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Design of highly capacitive and durable supercapacitors using activated carbons with different pore structures: Petroleum coke and oil palm
Journal of Industrial and Engineering Chemistry 80 (2019) 301–310
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Design of highly capacitive and durable supercapacitors using activated carbons with different pore structures: Petroleum coke and oil palm Habin Parka , Jaewon Chunga , Byong-il Limb , Cheolsoo Junga,* a b
Department of Chemical Engineering, University of Seoul, 163 Shiripdae-ro, Dongdaemun-gu, Seoul, 02504, Republic of Korea Korchip B/D, 359 Manan-ro, Anyang-si, 13966, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2019 Received in revised form 29 July 2019 Accepted 2 August 2019 Available online 9 August 2019
Four types of supercapacitors (SCs), assembled by activated carbons (ACs) with different pore structures derived from petroleum coke (CK) and oil palm (OP), are analyzed to investigate the mechanism for improvement of cycle durability and capacitance of 3.0 V SCs using 1.0 M spiro-(1,10)-bipyrrolidinium tetrafluoroborate (SBPBF4) in acetonitrile. Although there have been many studies about activated carbons, few researches have investigated the relationship between the pore structure and the electrochemical properties using the commercially available activated carbon electrodes. The outstanding performances are determined from a combination of negative OP and positive CK AC electrodes. From analyses of surface area and porosity of ACs, potential and capacitance distributions of both electrodes, and impedance components, the effective idea for designing the superior SC is identified as higher mesopore portion of negative electrode and larger specific surface area of positive electrode than its counter electrode. SCs composed of negative OP AC electrode with higher mesopore portion show stable potential and capacitance of positive and negative electrodes during cycling. These results are derived from alleviation of crack or delamination of active materials and suppressed increments of charge transfer or diffusion resistance, unlike in SCs with negative CK AC electrode having higher micropore portion. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Activated carbon Petroleum coke Oil palm Capacitance Pore structure
Introduction Recently, the importance of research into energy storages has been emphasized due to the development of various electronic mobile devices. Supercapacitors (SCs) are representative electrochemical energy storage cells with a superior power density and cycle life but have lower energy densities than lithium ion batteries (LIBs) [1–3]. SCs are divided into two categories: pseudocapacitors (PsCs) using the faradaic reactions [4] and electric double layer capacitors (EDLCs) which store energy only by the non-faradaic electrical physisorption of electrolyte ions [5]. In the case of PsCs, capacitance may be large by adding faradaic reactions at properly maintained potential, but their cycle durability may not be good due to their relatively poor electrochemical stabilities [6,7]. Since EDLCs operate at potentials that induce only electrical physisorption without faradaic reactions, the lifetime characteristics are excellent, although their capacitance contributions due to nonfaradaic reactions are small.
* Corresponding author. E-mail address: [email protected] (C. Jung).
Various materials have been studied as electrode materials in PsCs, such as transition metals [8] and conducting polymers [9], but EDLCs have been researched using various types of carbon materials like graphene, carbon nanotube (CNT) [10], onion-like carbon (OLC) [11], and activated carbon (AC) or modified AC [12–14]. Among the materials, ACs have a merit of moderate cost and large specific surface area (SSA), which is advantageous for achieving high capacitance [1,15]. There are also many types of precursor of ACs such as cellulose [16], rice husk [17], oil palm [18], anthracite [19], and petroleum coke [20]. The coke-based carbon materials have been commonly used, and the activation involves physical activation using steam or CO2 and chemical activation under acidic or basic condition [21–23]. Generally, ACs with high SSA and pore volumes can be obtained by chemical activation. Many reports on chemical activation process have mainly focused on the discussion of acidic or alkaline functional groups on AC surfaces, which can affect the performances of SCs [22]. In this study, the electrochemical performances of four types of SCs, composed of two AC electrodes having different pore structures, petroleum coke (CK) and oil palm precursor (OP) electrodes, were compared and analyzed to investigate the mechanism for better cycle durability and higher capacitance of
https://doi.org/10.1016/j.jiec.2019.08.008 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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SCs at 3.0 V. Several studies on SCs have used various types of AC electrodes [24–26]. However, although there are already commercial supercapacitors using the CK and OP AC electrodes, there are few studies on the mechanism for better cycle durability and higher capacitance of SCs at 3.0 V using the above CK and OP AC electrodes as positive and negative electrodes separately. Here, we tried to find out the mechanism to improve the electrochemical performances of SCs by exploring the relationships between pore structures of CK and OP ACs, which are the only variables in this experiment, and electrochemical properties. The electrochemical properties such as potential and capacitance distributions of positive and negative electrodes were compared and analyzed with their surface states and impedance components of their SCs. The highly capacitive and durable SC could be obtained by designing with negative OP AC electrode containing higher mesopore portion and positive CK AC electrode containing larger SSA. Experimental section Materials preparation and characterization Two types of AC electrodes, derived from CK and OP, were obtained from Korchip Co. (Anyang, Korea). These were massproduced electrodes coated with the same amounts and thickness of active materials (10% carbon black and 3% styrene butadiene rubber (SBR) for the both ACs) on an aluminum current collector. The electrolyte, 1.0 M spiro-(1,10)-bipyrrolidinium tetrafluoroborate (SBPBF4) in acetonitrile (ACN) was purchased from Enchem Co., Ltd. (Jecheon, Korea). Surface areas, pore volumes, and pore diameter distributions of the CK and OP ACs were determined from their N2 adsorptiondesorption data at 77 K obtained using 3 Flex (Micromeritics Instrument Corp., USA) after treating the ACs at 150 C for 24 h. Micropore surface areas and volumes were determined with t-plot method and pore size distributions were evaluated with micropore analysis (MP) method. Mesopore surface area, volume, and size distribution were determined using the Barrett-Joyner-Halenda (BJH) method. The resistances of both AC electrodes were obtained using a 3555 battery hitester (Hioki, Japan), and were measured between the longest two vertex points of electrodes. Microscopic images of pristine AC electrodes and degraded AC electrodes after electrochemical cycling were obtained using Mini-SEM (SEC Co., Suwon, Korea). Functional groups on the CK and OP AC electrodes were identified using PHI 5000 VersaProbe (ULVAC-PHI, Japan). Cell fabrication and electrochemical characterization Four types of SCs were fabricated as pouch type cells using CK and OP AC electrodes (0.0545 g of active material on 2:5 cm 2:5 cm), a cellulose separator, and 0.5 g of electrolyte, 1.0 M SBPBF4 in ACN. As indicated in Scheme 1, four types of SCs were fabricated and the compositions of their electrodes mean positive CK and negative CK electrodes for PCNC, positive OP and negative OP electrodes for PONO, positive OP and negative CK electrodes for PONC, and positive CK and negative OP electrodes for PCNO. All fabricated cells were charged to 3.0 V and discharged to 0 V at various current rates galvanostatically using WBCS 3000 (WonATech Co., Ltd., Korea) in chamber sustained at 20 C. Constant voltage tests were held using the CK and OP AC electrodes with each voltage for 8 h. After cycling tests, cell thicknesses were measured using Vernier calipers (CD-15CPX, Mitutoyo, Japan) to estimate extents of gaseous material evolution. To measure electrochemical impedance spectra, IM6ex (Zahner, German) was utilized in a fully charged state of 3.0 V with an amplitude
Scheme 1. Electrode combinations of AC supercapacitors tested in this research (CK, coke; OP, oil palm).
of 20 mV from 200 KHz to 30 mHz. Nyquist plots of the cells were fitted and analyzed using Zman software (WonATech, Korea). Electrochemical stability of the liquid electrolyte was tested on a glassy carbon (GC) electrode (MF-2012) purchased from BASi Co. (West Lafayette, USA) by cyclic voltammetry using a potentiostat device 920D (CH Instruments, Inc., USA) with 3-electrode beaker cell system (Working (W): GC, Counter (C): AC, Reference (R): AC) and the experiment was performed at a scan rate of 10 mV s1. Cyclic voltammograms of fabricated two electrodes pouch cells were obtained using ZIVE Sp1 (WonATech Co., Ltd., Korea) at various scan rates. Results and discussion Pore structure of ACs produced from CK and OP precursors The pore structure of AC electrode is pertinent to the electrochemical performances of SCs such as capacitances with current rates and cycle performance [27,28]. Surface area and pore volume distributions of ACs obtained from CK and OP precursors were determined using the Brunauer–Emmett–Teller (BET) method. As shown in Fig. 1a, the CK AC isotherm showed a largely microporous structure with a relative pressure of up to 1.0, whereas OP AC isotherm showed slight hysteresis from a relative pressure of about 0.5. These results suggest that OP AC had a higher mesoporous portion than CK AC. Pore size distributions of OP and CK ACs were calculated using MP method to investigate the micropore distribution and Barrett–Joyner–Halenda (BJH) method to investigate the mesopore distribution as shown in Fig. 1b and c, respectively. In the case of CK AC, micropores of from 0.3 to 0.5 nm were more abundant than in OP AC, but mesopores of from 3 to 5 nm were slightly more abundant in OP AC. In addition, as summarized in Table 1, both samples showed higher portions of micropore than mesopore on a surface area and volume basis, but the proportions of SSA and micropore volume in CK AC were higher than those in OP AC and mesopore volume ratio in OP AC was higher than that in CK AC. In this study, four types of SCs composed of CK and OP electrodes, PCNC, PONO, PONC, and PCNO, were fabricated and their electrochemical behaviors induced from the ionic physisorption onto the both porous AC electrodes during charging and discharging were analyzed with these BET data in an
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effort to find out the mechanism for the improvement of the electrochemical performances of SCs at 3.0 V. Capacitances and potential balances at various current rates As shown in Fig. 2a, the specific capacitance of PCNC was the largest at all current rates, followed by PONC, PCNO, and PONO. The specific capacitance of a SC is explained by the following definition 1
Fig. 1. (a) Nitrogen adsorption-desorption isotherms obtained from BET measurements. The inset shows the magnification of the marked area. Pore size distributions calculated using (b) t-plot MP method and (c) BJH method, respectively of CK AC and OP AC.
of the capacitance (C ¼ eA d , where C is the specific capacitance in F g1, e is the dielectric constant of the liquid electrolyte in F m1, A is the specific surface area (SSA) of two electrodes in m2 g1, and d is the distance between electrode material and the electrolyte ions in m) [29]. Since the electrolyte was fixed in this study (1.0 M SBPBF4 in ACN), the variation range of d is limited. Therefore, the capacitance difference was obviously caused by the difference between the surface areas of the ACs used for each electrode, especially of the negative electrode. That is, SCs with a negative CK AC electrode with larger total SSA such as PCNC and PONC showed higher capacitance than those with a negative OP AC electrode, PONO and PCNO. However, PCNC and PONC showed larger capacitance reductions as the current rate increased (Fig. 2b). As discussed on the pore structure of AC particles (Fig. 1 and Table 1), the CK AC had a higher portion of micropore volume, which could not be facilitated to induce the physisorption of larger cation, SBP+ ions, than its counter ions, BF4 ions easily, especially at high current rate. Correspondingly, the cells composed of OP AC electrodes with a higher portion of mesopore volume showed greater relative capacitance at high current rate than the cells with CK AC electrodes. Although AC electrodes are also known to exhibit a pseudocapacitance due to the presence of surface functional groups [15] especially at high voltage, all cells prepared in this study showed non-faradaic currents dominantly in cyclic voltammetry (CV) (Fig. S1, see SI). Therefore, it was apparent that the capacitance differences of the SCs were due to the porous structures of AC electrodes, especially the negative electrode where SBP+ ions having bulkier size than BF4 ions are adsorbed during charging. Fig. 2c shows the potential distributions of the positive and negative electrodes of all cells charged to 3.0 V. The potentials of negative electrodes were higher than those of positive electrodes in all cells except PONC and the potential difference increased at high-rate condition. Generally, the overpotential in SC depends on electrolyte system and the structure or amount of active materials coated on each electrode [25,26,30]. Since the high negative potential was confirmed even in the PCNC and PONO, it was clear that the potential difference of each electrode in SCs was related to not only the pore structures of ACs but also the electrochemical environment of electrolyte components absorbing on porous AC electrode, such as electronic spatial extent (ESE) [30] and solvation behavior [31] of ions. That is, as shown in Fig. 2c, the higher overpotential at negative electrode was assumed to be formed by the lower electron density induced from the larger ESE value of SBP+ ion and its stronger solvation force with polar solvent ACN
Table 1 Various calculated parameters of the ACs (Smicro, micropore surface area; Smeso, mesopore surface area, Stotal, total surface area = Smicro + Smeso; Vmicro, volume of micropore; Vmeso, volume of mesopore; Vtotal, volume of total pore = Vmicro + Vmeso). Samples
Smicro (m2 g1, t-plot)
Smeso (m2 g1, BJH)
Stotal (m2 g1)
Vmicro (cm3 g1, t-plot)
Vmeso (cm3 g1, BJH)
Vtotal (cm3 g1)
CK AC OP AC
2138 1713
348 306
2486 2019
0.97 0.76
0.21 0.22
1.18 0.98
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Fig. 2. (a) Specific capacitances, (b) relative capacitances of all cells at various current rates. (c) Potential, (d) capacitance distributions of positive and negative electrodes at different current rates (solid: positive potential; shaded: negative potential).
than BF4 ion or from even naked SBP+ ion at high potential [32]. As the current rate increased, the difference of total amount of ions adsorbed onto limited pore surface or of their mobilities induced by the size effect [11,30,33] could cause larger overpotential of negative electrode, which is assumed to be a driving force to make much higher potential gap between the negative and positive electrodes. In addition, PONO shows smaller increment of potential difference as the current rate increased than that in PCNC. This could be analyzed due to the smaller micropore portion in OP AC as in Fig. 2b. In PONC and PCNO, it is hard to deduce overpotential mechanism at the negative electrode based only on the electrochemical properties of ions because the difference of pore structure between CK and OP ACs was included as another variable of capacitances and electrode potentials (Fig. 2b and c). In the case of PCNO, SBP+ ions having greater solvation energy with ACN [31] than BF4 ion would be adsorbed at the charged negative OP AC electrode containing a larger mesopore portion but smaller SSA than CK AC electrode. In addition, the positive electrode of PCNO did not have so powerful electrochemical factors to increase the overpotential, because smaller BF4 ions with weaker solvating energy adsorb at CK AC with larger SSA in charging. The difference between these electrochemical environments of the positive CK and negative OP AC electrodes resulted in large potential difference in the PCNO system. On the other hand, the potential difference between two electrodes in PONC was much smaller than the others and showed as the reversed overpotential pattern between two electrodes with other cell systems, because the bulkier SBP+ ions were adsorbed at CK AC electrode with a larger SSA. Although CK AC had an electrochemical factor to increase the negative overpotential such as larger micropore ratio than OP AC, adsorption of SBP+ ion onto the CK AC electrode with larger SSA
could decrease effectively the overpotential of the negative electrode. In addition, the small potential difference between the positive and negative electrodes was sustained even as the current rate increased. As shown in Fig. 2d, the capacitance distributions of the both electrodes were calculated based on the mass balancing equation (Q ¼ mþ Cþ DVþ ¼ m C DV , where Q is the electric charge in A s, m is the mass of the active material of each electrode in g, C is the specific capacitance of each electrode in F g1, DV is the change of potential in each electrode in V) [25]. The capacitance behaviors show opposite patterns to the potential distributions (Fig. 2c), such as higher positive capacitance than negative capacitance due to lower positive potential in all cells except PONC. Lower positive capacitance of PONO than of PCNC and PCNO was derived from using OP AC with small SSA in positive electrodes. Although PONC had a same system in positive electrode as in PONO, smaller positive capacitance of PONC than of PONO was observed due to higher positive potential of PONC and larger negative capacitance than other SCs was observed due to smaller negative potential of PONC. Large decrements of negative capacitance as the current rate increased were observed in all cells, but PONO and PCNO showed suppressed decrement of negative capacitance than PCNC (approximately PCNC: 37%, PONO: 25%, PCNO: 27%). This electrochemical performance showed substantial relevance with analysis in Fig. 2b. PONO and PCNO, having larger relative capacitance at high current rate than PCNC and PONC, had negative OP AC electrode with small micropore portion and this factor could suppress the decrement of negative capacitance. Although PONC showed the smallest change in positive and negative potentials, the inferior relative capacitance (Fig. 2b) appeared due to large decrease in positive capacitance (approximately 23%) unlike other SCs with decrease in negative capacitance (approximately 18%).
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These electrochemical properties are important factors in the following analyses, such as cycle durability and impedance components. Cycle performance and capacitance recovery after cycling Fig. 3a shows the cycle durability of each cell at a rate of 3.2 A g1. The cells with CK AC as the negative electrode, PCNC and PONC, showed a drastic decrease in their cycling performances and PCNC showed more decrease than PONC. However, the cells with OP AC as the negative electrode, PONO and PCNO, showed almost no change. This classification of the two groups is also associated with their relative capacitances at 3.2 A g1 in Fig. 2b. That is, the SCs composed of OP AC as the negative electrode showed better relative capacitance at high rates and better capacitance stability than the SCs with CK AC as the negative electrode. More meaningful data was obtained from the capacitance recovery at 0.1 A g1 after cycling. As shown in Fig. 3b, the cells with CK AC as the negative electrode showed approximately up to 15% of unrecoverable performance reduction, but the cells with OP AC as the negative electrode showed approximately 98% of recovery ability. The cycle durability of PCNC could be improved using OP AC as the positive electrode like the system of PONC and this factor affected the recovery ability to enhance from 85 to 90%. These results suggest that although SCs with CK AC as the negative electrode can produce a larger initial capacitance, they showed drastic decreases in the cell durability as well as poor recovery performances unlike the SCs with the negative OP AC electrode.
Fig. 3. (a) Cycle stability tests at 3.2 A g1 during 5000 cycles and (b) capacitance recovery ratio at 0.1 A g1 after 5000 cycles.
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The galvanostatic charge and discharge curves were compared before and after 5000 cycles to investigate the degradation mechanism of the SCs, as shown in Fig. S2 (See SI). The PCNC and PONC showed a slight iR-drop at the initial cycle, but their curves after 5000 cycles showed only a change in slope in both charge-discharge processes without a serious increase in iR-drop. In both PONO and PCNO, however, the iR-drop was almost imperceptible after 5000 cycles and the capacitance was maintained compared to PCNC and PONC. In the galvanostatic cycle, the slope of the curve is proportional to the reciprocal of the capacitance, so the increase of slope implies a decrease of capacitance. Since there was no peculiarity on the slope and iRdrop during the galvanostatic cycles of all cells, it was predicted that there would be no serious changes in the electrolyte, regardless of the electrode-pair. Potential and capacitance balances of positive and negative electrodes and deterioration mechanism of the electrodes during cycling The mechanism for the better cycle performance of the cells with OP AC as the negative electrode was examined by monitoring the electrode potentials during the cycle tests using a threeelectrode system (Fig. 4a). Interestingly, the electrode potentials of the cells with CK AC as the negative electrode, PCNC and PONC, which showed poorer performances not only in the cycle test but also in capacitance recovery after 5000 cycles, were changed drastically during cycling, compared to the cells of PONO and PCNO. To investigate the mechanism on their decreased capacitances during cycling, the capacitance changes in positive and negative electrodes were also monitored as shown in Fig. 4b and c. There were relatively more changes in the negative capacitance in all cells and PCNC showed the largest decrease in its positive capacitance. PONC had the same negative electrode as in PCNC but the change in negative capacitance of PONC was suppressed due to its lower negative potential than of PCNC during cycling. In addition, the change in the positive capacitance of PONC was more stabilized than PCNC and the only difference between PONC and PCNC was the positive electrode. That means that the OP AC electrodes could endure even higher positive potential during cycling (Fig. 4a). SCs using negative OP AC electrode, PONO and PCNO, showed superb stability in the positive and negative capacitances during cycling. In conclusion, the OP AC electrode could easily stand higher positive and negative potentials than CK AC electrode (Fig. 4). In this study, two possible mechanisms on the change in the positive and negative electrodes potentials and capacitances of PCNC and PONC during cycling were investigated: electrochemical reaction at the charged AC electrodes and deformation of the active materials. The first possibility is the change in the electrode potential due to the electrochemical reaction at the AC electrode charged to 3.0 V. As shown in Fig. 5a on CV of the liquid electrolyte, the oxidation of the electrolyte increased suddenly from approximately 1.5 V, whereas the reduction current was relatively flat until 2.0 V. The oxidation of the electrolyte itself may occur more easily, but severe oxidation of the electrolyte in the cells was not expected except for PONC, as shown in Fig. 4a. Because the negative electrode was charged to approximately 2.0 V, the possibility for the reduction of the electrolyte was higher in these systems. In this paper, constant voltage tests of the electrolyte for each AC electrode were also performed to check the extent of the electrochemical reaction on the AC surface. As shown in Fig. 5b and c, the reduction current on the CK AC electrode at 2.0 V showed relatively unstable figure, unlike that on the OP AC electrode, which indicates the electrochemical instability of the CK AC electrode. According to several reports [34,35], solid deposition can occur due to the functional groups of AC with an ACN-based electrolyte and trace
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Fig. 4. (a) Potential distribution change of positive and negative electrodes and retention ratio of (b) positive and (c) negative capacitance during 5000 cycles.
amounts of water. In this study, there were slightly more surface oxides in CK AC than OP AC, as shown in Fig. S3 and Table S1 (See SI) on the deconvoluted C1s XPS spectra. Therefore, more intense electrochemical reactions may occur on CK AC and its product may hinder the adsorption of SBP+ ions. The amount of current in the saturation region was also larger in the CK AC, which can be interpreted as the electronic current of the cell [36,37] and be determined by the resistance of the cell, or the presence of a sustained consumption of electrons. Since the CK AC electrode has a relatively small resistance (Table S2, see SI), its saturated electronic current could be larger than that of the OP AC electrode according to the Ohm’s law. In addition, there could be more continuous electron consumption on the CK AC electrode, indicating an electrochemical instability, unlike the OP AC electrode. The oxidation current at +1.5 V was relatively stable compared to the reduction current at 2.0 V for CK AC electrode. The OP AC electrode appeared to be a few more stable than CK AC electrode at both potentials. Therefore, in the case of PCNC and PONC, where the CK AC was used as the negative electrode, more electrochemical reaction products interfering with the adsorption of SBP+ ions could be generated at the negative electrode. Owing to these mechanisms, the potential balance of both electrodes was shifted gradually toward the more negative direction during cycling due to the decrease in effective surface area. For PONC, the potential of the positive electrode was relatively higher than the other cells, so that oxidation of the electrolyte could not be ignored. On the other hand, because the electrode potential was changed toward the more negative direction during cycling, the deterioration of positive electrode occurred only for the first few cycles. In addition, although there are some researches that gas may be generated during the electrolytic reaction for the ACN-based quaternary ammonium electrolyte [38,39], the possibility on gas generation was not accepted as a major mechanism on the change of the electrode potential because the thickness variation of pouch cells was quite small (approximately 1–2%, Table S3, see SI) after cycling. The second reason for the change in the electrode potentials of PCNC and PONC during cycling was verified by surface images of positive and negative electrodes. The surface images of the pristine CK and OP ACs showed uniformly dispersed AC particles without any defects (Fig. 6a, b, and S5). After cycling under the severe conditions of 3.2 A g1 and 3.0 V, the positive electrode states in all cells were similar to the pristine ACs states (Fig. 6d, f, h, j, and S5), regardless of the decrement of their positive capacitances. On the other hand, lots of deformations such as delamination or crack on the negative electrode were observed only in the cells with negative CK AC electrode, PCNC and PONC (Fig. 6c, d, g, h, and S5), where the capacitance degradation and aggravation of potential imbalance during cycling were severe. As reported on the damage
Fig. 5. (a) Cyclic voltammetry of 1.0 M SBPBF4 in ACN on glassy carbon at scan rate of 10 mV s1 and electrochemical stability of CK and OP AC electrodes at (b) 2.0 V and (c) +1.5 V vs. AC.
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Fig. 6. SEM images before and after cycling. (a, b) pristine CK and OP AC electrodes. (c, d) PCNC, (e, f) PONO, (g, h) PONC, (i, j) PCNO negative and positive electrode after cycling.
of active materials of electrochemical cells [40], the electrode damage might be due to the repulsion of excessive electrons among active materials in the negative CK AC electrode, as well as the decrease in available pore surface area to induce the decrease
in negative capacitance. Damages in the activated carbon structure, especially in the CK AC could easily lead deformation of an active material particle and, severely, crack or delamination of the active materials in the electrode gradually. As the effective
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surface area of the active materials in the negative electrode decreased due to the damages during cycling, more negative potential would accumulate than positive potential. On the other hand, PONO and PCNO showed no deformation of the active materials in both positive and negative electrodes even at a higher negative potential, which means that the electrode composed of OP AC is more durable at a high negative potential than of CK AC. In the cells where the cycle performance degradation occurred, the reduction of the electrolyte could arise at the negative CK AC electrode due to its more oxide functional groups than at OP AC electrode. Therefore, a product formed by an electrochemical reaction, which could interrupt the adsorption of SBP+ ions, could accumulate on the CK AC surface. However, owing to the slight differences in the current shape and compositions of functional groups between CK and OP ACs, the crack or delamination of active material by the strong repulsion of accumulated negative charge could be the main reason for the aggravation of potential imbalance and severe reduction of negative capacitance during cycling, which degraded the cycle performances of PCNC and PONC. EIS before and after cycle tests To investigate further the mechanism of the superior cycle durability of the SCs with the negative OP AC electrode, the impedance components of the SCs were analyzed at 3.0 V before
Table 2 EIS parameters before and after cycling at 3.0 V charged state. Cells
PCNC PONO PONC PCNO
Before cycling
After cycling
Rb (Ohm)
Rct (Ohm)
W (Ohm s1/2)
Rb (Ohm)
Rct (Ohm s1/2)
W (Ohm s1/2)
0.098 0.083 0.101 0.080
0.038 – 0.044 0.021
0.581 0.403 0.556 0.455
0.112 0.092 0.115 0.089
0.131 0.031 0.109 0.056
0.658 0.417 0.606 0.471
and after 5000 cycles (Fig. 7). As shown in the inset in Fig. 7a, the impedance components were simulated using Zman software as the bulk electrolyte resistance (Rb), charge transfer resistance (Rct) [15,41], which depends on the pore structure and texture of the AC electrode interface, coupled with the constant phase element (CPE) at the double layer of the AC electrode, and a Warburg resistance (Zw) for ion diffusion in the pores of the AC electrode [10,42]. Table 2 lists the parameters, Rb, Rct, and Zw. As shown in Fig. 7a, the SCs containing CK AC as the negative electrode, PCNC and PONC, were found to have a larger Rct than PONO and PCNO, containing OP AC as the negative electrode (Table 2). These results are in agreement with the discussion on the desolvation energy in the micropores at charged state of SCs [43]. That is, the solvated SBP+ ions should be desolvated in the micropores at high negative potentials and the difference of their desolvation energies cannot be ignored when discussing the
Fig. 7. (a) Nyquist plots with equivalent circuit and fitted lines. (b) Bode plots with calculated time constants. (c, d) Nyquist plots before and after cycling with fitted lines (solid: before cycles; open: after cycles).
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electrochemical performance among the cells containing various forms of the micropore. This electrochemical behavior was appeared to be remarkable on the cells with the CK negative electrode containing a higher portion of micropore volume because the SBP+ ion is larger and has a stronger solvation force with ACN than BF4 ions. On the other hand, the difference in Rb among the cells is considerably related to the ion concentration in the bulk electrolyte because the combination of CK and OP AC electrodes was the only variable in this research and the quantity of the adsorbed ions, which indicates the charging capacitance, is dependent on the SSA of the AC electrode. That is, the ion concentration in the bulk electrolyte of PCNC and PONC with a larger initial capacitance would be reduced more than that of PONO and PCNO with a smaller capacitance (Fig. 2a). Actually, the ionic conductivity in the bulk electrolyte of PCNC and PONC would be much lower than that of PONO and PCNO at their fully charged state (Fig. S4, See SI). As shown in Fig. 1b, the CK AC has a larger pore volume than the OP AC, particularly in the micropore zone. This structural difference of AC particles could also make a difference in the diffusion resistance, Zw, in the pores of PCNC and PONO, and similarly of PONC and PCNO, which may affect the rate performance (Fig. 2b). As Wang et al. reported that the cell system with small pore diffusion resistance, namely, the Warburg impedance, showed superior rate capability [42]. The PONO and PCNO with relatively good rate performance had a small Zw in this study (Table 2). In addition, Fig. 7b shows the relaxation time constants, which are important factors to determine the response speed of capacitors in a RC parallel circuit. The frequency, where the phase angle appears at 45 , is called the characteristic frequency (f0), and its reciprocal is called the characteristic relaxation time constant (t0) [10,44]. A comparison of the time constants of the cells revealed that the response time of PONO was 1.23 s faster than PCNC and the time constants of PONC and PCNO were similar to PCNC and PONO, respectively. These results mean that the OP AC as the negative electrode in PONO and PCNO was composed of a relatively less complicated pore structure that facilitates ion diffusion easily. Actually, the Rct and Zw of SCs with negative OP AC electrode were smaller than those of SCs with negative CK AC electrode so that the response speed was relatively fast. In this study, the changes in Rct and Zw were discussed as the major impedance components for the decreased capacitance in cycling and were attributed to two factors: the solid deposition at the electrode surface by electrolyte decomposition and the deformation of pore structures by repulsive force among same charges in the micropores at a high negative potential. First, the surface of the AC particles could be deformed by solid deposition due to electrochemical reactions of the quaternary ammonium salt, such as SBPBF4 used in this study with ACN [34]. It is clear, however, that the solid deposition was a minor factor to increase the Rct in this condition because the electrochemical reaction was not too severe. (Fig. 5b and c). Second, as shown in the analysis of the change in potential and capacitance distributions between the positive and negative electrodes it was predicted that the potential of the negative electrode increased (Fig. 4a) so that the repulsive force between the electrons became severe in the narrow micropores and the solvated or naked SBP+ ions, having a relatively large ion size, penetrated continuously into the AC pore during cycling. Therefore, there was severe deterioration of negative capacitance (Fig. 4c) than positive capacitance (Fig. 4b) in PCNC and PONC, which decreased the total capacitance and recovery performance (Fig. 3) during cycling. In addition, the cells, composed of the negative CK AC electrode containing a larger micropore portion, could be collapsed easily compared to the cells
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with the smaller proportion of micropore. These factors could affect the deformation of the porous AC structure, which increased the Rct and Zw [40,41]. Although capacitance degradation and electrode deformation were not observed in PONO (Fig. 3, Fig. 6e, f, and S5), Rct of the PONO was formed to be a relatively small value after cycling (Fig. 7c and Table 2). Some resistance elements could be formed by the slight change in pore texture, but the factors were not effective to aggravate the cycle performance as with PCNC or PONC. In the case of PCNC, however, an approximately 245% increase in Rct and a 13% increase in Zw were observed after cycling, which was caused mainly by the cracks or delaminations of the active materials (Fig. 6c and S5). The increments of Rct and Zw in PONC, which showed large capacitance degradation, were larger than in PCNO. Although there was a slight change in Rct and Zw in PCNO owing to minor deformation of active materials, there was no aggravation of the potential imbalance and cycle durability in PCNO as in PONO. Comparing between PCNC and PONC, Rct and Zw of PCNC largely increased than those of PONC after cycling, indicating that the increase in impedance after cycling was greater when the negative potential was higher so that the active materials were severely deteriorated to decrease the negative capacitance more (Fig. 4a, c and Table 2). In the Nyquist plot, the imaginary axis (Z’’) component represents the capacitance and the ideal capacitor shows a capacitance close to 90 in the low frequency region. PCNC and PONC, which showed the most deterioration of the cycle performance, had less ideal capacitance in the low frequency region after cycling than PONO and PCNO. These results could also be interpreted as a result of the above factors deforming the active materials, such as crack or delamination that contributed to the increase in impedance components (Fig. 7c, and d). Conclusion In this study, four types of SCs composed of petroleum coke (CK) and oil palm (OP) AC electrodes, which were commercially available electrodes and the only variables, different pore structures, were studied to design the superior SC having enhanced capacitance and cycle durability at 3.0 V. The SCs with negative OP AC electrode, PONO and PCNO, showed stable cycle durability and superior recovery performance, unlike SCs with the negative CK AC electrode, PCNC and PONC. In addition, the electrode potentials and capacitance of the SCs containing the negative OP AC electrode were very stable during cycling unlike the SCs with the negative CK AC electrode. The Rct and Zw values of PCNC and PONC increased drastically after cycling, which were contrary to those of PONO and PCNO. The electrode surfaces cracked or delaminated remarkably after cycling only on the negative CK AC electrode. From the results of BET analysis and electrochemical decomposition, it was concluded that the OP AC electrode, containing a smaller micropore portion and showing better electrochemical stability, was suited for the negative electrode for a high voltage SC. The PCNO-like system, which is composed of ACs with a large SSA in the positive electrode and a small micropore portion in the negative electrode, will be a suitable candidate for a highly capacitive and voltage-durable SC. Acknowledgement This work was supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF 2017H1D8A1030582) and by the 2019 Research Fund of the University of Seoul.
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Design of highly capacitive and durable supercapacitors using activated carbons with different pore structures: Petroleum coke and oil palm Habin Parka , Jaewon Chunga , Byong-il Limb , Cheolsoo Junga,* a b
Department of Chemical Engineering, University of Seoul, 163 Shiripdae-ro, Dongdaemun-gu, Seoul, 02504, Republic of Korea Korchip B/D, 359 Manan-ro, Anyang-si, 13966, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 March 2019 Received in revised form 29 July 2019 Accepted 2 August 2019 Available online 9 August 2019
Four types of supercapacitors (SCs), assembled by activated carbons (ACs) with different pore structures derived from petroleum coke (CK) and oil palm (OP), are analyzed to investigate the mechanism for improvement of cycle durability and capacitance of 3.0 V SCs using 1.0 M spiro-(1,10)-bipyrrolidinium tetrafluoroborate (SBPBF4) in acetonitrile. Although there have been many studies about activated carbons, few researches have investigated the relationship between the pore structure and the electrochemical properties using the commercially available activated carbon electrodes. The outstanding performances are determined from a combination of negative OP and positive CK AC electrodes. From analyses of surface area and porosity of ACs, potential and capacitance distributions of both electrodes, and impedance components, the effective idea for designing the superior SC is identified as higher mesopore portion of negative electrode and larger specific surface area of positive electrode than its counter electrode. SCs composed of negative OP AC electrode with higher mesopore portion show stable potential and capacitance of positive and negative electrodes during cycling. These results are derived from alleviation of crack or delamination of active materials and suppressed increments of charge transfer or diffusion resistance, unlike in SCs with negative CK AC electrode having higher micropore portion. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Activated carbon Petroleum coke Oil palm Capacitance Pore structure
Introduction Recently, the importance of research into energy storages has been emphasized due to the development of various electronic mobile devices. Supercapacitors (SCs) are representative electrochemical energy storage cells with a superior power density and cycle life but have lower energy densities than lithium ion batteries (LIBs) [1–3]. SCs are divided into two categories: pseudocapacitors (PsCs) using the faradaic reactions [4] and electric double layer capacitors (EDLCs) which store energy only by the non-faradaic electrical physisorption of electrolyte ions [5]. In the case of PsCs, capacitance may be large by adding faradaic reactions at properly maintained potential, but their cycle durability may not be good due to their relatively poor electrochemical stabilities [6,7]. Since EDLCs operate at potentials that induce only electrical physisorption without faradaic reactions, the lifetime characteristics are excellent, although their capacitance contributions due to nonfaradaic reactions are small.
* Corresponding author. E-mail address: [email protected] (C. Jung).
Various materials have been studied as electrode materials in PsCs, such as transition metals [8] and conducting polymers [9], but EDLCs have been researched using various types of carbon materials like graphene, carbon nanotube (CNT) [10], onion-like carbon (OLC) [11], and activated carbon (AC) or modified AC [12–14]. Among the materials, ACs have a merit of moderate cost and large specific surface area (SSA), which is advantageous for achieving high capacitance [1,15]. There are also many types of precursor of ACs such as cellulose [16], rice husk [17], oil palm [18], anthracite [19], and petroleum coke [20]. The coke-based carbon materials have been commonly used, and the activation involves physical activation using steam or CO2 and chemical activation under acidic or basic condition [21–23]. Generally, ACs with high SSA and pore volumes can be obtained by chemical activation. Many reports on chemical activation process have mainly focused on the discussion of acidic or alkaline functional groups on AC surfaces, which can affect the performances of SCs [22]. In this study, the electrochemical performances of four types of SCs, composed of two AC electrodes having different pore structures, petroleum coke (CK) and oil palm precursor (OP) electrodes, were compared and analyzed to investigate the mechanism for better cycle durability and higher capacitance of
https://doi.org/10.1016/j.jiec.2019.08.008 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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SCs at 3.0 V. Several studies on SCs have used various types of AC electrodes [24–26]. However, although there are already commercial supercapacitors using the CK and OP AC electrodes, there are few studies on the mechanism for better cycle durability and higher capacitance of SCs at 3.0 V using the above CK and OP AC electrodes as positive and negative electrodes separately. Here, we tried to find out the mechanism to improve the electrochemical performances of SCs by exploring the relationships between pore structures of CK and OP ACs, which are the only variables in this experiment, and electrochemical properties. The electrochemical properties such as potential and capacitance distributions of positive and negative electrodes were compared and analyzed with their surface states and impedance components of their SCs. The highly capacitive and durable SC could be obtained by designing with negative OP AC electrode containing higher mesopore portion and positive CK AC electrode containing larger SSA. Experimental section Materials preparation and characterization Two types of AC electrodes, derived from CK and OP, were obtained from Korchip Co. (Anyang, Korea). These were massproduced electrodes coated with the same amounts and thickness of active materials (10% carbon black and 3% styrene butadiene rubber (SBR) for the both ACs) on an aluminum current collector. The electrolyte, 1.0 M spiro-(1,10)-bipyrrolidinium tetrafluoroborate (SBPBF4) in acetonitrile (ACN) was purchased from Enchem Co., Ltd. (Jecheon, Korea). Surface areas, pore volumes, and pore diameter distributions of the CK and OP ACs were determined from their N2 adsorptiondesorption data at 77 K obtained using 3 Flex (Micromeritics Instrument Corp., USA) after treating the ACs at 150 C for 24 h. Micropore surface areas and volumes were determined with t-plot method and pore size distributions were evaluated with micropore analysis (MP) method. Mesopore surface area, volume, and size distribution were determined using the Barrett-Joyner-Halenda (BJH) method. The resistances of both AC electrodes were obtained using a 3555 battery hitester (Hioki, Japan), and were measured between the longest two vertex points of electrodes. Microscopic images of pristine AC electrodes and degraded AC electrodes after electrochemical cycling were obtained using Mini-SEM (SEC Co., Suwon, Korea). Functional groups on the CK and OP AC electrodes were identified using PHI 5000 VersaProbe (ULVAC-PHI, Japan). Cell fabrication and electrochemical characterization Four types of SCs were fabricated as pouch type cells using CK and OP AC electrodes (0.0545 g of active material on 2:5 cm 2:5 cm), a cellulose separator, and 0.5 g of electrolyte, 1.0 M SBPBF4 in ACN. As indicated in Scheme 1, four types of SCs were fabricated and the compositions of their electrodes mean positive CK and negative CK electrodes for PCNC, positive OP and negative OP electrodes for PONO, positive OP and negative CK electrodes for PONC, and positive CK and negative OP electrodes for PCNO. All fabricated cells were charged to 3.0 V and discharged to 0 V at various current rates galvanostatically using WBCS 3000 (WonATech Co., Ltd., Korea) in chamber sustained at 20 C. Constant voltage tests were held using the CK and OP AC electrodes with each voltage for 8 h. After cycling tests, cell thicknesses were measured using Vernier calipers (CD-15CPX, Mitutoyo, Japan) to estimate extents of gaseous material evolution. To measure electrochemical impedance spectra, IM6ex (Zahner, German) was utilized in a fully charged state of 3.0 V with an amplitude
Scheme 1. Electrode combinations of AC supercapacitors tested in this research (CK, coke; OP, oil palm).
of 20 mV from 200 KHz to 30 mHz. Nyquist plots of the cells were fitted and analyzed using Zman software (WonATech, Korea). Electrochemical stability of the liquid electrolyte was tested on a glassy carbon (GC) electrode (MF-2012) purchased from BASi Co. (West Lafayette, USA) by cyclic voltammetry using a potentiostat device 920D (CH Instruments, Inc., USA) with 3-electrode beaker cell system (Working (W): GC, Counter (C): AC, Reference (R): AC) and the experiment was performed at a scan rate of 10 mV s1. Cyclic voltammograms of fabricated two electrodes pouch cells were obtained using ZIVE Sp1 (WonATech Co., Ltd., Korea) at various scan rates. Results and discussion Pore structure of ACs produced from CK and OP precursors The pore structure of AC electrode is pertinent to the electrochemical performances of SCs such as capacitances with current rates and cycle performance [27,28]. Surface area and pore volume distributions of ACs obtained from CK and OP precursors were determined using the Brunauer–Emmett–Teller (BET) method. As shown in Fig. 1a, the CK AC isotherm showed a largely microporous structure with a relative pressure of up to 1.0, whereas OP AC isotherm showed slight hysteresis from a relative pressure of about 0.5. These results suggest that OP AC had a higher mesoporous portion than CK AC. Pore size distributions of OP and CK ACs were calculated using MP method to investigate the micropore distribution and Barrett–Joyner–Halenda (BJH) method to investigate the mesopore distribution as shown in Fig. 1b and c, respectively. In the case of CK AC, micropores of from 0.3 to 0.5 nm were more abundant than in OP AC, but mesopores of from 3 to 5 nm were slightly more abundant in OP AC. In addition, as summarized in Table 1, both samples showed higher portions of micropore than mesopore on a surface area and volume basis, but the proportions of SSA and micropore volume in CK AC were higher than those in OP AC and mesopore volume ratio in OP AC was higher than that in CK AC. In this study, four types of SCs composed of CK and OP electrodes, PCNC, PONO, PONC, and PCNO, were fabricated and their electrochemical behaviors induced from the ionic physisorption onto the both porous AC electrodes during charging and discharging were analyzed with these BET data in an
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effort to find out the mechanism for the improvement of the electrochemical performances of SCs at 3.0 V. Capacitances and potential balances at various current rates As shown in Fig. 2a, the specific capacitance of PCNC was the largest at all current rates, followed by PONC, PCNO, and PONO. The specific capacitance of a SC is explained by the following definition 1
Fig. 1. (a) Nitrogen adsorption-desorption isotherms obtained from BET measurements. The inset shows the magnification of the marked area. Pore size distributions calculated using (b) t-plot MP method and (c) BJH method, respectively of CK AC and OP AC.
of the capacitance (C ¼ eA d , where C is the specific capacitance in F g1, e is the dielectric constant of the liquid electrolyte in F m1, A is the specific surface area (SSA) of two electrodes in m2 g1, and d is the distance between electrode material and the electrolyte ions in m) [29]. Since the electrolyte was fixed in this study (1.0 M SBPBF4 in ACN), the variation range of d is limited. Therefore, the capacitance difference was obviously caused by the difference between the surface areas of the ACs used for each electrode, especially of the negative electrode. That is, SCs with a negative CK AC electrode with larger total SSA such as PCNC and PONC showed higher capacitance than those with a negative OP AC electrode, PONO and PCNO. However, PCNC and PONC showed larger capacitance reductions as the current rate increased (Fig. 2b). As discussed on the pore structure of AC particles (Fig. 1 and Table 1), the CK AC had a higher portion of micropore volume, which could not be facilitated to induce the physisorption of larger cation, SBP+ ions, than its counter ions, BF4 ions easily, especially at high current rate. Correspondingly, the cells composed of OP AC electrodes with a higher portion of mesopore volume showed greater relative capacitance at high current rate than the cells with CK AC electrodes. Although AC electrodes are also known to exhibit a pseudocapacitance due to the presence of surface functional groups [15] especially at high voltage, all cells prepared in this study showed non-faradaic currents dominantly in cyclic voltammetry (CV) (Fig. S1, see SI). Therefore, it was apparent that the capacitance differences of the SCs were due to the porous structures of AC electrodes, especially the negative electrode where SBP+ ions having bulkier size than BF4 ions are adsorbed during charging. Fig. 2c shows the potential distributions of the positive and negative electrodes of all cells charged to 3.0 V. The potentials of negative electrodes were higher than those of positive electrodes in all cells except PONC and the potential difference increased at high-rate condition. Generally, the overpotential in SC depends on electrolyte system and the structure or amount of active materials coated on each electrode [25,26,30]. Since the high negative potential was confirmed even in the PCNC and PONO, it was clear that the potential difference of each electrode in SCs was related to not only the pore structures of ACs but also the electrochemical environment of electrolyte components absorbing on porous AC electrode, such as electronic spatial extent (ESE) [30] and solvation behavior [31] of ions. That is, as shown in Fig. 2c, the higher overpotential at negative electrode was assumed to be formed by the lower electron density induced from the larger ESE value of SBP+ ion and its stronger solvation force with polar solvent ACN
Table 1 Various calculated parameters of the ACs (Smicro, micropore surface area; Smeso, mesopore surface area, Stotal, total surface area = Smicro + Smeso; Vmicro, volume of micropore; Vmeso, volume of mesopore; Vtotal, volume of total pore = Vmicro + Vmeso). Samples
Smicro (m2 g1, t-plot)
Smeso (m2 g1, BJH)
Stotal (m2 g1)
Vmicro (cm3 g1, t-plot)
Vmeso (cm3 g1, BJH)
Vtotal (cm3 g1)
CK AC OP AC
2138 1713
348 306
2486 2019
0.97 0.76
0.21 0.22
1.18 0.98
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Fig. 2. (a) Specific capacitances, (b) relative capacitances of all cells at various current rates. (c) Potential, (d) capacitance distributions of positive and negative electrodes at different current rates (solid: positive potential; shaded: negative potential).
than BF4 ion or from even naked SBP+ ion at high potential [32]. As the current rate increased, the difference of total amount of ions adsorbed onto limited pore surface or of their mobilities induced by the size effect [11,30,33] could cause larger overpotential of negative electrode, which is assumed to be a driving force to make much higher potential gap between the negative and positive electrodes. In addition, PONO shows smaller increment of potential difference as the current rate increased than that in PCNC. This could be analyzed due to the smaller micropore portion in OP AC as in Fig. 2b. In PONC and PCNO, it is hard to deduce overpotential mechanism at the negative electrode based only on the electrochemical properties of ions because the difference of pore structure between CK and OP ACs was included as another variable of capacitances and electrode potentials (Fig. 2b and c). In the case of PCNO, SBP+ ions having greater solvation energy with ACN [31] than BF4 ion would be adsorbed at the charged negative OP AC electrode containing a larger mesopore portion but smaller SSA than CK AC electrode. In addition, the positive electrode of PCNO did not have so powerful electrochemical factors to increase the overpotential, because smaller BF4 ions with weaker solvating energy adsorb at CK AC with larger SSA in charging. The difference between these electrochemical environments of the positive CK and negative OP AC electrodes resulted in large potential difference in the PCNO system. On the other hand, the potential difference between two electrodes in PONC was much smaller than the others and showed as the reversed overpotential pattern between two electrodes with other cell systems, because the bulkier SBP+ ions were adsorbed at CK AC electrode with a larger SSA. Although CK AC had an electrochemical factor to increase the negative overpotential such as larger micropore ratio than OP AC, adsorption of SBP+ ion onto the CK AC electrode with larger SSA
could decrease effectively the overpotential of the negative electrode. In addition, the small potential difference between the positive and negative electrodes was sustained even as the current rate increased. As shown in Fig. 2d, the capacitance distributions of the both electrodes were calculated based on the mass balancing equation (Q ¼ mþ Cþ DVþ ¼ m C DV , where Q is the electric charge in A s, m is the mass of the active material of each electrode in g, C is the specific capacitance of each electrode in F g1, DV is the change of potential in each electrode in V) [25]. The capacitance behaviors show opposite patterns to the potential distributions (Fig. 2c), such as higher positive capacitance than negative capacitance due to lower positive potential in all cells except PONC. Lower positive capacitance of PONO than of PCNC and PCNO was derived from using OP AC with small SSA in positive electrodes. Although PONC had a same system in positive electrode as in PONO, smaller positive capacitance of PONC than of PONO was observed due to higher positive potential of PONC and larger negative capacitance than other SCs was observed due to smaller negative potential of PONC. Large decrements of negative capacitance as the current rate increased were observed in all cells, but PONO and PCNO showed suppressed decrement of negative capacitance than PCNC (approximately PCNC: 37%, PONO: 25%, PCNO: 27%). This electrochemical performance showed substantial relevance with analysis in Fig. 2b. PONO and PCNO, having larger relative capacitance at high current rate than PCNC and PONC, had negative OP AC electrode with small micropore portion and this factor could suppress the decrement of negative capacitance. Although PONC showed the smallest change in positive and negative potentials, the inferior relative capacitance (Fig. 2b) appeared due to large decrease in positive capacitance (approximately 23%) unlike other SCs with decrease in negative capacitance (approximately 18%).
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These electrochemical properties are important factors in the following analyses, such as cycle durability and impedance components. Cycle performance and capacitance recovery after cycling Fig. 3a shows the cycle durability of each cell at a rate of 3.2 A g1. The cells with CK AC as the negative electrode, PCNC and PONC, showed a drastic decrease in their cycling performances and PCNC showed more decrease than PONC. However, the cells with OP AC as the negative electrode, PONO and PCNO, showed almost no change. This classification of the two groups is also associated with their relative capacitances at 3.2 A g1 in Fig. 2b. That is, the SCs composed of OP AC as the negative electrode showed better relative capacitance at high rates and better capacitance stability than the SCs with CK AC as the negative electrode. More meaningful data was obtained from the capacitance recovery at 0.1 A g1 after cycling. As shown in Fig. 3b, the cells with CK AC as the negative electrode showed approximately up to 15% of unrecoverable performance reduction, but the cells with OP AC as the negative electrode showed approximately 98% of recovery ability. The cycle durability of PCNC could be improved using OP AC as the positive electrode like the system of PONC and this factor affected the recovery ability to enhance from 85 to 90%. These results suggest that although SCs with CK AC as the negative electrode can produce a larger initial capacitance, they showed drastic decreases in the cell durability as well as poor recovery performances unlike the SCs with the negative OP AC electrode.
Fig. 3. (a) Cycle stability tests at 3.2 A g1 during 5000 cycles and (b) capacitance recovery ratio at 0.1 A g1 after 5000 cycles.
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The galvanostatic charge and discharge curves were compared before and after 5000 cycles to investigate the degradation mechanism of the SCs, as shown in Fig. S2 (See SI). The PCNC and PONC showed a slight iR-drop at the initial cycle, but their curves after 5000 cycles showed only a change in slope in both charge-discharge processes without a serious increase in iR-drop. In both PONO and PCNO, however, the iR-drop was almost imperceptible after 5000 cycles and the capacitance was maintained compared to PCNC and PONC. In the galvanostatic cycle, the slope of the curve is proportional to the reciprocal of the capacitance, so the increase of slope implies a decrease of capacitance. Since there was no peculiarity on the slope and iRdrop during the galvanostatic cycles of all cells, it was predicted that there would be no serious changes in the electrolyte, regardless of the electrode-pair. Potential and capacitance balances of positive and negative electrodes and deterioration mechanism of the electrodes during cycling The mechanism for the better cycle performance of the cells with OP AC as the negative electrode was examined by monitoring the electrode potentials during the cycle tests using a threeelectrode system (Fig. 4a). Interestingly, the electrode potentials of the cells with CK AC as the negative electrode, PCNC and PONC, which showed poorer performances not only in the cycle test but also in capacitance recovery after 5000 cycles, were changed drastically during cycling, compared to the cells of PONO and PCNO. To investigate the mechanism on their decreased capacitances during cycling, the capacitance changes in positive and negative electrodes were also monitored as shown in Fig. 4b and c. There were relatively more changes in the negative capacitance in all cells and PCNC showed the largest decrease in its positive capacitance. PONC had the same negative electrode as in PCNC but the change in negative capacitance of PONC was suppressed due to its lower negative potential than of PCNC during cycling. In addition, the change in the positive capacitance of PONC was more stabilized than PCNC and the only difference between PONC and PCNC was the positive electrode. That means that the OP AC electrodes could endure even higher positive potential during cycling (Fig. 4a). SCs using negative OP AC electrode, PONO and PCNO, showed superb stability in the positive and negative capacitances during cycling. In conclusion, the OP AC electrode could easily stand higher positive and negative potentials than CK AC electrode (Fig. 4). In this study, two possible mechanisms on the change in the positive and negative electrodes potentials and capacitances of PCNC and PONC during cycling were investigated: electrochemical reaction at the charged AC electrodes and deformation of the active materials. The first possibility is the change in the electrode potential due to the electrochemical reaction at the AC electrode charged to 3.0 V. As shown in Fig. 5a on CV of the liquid electrolyte, the oxidation of the electrolyte increased suddenly from approximately 1.5 V, whereas the reduction current was relatively flat until 2.0 V. The oxidation of the electrolyte itself may occur more easily, but severe oxidation of the electrolyte in the cells was not expected except for PONC, as shown in Fig. 4a. Because the negative electrode was charged to approximately 2.0 V, the possibility for the reduction of the electrolyte was higher in these systems. In this paper, constant voltage tests of the electrolyte for each AC electrode were also performed to check the extent of the electrochemical reaction on the AC surface. As shown in Fig. 5b and c, the reduction current on the CK AC electrode at 2.0 V showed relatively unstable figure, unlike that on the OP AC electrode, which indicates the electrochemical instability of the CK AC electrode. According to several reports [34,35], solid deposition can occur due to the functional groups of AC with an ACN-based electrolyte and trace
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Fig. 4. (a) Potential distribution change of positive and negative electrodes and retention ratio of (b) positive and (c) negative capacitance during 5000 cycles.
amounts of water. In this study, there were slightly more surface oxides in CK AC than OP AC, as shown in Fig. S3 and Table S1 (See SI) on the deconvoluted C1s XPS spectra. Therefore, more intense electrochemical reactions may occur on CK AC and its product may hinder the adsorption of SBP+ ions. The amount of current in the saturation region was also larger in the CK AC, which can be interpreted as the electronic current of the cell [36,37] and be determined by the resistance of the cell, or the presence of a sustained consumption of electrons. Since the CK AC electrode has a relatively small resistance (Table S2, see SI), its saturated electronic current could be larger than that of the OP AC electrode according to the Ohm’s law. In addition, there could be more continuous electron consumption on the CK AC electrode, indicating an electrochemical instability, unlike the OP AC electrode. The oxidation current at +1.5 V was relatively stable compared to the reduction current at 2.0 V for CK AC electrode. The OP AC electrode appeared to be a few more stable than CK AC electrode at both potentials. Therefore, in the case of PCNC and PONC, where the CK AC was used as the negative electrode, more electrochemical reaction products interfering with the adsorption of SBP+ ions could be generated at the negative electrode. Owing to these mechanisms, the potential balance of both electrodes was shifted gradually toward the more negative direction during cycling due to the decrease in effective surface area. For PONC, the potential of the positive electrode was relatively higher than the other cells, so that oxidation of the electrolyte could not be ignored. On the other hand, because the electrode potential was changed toward the more negative direction during cycling, the deterioration of positive electrode occurred only for the first few cycles. In addition, although there are some researches that gas may be generated during the electrolytic reaction for the ACN-based quaternary ammonium electrolyte [38,39], the possibility on gas generation was not accepted as a major mechanism on the change of the electrode potential because the thickness variation of pouch cells was quite small (approximately 1–2%, Table S3, see SI) after cycling. The second reason for the change in the electrode potentials of PCNC and PONC during cycling was verified by surface images of positive and negative electrodes. The surface images of the pristine CK and OP ACs showed uniformly dispersed AC particles without any defects (Fig. 6a, b, and S5). After cycling under the severe conditions of 3.2 A g1 and 3.0 V, the positive electrode states in all cells were similar to the pristine ACs states (Fig. 6d, f, h, j, and S5), regardless of the decrement of their positive capacitances. On the other hand, lots of deformations such as delamination or crack on the negative electrode were observed only in the cells with negative CK AC electrode, PCNC and PONC (Fig. 6c, d, g, h, and S5), where the capacitance degradation and aggravation of potential imbalance during cycling were severe. As reported on the damage
Fig. 5. (a) Cyclic voltammetry of 1.0 M SBPBF4 in ACN on glassy carbon at scan rate of 10 mV s1 and electrochemical stability of CK and OP AC electrodes at (b) 2.0 V and (c) +1.5 V vs. AC.
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Fig. 6. SEM images before and after cycling. (a, b) pristine CK and OP AC electrodes. (c, d) PCNC, (e, f) PONO, (g, h) PONC, (i, j) PCNO negative and positive electrode after cycling.
of active materials of electrochemical cells [40], the electrode damage might be due to the repulsion of excessive electrons among active materials in the negative CK AC electrode, as well as the decrease in available pore surface area to induce the decrease
in negative capacitance. Damages in the activated carbon structure, especially in the CK AC could easily lead deformation of an active material particle and, severely, crack or delamination of the active materials in the electrode gradually. As the effective
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surface area of the active materials in the negative electrode decreased due to the damages during cycling, more negative potential would accumulate than positive potential. On the other hand, PONO and PCNO showed no deformation of the active materials in both positive and negative electrodes even at a higher negative potential, which means that the electrode composed of OP AC is more durable at a high negative potential than of CK AC. In the cells where the cycle performance degradation occurred, the reduction of the electrolyte could arise at the negative CK AC electrode due to its more oxide functional groups than at OP AC electrode. Therefore, a product formed by an electrochemical reaction, which could interrupt the adsorption of SBP+ ions, could accumulate on the CK AC surface. However, owing to the slight differences in the current shape and compositions of functional groups between CK and OP ACs, the crack or delamination of active material by the strong repulsion of accumulated negative charge could be the main reason for the aggravation of potential imbalance and severe reduction of negative capacitance during cycling, which degraded the cycle performances of PCNC and PONC. EIS before and after cycle tests To investigate further the mechanism of the superior cycle durability of the SCs with the negative OP AC electrode, the impedance components of the SCs were analyzed at 3.0 V before
Table 2 EIS parameters before and after cycling at 3.0 V charged state. Cells
PCNC PONO PONC PCNO
Before cycling
After cycling
Rb (Ohm)
Rct (Ohm)
W (Ohm s1/2)
Rb (Ohm)
Rct (Ohm s1/2)
W (Ohm s1/2)
0.098 0.083 0.101 0.080
0.038 – 0.044 0.021
0.581 0.403 0.556 0.455
0.112 0.092 0.115 0.089
0.131 0.031 0.109 0.056
0.658 0.417 0.606 0.471
and after 5000 cycles (Fig. 7). As shown in the inset in Fig. 7a, the impedance components were simulated using Zman software as the bulk electrolyte resistance (Rb), charge transfer resistance (Rct) [15,41], which depends on the pore structure and texture of the AC electrode interface, coupled with the constant phase element (CPE) at the double layer of the AC electrode, and a Warburg resistance (Zw) for ion diffusion in the pores of the AC electrode [10,42]. Table 2 lists the parameters, Rb, Rct, and Zw. As shown in Fig. 7a, the SCs containing CK AC as the negative electrode, PCNC and PONC, were found to have a larger Rct than PONO and PCNO, containing OP AC as the negative electrode (Table 2). These results are in agreement with the discussion on the desolvation energy in the micropores at charged state of SCs [43]. That is, the solvated SBP+ ions should be desolvated in the micropores at high negative potentials and the difference of their desolvation energies cannot be ignored when discussing the
Fig. 7. (a) Nyquist plots with equivalent circuit and fitted lines. (b) Bode plots with calculated time constants. (c, d) Nyquist plots before and after cycling with fitted lines (solid: before cycles; open: after cycles).
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electrochemical performance among the cells containing various forms of the micropore. This electrochemical behavior was appeared to be remarkable on the cells with the CK negative electrode containing a higher portion of micropore volume because the SBP+ ion is larger and has a stronger solvation force with ACN than BF4 ions. On the other hand, the difference in Rb among the cells is considerably related to the ion concentration in the bulk electrolyte because the combination of CK and OP AC electrodes was the only variable in this research and the quantity of the adsorbed ions, which indicates the charging capacitance, is dependent on the SSA of the AC electrode. That is, the ion concentration in the bulk electrolyte of PCNC and PONC with a larger initial capacitance would be reduced more than that of PONO and PCNO with a smaller capacitance (Fig. 2a). Actually, the ionic conductivity in the bulk electrolyte of PCNC and PONC would be much lower than that of PONO and PCNO at their fully charged state (Fig. S4, See SI). As shown in Fig. 1b, the CK AC has a larger pore volume than the OP AC, particularly in the micropore zone. This structural difference of AC particles could also make a difference in the diffusion resistance, Zw, in the pores of PCNC and PONO, and similarly of PONC and PCNO, which may affect the rate performance (Fig. 2b). As Wang et al. reported that the cell system with small pore diffusion resistance, namely, the Warburg impedance, showed superior rate capability [42]. The PONO and PCNO with relatively good rate performance had a small Zw in this study (Table 2). In addition, Fig. 7b shows the relaxation time constants, which are important factors to determine the response speed of capacitors in a RC parallel circuit. The frequency, where the phase angle appears at 45 , is called the characteristic frequency (f0), and its reciprocal is called the characteristic relaxation time constant (t0) [10,44]. A comparison of the time constants of the cells revealed that the response time of PONO was 1.23 s faster than PCNC and the time constants of PONC and PCNO were similar to PCNC and PONO, respectively. These results mean that the OP AC as the negative electrode in PONO and PCNO was composed of a relatively less complicated pore structure that facilitates ion diffusion easily. Actually, the Rct and Zw of SCs with negative OP AC electrode were smaller than those of SCs with negative CK AC electrode so that the response speed was relatively fast. In this study, the changes in Rct and Zw were discussed as the major impedance components for the decreased capacitance in cycling and were attributed to two factors: the solid deposition at the electrode surface by electrolyte decomposition and the deformation of pore structures by repulsive force among same charges in the micropores at a high negative potential. First, the surface of the AC particles could be deformed by solid deposition due to electrochemical reactions of the quaternary ammonium salt, such as SBPBF4 used in this study with ACN [34]. It is clear, however, that the solid deposition was a minor factor to increase the Rct in this condition because the electrochemical reaction was not too severe. (Fig. 5b and c). Second, as shown in the analysis of the change in potential and capacitance distributions between the positive and negative electrodes it was predicted that the potential of the negative electrode increased (Fig. 4a) so that the repulsive force between the electrons became severe in the narrow micropores and the solvated or naked SBP+ ions, having a relatively large ion size, penetrated continuously into the AC pore during cycling. Therefore, there was severe deterioration of negative capacitance (Fig. 4c) than positive capacitance (Fig. 4b) in PCNC and PONC, which decreased the total capacitance and recovery performance (Fig. 3) during cycling. In addition, the cells, composed of the negative CK AC electrode containing a larger micropore portion, could be collapsed easily compared to the cells
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with the smaller proportion of micropore. These factors could affect the deformation of the porous AC structure, which increased the Rct and Zw [40,41]. Although capacitance degradation and electrode deformation were not observed in PONO (Fig. 3, Fig. 6e, f, and S5), Rct of the PONO was formed to be a relatively small value after cycling (Fig. 7c and Table 2). Some resistance elements could be formed by the slight change in pore texture, but the factors were not effective to aggravate the cycle performance as with PCNC or PONC. In the case of PCNC, however, an approximately 245% increase in Rct and a 13% increase in Zw were observed after cycling, which was caused mainly by the cracks or delaminations of the active materials (Fig. 6c and S5). The increments of Rct and Zw in PONC, which showed large capacitance degradation, were larger than in PCNO. Although there was a slight change in Rct and Zw in PCNO owing to minor deformation of active materials, there was no aggravation of the potential imbalance and cycle durability in PCNO as in PONO. Comparing between PCNC and PONC, Rct and Zw of PCNC largely increased than those of PONC after cycling, indicating that the increase in impedance after cycling was greater when the negative potential was higher so that the active materials were severely deteriorated to decrease the negative capacitance more (Fig. 4a, c and Table 2). In the Nyquist plot, the imaginary axis (Z’’) component represents the capacitance and the ideal capacitor shows a capacitance close to 90 in the low frequency region. PCNC and PONC, which showed the most deterioration of the cycle performance, had less ideal capacitance in the low frequency region after cycling than PONO and PCNO. These results could also be interpreted as a result of the above factors deforming the active materials, such as crack or delamination that contributed to the increase in impedance components (Fig. 7c, and d). Conclusion In this study, four types of SCs composed of petroleum coke (CK) and oil palm (OP) AC electrodes, which were commercially available electrodes and the only variables, different pore structures, were studied to design the superior SC having enhanced capacitance and cycle durability at 3.0 V. The SCs with negative OP AC electrode, PONO and PCNO, showed stable cycle durability and superior recovery performance, unlike SCs with the negative CK AC electrode, PCNC and PONC. In addition, the electrode potentials and capacitance of the SCs containing the negative OP AC electrode were very stable during cycling unlike the SCs with the negative CK AC electrode. The Rct and Zw values of PCNC and PONC increased drastically after cycling, which were contrary to those of PONO and PCNO. The electrode surfaces cracked or delaminated remarkably after cycling only on the negative CK AC electrode. From the results of BET analysis and electrochemical decomposition, it was concluded that the OP AC electrode, containing a smaller micropore portion and showing better electrochemical stability, was suited for the negative electrode for a high voltage SC. The PCNO-like system, which is composed of ACs with a large SSA in the positive electrode and a small micropore portion in the negative electrode, will be a suitable candidate for a highly capacitive and voltage-durable SC. Acknowledgement This work was supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF 2017H1D8A1030582) and by the 2019 Research Fund of the University of Seoul.
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