Effects of activating agents of acids and alkalis on electrochemical properties of carbon spheres

Activation of carbon spheres (CSs) is an important process for the fabrication of CS electrodes for supercapacitor applications. To investigate the effects of different activating agents on CSs, we have synthesized and characterized the structure, morphology, and electrochemical properties of the CSs treated in nitric acid (ACS-HNO3) for different periods or potassium hydroxide (ACS-KOH) at different temperatures. The activated CSs have nanospherical morphology with the diameter of about 180 nm. The studies reveal that the activating agents differ in their effects on surface area and in generation of surface functional groups. The HNO3 activations decrease slightly the surface area and generate pyrone-like groups (C-O and C=O), while the KOH activations increase significantly the porosity and surface area and generate only a small amount of oxygen-containing surface groups. Both activation processes improve the specific capacitance from 113 F g(-1) of unactivated CS to approximately 200 F g(-1) at 0.5 A g(-1) in 3 M KOH electrolyte. The cycling tests show retention ratios of 89.3 and 85.6 %, respectively, after 500 cycles for the electrodes treated by HNO3 and KOH. The facile synthesis, good electrochemical performance, and cycling stability of the activated CS electrodes are very promising for applications in high-performance supercapacitors.