Fly ash electrodes fabricated by an acid-assisted subcritical water extraction method for supercapacitor applications

This work explores the utilization of coal fly ash (FA) for supercapacitor (SC) applications. Subcritical water extraction (SWE), a pressurized hydrothermal process, under acidic conditions was used to prepare FA as a SC electrode material; this is essential to remove the undesired compounds from FA powder as well as to improve its electrical conductivity. The effect of acid concentration in SWE process on the SC performance of the FA electrode was investigated. Characterizations such as galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were carried out to determine the electrochemical properties of samples. The FA electrode with 0.5 M acid pretreatment in SWE (FA-0.5) resulted in the best performance of the SC. The specific capacitance (C_s), energy density (E_d), and power density (P_d) of FA-0.5 as an electrode are calculated to be 620 F g⁻¹, 22 W h kg⁻¹, and 500 W kg⁻¹ at 2 A g⁻¹, respectively, in 3 M KOH alkaline solution. The energy storage capability might originate from the contained silicon dioxide and metal oxides in the FA-0.5 sample. The demonstration of a full cell supercapacitor was also performed by assembling FA-0.5 and activated carbon (AC) as the cathode and anode, respectively. Under a current density of 2 A g⁻¹, the asymmetrical FA-0.5//AC cell exhibits an E_d of 99 W h kg⁻¹ and P_d of 1600 W kg⁻¹ in the PVA-KOH gel electrolyte. This work provides an alternative valorization process to convert solid waste of coal FA by a novel SWE technique to an advanced energy storage application.