Optimizing surface properties and porosity of carbonized waste coffee grounds via molten base activation for enhanced zinc supercapattery performance

Zinc Supercapatteries (ZnSCs) are promising, low-cost and environmentally friendly electrochemical energy storage devices, with biomass-derived carbon showing promising results as its positrode active material. However, achieving sustainable biomass-derived carbon via a simple, single-stage and low-temperature process with properties for optimum electrochemical performance is still a challenge. In this study, we show that ultramicroporous and self-doped carbon which are critical electrode properties can be obtained from wet waste coffee grounds via a single-stage thermal process termed molten base carbonization and activation. The process integrates carbonization, in-situ activation and self-doping in one thermal step, driven by the catalytic reactivity of intercalated potassium ion within various components. Temperature control during the process resulted to changes in the microstructure, hierarchical porosity, oxygen and nitrogen functionalities affecting electrochemical performance. The carbon obtained at 700 °C (CGZ-700) as positive electrode active material in Zinc supercapattery achieved specific capacity of 361 mAh g⁻¹, specific capacitance of 204F g⁻¹ at 0.1 A g⁻¹, alongside a specific energy and power of 91.99 Wh kg⁻¹ and 89.99 W kg⁻¹. At 1.0 A g⁻¹, it achieved a coulombic efficiency of 99.8 % and 78 % capacity retention after 10,000 cycles. This study offers a facile, low temperature single-stage thermal conversion process for low-cost waste biomass, advancing the application of waste-derived carbon in ZnSCs and sustainable energy storage devices.