Synergistic mix design strategy for carbon black–modified metakaolin geopolymer composites toward structural energy-storage applications

This study proposed a synergistic mix-design strategy for metakaolin-based geopolymer composites for structural energy storage, addressing the trade-off between mechanical strength and electrochemical performance. Carbon black was used as a low-cost conductive filler, while the sodium hydroxide (NaOH)-sodium silicate (Na₂SiO₃) alkali activator system was optimized to simultaneously form a conductive electron network and intrinsic sodium ion transport channels during geopolymerization, without external electrolyte infiltration. The effects of carbon black content, NaOH-to-metakaolin ratio and activator dosage were systematically investigated using mechanical testing and electrochemical characterization, including electrochemical impedance spectroscopy, cyclic voltammetry (CV) and galvanostatic charge-discharge. The optimal formulation (5% carbon black, NaOH: metakaolin = 8.4%:41.6% and 53.4% activator content) achieved the best balance between strength and energy storage performance. Under this condition, a continuous carbon black conductive network and a dense geopolymer gel matrix were formed, enabling efficient coupled electron-ion transport. The composites exhibited a compressive strength of 31.7-33.2 MPa, specific capacitance of 72.5-78.3 F/g, ionic conductivity of 1.0 × 10⁻³ S/m and CV rectangularity of 8.8-9.2. After 1000 cycles, capacitance retention remained 94.2%-94.6%, with only a 3.6% reduction in compressive strength compared with low-carbon-black mixes. Overall, rational mix-design optimization enables simultaneous improvement of mechanical and electrochemical performance in geopolymer systems, providing a scalable and cost-effective route toward geopolymer-based structural supercapacitors for multifunctional infrastructure applications.