Fabrication and characterization of flexible supercapacitor electrodes from spent filter coffee grounds

The increasing demand for sustainable energy storage has driven interest in eco-friendly electrode materials with high stability and low environmental impact. In this study, activated carbon derived from coffee waste was functionalized using phosphoric and citric acids, providing a mild and green alternative to conventional strong-acid treatments. The dual-acid modification introduced oxygen and phosphorus-containing functional groups that enhanced surface wettability and electrochemical activity. The functionalized carbon was deposited onto a carbon felt substrate via electrophoretic deposition (EPD), a binder-free, uniform, and scalable coating method. Under optimized conditions, CPFAC exhibited a specific capacitance of 27.43 F g⁻¹ at 10 mV s⁻¹ and 29.77 F g⁻¹ at 1 A g⁻¹, and retained 120% of its initial capacitance after 10,000 cycles. In addition, the electrode preserved 95.9% and 83.3% of its capacitance after 1000 bending and twisting cycles, respectively, confirming excellent mechanical resilience. The mesoporous structure (2–10 nm) and improved electrolyte accessibility contributed to efficient ion transport, mechanical robustness, and stable capacitive behavior. These results demonstrate that the combination of dual mild-acid functionalization and EPD provides a green and cost-effective pathway for fabricating flexible supercapacitor electrodes with superior long-term electrochemical and mechanical stability.