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Electrochemical study on polypyrrole microparticle suspension as flowing anode for manganese dioxide rechargeable flow battery
Journal of Power Sources 248 (2014) 962e968
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Electrochemical study on polypyrrole microparticle suspension as flowing anode for manganese dioxide rechargeable flow battery Yongfu Zhao a, b, Shihui Si a, *, Lu Wang a, Cui Liao a, Ping Tang a, Huijun Cao a a b
College of Chemistry and Chemical Engineering, Central South University, No. 932 Lushan South Road, Changsha 410083, China College of Chemistry and Biological Engineering, Hezhou University, Hezhou 542800, China
h i g h l i g h t s The electrochemical behavior of PPy suspension was investigated in detail. Polypyrrole suspension was, for the first time, reported in the design of flow battery. Manganese dioxide was demonstrated to be an excellent cathode material for acidic battery. The PPy suspension battery shows improved current density relative to conventional film battery.
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Article history: Received 2 August 2013 Received in revised form 16 September 2013 Accepted 3 October 2013 Available online 16 October 2013
Comparison study on the electrochemical behaviors of PPy microparticle suspension and PPy film in the electrolytic solution of MnSO4 + H2SO4 has been done. The cyclic voltammogram of PPy suspension is almost similar to that of PPy film, suggesting the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution. The electrochemical response of PPy microparticle suspension in flowing mode obviously differs from that under static state, exhibiting good charge transfer property, therefore PPy microparticle suspension is used to fabricate PPy suspension//manganese dioxide rechargeable flow battery. The present system exhibits a significant improvement on the capacity density and cycle performance in comparison with PPy-based film batteries. The discharge capacity density of PPy reaches 132.3 mA h g1, and it still remains 97.2% of original value after 90 cycles. The coulombic efficiency shows no significant change with an average value of 92.1% over the experiment. Moreover, the system in the flow mode shows the improved discharge current, which reaches 15 mA cm2. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry of polypyrrole microparticle suspension Electrochemical quartz crystal microbalance Manganese dioxide Single flow battery
1. Introduction Polypyrrole (PPy) has attracted considerable attention for electrochemical application due to its high conductivity, good redox reversibility and environmental stability. These features make it an interesting candidate for electrode materials in rechargeable electrical energy storage devices. However, main investigations on PPy as the electrode material are focused on its application in aprotic electrolytes and lithium systems [1e5]. There are few attempts in the use of PPy as cathode material for aqueous-based rechargeable batteries due to the limitation of the low discharge voltage and the overoxidation degradation of PPy [6e9]. PPy, with the lower redox potential, promises to be used as anode material for the fabrication of rechargeable battery when it * Corresponding author. Tel.: þ86 73188876490, fax: þ86 73188879616. E-mail address: [email protected] (S. Si). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.10.008
is combined with high oxidation potential materials in aqueous electrolytes [10,11]. Among the high oxidation potential materials, manganese dioxide is considered to be one of the best cathode materials for rechargeable batteries due to its features of low toxicity and rich resource, and up to now some rechargeable alkaline manganese dioxide batteries have been developed [12e 17]. Though manganese dioxide can be electrochemically deposited on the conductive substrate and exhibits good redox property in the acidic media [18,19], there are few reports on manganese dioxide used as cathode material of rechargeable redox battery in acidic media [20e23]. In PPy-based batteries, PPy was typically made in the form of a thin coating on conductive substrates by using electrochemical synthesis, and the charge storage capacity was low due to the added weight of the substrate [24,25]. Other chemically synthesized PPy composites required an insulating binder to coat them on a current collector, which also lowered the energy and power
Y. Zhao et al. / Journal of Power Sources 248 (2014) 962e968
densities [26,27]. Unlike conventional rechargeable batteries that stored the electrical energy within electrode materials, the redox flow battery (RFB) stored the electrical energy in the external reservoirs. And the conversion between the electrical energy and chemical (or electrochemical) energy occurred as the electrolytes flow though electrodes in a cell stack [28e30]. Therefore, transition from solid PPy film to flowable suspension mode for the fabrication of a rechargeable flow battery would be a new route to overcome the aforementioned limitations. In the PPy suspension flow battery, the inherent advantages of flow architecture were retained while dramatically increasing energy density by using suspensions of energy-dense active materials and eliminating the substrate and additives required by film battery. In this paper, comparison study on the electrochemical behaviors of PPy microparticle suspension and PPy film in the electrolytic solution of MnSO4 þ H2SO4 has been done, and cyclic voltammogram of PPy suspension was almost similar to that of PPy film, suggesting the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution. Electrochemical quartz crystal microbalance (EQCM) was employed to characterize the doping/dedoping of anions during polypyrroble (PPy) redox process and the charge storage capacity of PPy. The electrochemical response of PPy microparticle suspension in flowing mode obviously differed from that under static state, exhibiting good charge transfer property due to continuous regeneration of PPy microparticles by the flow of suspension. Therefore, PPy microparticle suspension was used as flowing anode to fabricate a PPy microparticle suspension//manganese dioxide rechargeable flow battery, and its schematic illustration is shown in Fig. 1. Manganese dioxide deposited on porous carbon substrate was used as cathode. In the anode compartment a smooth graphite plate was used as current collector and PPy suspension played the role as anode and anolyte. Between the two compartments, a polypropylene microporous membrane was employed as separator to prevent PPy particles from getting into the cathode compartment. During the charge/discharge process, the PPy suspension was pumped into the anode compartment, where part of electrolyte penetrated into the cathode compartment via the microporous membrane, and the remained fluid containing PPy particles flowed through the anode compartment. Consequently, the anodic and
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cathodic reactions occurred in two electrode compartments respectively. 2. Experimental section 2.1. Preparation of materials PPy powder was synthesized galvanostatically on graphite plate electrode in a solution of 0.2 M pyrrole and 0.5 M HClO4. Prior to experiments, graphite electrode was mechanically polished with alumina of 0.5 mm on the polishing cloths, then cleaned in an ultrasonic bath in water/ethanol mixture. After synthesis, the PPy powder was collected by scratching with plastic knife, and then washed with dilute HClO4 and deionized water. In order to obtain PPy microparticles of various sizes, the obtained PPy powder was subjected to grinding using cutting mill. The PPy suspensions were prepared by dispersing the as-prepared PPy microparticles into solution of 1.0 M MnSO4 þ 0.2 M H2SO4. PPy film was electrochemically grown onto quartz crystal with Au electrode with various thickness by cyclic voltammetry (v ¼ 50 mV s1) between 0.4 V and 0.75 V in a solution of 0.2 M pyrrole and 0.5 M HClO4. After polymerization, the PPy film deposited on the surface of QCM was rinsed with dilute HClO4 and deionized water. According to Sauerbrey’s equation the relationship between polymer mass and frequency shift, the mass and thickness of PPy film was estimated [31].
f02 Dm Df ¼ p ffiffiffiffiffiffiffiffiffiffi A mq rq
(1)
Where f0 is the fundamental frequency of the QCM device (7.995 MHz), A the electrode area (0.2826 cm2), rq the density of quartz (2.648 g cm3), and mq is the shear modulus of quartz (2.947 1011 g s2 cm1). 2.2. Materials characterization Particle sizes of PPy particles were analyzed by laser particle size analyzer (Mastersizer 3000, Malvern, England).
Fig. 1. Schematic illustration of PPy microparticle suspension//manganese dioxide rechargeable flow battery.
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2.3. Flow battery manufacturing A porous carbon substrate (apparent area 9.0 cm 1.0 cm) recessed in groove of polytetrafluoroethylene (PTFE) plate was used as positive current collector and the cathode compartment was 9.0 1.0 0.2 cm3. A smooth graphite plate (area 9.0 cm 1.0 cm) recessed in groove of PTFE plate was used as negative current collector in the anode compartment (9.0 cm 1.0 cm 0.2 cm) and PPy suspension (containing 1% acetylene black) played the role as anode and anolyte. Between the two compartments, a polypropylene microporous membrane (with a mean pore size of 0.2 mm, thickness of 0.18 mm and active area of 9.0 cm 1.0 cm) serves as separator. The electrolyte in the battery is an aqueous solution of 1.0 M MnSO4 and 0.2 M H2SO4. The PPy suspension was pumped into the anode compartment at 37.5 mL min1, where part of electrolyte penetrated into the cathode compartment via the microporous membrane. 2.4. Electrochemical characterization EQCM experiments were carried out on a CHI420A electrochemical quartz crystal microbalance (Chenhua Instrument Co., China). Mechanically polished AT-cut 8 MHz quartz crystal (diameter of 12.5 mm, Beijing Chenjing Co., Beijing, China) was vacuum deposited with gold electrodes (6.0 mm in diameter) on both sides of the surface. Other electrochemical experiments were carried out with a CHI660 electrochemical analyzer (Chenhua Instrument Co., China). The working electrode was a glassy carbon electrode with diameter of 2 mm. A platinum wire was used as counter electrode and all potentials are referred to saturated calomel electrode (SCE). In the process of charge/discharge cycles, a SCE as reference was situated between the two electrodes to monitor the variation of potentials of positive and negative electrode. All charge/discharge experiments on battery were conducted in the potential range between 1.6 V and 0.7 V with a BTS battery test system (Neware Ltd., China). 3. Results and discussion 3.1. Comparison study on electrochemical behaviors of PPy film and PPy microparticle suspension Cyclic voltammogram and QCM response of PPy film in the solution of 1.0 M MnSO4 þ 0.2 M H2SO4 are shown in Fig. 2. The couple of redox peaks (ca. 0.17 V in the positive scan and 0.15 V in
the negative scan) were related to the electron transfer from/to the PPy film, and at the same time the doping/dedoping of SO4 2 anions to/from PPy film occurred. As shown in the frequency response of QCM, the decrease of frequency in the positive scan corresponded to the doping process of SO4 2 anions and the increase of frequency in the negative scan was attributed to the dedoping process of SO4 2 anions. The potentiostatic experiments combined with QCM have been carried out to determine the charge storage capacity of PPy doped with SO4 2 . The mass was estimated from the frequency response of QCM during the electropolymerization of PPy film, and the thickness of PPy film was calculated assuming the density of PPy film was ～1.08 g cm3. The charge storage capacity of PPy films with various thicknesses is shown in Table 1. The specific capacity of the PPy film decreased with the increase of film thickness, which ranged from 113.4 to 148.6 mAh g1. For the thick PPy film, the inner layer PPy materials could not take part in the redox reaction due to the unfavorable diffusion of anions, leading to the low specific capacity. The discharge capacity of the thin film was higher than the theoretical capacity of PPy (～136 mA h g1 [32]), and this phenomenon could be attributed to the electric double-layer capacitance of PPy film [33]. Cyclic voltammogram of PPy microparticle suspension is shown in Fig. 3. As can be seen, the peaks at the potential of 0.22 V and 0.23 V for PPy suspension, corresponding to the redox peaks at 0.17 V and 0.15 V of PPy film, were attributed to doping/ dedoping of SO4 2 anions to/from PPy particles, and the doping and dedoping process of SO4 2 were fairly reversible. The voltammogram curves of PPy suspension were almost similar to that of PPy film, and the doping/dedoping process for PPy particles dispersed in a solution of MnSO4 þ H2SO4 was expressed as follows: doping h
2nPPy þ nySO4 2 #
doping
yþ
PPy2
SO4 2
i y n
þ 2nye
(2)
However, in comparison with PPy film, the anodic peak potential shifted positively by 0.06 V and the cathodic peak potential shifted negatively by 0.08 V for PPy microparticles suspended in the same electrolyte solution, which indicated the presence of the charge transfer resistance between PPy microparticles and electrode substrate. In the experiments, the acetylene black (1%, V/V) was added into the PPy suspension to improve the conductivity of the system. It was found that the peak current in cyclic voltammogram test was almost independent on the concentration of PPy suspension without stirring (data not shown). The peak current was probably controlled by the PPy microparticles in contact with the electrode surface, on which the PPy microparticles formed a mono-particle layer with the nearly 100% coverage for particle concentrations ranging from 100 to 400 g L1. In Fig. 4, the anodic peak current of cyclic voltammograms was plotted against the square root of the scan rate, v1/2, for 150 g L1 of the PPy suspension under static condition. As can be seen, the peak current was proportional to v1/2,
Table 1 Effect of the PPy film thickness on the discharge capacity density.
Fig. 2. Cyclic voltammogram and EQCM response of PPy film in the solution of 1.0 M MnSO4 and 0.2 M H2SO4, scan rate: 10 mV s1.
Frequency shift (Hz) PPy film thicknessa (mm) Amount of charge transferred (C) Capacity density (mA h g1) a
6120
17241
26703
36983
49025
0.4
1.1
1.8
2.4
3.2
6.01 103 1.78 102 2.68 102 3.36 102 3.87 102 141.2
148.6
144.3
130.7
113.4
The PPy film thickness was estimated according to the Sauerbrey’s equation.
Y. Zhao et al. / Journal of Power Sources 248 (2014) 962e968
Fig. 3. Cyclic voltammogram of PPy suspension, scan rate: 50 mV s1, the concentration: 150 g L1.
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suggesting a diffusion control of counter ions. Consequently, the PPy suspension under static condition exhibited a similar electrochemical behavior as PPy film. In contrast, potentiostatic test of PPy suspension under stirring condition, which directly reflected the charge-discharge behavior of flowing suspension, displayed significant difference in electrochemical response from that under static condition. The potentiostatic test of PPy suspension under stirring condition (in flow mode) is shown in Fig. 5, the current increased with the concentration of PPy suspension and the currents for 100, 200 and 400 g L1 PPy suspension were about 0.12, 0.26 and 0.41 mA respectively. In flow mode, the PPy particles in contact with electrode surface were continuously regenerated by the flow of suspension, therefore the redox current of PPy particles at glassy carbon electrode depended on the concentration of PPy suspension. The electrochemical behavior of the PPy microparticles with various sizes dispersed in solution of MnSO4 þ H2SO4 was investigated. As shown in Fig. 6, for a given volume content (150 g L1), the voltammetric peak current increased with the decrease of the particle size, and the current in potentiostatic test showed the same trend (the inset in Fig. 6). The small PPy particles exhibited higher redox switching rate relative to the large ones due to the favorable diffusion and migration of ions, which originated from the increased surface area and the shortened path of ion diffusion [34]. However, for a given volume content, the smaller the size of PPy particle was, the higher viscosity the PPy suspensions possessed, therefore, the upper limit concentration of the flowable suspension was lower. Considering both the electrochemical response and the concentration of PPy suspension, PPy microparticles with the average size of 10.3 mm (bulk density of ～1.08 g cm3) was chosen as anode material with the upper limit content of about 435 g L1 in the flowable suspension. 3.2. Charge-discharge performance of PPy microparticle suspension flowing electrode
Fig. 4. Dependence of the anodic peak current of cyclic voltammograms on the square root of the scan rate, v1/2.
Charge-discharge performance of the anode and cathode for the flow battery was investigated. The galvanostatic charge-discharge curves for flowing PPy anode are shown in Fig. 7. In continuous circulating mode, PPy microparticles were only partially charged/ discharged during theirs residence time in the flow compartment. During the charge/discharge process, both the charging potential
Fig. 5. Effect of PPy suspension concentrations on the potentiostatic currents; concentration (g L1) : (1) 100, (2) 200, (3) 400; applying potential at 0.3 V, stirred with magnetic stirrer.
Fig. 6. Dependence of the particle size on the electrochemical behaviors of the PPy suspensions; the average size of particle (mm): (1) 28.6, (2) 10.3, (3) 3.2; Cyclic voltammograms, scan rate: 50 mV s1; (The inset) Potentiostatic test at potential of 0.3 V, stirred with magnetic stirrer.
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Mn4þ þ 2H2O / MnO2 þ 4Hþ
Fig. 7. Charge/discharge curves of PPy suspension; PPy suspension: 150 g L1, current density: 15 mA cm2.
and the discharging potential gradually changed, however, the overpotential for the charge process was over 0.3 V, and it was only 0.05 V for the discharge process. The net value of overpotential for the charge process was larger than that obtained in the discharge process, which could be attributed to the low conductivity of the reduced PPy particles.
(5)
Similarly, the MnO2 was reduced into Mn2þ through the MnOOH intermediate in the acidic medium [37]. The evident evolution of oxygen occurred above 1.5 V due to its overpotential, suggesting that the application of MnO2 as cathode material in the acidic medium was feasible. MnO2 was reduced into Mn2þ during discharge process in acidic media, whereas, in basic rechargeable batteries MnO2 was generally converted into MnOOH [12]. Therefore, the specific capacity of MnO2 in the basic solution was only half of that in acidic media. During the charge process, MnO2 was directly deposited on the porous carbon with the pore size of 0.4e0.6 mm from the electrolytic solution of 1.0 M MnSO4 þ 0.2 M H2SO4 at constant current ( the current density about 0.4 mA cm2). The thickness of the deposit was below 0.1 mm and the experimental temperature was ～ 25 1 C. The charge-discharge curves of manganese dioxide electrode are shown in Fig. 9. Charging of the electrode started at 1.16 V, followed by a slow increase of the potential, of which a small increase of potential was attributed to the fast charge reaction of the MnO2/Mn2þcouple in acidic solution. In the process of discharge, the voltage decreased quickly until the manganese dioxide completely changed into its reduced form, and the rapid change of discharging potentials maybe resulted from the occurrence of overpotential during the phrase transformation of manganese dioxide [35]. 3.4. Charge-discharge performance of the PPy microparticle suspension//manganese dioxide flow battery
3.3. Redox behavior and charge-discharge performance of manganese dioxide electrode in flowing battery Fig. 8 shows the cyclic voltammogram of Mn2þ at glassy carbon electrode in electrolytic solution of 1.0 M MnSO4 þ 0.2 M H2SO4. The oxidation of Mn2þ showed a conventional anodic wave at 1.2 V superimposed on the start of another wave corresponding to oxygen evolution [35,36]. And the cathodic peak at 1.05 V was corresponding to the reduction of MnO2 to Mn2þ. The electrodeposition of MnO2 has been proposed to proceed through the following pathway in the acidic medium [18,35]: Mn2þ / Mn3þ þ e
(3)
Mn3þ / Mn4þ þ Mn2þ
(4)
Fig. 8. Cyclic voltammogram of the manganese sulfate in the solution of 1.0 M MnSO4 and 0.2 M H2SO4; scan rate: 50 mV s1.
Fig. 10 shows the charge-discharge curve of the present flow battery at current density of 15 mA cm2. As can be seen, the voltage of the battery gradually increased from 0.9 V to 1.6 V during the charge process. A turning point occurred at the charge voltage of about 1.2 V, and then the charge curve rose swiftly. At the start of discharge, there was an IR drop of about 0.35 V, and then the cell voltage changed smoothly. As a result, the average charge voltage of the PPy flow battery was 1.3 V and the average discharge voltage was 0.95 V. The open circuit voltage was around 1.25 V. As seen from the voltage profile, the present flow cell suffered from low voltage efficiency. This meant that there was a big voltage difference between the charge and discharge processes, which resulted
Fig. 9. Charge/discharge curves of MnO2 electrode in solution of 1.0 M MnSO4 and 0.2 M H2SO4.
Y. Zhao et al. / Journal of Power Sources 248 (2014) 962e968
Fig. 10. Charge and discharge curve of the PPy microparticle suspension//manganese dioxide flow battery.
from the larger overpotential for two electrodes. On one hand, during the charge/discharge process of flowing PPy anode, there existed the large overpotential (over 0.3 V) for the charge process due to the low conductivity of the reduced PPy particles. On the other hand, in the discharge process of MnO2 electrode, the phrase transformation of manganese dioxide resulted in the occurrence of overpotential. In addition, there was the charge transfer resistance between PPy microparticles and electrode substrate, which led to a big challenge of charge transfer. The discharge capacity density reached 132.3 mA h g1 in the present battery system (calculated by the weight of PPy), exhibiting a significant improvement on the discharge capacity density in comparison with the conventional PPy film batteries. Calculated by the 435 g L1 upper limit content of the suspension and the 0.95 V average discharge voltage, the highest energy density of the Ppy tank could reach 54.7 W h L1 (39.1 W h kg1).The electrochemical experiments indicated that the PPy film possessed the high electric double-layer capacitance, (Reversible polarization led to formation of the electric double-layer by counterbalancing the surface charges of the polarized PPy particles.) and it could be expected that the PPy microparticles possessed the higher electric double-layer capacitance relative to PPy film due to its higher specific area [38]. The high capacity density of the whole battery could also be expected in comparison with PPy film battery since large amount of PPy materials was stored within the external reservoir in flow system, taking more percentage of the total mass due to no need of substrate and conducting additives. Additionally, in the small laboratory flow cell the current density of 15 mA cm2 has been achieved, which almost approached to the current density level of conventional RFB (25～100 mA cm2) [39]. Therefore, under optimized conditions, high power rates can be achieved in the present flow system. Fig. 11 exhibits the charge/discharge cycling performance of the present battery. After 90 cycles, its discharge capacity still remained 97.2% of the original value. And the coulombic efficiency showed no significant change with an average value of 92.1% over the experiment. As for the slightly low coulombic efficiency, it might be originated from the evolution of oxygen above the cathode. On the other hand, small PPy particles below pore size of membrane passed through the microporous membrane to transfer into cathode compartment and were oxidized at the cathode side, leading to self-discharging and the decrease of coulombic efficiency.
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Fig. 11. Cycling performance of the PPy microparticle suspension//manganese dioxide flow battery.
4. Conclusions We have demonstrated the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution, and the marked difference of electrochemical response for PPy suspension in flowing mode from that under static state, exhibiting good charge transfer property. The battery tests have showed that both electrodeposited MnO2 cathode and PPy slurry anode possessed good electrochemical properties and stable cycling performance. The present PPy microparticle suspension// manganese dioxide flow battery displayed a significant improvement on cycle performance. After 90 cycles, its discharge capacity still remained 97.2% of the original value. And the coulombic efficiency showed no significant change with an average value of 92.1% over the experiment. Transition from solid PPy film to a flowthrough mode for the PPy anode enabled PPy-based battery to operate at higher power rates and higher energy densities. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities of Central South University (No. 2013zzts016). References [1] Jong-Won Lee, B.N. Popov, J. Power Sources 161 (2006) 565e572. [2] R.P. Ramasamy, B. Veeraraghavan, B. Haran, B.N. Popov, J. Power Sources 124 (2003) 197e203. [3] J. Wang, C.O. Too, D. Zhou, G.G. Wallace, J. Power Sources 140 (2005) 162e167. [4] S.Y. Chew, Z.P. Guo, J.Z. Wang, J. Chen, P. Munroe, S.H. Ng, L. Zhao, H.K. Liu, Electrochem. Commun. 9 (2007) 941e946. [5] K. Park, S.B. Schougaard, J.B. Goodenough, Adv. Mater. 19 (2007) 848e851. [6] B.N. Grgur, M.M. Gvozdenovíc, J. Stevanovíc, B.Z. Jugovíc, V.M. Marinovíc, Electrochim. Acta 53 (2008) 4627e4632. [7] Y. Li, R. Qian, Electrochim. Acta 45 (2000) 1727e1731. [8] J.B. Schlenoff, H. Xu, J. Electrochem. Soc. 139 (1992) 2397e2401. [9] J. Mostany, B.R. Scharifker, Electrochim. Acta 42 (1997) 291e298. [10] G. Wang, Q. Qu, B. Wang, Y. Shi, S. Tian, Y. Wu, ChemPhysChem 9 (2008) 2299e2301. [11] Z. Yin, Y. Ding, Q. Zheng, L. Guan, Electrochem. Commun. 20 (2012) 40e43. [12] B. Sajdl, K. Micka, P. Krtil, Electrochim. Acta 40 (12) (1995) 2005e2011. [13] H. Adelkhani, M. Ghaemi, J. Alloys Compd. 481 (2009) 446e449. [14] X. Li, Z. Li, T. Xia, H. Dong, Y. Song, L. Wang, J. Phys. Chem. Solids 73 (2012) 1229e1234. [15] A.P. Malloy, S.W. Donne, J. Electroanal. Chem. 621 (2008) 83e90. [16] M. Manickam, P. Singh, T.B. Issa, S. Thurgate, R. De Marco, J. Power Sources 130 (2004) 254e259. [17] M. Manickam, D.R.G. Mitchell, P. Singh, Electrochim. Acta 52 (2007) 3294e 3298. [18] Suhasini, J. Electroanal. Chem. 690 (2013) 13e18.
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Short communication
Electrochemical study on polypyrrole microparticle suspension as flowing anode for manganese dioxide rechargeable flow battery Yongfu Zhao a, b, Shihui Si a, *, Lu Wang a, Cui Liao a, Ping Tang a, Huijun Cao a a b
College of Chemistry and Chemical Engineering, Central South University, No. 932 Lushan South Road, Changsha 410083, China College of Chemistry and Biological Engineering, Hezhou University, Hezhou 542800, China
h i g h l i g h t s The electrochemical behavior of PPy suspension was investigated in detail. Polypyrrole suspension was, for the first time, reported in the design of flow battery. Manganese dioxide was demonstrated to be an excellent cathode material for acidic battery. The PPy suspension battery shows improved current density relative to conventional film battery.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2013 Received in revised form 16 September 2013 Accepted 3 October 2013 Available online 16 October 2013
Comparison study on the electrochemical behaviors of PPy microparticle suspension and PPy film in the electrolytic solution of MnSO4 + H2SO4 has been done. The cyclic voltammogram of PPy suspension is almost similar to that of PPy film, suggesting the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution. The electrochemical response of PPy microparticle suspension in flowing mode obviously differs from that under static state, exhibiting good charge transfer property, therefore PPy microparticle suspension is used to fabricate PPy suspension//manganese dioxide rechargeable flow battery. The present system exhibits a significant improvement on the capacity density and cycle performance in comparison with PPy-based film batteries. The discharge capacity density of PPy reaches 132.3 mA h g1, and it still remains 97.2% of original value after 90 cycles. The coulombic efficiency shows no significant change with an average value of 92.1% over the experiment. Moreover, the system in the flow mode shows the improved discharge current, which reaches 15 mA cm2. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry of polypyrrole microparticle suspension Electrochemical quartz crystal microbalance Manganese dioxide Single flow battery
1. Introduction Polypyrrole (PPy) has attracted considerable attention for electrochemical application due to its high conductivity, good redox reversibility and environmental stability. These features make it an interesting candidate for electrode materials in rechargeable electrical energy storage devices. However, main investigations on PPy as the electrode material are focused on its application in aprotic electrolytes and lithium systems [1e5]. There are few attempts in the use of PPy as cathode material for aqueous-based rechargeable batteries due to the limitation of the low discharge voltage and the overoxidation degradation of PPy [6e9]. PPy, with the lower redox potential, promises to be used as anode material for the fabrication of rechargeable battery when it * Corresponding author. Tel.: þ86 73188876490, fax: þ86 73188879616. E-mail address: [email protected] (S. Si). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.10.008
is combined with high oxidation potential materials in aqueous electrolytes [10,11]. Among the high oxidation potential materials, manganese dioxide is considered to be one of the best cathode materials for rechargeable batteries due to its features of low toxicity and rich resource, and up to now some rechargeable alkaline manganese dioxide batteries have been developed [12e 17]. Though manganese dioxide can be electrochemically deposited on the conductive substrate and exhibits good redox property in the acidic media [18,19], there are few reports on manganese dioxide used as cathode material of rechargeable redox battery in acidic media [20e23]. In PPy-based batteries, PPy was typically made in the form of a thin coating on conductive substrates by using electrochemical synthesis, and the charge storage capacity was low due to the added weight of the substrate [24,25]. Other chemically synthesized PPy composites required an insulating binder to coat them on a current collector, which also lowered the energy and power
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densities [26,27]. Unlike conventional rechargeable batteries that stored the electrical energy within electrode materials, the redox flow battery (RFB) stored the electrical energy in the external reservoirs. And the conversion between the electrical energy and chemical (or electrochemical) energy occurred as the electrolytes flow though electrodes in a cell stack [28e30]. Therefore, transition from solid PPy film to flowable suspension mode for the fabrication of a rechargeable flow battery would be a new route to overcome the aforementioned limitations. In the PPy suspension flow battery, the inherent advantages of flow architecture were retained while dramatically increasing energy density by using suspensions of energy-dense active materials and eliminating the substrate and additives required by film battery. In this paper, comparison study on the electrochemical behaviors of PPy microparticle suspension and PPy film in the electrolytic solution of MnSO4 þ H2SO4 has been done, and cyclic voltammogram of PPy suspension was almost similar to that of PPy film, suggesting the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution. Electrochemical quartz crystal microbalance (EQCM) was employed to characterize the doping/dedoping of anions during polypyrroble (PPy) redox process and the charge storage capacity of PPy. The electrochemical response of PPy microparticle suspension in flowing mode obviously differed from that under static state, exhibiting good charge transfer property due to continuous regeneration of PPy microparticles by the flow of suspension. Therefore, PPy microparticle suspension was used as flowing anode to fabricate a PPy microparticle suspension//manganese dioxide rechargeable flow battery, and its schematic illustration is shown in Fig. 1. Manganese dioxide deposited on porous carbon substrate was used as cathode. In the anode compartment a smooth graphite plate was used as current collector and PPy suspension played the role as anode and anolyte. Between the two compartments, a polypropylene microporous membrane was employed as separator to prevent PPy particles from getting into the cathode compartment. During the charge/discharge process, the PPy suspension was pumped into the anode compartment, where part of electrolyte penetrated into the cathode compartment via the microporous membrane, and the remained fluid containing PPy particles flowed through the anode compartment. Consequently, the anodic and
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cathodic reactions occurred in two electrode compartments respectively. 2. Experimental section 2.1. Preparation of materials PPy powder was synthesized galvanostatically on graphite plate electrode in a solution of 0.2 M pyrrole and 0.5 M HClO4. Prior to experiments, graphite electrode was mechanically polished with alumina of 0.5 mm on the polishing cloths, then cleaned in an ultrasonic bath in water/ethanol mixture. After synthesis, the PPy powder was collected by scratching with plastic knife, and then washed with dilute HClO4 and deionized water. In order to obtain PPy microparticles of various sizes, the obtained PPy powder was subjected to grinding using cutting mill. The PPy suspensions were prepared by dispersing the as-prepared PPy microparticles into solution of 1.0 M MnSO4 þ 0.2 M H2SO4. PPy film was electrochemically grown onto quartz crystal with Au electrode with various thickness by cyclic voltammetry (v ¼ 50 mV s1) between 0.4 V and 0.75 V in a solution of 0.2 M pyrrole and 0.5 M HClO4. After polymerization, the PPy film deposited on the surface of QCM was rinsed with dilute HClO4 and deionized water. According to Sauerbrey’s equation the relationship between polymer mass and frequency shift, the mass and thickness of PPy film was estimated [31].
f02 Dm Df ¼ p ffiffiffiffiffiffiffiffiffiffi A mq rq
(1)
Where f0 is the fundamental frequency of the QCM device (7.995 MHz), A the electrode area (0.2826 cm2), rq the density of quartz (2.648 g cm3), and mq is the shear modulus of quartz (2.947 1011 g s2 cm1). 2.2. Materials characterization Particle sizes of PPy particles were analyzed by laser particle size analyzer (Mastersizer 3000, Malvern, England).
Fig. 1. Schematic illustration of PPy microparticle suspension//manganese dioxide rechargeable flow battery.
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2.3. Flow battery manufacturing A porous carbon substrate (apparent area 9.0 cm 1.0 cm) recessed in groove of polytetrafluoroethylene (PTFE) plate was used as positive current collector and the cathode compartment was 9.0 1.0 0.2 cm3. A smooth graphite plate (area 9.0 cm 1.0 cm) recessed in groove of PTFE plate was used as negative current collector in the anode compartment (9.0 cm 1.0 cm 0.2 cm) and PPy suspension (containing 1% acetylene black) played the role as anode and anolyte. Between the two compartments, a polypropylene microporous membrane (with a mean pore size of 0.2 mm, thickness of 0.18 mm and active area of 9.0 cm 1.0 cm) serves as separator. The electrolyte in the battery is an aqueous solution of 1.0 M MnSO4 and 0.2 M H2SO4. The PPy suspension was pumped into the anode compartment at 37.5 mL min1, where part of electrolyte penetrated into the cathode compartment via the microporous membrane. 2.4. Electrochemical characterization EQCM experiments were carried out on a CHI420A electrochemical quartz crystal microbalance (Chenhua Instrument Co., China). Mechanically polished AT-cut 8 MHz quartz crystal (diameter of 12.5 mm, Beijing Chenjing Co., Beijing, China) was vacuum deposited with gold electrodes (6.0 mm in diameter) on both sides of the surface. Other electrochemical experiments were carried out with a CHI660 electrochemical analyzer (Chenhua Instrument Co., China). The working electrode was a glassy carbon electrode with diameter of 2 mm. A platinum wire was used as counter electrode and all potentials are referred to saturated calomel electrode (SCE). In the process of charge/discharge cycles, a SCE as reference was situated between the two electrodes to monitor the variation of potentials of positive and negative electrode. All charge/discharge experiments on battery were conducted in the potential range between 1.6 V and 0.7 V with a BTS battery test system (Neware Ltd., China). 3. Results and discussion 3.1. Comparison study on electrochemical behaviors of PPy film and PPy microparticle suspension Cyclic voltammogram and QCM response of PPy film in the solution of 1.0 M MnSO4 þ 0.2 M H2SO4 are shown in Fig. 2. The couple of redox peaks (ca. 0.17 V in the positive scan and 0.15 V in
the negative scan) were related to the electron transfer from/to the PPy film, and at the same time the doping/dedoping of SO4 2 anions to/from PPy film occurred. As shown in the frequency response of QCM, the decrease of frequency in the positive scan corresponded to the doping process of SO4 2 anions and the increase of frequency in the negative scan was attributed to the dedoping process of SO4 2 anions. The potentiostatic experiments combined with QCM have been carried out to determine the charge storage capacity of PPy doped with SO4 2 . The mass was estimated from the frequency response of QCM during the electropolymerization of PPy film, and the thickness of PPy film was calculated assuming the density of PPy film was ～1.08 g cm3. The charge storage capacity of PPy films with various thicknesses is shown in Table 1. The specific capacity of the PPy film decreased with the increase of film thickness, which ranged from 113.4 to 148.6 mAh g1. For the thick PPy film, the inner layer PPy materials could not take part in the redox reaction due to the unfavorable diffusion of anions, leading to the low specific capacity. The discharge capacity of the thin film was higher than the theoretical capacity of PPy (～136 mA h g1 [32]), and this phenomenon could be attributed to the electric double-layer capacitance of PPy film [33]. Cyclic voltammogram of PPy microparticle suspension is shown in Fig. 3. As can be seen, the peaks at the potential of 0.22 V and 0.23 V for PPy suspension, corresponding to the redox peaks at 0.17 V and 0.15 V of PPy film, were attributed to doping/ dedoping of SO4 2 anions to/from PPy particles, and the doping and dedoping process of SO4 2 were fairly reversible. The voltammogram curves of PPy suspension were almost similar to that of PPy film, and the doping/dedoping process for PPy particles dispersed in a solution of MnSO4 þ H2SO4 was expressed as follows: doping h
2nPPy þ nySO4 2 #
doping
yþ
PPy2
SO4 2
i y n
þ 2nye
(2)
However, in comparison with PPy film, the anodic peak potential shifted positively by 0.06 V and the cathodic peak potential shifted negatively by 0.08 V for PPy microparticles suspended in the same electrolyte solution, which indicated the presence of the charge transfer resistance between PPy microparticles and electrode substrate. In the experiments, the acetylene black (1%, V/V) was added into the PPy suspension to improve the conductivity of the system. It was found that the peak current in cyclic voltammogram test was almost independent on the concentration of PPy suspension without stirring (data not shown). The peak current was probably controlled by the PPy microparticles in contact with the electrode surface, on which the PPy microparticles formed a mono-particle layer with the nearly 100% coverage for particle concentrations ranging from 100 to 400 g L1. In Fig. 4, the anodic peak current of cyclic voltammograms was plotted against the square root of the scan rate, v1/2, for 150 g L1 of the PPy suspension under static condition. As can be seen, the peak current was proportional to v1/2,
Table 1 Effect of the PPy film thickness on the discharge capacity density.
Fig. 2. Cyclic voltammogram and EQCM response of PPy film in the solution of 1.0 M MnSO4 and 0.2 M H2SO4, scan rate: 10 mV s1.
Frequency shift (Hz) PPy film thicknessa (mm) Amount of charge transferred (C) Capacity density (mA h g1) a
6120
17241
26703
36983
49025
0.4
1.1
1.8
2.4
3.2
6.01 103 1.78 102 2.68 102 3.36 102 3.87 102 141.2
148.6
144.3
130.7
113.4
The PPy film thickness was estimated according to the Sauerbrey’s equation.
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Fig. 3. Cyclic voltammogram of PPy suspension, scan rate: 50 mV s1, the concentration: 150 g L1.
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suggesting a diffusion control of counter ions. Consequently, the PPy suspension under static condition exhibited a similar electrochemical behavior as PPy film. In contrast, potentiostatic test of PPy suspension under stirring condition, which directly reflected the charge-discharge behavior of flowing suspension, displayed significant difference in electrochemical response from that under static condition. The potentiostatic test of PPy suspension under stirring condition (in flow mode) is shown in Fig. 5, the current increased with the concentration of PPy suspension and the currents for 100, 200 and 400 g L1 PPy suspension were about 0.12, 0.26 and 0.41 mA respectively. In flow mode, the PPy particles in contact with electrode surface were continuously regenerated by the flow of suspension, therefore the redox current of PPy particles at glassy carbon electrode depended on the concentration of PPy suspension. The electrochemical behavior of the PPy microparticles with various sizes dispersed in solution of MnSO4 þ H2SO4 was investigated. As shown in Fig. 6, for a given volume content (150 g L1), the voltammetric peak current increased with the decrease of the particle size, and the current in potentiostatic test showed the same trend (the inset in Fig. 6). The small PPy particles exhibited higher redox switching rate relative to the large ones due to the favorable diffusion and migration of ions, which originated from the increased surface area and the shortened path of ion diffusion [34]. However, for a given volume content, the smaller the size of PPy particle was, the higher viscosity the PPy suspensions possessed, therefore, the upper limit concentration of the flowable suspension was lower. Considering both the electrochemical response and the concentration of PPy suspension, PPy microparticles with the average size of 10.3 mm (bulk density of ～1.08 g cm3) was chosen as anode material with the upper limit content of about 435 g L1 in the flowable suspension. 3.2. Charge-discharge performance of PPy microparticle suspension flowing electrode
Fig. 4. Dependence of the anodic peak current of cyclic voltammograms on the square root of the scan rate, v1/2.
Charge-discharge performance of the anode and cathode for the flow battery was investigated. The galvanostatic charge-discharge curves for flowing PPy anode are shown in Fig. 7. In continuous circulating mode, PPy microparticles were only partially charged/ discharged during theirs residence time in the flow compartment. During the charge/discharge process, both the charging potential
Fig. 5. Effect of PPy suspension concentrations on the potentiostatic currents; concentration (g L1) : (1) 100, (2) 200, (3) 400; applying potential at 0.3 V, stirred with magnetic stirrer.
Fig. 6. Dependence of the particle size on the electrochemical behaviors of the PPy suspensions; the average size of particle (mm): (1) 28.6, (2) 10.3, (3) 3.2; Cyclic voltammograms, scan rate: 50 mV s1; (The inset) Potentiostatic test at potential of 0.3 V, stirred with magnetic stirrer.
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Mn4þ þ 2H2O / MnO2 þ 4Hþ
Fig. 7. Charge/discharge curves of PPy suspension; PPy suspension: 150 g L1, current density: 15 mA cm2.
and the discharging potential gradually changed, however, the overpotential for the charge process was over 0.3 V, and it was only 0.05 V for the discharge process. The net value of overpotential for the charge process was larger than that obtained in the discharge process, which could be attributed to the low conductivity of the reduced PPy particles.
(5)
Similarly, the MnO2 was reduced into Mn2þ through the MnOOH intermediate in the acidic medium [37]. The evident evolution of oxygen occurred above 1.5 V due to its overpotential, suggesting that the application of MnO2 as cathode material in the acidic medium was feasible. MnO2 was reduced into Mn2þ during discharge process in acidic media, whereas, in basic rechargeable batteries MnO2 was generally converted into MnOOH [12]. Therefore, the specific capacity of MnO2 in the basic solution was only half of that in acidic media. During the charge process, MnO2 was directly deposited on the porous carbon with the pore size of 0.4e0.6 mm from the electrolytic solution of 1.0 M MnSO4 þ 0.2 M H2SO4 at constant current ( the current density about 0.4 mA cm2). The thickness of the deposit was below 0.1 mm and the experimental temperature was ～ 25 1 C. The charge-discharge curves of manganese dioxide electrode are shown in Fig. 9. Charging of the electrode started at 1.16 V, followed by a slow increase of the potential, of which a small increase of potential was attributed to the fast charge reaction of the MnO2/Mn2þcouple in acidic solution. In the process of discharge, the voltage decreased quickly until the manganese dioxide completely changed into its reduced form, and the rapid change of discharging potentials maybe resulted from the occurrence of overpotential during the phrase transformation of manganese dioxide [35]. 3.4. Charge-discharge performance of the PPy microparticle suspension//manganese dioxide flow battery
3.3. Redox behavior and charge-discharge performance of manganese dioxide electrode in flowing battery Fig. 8 shows the cyclic voltammogram of Mn2þ at glassy carbon electrode in electrolytic solution of 1.0 M MnSO4 þ 0.2 M H2SO4. The oxidation of Mn2þ showed a conventional anodic wave at 1.2 V superimposed on the start of another wave corresponding to oxygen evolution [35,36]. And the cathodic peak at 1.05 V was corresponding to the reduction of MnO2 to Mn2þ. The electrodeposition of MnO2 has been proposed to proceed through the following pathway in the acidic medium [18,35]: Mn2þ / Mn3þ þ e
(3)
Mn3þ / Mn4þ þ Mn2þ
(4)
Fig. 8. Cyclic voltammogram of the manganese sulfate in the solution of 1.0 M MnSO4 and 0.2 M H2SO4; scan rate: 50 mV s1.
Fig. 10 shows the charge-discharge curve of the present flow battery at current density of 15 mA cm2. As can be seen, the voltage of the battery gradually increased from 0.9 V to 1.6 V during the charge process. A turning point occurred at the charge voltage of about 1.2 V, and then the charge curve rose swiftly. At the start of discharge, there was an IR drop of about 0.35 V, and then the cell voltage changed smoothly. As a result, the average charge voltage of the PPy flow battery was 1.3 V and the average discharge voltage was 0.95 V. The open circuit voltage was around 1.25 V. As seen from the voltage profile, the present flow cell suffered from low voltage efficiency. This meant that there was a big voltage difference between the charge and discharge processes, which resulted
Fig. 9. Charge/discharge curves of MnO2 electrode in solution of 1.0 M MnSO4 and 0.2 M H2SO4.
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Fig. 10. Charge and discharge curve of the PPy microparticle suspension//manganese dioxide flow battery.
from the larger overpotential for two electrodes. On one hand, during the charge/discharge process of flowing PPy anode, there existed the large overpotential (over 0.3 V) for the charge process due to the low conductivity of the reduced PPy particles. On the other hand, in the discharge process of MnO2 electrode, the phrase transformation of manganese dioxide resulted in the occurrence of overpotential. In addition, there was the charge transfer resistance between PPy microparticles and electrode substrate, which led to a big challenge of charge transfer. The discharge capacity density reached 132.3 mA h g1 in the present battery system (calculated by the weight of PPy), exhibiting a significant improvement on the discharge capacity density in comparison with the conventional PPy film batteries. Calculated by the 435 g L1 upper limit content of the suspension and the 0.95 V average discharge voltage, the highest energy density of the Ppy tank could reach 54.7 W h L1 (39.1 W h kg1).The electrochemical experiments indicated that the PPy film possessed the high electric double-layer capacitance, (Reversible polarization led to formation of the electric double-layer by counterbalancing the surface charges of the polarized PPy particles.) and it could be expected that the PPy microparticles possessed the higher electric double-layer capacitance relative to PPy film due to its higher specific area [38]. The high capacity density of the whole battery could also be expected in comparison with PPy film battery since large amount of PPy materials was stored within the external reservoir in flow system, taking more percentage of the total mass due to no need of substrate and conducting additives. Additionally, in the small laboratory flow cell the current density of 15 mA cm2 has been achieved, which almost approached to the current density level of conventional RFB (25～100 mA cm2) [39]. Therefore, under optimized conditions, high power rates can be achieved in the present flow system. Fig. 11 exhibits the charge/discharge cycling performance of the present battery. After 90 cycles, its discharge capacity still remained 97.2% of the original value. And the coulombic efficiency showed no significant change with an average value of 92.1% over the experiment. As for the slightly low coulombic efficiency, it might be originated from the evolution of oxygen above the cathode. On the other hand, small PPy particles below pore size of membrane passed through the microporous membrane to transfer into cathode compartment and were oxidized at the cathode side, leading to self-discharging and the decrease of coulombic efficiency.
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Fig. 11. Cycling performance of the PPy microparticle suspension//manganese dioxide flow battery.
4. Conclusions We have demonstrated the feasibility of charge transfer between the conductive substrate and PPy particles dispersed in the solution, and the marked difference of electrochemical response for PPy suspension in flowing mode from that under static state, exhibiting good charge transfer property. The battery tests have showed that both electrodeposited MnO2 cathode and PPy slurry anode possessed good electrochemical properties and stable cycling performance. The present PPy microparticle suspension// manganese dioxide flow battery displayed a significant improvement on cycle performance. After 90 cycles, its discharge capacity still remained 97.2% of the original value. And the coulombic efficiency showed no significant change with an average value of 92.1% over the experiment. Transition from solid PPy film to a flowthrough mode for the PPy anode enabled PPy-based battery to operate at higher power rates and higher energy densities. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities of Central South University (No. 2013zzts016). References [1] Jong-Won Lee, B.N. Popov, J. Power Sources 161 (2006) 565e572. [2] R.P. Ramasamy, B. Veeraraghavan, B. Haran, B.N. Popov, J. Power Sources 124 (2003) 197e203. [3] J. Wang, C.O. Too, D. Zhou, G.G. Wallace, J. Power Sources 140 (2005) 162e167. [4] S.Y. Chew, Z.P. Guo, J.Z. Wang, J. Chen, P. Munroe, S.H. Ng, L. Zhao, H.K. Liu, Electrochem. Commun. 9 (2007) 941e946. [5] K. Park, S.B. Schougaard, J.B. Goodenough, Adv. Mater. 19 (2007) 848e851. [6] B.N. Grgur, M.M. Gvozdenovíc, J. Stevanovíc, B.Z. Jugovíc, V.M. Marinovíc, Electrochim. Acta 53 (2008) 4627e4632. [7] Y. Li, R. Qian, Electrochim. Acta 45 (2000) 1727e1731. [8] J.B. Schlenoff, H. Xu, J. Electrochem. Soc. 139 (1992) 2397e2401. [9] J. Mostany, B.R. Scharifker, Electrochim. Acta 42 (1997) 291e298. [10] G. Wang, Q. Qu, B. Wang, Y. Shi, S. Tian, Y. Wu, ChemPhysChem 9 (2008) 2299e2301. [11] Z. Yin, Y. Ding, Q. Zheng, L. Guan, Electrochem. Commun. 20 (2012) 40e43. [12] B. Sajdl, K. Micka, P. Krtil, Electrochim. Acta 40 (12) (1995) 2005e2011. [13] H. Adelkhani, M. Ghaemi, J. Alloys Compd. 481 (2009) 446e449. [14] X. Li, Z. Li, T. Xia, H. Dong, Y. Song, L. Wang, J. Phys. Chem. Solids 73 (2012) 1229e1234. [15] A.P. Malloy, S.W. Donne, J. Electroanal. Chem. 621 (2008) 83e90. [16] M. Manickam, P. Singh, T.B. Issa, S. Thurgate, R. De Marco, J. Power Sources 130 (2004) 254e259. [17] M. Manickam, D.R.G. Mitchell, P. Singh, Electrochim. Acta 52 (2007) 3294e 3298. [18] Suhasini, J. Electroanal. Chem. 690 (2013) 13e18.
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