Preliminary investigation of electrical conductivity of monolithic biochar

Monolithic biochar is explored as electrode material in supercapacitor - a fast-charging, long-lasting energy storage device. Electrical conductivity of electrode is critical to supercapacitor's performance. Traditionally, biochar is used as a solid fuel for combustion where electrical conductivity is largely irrelevant and ignored. This work explores electrical conductivity of monolithic biochar and elucidates its dependence on micro and macro structures of biochar. Electrical conductivity of biochar was found to be highly dependent on its degree of carbonization. Bulk conductivity of biochar can increase by over six orders of magnitudes when its carbon content changes from 86.8 to 93.7 wt%. Transmission electronic microscope and x-ray diffraction analyses revealed the presence of graphite nanocrystals (∼3 nm) and the growth of biochar crystallinity after heat treatment at 950 °C. The highest skeletal conductivity of carbon was 343.2 S/m, found in a heat-treated sugar maple biochar with 96.2 wt% of carbon. It is higher than that of graphite single crystal in direction perpendicular to graphene plane (333.3 S/m). Moreover, it was observed conductivity of monolithic biochar increased with compressive loading and dropped to the pre-compression level when the loading was released - a new phenomenon termed “elastic behavior of electrical conductivity of biochar”.

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