A Soluble Lead Flow Battery Ewan Fraser Word Count 2983 1 Introduction In recent years, it has become apparent that human activity has caused the concentration of greenhouse gases in the atmosphere to increase. Currently, electricity generation emits large amounts of greenhouse gases and as electricity consumption is set to continue to rise, renewable, low-carbon generation sources, such as wind and solar power, have become popular technologies for new, low carbon, electricity generation. These renewable sources of electricity, however, have an intermittent generation profile. Methods including interconnection and energy storage, will be required to ensure stability with high levels of renewables capacity on a grid [1]. The European Commission and the US Department of Energy have stated the requirement of energy storage to maintain stable electricity supply [2, 3]. It is estimated that 150 GW of battery storage will be required globally by 2030 [4]. Energy storage comes in many different forms. No single form of energy storage can provide all required services to the grid. Technologies for grid storage range from fast acting technologies that deliver high power for a short period of time, such as supercapacitors and flywheels [5], to large, high energy stores, such as compressed air [6], thermal storage [7] and pumped hydro [8]. Globally, pumped hydro accounts for the majority of energy storage for grid support, forming 96%, by rated power, of the 176 GW of operational storage capacity [3]. The first pumped hydro plant was installed in 1929 and is still operational today [9]. Clearly, the maturity and lifetime are significant benefits to using this technology. However, locations for pumped hydro installation are geographically restricted and there are significant capital costs. Often areas must be flooded to install such systems, potentially causing harm to the local ecological system. The emissions from the vast amounts of concrete used combined with those from the construction of such systems can be significant, also [10]. Clearly, there needs to be an alternative. In terms of electrochemical devices, when energy is required to be stored for periods of four hours or more, redox flow batteries come into their own [11]. 1.1 Redox Flow Batteries A redox flow battery (RFB) is a secondary electrochemical system that, unlike conventional batteries, stores energy within liquid electrolyte that is held in external reservoirs [12]. These batteries allow for energy and power to be decoupled. The power of a system can be increased by increasing the electrode area, or by increasing the number of cells in a stack and the capacity, or amount of energy stored in the system, can be increased by increasing, either the concentration, of the active species, or by increasing the volume of the stored electrolyte [13]. Flow batteries also have the potential for high cycle-lifetimes, with some systems exhibiting lifetimes of over 15,000 cycles [14, 15]. They can also exhibit fast response times, in some cases less than 1 ms [15], making them an ideal technology for grid scale storage. RFBs have had some commercial success, particularly the most developed system, the all-vanadium flow battery [16]. 47 MW of flow batteries has been installed in 101 global projects [3]. However, RFBs are not without limitations. Many cell chemistries have been proposed and researched to varying degrees. However, engineering challenges, such as the flow of electrolyte through the cells have yet to be solved [12]. A major inhibiting factor for flow batteries is cost. The Advanced Research Project Agency for Energy (ARPA- E), part of the US Department of Energy has set a target $100 / kWh for battery pack costs [17]. This target would make a system a competitive technology allowing them to become a commercial success. Some of the most expensive parts of RFBs are the pumping and ancillary apparatus and the membranes that divide the cells. Undivided systems are, therefore, a promising option. Undivided systems are those without a membrane dividing the cell. Membranes usually have a shorter lifespan than the electrolyte and the unit price is expensive Page | 1 A Soluble Lead Flow Battery | Ewan Fraser [18]. These systems not only do not require the membrane, but they also only require a single electrolyte, so the cost of the reservoir and pumping systems is virtually halved. One such system is the soluble lead flow battery. 1.1.1 The Soluble Lead Flow Battery The soluble lead flow battery has a similar chemistry to that of the lead battery. See Equations 1-3 [19]. The SLFB differs from the lead-acid battery, in that the Pb2+ ions are soluble in the SLFB. In the conventional lead- acid system, all lead and lead compounds remain insoluble. Unlike a conventional redox flow battery, the soluble lead system requires phase change at both electrodes, with lead metal being formed at the negative electrode, and lead dioxide at the positive electrode. 2+ - charge 0 Pb (aq) + 2e Pb(s) E =-0.13 V vs SHE Equation 1 discharge 2+ - charge + 0 Pb (aq) + 2H2O(l) - 2e PbO2(s) + 4H (aq) E =+1.46 V vs SHE Equation 2 discharge 2+ charge + 0 2Pb (aq) + 2H2O(l) Pb(s) + PbO2(s) + 4H (aq) E cell=+1.59 V vs SHE Equation 3 discharge It is this deposition process that causes the most significant problems with the SLFB. At the negative electrode, the electrode reaction is highly efficient [20]. However, the lead can form in a dendritic manner, if these lead dendrites grow too large, they can grow across the inter-electrode gap, generally along the walls, to the opposite electrode and can short circuit that cell. These dendrites are also prone to falling away from the electrode as the electrolyte flows over them, causing a loss in capacity [21]. Deposition of lead dioxide causes the main issues with the SLFB system. Less favourable kinetics, overpotentials and reversibility usually cause the positive electrode reactions to be the limiting factor in the life of the battery [20]. Lead dioxide exists in two forms, α and β. In a practical battery, a mixture of both forms is deposited on the positive electrode. α lead dioxide usually bonds well to the electrode surface, forming a dense, solid layer. β lead dioxide, however, is a less dense material that is often dislodged from the electrode surface [22]. There is an optimal composition for the ratio of α to β allowing for a high surface area whilst keeping the structural properties to keep the lead dioxide adhered to the electrode surface [22]. The material that falls away from the electrode forms a sludge, which reduces the capacity of the cell because it cannot be reused during discharge, it can also cause a short circuit within the cell and can block the flow of electrolyte within the flow battery stack. Despite these problems, there have a series of preliminary studies that show SLFBs have the potential to be used as a large-scale energy storage device [19, 21, 23-29]. 2 Aims & Objectives -To investigate the effect of a range of additives on the deposited layers in static electrolyte cells. -If sludge is formed, to explore methods to mitigate its effects -Adding hydrogen peroxide to remove sludge and restore capacity -Use the results from static electrolyte cells to design a prototype 6V soluble lead flow battery. -Perform optimisation of the flow field within the flow battery using CFD (computational fluid dynamics) with the aim of producing an even flow distribution over the electrodes whilst maximising the electrode area for given cell dimensions. -Compare results to a 6V conventional lead-acid battery and to a 6V static electrolyte soluble lead cell using the same electrolyte. -Assess the feasibility and design of a gravity fed, low-cost soluble lead flow battery for use with photovoltaic panels. 3 Method Firstly, static, single cells with electrode dimensions of 5 cm x 5 cm were manufactured using laser cut Perspex, which was glued and clamped between graphite polymer electrodes with copper current collectors at either end. A solution of 0.7 mol dm-3 Pb2+, 1.0 mol dm-3 methanesulfonic acid (MSA), 1 g dm-3 lignosulfonate and 15 Page | 2 A Soluble Lead Flow Battery | Ewan Fraser mmol dm-3 Bi3+ was used within these cells. The first cell was tested with no other additives or additions to this basic design. A second cell was constructed with the addition of reticulated vitreous carbon (RVC) to both electrodes and a plastic mesh separator was added between the electrodes. The same solution was used in this cell. To ensure this configuration worked, the first cell was tested by charging for 15 minutes then discharging for a further 15 minutes or until the cell voltage reached 0.3 V at a constant current of 1 mA cm-2 (25 mA). Both 25 cm2 cells were then tested at a higher current of 15 mA cm-2 (375 mA), again charging the first cell for 15 minutes and discharging for 15 minutes or until the voltage reached 0.3 V. Due to the smaller volume of the second cell, because of the RVC and the plastic mesh, it was charged for 10 minutes and discharged for 10 minutes. This testing was completed using an MTI 8 channel battery analyser. After testing each cell to failure, the cells were deconstructed for analysis. The static cells were then run at a constant current of 10 mA cm-2 and 5 mA cm-2. The other variables were unchanged. A 6 V, 4-cell battery with an electrode area of 15 cm2 per electrode was then constructed using the same components as the small cells.