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Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
9
H. Zhang Chinese Academy of Sciences, Dalian, China
9.1
Introduction
This chapter reviews key aspects of polysulfide-bromine batteries (PBBs) as a candidate energy storage technology including their: l
l
l
l
working principles, technological development, key materials (membrane separator, electrolyte solutions, and electrodes), performance and applications.
The low cost of electrolyte is an advantage of PBBs provided that expensive bromine complexing agents are not employed to bind bromine vapor that can form during battery operation. However, there are several issues restricting practical and widespread application of PBBs: l
l
l
first, its electrolytes contain different types of active species, which may result in serious cross-contamination, and consequently low efficiency and device capacity; second, sodium ion transport through the membrane separator experiences large resistance compared with proton transport, leading to low power density of battery, and therefore relatively more material is needed to construct large-scale PBB systems, making this technology disadvantageous in terms of cost; and lastly, bromine in the catholyte and hydrogen sulfide evolved in the anolyte can lead to environmental pollution.
For these reasons, most R&D efforts in this area have ceased and more attention has focused on other flow cell chemistries. Promise of PBB technology will rely on breakthroughs in the development of: l
l
l
l
low-cost, durable membranes with high sodium ion conductivity and selectivity, electrode materials with high electrochemical activity and stability, suitable seal material, optimization of electrode structure and stack design.
This chapter reviews key research in these areas. Advances in Batteries for Medium- and Large-scale Energy Storage. http://dx.doi.org/10.1016/B978-1-78242-013-2.00009-1 Copyright © 2015 Elsevier Ltd. All rights reserved.
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9.2
Advances in Batteries for Medium- and Large-scale Energy Storage
PBBs: principles and technologies
First conceptualized by Thaller in 1976, the flow battery is known as an electrochemical device for conversion between chemical energy and electricity via the redox reactions of different active species stored in electrolytes. Its charge process involves an oxidation reaction occurring on the cathode and a reduction reaction on the anode; these reactions reverse during discharge. The catholyte and anolyte of flow batteries, both containing active species (ions of different valence states) for the electrode reactions, are stored in two tanks and circulated within the battery. Based on the states of active species in the anode and cathode half-cells, flow batteries can be categorized into all-liquid types (including the all-vanadium flow battery (VFB), iron-chromium flow battery, and polysulfide-bromine flow battery) and deposition (or half-liquid) types (such as the zinc-nickel flow battery). Unlike conventional secondary batteries with active electrodes, all-liquid flow batteries use inert electrodes that are simply placed on which the electrode reactions occur; such batteries feature state-of-valence changes but no state-of-condensation change of the active species. By contrast, a deposition flow battery involves alternate deposition and dissolution of metals during battery operation. The PBB falls within the all-flow category. The PBB employs aqueous alkaline solutions of sodium polysulfide (Na2Sx) and sodium bromide (NaBr) as the anolyte and catholyte, respectively. Its redox couples are Sx þ 1 2 =Sx 2 at the anode and Br2/Br at the cathode. Br2 exists as Br3 in the catholyte, and the elemental sulfur combines with sulfide anions to form polysulfides in the anolyte. The PBB works via the redox reaction of anions, rather than cations. The catholyte and the anolyte are separated with an ion exchange membrane, which transports sodium ions between them to complete the battery circuit. The electrode reactions during charge and discharge are shown in Figure 9.1 and described as follows:
Negative electrode
Ion exchange membrane
Positive electrode
Charge Na+
Na+ Discharge
Br3−
Sn2− −2e−
+2e− Sn+12−
+2e−
−2e− 3Br −
Figure 9.1 Redox reactions on the positive and negative electrodes of PBB.
Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
319
At the cathode : 2NaBr , 2e þ Br2 þ 2Naþ
(9.1)
At the anode : 2Naþ þ ðx 1ÞNa2 Sx þ 2e , xNa2 Sx1 , x ¼ 2-4
(9.2)
Overall : 2NaBr þ ðx 1ÞNa2 Sx , Br2 þ xNa2 Sx1 , x ¼ 2-4
(9.3)
The standard electrode potential of reaction (9.1) is 1.087 V, while that of (9.2) is 0.428 V, so the standard EMF of the PBB is 1.515 V. Due to variations in the electrolyte concentrations and the state of charge or discharge, a PBB typically shows an open circuit voltage of 1.54-1.60 V. Compared with an all-VFB, a PBB uses low-cost materials such as polysulfide and bromine, and therefore it has aroused considerable research interest and funding in the past. Remick (Remick and Ang, 1984) was the first to propose flow batteries with polysulfide as the anode redox couple and halide as the cathode redox couple. Innogy (Price et al., 1999), a British company, registered Regenesys™ as the trademark for PBB energy storage technology, and has developed three PBB stacks with different powers. The stack structure is similar to that of a fuel cell stack, which consists of electrodes, bipolar plates, ion exchange membranes, electrode frames, and end plates. The PBB employs anion redox couples of different elements for energy conversion; this often causes problems because anion cross-contamination can hardly be avoided with all available membranes, including the perfluorinated sulfonic Nafion membrane, which results in rapid fading of the batteries’ capacity. To overcome this drawback, Innogy developed methodologies such as sulfate-based removal of contaminant, pH-based recovery of active species, and water control. These methods have not been reported to result in electrolyte imbalance or battery performance decay, but their influences on PBB performance must be studied to achieve a thorough understanding of the operational characteristics of the system and provide a good basis for system design and choice of operating conditions.
9.3
Electrolyte solution and its chemistry
Electrolyte is the medium for energy storage. Typically, it consists of active species dissolved in a supporting electrolyte. The electrolyte solution is stored in a tank, pumped into the battery and then goes back to the tank. It meets the porous electrodes inside the battery and undergoes electrochemical reactions in response to the external electric current changes; this is accompanied with changes in the valence states of the active species so that energy storage and release are achieved. In the above process, electrons released travel through the external circuit, and the sodium ions permeate the membrane to achieve electroneutrality. Common catholytes and anolytes for PBBs are NaBr (typically 4 mol L1) and alkaline Na2S (typically 1.5 mol L1) solutions, respectively, with de-ionized water as the supporting electrolyte; their volume and concentration can be adjusted according to the PBB capacity requirements.
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9.3.1
Advances in Batteries for Medium- and Large-scale Energy Storage
The solution chemistry and electrochemistry of bromide ions
The Br– ion’s state in the catholyte and its electrochemistry is relatively simple. During charge, Br– ions release electrons and form Br2, whose solubility is low in water but high in NaBr solution because Br2 can complex with Br– to form Br3 or Br5 . Such complex anions, driven by the electric field and concentration gradient, can diffuse through the ion exchange membrane to the anode and react with the active species therein; this phenomenon, known as self-discharge, is undesirable because it leads to energy efficiency decay and capacity fading. Meanwhile, the differing pH values of the catholyte (initial pH ¼ 3) and the anolyte (initial pH ¼ 12) also affect the cycle life of the battery; this is because during charge/discharge the pH of the catholyte increases and results in loss of bromine as the active material. In zinc-bromine flow batteries, which also employ Br2/Br– as the cathode redox couple, a quaternary ammonium is often added to the catholyte as a complexing agent. The resulting Q Br3 complex is a solid and can deposit at the bottom of catholyte so that Br3 diffusion and battery self-discharge can be suppressed. But addition of a complexing agent may lower the power density and increase battery cost. Meanwhile, complexing agents are often organic materials with toxicity, which may lead to battery failure and environment issues. Therefore, they are not used in PBB, and Regenesys technology simply relies on sealing the stacks and tanks to prevent bromine vapors. In view of the low energy density of the bromine cathode, Zhang et al. (2012a) have extended the cathode reaction to a polyhalide system (Br– redox being predominant) and improved the cathode power density significantly.
9.3.2
Solution chemistry and electrochemistry of polysulfides
There are a number of ion equilibriums in the polysulfide solution. According to Licht’s model (Giggenbach, 1971), the main anions in the polysulfide solution are OH–, HS–, S4 2 , and S3 2 . This point is derived from the relative activity of H2O, alkali-metal cations, OH–, Hþ, H2S, HS–, and Sn 2 (n ¼ 1-5) calculated on the basis of given concentrations of sulfur, sulfides, and hydroxide ions. Polysulfide may decompose into sulfide and thiosulfate with a decomposition rate closely related to temperature, ratio of elemental sulfur to total sulfur, and pH value of the solution. It decomposes faster at temperatures above 150 C and is even metastable at room temperature, with a pH value higher than 8. With electrode overpotential above 200 mV, sulfide can be partially oxidized to thiosulfate and sulfate in addition to polysulfides. In addition, protons in the catholyte may diffuse through the ion exchange membrane into the anolyte, resulting in its pH drop and change of composition, with the primary change being the increase of hydrogen sulfide (H2S) content. Lessner (1980) proposed a formula describing the relationship between the pH value and the maximum sulfur-solvation number (Xmax) of the polysulfide solution, which is shown as follow:
Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
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X5
ðn 1Þ Sn 2 6 Xmax ¼ 2 X5 2 6 6 10 ½H2 S þ ½HS þ S þ S þ 1:75 n¼2 n ½OH n¼2
When the pH drops below 9.5, the sulfur-solvation number will be smaller than 3 and the Xmax will decrease rapidly with pH. If the sulfur content in the catholyte exceeds Xmax, sulfur will precipitate and deposit on the electrode surface, resulting in the rapid increase of charge overpotential. Ideally, only Naþ ions transport through the cation exchange membrane during charge/discharge of PBB; this, however, can hardly be achieved in practice because Hþ size is much smaller than that of Naþ, and thus Hþ diffusion through the membrane is inevitable. Therefore, pH imbalance during PBB operation cannot be avoided, and for prolonged cycling, the pH of the anolyte must be adjusted regularly. One thing for sure is that, at the early stage of PBB operation, [OH] in the anolyte should be kept at an adequately high level to ensure normal operation of the battery. The multiple equilibriums in the polysulfide solution make the redox reaction of polysulfide very complicated; any factor influencing the equilibrium will influence the electrochemical reactions. Take the sulfide oxidation on a platinum electrode as an example: sulfide is first converted to PtS2 and S at the electrode surface, forming a passivating layer that must be oxidized or reduced before sulfide oxidation proceeds further. Szynkarczuk et al. (1994, 1995) carried out a cyclic voltammetry study on the redox of HS– on a platinum electrode and found that the reaction proceeds in two steps: HS– is first oxidized to intermediate polysulfide, and further to elemental sulfur. Formation of an elemental sulfur passivation layer, confirmed by electrochemical impedance spectroscopy, dramatically lowers the overall oxidation rate. Dissolution or removal of the elemental sulfur passivation layer is the ratecontrolling step for sulfide oxidation. Scientists have proposed various methods to tackle this bottleneck, including chemical self-dissolution by polysulfide formation between sulfide and elemental sulfur, indirect electrolysis, and organic solvent dissolution. Generally speaking, the extent of electrode passivation is closely related to the electrode properties and the solubility of elemental sulfur in the electrolyte solution.
9.4
Electrode materials
The electrode is one of the key components for a PBB. Electrode properties such as electrochemical activity, stability, mass-transfer structure, conductivity, mechanical strength, and cost are all important factors that determine whether PBB can find practical applications. In fact, the positive and negative redox couples of PBB have been employed in other energy devices, including zinc-bromine and hydrogen-bromine batteries as well as solar cell and metal-sulfur batteries, so the electrode materials have been well studied.
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9.4.1
Advances in Batteries for Medium- and Large-scale Energy Storage
Bromine cathode materials
Bromine is strongly corrosive, so the material for the bromine electrode should be highly corrosion resistant in addition to being adequately active. Bromine electrode materials mainly include carbon felt, graphite felt, carbon cloth, and carbon paper, which are highly abundant and easily available. These materials possess large porosity and high activity for electrode reactions, and can be used to fabricate threedimensional electrodes. Their compressibility also allows for electrode integration with carbon-plastic bipolar plates, which can reduce interface resistance. Meanwhile, these materials are of low cost and often used as the cathode of a PBB (Zhou et al., 2006). Shao et al. (2009) have explored the possibility of using carbon nanotube as the catalytic material for a vanadium-bromine flow battery, finding that it can dramatically improve the electrochemical activity of the Br2/Br– reaction. Carbon felt is a porous material that can provide high surface area for redox reactions. The main parameters for assessing carbon felt electrode performance include the electrode thickness, length and width, pore diameter, porosity, diffusion coefficient, and flow rate. Surface modifications of carbon felt by different chemical methods are necessary to achieve favorable electrode properties. These methods include oxidative treatment with concentrated alkali, KMnO4, concentrated HNO3, and H2O2; modifications can also be done with plasma treatment, electrochemical oxidation, thermal treatment, and so on. It is shown that thermally and acid-treated carbon felts have more abundant CdOH and dCOOH moieties, the former being predominant among all the functional groups on the electrode surface. CdOH can facilitate electron transfer and thus improve the electrochemical activity of carbon felt significantly.
9.4.2
Sulfur anode materials
The sulfur anode is the place where the polysulfide ions undergo electrochemical reactions. The materials for the sulfur anode include CoS, PbS, CuS, NiS, and Ni3S2. The electrode can be fabricated via three methods: (1) thermal treating of a thin metal foil in an inert gas followed by reaction with mixed H2S/H2, (2) transition metal electroplating on a metal electrode followed by vulcanization, and (3) deposition of Ni, Co, Mo, or their sulfides on high-surface-area metal mesh to produce an active layer. Morrissey and Ward (2002) have developed a method using soluble salt such as sodium chloride as a porogen to fabricate a three-dimensional (3D) CuS or Ni3S2 electrode. In their method, the porogen is mixed with a metal sulfide; the resulting mixture is then compressed and thermally molded; subsequent porogen removal by dissolution gives the formation of a 3D porous electrode. With this electrode, the PBB shows charge and discharge overpotential of 100 and 30 mV, respectively, both at 40 mA cm–2. Activated carbon can be another important class of material for anode fabrication. Calver et al. (1999) have developed a shuck-type activated carbon for sulfur reduction at the anode. Its specific surface area and pore volume are 1000-1100
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and 0.6-0.7 cm3 g–1, respectively. With poly(vinylidene fluoride) or high-density polyethylene as a binder (content below 25%), the above carbon material was made into an electrode, which produced an overpotential of 40-75 mV at 40 mA cm–2 for sulfur reduction. However, the PBB assembled with this electrode only yielded an energy efficiency of 56% and a voltage efficiency of 50% at 60 mA cm–2. Zhao et al. (2005) studied the activity of iron, nickel, cobalt, lead, and graphite for the redox reaction of polysulfide. Electrochemical studies show that cobalt, nickel, and lead outperform iron and graphite in catalytic activity. Single cells using nickel foam or nickel- or cobalt-coated carbon felt as anode materials yielded an energy efficiency 20% higher than that of the cells using pure carbon felt without a catalytic layer. This means the cobalt and nickel catalysts can lower the polarization of polysulfide redox reactions and improve cell performance significantly.
9.5
Ion-conductive membrane separators for PBBs
An ionic exchange membrane is typically used in PBB as the anolyte-catholyte separator to prevent self-discharge of the battery, and to allow selective transport of sodium ions for charge balance during battery operation. Conductivity and selectivity of the membrane are equally important: high conductivity helps reduce ohmic polarization and thus improves voltage efficiency of the cell, while high selectivity improves the cell’s coulombic efficiency and helps maintain good capacity retention. The charge process of a PBB produces strongly corrosive bromine at the cathode, so the membrane separator should be highly stable against the oxidation by bromine. A perfluorinated sulfonic membrane such as DuPont’s Nafion, widely used in fuel cells, possesses excellent proton conductivity and chemical stability and therefore is often used in PBBs. However, this membrane swells much, and therefore anions in the catholyte and anolyte may diffuse across the membrane together with sodium cations, resulting in reduced coulombic efficiency and also lowered capacity due to cross-contamination of electrolyte solutions. Zhao et al. carried out TG-MS (m/z ¼ 64, SO2) studies on the Nafion-117 membrane before and after use in a PBB to ascertain the anion transport phenomenon. As shown in Figure 9.2, the SO2 signal detected from the pristine membrane reaches its peak value at around 520 C and is attributed to the sulfonate groups, while SO2 from the used membrane starts to appear at 200 C and reaches its first peak value at around 380 C, which is supposed to come from sources other than the membrane itself. The SO2 should be produced by the reaction of the Br3– ions, which permeate the membrane, with the sulfonate groups. An elemental analysis gives a sulfur content of 2.97 wt% in the pristine Nafion membrane, and this value increases to 3.25 wt% in the used membrane. To address the above issue, researchers from the U.S. State Power Corporation successfully modified the perfluorinated sulfonic membrane by surface attachment of insoluble bromides or sulfides of silver, tungsten, or aluminum. The modified membrane reduced anion crossover to some extent (Cooley and D’agostino, 1995).
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1´10-11
1.0 1 2
Line 1,3: new Nafion-117 Line 2,4: used Nafion-117
0.6
1´10-12
4 0.4
Intensity / A
Weight ratio
0.8
3 0.2
0.0
0
400 800 Temperature (°C)
1´10-13 1200
Figure 9.2 TG-MS curves of new and used Nafion-117 membrane.
However, such crossover still persists. Considering this drawback and its high cost, the Nafion membrane is not an ideal choice for practical application in PBBs. It is imperative to develop membranes with high sodium ion conductivity and selectivity and low cost for improvement of PBB performance. Recently, Professor Zhang and his team at the Chinese Academy of Sciences achieved a success, after years of efforts, in the development of a low-cost, porous ion-conductive membrane as a new generation separator for all-VFBs (Zhang et al., 2011, 2012b). They exhibited high ion conductivity and high selectivity, and therefore yielded higher coulombic efficiency, voltage efficiency, and energy efficiency in VFBs than Nafion. Such membranes work on the basis of size exclusion and charge repulsion effects, and therefore allow passage of protons only and prevent transport of vanadium ions, which are larger in size than protons. Professor Zhang’s achievement is highly meaningful and instructive for the development of highperformance PBB membranes.
9.6
PBB applications and performance
In 1984, the PBB was invented by Remick in the U.S. In the early 1990s, a UK company, Innogy, invested a lot in this technology for development of a large-scale energy storage system; they have successfully developed 5-, 20-, and 100-kW PBB stacks and energy storage systems to demonstrate the application of PBB technology. In 1996, a 1 MW PBB energy storage system was demonstrated at the Aberthaw power station in South Wales. Innogy constructed the world’s first 12 MW/120 MWh energy storage system in August 2000; it was linked with a 680 MW gas turbine power station and can meet the daily power demand for 10,000 homes. This company also signed a
Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
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Figure 9.3 The one-kilowatt scale PBB battery energy storage system from DICP.
contract with the U.S. Tennessee Valley Authority in 2001 to construct a 12 MW/ 120 MWh PBB system as an unusual-time power supply for the Columbian Air Base. Dalian Institute of Chemical Physics (DICP) under the Chinese Academy of Sciences started PBB research and development in 2000. DICP has developed highactivity electrocatalysts, low-cost electrode materials, and highly stable electrolyte solutions. With these materials, the 100 and 1000 W PBB stacks have been constructed (Figure 9.3), which have yielded good cycle performance. Their coulombic efficiency, voltage efficiency, and energy efficiency at 40 mA cm–2 has reached 96.1%, 84.3%, and 81%, respectively. In particular, the stack was able to deliver a maximum power of 4 kW; its single cells showed uniform performance with a voltage deviation of less than 1.6%. Despite this progress, DICP later stopped working on PBBs due to technical and environmental issues such as electrolyte crossover, strong permeability and corrosion of bromine, and the bad odor of sulfide materials. Regenesys PBB modules of series XL200.911 and XL200.921 have successfully achieved over 1500 operational hours (Regenesys utility scale energy storage). Another series (XL200.769i) had voltage and current profiles reported, which did not simply follow charge and discharge patterns but also increased and decreased levels of charge according to practical requirements; the sub-stacks within module XL200.769i showed smaller differences in voltages for cells 2-22 through cells 182-202 during the first 150 min of operation (Regenesys utility scale energy storage).
9.7
Summary and future trends
Low cost of the electrolyte makes PBBs a viable technology for large-scale energy storage, but only if expensive bromine complexing agents are not employed to bind bromine vapor that can form during battery operation. However, a PBB employs electrolytes containing different types of active species; this produces serious crosscontamination of electrolytes and lowers the efficiency and capacity for energy
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Advances in Batteries for Medium- and Large-scale Energy Storage
storage. Meanwhile, charge balance and circuit completion are accomplished with the transport of sodium cations, which are larger in size than protons and thus experience higher resistance when traveling through the membrane separator. This problem leads to low power density of the battery; therefore, relatively more material is needed to construct a large-scale PBB system, making this technology disadvantageous in terms of fabrication cost. Meanwhile, bromine in the catholyte and hydrogen sulfide evolved in the anolyte can lead to environmental pollution. For these reasons, most R&D efforts in this area have ceased and more attention has focused on other flow cell chemistries. Practical application and commercialization of PBBs relies heavily on the development of low-cost, durable membranes with high sodium ion conductivity and high sodium ion selectivity. It also relies on electrode materials of high electrochemical activity and chemical stability. Development of suitable seal material and optimization of electrode structure and stack design are also important tasks.
References Calver, T.J., Male, S.E., Mitchell, P.J., Whyte, I., 1999. Carbon based electrodes. GB Patent, 2337150. Cooley, G.E., D’agostino, V.F, 1995. A modified cation exchange membrane for electrochemical cells and method for the preparation of such membrane. Patent WO:9528745, October 26. Giggenbach, W., 1971. Optical spectra of highly alkaline sulfide solutions and the second dissociation constant of hydrogen sulfide. Inorg. Chem. 10 (7), 1333. Innogy to commercialize energy storage on both side of the Atlantic, http://www.investegate.co. uk/ArticlePrint.aspx?id=200008210858137279P. Lessner, P.M., 1980. Kinetics and transport processes in aqueous polysulfide electrode reactions (Ph.D. thesis). University of California, Berkeley, USA. Morrissey, P.J., Ward, N.J, 2002. Regenerative fuel cell with pH control. GB Patent, 2374722. Price, A., Bartley, S., Male, S., Cooley, G., 1999. Novel approach to utility scale energy storage. Power Eng. J. 13 (3), 122–129. Regenesys utility scale energy storage, http://www.dti.gov.uk/renewables/publications/pdfs/ kel00246moduletestprogramme.pdf. Remick, R.J., Ang, P.G.P., 1984. Electrically rechargeable anionically active reduction-oxidation electrical storage-supply system. US Patent, 4485154. Shao, Y.Y., Engelhard, M., Lin, Y.H., 2009. Electrochemical investigation of polyhalide ion oxidation–reduction on carbon nanotube electrodes for redox flow batteries. Electrochem. Commun. 11, 2064. Szynkarczuk, J., Komorowski, P.G., Donini, J.C., 1994. Redox reactions of hydrosulphide ions on the platinum electrode—I. The presence of intermediate polysulphide ions and sulphur layers. Electrochim. Acta 39 (15), 2285. Szynkarczuk, J., Komorowski, P.G., Donini, J.C., 1995. Redox reactions of hydrosulphide ions on the platinum electrode—II. An impedance spectroscopy study and identification of the polysulfide intermediate. Electrochim. Acta 40 (4), 487. Thaller, L.H., 1976. Electrically rechargeable redox flow cell. US Patent, 3996064. Zhang, H.Z., Zhang, H.M., Li, X.F., 2011. Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs)? Energy Environ. Sci. 4, 1676–1679.
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Zhang, L.Q., Lai, Q.Z., Zhang, J.L., Zhang, H.M., 2012a. A high-energy-density redox flow battery based on zinc/polyhalide chemistry. ChemSusChem 5, 867–869. Zhang, H.Z., Zhang, H.M., Li, X.F., Mai, Z.S., Wei, W.P., 2012b. Silica modified nanofiltration membranes with improved selectivity for redox flow battery application. Energy Environ. Sci. 5, 6299–6303. Zhao, P., Zhang, H.M., Zhou, H.T., Yi, B.L., 2005. Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes. Electrochim. Acta 51 (6), 1091. Zhou, H.T., Zhang, H.M., Zhao, P., Yi, B.L., 2006. Novel cobalt coated carbon felt as high performance negative electrode in sodium polysulfide/bromine redox flow battery. Electrochemistry 74 (4), 296.
9
H. Zhang Chinese Academy of Sciences, Dalian, China
9.1
Introduction
This chapter reviews key aspects of polysulfide-bromine batteries (PBBs) as a candidate energy storage technology including their: l
l
l
l
working principles, technological development, key materials (membrane separator, electrolyte solutions, and electrodes), performance and applications.
The low cost of electrolyte is an advantage of PBBs provided that expensive bromine complexing agents are not employed to bind bromine vapor that can form during battery operation. However, there are several issues restricting practical and widespread application of PBBs: l
l
l
first, its electrolytes contain different types of active species, which may result in serious cross-contamination, and consequently low efficiency and device capacity; second, sodium ion transport through the membrane separator experiences large resistance compared with proton transport, leading to low power density of battery, and therefore relatively more material is needed to construct large-scale PBB systems, making this technology disadvantageous in terms of cost; and lastly, bromine in the catholyte and hydrogen sulfide evolved in the anolyte can lead to environmental pollution.
For these reasons, most R&D efforts in this area have ceased and more attention has focused on other flow cell chemistries. Promise of PBB technology will rely on breakthroughs in the development of: l
l
l
l
low-cost, durable membranes with high sodium ion conductivity and selectivity, electrode materials with high electrochemical activity and stability, suitable seal material, optimization of electrode structure and stack design.
This chapter reviews key research in these areas. Advances in Batteries for Medium- and Large-scale Energy Storage. http://dx.doi.org/10.1016/B978-1-78242-013-2.00009-1 Copyright © 2015 Elsevier Ltd. All rights reserved.
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9.2
Advances in Batteries for Medium- and Large-scale Energy Storage
PBBs: principles and technologies
First conceptualized by Thaller in 1976, the flow battery is known as an electrochemical device for conversion between chemical energy and electricity via the redox reactions of different active species stored in electrolytes. Its charge process involves an oxidation reaction occurring on the cathode and a reduction reaction on the anode; these reactions reverse during discharge. The catholyte and anolyte of flow batteries, both containing active species (ions of different valence states) for the electrode reactions, are stored in two tanks and circulated within the battery. Based on the states of active species in the anode and cathode half-cells, flow batteries can be categorized into all-liquid types (including the all-vanadium flow battery (VFB), iron-chromium flow battery, and polysulfide-bromine flow battery) and deposition (or half-liquid) types (such as the zinc-nickel flow battery). Unlike conventional secondary batteries with active electrodes, all-liquid flow batteries use inert electrodes that are simply placed on which the electrode reactions occur; such batteries feature state-of-valence changes but no state-of-condensation change of the active species. By contrast, a deposition flow battery involves alternate deposition and dissolution of metals during battery operation. The PBB falls within the all-flow category. The PBB employs aqueous alkaline solutions of sodium polysulfide (Na2Sx) and sodium bromide (NaBr) as the anolyte and catholyte, respectively. Its redox couples are Sx þ 1 2 =Sx 2 at the anode and Br2/Br at the cathode. Br2 exists as Br3 in the catholyte, and the elemental sulfur combines with sulfide anions to form polysulfides in the anolyte. The PBB works via the redox reaction of anions, rather than cations. The catholyte and the anolyte are separated with an ion exchange membrane, which transports sodium ions between them to complete the battery circuit. The electrode reactions during charge and discharge are shown in Figure 9.1 and described as follows:
Negative electrode
Ion exchange membrane
Positive electrode
Charge Na+
Na+ Discharge
Br3−
Sn2− −2e−
+2e− Sn+12−
+2e−
−2e− 3Br −
Figure 9.1 Redox reactions on the positive and negative electrodes of PBB.
Polysulfide-bromine flow batteries (PBBs) for medium- and large-scale energy storage
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At the cathode : 2NaBr , 2e þ Br2 þ 2Naþ
(9.1)
At the anode : 2Naþ þ ðx 1ÞNa2 Sx þ 2e , xNa2 Sx1 , x ¼ 2-4
(9.2)
Overall : 2NaBr þ ðx 1ÞNa2 Sx , Br2 þ xNa2 Sx1 , x ¼ 2-4
(9.3)
The standard electrode potential of reaction (9.1) is 1.087 V, while that of (9.2) is 0.428 V, so the standard EMF of the PBB is 1.515 V. Due to variations in the electrolyte concentrations and the state of charge or discharge, a PBB typically shows an open circuit voltage of 1.54-1.60 V. Compared with an all-VFB, a PBB uses low-cost materials such as polysulfide and bromine, and therefore it has aroused considerable research interest and funding in the past. Remick (Remick and Ang, 1984) was the first to propose flow batteries with polysulfide as the anode redox couple and halide as the cathode redox couple. Innogy (Price et al., 1999), a British company, registered Regenesys™ as the trademark for PBB energy storage technology, and has developed three PBB stacks with different powers. The stack structure is similar to that of a fuel cell stack, which consists of electrodes, bipolar plates, ion exchange membranes, electrode frames, and end plates. The PBB employs anion redox couples of different elements for energy conversion; this often causes problems because anion cross-contamination can hardly be avoided with all available membranes, including the perfluorinated sulfonic Nafion membrane, which results in rapid fading of the batteries’ capacity. To overcome this drawback, Innogy developed methodologies such as sulfate-based removal of contaminant, pH-based recovery of active species, and water control. These methods have not been reported to result in electrolyte imbalance or battery performance decay, but their influences on PBB performance must be studied to achieve a thorough understanding of the operational characteristics of the system and provide a good basis for system design and choice of operating conditions.
9.3
Electrolyte solution and its chemistry
Electrolyte is the medium for energy storage. Typically, it consists of active species dissolved in a supporting electrolyte. The electrolyte solution is stored in a tank, pumped into the battery and then goes back to the tank. It meets the porous electrodes inside the battery and undergoes electrochemical reactions in response to the external electric current changes; this is accompanied with changes in the valence states of the active species so that energy storage and release are achieved. In the above process, electrons released travel through the external circuit, and the sodium ions permeate the membrane to achieve electroneutrality. Common catholytes and anolytes for PBBs are NaBr (typically 4 mol L1) and alkaline Na2S (typically 1.5 mol L1) solutions, respectively, with de-ionized water as the supporting electrolyte; their volume and concentration can be adjusted according to the PBB capacity requirements.
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The solution chemistry and electrochemistry of bromide ions
The Br– ion’s state in the catholyte and its electrochemistry is relatively simple. During charge, Br– ions release electrons and form Br2, whose solubility is low in water but high in NaBr solution because Br2 can complex with Br– to form Br3 or Br5 . Such complex anions, driven by the electric field and concentration gradient, can diffuse through the ion exchange membrane to the anode and react with the active species therein; this phenomenon, known as self-discharge, is undesirable because it leads to energy efficiency decay and capacity fading. Meanwhile, the differing pH values of the catholyte (initial pH ¼ 3) and the anolyte (initial pH ¼ 12) also affect the cycle life of the battery; this is because during charge/discharge the pH of the catholyte increases and results in loss of bromine as the active material. In zinc-bromine flow batteries, which also employ Br2/Br– as the cathode redox couple, a quaternary ammonium is often added to the catholyte as a complexing agent. The resulting Q Br3 complex is a solid and can deposit at the bottom of catholyte so that Br3 diffusion and battery self-discharge can be suppressed. But addition of a complexing agent may lower the power density and increase battery cost. Meanwhile, complexing agents are often organic materials with toxicity, which may lead to battery failure and environment issues. Therefore, they are not used in PBB, and Regenesys technology simply relies on sealing the stacks and tanks to prevent bromine vapors. In view of the low energy density of the bromine cathode, Zhang et al. (2012a) have extended the cathode reaction to a polyhalide system (Br– redox being predominant) and improved the cathode power density significantly.
9.3.2
Solution chemistry and electrochemistry of polysulfides
There are a number of ion equilibriums in the polysulfide solution. According to Licht’s model (Giggenbach, 1971), the main anions in the polysulfide solution are OH–, HS–, S4 2 , and S3 2 . This point is derived from the relative activity of H2O, alkali-metal cations, OH–, Hþ, H2S, HS–, and Sn 2 (n ¼ 1-5) calculated on the basis of given concentrations of sulfur, sulfides, and hydroxide ions. Polysulfide may decompose into sulfide and thiosulfate with a decomposition rate closely related to temperature, ratio of elemental sulfur to total sulfur, and pH value of the solution. It decomposes faster at temperatures above 150 C and is even metastable at room temperature, with a pH value higher than 8. With electrode overpotential above 200 mV, sulfide can be partially oxidized to thiosulfate and sulfate in addition to polysulfides. In addition, protons in the catholyte may diffuse through the ion exchange membrane into the anolyte, resulting in its pH drop and change of composition, with the primary change being the increase of hydrogen sulfide (H2S) content. Lessner (1980) proposed a formula describing the relationship between the pH value and the maximum sulfur-solvation number (Xmax) of the polysulfide solution, which is shown as follow:
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X5
ðn 1Þ Sn 2 6 Xmax ¼ 2 X5 2 6 6 10 ½H2 S þ ½HS þ S þ S þ 1:75 n¼2 n ½OH n¼2
When the pH drops below 9.5, the sulfur-solvation number will be smaller than 3 and the Xmax will decrease rapidly with pH. If the sulfur content in the catholyte exceeds Xmax, sulfur will precipitate and deposit on the electrode surface, resulting in the rapid increase of charge overpotential. Ideally, only Naþ ions transport through the cation exchange membrane during charge/discharge of PBB; this, however, can hardly be achieved in practice because Hþ size is much smaller than that of Naþ, and thus Hþ diffusion through the membrane is inevitable. Therefore, pH imbalance during PBB operation cannot be avoided, and for prolonged cycling, the pH of the anolyte must be adjusted regularly. One thing for sure is that, at the early stage of PBB operation, [OH] in the anolyte should be kept at an adequately high level to ensure normal operation of the battery. The multiple equilibriums in the polysulfide solution make the redox reaction of polysulfide very complicated; any factor influencing the equilibrium will influence the electrochemical reactions. Take the sulfide oxidation on a platinum electrode as an example: sulfide is first converted to PtS2 and S at the electrode surface, forming a passivating layer that must be oxidized or reduced before sulfide oxidation proceeds further. Szynkarczuk et al. (1994, 1995) carried out a cyclic voltammetry study on the redox of HS– on a platinum electrode and found that the reaction proceeds in two steps: HS– is first oxidized to intermediate polysulfide, and further to elemental sulfur. Formation of an elemental sulfur passivation layer, confirmed by electrochemical impedance spectroscopy, dramatically lowers the overall oxidation rate. Dissolution or removal of the elemental sulfur passivation layer is the ratecontrolling step for sulfide oxidation. Scientists have proposed various methods to tackle this bottleneck, including chemical self-dissolution by polysulfide formation between sulfide and elemental sulfur, indirect electrolysis, and organic solvent dissolution. Generally speaking, the extent of electrode passivation is closely related to the electrode properties and the solubility of elemental sulfur in the electrolyte solution.
9.4
Electrode materials
The electrode is one of the key components for a PBB. Electrode properties such as electrochemical activity, stability, mass-transfer structure, conductivity, mechanical strength, and cost are all important factors that determine whether PBB can find practical applications. In fact, the positive and negative redox couples of PBB have been employed in other energy devices, including zinc-bromine and hydrogen-bromine batteries as well as solar cell and metal-sulfur batteries, so the electrode materials have been well studied.
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Bromine cathode materials
Bromine is strongly corrosive, so the material for the bromine electrode should be highly corrosion resistant in addition to being adequately active. Bromine electrode materials mainly include carbon felt, graphite felt, carbon cloth, and carbon paper, which are highly abundant and easily available. These materials possess large porosity and high activity for electrode reactions, and can be used to fabricate threedimensional electrodes. Their compressibility also allows for electrode integration with carbon-plastic bipolar plates, which can reduce interface resistance. Meanwhile, these materials are of low cost and often used as the cathode of a PBB (Zhou et al., 2006). Shao et al. (2009) have explored the possibility of using carbon nanotube as the catalytic material for a vanadium-bromine flow battery, finding that it can dramatically improve the electrochemical activity of the Br2/Br– reaction. Carbon felt is a porous material that can provide high surface area for redox reactions. The main parameters for assessing carbon felt electrode performance include the electrode thickness, length and width, pore diameter, porosity, diffusion coefficient, and flow rate. Surface modifications of carbon felt by different chemical methods are necessary to achieve favorable electrode properties. These methods include oxidative treatment with concentrated alkali, KMnO4, concentrated HNO3, and H2O2; modifications can also be done with plasma treatment, electrochemical oxidation, thermal treatment, and so on. It is shown that thermally and acid-treated carbon felts have more abundant CdOH and dCOOH moieties, the former being predominant among all the functional groups on the electrode surface. CdOH can facilitate electron transfer and thus improve the electrochemical activity of carbon felt significantly.
9.4.2
Sulfur anode materials
The sulfur anode is the place where the polysulfide ions undergo electrochemical reactions. The materials for the sulfur anode include CoS, PbS, CuS, NiS, and Ni3S2. The electrode can be fabricated via three methods: (1) thermal treating of a thin metal foil in an inert gas followed by reaction with mixed H2S/H2, (2) transition metal electroplating on a metal electrode followed by vulcanization, and (3) deposition of Ni, Co, Mo, or their sulfides on high-surface-area metal mesh to produce an active layer. Morrissey and Ward (2002) have developed a method using soluble salt such as sodium chloride as a porogen to fabricate a three-dimensional (3D) CuS or Ni3S2 electrode. In their method, the porogen is mixed with a metal sulfide; the resulting mixture is then compressed and thermally molded; subsequent porogen removal by dissolution gives the formation of a 3D porous electrode. With this electrode, the PBB shows charge and discharge overpotential of 100 and 30 mV, respectively, both at 40 mA cm–2. Activated carbon can be another important class of material for anode fabrication. Calver et al. (1999) have developed a shuck-type activated carbon for sulfur reduction at the anode. Its specific surface area and pore volume are 1000-1100
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and 0.6-0.7 cm3 g–1, respectively. With poly(vinylidene fluoride) or high-density polyethylene as a binder (content below 25%), the above carbon material was made into an electrode, which produced an overpotential of 40-75 mV at 40 mA cm–2 for sulfur reduction. However, the PBB assembled with this electrode only yielded an energy efficiency of 56% and a voltage efficiency of 50% at 60 mA cm–2. Zhao et al. (2005) studied the activity of iron, nickel, cobalt, lead, and graphite for the redox reaction of polysulfide. Electrochemical studies show that cobalt, nickel, and lead outperform iron and graphite in catalytic activity. Single cells using nickel foam or nickel- or cobalt-coated carbon felt as anode materials yielded an energy efficiency 20% higher than that of the cells using pure carbon felt without a catalytic layer. This means the cobalt and nickel catalysts can lower the polarization of polysulfide redox reactions and improve cell performance significantly.
9.5
Ion-conductive membrane separators for PBBs
An ionic exchange membrane is typically used in PBB as the anolyte-catholyte separator to prevent self-discharge of the battery, and to allow selective transport of sodium ions for charge balance during battery operation. Conductivity and selectivity of the membrane are equally important: high conductivity helps reduce ohmic polarization and thus improves voltage efficiency of the cell, while high selectivity improves the cell’s coulombic efficiency and helps maintain good capacity retention. The charge process of a PBB produces strongly corrosive bromine at the cathode, so the membrane separator should be highly stable against the oxidation by bromine. A perfluorinated sulfonic membrane such as DuPont’s Nafion, widely used in fuel cells, possesses excellent proton conductivity and chemical stability and therefore is often used in PBBs. However, this membrane swells much, and therefore anions in the catholyte and anolyte may diffuse across the membrane together with sodium cations, resulting in reduced coulombic efficiency and also lowered capacity due to cross-contamination of electrolyte solutions. Zhao et al. carried out TG-MS (m/z ¼ 64, SO2) studies on the Nafion-117 membrane before and after use in a PBB to ascertain the anion transport phenomenon. As shown in Figure 9.2, the SO2 signal detected from the pristine membrane reaches its peak value at around 520 C and is attributed to the sulfonate groups, while SO2 from the used membrane starts to appear at 200 C and reaches its first peak value at around 380 C, which is supposed to come from sources other than the membrane itself. The SO2 should be produced by the reaction of the Br3– ions, which permeate the membrane, with the sulfonate groups. An elemental analysis gives a sulfur content of 2.97 wt% in the pristine Nafion membrane, and this value increases to 3.25 wt% in the used membrane. To address the above issue, researchers from the U.S. State Power Corporation successfully modified the perfluorinated sulfonic membrane by surface attachment of insoluble bromides or sulfides of silver, tungsten, or aluminum. The modified membrane reduced anion crossover to some extent (Cooley and D’agostino, 1995).
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1´10-11
1.0 1 2
Line 1,3: new Nafion-117 Line 2,4: used Nafion-117
0.6
1´10-12
4 0.4
Intensity / A
Weight ratio
0.8
3 0.2
0.0
0
400 800 Temperature (°C)
1´10-13 1200
Figure 9.2 TG-MS curves of new and used Nafion-117 membrane.
However, such crossover still persists. Considering this drawback and its high cost, the Nafion membrane is not an ideal choice for practical application in PBBs. It is imperative to develop membranes with high sodium ion conductivity and selectivity and low cost for improvement of PBB performance. Recently, Professor Zhang and his team at the Chinese Academy of Sciences achieved a success, after years of efforts, in the development of a low-cost, porous ion-conductive membrane as a new generation separator for all-VFBs (Zhang et al., 2011, 2012b). They exhibited high ion conductivity and high selectivity, and therefore yielded higher coulombic efficiency, voltage efficiency, and energy efficiency in VFBs than Nafion. Such membranes work on the basis of size exclusion and charge repulsion effects, and therefore allow passage of protons only and prevent transport of vanadium ions, which are larger in size than protons. Professor Zhang’s achievement is highly meaningful and instructive for the development of highperformance PBB membranes.
9.6
PBB applications and performance
In 1984, the PBB was invented by Remick in the U.S. In the early 1990s, a UK company, Innogy, invested a lot in this technology for development of a large-scale energy storage system; they have successfully developed 5-, 20-, and 100-kW PBB stacks and energy storage systems to demonstrate the application of PBB technology. In 1996, a 1 MW PBB energy storage system was demonstrated at the Aberthaw power station in South Wales. Innogy constructed the world’s first 12 MW/120 MWh energy storage system in August 2000; it was linked with a 680 MW gas turbine power station and can meet the daily power demand for 10,000 homes. This company also signed a
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Figure 9.3 The one-kilowatt scale PBB battery energy storage system from DICP.
contract with the U.S. Tennessee Valley Authority in 2001 to construct a 12 MW/ 120 MWh PBB system as an unusual-time power supply for the Columbian Air Base. Dalian Institute of Chemical Physics (DICP) under the Chinese Academy of Sciences started PBB research and development in 2000. DICP has developed highactivity electrocatalysts, low-cost electrode materials, and highly stable electrolyte solutions. With these materials, the 100 and 1000 W PBB stacks have been constructed (Figure 9.3), which have yielded good cycle performance. Their coulombic efficiency, voltage efficiency, and energy efficiency at 40 mA cm–2 has reached 96.1%, 84.3%, and 81%, respectively. In particular, the stack was able to deliver a maximum power of 4 kW; its single cells showed uniform performance with a voltage deviation of less than 1.6%. Despite this progress, DICP later stopped working on PBBs due to technical and environmental issues such as electrolyte crossover, strong permeability and corrosion of bromine, and the bad odor of sulfide materials. Regenesys PBB modules of series XL200.911 and XL200.921 have successfully achieved over 1500 operational hours (Regenesys utility scale energy storage). Another series (XL200.769i) had voltage and current profiles reported, which did not simply follow charge and discharge patterns but also increased and decreased levels of charge according to practical requirements; the sub-stacks within module XL200.769i showed smaller differences in voltages for cells 2-22 through cells 182-202 during the first 150 min of operation (Regenesys utility scale energy storage).
9.7
Summary and future trends
Low cost of the electrolyte makes PBBs a viable technology for large-scale energy storage, but only if expensive bromine complexing agents are not employed to bind bromine vapor that can form during battery operation. However, a PBB employs electrolytes containing different types of active species; this produces serious crosscontamination of electrolytes and lowers the efficiency and capacity for energy
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storage. Meanwhile, charge balance and circuit completion are accomplished with the transport of sodium cations, which are larger in size than protons and thus experience higher resistance when traveling through the membrane separator. This problem leads to low power density of the battery; therefore, relatively more material is needed to construct a large-scale PBB system, making this technology disadvantageous in terms of fabrication cost. Meanwhile, bromine in the catholyte and hydrogen sulfide evolved in the anolyte can lead to environmental pollution. For these reasons, most R&D efforts in this area have ceased and more attention has focused on other flow cell chemistries. Practical application and commercialization of PBBs relies heavily on the development of low-cost, durable membranes with high sodium ion conductivity and high sodium ion selectivity. It also relies on electrode materials of high electrochemical activity and chemical stability. Development of suitable seal material and optimization of electrode structure and stack design are also important tasks.
References Calver, T.J., Male, S.E., Mitchell, P.J., Whyte, I., 1999. Carbon based electrodes. GB Patent, 2337150. Cooley, G.E., D’agostino, V.F, 1995. A modified cation exchange membrane for electrochemical cells and method for the preparation of such membrane. Patent WO:9528745, October 26. Giggenbach, W., 1971. Optical spectra of highly alkaline sulfide solutions and the second dissociation constant of hydrogen sulfide. Inorg. Chem. 10 (7), 1333. Innogy to commercialize energy storage on both side of the Atlantic, http://www.investegate.co. uk/ArticlePrint.aspx?id=200008210858137279P. Lessner, P.M., 1980. Kinetics and transport processes in aqueous polysulfide electrode reactions (Ph.D. thesis). University of California, Berkeley, USA. Morrissey, P.J., Ward, N.J, 2002. Regenerative fuel cell with pH control. GB Patent, 2374722. Price, A., Bartley, S., Male, S., Cooley, G., 1999. Novel approach to utility scale energy storage. Power Eng. J. 13 (3), 122–129. Regenesys utility scale energy storage, http://www.dti.gov.uk/renewables/publications/pdfs/ kel00246moduletestprogramme.pdf. Remick, R.J., Ang, P.G.P., 1984. Electrically rechargeable anionically active reduction-oxidation electrical storage-supply system. US Patent, 4485154. Shao, Y.Y., Engelhard, M., Lin, Y.H., 2009. Electrochemical investigation of polyhalide ion oxidation–reduction on carbon nanotube electrodes for redox flow batteries. Electrochem. Commun. 11, 2064. Szynkarczuk, J., Komorowski, P.G., Donini, J.C., 1994. Redox reactions of hydrosulphide ions on the platinum electrode—I. The presence of intermediate polysulphide ions and sulphur layers. Electrochim. Acta 39 (15), 2285. Szynkarczuk, J., Komorowski, P.G., Donini, J.C., 1995. Redox reactions of hydrosulphide ions on the platinum electrode—II. An impedance spectroscopy study and identification of the polysulfide intermediate. Electrochim. Acta 40 (4), 487. Thaller, L.H., 1976. Electrically rechargeable redox flow cell. US Patent, 3996064. Zhang, H.Z., Zhang, H.M., Li, X.F., 2011. Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs)? Energy Environ. Sci. 4, 1676–1679.
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Zhang, L.Q., Lai, Q.Z., Zhang, J.L., Zhang, H.M., 2012a. A high-energy-density redox flow battery based on zinc/polyhalide chemistry. ChemSusChem 5, 867–869. Zhang, H.Z., Zhang, H.M., Li, X.F., Mai, Z.S., Wei, W.P., 2012b. Silica modified nanofiltration membranes with improved selectivity for redox flow battery application. Energy Environ. Sci. 5, 6299–6303. Zhao, P., Zhang, H.M., Zhou, H.T., Yi, B.L., 2005. Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes. Electrochim. Acta 51 (6), 1091. Zhou, H.T., Zhang, H.M., Zhao, P., Yi, B.L., 2006. Novel cobalt coated carbon felt as high performance negative electrode in sodium polysulfide/bromine redox flow battery. Electrochemistry 74 (4), 296.