Bio-Polymer Derived Activated Carbons for Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (EDLCs), are energy storage devices featuring many advantages, including long cycle life, low internal resistance, and high power densities (i.e. fast charge/discharge rates). Due to these advantages EDLCs have drawn much attention recently, and have been applied to electric/hybrid vehicles, heavy-construction equipment, electronics, and grid utility storage. The most common electrode material for EDLCs is porous activated carbon with the advantages of low cost, abundant resources, and simple processing. In this present work, carbon materials for EDLCs were hydrothermally synthesized from low-cost bio-polymer precursors. Hydrothermal synthesis (HTS) uses simple equipment and runs under mild conditions; the process also uses environmentally friendly precursors and does not produce any hazardous waste. HTS provides a facile method for the development of inexpensive, high- performance carbon materials.

In this work, low cost precursors were obtained locally. The precursors were ground and mixed with DI water, and the solution was then sealed in a Teflon lined pressure vessel. The pressure vessel was heated to 200 °C for 24 hours. After the reactor cooled down, the as-synthesized carbons were harvested by filtration and dried in an oven at 120 °C overnight. The morphology of the as-synthesized carbons was characterized by scanning electron microscopy (SEM) as shown in Figure 1. The as-synthesized carbons went through a one-step activation,  which increased the surface area to over 1000 m²/g, with 70% micro porosity. Electrodes were made, and tested in 1.8M TEMABF₄in PC and 38% sulfuric acid. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests were run on these activated carbons to study the electrochemical performance. The activated carbons showed very nice capacitor characteristics such as rectangular CV curves and very fast charge-discharge rates in both organic and aqueous electrolytes, as shown in Fig 2. The specific capacitance of these materials in organic electrolyte was ~90-120 F/g and had a volumetric capacitance of ~40-60 F/cc, while in aqueous electrolyte the gravimetric capacitance ranged from 180-200 F/g and 100-120 F/cc.