With the increasing drive towards renewable energy sources, the need for further development in electrical energy storage is apparent. As it stands, the temporal misalignment between energy generation and electrical use or demand gives rise to grid-scale issues that can only be addressed by the de-coupling of energy generation and energy demand [1]. This is achievable through improved electrical energy storage technologies, where supercapacitors and batteries stand at the forefront.

Electrochemical double layer capacitors (EDLCs) store charge physically using the reversible adsorption of ions onto materials that are often highly conductive, and porous with high specific surface areas. The clear majority of EDLCs both in research and in commercial use utilize porous carbon powder electrodes, held together with binder materials and compressed into thin-films [2]. These would typically be made of graphene, carbon nanotubes or activated carbons. Our group investigates biochar, or biomass-derived carbon, as potential monolithic electrodes rather than thin-film powders now used.

Through a controlled pyrolysis process, these monolithic biochar electrodes can retain the internal macrostructures of the sugar-maple (acer saccharum) hardwood, the precursor material of this study. As such, natural mass-transport pathways are available to facilitate electrolyte and ion transport. Monolithic electrodes also provide the benefits of reducing non-active materials such as non-conductive binder materials, and allows the simple construction of larger electrodes that are structurally robust.

The scalability of supercapacitors is an important aspect for tackling the large-scale issues in energy storage. This study investigates biochar electrode scalability by pushing the limits of electrode size and analyzing electrochemical performance as a supercapacitor. Previous studies on monolithic biochar supercapacitors explored electrodes in the millimetre size range, with masses of approximately 1 milligram [3]. This study uses electrodes orders of magnitude larger, measuring 2.3 cm x 1.3 cm x 9.6 cm, with masses of 1.4 grams.

Preliminary testing was carried out in a two-cell setup in 4M KOH electrolyte with cyclic voltammetry, galvanostatic charge and discharge cycling and chronoamperometric charge and discharge. The results of these experiments are promising, showing high specific capacitances of 35 F/g at 5 mA/g, and 28 F/g at 200 mA/g. Energy storage capability is shown through chronoamperometric charge and discharge to be 83 Joules or 8.23 Wh/kg with a peak power output of approximately 285 W/kg.

References

[1] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials. 2008;7:845-854.

[2] Wang Q, Yan J, Fan Z. Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities. Energy and Environmental Science. 2016;9(3):729-762

[3] Zhang L, Jiang J, Holm N, Chen F. Mini-chunk biochar supercapacitors. Journal of Applied Electrochemistry. 2014;44:1145-51