Electrical conductivity of wood biochar monoliths and its dependence on pyrolysis temperature

Abstract

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.

Incorporation of Biochar to Improve Mechanical, Thermal and Electrical Properties of Polymer Composites

“…Biochar when carbonized at high treatment temperatures above 500 °C shows electrical conductivity. Gabhi, Kirk, and Jia (2017) [ 104 ], R. Gabhi et al (2020) [ 105 ] in their studies observed that carbonization of wood blocks at higher temperatures of 1000 °C leads to a subsequent increase in carbon content in the char. This temperature of pyrolysis and carbon content plays an important role in electrical conductivity of biochar.…”

“…This temperature of pyrolysis and carbon content plays an important role in electrical conductivity of biochar. The study reported that biochar monoliths carbonized at 1000 °C had electrical conductivity between the range of 2300–3300 S/m [ 105 ]. Electrically conductive composites developed by Nan et al (2016) [ 33 ] reported electrical conductivity in biochar filled composites fabricated using solution casting method.…”

Industrial biochar systems for atmospheric carbon removal: a review

“…For example, for agronomic applications, a high electrical conductivity value indicates the presence of salts, where a high biochar application rate may have negative effects on salt-sensitive plants (Singh et al 2017). However, a high electrical conductivity value would be favourable for other applications such as supercapacitors for energy storage (Gabhi et al 2020;Abdel Maksoud et al 2020). Similar to pH, the type of feedstock and the processing conditions are determinants for the electrical conductivity values measured in the resulting biochar.…”

A nanoscale observation to explain the discrepancy of electron exchange capacities between biochar containing comparable surface redox-active moieties

“…The poor conductivity of ind400ex resulting from the low carbonization and graphitization degree (Gabhi et al 2020(Gabhi et al , 2017) could be the reason for the weak current. Meanwhile, the composition characterization showed higher contents of amorphous aromatics with heteroatoms in st700ex (than st700re) and ind400re (than ind400ex), more condensed structures with fewer heteroatoms in st700re than in st700ex, and fewer aromatized carbons with more heteroatoms in ind400ex than in ind400re.…”