using 2 M KOH, a capacitance of 54 F g −1 at 0.1 A g −1 , a very high rate capability of 48.7% at 10 A g −1 , and a superior rate performance of 82% after 10000 cycles.

This paper reports the development of a safe carbon-based supercapacitor, which is based on a green biodegradable hydrogel electrolyte that is prepared from chitosan biopolymer and KOH as the electrolyte...