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An asymmetric electric double-layer capacitor with a janus membrane and two different aqueous electrolytes
Journal of Power Sources 423 (2019) 68–71
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
An asymmetric electric double-layer capacitor with a janus membrane and two different aqueous electrolytes
T
Na Liang1, Yongsheng Ji1, Jing Xu, Danying Zuo, Dongzhi Chen, Hongwei Zhang∗ College of Materials Science and Engineering, Wuhan Textile University, WuHan, 430073, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
asymmetric EDLC with a new • An configuration is proposed. and alkaline aqueous solutions • Acidic are simultaneously used in such an EDLC.
EDLC has a cell voltage of 2.2 V • The and a specific energy of 20.06 Wh kg−1.
A R T I C LE I N FO
A B S T R A C T
Keywords: Asymmetric EDLC Janus membrane Aqueous electrolyte Specific energy Voltage
An asymmetric aqueous electric double-layer capacitor is presented, which consists of two activated carbon electrodes, a compact Janus membrane with sulfonated polystyrene and quaternized polystyrene, aqueous acidic and alkaline electrolyte solutions. The carbon-based aqueous supercapacitor can not only run at a high cell voltage of 2.2 V, but also deliver a specific energy of 20.06 Wh kg−1. Moreover, such a supercapacitor with specific energy of 40–50 Wh kg−1 can be predicted after further optimization.
1. Introduction With the rapid increment of energy demands in the last decades, a new energy system is necessary to meet various energy applications, such as portable electronics, electric vehicles and grid storage [1–6]. As green energy storage devices, supercapacitors and Li-ion batteries (LIBs) are two highly desirable electrochemical devices at present [2]. Compare with LIBs, supercapacitors have a high specific power (10 kW kg−1) and long cycle life but suffer from low specific energy (only 5–10 W h kg−1) [6]. As a result, there is a worldwide effort to resolve the specific energy issue of supercapacitors. According to the formula: E = 1/2CV2, there are two strategies to
boost the specific energy of supercapacitors [7]. One is to increase the capacitance of supercapacitors, including exploring new carbon-based materials with large surface area and high N doping level [8–10], adopting pseudocapacitive materials with high specific capacitance [11], and adding redox species in the electrolyte [12–14]. The other is to increase the voltage of supercapacitors, including using non-aqueous electrolytes to obtain a high operating voltage of above 3.0 V [15–17], assembling asymmetric supercapacitors with a capacitor-type electrode and a battery-type electrode [18–20], balancing the loading mass of two electrodes [21], and adjusting the cutoff potential for two electrodes [22]. However, these methods to increase the capacitance and/ or enlarge the cell voltage have their respective disadvantages, such as
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhang). 1 The two authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.03.053 Received 2 January 2019; Received in revised form 7 March 2019; Accepted 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 423 (2019) 68–71
N. Liang, et al.
flammability, low ionic mobility, time-consuming fabricating process or high cost. Consequently, a facile approach of boosting the specific energy and voltage of aqueous supercapacitors still remains a great challenge. Janus membrane is a membrane with opposing properties (e.g. hydrophilicity/hydrophobicity and positive/negative charges) on each side, which has a promising potential for various applications, such as nanofiltration, catalytic contactor, demulsification, emulsification, aeration, fog harvesting, osmosis energy harvesting, battery [23,24]. It is well-known that a supercapacitor has three key components: electrode, electrolyte and separator. All methods mentioned above are based on the modification of electrode and electrolyte. It is also possible to construct electrical double layer capacitors (EDLCs) by asymmetric electrolytes and separators [25–33]. But most cell voltage of these EDLCs used the Nafion 212 membrane to separate different electrolytes, which is expensive and inadequate to prevent different electrolytes from neutralization. Herein, we proposed a concept of asymmetric EDLCs to increase the specific energy and the cell voltage. The asymmetric EDLC is formed by two activated carbon electrodes, a compact Janus membrane (also denoted as bipolar membrane in such a case) as the separator, which consists of sulfonated polystyrene (SPS) on one side and quaternized polystyrene (QPS) on the other side, and two different aqueous electrolytes (Scheme 1). As a proof-of-concept configuration, two activated carbon electrodes are separated by the compact Janus membrane. The spaces on the SPS side and the QPS side are filled with 1 M H2SO4 and 2 M KOH aqueous solutions, respectively. It is obvious that the supercapacitor is still based on the electrical double layer mechanism. As shown in Scheme 1, when it is charged, K+ cations and SO42− anions are accumulated at the surfaces of activated carbons on the negative electrode and the positive electrode, respectively. At the same time, OH− and H+ transfer towards the Janus membrane and meet in the interface of SPS and QPS to generate H2O molecules. When it is discharged, the process is converse. The asymmetric EDLC can utilize the different stable potential
windows of positive and negative electrodes and their over-potentials, which results in a high operating voltage of up to 2.2 V. Consequently, its specific energy reaches 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH aqueous solution at least. The stable potential windows of an activated carbon electrode in 2 M KOH and 1 M H2SO4 solutions are −1.2 -0 V (vs. NHE) and 0–1.0 V (vs. NHE), respectively (Fig. S1). For the assembly of such an asymmetric EDLC, it is worth mentioning that the electrodes of the EDLC on the SPS side and the QPS side can be only served as positive electrode and negative electrode, respectively. Otherwise the EDLC does not work well and has a very high voltage drop (∼1 V) because the transference of K+ cations and SO42− anions through the Janus membrane is blocked by the Janus membrane (Fig. S2). The capacitive performances of such an asymmetric EDLC are displayed in Fig. 1. Comparison of the cyclic voltammetry (CV) curves of the asymmetric EDLC under different voltage windows at a scan rate of 50 mV s−1 demonstrates that the asymmetric EDLC can harness the potential windows of activated carbon electrodes in 2 M KOH and 1 M H2SO4 solutions to reach a high cell voltage of 2.4 V (Fig. 1a). The asymmetric EDLC exhibits a rectangle-like CV curves in a large cell voltage range from 0 to 2.2 V, which is twice as that of conventional carbon-based EDLCs using KOH or H2SO4 solutions (∼1 V). However, the current of the asymmetric EDLC significantly increases when the cell voltage exceeds 2.3 V, which is related with the decomposition of water [25]. Galvanostatic charge-discharge (GCD) curves of the asymmetric EDLC under different voltage windows at a specific current of 1 A g−1 are also collected to further evaluate the electrochemical performance (Fig. 1b). The discharge curves of the asymmetric EDLC demonstrate a nearly linear relation of discharge/charge voltage versus time, suggesting a capacitor-like behavior. The absence of obvious voltage drop means the low internal resistance of the asymmetric EDLC. Furthermore, all the specific capacitances of the asymmetric EDLC calculated from these GCD curves are close to 34 F g−1 (based on the total weight of the positive electrode and negative electrode active materials). Fig. 1c presents CV curves of the asymmetric EDLC at the scan rates ranging from 5 to 100 mV s−1 under 2.2 V. The CV curve at the scan rate of 5 mV s−1 is a rectangular shape without noticeable redox peaks, revealing an ideal capacitive behavior. In addition, the CV curve still remains a definite rectangular shape without obvious distortion even at the scan rate of 100 mV s−1, implying an excellent reversibility and good rate performance. It is consistent with the nearly symmetric and linear GCD curves. The specific capacitances of the asymmetric EDLC at various current densities under 2.2 V are plotted in Fig. 1d. It can be found that the specific capacitance gradually decreases with the increase of the current densities because electrolyte ions has not enough time to access the interior parts of activated carbons and only the outer active surfaces of activated carbons are used for charge storage at high current densities. The specific capacitances are 45.8, 36.9, 34.4, 31.4 and 29.4 F g−1 at the different current densities of 0.1, 0.5, 1, 2, and 3 A g−1, respectively. It illustrates the specific capacitance retains about 64% of the original capacitance when the specific current increases from 0.1 to 3 A g−1, exhibiting a high rate capability. Fig. 1e provides the Ragone plot of the asymmetric EDLC under 2.2 V. The asymmetric EDLC displays specific energy of 12.65–28.07 Wh kg−1 and specific power of 36.46–916.56 W kg−1. At the specific current of 1 A g−1, the specific energy of the asymmetric EDLC is 20.06 Wh kg−1, which is more than 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH solution (Fig. S3). The value is one of the highest specific energy of EDLCs based on carbon//carbon in literature (Table S1). One charged asymmetric EDLC can light a green light-emitting diode (LED) for 80 s (inset of Fig. 1e and Video S1) or drive a mini-fan for 39 s (Video S2). While one charged conventional EDLC with 1 M H2SO4 can only drive the mini-fan for 14 s (Video S3). The long-term cycle stability of the asymmetric EDLC is evaluated at the specific current of 1 A g−1 in the voltage window of
Scheme 1. Schematic of the asymmetric EDLC using Janus membrane and asymmetric electrolytes. 69
Journal of Power Sources 423 (2019) 68–71
N. Liang, et al.
Fig. 1. (a) CV curves of the asymmetric EDLC using a Janus membrane under different voltage windows at a scan rate of 50 mV s−1. (b) GCD curves of the EDLC under different voltage windows at a specific current of 1 A g−1. (c) CV curves of the asymmetric EDLC at different scan rates. (d) Variation of specific capacitance with current densities of the asymmetric EDLC (Inset: GCD curves of the EDLC at different current densities). (e) Ragone plot of the asymmetric EDLC and inset: a green LED lighted by one EDLC device. (f) The cycling performance and coulombic efficiency of the asymmetric EDLC at a specific current of 1 A g−1 (Inset: Nyquist plots of the asymmetric EDLC before and after cycling test). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the straight line nearly parallel to the virtual axis represents the ideal capacitive behavior and the good ion diffusion. In the high-frequency region, the equivalent series resistance increases from 3.85 to 4.89 Ω after cycling test. It may be also attributed to the degradation of quaternary-ammonium groups on the Janus membrane, which results in higher resistance. But the charge transfer resistance is almost unchanged (0.82–0.84 Ω) after cycling test, which indicates the retained high charge transfer rate between the electrolyte and active materials. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.053. It has been proved that the addition of redox-active mediators to electrolytes can boost the specific energy of supercapacitors [13,14]. As a consequence, p-phenylenediamine and hydroquinone are added in
0–2.2 V (Fig. 1f). Clearly, the specific capacitance reduces to 96% of the initial specific capacitance in the first 100 cycles, then the value still retains over 90% of the initial specific capacitance after 1500 cycles. Thereafter, the specific capacitance shows considerable reduction and only 62% of the initial specific capacitance after 2200 cycles. The Coulombic efficiency keeps in the range between 91 and 97% before 1200 cycles. Afterward, the value decreases gradually to 70.26% after 2200 cycles. The reason may be mainly attributed to the degradation of quaternary-ammonium groups on the Janus membrane due to the Hofmann degradation or SN2 substitution under strongly alkaline environments [34]. Nyquist plots of the asymmetric EDLC before and after cycling test are obtained from electrochemical impedance spectroscopy (EIS) and displayed in the inset of Fig. 1f. In the low-frequency region, 70
Journal of Power Sources 423 (2019) 68–71
N. Liang, et al.
2 M KOH and1 M H2SO4 of the asymmetric EDLC in order to further increase the specific energy. This revised EDLC delivers a specific energy of 48.4 Wh kg−1 at the specific current of 1 A g−1 in the voltage window of 0–2.2 V (Fig. S4a) and can drive the mini-fan for 112 s (Video S4). Unfortunately, its capacitive performance seriously deteriorates and specific capacitance retention is less than 80% after 700 cycles at the specific current of 1 A g−1 (Fig. S4b). Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.053. In summary, the presented concept of asymmetric EDLC simultaneously uses a compact Janus membrane and KOH and H2SO4 electrolytic solutions. This EDLC not only retains high ionic mobility of two electrolytic solutions, but also utilizes the different stable potential windows of positive and negative electrodes. It can be successfully operated at 2.2 V and delivers a specific energy of 20.06 Wh kg−1 at the specific current of 1 A g−1, which is over 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH aqueous solution. Therefore, this investigation provides a new avenue toward boosting the specific energy and voltage of aqueous supercapacitors based on active carbon electrodes.
[11] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597–1614. [12] S.-E. Chun, B. Evanko, X. Wang, D. Vonlanthen, X. Ji, G.D. Stucky, S.W. Boettcher, Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge, Nat. Commun. 6 (2015) 7818. [13] R. Silvia, B. Clara, G. Marcos, M. Rosa, S. Ricardo, Towards a further generation of high-energy carbon-based capacitors by using redox-active electrolytes, Angew. Chem. Int. Ed. 50 (2011) 1699–1701. [14] J. Wu, H. Yu, L. Fan, G. Luo, J. Lin, M. Huang, A simple and high-effective electrolyte mediated with p-phenylenediamine for supercapacitor, J. Mater. Chem. 22 (2012) 19025–19030. [15] E. Mourad, L. Coustan, P. Lannelongue, D. Zigah, A. Mehdi, A. Vioux, Stefan A. Freunberger, F. Favier, O. Fontaine, Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors, Nat. Mater. 16 (2016) 446. [16] R. Na, C.-W. Su, Y.-H. Su, Y.-C. Chen, Y.-M. Chen, G. Wang, H. Teng, Solvent-free synthesis of an ionic liquid integrated ether-abundant polymer as a solid electrolyte for flexible electric double-layer capacitors, J. Mater. Chem. 5 (2017) 19703–19713. [17] H.-C. Huang, Y.-C. Yen, J.-C. Chang, C.-W. Su, P.-Y. Chang, I.W. Sun, C.-T. Hsieh, Y.L. Lee, H. Teng, An ether bridge between cations to extend the applicability of ionic liquids in electric double layer capacitors, J. Mater. Chem. 4 (2016) 19160–19169. [18] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Advanced asymmetric supercapacitors based on Ni(OH)2/Graphene and porous graphene electrodes with high energy density, Adv. Funct. Mater. 22 (2012) 2632–2641. [19] K. Qingqing, G. Cao, Z. Xiao, Z. Minrui, Z. Yong-Wei, C. Yongqing, Z. Hua, W. John, Surface-charge-Mediated formation of H-TiO2@Ni(OH)2 heterostructures for highperformance supercapacitors, Adv. Mater. 29 (2017) 1604164. [20] N. Jabeen, A. Hussain, Q. Xia, S. Sun, J. Zhu, H. Xia, High-performance 2.6 V aqueous asymmetric supercapacitors based on in situ formed Na0.5MnO2 nanosheet assembled nanowall arrays, Adv. Mater. 29 (2017) 1700804. [21] J.H. Chae, G.Z. Chen, 1.9 V aqueous carbon–carbon supercapacitors with unequal electrode capacitances, Electrochim. Acta 86 (2012) 248–254. [22] M. Yu, D. Lin, H. Feng, Y. Zeng, Y. Tong, X. Lu, Boosting the energy density of carbon-based aqueous supercapacitors by optimizing the surface charge, Angew. Chem. Int. Ed. 56 (2017) 5454–5459. [23] Y. Hao-Cheng, H. Jingwei, C. Vicki, X. Zhi-Kang, Janus membranes: exploring duality for advanced separation, Angew. Chem. Int. Ed. 55 (2016) 13398–13407. [24] H.-C. Yang, Y. Xie, J. Hou, A.K. Cheetham, V. Chen, S.B. Darling, Janus membranes: creating asymmetry for energy efficiency, Adv. Mater. 30 (2018) 1801495. [25] K. Fic, M. Meller, E. Frackowiak, Interfacial redox phenomena for enhanced aqueous supercapacitors, J. Electrochem. Soc. 162 (2015) A5140–A5147. [26] K. Fic, M. Meller, J. Menzel, E. Frackowiak, Around the thermodynamic limitations of supercapacitors operating in aqueous electrolytes, Electrochim. Acta 206 (2016) 496–503. [27] X. Wang, R.S. Chandrabose, Z. Jian, Z. Xing, X. Ji, A 1.8 V aqueous supercapacitor with a bipolar assembly of ion-exchange membranes as the separator, J. Electrochem. Soc. 163 (2016) A1853–A1858. [28] P. Ratajczak, F. Béguin, A high-voltage electrochemical cell operating with two aqueous electrolytes of different pH values, ChemElectroChem 5 (2018) 2518–2521. [29] L.-Q. Fan, J. Zhong, C.-Y. Zhang, J.-H. Wu, Y.-L. Wei, Improving the energy density of quasi-solid-state supercapacitors by assembling two redox-active gel electrolytes, Int. J. Hydrogen Energy 41 (2016) 5725–5732. [30] P.F.R. Ortega, Z. González, C. Blanco, G.G. Silva, R.L. Lavall, R. Santamaría, Biliquid supercapacitors: a simple and new strategy to enhance energy density in asymmetric/hybrid devices, Electrochim. Acta 254 (2017) 384–392. [31] C. Li, W. Wu, P. Wang, W. Zhou, J. Wang, Y. Chen, L. Fu, Y. Zhu, Y. Wu, W. Huang, Fabricating an aqueous symmetric supercapacitor with a stable high working voltage of 2 V by using an alkaline–acidic electrolyte, Adv. Sci. 6 (2019) 1801665. [32] M. Tian, J. Wu, R. Li, Y. Chen, D. Long, Fabricating a high-energy-density supercapacitor with asymmetric aqueous redox additive electrolytes and free-standing activated-carbon-felt electrodes, Chem. Eng. J. 363 (2019) 183–191. [33] C. Li, W. Wu, S. Zhang, L. He, Y. Zhu, J. Wang, L. Fu, Y. Chen, Y. Wu, W. Huang, A high-voltage aqueous lithium ion capacitor with high energy density from an alkaline–neutral electrolyte, J. Mater. Chem. 7 (2019) 4110–4118. [34] G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Polymeric materials as anionexchange membranes for alkaline fuel cells, Prog. Polym. Sci. 36 (2011) 1521–1557.
Acknowledgments This work was supported by the Hubei Provincial Natural Science Foundation of China (2018CFB267). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.03.053. References [1] M. Sessolo, H.J. Bolink, Perovskite solar cells join the major league, Science 350 (2015) 917 917. [2] N.-S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.-K. Sun, K. Amine, et al., Challenges facing lithium batteries and electrical double-layer capacitors, Angew. Chem. Int. Ed. 51 (2012) 9994–10024. [3] S. Jing, J. Lu, G. Yu, S. Yin, L. Luo, Z. Zhang, Y. Ma, W. Chen, P.K. Shen, Carbonencapsulated WOx hybrids as efficient catalysts for hydrogen evolution, Adv. Mater. 30 (2018) 1705979. [4] L. Zhang, J. Lu, S. Yin, L. Luo, S. Jing, A. Brouzgou, J. Chen, P.K. Shen, P. Tsiakaras, One-pot synthesized boron-doped RhFe alloy with enhanced catalytic performance for hydrogen evolution reaction, Appl. Catal. B Environ. 230 (2018) 58–64. [5] S. Jing, L. Zhang, L. Luo, J. Lu, S. Yin, P.K. Shen, P. Tsiakaras, N-doped porous molybdenum carbide nanobelts as efficient catalysts for hydrogen evolution reaction, Appl. Catal. B Environ. 224 (2018) 533–540. [6] Y. Ma, H. Chang, M. Zhang, Y. Chen, Graphene-based materials for lithium-ion hybrid supercapacitors, Adv. Mater. 27 (2015) 5296–5308. [7] Z. Huaping, L. Long, V. Ranjith, L. Yong, Recent advances in designing and fabricating self-supported nanoelectrodes for supercapacitors, Adv. Sci. 4 (2017) 1700188. [8] T. Lin, I.-W. Chen, F. Liu, C. Yang, H. Bi, F. Xu, F. Huang, Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage, Science 350 (2015) 1508–1513. [9] Y. Mei, Z. Zhen, Recent breakthroughs in supercapacitors boosted by nitrogen-rich porous carbon materials, Adv. Sci. 4 (2017) 1600408. [10] Z. Chen, Y. Yuan, H. Zhou, X. Wang, Z. Gan, F. Wang, Y. Lu, 3D Nanocomposite architectures from carbon-nanotube-threaded nanocrystals for high-performance electrochemical energy storage, Adv. Mater. 26 (2014) 339–345.
71
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
An asymmetric electric double-layer capacitor with a janus membrane and two different aqueous electrolytes
T
Na Liang1, Yongsheng Ji1, Jing Xu, Danying Zuo, Dongzhi Chen, Hongwei Zhang∗ College of Materials Science and Engineering, Wuhan Textile University, WuHan, 430073, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
asymmetric EDLC with a new • An configuration is proposed. and alkaline aqueous solutions • Acidic are simultaneously used in such an EDLC.
EDLC has a cell voltage of 2.2 V • The and a specific energy of 20.06 Wh kg−1.
A R T I C LE I N FO
A B S T R A C T
Keywords: Asymmetric EDLC Janus membrane Aqueous electrolyte Specific energy Voltage
An asymmetric aqueous electric double-layer capacitor is presented, which consists of two activated carbon electrodes, a compact Janus membrane with sulfonated polystyrene and quaternized polystyrene, aqueous acidic and alkaline electrolyte solutions. The carbon-based aqueous supercapacitor can not only run at a high cell voltage of 2.2 V, but also deliver a specific energy of 20.06 Wh kg−1. Moreover, such a supercapacitor with specific energy of 40–50 Wh kg−1 can be predicted after further optimization.
1. Introduction With the rapid increment of energy demands in the last decades, a new energy system is necessary to meet various energy applications, such as portable electronics, electric vehicles and grid storage [1–6]. As green energy storage devices, supercapacitors and Li-ion batteries (LIBs) are two highly desirable electrochemical devices at present [2]. Compare with LIBs, supercapacitors have a high specific power (10 kW kg−1) and long cycle life but suffer from low specific energy (only 5–10 W h kg−1) [6]. As a result, there is a worldwide effort to resolve the specific energy issue of supercapacitors. According to the formula: E = 1/2CV2, there are two strategies to
boost the specific energy of supercapacitors [7]. One is to increase the capacitance of supercapacitors, including exploring new carbon-based materials with large surface area and high N doping level [8–10], adopting pseudocapacitive materials with high specific capacitance [11], and adding redox species in the electrolyte [12–14]. The other is to increase the voltage of supercapacitors, including using non-aqueous electrolytes to obtain a high operating voltage of above 3.0 V [15–17], assembling asymmetric supercapacitors with a capacitor-type electrode and a battery-type electrode [18–20], balancing the loading mass of two electrodes [21], and adjusting the cutoff potential for two electrodes [22]. However, these methods to increase the capacitance and/ or enlarge the cell voltage have their respective disadvantages, such as
∗
Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhang). 1 The two authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.03.053 Received 2 January 2019; Received in revised form 7 March 2019; Accepted 14 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 423 (2019) 68–71
N. Liang, et al.
flammability, low ionic mobility, time-consuming fabricating process or high cost. Consequently, a facile approach of boosting the specific energy and voltage of aqueous supercapacitors still remains a great challenge. Janus membrane is a membrane with opposing properties (e.g. hydrophilicity/hydrophobicity and positive/negative charges) on each side, which has a promising potential for various applications, such as nanofiltration, catalytic contactor, demulsification, emulsification, aeration, fog harvesting, osmosis energy harvesting, battery [23,24]. It is well-known that a supercapacitor has three key components: electrode, electrolyte and separator. All methods mentioned above are based on the modification of electrode and electrolyte. It is also possible to construct electrical double layer capacitors (EDLCs) by asymmetric electrolytes and separators [25–33]. But most cell voltage of these EDLCs used the Nafion 212 membrane to separate different electrolytes, which is expensive and inadequate to prevent different electrolytes from neutralization. Herein, we proposed a concept of asymmetric EDLCs to increase the specific energy and the cell voltage. The asymmetric EDLC is formed by two activated carbon electrodes, a compact Janus membrane (also denoted as bipolar membrane in such a case) as the separator, which consists of sulfonated polystyrene (SPS) on one side and quaternized polystyrene (QPS) on the other side, and two different aqueous electrolytes (Scheme 1). As a proof-of-concept configuration, two activated carbon electrodes are separated by the compact Janus membrane. The spaces on the SPS side and the QPS side are filled with 1 M H2SO4 and 2 M KOH aqueous solutions, respectively. It is obvious that the supercapacitor is still based on the electrical double layer mechanism. As shown in Scheme 1, when it is charged, K+ cations and SO42− anions are accumulated at the surfaces of activated carbons on the negative electrode and the positive electrode, respectively. At the same time, OH− and H+ transfer towards the Janus membrane and meet in the interface of SPS and QPS to generate H2O molecules. When it is discharged, the process is converse. The asymmetric EDLC can utilize the different stable potential
windows of positive and negative electrodes and their over-potentials, which results in a high operating voltage of up to 2.2 V. Consequently, its specific energy reaches 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH aqueous solution at least. The stable potential windows of an activated carbon electrode in 2 M KOH and 1 M H2SO4 solutions are −1.2 -0 V (vs. NHE) and 0–1.0 V (vs. NHE), respectively (Fig. S1). For the assembly of such an asymmetric EDLC, it is worth mentioning that the electrodes of the EDLC on the SPS side and the QPS side can be only served as positive electrode and negative electrode, respectively. Otherwise the EDLC does not work well and has a very high voltage drop (∼1 V) because the transference of K+ cations and SO42− anions through the Janus membrane is blocked by the Janus membrane (Fig. S2). The capacitive performances of such an asymmetric EDLC are displayed in Fig. 1. Comparison of the cyclic voltammetry (CV) curves of the asymmetric EDLC under different voltage windows at a scan rate of 50 mV s−1 demonstrates that the asymmetric EDLC can harness the potential windows of activated carbon electrodes in 2 M KOH and 1 M H2SO4 solutions to reach a high cell voltage of 2.4 V (Fig. 1a). The asymmetric EDLC exhibits a rectangle-like CV curves in a large cell voltage range from 0 to 2.2 V, which is twice as that of conventional carbon-based EDLCs using KOH or H2SO4 solutions (∼1 V). However, the current of the asymmetric EDLC significantly increases when the cell voltage exceeds 2.3 V, which is related with the decomposition of water [25]. Galvanostatic charge-discharge (GCD) curves of the asymmetric EDLC under different voltage windows at a specific current of 1 A g−1 are also collected to further evaluate the electrochemical performance (Fig. 1b). The discharge curves of the asymmetric EDLC demonstrate a nearly linear relation of discharge/charge voltage versus time, suggesting a capacitor-like behavior. The absence of obvious voltage drop means the low internal resistance of the asymmetric EDLC. Furthermore, all the specific capacitances of the asymmetric EDLC calculated from these GCD curves are close to 34 F g−1 (based on the total weight of the positive electrode and negative electrode active materials). Fig. 1c presents CV curves of the asymmetric EDLC at the scan rates ranging from 5 to 100 mV s−1 under 2.2 V. The CV curve at the scan rate of 5 mV s−1 is a rectangular shape without noticeable redox peaks, revealing an ideal capacitive behavior. In addition, the CV curve still remains a definite rectangular shape without obvious distortion even at the scan rate of 100 mV s−1, implying an excellent reversibility and good rate performance. It is consistent with the nearly symmetric and linear GCD curves. The specific capacitances of the asymmetric EDLC at various current densities under 2.2 V are plotted in Fig. 1d. It can be found that the specific capacitance gradually decreases with the increase of the current densities because electrolyte ions has not enough time to access the interior parts of activated carbons and only the outer active surfaces of activated carbons are used for charge storage at high current densities. The specific capacitances are 45.8, 36.9, 34.4, 31.4 and 29.4 F g−1 at the different current densities of 0.1, 0.5, 1, 2, and 3 A g−1, respectively. It illustrates the specific capacitance retains about 64% of the original capacitance when the specific current increases from 0.1 to 3 A g−1, exhibiting a high rate capability. Fig. 1e provides the Ragone plot of the asymmetric EDLC under 2.2 V. The asymmetric EDLC displays specific energy of 12.65–28.07 Wh kg−1 and specific power of 36.46–916.56 W kg−1. At the specific current of 1 A g−1, the specific energy of the asymmetric EDLC is 20.06 Wh kg−1, which is more than 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH solution (Fig. S3). The value is one of the highest specific energy of EDLCs based on carbon//carbon in literature (Table S1). One charged asymmetric EDLC can light a green light-emitting diode (LED) for 80 s (inset of Fig. 1e and Video S1) or drive a mini-fan for 39 s (Video S2). While one charged conventional EDLC with 1 M H2SO4 can only drive the mini-fan for 14 s (Video S3). The long-term cycle stability of the asymmetric EDLC is evaluated at the specific current of 1 A g−1 in the voltage window of
Scheme 1. Schematic of the asymmetric EDLC using Janus membrane and asymmetric electrolytes. 69
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Fig. 1. (a) CV curves of the asymmetric EDLC using a Janus membrane under different voltage windows at a scan rate of 50 mV s−1. (b) GCD curves of the EDLC under different voltage windows at a specific current of 1 A g−1. (c) CV curves of the asymmetric EDLC at different scan rates. (d) Variation of specific capacitance with current densities of the asymmetric EDLC (Inset: GCD curves of the EDLC at different current densities). (e) Ragone plot of the asymmetric EDLC and inset: a green LED lighted by one EDLC device. (f) The cycling performance and coulombic efficiency of the asymmetric EDLC at a specific current of 1 A g−1 (Inset: Nyquist plots of the asymmetric EDLC before and after cycling test). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the straight line nearly parallel to the virtual axis represents the ideal capacitive behavior and the good ion diffusion. In the high-frequency region, the equivalent series resistance increases from 3.85 to 4.89 Ω after cycling test. It may be also attributed to the degradation of quaternary-ammonium groups on the Janus membrane, which results in higher resistance. But the charge transfer resistance is almost unchanged (0.82–0.84 Ω) after cycling test, which indicates the retained high charge transfer rate between the electrolyte and active materials. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.053. It has been proved that the addition of redox-active mediators to electrolytes can boost the specific energy of supercapacitors [13,14]. As a consequence, p-phenylenediamine and hydroquinone are added in
0–2.2 V (Fig. 1f). Clearly, the specific capacitance reduces to 96% of the initial specific capacitance in the first 100 cycles, then the value still retains over 90% of the initial specific capacitance after 1500 cycles. Thereafter, the specific capacitance shows considerable reduction and only 62% of the initial specific capacitance after 2200 cycles. The Coulombic efficiency keeps in the range between 91 and 97% before 1200 cycles. Afterward, the value decreases gradually to 70.26% after 2200 cycles. The reason may be mainly attributed to the degradation of quaternary-ammonium groups on the Janus membrane due to the Hofmann degradation or SN2 substitution under strongly alkaline environments [34]. Nyquist plots of the asymmetric EDLC before and after cycling test are obtained from electrochemical impedance spectroscopy (EIS) and displayed in the inset of Fig. 1f. In the low-frequency region, 70
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2 M KOH and1 M H2SO4 of the asymmetric EDLC in order to further increase the specific energy. This revised EDLC delivers a specific energy of 48.4 Wh kg−1 at the specific current of 1 A g−1 in the voltage window of 0–2.2 V (Fig. S4a) and can drive the mini-fan for 112 s (Video S4). Unfortunately, its capacitive performance seriously deteriorates and specific capacitance retention is less than 80% after 700 cycles at the specific current of 1 A g−1 (Fig. S4b). Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.053. In summary, the presented concept of asymmetric EDLC simultaneously uses a compact Janus membrane and KOH and H2SO4 electrolytic solutions. This EDLC not only retains high ionic mobility of two electrolytic solutions, but also utilizes the different stable potential windows of positive and negative electrodes. It can be successfully operated at 2.2 V and delivers a specific energy of 20.06 Wh kg−1 at the specific current of 1 A g−1, which is over 5 times as high as that of a conventional EDLC with sole 1 M H2SO4 or 2 M KOH aqueous solution. Therefore, this investigation provides a new avenue toward boosting the specific energy and voltage of aqueous supercapacitors based on active carbon electrodes.
[11] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597–1614. [12] S.-E. Chun, B. Evanko, X. Wang, D. Vonlanthen, X. Ji, G.D. Stucky, S.W. Boettcher, Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge, Nat. Commun. 6 (2015) 7818. [13] R. Silvia, B. Clara, G. Marcos, M. Rosa, S. Ricardo, Towards a further generation of high-energy carbon-based capacitors by using redox-active electrolytes, Angew. Chem. Int. Ed. 50 (2011) 1699–1701. [14] J. Wu, H. Yu, L. Fan, G. Luo, J. Lin, M. Huang, A simple and high-effective electrolyte mediated with p-phenylenediamine for supercapacitor, J. Mater. Chem. 22 (2012) 19025–19030. [15] E. Mourad, L. Coustan, P. Lannelongue, D. Zigah, A. Mehdi, A. Vioux, Stefan A. Freunberger, F. Favier, O. Fontaine, Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors, Nat. Mater. 16 (2016) 446. [16] R. Na, C.-W. Su, Y.-H. Su, Y.-C. Chen, Y.-M. Chen, G. Wang, H. Teng, Solvent-free synthesis of an ionic liquid integrated ether-abundant polymer as a solid electrolyte for flexible electric double-layer capacitors, J. Mater. Chem. 5 (2017) 19703–19713. [17] H.-C. Huang, Y.-C. Yen, J.-C. Chang, C.-W. Su, P.-Y. Chang, I.W. Sun, C.-T. Hsieh, Y.L. Lee, H. Teng, An ether bridge between cations to extend the applicability of ionic liquids in electric double layer capacitors, J. Mater. Chem. 4 (2016) 19160–19169. [18] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Advanced asymmetric supercapacitors based on Ni(OH)2/Graphene and porous graphene electrodes with high energy density, Adv. Funct. Mater. 22 (2012) 2632–2641. [19] K. Qingqing, G. Cao, Z. Xiao, Z. Minrui, Z. Yong-Wei, C. Yongqing, Z. Hua, W. John, Surface-charge-Mediated formation of H-TiO2@Ni(OH)2 heterostructures for highperformance supercapacitors, Adv. Mater. 29 (2017) 1604164. [20] N. Jabeen, A. Hussain, Q. Xia, S. Sun, J. Zhu, H. Xia, High-performance 2.6 V aqueous asymmetric supercapacitors based on in situ formed Na0.5MnO2 nanosheet assembled nanowall arrays, Adv. Mater. 29 (2017) 1700804. [21] J.H. Chae, G.Z. Chen, 1.9 V aqueous carbon–carbon supercapacitors with unequal electrode capacitances, Electrochim. Acta 86 (2012) 248–254. [22] M. Yu, D. Lin, H. Feng, Y. Zeng, Y. Tong, X. Lu, Boosting the energy density of carbon-based aqueous supercapacitors by optimizing the surface charge, Angew. Chem. Int. Ed. 56 (2017) 5454–5459. [23] Y. Hao-Cheng, H. Jingwei, C. Vicki, X. Zhi-Kang, Janus membranes: exploring duality for advanced separation, Angew. Chem. Int. Ed. 55 (2016) 13398–13407. [24] H.-C. Yang, Y. Xie, J. Hou, A.K. Cheetham, V. Chen, S.B. Darling, Janus membranes: creating asymmetry for energy efficiency, Adv. Mater. 30 (2018) 1801495. [25] K. Fic, M. Meller, E. Frackowiak, Interfacial redox phenomena for enhanced aqueous supercapacitors, J. Electrochem. Soc. 162 (2015) A5140–A5147. [26] K. Fic, M. Meller, J. Menzel, E. Frackowiak, Around the thermodynamic limitations of supercapacitors operating in aqueous electrolytes, Electrochim. Acta 206 (2016) 496–503. [27] X. Wang, R.S. Chandrabose, Z. Jian, Z. Xing, X. Ji, A 1.8 V aqueous supercapacitor with a bipolar assembly of ion-exchange membranes as the separator, J. Electrochem. Soc. 163 (2016) A1853–A1858. [28] P. Ratajczak, F. Béguin, A high-voltage electrochemical cell operating with two aqueous electrolytes of different pH values, ChemElectroChem 5 (2018) 2518–2521. [29] L.-Q. Fan, J. Zhong, C.-Y. Zhang, J.-H. Wu, Y.-L. Wei, Improving the energy density of quasi-solid-state supercapacitors by assembling two redox-active gel electrolytes, Int. J. Hydrogen Energy 41 (2016) 5725–5732. [30] P.F.R. Ortega, Z. González, C. Blanco, G.G. Silva, R.L. Lavall, R. Santamaría, Biliquid supercapacitors: a simple and new strategy to enhance energy density in asymmetric/hybrid devices, Electrochim. Acta 254 (2017) 384–392. [31] C. Li, W. Wu, P. Wang, W. Zhou, J. Wang, Y. Chen, L. Fu, Y. Zhu, Y. Wu, W. Huang, Fabricating an aqueous symmetric supercapacitor with a stable high working voltage of 2 V by using an alkaline–acidic electrolyte, Adv. Sci. 6 (2019) 1801665. [32] M. Tian, J. Wu, R. Li, Y. Chen, D. Long, Fabricating a high-energy-density supercapacitor with asymmetric aqueous redox additive electrolytes and free-standing activated-carbon-felt electrodes, Chem. Eng. J. 363 (2019) 183–191. [33] C. Li, W. Wu, S. Zhang, L. He, Y. Zhu, J. Wang, L. Fu, Y. Chen, Y. Wu, W. Huang, A high-voltage aqueous lithium ion capacitor with high energy density from an alkaline–neutral electrolyte, J. Mater. Chem. 7 (2019) 4110–4118. [34] G. Couture, A. Alaaeddine, F. Boschet, B. Ameduri, Polymeric materials as anionexchange membranes for alkaline fuel cells, Prog. Polym. Sci. 36 (2011) 1521–1557.
Acknowledgments This work was supported by the Hubei Provincial Natural Science Foundation of China (2018CFB267). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.03.053. References [1] M. Sessolo, H.J. Bolink, Perovskite solar cells join the major league, Science 350 (2015) 917 917. [2] N.-S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.-K. Sun, K. Amine, et al., Challenges facing lithium batteries and electrical double-layer capacitors, Angew. Chem. Int. Ed. 51 (2012) 9994–10024. [3] S. Jing, J. Lu, G. Yu, S. Yin, L. Luo, Z. Zhang, Y. Ma, W. Chen, P.K. Shen, Carbonencapsulated WOx hybrids as efficient catalysts for hydrogen evolution, Adv. Mater. 30 (2018) 1705979. [4] L. Zhang, J. Lu, S. Yin, L. Luo, S. Jing, A. Brouzgou, J. Chen, P.K. Shen, P. Tsiakaras, One-pot synthesized boron-doped RhFe alloy with enhanced catalytic performance for hydrogen evolution reaction, Appl. Catal. B Environ. 230 (2018) 58–64. [5] S. Jing, L. Zhang, L. Luo, J. Lu, S. Yin, P.K. Shen, P. Tsiakaras, N-doped porous molybdenum carbide nanobelts as efficient catalysts for hydrogen evolution reaction, Appl. Catal. B Environ. 224 (2018) 533–540. [6] Y. Ma, H. Chang, M. Zhang, Y. Chen, Graphene-based materials for lithium-ion hybrid supercapacitors, Adv. Mater. 27 (2015) 5296–5308. [7] Z. Huaping, L. Long, V. Ranjith, L. Yong, Recent advances in designing and fabricating self-supported nanoelectrodes for supercapacitors, Adv. Sci. 4 (2017) 1700188. [8] T. Lin, I.-W. Chen, F. Liu, C. Yang, H. Bi, F. Xu, F. Huang, Nitrogen-doped mesoporous carbon of extraordinary capacitance for electrochemical energy storage, Science 350 (2015) 1508–1513. [9] Y. Mei, Z. Zhen, Recent breakthroughs in supercapacitors boosted by nitrogen-rich porous carbon materials, Adv. Sci. 4 (2017) 1600408. [10] Z. Chen, Y. Yuan, H. Zhou, X. Wang, Z. Gan, F. Wang, Y. Lu, 3D Nanocomposite architectures from carbon-nanotube-threaded nanocrystals for high-performance electrochemical energy storage, Adv. Mater. 26 (2014) 339–345.
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