Synthesis of size-controlled carbon microspheres from resorcinol/formaldehyde for high electrochemical performance

Nanostructured phenolic resin-based carbon aerogels with an extensive network structure are regarded as ideal energy storage materials for supercapacitors. However, the initial bulk form and low capacitance of previously reported porous carbon aerogels are problematic for practical use. Phenolic resin-based porous carbon spheres were synthesized by a simple hydrothermal process using ammonia, ethylenediamine or hexylenediamine as a catalyst. The porous carbon spheres were investigated by SEM, BET, XPS, etc. It was found that the number of ammonium groups, length of the alkyl chain and processing temperature play vital roles in determining the pore structure, size and uniformity of the carbon spheres. NH₄⁺ is necessary to obtain the carbon spheres and but changing the other parameters has no obvious effect on their crystal structure. The sample prepared at a hydrothermal temperature of 80 °C using ammonia as the catalyst has the highest specific capacitance of 233.8 F g⁻¹ at a current density of 1.0 A g⁻¹. It has an excellent capacitance retention of 98% after 10 000 charge/discharge cycles at 7 A g⁻¹, indicating its good cycling stability and rate capability. This result shows that a higher specific surface area, porosity and defect density are probably the crucial factors in improving the electrochemical capacitance.