Zinc bromide in aqueous solutions of ionic liquid bromide salts: the interplay between complexation and electrochemistry

Abstract

Voltammetric studies of ZnBr₂ (50 mM) in aqueous equimolar solutions of ionic liquid bromide salts (1-alkyl-1-methylpyrrolidinium bromide, [C_nMPyrr], and tetraalkylammonium bromide, [N_n,n,n,n]Br, 50 mM, where n = 2, 4 or 6) are reported. A simple increase in alkyl chain length of the cation of the bromide salt greatly altered the mechanism of zinc deposition as a result of the complexation of bromozincate anions. The structures of two such bromozincates were isolated and their structures determined by single-crystal X-ray diffraction. These findings provide an additional strategic basis for the design of alternate bromine sequestering agents for aqueous zinc bromide flow cells.

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1. Introduction

The performance of the Zn/Zn(II) redox couple in novel electrolytes has been of significant research interest in the fields of electrodeposition and electrochemical storage devices. In the latter case, zinc is a promising candidate in half cells due to its abundance and low cost, as evidenced by its use in a number of electrochemical storage devices including the zinc–air batteries,¹ the sodium–zinc chloride battery,² the zinc bromide hybrid flow battery (ZBB)^3,4 and batteries employing various other zinc-based chemistries.^5,6 While research is often focused on conventional electrolytes,^7,8 several publications have recently employed various classes of ionic liquids as components of new electrolyte systems. These ionic liquids are often based on imidazolium and pyrrolidinium cations (the latter of which have been in use since the 1970s (ref. 9–11)) paired with triflate, bis(trifluoromethanesulfonyl)imide or dicyanamide anions.^12–16

The movement towards ionic liquids as electrolytic media is primarily motivated by their wide electrochemical windows, low vapour pressures (preventing drying out of the electrolyte) and the exclusion of potential-limiting water reactions.^17–20 Recent work has shown high cyclability and energy current densities for Zn-air batteries in IL electrolytes,^13,15,21 while in the case of the ZBB, IL-like bromide salts have long been used as additives to the aqueous electrolytes in order to sequester the otherwise volatile and corrosive bromine evolved at the battery's anode.^9,11,22 This so-called bromine sequestering agent (BSA) is typically the bromide salt of the 1-ethyl-1-methylpyrrolidinium cation. The primary requirement for a BSA is the presence of the bromide anion, which can be matched with numerous cationic species.

Recent work for the Zn/Zn(II) redox couple in ILs has demonstrated favourable cyclability and zinc deposition morphology with an increase in the concentration of water in the electrolyte.^13,15,23 Xu et al.¹³ proposed an optimised electrolyte composition for zinc–air batteries consisting of molar ratios of IL : H₂O : DMSO of 1 : 1.1 : 2.3, which can alternatively be viewed as a concentrated IL solution (∼5 M). Further work showed the advantages of low concentrations of IL (0.5 wt%) as additives to influence the arrangement of the electric surface layer,²⁴ demonstrating the controllable nature of an electrolyte system with careful tailoring of its components. Further, this demonstrates a shift in the battery literature from aqueous battery electrolytes, via all-IL electrolytes, to a median point that takes advantages of each regime.

In aqueous electrolytes, IL-like additives have been shown to favourably influence zinc deposition rates and morphologies, as well as battery performance.^13,25 Despite IL BSAs being a significant component of the ZBB electrolyte (1.0 M), little work has been performed to screen the influence of such IL BSAs on zinc deposition behavior outside of the determination of general physical properties.¹¹ Similarly, recent studies of ILs for the zinc-containing cells typically focus on anion variation rather than the influence of cationic species on zinc speciation.^26,27

Conventional cyclic voltammetry (CV) for metallic electrodeposition involves a reduction of metal ions to the neutrally charged metal, electrodeposited on the electrode surface. On the return sweep, a potential crossover occurs (referred to as the nucleation loop) due to a change in the nature of the electrode surface as a consequence of metal deposition, which results in oxidation of the metal (the stripping peak) at more positive potentials than that of the reductive peak.^28,29 These conventional CVs for electrodeposited metals are observed where minimally complexed metal ions are freely available for electrodeposition from their electrolyte system. However, some ILs are capable of complexing metallic ions, resulting in an alteration of the observed electrochemistry.^14,24 Thus, this complexation may also provide opportunities to direct the kinetics and morphologies of zinc electrodeposits.^30,31

An unconventional deposition pattern involving ‘blocking’ of the electrode from reduction of metal ions during a forward (reducing) voltammetric scan has recently been observed for the zinc redox couple in neat ionic liquids and aqueous media,^12,14,15,30 as well as in deep eutectic solvents incorporating choline chloride and ethylene glycol.^8,32 This behavior has been ascribed to one of four general schemes. Simons et al. suggested the pre-formation of an IL double-layer on polarization of the electrode, preventing the approach of electroactive species.^14,15 By this mechanism, on the return positive scan a relaxation of this double-layer was proposed, allowing the complexed zinc species to be reduced at the still reducing return potentials. Similarly, Whitehead et al. pointed to the assembly of a positively charged solvent species (in their case, choline cations), resulting in the blocking and passivation of the electrode surface on the forward, negative scan.^33,34 Xu et al. demonstrated preferential electrodeposition of zinc with the addition of water to an all-IL electrolyte, complexing zinc as positively charged zinc aqua cations rather than as the negatively charged complex from the dicyanamide anions studied.^12,13,24 Alternatively, the work of Vieira et al. proposed the formation of an intermediate RO⁻ species produced by deprotonation of the solvent (in their case, ethylene glycol) at reducing potentials, with favourable deposition occurring from an intermediate Zn(OR)_n species.^8,32

The work reported here outlines the varying electrochemistry of the Zn/Zn(II) redox couple in solutions of zinc bromide and the aqueous bromide salts of interest in zinc bromine flow cell applications. New electrochemical signals for the Zn/Zn(II) redox couple resulted from only minor structural alterations of the BSA employed as a supporting electrolyte in zinc bromine flow cells. These alterations are ascribed to complexation of bromozincate intermediates which were isolated and structurally characterised. This work aids in developing structural criteria for the selection of new BSAs, and provides supportive explanations for various zinc deposition mechanisms induced by complexation phenomena, enriching understanding of zinc electrodeposition from aqueous systems, and enriching the understanding of bromozincate chemistry.

2. Experimental section

2.1. Chemical reagents

Zinc bromide dihydrate (ZnBr₂·2H₂O) was used as received (Aldrich). 1-Alkyl-1-methylpyrrolidinum bromide salts ([C_nMPyrr]Br, n = ethyl or hexyl) were prepared by quaternisation of 1-methylpyrrolidine (Alfa-Aesar) with the required bromoalkane (bromoethane, BDH or bromohexane, Merck) by standard literature methods.³⁵ The resulting white solid was recrystallized from its acetonitrile solution by addition of ethyl acetate and was dried under vacuum at 60 °C prior to use. Tetraethylammonium bromide ([N_2,2,2,2]Br, Merck), tetrabutylammonium bromide ([N_4,4,4,4]Br, Merck), potassium chloride (KCl, Ajax), ferrocene, (Aldrich), and tetrabutylammonium perchlorate ([TBAP], Aldrich) were used as received. FTO glass (TEC-15 glass plate, Dyesol Industries) was pre-treated by three rinses in a sonicating bath with acetone, isopropanol and deionized water, respectively. The treated FTO glass was dried under nitrogen prior to use.

2.2. Voltammetric methods

All voltammetric experiments were performed on an edaq ER466 Integrated Potentiostat System (edaq, Sydney, Australia) using a conventional three-electrode configuration. Cyclic voltammograms were obtained using a 1 mm diameter glassy carbon working electrode (edaq) with a Pt wire as counter electrode and a leakless Ag/AgCl reference electrode. Working electrodes were polished on a succession of alumina slurries (1 μM and 0.3 μM) prior to each experiment and treated with oxidizing potentials following each experiment to remove any deposited zinc. The electroactive surface area was calibrated from the oxidation of a 5 mM ferrocene solution in 0.1 M TBAP in acetonitrile, assuming the diffusion coefficient to be 2.4 × 10⁻⁷ cm s⁻¹ at 25 °C.³⁶ All solutions were purged with argon prior to analysis and the experiments were conducted at 25 ± 2 °C.

Chronoamperometry was performed in refreshed electrolyte solutions of identical compositions to the cyclic voltammetric studies. Current–time transients were collected via two-step chronoamperometry, with 3 s equilibration time at −0.6 V preceding reduction at −1.25 V vs. Ag/AgCl reference. Similarly, zinc electrodeposits were prepared on an FTO glass working electrode substrate by the same methodology. These electrodeposits were rinsed with deionized water and then acetone, prior to drying under nitrogen flow and mounting for Scanning Electron Microscopy (SEM) analysis. SEM was performed with a Zeiss ULTRA+ instrument operating at 5.0 kV.

2.3. Crystallographic methods

For the [N_2,2,2,2]₂[Zn₂Br₆] salt, colourless, prismatic crystals were collected on standing of an equimolar 1.0 M aqueous solution of [N_2,2,2,2]Br and ZnBr₂ for 24 h. The crystal structure was solved in the space group P2_1/n (#14) by direct methods and the formula designated [N_2,2,2,2]₂[Zn₂Br₆] (see ESI† for further details). For the [C₆MPyrr]₃[Zn₂Br₇] compound, a colourless solid precipitated rapidly from a 1.0 M aqueous solution of [C₆MPyrr]Br and ZnBr₂. This solid was removed, dried and dissolved in acetone, with recrystallization of irregular shaped crystals via slow evaporation of the solvent. The structure was solved in the space group P31c (#159) by direct methods prior to refinement and treatment for twinning (see ESI† for further details).

3. Results and discussion

3.1. Structural characterization of bromozincate precipitates

High concentration aqueous solutions of zinc bromide and some bromide salts (QBr, where Q = cation) can yield a precipitate that was proposed in the early zinc bromide battery literature to be of stoichiometry ZnBr₂·2QBr.^10,37 At high concentrations (i.e., 1 M each of ZnBr₂·2H₂O and QBr) of the salts in this study, such white precipitates were formed immediately for mixtures of the longer alkyl chain salts ([C₆MPyrr]Br and [N_4,4,4,4]Br), while the solution containing [C₂MPyrr]Br exhibited no obvious evidence of precipitation at high concentrations. At a concentration of 1 M, an aqueous solution of equimolar amounts of ZnBr₂ and [N_2,2,2,2]Br could be prepared, which slowly precipitated large, prismatic crystals of a ZnBr₂·[N_2,2,2,2]Br salt over 12 h. In the case of [C₆MPyrr]Br, a colourless solid precipitated rapidly, but required recrystallization for further analysis (see Experimental section). Selected crystals from these two solids were examined by single crystal X-ray diffraction, vide infra. By contrast, the rapid precipitation of solid masses from aqueous ZnBr₂ solutions containing [N_4,4,4,4]Br produced a solid, the crystals of which were of insufficient quality for crystallographic analysis.

Single crystal X-ray diffraction analysis of the crystalline solid precipitated from the aqueous solution of [N_2,2,2,2]Br and ZnBr₂ revealed a salt of the formula [N_2,2,2,2]₂[Zn₂Br₆] (Fig. 1, see ESI† for crystallographic details). The [N_2,2,2,2]₂[Zn₂Br₆] moieties stack in layers, similar to those of the dizinchexabromide dianion previously characterised in the literature in the presence of other cations.^38,39 The closely related ZnBr₃⁻ salt of [N_3,3,3,3]⁺ has been spectroscopically identified⁴⁰ but not structurally characterized. The [Zn₂Br₆]²⁻ anion can be considered a dimer of two ZnBr₃⁻ units, a ZnBr₂ adduct of a tetrahedral [ZnBr₄]²⁻ ion, or as two edge-sharing [ZnBr₄]²⁻ units. The crystal structure of Fig. 1a most closely resembles the latter case, and is thus described as [(ZnBr₃)₂(μ-Br)₂]²⁻. The depiction of the crystal packing in Fig. 1b shows layers of tetraethylammonium cations that encapsulate the bromozincate dianion.

Fig. 1 The structure of [N_2,2,2,2]₂[Zn₂Br₆] determined by single crystal X-ray diffraction: (a) showing the repeating unit of two tetraethylammonium cations and the dizinchexabromide anion, and; (b) extended lattice looking along the c-axis to show encapsulation of the bromozincate dianions. Red spheres indicate the positions of bromine atoms, purple, those of zinc atoms, orange those of nitrogen, dark grey those of carbon and light grey those of hydrogen, atoms. See ESI† for ORTEP representation and further information.

The solid obtained by precipitation from the solution of [C₆MPyrr]Br and ZnBr₂ was also structurally characterized, and designated the formula [C₆MPyrr]₃[Zn₂Br₇] (Fig. 2, see ESI† for crystallographic details). Unlike the dizinchexabromide dianion (Fig. 1a), a bromine atom (shown in Fig. 2a displaced from, and disordered around the Zn–Br–Zn axis in three positions) can be considered to bridge two [ZnBr₃]⁻ moieties, resulting in two four-coordinate zinc sites. A dizincheptabromide trianion has been assigned previously as a dimer of [ZnBr₄]²⁻ by Raman spectroscopy, but not structurally characterized.^41,42 Consistent with the previous structural description, the [Zn₂Br₇]³⁻ trianion can be considered as comprising two corner-sharing [ZnBr₄]²⁻ tetrahedra, and can be described as [{ZnBr₃}₂(μ-Br)]³⁻. The crystal packing depicted in Fig. 2b shows encapsulation of the bromozincate ions by the [C₆MPyrr]⁺ cations.

Fig. 2 The structure of [C₆MPyrr]₃[Zn₂Br₇] determined by single crystal X-ray diffraction: (a) showing the repeating unit of three 1-hexyl-1-methylpyrrolidinium cations and the dizincheptabromide anion, and; (b) extended lattice looking along the c-axis to demonstrate the encapsulation of the dianions. Red spheres indicate the positions of bromine atoms, purple, those of zinc atoms, orange those of nitrogen, dark grey those of carbon and light grey those of hydrogen, atoms. See ESI† for ORTEP representation and further information.

The characterization of the bromozincate salts as either a [{ZnBr₂}₂(μ-Br)₂]²⁻ dianion or [{ZnBr₃}₂(μ-Br)]³⁻ trianion suggests the possibility of related monovalent ion. The dianion structure has been interpreted as two [ZnBr₄]²⁻ tetrahedra that share an edge, whilst in the trianion structure a vertex is shared to form the trianion. This suggests the possibility of a shared face between the two tetrahedra resulting in a [Zn₂Br₅]⁻ monoanion. Although such a structure has not yet been directly characterised, the [Zn₂Br₅]⁻ has been observed by high resolution mass spectrometry as a component of a cyanamide–zinc complex.⁴³ The present work suggests that this ion might be isolated by strategic variation of the IL counterion.

The structural determination of the two bromozincate compounds above demonstrates the complexation behavior that is possible in solutions of zinc bromide and IL bromide salts. For the ensuing voltammetric study, solutions at these concentrations were unable to be studied due to precipitation of the electroactive zinc salt. Thus, concentrations were decreased significantly (from 1 M to 50 mM) to a level where no precipitation occurred in order to study a homogenous aqueous medium.

3.2. Cyclic voltammetry of zinc bromide in the presence of ionic liquid bromide salts

Cyclic voltammetry experiments at a glassy carbon electrode were performed by scanning potentials in a negative direction from a resting potential at which no electrochemical reaction occurs (−0.6 V vs. Ag/AgCl), to a potential beyond the reducing peak current (−1.4 V vs. Ag/AgCl), before scanning back to the initial potential value. In each experiment, equimolar aqueous solutions of zinc bromide and a bromide salt were prepared at concentrations of 50 mM each in a supporting electrolyte of 0.4 M KCl. The four bromide salts studied fall into two classes, where the cations were: tetraalkylammonium ([N_n,n,n,n]) or 1-alkyl-1-methylpyrrolidinium ([C_nMPyrr]) where n = 2 or 4 (linear tetraethyl or tetrabutyl) for the former, and 2 or 6 (linear ethyl or hexyl chains) for the latter salts.

A clear influence on the reversible deposition of zinc on a glassy carbon working electrode (Fig. 3) is observed on alteration of the cation of the bromide salt additive. The CVs of Fig. 3a and b (new redox behaviour in the presence of [C₂MPyrr]Br or [N_2,2,2,2]Br respectively) are typical for metallic deposition and stripping in aqueous solutions. These sequences consist of a reductive, deposition current (in this case, beginning at approximately −1.15 V vs. Ag/AgCl), with the oxidative, stripping, current occurring at a more positive potential than that of deposition onset, which is consistent with the nucleation of a solid species on the working electrode (the ‘nucleation loop’).

Fig. 3 Cyclic voltammograms (selections from five cycles) of equimolar 50 mM aqueous solutions of ZnBr₂·2H₂O and: (a) [C₂MPyrr]Br; (b) [N_2,2,2,2]Br; (c) [C₆MPyrr]Br; (d) [N_4,4,4,4]Br (scan rate 100 mV s⁻¹; supporting electrolyte 0.4 M aqueous KCl).

On increasing the alkyl chain lengths of the bromide salts added, a significant change in zinc redox behavior is observed for both classes of bromide salts. For the change from [N_2,2,2,2]Br to [N_4,4,4,4]Br additives, at the same zinc concentration, the most prominent peak current magnitudes are decreased from 0.66 and 0.33 mA to 0.04 and 0.007 mA, respectively. Notably, an unconventional deposition pattern occurs for zinc deposition in the presence of the larger cation, with minimal deposition occurring on the forward (negative) scan, and the largest deposition current observed for the reverse sweep, with as many as four distinct deposition currents observed. This behaviour persists for repeating cycles, with the stripping peak current decreasing substantially from the 1^st to 5^th scan.

Similar behaviour is observed on comparison of the electrolytes containing ethyl-([C₂MPyrr]⁺) to hexyl-substituted ([C₆MPyrr]⁺) cations. The CV in the presence of [C₆MPyrr]Br demonstrates similar unconventional deposition behaviour to that observed in the presence of [N_4,4,4,4]Br, although a significant amount of deposition on the forward scan (1^st cycle only) is apparent prior to an additional deposition current observed on the return sweep. On repeat cycles, an alteration of the deposition pattern is observed that is repeated for the next five cycles, whereby deposition on the reverse sweep is the dominant reductive process observed.

The increase in alkyl-chain length of the cations of the accompanying bromide salts is shown to influence scan rate dependence as well as the cycling behaviour of the voltammograms (Fig. 4). For the ethyl-substituted cations, zinc deposition is shown to be diffusion controlled and minimally altered by repeat scans, as is expected for free deposition and stripping. Conversely, voltammetry in the presence of [N_4,4,4,4]Br and [C₆MPyrr]Br demonstrates no clear relationship between peak current and scan rate. In the case of voltammetry in the presence of [N_4,4,4,4]Br, deposition and stripping currents appear to increase with decreasing scan rate, suggesting a deposition mechanism which is time-dependent rather than diffusion related. It is suggested that the slower scan rates result in an increase in the voltammetric response due to a time and potential dependent arrangement of complexed ions in the electrolyte, in which a relaxation of blocking cation layers on the reverse sweep allows for further electrodeposition (vide infra).

Fig. 4 Cyclic voltammograms of equimolar 50 mM aqueous solutions of ZnBr₂·2H₂O and: (a) [C₂MPyrr]Br; (b) [N_2,2,2,2]Br; (c) [C₆MPyrr]Br; (d) [N_4,4,4,4]Br (varying scan rates; supporting electrolyte 0.4 M KCl).

At faster scan rates, such a relaxation process does not occur quickly enough, so that the electrode passivation is not overcome within the timescale of the CV scan, and thus, the observed peak currents are decreased.

In the presence of [C₆MPyrr]Br, decreasing scan rates result in mixed electrodeposition modes (on both forward and reverse scans) with equivalent stripping peak currents and the appearance of a second stripping response at a scan rate of 20 mV s⁻¹. Multiple zinc stripping peaks such as these have been attributed in the past to alloyed metal deposits (e.g. Au/Zn)²³ or the influence of high electrolyte viscosity,¹² which are not plausible explanations for this study. In the present case, it is likely to be a result of two distinct zinc deposition modes, for example, zinc-on-glassy carbon and zinc-on-zinc. This behaviour is an established phenomenon in metal stripping voltammetry,⁴⁴ however no further evidence of such phenomena was observed by SEM, vide infra.

3.3. Deposition mechanisms

Both of the observed zinc deposition mechanisms have literature precedence, with conventional deposition modes associated with freely mobile metal ions, and the alternative mechanism (dominated by reduction on the reverse sweep) present in cases where significant complexation or electrolyte viscosity dominates the metal speciation in solution. This work examines the use of structurally similar additives (altered only by the substituted alkyl-chain length of the additive's cations) to form solutions that are susceptible to both mechanisms of deposition. Thus, neither one of the literature descriptions of deposition is by itself sufficient to explain the behavior observed here. The presence or absence of an IL double layer¹⁵ or formation of an intermediate deprotonated species³² would be dependent upon the alkyl-chain length of the cation, which is unlikely to be the sole determinant for the observed behaviour, but serves as a useful point of departure for the following discussion. Further, the susceptibility to electrode passivation on the forward (reducing) scan due to the pre-formation of an intermediate RO⁻ species cannot be an isolated solvent phenomenon in this case (as the solvent, water, is constant). Thus, for the data observed a combination of both the IL double layer proposed by Simons et al. and the complexation of metal ions described by Xu et al. are likely the most valid.^12–15

Zinc can be complexed by negative bromide, neutral water ligands, or both, influencing the arrangement of the electric double layer (EDL) at the polarized working electrode. Such an EDL arrangement is a well-known phenomenon in aqueous electrolytes,⁴⁵ with similar behaviour also observed in detail by atomic force microscopy (AFM) and other physical methods in the ionic liquid literature.^46,47 It is suggested that in the presence of the longer alkyl-chain containing cations, zinc ions are coordinated preferentially as an anionic bromozincate species due to the lipophilic association of the larger cations, which were shown by structural characterization to ‘sandwich’ the complexed bromozincate anion. Thus, we propose the longer alkyl-chain length results in a greater association between the organic bromide salt and the zinc bromide ions in solution, increasing the likelihood of zinc ions coordinating bromine ligands relative to water.

Since the forward scanning reducing potentials occur concurrently with negative polarization of the working electrode, the formation of anionic zinc ions will disfavour electrodeposition due to repulsion of the zincate anions by the polarized electrode. As seen in the CV study, when the working electrode is positively polarized (following the switching potential), a depolarization of the EDL occurs, and electrostatic attraction of the anionic zinc species promotes deposition. In the case of the additives, [Q]Br (where Q = cation), that are poor donors of bromide ligands toward zinc ions, electrodeposition occurs readily on the forward scan. This demonstrates that neutral or uncomplexed zinc species are able to more readily penetrate the EDL. In aqueous, bromide-containing media, these two deposition mechanisms can be summarized by eqn (1) (unconventional, unfavourable deposition) and (2) (conventional, favourable deposition), respectively. Although these effects likely act in concert in the presence of each additive, QBr, the predominance of one mechanism is suggested by the voltammetric observations.

Zn(Br)_x^(x−2)− + 2e⁻ → Zn⁰ + _xBr⁻ (1)

Zn(H₂O)_y²⁺ + 2e⁻ → Zn⁰ + _yH₂O (2)

A situation similar to that described in eqn (1) and (2) was proposed for zinc electrochemistry in dicyanamide (DCA) ionic liquids by Xu et al., in which water-containing electrolytes showed preferential electrodeposition due to the formation of positively charged hydrated complexes, rather than the negatively charged Zn(DCA)_n⁽ⁿ⁻²⁾⁻ complexes.^13,24 Further evidence of such behaviour can be obtained through the observation of current transients and the resulting deposition morphologies during the constant potential electrolysis of zinc in each solution.

3.4. Chronoamperometry and zinc deposition morphologies

Fig. 5 demonstrates overlaid current transients for each sample studied by chronoamperometric methods, in which a constant depositing potential of −1.25 V vs. Ag/AgCl was applied to examine the resulting current response. Significantly larger peak deposition currents (280 and 300 mA) are observed for the additives with ethyl-substituents than are observed when using their long alkyl-chain congeners, for which significantly hindered rates of deposition ([C₆MPyrr]⁺ – reaching a peak deposition potential of 100 mA at 6 s), or very minor deposition currents are observed (10 mA for the [N_4,4,4,4]⁺).

Fig. 5 Chronoamperometric transients for zinc deposition (−1.25 V vs. Ag/AgCl) in the presence of IL bromide salts (indicated in the legend).

This resistance to immediate deposition is suggested to be a result of the complexation mode of eqn (1), resulting in repulsion of zincate anions approaching the negatively polarized electrode. Precise determination of nucleation mechanisms and the resulting diffusion coefficients by the methods of Scharifker and Hills,⁴⁸ were inaccessible, since the current transients failed to accurately reflect the model transients for neither instantaneous nor progressive nucleation (see ESI†). In this case, it appears a modified nucleation mode resembling progressive nucleation is most likely, but because experimental dimensionless transients are only poorly fitted by the calculated transients, this cannot be stated with confidence.

Nevertheless, the effect of this hindered deposition mode on the size and morphology of the corresponding zinc deposits can be observed via SEM analyses (Fig. 6, see ESI† for EDX spectra, confirming the identity of the electrodeposit). The SEM images demonstrate a denser arrangement of deposited zinc when the deposition occurs in the presence of the ‘non-complexing’ additives as compared to what is the case when the ‘complexing’ additives are employed. This is consistent with the higher deposition currents observed in the chronoamperometric transients, which demonstrate a greater charge passed per unit time, and thus more deposited zinc. The hexagonal deposition morphologies are shown to be related for each sample, with more rapid electrodeposition resulting in, for example, hexagonal columns of about 1 μm in length for deposition in the presence of [N_2,2,2,2]Br (Fig. 6b) compared to thin (50 nm) hexagonal discs for the low-current deposition in the presence of [N_4,4,4,4]Br (Fig. 6d) induced by complexation of the zinc cation by negative bromide ligands.

Fig. 6 SEM images of zinc electrodeposits from equimolar aqueous solutions of ZnBr₂·2H₂O and: (a) [C₂MPyrr]Br; (b) [N_2,2,2,2]Br; (c) [C₆MPyrr]Br; (d) [N_4,4,4,4]Br (5 min @ −1.25 V on FTO glass).

4. Conclusions

Examination of the Zn/Zn(II) redox couple in the presence of equimolar concentrations of various bromide salts has identified two different electrochemical deposition mechanisms. The bromide salts that were shown to complex zinc cations to form anionic bromozincate species of the general formula Zn_nBr_n·2(QBr) experienced minimal electrodeposition on the forward (reducing) scans, due to the unfavourable negative charge of the [Zn_nBr_(n+2)]^m− anion inhibiting reduction on a negatively polarized electrode. Conversely, conventional cyclic voltammograms were observed in the presence of those bromide salts that did not significantly form negative bromozincate complexes. The structures of a dizinchexabromide dianion and dizincheptabromide trianion were also determined, the latter of which represents the first structural characterization of such an ion. The common structural motifs of these ions – condensed [ZnBr₄]²⁻ tetrahedra, suggest the existence of a [Zn₂Br₅]⁻ anion, composed of two face-sharing tetrahedra. This work provides additional structural criteria for aqueous electrolyte additives for technologies such as the zinc bromide flow battery. This work also provides an additional explanation for the conflicting mechanistic literature discussions of the Zn/Zn(II) redox behaviour in the presence of complexing additives and expands upon general bromozincate chemistry.

Acknowledgements

The authors acknowledge the Australian Research Council, Redflow Ltd, Alpha Chemicals and the Henri Bertie and Florence Mabel Gritten Postgraduate Scholarship (MEE) for sources of funding. The use of microscopy facilities of the Australian Centre for Microscopy and Microanalysis (ACMM) at the University of Sydney is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1416613 and 1416614. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra15736f

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