Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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High Zn Concentration Pyrrolidinium-Dicyanamide-Based Ionic Liquid Electrolytes for Zn2+/Zn0 Electrochemistry in a Flow Environment Kalani Periyapperuma,§ Cristina Pozo-Gonzalo,§ Douglas R. MacFarlane,¥ Maria Forsyth,§ and Patrick C. Howlett*,§
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ARC Centre of Excellence for Electromaterials Science, Institute for Frontier Materials, Deakin University, Melbourne, Victoria 3125, Australia ¥ ARC Centre of Excellence for Electromaterials Science, School of Chemistry, Monash University, Wellington Road, Clayton 3800, Australia S Supporting Information *
ABSTRACT: The cycling performance of N-butyl-N-methylpyrrolidinium dicyanamide [C4mpyr][dca] ionic liquid (IL) with H2O additive for application in a Zn2+/Zn0 redox couple is reported for the first time under realistic flow conditions using a 3D printed flow half-cell prototype. This IL electrolyte displayed a superior performance at high current densities (3 mA cm−2) under the flow condition, in terms of cycling efficiency (60 ± 2% vs 45 ± 3%) and long-term cycling stability (>200 cycles) in contrast to similar experiments performed under a static or “no flow” condition. This is possibly due to different Zn2+ speciation mechanisms and/or different structuring of the IL cation and anion at the electrode/electrolyte interface under static and dynamic conditions. Significantly, [C4mpyr][dca] IL allowed a high solubility of the Zn(dca)2 salt, up to a ∼1:1 molar ratio, which is desirable for achieving a high energy density. This high concentration IL electrolyte composition, which has been studied here for the first time, displayed the steadiest long-term cycling stability (>100 cycles), a compact and dendrite-free Zn morphology, as well as a high volumetric capacity (ca. 1.6 Ah/L) at higher current density (3 mA cm−2). It was also revealed that H2O is essential in the electrolyte to achieve an improved cycling efficiency (65 ± 2%), and more than 1 wt % H2O is essential to attain a uniform well adhered Zn deposit. The dendritefree Zn morphology, even at higher water contents (10 wt %), allows this system to work successfully in ambient atmospheric conditions. However, considering both the cycling efficiency and Zn deposition morphology, the optimized H2O content in the electrolyte was ∼3 wt %. KEYWORDS: redox flow batteries, ionic liquids, Zn electrochemistry, pyrrolidinium dicyanamide, concentration
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INTRODUCTION Recently, research attention has aimed at improving the state of the art energy storage technologies including Li-ion, metal− air, and redox flow batteries (RFBs) to make them efficient and cost-effective.1 RFBs in particular stand out because of their characteristic feature of independent control of energy storage capacity (proportional to electrolyte volume) and power output (proportional to electrode area).2 From the redox flow technologies investigated in the past, Zn-anode-based systems have captured great attention owing to a number of benefits including high negative potential, safety, cost effectiveness, and an abundant supply of Zn.1,3−5 Among various cathodic species that a Zn anode can be coupled with in aqueous electrolytes, Zn−Br is one of the most developed, possessing a theoretical capacity of 440 Wh kg−1 and a cycling efficiency of 75−80%.2,6 However, the low solubility of active species in aqueous solvents has limited the practical energy density of this system to 65−75 Wh kg−1.2,7 Also, Zn dendrite © XXXX American Chemical Society
formation during charging is another issue yet to be solved, as it can lead to capacity fading or short circuit.5 Similar phenomena can be seen with commercialized aqueous allvanadium RFBs, where their energy density is limited 99.9% H
DOI: 10.1021/acsaem.8b00742 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 10. SEM micrographs of Zn deposits after 1 h of electrodeposition on a GC electrode with (a) 18, (b) 30, (c) 40, and (d) 46 mol % Zn(dca)2 in [C4mpyr][dca] and 3 wt % H2O at an applied current density of −3 mA cm−2 and 11 mL min−1 flow rate.
Figure 11. FT-IR (normalized-ATR) spectra of the C−N and CN peaks of [C4mpyr][dca] + 3 wt % H2O + varying Zn(dca)2 concentration in the ranges of (a) 1200−1450 and (b) 2000−2340 cm−1.
the solvent mixture, five bands emerge at 2127, 2167, 2191, 2230, and 2299 cm−1 frequencies. With increasing salt content, the band at 2191 cm−1 rapidly disappears, while the peak at 2167 cm−1 gradually grows while merging with the band at 2127 cm−1. This characteristic agrees well with the previous explanation suggesting a net consumption of free [dca]− from the solution. In addition, the band at 2127 cm−1 shifts to
higher frequency with increasing Zn2+ concentration. In the literature, it has been shown that shifts to higher wavenumbers for [dca]− vibrations are usually because of the stronger binding of the anion to the metal center.29 It is possible that the shift in the 2167 cm−1 band to a lower frequency with increasing Zn(dca)2 concentration is because of [dca]− acting as a bridging ligand between two Zn2+ ions, implying the I
DOI: 10.1021/acsaem.8b00742 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials existence of larger complex ions of Zn2+.28 The appearance of new peaks in conjunction with the loss of intensity of characteristic peaks and peaks shifts in the C−N and CN regions with the addition of Zn(dca)2 suggest possible changes in Zn2+ speciation with varying Zn salt concentration in the electrolyte. Such changes in Zn2+ speciation can affect ion diffusion, contributing to the differences in Zn morphology observed in Figure 10. It should also be noted that with increasing Zn(dca)2 concentration, other factors such as slower mass transport because of increase in electrolyte viscosity and/ or possible changes in ionic layer arrangement at the electrode/electrolyte interface may also affect the charge transfer rate and can contribute to changes in the Zn morphology at various Zn2+ concentrations.
beneficial for such applications because of their relatively low cost compared to fluorinated-anion-based ILs.16 The demonstration of a high concentration, high volumetric capacity, and safe Zn redox flow analyte is an important step toward a high energy density redox flow battery. In particular, the high solubility of Zn salts and use of water addition to improve cyclability are useful strategies to enhance redox flow battery performance.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00742.
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CONCLUSIONS Here we have investigated the applicability of Zn(dca)2/ [C4mpyr][dca]/H2O as a potential electrolyte for the Zn anode in redox flow batteries for the first time under a realistic flow configuration. Although previously this electrolyte system was shown to exhibit inferior performance compared to [C2mim][dca] in terms of cyclability and Zn morphology under static or “no flow” conditions, the results obtained here under the flow configuration were reversed. We speculate this may be a result of different Zn2+ speciation mechanisms involved with [C4mpyr][dca] and [C2mim][dca] systems and/ or the effect of distinct IL-ion arrangements at the electrode surfaces. Further, similar to the imidazolium system, a dendrite-free, uniform Zn morphology was also obtained with [C4mpyr][dca] under the flow configuration. The concentration of water in the electrolyte was found to directly affect both the cycling efficiency and Zn deposition morphology. With increasing water content, the cycling efficiency slightly decreased from 65 to 50 ± 2%. However, water is essential to overcome the cycling inefficiencies caused by possible mass transport limitations. Also, more than 1 wt % of H2O is required in the electrolyte to achieve a compact and dendrite-free Zn deposit that is well adhered to the electrode surface. Nevertheless, all of the compositions showed stable long-term cyclability (>100 cycles) under the flow configuration in ambient atmospheric conditions. Another highlight of the [C4mpyr][dca] IL solvent system is that it enables a high solubility of active species with a ∼1:1 molar ratio, which is pivotal for improving the energy density of redox flow systems. Regardless of Zn2+ concentration, all of the electrolyte compositions showed a similar cycling efficiency and discharge capacity. Nevertheless, the higher concentration systems (30−46 mol %) showed a steadier and more stable long-term cyclability (>100 cycles) compared to the erratic cycling behavior of lower concentration systems. Furthermore, the highest concentration system (46 mol %) showed a compact and uniform Zn morphology without any indications of dendrite formation, which is one of the major issues yet to be overcome in current aqueous RFBs. It is also important to note that there is more room for improving the cycling performance of the high concentration electrolyte by optimizing operating parameters including the water content, flow rate, and current density. Nonetheless, this work under the flow configuration demonstrates the stability and reliability of the Zn2+/Zn0 redox couple in a [C4mpyr][dca] IL in ambient atmospheric conditions, providing the basis for future investigations of high energy IL/aqueous Zn-based flow batteries. In addition, pyrrolidinium dicyanamide ILs are
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A table summarizing the performance of the 3D printed flow half-cell prototype; the voltage profile for initial Zn deposition on the pristine glassy carbon electrode; SEM and EDX of remaining Zn deposit after exhaustion of initial Zn deposit; FT-IR spectra of O−H region for 18 mol % Zn(dca)2 with 1, 3, 6, and 10 wt % H2O; SEM and EDX analysis of Zn electrodeposit on the thin surface film and the less compact deposit obtained by 40 mol % Zn(dca)2 electrolyte (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cristina Pozo-Gonzalo: 0000-0002-7890-6457 Patrick C. Howlett: 0000-0002-2151-2932 Author Contributions
All authors have given approval to the final version of the manuscript. Funding
The authors gratefully acknowledge financial support from the Australian Research Council through the Centre of Excellence for Electromaterials Science (ACES - Project ID CE140100012). Prof. Douglas MacFarlane acknowledges support from the ARC Laureate program. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank our collaborators Prof. Brett Paul and Dr. Niall Macdonald from the University of Tasmania for 3D printing capability. We also thank the Australian Synchrotron for the access to the powder diffraction beamline as well as Dr. Helen Brand and Dr. Justin Kimpton for their assistance with the measurements.
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REFERENCES
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DOI: 10.1021/acsaem.8b00742 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaem.8b00742 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
