Recycled paper fiber as a 3D network builder for high-performance carbon-cement structural supercapacitors

To advance sustainable building–energy integration, structural materials that combine load-bearing capacity with efficient energy storage are urgently needed. Conventional carbon–cement composites suffer from a strength–capacitance trade-off, in which increased carbon content enhances conductivity but degrades mechanical integrity. This study proposes a sustainable strategy using recycled paper fibers as a three-dimensional (3D) network builder to overcome this limitation. Owing to their hydrophilicity and strong affinity for carbon surfaces, paper fibers form a continuous conductive network within the cement matrix at low carbon content, markedly improving carbon utilization and ion transport. At a carbon–cement ratio of only 5%, the incorporation of recycled paper fibers increases carbon utilization from 1.49% to 98.39%, while simultaneously achieving a volumetric capacitance of 287.4 kF m⁻³ and a compressive strength of 29.46 MPa. Microstructural characterization and first-principles calculations reveal that the fibers create interconnected pore channels and conductive bridges, thereby optimizing pore connectivity and interfacial charge transfer. This work provides a facile and effective material-design route for fabricating high-performance carbon–cement structural supercapacitors. More broadly, it highlights the potential of waste-derived paper fibers for developing sustainable, multifunctional cementitious materials for future energy–structure integration.