Influence of Phosphoric Acid Activation on Physiochemical Characteristics of Activated Carbons and Their Performance as Supercapacitor

Abstract

We investigate the influence of phosphoric acid (H₃PO₄) activation on physiochemical characteristics of activated carbons (ACs) as a function of number of activation steps such as two-step and three-step and impregnation ratio (IR). Scanning electron microscopy observations identified that different morphologies in the forms of graphene sheet-like, nano-granular, and flake-like carbon structure in the cases of ACs. FTIR spectroscopy confirmed that successful incorporation of phosphorous group in the ACs by H₃PO₄ activation. X-ray diffraction (XRD) profile exposed that irrespective of the activation method and IR, all AC samples showed narrow and sharp XRD crystalline peak along with the amorphous signals. Raman scattering analysis suggested that three-step activation create more defective structure as compared to two-step activation route. Nitrogen adsorption–desorption isotherm measurements indicated that upon fabricating ACs via three-step and two-step activation approach, around 6.5 times and 3-fold enhancement in the value of surface area of ACs as compared to that of carbon before activation. In addition, higher the IR value, lower the textural properties of ACs. This study demonstrated that three-step activation methodology is capable of generating highly porous AC when compared to two-step activation route. Cyclic voltammetry analysis showed that for the electrode developed from AC that fabricated via three-step activation, capacitance retention of 50% is achieved upon tuning the scan rate by 10 times. The same electrode exhibited the capacitance retention of 45% upon increasing the current density by 10 times. We have also compared the electrochemical performance of symmetric and asymmetric supercapacitors. Electrochemical capacitance retention of symmetric and asymmetric supercapacitors is determined to be 100% and 92% respectively after 1000 cycles at the current density of 1 A g⁻¹. Based on the Ragone plot study, it is observed that the maximum energy density of 5 W h kg⁻¹ and the maximum power density of 943 W kg⁻¹ are attained for the case of symmetric supercapacitors. Asymmetric supercapacitor displayed improved energy density of 7.15 W h kg⁻¹ and modest power density of 432 W kg⁻¹.