From dead leaves to high energy density supercapacitors

Abstract

Functional microporous conducting carbon with a high surface area of about 1230 m² g⁻¹ is synthesized by single-step pyrolysis of dead plant leaves (dry waste, ground powder) without any activation and studied for supercapacitor application. We suggest that the activation is provided by the natural constituents in the leaves composed of soft organics and metals. Although the detailed study performed and reported here is on dead Neem leaves (Azadirachta indica), the process is clearly generic and applicable to most forms of dead leaves. Indeed we have examined the case of dead Ashoka leaves as well. The comparison between the Neem and Ashoka leaves brings out the importance of the constitution and composition of the bio-source in the nature of carbon formed and its properties. We also discuss and compare the cases of pyrolysis of green leaves as well as un-ground dead leaves with that of ground dead leaf powder studied in full detail. The concurrent high conductivity and microporosity realized in our carbonaceous materials are key to the high energy supercapacitor application. Indeed, our synthesized functional carbon exhibits a very high specific capacitance of 400 F g⁻¹ and an energy density of 55 W h kg⁻¹ at a current density of 0.5 A g⁻¹ in aqueous 1 M H₂SO₄. The areal capacitance value of the carbon derived from dead (Neem) plant leaves (CDDPL) is also significantly high (32 μF cm⁻²). In an organic electrolyte the material shows a specific capacitance of 88 F g⁻¹ at a current density of 2 A g⁻¹.