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Biochar, also known as black carbon, is a byproduct of biomass pyrolysis. As a low-cost, environmental-friendly material, biochar has the potential to replace more expensive synthesized carbon nanomaterials (e.g. carbon nanotubes) for use in future supercapacitors. To achieve high capacitance, biochar requires proper activation. A conventional approach involves mixing biochar with a strong base and baking at a high temperature. However, this process is time consuming and energy inefficient (requiring temperatures >900 °C). This work demonstrates a low-temperature (<150 °C) plasma treatment that efficiently activates a yellow pine biochar. Particularly, the effects of oxygen plasma on the biochar microstructure and supercapacitor characteristics are studied. Significant enhancement of the capacitance is achieved: 171.4 F g −1 for a 5-min oxygen plasma activation, in comparison to 99.5 F g −1 for a conventional chemical activation and 60.4 F g −1 for untreated biochar. This enhancement of the charge storage capacity is attributed to the creation of a broad distribution in pore size and a larger surface area. The plasma activation mechanisms in terms of the evolution of the biochar surface and microstructure are further discussed. [ABSTRACT FROM AUTHOR]