Effects of physical and chemical activations on the performance of biochar applied in supercapacitors

[Display omitted] •To reduce the cost of biochar, Sapindus peels and oyster shell waste are used.•The effects of physical, chemical and hybrid activation are discussed.•Activated carbons, rich in 4 nm mesopores, can be obtained by physical activation.•The asymmetric supercapacitor exhibits a high power density of 2200 W kg−1. Recently, there has been a growing concern for environmental protection. Herein, an economical, sustainable, and green method for synthesizing activated carbon from waste using Sapindus peels and oyster shell powder is reported. Oyster shell powder decomposes into CO2 at high temperatures and can be used as a physical activator. In this study, we discuss the difference between physical, chemical, and hybrid activation. Through proper treatment, the original specific surface area of 12.8 m2 g−1 for Sapindus peel biochar was increased to 1175–1537 m2 g−1. In addition, activated carbons rich in 4 nm mesopores can be obtained by simple physical activation with oyster shell powder. In applications, the prepared electrodes showed a high specific capacitance of 258.1 F g−1 at 5 mV s−1. The assembled supercapacitor also exhibited excellent electrochemical performance with a capacitance retention of 81.9 % after 5000 test cycles and a coulombic efficiency close to 100 % during the test.