From dead leaves to high energy density supercapacitors† Mandakini Biswal, ab Abhik Banerjee, ab Meenal Deo ab and Satishchandra Ogale * ab Functional microporous conducting carbon with a high surface area of about 1230 m 2 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 H 2 SO 4 . The areal capacitance value of the carbon derived from dead (Neem) plant leaves (CDDPL) is also significantly high (32 mF 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 . Broader context Waste management has always been a big problem in big cities. Most such waste is a rich source of carbon but may contain other elements in different proportions. Usually the waste from natural sources is just burnt producing ash and hazardous gaseous pollution products. If instead it is harnessed to synthesize electronically active carbon, one could use it for value-added products such as materials for supercapacitor electrodes. Supercapacitors have been attracting signicant interest due to their applications in electrical vehicles, digital devices, pulsing techniques etc. In this work we demonstrate the synthesis of high surface area microporous conducting carbon by one-step pyrolysis of dead plant leaves (abundant waste material) without any chemical or physical activation and have examined its properties for supercapacitor application. 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 too. With dead Neem leaves we have achieved a high specic capacitance of 400 F g 1 and a energy density of 55 W h kg 1 at 0.5 A g 1 . Moreover, in an organic electrolyte the material shows a specic capacitance of 88 F g 1 at 2 A g 1 . Introduction Waste management has always been a big problem in big cities. Fortunately, through several environmental initiatives communities have begun to separate dry and wet waste matter. Most such waste is a rich source of carbon but may contain other elements in different proportions depending on the source of the waste. Usually the waste from natural sources is just burnt producing ash and hazardous gaseous pollution products. There have been some initiatives to employ the ash in certain applications but in most situations the use is in the form of passive llers. If the natural waste and some of the man-made waste are harnessed to synthesize electronically active func- tional forms of carbon, one could get value-added products for diverse and growing carbon-based applications. Carbon is the most naturally occurring abundant material exhibiting a variety of molecular and structural forms such as graphite, diamond, nanotubes, graphene, fullerene, nano- diamonds, amorphous carbon, porous carbon, etc. with various applications. 1–6 Amongst these porous carbon and graphene have high surface area, chemical inertness, and synthetically tunable electrical, thermal and optical properties. These versa- tile properties of specic carbon based materials make them efficient to be used in supercapacitors and batteries. 7–9 Supercapacitors have been attracting signicant research interest lately due to their wide range of applications in a Centre of Excellence in Solar Energy, National Chemical Laboratory (CSIR-NCL), Dr Homi Bhabha Road, Pune, India. E-mail: sb.ogale@ncl.res.in; m.biswal@ncl.res.in; abh.banerjee@ncl.res.in; Fax: +91 20 2590 2636; Tel: +91 20 2590 2260 b Network Institute of Solar Energy (CSIR-NISE), New Delhi, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ee22325f Cite this: Energy Environ. Sci., 2013, 6, 1249 Received 24th May 2012 Accepted 31st January 2013 DOI: 10.1039/c3ee22325f www.rsc.org/ees This journal is ª The Royal Society of Chemistry 2013 Energy Environ. Sci., 2013, 6, 1249–1259 | 1249 Energy & Environmental Science PAPER Published on 01 February 2013. Downloaded by Ulsan National Institute of Science & Technology (UNIST) on 24/07/2014 03:37:42. View Article Online View Journal | View Issue