Modulus-regulated alkali-activated geopolymers for structural supercapacitors

This study reports a bifunctional structural supercapacitor incorporating a metakaolin-based geopolymer as a mechanically robust electrolyte separator and activated carbon as the electrode material. Geopolymer separators were prepared using a mixed sodium hydroxide (NaOH)–sodium metasilicate (Na2SiO3) activator with different activator modulus values (Ms), and their pore structure, mechanical properties and electrochemical performance were systematically investigated. Increasing Ms from 0.6 to 1.4 reduced the accessible porosity to approximately 40% and refined the most probable pore size from about 430 nm to 120–150 nm, indicating the formation of a denser and narrower multiscale pore network. This refinement enhanced the 28-day compressive strength, which reached 15.3 MPa at Ms = 1.4. All assembled symmetric structural supercapacitors with sodium sulfate (Na2SO4) electrolyte exhibited nearly rectangular cyclic voltammetry (CV) curves and quasi-linear galvanostatic charge–discharge (GCD) profiles, confirming electric double-layer capacitive behaviour. The device with Ms = 0.6 delivered the highest areal capacitance of 232.51 mF/cm2, whereas Ms = 1.2 achieved a favourable balance between mechanical and electrochemical performance, with a compressive strength of approximately 12.5 MPa and an areal capacitance of 191.68 mF/cm2. These results demonstrate that activator modulus regulates mechanical–electrochemical coupling by tailoring geopolymer pore structure.