The influence of novel bromine sequestration agents on zinc/bromine flow battery performance

Abstract

This study benchmarks cycle performance of electrolyte solutions containing novel bromine sequestration agents (BSA) in a zinc bromine flow battery. Five alternative BSA candidates – 1-ethyl-1-methylpiperidinium bromide ([C₂MPip]Br), 1-ethylpyridinium bromide ([C₂Py]Br), 1-(2-hydroxyethyl)-pyridinium bromide ([C₂OHPy]Br), 1-ethyl-3-methylimidazolium bromide ([C₂MIm]Br) and 1-(2-hydroxyethyl)-3-methylimidazolium bromide ([C₂OHMIm]Br) were investigated under operational conditions typical for zinc bromine flow batteries. Results were compared to the conventional BSA, 1-ethyl-1-methylpyrrolidinium bromide ([C₂MPyrr]Br). Electrolytes containing the various alternative BSAs were tested at bench scale in a full-cell battery setup under controlled electrolyte flow and temperature. The evaluated performance parameters were: voltaic efficiency, coulombic efficiency, energy efficiency and recoverable charge. A correlation between BSA–bromine bond strength and cycle performance was observed. Performance of the tested electrolytes varied widely, and gains in coulombic efficiency were generally offset by losses in voltaic efficiency. [C₂Py]Br and [C₂MIm]Br produced cycle performance improvements compared to the other BSAs studied.

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1 Introduction

Fossil fuel resources are inherently finite and their combustion releases carbon dioxide, which is known to be the main contributor to human induced climate change. As our electricity supply currently relies heavily on the burning of these fuels, it is necessary to transform it in order to make it more sustainable. Contrary to fossil fuels, wind energy (WE) and photovoltaic energy (PV) are renewable energy sources and due to the abundance of wind and sun, they are considered promising alternatives to fossil fuels. However, they also suffer from the problem of intermittence that makes it desirable to implement corresponding energy storage solutions into the electricity grid. Among the various available technologies for this purpose (e.g. pumped hydro, fly wheels, compressed air storage etc.) secondary batteries and zinc bromine redox flow batteries (ZBB) in particular, can play a vital role. They have advantages such as an abundance of the active material, high theoretical energy density of 440 W h kg⁻¹, good scalability and cycle efficiencies of around 80%.^1–5 These properties make ZBB suitable for load levelling applications, power quality control and renewable energy deployment.⁶

As one of many redox flow batteries, the ZBB is characterized by circulation of electrolyte from external reservoirs through the electrochemical cell (see Fig. 1). This allows for the decoupling of battery capacity (depending on the electrolyte volume) and power output (depending on electrode area) and makes the system scalable. The electrochemical cell consists of zinc and bromine half-cells which are separated by a porous membrane. Consequently, the battery has two separate hydraulic circuits each with its own pump, storage tank and auxiliary instrumentation (flow meters, filters etc.) if required.

Fig. 1 Schematic of a zinc bromine redox flow battery during charging (left) and discharging (right).

The electrolyte is an aqueous solution containing the active ingredient zinc bromide, a supporting electrolyte and a bromine (Br₂) sequestration agent (BSA). During charging a DC power source is connected to the cell. Zinc ions are reduced to zinc and plated onto the surface of the negative electrode, eqn (1). At the same time bromide ions are oxidized to bromine on the positive electrode, eqn (2).

Zn_aq²⁺ + 2e⁻ ↔ Zn_s | E⁰ = −0.78 V vs. SHE* (1)

2Br_aq⁻ ↔ Br₂ + 2e⁻ | E⁰ = +1.09 V vs. SHE* (2)

*(SHE: standard hydrogen electrode).

In subsequent reactions the evolved bromine reacts either with bromide ions to form aqueous polybromide ions, eqn (3), or with a BSA (QBr_(x)) to form a complex, eqn (4). The complex sequesters in a separate phase and forms an emulsion during electrolyte cycling.

Br₂ + Br_(x)aq⁻ ↔ Br_(x+2)aq⁻ (3)

Br₂ + QBr_(x) ↔ QBr_(x+2)seq (4)

Aqueous polybromide ions can diffuse to the zinc electrode where they directly react with the plated zinc and therefore trigger self-discharge. To mitigate this undesirable reaction, the cell membrane largely restricts the sequestered bromine phase to the bromine half-cell and greatly reduces the self-discharge reaction. During discharge, the processes described above are reversed. Bromine supply is the limiting factor for the discharge reactions, and it can be supplied either directly from the sequestered phase or indirectly from the aqueous phase, both phases of which are in solution equilibrium.⁷

A suitable BSA should thus reduce the aqueous bromine content, and remain liquid upon bromine uptake. At the same time, bromine should easily be released from the BSA–bromine complex during discharge, to minimize voltaic losses and enable full discharge of the cell. Historically, suitable BSA candidates have been found to be the quaternary ammonium ionic liquids.⁸

During the development of ZBB systems in the 1980s and 1990s screening tests on possible BSA candidates were performed.^8–11 1-Ethyl-1-methylpyrrolidinium bromide ([C₂MPyrr]Br) emerged as the best candidate at the time. Since then, new BSA candidates were discovered justifying further investigation into alternatives. In related work by some of the authors, density functional theory (DFT) modelling and spectroscopic investigations in acetonitrile solutions were used to investigate key factors which determine the stability of BSA–polybromide complexes.¹² It was found that weaker interionic interactions within the starting monobromide salt lead to more stable higher order BSA–polybromide complexes, and in turn higher bromine sequestration efficiency.

The BSAs studied in the present work have been previously investigated for their effects on zinc half-cell performance and found to influence both the physical and electrochemical behavior observed during voltammetry, impedance spectroscopy and zinc electrodeposition during charging of the system.¹³ In that work, it was proposed that the strength of interaction between the BSA cation and its bromide counter-ion influences the strength of EDL assembly and the concentration of bromide ligands available to complex zinc ions in solution. Similar studies have been investigated by the authors for the bromine half-cell.¹⁴

In this study, we have investigated the full-cell performance of electrolyte solutions containing novel BSA candidates under full operating flow conditions. The aim was to establish how the previous findings would influence the performance of a flow battery and to what extent gains or losses in coulombic efficiency would be offset by changes in voltaic efficiency. The five ionic liquids 1-ethyl-1-methylpiperidinium bromide ([C₂MPip]Br), 1-ethylpyridinium bromide ([C₂Py]Br), 1-(2-hydroxyethyl)-pyridinium bromide ([C₂OHPy]Br), 1-ethyl-3-methylimidazolium bromide ([C₂MIm]Br) and 1-(2-hydroxyethyl)-3-methylimidazolium bromide ([C₂OHMIm]Br) were selected for testing against [C₂MPyrr]Br and a BSA free electrolyte as a benchmark (see Fig. 2 for chemical structures).

Fig. 2 Lewis structures of investigated bromine sequestration agents.

The data were analyzed qualitatively with regards to representative cell voltage progressions and quantitatively via derived performance figures. Parameters assessed were:

• Voltaic Efficiency (VE): ratio of averaged voltage during discharge phase voltage to averaged voltage during charge phase.

• Coulombic efficiency (CE): ratio of electric charge which can be drawn from a battery (at given current density) to electric charge used for charging of the battery.

• Recoverable charge (RC): ratio of maximum electric charge which can be drawn from a battery (including stripping cycle) to electric charge used for charging of the battery.

• Energy efficiency (EE): ratio of electric energy which can be drawn from a battery (at given current density) to ratio of electric energy used for charging of the battery.

2 Methodology

2.1 Electrolyte preparation

Zinc bromide (as ZnBr₂, Aldrich, >98%), zinc chloride (ZnCl₂, Ajax, >99.8%), potassium chloride (KCl, Aldrich) and bromine (Br₂, BDH, >99%) were sourced commercially and used without further purification. Electrolyte solutions were prepared according to a previous study by the authors and contained 1 M of the BSA, 2.5 M of zinc bromide, zinc chloride and potassium chloride.¹³ The different BSAs were synthesized by alkylation of the corresponding tertiary amines with bromoethane or bromoethanol (where appropriate), with purification and decolorization performed by standard literature methods.¹⁵

Conductivity and pH values of the electrolyte solutions were measured using an Accumet XL600 (Fisher Scientific) conductivity meter and are summarized in Table 1.

Table 1 Properties of the electrolyte solutions prepared in this study¹³

Identifier	BSA	pH	Conductivity (mS cm^−1)
EL0	None	3.9	132
EL1	[C2MPyrr]Br	3.7	92
EL2	[C2MPip]Br	3.6	84
EL3	[C2Py]Br	3.7	99
EL4	[C2OHPy]Br	3.4	97
EL5	[C2MIm]Br	3.8	97
EL6	[C2OHMIm]Br	3.3	96

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2.2 Electrochemical cell configuration and operation

Tests were conducted in a single flow cell setup in a vertical orientation (Fig. 3). Electrolyte was fed in from the bottom and drained from the top. The porous membrane which separated the zinc and bromine half-cell was held in place by two frames which incorporated flow paths for even electrolyte distribution.

Fig. 3 Exploded view of single cell setup showing (from left to centre) the cell body, electrode, cell frame, membrane frames with embedded flow paths, and membrane.

The electrodes had a square geometric area of 40 mm × 40 mm size (16 cm²). The bulk electrode material was commercial grade conductive carbon plastic made from high density polyethylene (HDPE) filled with carbon nanotubes.¹⁶ A copper mesh was incorporated within the bulk material as the current collector, which was then soldered to a copper stub which acted as connector to external circuitry. An external thread allowed firm mounting of the electrode to the cell body. A HDPE frame was molded around the active area for better mechanical stability and to prevent exposure of copper to the electrolyte.

To improve the electrode performance, the surfaces were coated in a heat press process. The zinc electrode was coated with graphite which guaranteed even charge distribution over the electrode surface and also improved the adherence of the zinc deposit.

The bromine electrode was coated with an active carbon material in order to increase the active surface area. This increased the bromine electrode reaction rate, which is considerably slower than the zinc reaction.¹⁷

Associated cell equipment on each site consisted of a tank, gear pump, turbine flow meter and thermostat (shared between both flow paths). Electrolyte flow was regulated via a feedback loop between pumps and flowmeters. Thermocouples were used for temperature recording and thermostat regulation.

Charging and discharging as well as recording of cell voltage and current were performed via a programmable battery cycler (Neware model CT-4008).

Cell components are able to absorb some amount of bromine and it is beneficial for test reproducibility to saturate surfaces prior to testing with bromine. For this reason, 200 mL of electrolyte was spiked with 200 μL bromine and the solution was shaken until fully dissolved.

Before each test, both flow paths were thoroughly flushed with deionized water. This procedure was repeated with 30 mL of the spiked electrolyte solution for each side. Both sides were then filled with 40 mL electrolyte solution. The flow rate was set to 300 mL min⁻¹ and the temperature controlled at 25 °C. On the bromine side an additional 1 mL bromine was added. This facilitated sufficient bromine supply for the full deplating of the zinc electrode. For EL 5, an additional 3 mL bromine had to be added as the BSA reacted with the bromine (resulting in decolorization). For EL1 to EL6 the bromine addition caused the immediate formation of a sequestered phase which emulsified in the aqueous phase. This emulsion was maintained during cycle experiments. In contrast EL0 did not form a separate phase at any time of the cycle experiments due to the absence of BSA (which is responsible for the observed phase separation). After both temperature and flow stabilized, the following cycle program was run:

(1) Constant current charge with 0.32 A (20 mA cm⁻² current density) for 2 hours.

(2) Five minute rest phase used for determination of open circuit voltage (OCV).

(3) Constant current discharge with 0.32 A until cell voltage drops below 0.1 V (set as a practical cut-off voltage limit).

(4) Stripping phase for removal of residual zinc (consisting of several constant discharge steps with progressively lower current density).

The stripping phase was important for the reproducibility of the cycles. Residual zinc deposits were found to be rough and unevenly distributed across the electrode surface, which would have been preferred sites for zinc deposition in subsequent cycles and promoted dendritic growth. Cell voltage, cell current, flow rates and temperatures were recorded in 10 s intervals. The complete charge/discharge cycle was repeated three times after an initial conditioning cycle in order to confirm reproducibility of results.

2.3 Data evaluation

From the recorded data, characteristic efficiencies, and voltages for each of the tested electrolytes were determined. Fig. 4 shows the typical course of a charge/discharge cycle for EL1 ([C₂MPyrr]Br).

Fig. 4 Exemplary test cycle showing battery voltage, battery current, charge balance and energy balance versus elapsed time. The intersection of the two tangents at the end of the discharging step indicates the inflection point.

Towards the end of the discharge cycle the cell voltage dropped sharply and no more usable power could be drawn from the battery. The end point of the discharge was defined as the inflexion point of the discharge cycle determined by the intersection of tangents applied to the voltage curve.

Charge balance and energy balance were calculated by eqn (5) and (6) respectively.

with C_i being instantaneous current balance of the cell, W_i instantaneous energy balance of the cell, I_i instantaneous current, E_i instantaneous voltage and Δt_m measurement interval.

The open circuit voltage, OCV, can be determined according to eqn (7).¹⁸

with being the standard potential of an electrochemical redox couple and a_X being the activity of an electroactive species.

VE, CE and EE were calculated according to eqn (8)–(10).

with η_E being VE, Ē_disch average voltage during discharge, Ē_ch average voltage during charge, η_C being CE, C_disch-e charge balance end of discharge, C_ch-e charge balance end of charge, η_W being EE, W_disch-e energy balance at end of discharge and W_ch-e energy balance at end of charge.

Recoverable charge (RC) is defined as the share of charge which could be recovered from the cell including the stripping cycle. The difference of this value from 100% is caused by competing reactions such as hydrogen evolution and especially self-discharge due to bromine diffusion through the membrane. It therefore represents the upper limit achievable for the coulombic efficiency η_C under the prevailing system conditions. RC can be calculated according to eqn (11).

with η_RC being RC and C_strp-e being the charge balance at the end of discharge.

2.4 Acquisition and investigation of zinc electrodeposits

Zinc electrodeposits were obtained at the end of the charging phase under the same conditions as with the cycle experiments. The cell was then flushed three times with de-ionized water before disassembly and removal of the coated zinc electrode from the cell chamber. The electrode was blown dry with nitrogen and then immediately transferred into a vacuum-desiccator to prevent oxidation of the electrodeposits. The deposits were analyzed with a JCM-6000 NeoScope Benchtop scanning electron microscope at 15 kV acceleration voltage with a secondary electron detector.

3 Results and discussion

3.1 Assessment of cell voltage progression

The ratio of cell voltages during charge and discharge determine voltaic efficiency (see eqn (7) and (8)). Cell polarization can be caused by different types of overpotentials in the electrochemical system.¹⁹ At lower current densities, electron transfer overpotentials (which depend on the nature of the species participating in the reaction as well as the properties of the electrolyte and the electrode) are dominant. At higher current densities, diffusion overpotentials and reaction overpotentials become dominant.¹⁸

Fig. 5 compares cell voltage progression for the fourth cycle of each tested electrolyte. It is evident that differences in cell polarization during the discharge phase were much larger than during the charge phase. EL0 (without BSA) had the lowest charge voltage and the highest discharge voltage and it can be concluded that the addition of BSA increases cell polarization.

Fig. 5 Comparison of final cycle voltage trends of electrolytes with different BSA. Enlargement A shows voltage differences during the charge phase and enlargement B shows differences in open circuit voltages.

Cell voltages generally remained stable throughout the charging phase. There was about 100 mV difference in charging voltages, with the lowest charging voltages for EL0, EL5 and EL6 having an over potential of about 300 mV above open circuit voltage due to the current flow (see Fig. 5 inset A). Charging voltage was highest for EL2 followed by EL1. Voltages for EL3 and EL4 were similar and between the two extremes. The electrode polarization for EL0 during charging can mainly be attributed to ohmic resistances of the electrolyte and the electrode.

The other electrolytes have lower conductivity (see Table 1) due to the BSA addition resulting in higher cell polarization. Some quaternary ammonium ions have been observed to be surface active and tend to form layers on zinc surfaces, influencing zinc deposition quality as well as zinc electrode polarization.^20–23 It has also been reported that an adherent sequestered phase is able to depolarize the bromine electrode.²⁴

In the subsequent rest phase the OCV could be determined (see Fig. 5 inset B). Since there was no current flow, OCV represents an equilibrium potential which depends on the activity of electroactive species (see eqn (7)). The difference between the highest and lowest OCV was approximately 40 mV. EL0 had the highest OCV. EL2 had the lowest OCV closely followed by EL5, EL1 and EL3. The OCV of EL4 and EL6 were in the middle of the range indicating that the contained BSA had a comparatively lower sequestration efficiency. The lower OCV in comparison to EL0 are caused by lower activity of the sequestered bromine. This leads in turn to a lower bromine electrode potential as can be inferred from eqn (7).

It was previously shown that the bromine discharge reaction can be maintained from both the aqueous and sequestered phase.^7,25 The initial cell polarization during the discharge phase was similar to the charge phase. However subsequent cell voltages started to decline and vary widely.

Cell voltages for EL0, EL4, EL5 and EL6 evolved in a slow but steady decline. EL0 showed no phase separation on bromine evolution (as mentioned in 4.2, due to the absence of sequestration agent) and maintained the highest discharge voltage. The slow decline in cell voltage can be attributed to diffusion limited transport to the electrode surface. Cell voltage progressions for EL4, EL5 and EL6 were slightly lower during discharge but had a similar decline. This indicates that aqueous bromine diffusion also dominated voltage progressions for these electrolytes. The lower voltage level was caused by lower aqueous bromine content due to bromine sequestration.

Unlike the other electrolytes, cell voltages for EL3, EL1 and especially EL2 clearly dropped during the first minutes of discharge before they also evolved into steady declines. After charging, sequestered phase adhered to the electrode surface.²⁴ Adherent sequestered phase was initially able to maintain sufficient bromine supply for the discharge reaction. However, when it was gradually consumed, bromine had to be increasingly supplied from the bulk of the electrolyte. It can be concluded that transport of emulsified sequestered phase to the electrode surface was insufficient and instead, the diffusion limited transport of bromine through the aqueous phase became dominant. This explains the lower discharge voltages due to lower aqueous bromine concentration and also the gradient of the voltage progressions of EL3 and EL1, which is similar to EL0 (without BSA). The gradient for EL2 however was much steeper compared to the other BSA indicating that a different process was dominant. It is likely that in this case due to inhibited bromine release into the aqueous phase (according to eqn (4)).

Coulombic efficiency is limited by the amount of zinc deposit available for discharge which in turn depends on the reaction rate of side reactions during cell operation. Likely side reactions are the hydrogen evolution reaction, and self-discharge due to bromine diffusion. The self-discharge reaction is determined by aqueous bromine concentration and therefore will be limited with increasing BSA efficiency, as previously stated.

The hydrogen evolution reaction is a known phenomenon during zinc plating.^26–28 However, only negligible gas evolution was observed during cell operation. This was expected since the pH values of the electrolytes (see Table 1) were towards the higher end of the range of 1 to 4 previously reported elsewhere.¹⁷

Towards the end of the discharge cycle, cell voltages dropped steeply. At this point the zinc electrodes were partly de-plated (see Fig. 6). This resulted in the spatial constraint of transport processes and electrode reactions causing increasing current densities and cell polarization. Finally, the cell current could no longer be maintained and the discharge was finished.

Fig. 6 Representative image of a zinc electrode after the discharge cycle. Zinc remained in an area in front of the outlet due to steadily increasing polarization once the electrode starts to get de-plated.

Cell voltage in EL0 and EL6 dropped earlier and more sharply than with the other electrolytes. In this case the end of discharge was determined by strong self-discharge due to poor bromine sequestration. Discharge of EL4 was finished later indicating better BSA efficiency.

For EL2 the fall in cell voltage was more protracted. In this case, the end of discharge was less determined by zinc availability, but by the previously mentioned slow bromine release which made it difficult to maintain bromine supply at the given current density.

EL1, EL3 and EL5 could be discharged for longer times, indicating that the proposed strength of bromine bonding to the (poly)bromide ions had a good middle ground between those two effects.

3.2 Cycle performance of electrolytes

An overview of calculated efficiencies for all electrolyte solutions is given in Fig. 7. The included standard deviations indicate good reproducibility of the results.

Fig. 7 Cycle performance of seven electrolytes with different bromine sequestration agents.

Energy efficiency (the product of voltaic and coulombic efficiencies) is an important battery performance parameter, indicating energy losses incurred during charging and discharging. A higher voltaic efficiency indicates that a battery suffers fewer losses from rapid charging and discharging and allows therefore a higher power draw. A higher coulombic efficiency (linked to a higher recoverable charge) indicates, for example, that a battery suffers lower self-discharge. The results obtained indicate that the BSAs investigated influence the various efficiencies in different ways.

Voltaic efficiency depends mostly on bromine supply to the electrode during discharge as shown above. EL0 without BSA performed best in terms of voltaic efficiency followed by EL6, EL5 and EL4. The voltaic efficiency of EL3 and EL1 was significant lower. EL2 had clearly the lowest voltaic efficiency of all electrolytes.

This trend was generally reversed for coulombic efficiency. EL1, EL3 and EL5 had the highest and EL0 and EL6 the lowest coulombic efficiency. Coulombic efficiency for EL4 and EL2 coulombic efficiency was between these extremes. However, for EL2 insufficient bromine release was the limiting factor while for all other electrolytes it was zinc availability as can be inferred from recoverable charge share.

Recoverable charge is a good indicator for sequestration efficiency since it is directly correlated with available zinc. Bromine induced self-discharge was deemed the dominant side reaction. As expected recoverable charge share for EL0 without BSA was lowest. EL2 had the highest recoverable charge share indicating it had the best sequestration efficiency. Notable is the difference of almost 17% between coulombic efficiency and recoverable charge share for EL2, whereas for all other electrolytes this difference is in the low percent range.

In terms of energy efficiency EL2 had the poorest performance, due to the previously discussed issues such as insufficient bromine release during discharge. It is also not surprising that EL5 had the highest energy efficiency since it had good performance in both parameters. EL3 and EL4 also performed better than the benchmark BSA, EL1 ([C₂MPyrr]Br). However, higher self-discharge of EL4 made it less suitable for energy storage compared to EL3.

The addition of BSA reduced self-discharge, but also caused cell polarization. Both findings appear to be related to the structure of the BSA (see Fig. 2).

[C₂MPip]Br and [C₂MPyrr]Br differ from the other BSAs studied in their localized cationic charge. They caused the strongest cell polarization, but better sequestration efficiency than the other electrolytes (as evident with voltaic efficiency and recoverable charge share values).

EL3 and EL5 with delocalized cationic charge demonstrated considerably lower cell polarization, but also reduced sequestration efficiency.

The presence of an OH-group increases these effects as evident with EL4 and EL6. These findings are in general agreement with previously published results.¹²

3.3 Electrolyte influence on zinc deposition quality

In previous zinc half-cell testing of the BSA under non-flow conditions it was found that the choice of BSA had significant influence on the morphology of the zinc electrodeposits.¹³

Scanning electron micrographs of zinc deposits obtained in this study are presented in Fig. 8 to assess the influence of various BSAs under flow conditions. The deposits showed a wide variety of morphologies, with some of the deposits also showing macroscopic surface features such as pores, crevices and ramified growth. This finding confirms that of the previous study regarding the strong influence of BSAs on the morphology of zinc electrodeposits, with a more detailed investigation into this phenomenon currently underway in an ongoing study.

Fig. 8 Scanning electron micrographs of deposits acquired with the various electrolytes. The images show significant influence of the BSA on deposition quality.

4 Conclusions

The performance of seven zinc bromine battery electrolytes was compared in charge/discharge cycling, five of which were tested in flow cell conditions for the first time. These novel BSAs were benchmarked against an electrolyte containing [C₂MPyrr]Br as BSA which is conventionally employed in most commercial zinc bromine flow batteries. The performances of the BSAs were determined by comparison of VE, CE, EE and RC values acquired via flow cell testing. Stronger ionic and other bonding interactions between (poly)bromide species and BSA cations reduce the aqueous bromine content and thus inhibits the self-discharge reaction in the battery. This was evident in the results of RC where EL2 (containing [C₂MPip]Br as BSA) had the best performance, while EL6 (with [C₂MIm]Br as BSA) performed only slightly better than EL0 without BSA.

However, strong bromine sequestration also constrains bromine transport to the electrode surface and therefore causes electrode polarization evident in lower voltaic efficiency. In the case of [C₂MPip]Br this prevents full discharge of the cell and losses in VE more than offset gains in CE.

The following conclusions can be drawn:

(1) Processes in the discharge phase have higher influences on cell performance compared to those in the charge phase.

(2) VE is inversely correlated to BSA cation interactions with (poly)bromide species.

(3) RC is directly correlated to BSA cation interactions with (poly)bromide species.

(4) CE also depends on BSA cation interactions with (poly)bromide species but goes through an optimum value.

(5) [C₂MIm]Br and [C₂Py]Br appear to be viable BSA alternatives to the standard [C₂MPyrr]Br and should be further investigated.

(6) The type of BSA strongly influences zinc deposition quality.

Acknowledgements

The authors thank the Australian Research Council and RedFlow Ltd (Brisbane, Australia) for research funding and support. They also acknowledge the Australian Postgraduate Award (MS, GPR) and the Henry Bertie and Florence Mabel Gritton Research Scholarship (MEE).

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