Coupled structural-electrochemical design of biochar-cement supercapacitors

Buildings are the largest engineered mass on earth. Cement-based supercapacitors promise distributed energy storage in structural elements, yet their design is governed by a coupled ionic-electronic transport problem that remains poorly resolved: conductive fillers that maximize electrochemical performance simultaneously dilute the load-bearing matrix. Here, these competing contributions are disentangled in carbon–cement composites, and the resulting framework is used to define the multifunctional optimum. Employing cassava ( Manihot esculenta ) leaf-derived biochar, an agricultural residue, as a model intrinsic-porosity carbon, the total conductivity is resolved into a finite ionic background and an electronic percolation branch, with an apparent threshold p c ≈ 13.3 wt% and a critical exponent t ≈ 2 consistent with a three-dimensional random conductive network. Micro-computed tomography, electrochemical impedance spectroscopy, and a Bruggeman-type ionic-tortuosity estimate are combined into a single semi-analytical model that links microstructure, transport, and mechanical response. The composites deliver areal capacitances approaching 180 mF cm − 2 , and power densities of 3– 4 kW kg − 1 on an active-carbon basis. On a total-device basis, capacitance reaches ≈ 35 mF g −1 and ≈ 71 mF cm −3 (BC80). Across loading, electronic connectivity, ionic accessibility, and paste-level strength (8–12 MPa at BC20-BC40) trade off in the manner the model predicts, identifying BC40–BC60 as the multifunctional window. The central result is a transferable design framework providing quantitative design guidance for the present material system for balancing percolation, ionic transport, and matrix continuity in structural energy-storage cement.