Activated Carbon Derived from Hemp and Its Use in Electrochemical Capacitors

Electrochemical capacitor based on the charge storage in the electric double layer have various potential applications, i.e. portable electronics, loading leveling, electric vehicles, etc.¹ Major goals in the development of the electrochemical capacitors are to make devices with high capacitance, high charge-discharge rates and suitable for applications under a variety of operational conditions. Activated carbonaceous material which have high surface area, suitable pore volume and tailorable pore size are promising candidates as electrode materials for electrochemical capacitors. One important issue affecting the application of high-performance activated carbonaceous materials (e.g. activated graphene) is cost.  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 various climates is a useful industrial raw material for preparing activated carbons. Hemp bast fiber has already been used as a precursor for the hydrothermal process to prepare interconnected carbon nanosheets which show excellent high rate performance in electrochemical capacitors.²  In this referenced work, only the outside part of the hemp stem (or bast) which makes up a minor percentage of the entire hemp stem was systematically investigated.  A 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 properties electrochemical performance of the activated carbons, especially when the raw biomass is utilized as the precursor.

In this study, the entire raw hemp stem is taken into full use; both bast (the outside fibrous part of the stem) and hurd (inside pulp of the stem) were prepared into high-performance activated carbons for electrochemical capacitors.  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, activation temperatures, etc.  Using a systematic study of the preparation conditions, we are able to control 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 hydrothermal and chemical activation processes using raw hemp as the precursor.  Activated carbon materials having high surface area and high mesoporosity, perform well as active electrode materials in electrochemical capacitors (Figures 1a and 1b). In each case, the hemp (hurd) biochar was prepared by a hydrothermal process at 200 ^oC for 24h in 1 wgt % H₂SO₄.  The resulting biochar was then activated with KOH at 800 ^oC for 1h.

Figure 1a. Cyclic voltammograms of KOH activated carbons prepared from hemp (hurd). The electrolyte is 1.8M TEMABF₄/PC. Figure 1b. Charge/discharge curves of KOH activated carbons prepared from hemp (hurd) at different current densities. The electrolyte is 1.8M TEMABF₄/PC.

References

(1)        Winter, M. and Brodd, R. J.,  What are batteries, fuel cells, and supercapacitors? Chemical reviews, 2004. 104(10): p. 4245-4270.

(2)        Wang, H., et al., Interconnected carbon nanosheets derived from hemp for ultrafast supercapacitors with high energy. ACS nano, 2013. 7(6): p. 5131-5141.