Solar-driven synthesis of CaS-decorated activated carbon from pecan nutshell agro-industrial waste to assemble green, stable, and electro-activated supercapacitors

Developing efficient energy storage devices is crucial in a growing trend toward using electromobility and renewable energy sources. This study focuses on preparing environmentally friendly supercapacitors by synthesizing activated carbon decorated with semispherical CaS nanoparticles of approximately 30 nm in size. Pecan nutshells (Carya illinoinensis) were used as a carbon and calcium source, whereas (NH₄)₂SO₃ was used both as a novel activating agent and a CaS precursor, enabling the formation of CaS-decorated activated carbons, as discussed in this work, for the first time. Activated carbon was prepared using a high-flux solar radiation furnace (SF) and a conventional tube furnace (TF). The synthesis temperature and concentration of the activating agent were determined using a factorial experimental design with central set points at 400, 600, and 800 °C and 1, 2, and 3 M. Physicochemical analysis revealed that temperature had a statistically significant effect on the surface area and degree of graphitization of the synthesized materials only when the TF method was used. The carbonaceous materials were electrochemically characterized in a 3-electrode cell to investigate their energy storage properties as supercapacitor electrodes. The highest electrochemical capacitance (≈180 Fg⁻¹) was obtained in a potential window of 2 V. In addition, the best carbon-based material was tested as a supercapacitor electrode and exhibited a specific energy of 14.42 Wh/kg, increasing by 50 % after 15,000 cycles, highlighting its electro-activation characteristics. Even though metal sulfides have been extensively used for supercapacitors, this is the pioneering work harnessing biomass-derived carbons decorated with calcium sulfide nanoparticles for supercapacitors.