Max E. Eastona,
Peter Turnera,
Anthony F. Mastersa and
Thomas Maschmeyer*ab
aSchool of Chemistry, University of Sydney, NSW 2006, Australia. E-mail: thomas.maschmeyer@sydney.edu.au
bAustralian Institute for Nanoscale Science and Technology, Australia
First published on 16th September 2015
Voltammetric studies of ZnBr2 (50 mM) in aqueous equimolar solutions of ionic liquid bromide salts (1-alkyl-1-methylpyrrolidinium bromide, [CnMPyrr], and tetraalkylammonium bromide, [Nn,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.
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.
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.
Single crystal X-ray diffraction analysis of the crystalline solid precipitated from the aqueous solution of [N2,2,2,2]Br and ZnBr2 revealed a salt of the formula [N2,2,2,2]2[Zn2Br6] (Fig. 1, see ESI† for crystallographic details). The [N2,2,2,2]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 [N3,3,3,3]+ 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 [N2,2,2,2]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.
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 [N2,2,2,2]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’).
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 [N2,2,2,2]Br to [N4,4,4,4]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 1st to 5th 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 [N4,4,4,4]Br, although a significant amount of deposition on the forward scan (1st 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 [N4,4,4,4]Br and [C6MPyrr]Br demonstrates no clear relationship between peak current and scan rate. In the case of voltammetry in the presence of [N4,4,4,4]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).
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.
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.
![]() | ||
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 [N2,2,2,2]Br (Fig. 6b) compared to thin (50 nm) hexagonal discs for the low-current deposition in the presence of [N4,4,4,4]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) [N2,2,2,2]Br; (c) [C6MPyrr]Br; (d) [N4,4,4,4]Br (5 min @ −1.25 V on FTO glass). |
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 |
This journal is © The Royal Society of Chemistry 2015 |