High-Strength Bauxite Residue (Red Mud)–Cement Composite for Integrated Bulk Electrode/Electrolyte Structural Capacitors

Cement-based structural capacitors (CSCs) combining load-bearing and energy-storage functions are promising for next-generation green buildings. However, the practical application of CSCs remains hindered by the trade-off between mechanical and electrochemical performance, together with challenges related to material complexity and scalability. To address these challenges, we propose a red mud-enabled ion–electron dual-conduction bulk-electrode concept by incorporating 25 wt% Bayer-process bauxite residue (red mud, RM) into the cement matrix, thereby forming a composite with efficient coupled ionic/electronic conduction while retaining robust mechanical performance. The resulting composite serves as an integrated electrode/electrolyte in CSCs, expanding the energy storage region from the conventional 2D planar interface to the 3D bulk electrode, increasing the areal capacitance from 0.588 mF cm−2 (pure cement) to 53.4 mF cm−2. This configuration also retains a compressive strength of 45.2 MPa, demonstrating a favorable balance between mechanical and electrochemical performance. Four building-component-scale RMCSCs (20 × 10 × 6 cm3) connected in series can power a 1 W, 3 V industrial light bulb for more than 20 min on a single charge. A building wall (100 × 90 × 10 cm3) assembled from these units can power more than ten industrial light bulbs.