Intrinsic electrical conductivity of monolithic biochar

While biochar is an effective and viable tool for alleviating climate change, monolithic biochar is emerging as a functional material for applications that enhance sustainability, such as renewable energy storage and low-cost, high-efficiency water purification. Its performance depends on its physical and chemical characteristics, including electrical conductivity. Monolithic biochar's bulk conductivity is expected to rely on the porosity and conductivity of its carbon matrix - intrinsic conductivity – a fundamental property that has not been systematically studied. The work discerns intrinsic conductivity and its dependence on biomass species and carbonization temperature. We carbonized four hardwoods, three softwoods, and bamboo following an ultra-slow pyrolysis procedure and characterized biochar's chemical and structural properties. We modified the two-probe method and measured bulk conductivity along the axial direction. The bulk conductivity and density followed the linear and the Reynolds-Hough relationships, allowing the determination of intrinsic conductivity. The intrinsic conductivity increased with pyrolysis temperatures and became independent of wood species at 1500 °C. According to the linear model, the highest value of wood biochar produced at 1500 °C was 14,600 S/m. Bamboo biochar had a higher intrinsic conductivity (21,000 S/m), attributed to bamboo's high cellulose content and large graphite nanocrystal size in its biochar. Moreover, the modified two-probe method minimized measurement uncertainty and enabled the quantification of natural variation in biochar conductivity, which was less than 10% for the same wood species. These findings will help strengthen monolithic biochar's potential as a low-cost, high-performance functional material.