Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India Design, Fabrication and Electrochemical performance of Soluble Lead Redox-Flow Battery for Energy Storage

Shivangi Kosta R Sneha Kuldeep Rana Electrical Appliances Technology Electrical Appliances Technology Electrical Appliances Technology Division Division Division Central Power Research Institute Central Power Research Institute Central Power Research Institute Bangalore, India Bangalore, India Bangalore, India [email protected] [email protected] [email protected]

Single electrolyte, single pump and absence of membrane Abstract - Flow batteries are being considered as a potential reduces the cost of the system and makes it less candidate for large scale energy storage system, however, complicated as compared with other flow batteries. It can main issue with them is their higher cost due to the electrode also be operated without any separable membrane leading materials and membrane used in them. In this regards, current work is focused on the development of cost effective to decrease in the cost as well as the usage of cell flow batteries. In order to reduce the cost of flow batteries, equipment; overall simplifies the cell design. The low cost graphitic carbon was used for both positive and performance of the cell is completely based on the negative electrodes to fabricate the soluble lead redox flow fabrication and structure of the electrodes and the batteries (100 cm2) without using any separator. Lead electrolyte composition [8]. The reaction site and the methane sulfonate (1M) and methane sulfonic acid (1M) were current collector membrane determine battery power used as electrolytes, which plays an important role in whereas volume of the electrolyte tank gives battery reduction and oxidation process of redox flow batteries. The energy. The reason why soluble lead redox flow batteries charge/discharge behavior has been studied at constant are uniquely opted using single electrolyte is because a current of 500 mA, which showed an average efficiency of 79 % with an initial discharge capacity of 200 mAh. The cell single species lead (II) ion is both oxidized and reduced capacity decreases during the 14-15th cycles and becomes 110 upon charging and discharging of the cell. The electrolyte mAh, this loss in capacity has been studied after cut down consists of high concentrated lead methanesulfonate (LMS) analysis of cell. and methanesulfonic acid (MSA) circulated between the electrodes by pumping it [2]. During the charge Pb2+ is Keywords – Energy Storage, Electrode Fabrication, Membrane oxidized leading to deposition of lead-oxide on positive free SLRFB, electrochemical performance. electrode surface and metallic lead at negative electrode. I. INTRODUCTION During the discharge, the electrodeposits dissolve back into electrolyte [9]. With the growing energy demand worldwide for The following reversible reactions take place at the cleaner and cheaper energy, energy storage plays a crucial electrodes as described above- role to facilitate the penetration of renewables into Negative: Pb2+ + 2e- → Pb E0 = -0.125 V (1) 2+ + electricity network. Redox flow batteries (RFB) are Positive: Pb + 2H2O - 2e- → PbO2 + 4H (2) becoming popular choice for large scale energy storage E0 = 1.468 V 2+ + including load levelling and reserve electricity supplies as Overall: 2Pb + 2H2O → Pb + PbO2 + 4H (3) well as power sources for traction. RFBs are well- E0 = 1.593 V established energy storage technologies which is accessible RFBs are comparatively different from other lead acid worldwide, due to its flexibility in decoupling energy and batteries as they have different chemistry and performance power. Various chemistries of flow batteries such as characteristics, as well, used for different industrial vanadium, Fe-Cr, Fe-V, Zn-Br, Br-polysulphide are applications. The two major problems associated with existing today, however among all other technologies, SLRFBs are dendrite growth of Pb at the negative vanadium redox system is more matured and developed electrode and partial irreversibility of PbO2 at the positive system [1]. All the flow battery system employed an ion- electrode, as reported in earlier papers [10]. The SLRFB exchange membrane, which are expensive and increases has been developed in different size and configuration, as the complexity associated with unwanted ion transport reported previously [11]. through this membrane. Further different electrolytes being Here we report the electrochemical behaviour of a used for the anode and cathode as anolyte and catholyte, single cell without using any separable membrane and any leading to increase in the complexity of the system. additives. The graphite electrodes of 10 x 10 cm were Therefore, they require further modification to make it as fabricated and designed to form a single cell. The higher commercial application by lowering the cost. concentration of electrolyte has been reported [2,3], Further attempts are being made to improve the therefore, 0.5 M of lead methanesulfonate and 0.5 M of performance and cost reduction of flow batteries. The methanesulfonic acid has been considered to eventually soluble lead redox flow battery (SLRFB) system has know the behaviour of lead ions with different shown the much promise of reducing cost. It is based on configurations. 2+ 2+/ the two redox couples Pb/Pb and Pb PbO2, which can be fabricated without using any membrane/separator [2-7].

978-1-5386-6159-8/18/$31.00 ©2018 IEEE Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India II. EXPERIMENT to the G-band, which arises from the in-plane vibrations of carbon atoms; the other peak, at which is around 1350 cm−1 Chemicals Used corresponds to the D-band, arises from the disorder in The electrolyte used in SLRFB has been synthesized graphitic structure. Thus the XRD and Raman spectrum using lead methane sulfonate (LMS, Pb(SO CH ) , 50% of 3 3 2 confirms that the electrode material used for cell H O) and methane sulfonic acid (MSA, HSO CH ). An 2 3 3 fabrication is well crystalline without having any aqueous solution of the electrolyte was prepared using de- impurities. ionised (DI) water and molarity concentration was kept at 0.5 M. The electrolyte was used as synthesized.

TABLE I DIMENSIONS OF SOLUBLE LEAD REDOX FLOW BATTERY Flow cell Width Length (cm) Thickness (mm) parts (cm) Graphite 12 (depth of 2 mm 12 12 Electrodes for each cell) Circulation 10 10 4 area Current 12 12 2.5 Collectors Aluminum 20 20 12 End Plate

Designing of Redox flow single cell A single flow cell of soluble lead redox flow battery (SLRFB) was designed and fabricated, which consists of graphite plates (electrodes), copper current collectors, insulating end plates insulated with current collector by Fig. 1. Optical image of (a) Al end plate (b) Cu current collector (c) Graphite plate (d) FRG gasket (e) Redox flow battery single cell (100 cm2 FRG sheets. The detail dimensions of the SLRFB are listed area) (f) Graphical Side view of Redox flow battery in Table 1. The A rotor pump (Ravel-Peristalic Pump) at a flow rate of 90 ml min-1 was operated for the circulation of electrolyte. The electrolyte was pumped inside the single cell assembly to check the electrochemical behaviour of the cell. The designing and fabrication details of the single cell have been demonstrated in Fig. 1. It consists of two graphite electrodes (Fig. 1c) which are assembled without using any separator. Both the graphite plates have been fabricated in such a way to provide the space for the flow of electrolyte. In this design two graphite plates are stacked with a FRG gasket in between and the active area for the 2 electrode is 100 cm . These graphite plates are provided with inlet and outlet connections for the flow of electrolyte. Fig. 2. (a) XRD pattern of Graphite (b) Raman spectra of The two graphite plates are attached with copper current Graphite. collectors as shown in Fig. 1 (f). Finally the fabricated cell has been connected with electrolyte tank through the Electrochemical Performance of SLRFB single cell peristatic pump. The battery was connected to a Battery The SLRFB single cell was assembled and tested with Life Cycle Tester (LCV16-100-12) and charge/discharge constant current mode. Each electrode had an exposed studies were carried out at electrolyte flow rate of 90 ml surface area of 10 cm x 10 cm. The galvanostatic cycling min-1. tests were performed using Bitrode battery testing system. The electrodes were consistently subjected to a constant III. RESULTS AND DISCUSSIONS current of 500 mA in voltage range of 1.0 to 1.98 V at the flow rate of 90 ml min-1. Before assembling the cell the electrode material used Initially in formation cycles the charge/discharge in cell has been characterized by XRD and Raman capacity was low as shown in Fig. 3 (b), which increases spectrum in order to confirm the crystal structure and and stabilized in next few cycles. The fourth quality of carbon materials used respectively. Carbon based charge/discharge behaviour of the cell is demonstrated in materials are already being used as the material of choice Fig. 3 (a). From the charging profile of cell two different for various energy storage devices [12,13,14]. The XRD charging plateau can be seen clearly, at constant current pattern of electrode materials is shown in Fig 2 (a), which voltage initially increases and then became constant (~1.98 shows a sharp peak at 26.6o which is characteristic V). After reaching the voltage up to 1.98 V, cell has been diffraction peak of crystalline graphite corresponding to discharge at constant current of 500 mA till end voltage of 002 plane, no peaks other than graphite has been observed 1.0 V. The discharge plateau has been observed at around ~ in XRD pattern. The Raman spectrum was analysed in 1.50 V. The charge and discharge capacity was observed order to confirm the disorder and impurity present in around 116 and 114 mAh respectively. However, in later graphite, as shown in Fig. 2 (b). The resulting spectrum cycles it reaches up to 220 mAh. The corresponding shows two sharp peaks: the peak at 1579 cm−1 corresponds coulombic efficiency was observed around 90% [8, 11]. Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India Initially for the first few charge cycle the cell voltage was approximate 1.98 V, in the starting, which became lesser approximate 1.90 V shown in Fig. 3 (b). This phenomenon is associated with the deposition of lead metal and lead oxide over the negative and positive electrodes respectively as reaction is not fully reversible and has been explained in earlier studies [8, 15]. At the end of 15th cycle the discharge capacity was reduced to 140 mAh as shown Fig. 3 (c). From this observation it is clear that the deposition of lead oxide and lead increases with number of charge/discharge cycles at the electrode surface. There are reports which indicates the same trend of phenomenal and mechanism for lead oxide formation and hence results in cell failure. This may be due to thermodynamics and/or kinetics involved within the redox reaction. The other reason of cell failure in this case was corrosion of metal current collector due the leakage of electrolyte during the charge/discharge process. Fig. 3 (d) shows the coulombic efficiency curve of SLRFB single cell. During the formation cycles the efficiency was found to be 40%. In the initial charge/discharge cycle efficiency reached to 90% followed Fig. 3. (a) Cell voltage vs time response for 15 cycles of SLRFB flow cell th by a sudden drop of 40% at the end of the 15 cycle. The at a flow rate of 90 mlmin-1 , degradation in efficiency after 10th cycle (as fall in discharge time create unbalancing of the redox encircled-green) (b) Cell voltage vs time response for initial charge reaction taking place inside the cell and leads to fall in discharge cycle (c) Efficiency vs number of cycles of single flow cell . (d) Discharge Capacity vs number of cycles of single flow cell. overall discharge capacity and efficiency after 15th cycle.

Even though the initial efficiency, and initial cell At the positive side, rough powdery deposit was obtained performance is good, however, in later cycle the due to overlapping of angular crystallites, as can be seen deterioration has been observed which is due to the from Fig. 4 (b) indicating more of β-PbPO deposits upon deposition of lead oxide/metal over the surface of the 2 α- PbPO deposits favored by higher temperature range 333 electrodes. 2 K- 348 K and low charging current density [17]. The X-ray

intensity peaks at ~25° [110], 32° [101], 36° [200], ~49.1° IV. FAILURE ANALYSIS [211] describes the formation of β-PbO as shown in fig. 5 2, (a). The early cycles leads to α-PbO deposits [2], as it is The cell failure just after 15th cycle has been studied 2 deposited from high pH solution it takes place during the by using cut-down analysis of cell in order to understand initial formation cycles. The appearance of α- PbPO is the deposition over both positive and negative electrode 2 very compact and shiny, can be clearly seem from the fig. surface. The cut down analysis of the cell was performed to 4 (c) The diffraction peaks around 31° [002] denotes the examine the change in the overall structure and behavior of formation of pure α- PbO2 [14], as shown in Fig. 5 (b). the cell by opening the cell removing the deposits from both electrode surface, dried the deposits for further To study the morphological behavior of the positive and analysis of the deposit. Fig. 4 shows the appearance of the negative deposited material SEM characterization was th deposited electrodes as well as current collectors after 15 performed. charge/discharge cycles. The surface of both the electrodes was completely covered with the deposition of lead oxide/metal as can be seen from the Fig. 4 (b) and 4 (c). The appearance of the deposited material over the positive (Fig. 4 (b)) and negative (Fig. 4 (c)) electrode was found to be slightly different. Therefore, to study the structural, compositional and morphological characteristics of both the electrodes, XRD, EDAX and SEM characterization was performed. The corrosion over the current collectors can also be easily observed from the Fig. 4 (e) and 4 (f). The lead deposition is polymorphic and consist of α- PbO2 having orthorhombic structure and β-PbO2 having tetragonal rutile structure. The identification of phase composition for α- PbO2 and β- PbO2 was determined by X-ray Diffraction. The XRD pattern for both the electrodes is shown in Fig. 5 (a) and (b). The phase identification was Fig. 4.Photographs of electrodes and current collector surface morphology performed using ICDD data base (α- PbO2 card no. 72- before and after performing the electrochemical test. (a) electrode before 2440 and β- PbO2 card no. 76-0564) [16]. The diffraction testing (b) positive side after testing (c) negative side after testing (d) peak of both the phases is clearly distinguishable within the current collector before testing (e) positive current collector after testing range of 20°- 40°. (f) negative current collector after testing

Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India V. CONCLUSION In this study, we have designed, fabricated and demonstrated the electrochemical performance of single cell based on soluble lead redox system for 15 cycles without using any separator membrane. Further the failure analysis of the cell has been carried out in order to understand the characteristics of the deposited material. The degradation of the electrochemical behaviour such as capacity and coulombic efficiency is caused due to the formation of metallic lead dendrites formation/deposition on the surface of the negative and positive electrodes, which may further lead to short circuiting, and also loss of Fig. 5. XRD data for dendrite formation over the electrode (a) positive active material from the electrolyte. Solving the problem of side (b) negative side (inset includes the optical image of dendrites) the cell failure in initial cycles will make this battery

chemistry more suitable as compared to other RFB system The SEM images of dendrite formation are shown in in terms of low cost and easy fabrication. Due to this it is Fig.6. The nano-crystalline structure of PbO deposition 2 suitable for renewable large scale energy storage has been reported previously in the range of 30-140 nm [19] and 30-50 nm [20]. At the positive side, random oval applications for eg. Solar photovoltaic applications. structure morphology was obtained and the maximum size of one particular oval structure was found to be around 60µm, as shown in fig. 6 (a). The deposited lead oxide was ACKNOWLEDGMENT highly dense in nature which caused the formation of powder layer over the positive electrode. According to This research work is funded and supported by Central XRD, the growth of larger crystallites leads to increase in Power Research Institute, under IHRD scheme of Ministry crystalline size corresponds to β-PbO [18]. 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Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India