ZBBs have a high open circuit voltage (1.82 V), a high theoretical energy (> 400 W h^-1 kg^-1) and high demonstrated power densities (> 100 mW cm^-2). Typically, ZBBs adopt a redox flow design involving the use of a Nafion membrane to separate aqueous zinc bromide anolyte and catholyte solutions [1]. In this study, the use of a membrane-free non-flow design was investigated for the purposes of recovering zinc from scrap and waste steel resources [2]. The rationale for this work stems from the greenhouse gas emissions produced by the iron and steel industry, which accounts for between 4-7 % of the anthropogenic CO₂ emissions globally [3]. Blast furnace technology is likely to account for most stainless steel production in the coming decades, and therefore a transition to low-carbon and green steel production will require increased steel recycling rates and significantly improved waste and scrap management. To the best of our knowledge, this is the first time a zinc-bromine battery has been investigated for the recovery of materials rather than energy storage.

Galvanization of steel is required to prevent rusting and degradation and involves coating the surface of steel in a protective layer of zinc. Galvanization processes account for over 50 % of global zinc consumption and by 2050 the demand for zinc will be 2.7 times greater than that of 2012 [4]. In order to enable recycling of scrap steel directly into blast furnaces, zinc is removed and recovered via mineral acid leaching. This method of recovery has a high zinc extraction efficiency but creates problematic waste streams and has poor energetic efficiency.

In this work, the use of a membrane-free zinc-bromine battery has been studied for the purposes of extracting zinc from steel substrates and subsequently re-electroplating onto a conventional carbon foam electrode. The electrical performance of the cell was characterised by charge-discharge profiles and I-V curves. Zinc removal and recovery onto electrodes was characterised using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The work successfully demonstrates that ZBB technology could enable efficient and clean recovery of zinc from metal and waste substrates including scrap steel, slurries generated from basic oxygen steelmaking processes, and secondary vent dust from the primary steelmaking off gas streams. The cell studied in this work enabled dipping of zinc-containing steel substrates directly into the electrolyte solution without disassembly of the battery housing. In addition, the design involved the use of low-cost materials and reagents and potentially offers low balance-of-plant costs.

The results show that zinc could be removed from steel surfaces during cell discharge with greater than 99.9 % yield. The Figure shows the extracted zinc could subsequently be re-electroplated onto a standard carbon foam electrode upon re-charging the cell. When a 0.5 V cut-off voltage was used upon discharge, the zinc was recovered selectively from the steel (see Fig. (a)); the surface elemental composition of the carbon electrode measured by EDS after charging was: carbon (26.79 wt%), oxygen (18.06 wt%), zinc (30.03 wt%) and bromine (25.11 wt%). Using a lower cut-off voltage (0.2 V) resulted in the co-extraction of iron from the substrate as well as zinc (see Fig. (b)); in this case, the elemental composition of the carbon electrode after charging was: carbon (27.53 wt%), oxygen (22.65 wt%), iron (4.72 wt%), zinc (24.44 wt%) and bromine (20.65 wt%). Provided a cut-off voltage of no less than 0.5 V was used for discharging, high purity zinc was recovered, and the cell showed good initial durability, with 30 cycles of charge-discharge demonstrated in this work.

[1] S. Suresh, M. Ulaganathan, N. Venkatesan, P. Periasamy, P. Ragupathy, High performance zinc-bromine redox flow batteries: Role of various carbon felts and cell configurations. J. Energy Storage, 2018. 20: pp. 134-139.

[2] S. Biswas, A. Senju, R. Mohr, T. Hodson, N. Karthikeyan, K. Knehr, A.G. Hsieh, X. Yang, B.E. Koel, D.A. Steingart, Minimal architecture zinc-bromine battery for low-cost electrochemical storage. Energy Environ. Sci., 2017. 10: pp. 114-120.

[3] Iron and Steel Technology Roadmap: Towards More Sustainable Steelmaking, Energy Technology Perspectives, International Energy Agency, IEA Publications, Paris, 2020. https://www.iea.org/reports/iron-and-steel-technology-roadmap [accessed 12 April 2022].

[4] K.S. Ng, I. Head, G.C. Premier, K. Scott. E. Yu, J. Lloyd, J. Sadhukhan. A multilevel sustainability analysis of zinc recovery from wastes. Resour. Conserv. Recycl., 2016. 113: pp. 88-105.

Figure. SEM images showing the carbon foam zinc electrodes after charging the cell. (a) is an electrode when a discharge cut-off voltage of 0.5 V was used, (b) is an electrode when a discharge cut-off voltage of 0.2 V was used.