There is a need for efficient energy storage with the capability of fast charging and discharging, and various supercapacitors satisfy many of these requirements [1]. A comprehensive selection of carbon-based materials including activated carbons (ACs), templated carbon, carbon nanofibers, nanotubes, and graphene have been utilized as an electrode material for EDLC applications [2]. The abundance and price, together with adequate capacitance performance in the general range from 40 to 100 F/g [3] [4], make ACs the choice of material for symmetrical supercapacitors. Hence, extensive research has been dedicated on new approaches for producing activated carbon with high porosity in a cost-effective way. A variety of cellulosic carbons from different sources including, but not limited, to sucrose, cellulose, corn grain, banana fiber, potato starch have been utilized as EDLC electrodes. The crucial aspects in using these precursors are the reduced cost and the lower ecological effect for using bio-wastes to produce the value-added products [5].

In the present work, ACs were synthesized by processing Eastern White Pine (Pinus strabus). The Pine particles screened with a 2 mm opening were used in the experiment. The moisture content of the wood samples was ~8%. The carbon samples were prepared in batches using a three-stage treatment which included drying, stabilization, and carbonization. For each batch, 20 g of wood particles were evenly spread in an alumina tray (8inch L x 4inch W x 1 inch H) within into 5-inch diameter tube furnace. The following heating schedule was used to produce the carbon samples: (1) heating the samples in air to 110°C with a heating ramp of 5°C/min and holding for 60 min; (2) increasing the temperature to 220°C with a heating ramp of 0.03°C/min; (3) cooling the furnace to room temperature (RT) and purging the tube with nitrogen; (4) heating the samples under a nitrogen flow of 100 mL/min to 1000°C at a heating ramp of 10°C/min and holding the temperature at 1000°C for 30 min; and (5) cooling the furnace to RT before turning off the nitrogen flow. When the carbon samples were activated, the samples were heated to 800°C with 10°C/min and kept for 60 min. A flow of CO₂at 100 mL/min was used through the process.

The microstructure of the samples was characterized with scanning electron microscopy (SEM). The chemical analysis of the functional groups and bonding structure of the carbon was characterized using x-ray photoelectron spectroscopy (XPS). The Brunauer-Emmett-Teller (BET) method, in addition to the nitrogen gas adsorption/desorption isotherms of the carbon materials, were used to quantify the porosity in the materials. Electrochemical tests were completed by a two-electrode assembly within a CR-2032 architecture [6]. The pine-derived materials were ball-milled and tape-casted onto a stainless steel to a thickness of ~800 µm with n-methyl-2-pyrrolidone as the binder. A 6 M aqueous solution of KOH was used as the electrolyte. A Cyclic Charge-Discharge method (CCD) was used to evaluate the performance by a using multi-channel Capacitor/Battery Analyzer (MTI Corp., USA). Each sample was tested for 5000 charge-discharge cycles from 0.1 V to 1 V using a 0.1 A current. Self-discharge tests were completed for each sample by charging to 1 V subsequently the current load removed and measuring the voltage decay for 150 min. The tests were repeated for 75 cycles. Figure 1 shows the constant current charge/discharge (CCD) profile for the supercapacitors with two different Pine samples. The constant current charge/discharge measurements for both materials showed repeatable cyclic behavior [2]. The CCD curves were almost linear and symmetrical. The specific capacitance (C_g) for the electrode system was ~70 and ~75 F/g over the 2500 cycles for Pine 1 and Pine 2, respectively. These results demonstrate that the activated Pine based supercapacitors preserved the desired electrochemical reversibility and charge/discharge capabilities.

References

[1] P. Simon and K. K. Gogotsi, Nat. Mater., vol. 7, 845-854, 2008.

[2] H. Wang, Z. Xu, A. Kohandehghan, Z. Li, K. Cui, X. Tan, T. J. Stephenson, C. King’ondu, C. M. Holt, B. C. Olsen, J. K. Tak, D. Harfield, A. O. Anyia and D. Mitlin, Nano, vol. 7, 5131-5141, 2013.

[3] E. Frackowiak and F. Beguin, Carbon, vol. 40, 1775, 2002.

[4] I. Tanahashi, J. Appl. Electrochem., vol. 35, 1067, 2005.

[5] L. Wei and G. Yushin, Nano Energy, vol. 1, 552-565, 2012.

[6] M. D. Stoller and R. S. Ruoff, Energy Environ. Sci., vol. 3, 1294-1301, 2010.