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

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[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

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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. This battery was then tested using the same test cycle at 15 mA cm-2 to failure using a BaSyTec battery analyser.

4 Deliverables A brief literature review of the soluble lead battery. A comparison of several designs of static electrolyte cells over varying current densities. A flow simulation study to achieve an optimised flow field within the flow battery.

5 Discussion Initially, to ensure the cell operated, the basic static soluble lead battery was tested at a very low current of 25 mA. At this low current, the cell is shown to operate without any degradation in performance for over 40 cycles. See Figure 5-1.

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Figure 5-1: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell running at a constant current of 1 mA cm-2.

Figure 5-2 shows both electrodes after cycling at 25 mA. On the negative electrode (a & b), the lead has deposited relatively evenly over the surface of the electrode, with slightly thicker deposits near the bottom of the cell (top of the figure). However, there are a few areas that could develop into much thicker layers of lead were the cell cycled further. Most notably, at the bottom of the cell (top left corner in Figure 5-2a), a white deposit has grown across the wall of the cell, towards the positive electrode. This may have electrically shorted the cell and caused it to fail after only a few further cycles. The white/yellow compound that formed was likely to be Lead Oxide (PbO).

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(a)

(b) (c)

Figure 5-2: Negative (a & b) and positive (c) electrodes of the Static Soluble Lead Cell after cycling at a constant current of 1 mA cm-2.

The cell was then run at a higher current of 375 mA. All other test conditions were the same. The efficiencies are seen in Figure 5-3. The efficiencies of this cell quickly degrade with the number of cycles at the higher current, with the cell losing all capacity after just 13 cycles.

Upon disassembling the cell, it was discovered that there was a deposit, likely PbO2 that had collected at the bottom of the cell, as shown in Figure 5-4. This deposit spanned the inter-electrode gap and likely caused a short circuit and ultimately failure of the cell.

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Figure 5-3: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell running at a constant current of 15 mA cm-2.

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(a) (b)

Figure 5-4: Positive (a) and Negative (b) electrodes of the Static Soluble Lead Cell after cycling at a constant current of 15 mA cm-2.

Adding RVC to the electrodes meant a reduction in the volume and hence the capacity of the cell. To compensate, the charge/discharge time was reduced to 10 minutes. Due to the high current density, the cells failed quickly. It is, therefore, difficult to compare the cells. However, comparing the efficiencies for the cell with RVC electrodes, Figure 5-5 with the efficiencies of the polymer-graphite electrodes only, Figure 5-3, it is seen that there is no significant difference in cycle life. The RVC cell displays a low efficiency in the early cycles. This may suggest that there was still air trapped within the electrodes. Figure 5-6 shows the electrodes after cycling, with the RVC foam shown above. There is very little lead remaining on the negative electrode, suggesting it is the positive electrode reaction that needs attention in this cell. It should also be noted that there is a small area of solid lead that has deposited on the positive electrode. This suggests that the RVC was not in complete contact with the electrode, effectively creating a bipolar cell in that region, leading to this small amount of lead to form between the RVC and the polymer graphite electrode.

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0% 0 5 10 15 20 Cycle Number Energy Efficiency Charge Efficiency Voltage Efficiency Figure 5-5: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell with RVC electrodes running at a constant current of 15 mA cm-2.

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(a) (b)

Figure 5-6: Negative (a) and Positive (b) electrodes with RVC of the Static Soluble Lead Cell after cycling at a constant current of 15 mA cm-2.

Due to the low cycle life displayed by the cells at 15 mA cm-2, the cells were run again at lower current densities of 10 mA cm-2 and 5 mA cm-2. It is seen in Figure 5-7 that the cycle life of the cell at 10 mA cm-1 is significantly higher, with the cell cycled 20 times before it started to degrade significantly.

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Figure 5-7: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell running at a constant current of 10 mA cm-2.

As expected, Figure 5-8 shows the cell performed consistently at a high efficiency for over 40 cycles at 5 mA cm-2.

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Figure 5-8: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell running at a constant current of 5 mA cm-2.

Another method that aimed to improve the cycle life of the cells was to produce a cell with an area of inactive material below the electrodes, i.e. a non-conductive region within the cell. This region would act as a volume to collect any sludge that formed and subsequently fell to the bottom of the cell. However, due to this region being non-conductive, the cell would not short circuit as this material gathered at the base of the cell. This method would not prevent capacity fade due to active material precipitating out of solution, but it should simply prevent the cell short-circuiting.

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Figure 5-9: Charge, Voltage and Energy Efficiencies of a Soluble Lead Static Cell running at a constant current of 15 mA cm-2. The electrodes were raised from the bottom of the cell to prevent a sludge build-up short-circuiting the cell.

Figure 5-9, however, shows no significant improvement in cycle life occurred compared to that of the standard test cell. Comparing Figure 5-10 with Figure 5-4, the amount of sludge that was formed in the cell with the insulated base formed was much higher in the modified cell. While shorting of the cell was prevented, there was increased capacity fade. Methods to remove this sludge chemically have been explored by adding hydrogen peroxide periodically to restore capacity [29]. This method proved successful in extending the cycle life of the system. However, remaining hydrogen peroxide in the solution caused a decrease in efficiency and it was

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deemed that the added cost of adding hydrogen peroxide did not improve the life sufficiently to prove economical.

(a) (b)

Figure 5-10: Positive (a) and Negative (b) electrodes with an insulated base of the Static Soluble Lead Cell after cycling at a constant current of 15 mA cm-2.

Figure 5-11 shows that the efficiencies of the system remained high at 5 mA cm-2 when cells were combined into a 4 cell stack. While the cycle life of this stack is unknown, the efficiencies are comparable to the single cell showing the scalability of this system.

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Figure 5-11: Charge, Voltage and Energy Efficiencies of a 4 Cell Soluble Lead Static Battery running at a constant current of 5 mA cm-2.

Comparing the electrode surfaces after cycling of all the cells, it is clear that, whilst generally the negative electrode was clean and lead free, there was, without fail, a layer of lead dioxide on the positive surface, indicating that the reaction at the positive electrode is the limiting factor in this chemistry.

6 Conclusions and Further Work The soluble lead chemistry has already been proven to have potential in flow battery applications. The high efficiencies of the 6V system produced in this work further support the scalability of this system. However, there is significant further work to be completed before this technology can become a commercial technology. As results from this work and others have shown, the positive electrode requires the most attention. With sludge formation reducing capacity and potentially causing a short circuit across the cell.

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Attempts to mitigate the effect of this sludge have proved unsuccessful. Initial results in this work have shown that mechanical removal, or collection of the sludge may prevent short circuiting, but the increased capacity fade caused no improvement in cycle life compared to the standard cell. However, further work in this area may yield a successful result. Further research is also required into the electrode material, particularly for the positive electrode. The electrodes used in this work could not provide a long cycle life with the soluble lead chemistry. Whilst unsuccessful in increasing cycle life at 15 mA cm-2, improvement in design and using lower currents, RVC may produce some increase in cycle lifetime. The next immediate steps would be to produce further data from the 6V static soluble lead battery before producing a comparable soluble lead flow battery.

7 References [1] M. Alexander and P. James (2015) Role of distributed storage in a 100% renewable UK network, Proceedings of the ICE-Energy, pp. 1-21. [2] European Commission (2016), Strategic Energy Technologies [Online]. Available: https://setis.ec.europa.eu/technologies [Accessed: 26/04/2017]. [3] Scandia National Laboratories and Department of Energy (2016), Global Energy Storage Database. Available: https://www.energystorageexchange.org/ [Accessed: 26/04/2017]. [4] Energy Storage Council, "ESC Global Energy Storage Report_2015," 2015. [5] R. Sebastián and R. Peña Alzola (2012) Flywheel energy storage systems: Review and simulation for an isolated wind power system, Renewable and Sustainable Energy Reviews, 16 (9), pp. 6803-6813. [6] J. Chou, A. Szulc, L. Tang and X.Y. Zeng (2014) Compressed Air Energy Storage. [7] European Association for Storage of Energy, "Pumped Heat Electrical Storage." [8] First Hydro Company, Dinorwig Power Station. Available: http://www.fhc.co.uk/dinorwig.htm [Accessed: 01/05/2017]. [9] J. Baker and A. Collinson (1999) Electrical energy storage at the turn of the millennium, Power Engineering Journal, 13 (3), pp. 107-112. [10] I.B. Lima, F.M. Ramos, L.A. Bambace and R.R. Rosa (2008) Methane emissions from large dams as renewable energy resources: a developing nation perspective, Mitigation and Adaptation Strategies for Global Change, 13 (2), pp. 193-206. [11] P. Alotto, M. Guarnieri, F. Moro and A. Stella (2012) Redox Flow Batteries for large scale energy storage, 2012 IEEE International Energy Conference and Exhibition, ENERGYCON 2012, September 9, 2012 - September 12, 2012, pp. 293-298. [12] P. Leung, X. Li, C. Ponce De Leon, L. Berlouis, C.T.J. Low and F.C. Walsh (2012) Progress in redox flow batteries, remaining challenges and their applications in energy storage, RSC Advances, 2 (27), pp. 10125- 10156. [13] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick and Q. Liu (2011) Redox flow batteries: a review, Journal of Applied Electrochemistry, 41 (10), pp. 1137-1164. [14] B. Yang, L. Hoober-Burkhardt, F. Wang, G.K.S. Prakash and S.R. Narayanan (2014) An inexpensive aqueous flow battery for large-scale electrical energy storage based onwater-soluble organic redox couples, Journal of the Electrochemical Society, 161 (9), pp. A1371-A1380. [15] X. Luo, J. Wang, M. Dooner and J. Clarke (2015) Overview of current development in electrical energy storage technologies and the application potential in power system operation, Applied Energy, 137 (pp. 511- 536. [16] M. Skyllas-Kazacos and J. McCann (2015) Vanadium redox flow batteries (VRBs) for medium-and large- scale energy storage, Advances in batteries for medium and large-scale energy storage, pp. 329-386. [17] ARPA-E, "ARPA-E: The First Seven Years: A Sampling of Project Outcomes," Advanced Research Project Agency for Energy2016. [18] Electric Power Research Institute, "Program on Technology Innovation: Assessment of Flow Battery Technologies for Stationary Applications," 2016. [19] A. Hazza, D. Pletcher and R. Wills (2004) A novel flow battery: A lead acid battery based on an electrolyte with soluble lead (II) Part I. Preliminary studies, Physical Chemistry Chemical Physics, 6 (8), pp. 1773-1778.

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[20] L. Wallis and R. Wills (2014) Membrane divided soluble lead battery utilising a bismuth electrolyte additive, Journal of Power Sources, 247 (pp. 799-806. [21] A. Hazza, D. Pletcher and R. Wills (2005) A novel flow battery—a lead acid battery based on an electrolyte with soluble lead (II): IV. The influence of additives, Journal of Power Sources, 149 (pp. 103-111. [22] X. Li, D. Pletcher and F.C. Walsh (2011) Electrodeposited lead dioxide coatings, Chemical Society Reviews, 40 (7), pp. 3879-3894. [23] D. Pletcher and R. Wills (2004) A novel flow battery: A lead acid battery based on an electrolyte with soluble lead (II) Part II. Flow cell studies, Physical Chemistry Chemical Physics, 6 (8), pp. 1779-1785. [24] D. Pletcher and R. Wills (2005) A novel flow battery—A lead acid battery based on an electrolyte with soluble lead(II): III. The influence of conditions on battery performance, Journal of Power Sources, 149 (pp. 96-102. [25] D. Pletcher, H. Zhou, G. Kear, C.J. Low, F.C. Walsh and R.G. Wills (2008) A novel flow battery—A lead- acid battery based on an electrolyte with soluble lead (II): V. Studies of the lead negative electrode, Journal of Power Sources, 180 (1), pp. 621-629. [26] D. Pletcher, H. Zhou, G. Kear, C.J. Low, F.C. Walsh and R.G. Wills (2008) A novel flow battery—A lead- acid battery based on an electrolyte with soluble lead (II): Part VI. Studies of the lead dioxide positive electrode, Journal of Power Sources, 180 (1), pp. 630-634. [27] X. Li, D. Pletcher and F.C. Walsh (2009) A novel flow battery: a lead acid battery based on an electrolyte with soluble lead (II): Part VII. Further studies of the lead dioxide positive electrode, Electrochimica Acta, 54 (20), pp. 4688-4695. [28] J. Collins, G. Kear, X. Li, C.T.J. Low, D. Pletcher, R. Tangirala, et al. (2010) A novel flow battery: A lead acid battery based on an electrolyte with soluble lead(II) Part VIII. The cycling of a 10 cm × 10 cm flow cell, Journal of Power Sources, 195 (6), pp. 1731-1738. [29] J. Collins, X. Li, D. Pletcher, R. Tangirala, D. Stratton-Campbell, F.C. Walsh, et al. (2010) A novel flow battery: A lead acid battery based on an electrolyte with soluble lead (II). Part IX: Electrode and electrolyte conditioning with hydrogen peroxide, Journal of Power Sources, 195 (9), pp. 2975-2978.

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Appendix

This appendix provides any recorded material that was not included in the main body of the report. Comments have been made as bullet points below figures where appropriate. It is not intended to be marked along with the main body, but is simply to document all material that may be of use for future work.

A 1: Solution before (a) and immediately after cycling (b) and after resting (c)

• The solution is initially yellow due to the additives. • Immediately after removal from the cell, the solution is cloudy • Once allowed to stand, the precipitate settles, showing it is not soluble in the solution.

A 2: positive electrode after cycling at 1 mA cm-2

• Loose, crystalline structure formed on the surface. • At the bottom of the cell, the white deposit can be seen growing towards the positive electrode.

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A 3: Negative electrode of cell with non-conductive area at base

• The large volume of sludge formed can be seen here. This is in contrast to the relatively small amount formed in the unmodified cell.

A 4: Three separate cells tested in close proximity. Material on top caused electrical contact between the cells.

• Solid material formed on top of the cells • This caused an electrical contact between the current collectors of separate cells. • Were a stack with the cells in parallel to be produced, an open system such as this could not be used due to the risk of the same problem occurring.

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400 Capacity to failure (mAh) 200

0 0 2 4 6 8 10 12 14 16 Current Density (mA cm-2)

A 5: The relationship between capacity to failure and current density of the 25 cm2 cell.

• Approximately linear relationship over measured range • Significant increase in capacity to failure cycle life is 2.75 times longer at 5 mA cm-2 than 15 mA cm-2 •

A 6: Flow conditions of previous flow field

• High velocity at base of electrode • Higher velocity in the centre compared with the edges of the electrode • Flow returns in bottom left corner

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A 7: Flow trajectories of iterations of flow field design

• Constant volumetric flow rate • Eddies almost eliminated by last iteration. Flow over electrode is approximately even.

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