Activated carbons (ACs) were prepared from a lignocellulosic-based waste material by a chemical impregnation method using KOH, NaOH or CaCl 2 as the activating agent. These ACs were characterized by different techniques such as N 2 adsorption, FTIR, XRD and SEM. Electrostatic properties viz. pH and pH pzc of AC suspensions in aqueous media were measured. The concentration of surface oxygenated functional groups of the ACs was estimated following the Boehm titration method. Cyclic voltammetry was conducted in H 2SO 4 after fabricating two-electrode capacitor cells of the ACs. The correlation of AC surface chemistry and morphology with electrochemical performance (capacitance) of powdered electrodes is analyzed and discussed.
Electrochemical double layer capacitors (EDLCs) also known as supercapacitors are promising high power energy sources for many different applications where high power density, high cycle efficiency and long cycle life is needed [
ACs are generally prepared by either physical or chemical activation methods from a wide range of precursor materials [
The principal objective of the present work is to explore the feasibility of using lignocellulosic-based ACs as an electrode material for EDLC application. The ACs are prepared following a chemical impregnation method from the testas of Mesua ferrea oil seed, extensively grown in the North Eastern part of India and considered as waste material in biodiesel production [
The precursor for production of porous carbon was waste material of the testa of an oil seed, Mesua ferrea used in the production of biodiesel. Scheme 1 shows a
Scheme 1. Schematic representation of the preparation of activated carbon from Mesua ferrea testa.
photograph of the testa and the detailed scheme of the preparation of activated carbon. The precursor material was first washed with distilled water to remove water-soluble impurities and surface adhered particles and then dried at 60˚C to remove moisture and other volatile impurities. The dried material was cut into small pieces. Prior to activation, the precursor was carbonized at 450˚C for 30 min under a nitrogen flow rate of 100 cm3∙min−1. The rate of heating and cooling was 5˚C min−1. The charcoal was ground and sieved to particle size range of 20 - 30 BS (British Standard). This step was intended to produce a dense carbonized material prior to impregnation and activation. ACs were prepared by chemical activation using KOH, NaOH or CaCl2 (activating agent, CAK, CAN and CAC respectively) following an impregnation procedure. 4 g of charcoal was well mixed by stirring with a solution that contained 20 ml water and 16 g of the activating agent for 2 h at 70˚C. The resulting slurry was dried at 110˚C for 24 h in an oven under air atmosphere. Activation was carried out in a vertical furnace by heating (15˚C min−1) from 27˚C to the final activation temperature (950˚C for 30 min) under a flow of nitrogen (100 cm3∙min−1) before cooling at the same rate and environment.
The product was washed with hot deionized water several times. This was followed by refluxing with 25% HNO3 for 2 h at ~100˚C before filtering and washing with hot deionized water for a number of times until the pH of the filtrate is neutral (and free from chloride in case of CaCl2 activation). Finally, the samples were washed with methanol and dried at 110˚C for 48 h in an oven. The samples were preserved in a desiccator. The yield of ACs was in the range of 30% - 35%. The proximate and ultimate analysis of the precursor and the ACs are given in
Analyses | Precursor | CAK | CAN | CAC |
---|---|---|---|---|
Proximate analysis (wt. %) | ||||
Moisture | 17.8 | 2.4 | 3.3 | 4.3 |
Ash (db)* | 8.3 | 0.1 | 0.2 | 0.2 |
Volatile matter (daf)† | 30.5 | 3.4 | 4.8 | 7.6 |
Fixed carbon (daf)† | 43.4 | 94.1 | 91.7 | 87.9 |
Ultimate analysis (wt. %) | ||||
Carbon (daf)† | 54.3 | 87.2 | 85.0 | 79.4 |
Hydrogen (daf)† | 10.6 | 3.2 | 3.9 | 5.1 |
Nitrogen (daf)† | 7.8 | 0.6 | 0.5 | 1.2 |
Sulphur (db)* | 0.2 | - | - | - |
Oxygen (daf)†, by difference | 27.1 | 8.9 | 10.6 | 14.3 |
*dry basis; †dry and ash free.
Surface area of the ACs were estimated by physical adsorption of N2 gas measured at 77 K with a Sorptomatic 1900 (Carlo Erba) using the BET model. Prior to the experiments, the samples were outgassed at 250˚C under vacuum for 24 h. Total pore volumes were calculated using the Horvath-Kawazoe (HK) approach [
Fourier transformed infrared (FTIR) spectra were acquired using a Perkin Elmer FTIR 1600 Spectrophotometer. The samples were mixed with KBr and a mixture containing 0.1 wt. % materials was pressed into pellets. Spectra were obtained by averaging 100 scans in the 4000 - 400 cm−1 spectral range at a resolution of 4 cm−1. Surface morphology of the ACs was observed using a FEI Inspect F Scanning Electron Micrograph (SEM) operating at an accelerating voltage of 10 kV.
The ACs were characterized by the pH of their aqueous suspension measured under standardized conditions [
Boehm titration method was used to determine the oxygenated functional groups of the ACs [
To prepare the electrodes, a slurry of AC and polytetrafluoroethylene (PTFE) binder (20:1 weight ratio) was pressed on a stainless-steel-foil current collector of 2 cm2 and subsequently dried at 100˚C for 12 h under vacuum. A two-elec- trode capacitor cell was fabricated to examine the electrochemical performance of the AC electrodes in a capacitor. The cell was assembled with two facing AC electrodes, sandwiching a piece of filter paper as separator. Cyclic voltammetric measurements were conducted between −1.0 and 1.0 V at different sweep rates in 1 M H2SO4. The capacitance of the AC electrodes were calculated from integration of the voltammogram within the potential range applied.
The N2 adsorption isotherms at 77 K were used to assess the pore characteristics of the ACs and are presented in
Carbon | SBET (m2∙g−1) | Vtotal (HK) (cm3∙g−1) | Vmic (DR) (cm3∙g−1) | Vmes (BJH) (cm3∙g−1) | Micropore (%) |
---|---|---|---|---|---|
CAK | 1604 | 1.27 | 0.86 | 0.32 | 67.7 |
CAN | 1367 | 0.88 | 0.64 | 0.17 | 72.7 |
CAC | 974 | 0.62 | 0.47 | 0.11 | 75.8 |
The pore size distribution graphs of the ACs are presented in
SEM was used to determine the morphology of the lignocellulosic derived ACs. Clearly, the distribution and sizes of the pores is critical in determining the applicability of ACs [
cell-like structure (note that a sizeable fraction of pores belongs to the macro domain (<30%)) but a range of pores of different sizes and shapes could be observed. The reason for the development of cavities is not clear but the range of porosities in these materials from the sub nm range through to micron sized pores is probably highly advantageous in terms of material transport. The cell- like structures may have arisen from the evaporation of the activating agent during activation leaving the large void structures observed here. As well as this, the activation temperature is relatively high (950˚C) and caking and agglomera- tion could have occurred on the char structure resulting in the formation of char with an intact external surface. The images further reveal that the surface morphology of the three carbons is different. Pores are well developed and relatively higher in CAK and CAN carbons. It appears that the porous texture characteristics are governed by the nature of the activating agent used in the chemical activation process.
FTIR spectroscopy provides valuable information on the chemical structures of the ACs.
The FTIR spectra of the ACs show absorption bands due to aliphatic C-H in −CH2− (2924 cm−1 and 2857 cm−1) and −CH2− deformation (1450 cm−1). Bands due to the C-H stretching mode (3045 cm−1 and 3025 cm−1) and out-of-plane deformation mode (886 cm−1 and 800 cm−1) of aromatic structures are identified in the spectra. The out-of-plane deformation modes derive from variously substituted benzene rings. The intensity of these bands is low and suggests the elimination of a large fraction of C-H bonds during pyrolysis and the formation
of the carbon network. This observation is in concordance with the results of elemental analysis (
The acid-base properties of AC are important when this class of material is used as electrode material/adsorbents. The pH of carbon suspension in de-ionized water is highly acidic as is evident from the results of
Carbon | pH | pHpzc | Acidic surface functional groups (mmol/g) | ||||
---|---|---|---|---|---|---|---|
Carboxylic | Phenolic | Lactonic | Carbonylic | Total | |||
CAK | 5.3 | 7.5 | 1.43 | 0.16 | 0.07 | 0.25 | 1.91 |
CAN | 4.8 | 6.7 | 1.57 | 0.21 | 0.07 | 0.36 | 2.21 |
CAC | 5.1 | 6.9 | 1.38 | 0.23 | 0.06 | 0.28 | 1.95 |
the pH scale for CAN and CAC carbons. This is consistent with the variation in the level of acidic surface functional groups (
The electrochemical properties of the ACs as electrode materials for EDLCs/su- percapacitors were evaluated using cyclic voltammetry (CV) measurements. The CVs of the AC electrodes are shown in
The specific capacitance value (expressed in Farad (F)) of the electrode materials is calculated from the cyclic voltammograms and the results at different sweep rates are listed in
Carbon | Capacitance (F/g) at different voltage sweep rates | |||
---|---|---|---|---|
5 mV/s | 10 mV/s | 20 mV/s | 50 mV/s | |
CAK | 177.8 | 116.2 | 75.6 | 43.2 |
CAN | 183.8 | 113.3 | 55.3 | 25.0 |
CAC | 172.4 | 101.3 | 37.8 | 15.3 |
decrease with the increase of sweep rate for the three ACs. Further, it indicates that there is always less electrochemically active surface area of pores being utilized at higher voltage sweep rate. The CAK carbon retained the highest of their initial capacitance even at the highest sweep rate of 50 mV∙s−1 amongst all the ACs. The relationship of capacitance verses the reciprocal of the sweep rate is shown in
The ability for charge accumulation depends upon the specific surface area of AC. A direct relationship between capacitance and surface area exists and is given by:
where, ε is permittivity of the electrolyte, S is surface area of the electrode-elec- trolyte interface, and d is distance between the polarized AC surface and the maximum charge density of solvated ions [
The functional groups may import an additive enhancement of the specific capacitance of polarizable electrode. The surface oxygen-containing moieties such as carbonyl, hydroxyl, etc. are electrochemically active in both the capaci- tive and Faradic sense. These Faradically active surface groups can contribute to the measured charge and consequently to the capacitance values (i.e., a pseudocapacitance) [
are indicative of the decisive role of surface concentration of oxygenated functional groups in double layer formation and consequently capacitance.
The ACs prepared here are predominantly microporous with highly developed surface area and porous texture. Further, the carbons possess sufficient oxygenated and acidic surface moieties. EDLCs performances of the ACs have demonstrated the dependence of capacitance on porous texture and concentration of surface oxygen-containing functional groups. There is little variation in the specific capacitance of the ACs with CAN carbon showing the highest capacitance value of 183.8 F∙g−1.
The authors wish to acknowledge SFI AMBER Centre Grant 12/RC/2278 for providing financial support to carry out the research.
Borah, D., Bharali, D.K. and Morris, M.A. (2017) Lignocellulosic-Based Activated Carbon Prepared by a Chemical Impregnation Method as Electrode Materials for Double Layer Capacitor. Advances in Chemical Engineering and Science, 7, 175-190. https://doi.org/10.4236/aces.2017.72013