Development of geopolymer-based electrodes and electrolytes for structural supercapacitors in energy storage

Supercapacitors have attracted widespread attention for their high-power density and long cycle life. Meanwhile, low-carbon structural materials with energy storage capability are emerging as promising candidates for sustainable civil infrastructure. Geopolymers exhibit a lower carbon footprint than conventional cementitious materials and show strong potential for structural energy storage owing to their relatively high ionic conductivity. In this study, we develop a geopolymer-based supercapacitor (GBSC) system that, unlike conventional structural supercapacitors relying on separate functional components, uses a fly ash–kaolin geopolymer as both electrolyte and electrode, enabling a simplified and multifunctional device. GBSCs are fabricated using fly ash–kaolin composites activated with alkaline activators containing different concentrations of sodium and potassium ions. A potassium-rich geopolymer enhances precursor dissolution and gel-network reconstruction, resulting in an ionic conductivity of 21.8 mS cm −1 after 28 days of curing, despite a moderate reduction in compressive strength. When used as the electrolyte, the optimized geopolymer achieved a specific capacitance of 1.2 mF cm −2 at 0.05 mA cm −2 , with 82.7% retention after 2000 cycles. When configured as electrode, it delivers a significantly higher capacitance of 6.0 mF cm −2 , along with an energy density of 94.6 μWh cm −2 and a power density of 1486.8 μW cm −2 , while retaining 78.8% capacitance after 2000 cycles. These findings demonstrate that geopolymer composites can function as both electrodes and electrolytes, allowing simplified and scalable GBSCs for structure-integrated energy-storage applications. They also highlight the previously underexplored electrochemical functionality of geopolymers and establish a new design strategy for simplified, scalable, and structure-integrated energy-storage systems.