Valorisation of wastewater sludge into activated carbon via H3PO4 activation for supercapacitor applications

Abstract

The valorisation of wastewater sludge (WWS) into high-value functional materials offers a sustainable route to address waste management and energy storage challenges. In this work, WWS was converted into activated carbon (WSAC-H) via a single-step carbonisation process using 30% H₃PO₄ as the activating agent at 600 °C for 4 h. Structural and surface analyses revealed a rough, functionalised morphology and well-developed porosity. Electrochemical evaluation in a three-electrode system showed that 1 M H₂SO₄ yielded the highest specific capacitance (114.96 F g⁻¹ at 0.5 A g⁻¹), outperforming 1 M Na₂SO₄ (41.40 F g⁻¹) and 1 M KOH (18.04 F g⁻¹) at the same current density. A symmetric device in 1 M H₂SO₄ delivered an energy density of 5.89 Wh kg⁻¹ and a power density of 1000 W kg⁻¹ at 0.5 A g⁻¹, retaining 84% of its capacitance after 20 000 charge–discharge cycles at 3.0 A g⁻¹. The performance is attributed to the synergistic effects of acid activation, intrinsic elemental composition, and high electrolyte conductivity. These findings demonstrate the potential of WWS-derived activated carbon as a cost-effective, high-performance supercapacitor electrode material, advancing both environmental remediation and sustainable energy storage.