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Biochar is traditionally used as solid fuel and for soil amendment where its electrical conductivity is largely irrelevant and unexplored. However, electrical conductivity is critical to biochar's performance in new applications such as supercapacitor energy storage and capacitive deionization of water. In this study, sugar maple and white pine were carbonized via a slow pyrolysis process at 600, 800 and 1000 °C and conductivities of monolithic biochar samples along the radial direction were measured using the 4-probe method. Biochars were characterized using an elemental analyzer, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The solid carbon in biochar samples was found to consist primarily of disordered carbon atoms with small graphitic nanocrystallites that grow with increasing temperature. The bulk conductivity of biochar was found to increase with pyrolysis temperature-1 to ~ 1000 S/m for maple and 1 to ~ 350 S/m for pine, which was accompanied by an increase in carbon content-91 to 97 wt% and 90 to 96 wt% for maple and pine, respectively. The skeletal conductivity of biochar samples carbonized at 1000 °C is about 3300 S/m and 2300 S/m for maple and pine, respectively (assuming solid carbon is amorphous); both values are above that of amorphous carbon (1250-2000 S/m). This work demonstrated the importance of carbonization and graphitization to electrical conductivity and suggested electron hopping as a likely mechanism for electric conduction in biochar-an amorphous carbon matrix embedded with graphitic nanocrystallites.

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The electrode material in commercial supercapacitors has high electrical resistivity, intrinsic to the activated carbon powder−organic binder mixture. Consequently, electrode materials are coated on current collectors as thin films to reduce the device resistance. However, the thin-film configuration limits the volume fraction of an active electrode material (activated carbon) and holds the volumetric energy density low. Wood biochar monoliths (WBMs) have a low-tortuosity porous structure and a conductive carbon matrix, a combination desirable for binder-free electrodes with high energy density. This article reports a novel approach for fabricating high-performance, WBM-based thick electrodes. The approach combines wood pulping, mechanical compression, and thermal carbonization, transforming a common softwood-white pine into an intensified wood biochar monolith (IWBM) with high bulk density (0.617 g/cm 3 ) and pore utilization efficiency (28 μF/cm 2 ) at the same time. Furthermore, the thick (1.2 mm) electrode made with the IWBM exhibited record-high areal capacitances (7.58−9.36 F/cm 2 ) and a high volumetric capacitance (78.0 F/cm 3 ) attributed to the densified, easily accessible pore network and the conductive carbon matrix in the IWBM. Moreover, the white pine WBM is readily augmented with pseudocapacitive materials (e.g., RuO 2 ). A supercapacitor cell with two symmetric pine WBM electrodes displayed no performance attenuation after 10,000 cycles. This study provides a practical approach to increasing the volumetric energy density, demonstrating the potential of WBMs as a low-cost, high-performance alternative to advanced nanoporous carbon materials such as graphene and carbon nanotubes.

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