Hemp-Derived Activated Carbons for Electrochemical Energy Storage

Abstract

Electrochemical double layer capacitors have many potential applications, including portable electronics, load leveling, and engine start-stop systems.1 Major goals in the development of electrochemical capacitors include the fabrication of devices which feature high capacitance, high power performance, long cycle life, and are suitable for applications under a variety of operational conditions. Activated carbonaceous materials which have high surface area, suitable pore volume, and tailorable pore size are promising candidates as electrode materials for electrochemical capacitors. Cost is one important issue affecting the application of high-performance activated carbonaceous materials to electrochemical energy storage applications. Hydrothermal processing, followed by either chemical or physical activation is an efficient and economical strategy to prepare activated carbonaceous materials with attractive properties. Hemp, which is an easily grown crop in certain climates, including Kentucky, is a useful industrial raw material for preparing activated carbons. Hemp bast fiber has already been utilized as a precursor for the hydrothermal process to prepare interconnected carbon nanosheets which show excellent high rate performance in electrochemical capacitors.2 Only the outside part of the hemp stem (or bast) which makes up a minor percentage of the entire hemp stem was systematically investigated.2A major portion of the hemp stem (or hurd) was not used. Moreover, a variety of synthesis parameters in the whole approach (both hydrothermal processing and activation) can influence the electrochemical performance properties of the activated carbons, especially when raw biomass is utilized as the precursor. In this study, the entire raw hemp stem is taken into full use; both activated bast (the outside fibrous part of the stem) and activated hurd (inside pulp of the stem) were prepared and demonstrated as high-performance activated carbons for electrochemical capacitors utilizing both hydrothermal and non-hydrothermal methods. Various processing factors were found to play important roles in the morphology, surface properties, and electrochemical performance of the activated carbons, i.e. hydrothermal solution, pre-hydrothermal treatment, ratio of KOH/carbon, and activation temperatures. Using a systematic study of the preparation conditions, we are able to control and tune the properties of the activated carbons and achieve excellent performance when used as electrode materials in electrochemical capacitors. This simple processing route presents great potential for preparing energy storage materials from raw biomass. In this work, activated carbonaceous materials were prepared through both hydrothermal and non-hydrothermal processing conditions, followed by either physical or chemical activation processes.3,4 Activated carbon materials having high surface area and high mesoporosity, perform well as active electrode materials in electrochemical capacitors (Figures 1a and 1b). The activated hurd achieved excellent electrochemical performance metrics, including a specific capacitance of 160 F/g, and a high energy density of 19.8 Wh/kg at a power density of 21 kW/kg. In addition, one activated hurd sample achieved > 80% specific capacitance at a discharge rate of 80 A/g, relative to the baseline achieved at 1 A/g, demonstrating the high power performance achievable with activated carbons derived from hemp.4 References 1) Winter, M.; Brodd, R. J. Chemical Reviews. 2004, 104, 4245. 2) Wang, H., et al. ACS Nano. 2013, 7 , 5131. 3) Sun, W.; Lipka, S. M.; Yang, F. “Activated Carbon Derived from hemp and Its Use in Electrochemical Capacitors.” Presented at the 228thMeeting of the Electrochemical Society, Phoenix, AZ, October 2015 (ECS abstract MA2015-02, 587). 4) Sun, W.; Lipka, S. M.; Swartz, C. R.; Williams, D.; Cheng, Y. T.; Yang, F. Hemp-derived activated carbons for supercapacitors. Carbon. 2016, 103, 181. Figure 1