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Aspects of electron transfer processes in vanadium redox-flow batteries
Journal Pre-proof Aspects of electron transfer processes in vanadium redox-flow batteries Nataliya Roznyatovskaya, Jens Noack, Karsten Pinkwart, Jens Tübke PII:
S2451-9103(19)30144-9
DOI:
https://doi.org/10.1016/j.coelec.2019.10.003
Reference:
COELEC 463
To appear in:
Current Opinion in Electrochemistry
Received Date: 13 August 2019 Revised Date:
27 September 2019
Accepted Date: 3 October 2019
Please cite this article as: Roznyatovskaya N, Noack J, Pinkwart K, Tübke J, Aspects of electron transfer processes in vanadium redox-flow batteries, Current Opinion in Electrochemistry, https:// doi.org/10.1016/j.coelec.2019.10.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V.
Aspects of electron transfer processes in vanadium redox-flow batteries. Nataliya Roznyatovskaya*1,2, Jens Noack1,2, Karsten Pinkwart1,2 and Jens Tübke1,2 1
Fraunhofer Institute for Chemical Technology, Applied Electrochemisrty, Joseph-von-FraunhoferStr. 7, Pfinztal 76327, Germany
2
German-Australian Alliance for Electrochemical Technologies for Storage of Renewable Energy (CENELEST), Mechanical and Manufacturing Engineering, University of New South Wales (UNSW), UNSW Sydney, NSW 2052, Australia
*
corresponding author:
Email: [email protected] Tel: +49-721-4640659 Keywords Vanadium redox-flow battery; vanadium electrode reactions; kinetics
Abstract The electrochemical processes in vanadium redox-flow batteries (VRFBs) include conversions of vanadium species in acidic electrolytes with total vanadium concentrations over molar range. The majority of currently available data on electrode kinetics of vanadium reactions, and on the role of electrode surface chemistry are obtained for diluted electrolyte solutions and are very controversial. In this minireview we consider the interpretations of electrochemical kinetic data for vanadium electrode reactions and mechanistic concepts, which have been reported in the literature. Thereby, the gap between electrochemical kinetics in „diluted“ and „concentrated“ solutions is in the focus of the review.
Introduction The electrode reactions of vanadium species attract increasing attention since the last two decades. This is due to the fact that the redox reactions of three vanadium couples (V(III)/V(II), V(IV)/V(III) and V(V)/V(IV)1) at the electrode-electrolyte interface define the chemistry and operation of VRFB, which have been developed and commercialized. Therefore, the kinetics of these reactions is of crucial importance for understanding and optimization of VRFB performance. To obtain the electrochemical rate parameters and ultimately increase the power density of VRFB the information about vanadium speciation in electrolyte and kinetic data are needed. This information is available for vanadium electrode reactions in diluted solutions at the electrodes with well characterized surface structure, i.e. 1
For brevity, vanadium species in different oxidation states are denoted here as V(II), V(III), V(IV) and V(V) respectively, unless the structure of vanadium species in solution is considered.
1
under well defined mass transport conditions. Concerning the VRFB application, the total vanadium concentration in electrolyte exceeds molar range and electrode reactions proceed under mixed control (convective-diffusion mass transport and charge transfer) at the porous electrode surface with poorly characterized surface structure. This review is focused on the recent investigations of vanadium electrode reactions. The results reported for concentrated electrolytes are considered in comparison with data for diluted solutions. The thermodynamics of the battery under standard conditions can be derived from the combination of the following half-reactions2: + 2 +
+
ℎ ⇌ + ℎ
ℎ ⇌ ℎ
(positive
half-cell)
(negative half-cell)
E0 = 1.0 V vs. SHE
(1)
E0 = -0.26 V vs. SHE
(2)
Since commercial electrolytes for VRFB commonly contain V(III) and V(IV) species in 1:1 molar ratio in a highly acidic matrix, the operation of a VRFB usually includes the initial charging (precharging) step to convert the mixture or initial electrolyte to anolyte and catholyte forms in battery compartments. The reactions of V(IV)/V(III) couple (equation 3) are known to have high activation energy, i.e. slow kinetics [1] and are not well characterized. + 2 +
ℎ ( ) ⇌ + ℎ (ℎ )
E0 = 0.337 V vs. SHE
(3)
The analysis of locally-resolved current density distribution at the cathode in a VRFB with segmented current collector during initial charging indicates that the vanadium conversion, i.e. pre-charging proceeds by EC’ (electrochemical-catalytic) mechanism [2]. Though the attempt to accelerate this reaction by electrocatalysis and to extend the theoretical volumetric capacity of VRFB by using the V(IV)/V(III) couple as energy bearing component of battery is recently reported in the work [3], the processes during the pre-charging of conventional VRFB are poorly investigated. Therefore, the electrode reactions of V(III)/V(II) and V(V)/V(IV) couples in acidic solutions are considered further. Reactions of V(III)/V(II) redox couple The V(III) and V(II) species at millimolar concentrations in non-complexing or weakly complexing acids (like HClO4) present as hexaaqua-cations of common structure [V(H2O)6]n+. V(III)/V(II) couple was used in the past as a model redox pair to study the effect of reactant solvation on the rate of electrode reaction at mercury electrodes [4,5]. Mercury surfaces are known to have only a small affinity to water (i.e. are relatively “hydrophobic”) and therefore may affect the structural environment of the reacting species to a small extent. The labilities of these aqua-ligands is directly related to the problem of differentiating between „outer“ and „inner-sphere“ oxidation-reduction process. In strongly acidic media the standard heterogeneous electron-transfer rate constant (ks) for the V(III)/(II) couple is high (from 1·10-3 to 3·10-3 cm/s, Figure 1a) with charge transfer coefficient of ca. 0.5 [5] indicating 2
The actual vanadium speciation in VRFB electrolytes and therefore reversible potentials can deviate from that ones, which are related to equations (1) and (2).
2
that the reduction of V(III) to V(II) in millimolar solutions is reversible at the dropping mercury electrode. The charge transfer process in H2SO4, HClO4 and HCl solutions of V(III) at this electrode was supposed to proceed through the following scheme [5]: ⇄ ⇄
(4)
→ → !"#
(5)
where X is complexing anion (counterion like Cl-, or HSO4-). The complexation equilibrium (4) was assumed to occur infinitely fast so that the reduction of noncomplexed V(III) cation was the rate-determining step (5) in these electrolytes. Thereby, no significant complexation of V(II) was expected in sulphate and chloride media (at millimolar vanadium concentrations). In case of carbon electrodes for V(III)/V(II) redox reactions in diluted vanadium solutions, inner-sphere route dominated the electron transfer unless the carbonyl coverage of electrode surface was very low [6]. The suggestion that V(II)/(III) is not necessarily outer-sphere at glassy carbon electrode activated potentiodynamically was made by Yamamura et al. [7]. This suggestion if the redox reaction proceeds on outer-sphere mechanism was based on the evaluation of ks in the context of Marcus theory for outer-sphere reactions3. At carbon electrodes used without activation (Figure 1a) [8, 9], at well-defined carbon surfaces with low oxide coverage (highly oriented pyrographite) or with oxygen-containing surface groups blocked by silanization [10] the V(III)/(II) reactions appeared to be outer-sphere and had low rates. The faster kinetics of V(III)/V(II) reactions in terms of higher exchange curent densities (j0) at glassy carbon electrode in 0.2 M vanadium electrolyte was recently reported for HCl matrix (1.6 M total chloride) compared to H2SO4 matrix (0.7 M total sulfate) [11]. Since the fraction of V(III) species complexed with counter ion was lower in HCl than in H2SO4, the inner-sphere bridging mechanism of V(II) electrooxidation through chloride adsorbed at the electrode surface was assumed [11]. A loss of catalytic activity of oxidized glassy carbon surface was reported for a long-term electrooxidation of concentrated (1 M) V(II) electrolyte solutions [12]. Under real VRFB operation conditions, the V(III)/V(II) reaction or electrode surface chemistry can be affected by side reaction of hydrogen evolution, which depends on the current density and proton activity [13]. Figure 1. Comparison of rate constants reported in the literature for electrode reactions of (a) V(III)/V(II) and (b) V(V)/V(IV) redox couples at different electrode materials (GC-glassy carbon, CPcarbon paper, CF-carbon fiber, ox-oxidized, red-reduced). Attention is given also to the concentration of solutions under investigation, thereby: sulfuric acid electrolytes only are considered in case of molar vanadium concentrations; in case of low vanadium concentrations the values are taken from studies on various electrolyte matrixes.
3
This treatment is based on the assumption that ks tracks the square root of the homogeneous self-exchange rate constant (kex), whereby kex is taken as a literature value or is determined independently.
3
In recent years the electrode pretreatment or modification for investigation of V(III)/V(II) redox reactions in concentrated electrolytes or characterization of VRFB electrode materials is taken into account and has been comprehensively reviewed by Cao et al. [23], Wu et al. [24*] and Le et al. [25]. As it can be seen at Figure 1a, the rate constant values reported for carbon electrodes in vanadium solutions of molar concentrations are scattered to large extent in the range from 10-4 to 10-8 cm/s. Though the oxidized electrode surfaces enhance the V(III)/V(II) redox kinetics, the large scattering of data is due to experimental complications in controlling the compositions of electrode materials, especially porous carbons, in the choice of the appropriate procedure to measure kinetic parameters (j0) and evaluate reaction rate constants. Electrocatalysis can be defined as increase of j0 or decrease of charge transfer overpotential at a constant current density which is not accompanied by increase of real surface (i.e. double layer capacitance CDL) [24*]. The necessity to estimate the real electrochemically active electrode surface or to determine CDL and lack of information on vanadium speciation (complexation, ion-pairing) at given concentration are the main obstacles for comparison of the reported data and their application to the development of battery materials or modelling of VRFB performance.
Reactions of V(V)/V(IV) redox couple The V(V) and V(IV) species in diluted solutions can be represented as aquated dioxy- (VO2+) and oxocations (VO2+) respectively. The water molecules from the first coordination shell are labile and can be replaced by sulphate or bisulphate as bi- or monodentate ligands [26,27]. In highly acidic media the dimerization equilibrium was assumed for V(IV) [27] even for solutions of low (millimolar) vanadium concentrations. The formation of dimers was detected in V(V) solutions [26]. Moreover, the presence of V(V) as solvated VO3+ ionic form in highly concentrated sulfuric acid and its reaction with VO2+ to form V2O34+ moiety was stated in the past [28]. Therefore in diluted acidic solutions, the 4
vanadium dioxy- and oxocation core are considered commonly in connection with protonation equlibria to propose the mechanistic pathways for V(V)/V(IV) redox reactions [29] (Figure 2a). Gattrell et al. examined extensively the kinetic mechanism of the V(IV)/(V) couple in H2SO4 electrolyte solutions with 50 mM total vanadium concentration at graphite electrode (pretreated by polishing or slightly oxidized) [29,30]. The reaction of V(IV) oxidation was expected to consist of three elementary steps: chemical-electrochemical-chemical (CEC) mechanism at low overpotentials4, which changes to a electrochemical-chemical-chemical (ECC) mechanism at higher anodic or cathodic overpotentials (Figure 2a). The polarization curves cannot be fit using a simple Butler-Volmer kinetics equation and unusually low charge transfer coefficient for reduction of V(V) at carbon (α< 0.13 or Tafel slope > 460 mV/decade) was reported. The formation of insoluble intermediate product (VO2·4H2O) and electron transfer through an adsorbed layer of V(IV) was suggested. Electron transfer through adsorption intermediate during reduction of 100 mM V(V) at high overpotentials at glassy carbon was also postulated recently in the study [21*] in H2SO4 and H3PO4 acids. In contrast to the pathways proposed by Gattrell et al., the ECC and CEC mechanisms were assumed for the V(IV) oxidation in H2SO4 and H3PO4 acids at low and high overpotentials respectively, whereby the E step is faster in H3PO4 and the first C step in CEC is similar in both acids [21*]. The participation of vanadium dimeric intermediates or reactants in V(V) electroreduction at glassy carbon was recently considered by Wang. et al. [31**] to explain the observed change of Tafel slope with overpotential in concentrated (1.2 M) vanadium electrolyte solutions. The formation of possible mixed-valence V2O33+ ions resulted in the change of the reduction mechanism of V(V) ions, from a VO2+ - centered reduction at low overpotentials (CEC mechanism) to a V2O33+ - centered reduction at high overpotentials (ECC mechanism) [31**] (Figure 2a). In contrast to it, no change in Tafel slope was reported for V(V) reduction in this electrolyte at a platinum electrode and the CECC mechanism including adsorption of V2O33+ at the electrode was proposed [31**]. The formation of vanadium concatenated, i.e. dimeric species was recently confirmed by Raman spectroscopy investigation of catholyte probes containing V(V) and V(IV) at 1 M total vanadium concentration [32*]. These vanadium dimers were supposed to be less electrochemically active than monomeric species [32*].
Figure 2. Schematic presentation of possible equilibria for the V(V)/V(IV) couple and mechanistic pathways for electrode reactions at low pH (a) for outer-sphere electron transfer, (b) example of electrocatalytic pathway in the presence of electrode surface groups proposed in the work [33]. Ion association or ligand exchange with counter ions like sulphate or bisulphate are omitted here.
4
Overpotentials are referred in this case to the formal potential of the redox couple, i.e. reversible potential under non-standard conditions.
5
The presence of carbon surface o-benzoquinone-type groups at the electrode was previously reported to suppress the reaction rate of V(V) reduction, though the reaction rate was largely recovered when these groups were reduced to their cathechol-type (hydroxylic) form [30,33] (Figure 2b). The opinions about the effect of oxygen-containing groups on the V(V)/V(IV) kinetics have recently become opposite. The catalytic effect of hydroxyl group for the V(V)/V(IV) redox reactions was reported by Noack et al. [34]. Bourke et al. [14] and Miller et al. [15] consider the electrode surface groups to be responsible for the catalytic effect and suggested that cathodic (reductive) treatment of glassy carbon, carbon fiber or carbon paper electrodes accelerated the V(V)/V(IV) redox reaction, whereby the anodic (oxidative) treatment slowed it down. The assumption about adsorption of vanadium species at the surface C-O groups as inner-sphere monodentate complexes during reduction of V(V) was supported by Car−Parrinello molecular dynamics-based metadynamics simulations study [35*]. Moreover, the chemisorption of the vanadium ions to the graphite surface and desorption from the surface after charge transfer was supposed to limit the overall redox reaction process for both V(III)/V(II) and V(V)/V(IV) couples [35*, 36]. Holland-Cunz et al. [21*], Fink et al. [37] and Friedl et al. [38**] state that oxygen-containing surface groups only increase the wetted surface area and impede the V(V)/V(IV) reaction. This suggestion is based on investigation of the V(V)/V(IV) reaction at glassy carbon (also modified by MWCNT) and carbon felt by EIS and chronoamperometry. Ruemmler et al. supported the idea that the activation provides a more hydrophilic surface, facilitating diffusion and adsorption of V(IV) ions. The differences in kind and amount of functional groups at mesoporous carbon electrodes was not observed to influence the kinetics of V(IV) conversion [39]. Kinetics of the V(V)/V(IV) reaction, 6
which was investigated in-situ of a VRFB by Mazur et. al, was found to be unaffected by oxygen functionalization [40]. The passivation of the carbon electrode by strong chemisorption of V2O33+ dimeric structures at surface C-O groups was also presumed by Wang et al. [31**]. Choi et al. [41] assumes on the basis of MD simulations and impedance analysis for symmetric cells that direct contact of V(V) and V(IV) ions and electrode surface (i.e. outer-sphere mechanism) is possible due to less-organized nature of their hydration shell, enabling electron transfer without the participation of OH groups. The comparison of these results or reported rate constants (Figure 1b) is complicated by the fact that the pretreatment protocols are very different and the nature of electrode surface groups remains often unspecified, can change with applied potential, thereby only the C:O ratio is discussed. It is interesting to note, that for electrooxidation of V(IV) in solid samples of VOSO4·xH2O (x=3 or 5) at carbon disk ultramicroelectrode reversible cyclic voltammogramms were obtained in contrast to solution-phase systems [42]. Considering the solid hydrates of vanadyl sulfate as a limiting case of a very concentrated solutions, where vanadium and sulfate ions are bound in a manner analogous to a contact ion-pair in liquid media of low dielectric permittivity, the stability of the coordination spheres of vanadium ions during electrolysis may account for the reversibility. The movement of the counterion (proton) is apparently the rate determining step [42]. Concluding remarks In concentrated i.e. VRFB relevant vanadium electrolyte solutions, hydrolytic, polymerization, and ion association processes can be generally very extensive. All these phenomena can be expected to affect the redox chemistry of vanadium species especially at porous carbon electrodes [43]. However, the exact correlation between vanadium speciation in VRFB electrolyte and kinetics of electrode reaction, i.e. ultimately power density of VRFB, and the role of electrode surface groups are not systematically investigated. Chemical stability as well as the oxidizing and reducing activity of VRFB electrolyte at high state-of-charge especially at high concentrations of acid [44] is to be taken into account for interpretation of its electrochemical response. Looking at the reported rate constants for both V(III)/V(II) and V(V)/V(IV) couples in concentrated solutions (Figure 1), it is difficult to state which half-cell reaction would limit the VRFB operation. However, the anodic reaction is suggested to limit the performance of a VRFB from kinetics aspects even at low current densities, thereby the cathodic reaction appears to exhibit mass transport effect at high current densities [45].
Conflict of interest statement The authors declare no conflict of interest. Acknowledgement This work is funded by the Fraunhofer Talenta programm for support of female scientists. References Papers of particular interest, published within the period of review have been highlighted as: * of special interest ** of outstanding interest
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VIV-VV to Electrochemical Pretreatment of Carbon. J. Electrochem. Soc 2016, 163:A5097A5105. 15. Miller, M. A., Bourke A, Quill N, Wainright JS, Lynch, R. P., Buckley, D. N., Savinell, R. F.: Kinetic Study of Electrochemical Treatment of Carbon Fiber Microelectrodes Leading to In Situ Enhancement of Vanadium Flow Battery Efficiency. J. Electrochem. Soc 2016, 163:A2095-A2102. 16. Goulet M-A, Eikerling M, Kjeang E: Direct measurement of electrochemical reaction kinetics in flow-through porous electrodes. Electrochemistry Communications 2015, 57:14–7. 17. Becker M, Bredemeyer N, Tenhumberg N, Turek T: Kinetic studies at carbon felt electrodes for vanadium redox-flow batteries under controlled transfer current density conditions. Electrochimica Acta 2017, 252:12–24. 18. Pezeshki AM, Sacci RL, Delnick FM, Aaron DS, Mench MM: Elucidating effects of cell architecture, electrode material, and solution composition on overpotentials in redox flow batteries. Electrochimica Acta 2017, 229:261–70. 19. Sum E, Rychcik M, Skyllas-Kazacos M: Investigation of the V(V)/V(IV) system for use in the positive half-cell of a redox battery. Journal of Power Sources 1985, 16:85–95. 20. Wen Y, Zhang H, Qian P, Zhao P, Zhou H, Yi B: Investigations on the Electrode Process of Concentrated V(IV)/V(V) Species in a Vanadium Redox Flow Battery. Acta Physico-Chimica Sinica 2006, 22:403–8. 21.* Holland-Cunz MV, Friedl J, Stimming U: Anion effects on the redox kinetics of positive electrolyte of the all-vanadium redox flow battery. Journal of Electroanalytical Chemistry 2018, 819:306–11. It is shown that the kinetics of V(V)/V(IV) redox pair at glassy carbon or carbon felt in 50 mM vanadium solutions is faster in case of H3PO4 electrolyte compared to H2SO4 as a matrix. The mechanistic pathways (ECC or CEC) proposed earlier for oxidation/reduction of V(V)/V(IV) in diluted solution in dependence of overpotential have been reconsidered. 22. Wang W, Fan X, Liu J, Yan C, Zeng C: A novel mechanism for the oxidation reaction of VO2+ on a graphite electrode in acidic solutions. Journal of Power Sources 2014, 261:212–20. 23. Cao L, Skyllas-Kazacos M, Wang D-W: Effects of Surface Pretreatment of Glassy Carbon on the Electrochemical Behavior of V(IV)/V(V) Redox Reaction. J. Electrochem. Soc 2016, 163:A1164-A1174. 24.* Wu Y, Holze R: Electrocatalysis at Electrodes for VanadiumRedox Flow Batteries. Batteries 2018, 4:47. Special attention is given to the methods for experimental determination of catalytic effects for vanadium electrode reactions. The effect of enlarged electrochemically active electrode surface and true (specific) electrocatalytic activity are to be separated. This is often not the case in recent studies on the electrodes for vanadium reactions. 25. Le TXH, Bechelany M, Cretin M: Carbon felt based-electrodes for energy and environmental applications: A review. Carbon 2017, 122:564–91. 9
26. Kausar N, Howe R, Skyllas-Kazacos M: Raman spectroscopy studies of concentrated vanadium redox battery positive electrolytes. Journal of Applied Electrochemistry 2001, 31:1327–32. 27. Wu X, Wang J, Liu S, Wu X, Li S: Study of vanadium(IV) species and corresponding electrochemical performance in concentrated sulfuric acid media. Electrochimica Acta 2011, 56:10197–203. 28. Richter K-L, Mattes R: Darstellung, Ramanspektren und Kristallstrukturen von V2O3(SO4)2, K[VO(SO4)2] und NH4[VO(SO4)2]. Z. anorg. allg. Chem. 1992, 611:158–64. 29. Gattrell M, Park J, MacDougall B, Apte J, McCarthy S, Wu, C. W.: Study of the Mechanism of the Vanadium 4+/5+ Redox Reaction in Acidic Solutions. J. Electrochem. Soc 2004, 151:A123A130. 30. Gattrell M, Qian J, Stewart C, Graham P, MacDougall B: The electrochemical reduction of VO2+ in acidic solution at high overpotentials. Electrochimica Acta 2005, 51:395–407. 31.** Wang W, Fan X, Qin Y, Liu J, Yan C, Zeng C: The reduction reaction kinetics of vanadium(V) in acidic solutions on a platinum electrode with unusual difference compared to carbon electrodes. Electrochimica Acta 2018, 283:1313–22. Changes in Tafel slope with overpotential for reduction of V(V) species in concentrated solutions at glassy carbon electrode are explained by changes of reduction mechanism: from a V(V) monomercentered one (CEC) to another one (ECC) with participation of vanadium mixed-valance complex V2O33+. To explain te absence of the change in Tafel slope at platinum electrode, a CECC reduction mechanism is proposed. 32.* Sun C, Vezzù K, Pagot G, Nale A, Bang YH, Pace G, Negro E, Gambaro C, Meda L, Zawodzinski TA, Di Noto V: Elucidation of the interplay between vanadium species and charge-discharge processes in VRFBs by Raman spectroscopy. Electrochimica Acta 2019, 318:913–21. The changes in catholyte composition during VRFB operation are investigated by Raman spectroscopy. Both of V(V) and V(IV) monomeric species are found to be dominant and able to form concatenated complexes, i.e. dimers. The SO42-/HSO4- anions are supposed to be unaffected by interaction with vanadium species and are hardly involved in electrochemical charge transfer process. 33. Maruyama J, Hasegawa T, Iwasaki S, Fukuhara T, Nogami M: Mechanism of Dioxovanadium Ion Reduction on Oxygen-Enriched Carbon Surface. J. Electrochem. Soc 2013, 160:A1293A1298. 34. Noack J, Roznyatovskaya N, Kunzendorf J, Skyllas-Kazacos M, Menictas C, Tübke J: The influence of electrochemical treatment on electrode reactions for vanadium redox-flow batteries. Journal of Energy Chemistry 2018, 27:1341–52. 35.* Jiang Z, Klyukin K, Alexandrov V: Ab Initio Metadynamics Study of the VO2+/VO2+ Redox Reaction Mechanism at the Graphite Edge/Water Interface. ACS Appl Mater Interfaces 2018, 10:20621–6.
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The activation barriers for adsorption/desorption of vanadium ions onto graphite surface are considered in CPMD simulations and desorption of reactants from the surface after electron transfer step is suggested to be the rate-determining during the discharge process for both V(III)/V(II) and V(V)/V(IV) couples. 36. Jiang Z, Klyukin K, Alexandrov V: First-principles study of adsorption-desorption kinetics of aqueous V2+/V3+ redox species on graphite in a vanadium redox flow battery. Phys Chem Chem Phys 2017, 19:14897–901. 37. Fink H, Friedl J, Stimming U: Composition of the Electrode Determines Which Half-Cell’s Rate Constant is Higher in a Vanadium Flow Battery. J. Phys. Chem. C 2016, 120:15893–901. 38.** Friedl J, Stimming U: Determining Electron Transfer Kinetics at Porous Electrodes. Electrochimica Acta 2017, 227:235–45. Clearly argumentated the inapplicability of cyclic voltammetry at porous electrodes to evaluate the rate constant (ks) of electrode reactions. The methods of ks estimate from EIS and chronoamperometry at porous electrodes are described. Values of ks given in the literature for V(III)/V(II) and V(V)/V(IV) reactions are critically evaluated. 39. Rümmler S, Steimecke M, Schimpf S, Hartmann M, Förster S, Bron M: Highly Graphitic, Mesoporous Carbon Materials as Electrocatalysts for Vanadium Redox Reactions in AllVanadium Redox-Flow Batteries. J. Electrochem. Soc 2018, 165:A2510-A2518. 40. Mazúr P, Mrlík J, Beneš J, Pocedič J, Vrána J, Dundálek J, Kosek J: Performance evaluation of thermally treated graphite felt electrodes for vanadium redox flow battery and their fourpoint single cell characterization. Journal of Power Sources 2018, 380:105–14. 41. Choi C, Noh H, Kim S, Kim R, Lee J, Heo J, Kim H-T: Understanding the redox reaction mechanism of vanadium electrolytes in all-vanadium redox flow batteries. Journal of Energy Storage 2019, 21:321–7. 42. Gorski W, Cox JA: Voltammetry of vanadyl sulfate hydrates in the absence of a deliberately added liquid phase. Journal of Electroanalytical Chemistry 1992, 323:163–78. 43. Gao Y, Huang J, Liu Y, Chen S: Charge transport in confined concentrated solutions: A minireview. Current Opinion in Electrochemistry 2019, 13:107–11. 44. Zhao Y, Le Liu, Qiu X, Xi J: Revealing sulfuric acid concentration impact on comprehensive performance of vanadium electrolytes and flow batteries. Electrochimica Acta 2019, 303:21– 31. 45. Cecchetti M, Casalegno A, Zago M: Local potential measurement through reference electrodes in vanadium redox flow batteries: Evaluation of overpotentials and electrolytes imbalance. Journal of Power Sources 2018, 400:218–24.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2451-9103(19)30144-9
DOI:
https://doi.org/10.1016/j.coelec.2019.10.003
Reference:
COELEC 463
To appear in:
Current Opinion in Electrochemistry
Received Date: 13 August 2019 Revised Date:
27 September 2019
Accepted Date: 3 October 2019
Please cite this article as: Roznyatovskaya N, Noack J, Pinkwart K, Tübke J, Aspects of electron transfer processes in vanadium redox-flow batteries, Current Opinion in Electrochemistry, https:// doi.org/10.1016/j.coelec.2019.10.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V.
Aspects of electron transfer processes in vanadium redox-flow batteries. Nataliya Roznyatovskaya*1,2, Jens Noack1,2, Karsten Pinkwart1,2 and Jens Tübke1,2 1
Fraunhofer Institute for Chemical Technology, Applied Electrochemisrty, Joseph-von-FraunhoferStr. 7, Pfinztal 76327, Germany
2
German-Australian Alliance for Electrochemical Technologies for Storage of Renewable Energy (CENELEST), Mechanical and Manufacturing Engineering, University of New South Wales (UNSW), UNSW Sydney, NSW 2052, Australia
*
corresponding author:
Email: [email protected] Tel: +49-721-4640659 Keywords Vanadium redox-flow battery; vanadium electrode reactions; kinetics
Abstract The electrochemical processes in vanadium redox-flow batteries (VRFBs) include conversions of vanadium species in acidic electrolytes with total vanadium concentrations over molar range. The majority of currently available data on electrode kinetics of vanadium reactions, and on the role of electrode surface chemistry are obtained for diluted electrolyte solutions and are very controversial. In this minireview we consider the interpretations of electrochemical kinetic data for vanadium electrode reactions and mechanistic concepts, which have been reported in the literature. Thereby, the gap between electrochemical kinetics in „diluted“ and „concentrated“ solutions is in the focus of the review.
Introduction The electrode reactions of vanadium species attract increasing attention since the last two decades. This is due to the fact that the redox reactions of three vanadium couples (V(III)/V(II), V(IV)/V(III) and V(V)/V(IV)1) at the electrode-electrolyte interface define the chemistry and operation of VRFB, which have been developed and commercialized. Therefore, the kinetics of these reactions is of crucial importance for understanding and optimization of VRFB performance. To obtain the electrochemical rate parameters and ultimately increase the power density of VRFB the information about vanadium speciation in electrolyte and kinetic data are needed. This information is available for vanadium electrode reactions in diluted solutions at the electrodes with well characterized surface structure, i.e. 1
For brevity, vanadium species in different oxidation states are denoted here as V(II), V(III), V(IV) and V(V) respectively, unless the structure of vanadium species in solution is considered.
1
under well defined mass transport conditions. Concerning the VRFB application, the total vanadium concentration in electrolyte exceeds molar range and electrode reactions proceed under mixed control (convective-diffusion mass transport and charge transfer) at the porous electrode surface with poorly characterized surface structure. This review is focused on the recent investigations of vanadium electrode reactions. The results reported for concentrated electrolytes are considered in comparison with data for diluted solutions. The thermodynamics of the battery under standard conditions can be derived from the combination of the following half-reactions2: + 2 +
+
ℎ ⇌ + ℎ
ℎ ⇌ ℎ
(positive
half-cell)
(negative half-cell)
E0 = 1.0 V vs. SHE
(1)
E0 = -0.26 V vs. SHE
(2)
Since commercial electrolytes for VRFB commonly contain V(III) and V(IV) species in 1:1 molar ratio in a highly acidic matrix, the operation of a VRFB usually includes the initial charging (precharging) step to convert the mixture or initial electrolyte to anolyte and catholyte forms in battery compartments. The reactions of V(IV)/V(III) couple (equation 3) are known to have high activation energy, i.e. slow kinetics [1] and are not well characterized. + 2 +
ℎ ( ) ⇌ + ℎ (ℎ )
E0 = 0.337 V vs. SHE
(3)
The analysis of locally-resolved current density distribution at the cathode in a VRFB with segmented current collector during initial charging indicates that the vanadium conversion, i.e. pre-charging proceeds by EC’ (electrochemical-catalytic) mechanism [2]. Though the attempt to accelerate this reaction by electrocatalysis and to extend the theoretical volumetric capacity of VRFB by using the V(IV)/V(III) couple as energy bearing component of battery is recently reported in the work [3], the processes during the pre-charging of conventional VRFB are poorly investigated. Therefore, the electrode reactions of V(III)/V(II) and V(V)/V(IV) couples in acidic solutions are considered further. Reactions of V(III)/V(II) redox couple The V(III) and V(II) species at millimolar concentrations in non-complexing or weakly complexing acids (like HClO4) present as hexaaqua-cations of common structure [V(H2O)6]n+. V(III)/V(II) couple was used in the past as a model redox pair to study the effect of reactant solvation on the rate of electrode reaction at mercury electrodes [4,5]. Mercury surfaces are known to have only a small affinity to water (i.e. are relatively “hydrophobic”) and therefore may affect the structural environment of the reacting species to a small extent. The labilities of these aqua-ligands is directly related to the problem of differentiating between „outer“ and „inner-sphere“ oxidation-reduction process. In strongly acidic media the standard heterogeneous electron-transfer rate constant (ks) for the V(III)/(II) couple is high (from 1·10-3 to 3·10-3 cm/s, Figure 1a) with charge transfer coefficient of ca. 0.5 [5] indicating 2
The actual vanadium speciation in VRFB electrolytes and therefore reversible potentials can deviate from that ones, which are related to equations (1) and (2).
2
that the reduction of V(III) to V(II) in millimolar solutions is reversible at the dropping mercury electrode. The charge transfer process in H2SO4, HClO4 and HCl solutions of V(III) at this electrode was supposed to proceed through the following scheme [5]: ⇄ ⇄
(4)
→ → !"#
(5)
where X is complexing anion (counterion like Cl-, or HSO4-). The complexation equilibrium (4) was assumed to occur infinitely fast so that the reduction of noncomplexed V(III) cation was the rate-determining step (5) in these electrolytes. Thereby, no significant complexation of V(II) was expected in sulphate and chloride media (at millimolar vanadium concentrations). In case of carbon electrodes for V(III)/V(II) redox reactions in diluted vanadium solutions, inner-sphere route dominated the electron transfer unless the carbonyl coverage of electrode surface was very low [6]. The suggestion that V(II)/(III) is not necessarily outer-sphere at glassy carbon electrode activated potentiodynamically was made by Yamamura et al. [7]. This suggestion if the redox reaction proceeds on outer-sphere mechanism was based on the evaluation of ks in the context of Marcus theory for outer-sphere reactions3. At carbon electrodes used without activation (Figure 1a) [8, 9], at well-defined carbon surfaces with low oxide coverage (highly oriented pyrographite) or with oxygen-containing surface groups blocked by silanization [10] the V(III)/(II) reactions appeared to be outer-sphere and had low rates. The faster kinetics of V(III)/V(II) reactions in terms of higher exchange curent densities (j0) at glassy carbon electrode in 0.2 M vanadium electrolyte was recently reported for HCl matrix (1.6 M total chloride) compared to H2SO4 matrix (0.7 M total sulfate) [11]. Since the fraction of V(III) species complexed with counter ion was lower in HCl than in H2SO4, the inner-sphere bridging mechanism of V(II) electrooxidation through chloride adsorbed at the electrode surface was assumed [11]. A loss of catalytic activity of oxidized glassy carbon surface was reported for a long-term electrooxidation of concentrated (1 M) V(II) electrolyte solutions [12]. Under real VRFB operation conditions, the V(III)/V(II) reaction or electrode surface chemistry can be affected by side reaction of hydrogen evolution, which depends on the current density and proton activity [13]. Figure 1. Comparison of rate constants reported in the literature for electrode reactions of (a) V(III)/V(II) and (b) V(V)/V(IV) redox couples at different electrode materials (GC-glassy carbon, CPcarbon paper, CF-carbon fiber, ox-oxidized, red-reduced). Attention is given also to the concentration of solutions under investigation, thereby: sulfuric acid electrolytes only are considered in case of molar vanadium concentrations; in case of low vanadium concentrations the values are taken from studies on various electrolyte matrixes.
3
This treatment is based on the assumption that ks tracks the square root of the homogeneous self-exchange rate constant (kex), whereby kex is taken as a literature value or is determined independently.
3
In recent years the electrode pretreatment or modification for investigation of V(III)/V(II) redox reactions in concentrated electrolytes or characterization of VRFB electrode materials is taken into account and has been comprehensively reviewed by Cao et al. [23], Wu et al. [24*] and Le et al. [25]. As it can be seen at Figure 1a, the rate constant values reported for carbon electrodes in vanadium solutions of molar concentrations are scattered to large extent in the range from 10-4 to 10-8 cm/s. Though the oxidized electrode surfaces enhance the V(III)/V(II) redox kinetics, the large scattering of data is due to experimental complications in controlling the compositions of electrode materials, especially porous carbons, in the choice of the appropriate procedure to measure kinetic parameters (j0) and evaluate reaction rate constants. Electrocatalysis can be defined as increase of j0 or decrease of charge transfer overpotential at a constant current density which is not accompanied by increase of real surface (i.e. double layer capacitance CDL) [24*]. The necessity to estimate the real electrochemically active electrode surface or to determine CDL and lack of information on vanadium speciation (complexation, ion-pairing) at given concentration are the main obstacles for comparison of the reported data and their application to the development of battery materials or modelling of VRFB performance.
Reactions of V(V)/V(IV) redox couple The V(V) and V(IV) species in diluted solutions can be represented as aquated dioxy- (VO2+) and oxocations (VO2+) respectively. The water molecules from the first coordination shell are labile and can be replaced by sulphate or bisulphate as bi- or monodentate ligands [26,27]. In highly acidic media the dimerization equilibrium was assumed for V(IV) [27] even for solutions of low (millimolar) vanadium concentrations. The formation of dimers was detected in V(V) solutions [26]. Moreover, the presence of V(V) as solvated VO3+ ionic form in highly concentrated sulfuric acid and its reaction with VO2+ to form V2O34+ moiety was stated in the past [28]. Therefore in diluted acidic solutions, the 4
vanadium dioxy- and oxocation core are considered commonly in connection with protonation equlibria to propose the mechanistic pathways for V(V)/V(IV) redox reactions [29] (Figure 2a). Gattrell et al. examined extensively the kinetic mechanism of the V(IV)/(V) couple in H2SO4 electrolyte solutions with 50 mM total vanadium concentration at graphite electrode (pretreated by polishing or slightly oxidized) [29,30]. The reaction of V(IV) oxidation was expected to consist of three elementary steps: chemical-electrochemical-chemical (CEC) mechanism at low overpotentials4, which changes to a electrochemical-chemical-chemical (ECC) mechanism at higher anodic or cathodic overpotentials (Figure 2a). The polarization curves cannot be fit using a simple Butler-Volmer kinetics equation and unusually low charge transfer coefficient for reduction of V(V) at carbon (α< 0.13 or Tafel slope > 460 mV/decade) was reported. The formation of insoluble intermediate product (VO2·4H2O) and electron transfer through an adsorbed layer of V(IV) was suggested. Electron transfer through adsorption intermediate during reduction of 100 mM V(V) at high overpotentials at glassy carbon was also postulated recently in the study [21*] in H2SO4 and H3PO4 acids. In contrast to the pathways proposed by Gattrell et al., the ECC and CEC mechanisms were assumed for the V(IV) oxidation in H2SO4 and H3PO4 acids at low and high overpotentials respectively, whereby the E step is faster in H3PO4 and the first C step in CEC is similar in both acids [21*]. The participation of vanadium dimeric intermediates or reactants in V(V) electroreduction at glassy carbon was recently considered by Wang. et al. [31**] to explain the observed change of Tafel slope with overpotential in concentrated (1.2 M) vanadium electrolyte solutions. The formation of possible mixed-valence V2O33+ ions resulted in the change of the reduction mechanism of V(V) ions, from a VO2+ - centered reduction at low overpotentials (CEC mechanism) to a V2O33+ - centered reduction at high overpotentials (ECC mechanism) [31**] (Figure 2a). In contrast to it, no change in Tafel slope was reported for V(V) reduction in this electrolyte at a platinum electrode and the CECC mechanism including adsorption of V2O33+ at the electrode was proposed [31**]. The formation of vanadium concatenated, i.e. dimeric species was recently confirmed by Raman spectroscopy investigation of catholyte probes containing V(V) and V(IV) at 1 M total vanadium concentration [32*]. These vanadium dimers were supposed to be less electrochemically active than monomeric species [32*].
Figure 2. Schematic presentation of possible equilibria for the V(V)/V(IV) couple and mechanistic pathways for electrode reactions at low pH (a) for outer-sphere electron transfer, (b) example of electrocatalytic pathway in the presence of electrode surface groups proposed in the work [33]. Ion association or ligand exchange with counter ions like sulphate or bisulphate are omitted here.
4
Overpotentials are referred in this case to the formal potential of the redox couple, i.e. reversible potential under non-standard conditions.
5
The presence of carbon surface o-benzoquinone-type groups at the electrode was previously reported to suppress the reaction rate of V(V) reduction, though the reaction rate was largely recovered when these groups were reduced to their cathechol-type (hydroxylic) form [30,33] (Figure 2b). The opinions about the effect of oxygen-containing groups on the V(V)/V(IV) kinetics have recently become opposite. The catalytic effect of hydroxyl group for the V(V)/V(IV) redox reactions was reported by Noack et al. [34]. Bourke et al. [14] and Miller et al. [15] consider the electrode surface groups to be responsible for the catalytic effect and suggested that cathodic (reductive) treatment of glassy carbon, carbon fiber or carbon paper electrodes accelerated the V(V)/V(IV) redox reaction, whereby the anodic (oxidative) treatment slowed it down. The assumption about adsorption of vanadium species at the surface C-O groups as inner-sphere monodentate complexes during reduction of V(V) was supported by Car−Parrinello molecular dynamics-based metadynamics simulations study [35*]. Moreover, the chemisorption of the vanadium ions to the graphite surface and desorption from the surface after charge transfer was supposed to limit the overall redox reaction process for both V(III)/V(II) and V(V)/V(IV) couples [35*, 36]. Holland-Cunz et al. [21*], Fink et al. [37] and Friedl et al. [38**] state that oxygen-containing surface groups only increase the wetted surface area and impede the V(V)/V(IV) reaction. This suggestion is based on investigation of the V(V)/V(IV) reaction at glassy carbon (also modified by MWCNT) and carbon felt by EIS and chronoamperometry. Ruemmler et al. supported the idea that the activation provides a more hydrophilic surface, facilitating diffusion and adsorption of V(IV) ions. The differences in kind and amount of functional groups at mesoporous carbon electrodes was not observed to influence the kinetics of V(IV) conversion [39]. Kinetics of the V(V)/V(IV) reaction, 6
which was investigated in-situ of a VRFB by Mazur et. al, was found to be unaffected by oxygen functionalization [40]. The passivation of the carbon electrode by strong chemisorption of V2O33+ dimeric structures at surface C-O groups was also presumed by Wang et al. [31**]. Choi et al. [41] assumes on the basis of MD simulations and impedance analysis for symmetric cells that direct contact of V(V) and V(IV) ions and electrode surface (i.e. outer-sphere mechanism) is possible due to less-organized nature of their hydration shell, enabling electron transfer without the participation of OH groups. The comparison of these results or reported rate constants (Figure 1b) is complicated by the fact that the pretreatment protocols are very different and the nature of electrode surface groups remains often unspecified, can change with applied potential, thereby only the C:O ratio is discussed. It is interesting to note, that for electrooxidation of V(IV) in solid samples of VOSO4·xH2O (x=3 or 5) at carbon disk ultramicroelectrode reversible cyclic voltammogramms were obtained in contrast to solution-phase systems [42]. Considering the solid hydrates of vanadyl sulfate as a limiting case of a very concentrated solutions, where vanadium and sulfate ions are bound in a manner analogous to a contact ion-pair in liquid media of low dielectric permittivity, the stability of the coordination spheres of vanadium ions during electrolysis may account for the reversibility. The movement of the counterion (proton) is apparently the rate determining step [42]. Concluding remarks In concentrated i.e. VRFB relevant vanadium electrolyte solutions, hydrolytic, polymerization, and ion association processes can be generally very extensive. All these phenomena can be expected to affect the redox chemistry of vanadium species especially at porous carbon electrodes [43]. However, the exact correlation between vanadium speciation in VRFB electrolyte and kinetics of electrode reaction, i.e. ultimately power density of VRFB, and the role of electrode surface groups are not systematically investigated. Chemical stability as well as the oxidizing and reducing activity of VRFB electrolyte at high state-of-charge especially at high concentrations of acid [44] is to be taken into account for interpretation of its electrochemical response. Looking at the reported rate constants for both V(III)/V(II) and V(V)/V(IV) couples in concentrated solutions (Figure 1), it is difficult to state which half-cell reaction would limit the VRFB operation. However, the anodic reaction is suggested to limit the performance of a VRFB from kinetics aspects even at low current densities, thereby the cathodic reaction appears to exhibit mass transport effect at high current densities [45].
Conflict of interest statement The authors declare no conflict of interest. Acknowledgement This work is funded by the Fraunhofer Talenta programm for support of female scientists. References Papers of particular interest, published within the period of review have been highlighted as: * of special interest ** of outstanding interest
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The activation barriers for adsorption/desorption of vanadium ions onto graphite surface are considered in CPMD simulations and desorption of reactants from the surface after electron transfer step is suggested to be the rate-determining during the discharge process for both V(III)/V(II) and V(V)/V(IV) couples. 36. Jiang Z, Klyukin K, Alexandrov V: First-principles study of adsorption-desorption kinetics of aqueous V2+/V3+ redox species on graphite in a vanadium redox flow battery. Phys Chem Chem Phys 2017, 19:14897–901. 37. Fink H, Friedl J, Stimming U: Composition of the Electrode Determines Which Half-Cell’s Rate Constant is Higher in a Vanadium Flow Battery. J. Phys. Chem. C 2016, 120:15893–901. 38.** Friedl J, Stimming U: Determining Electron Transfer Kinetics at Porous Electrodes. Electrochimica Acta 2017, 227:235–45. Clearly argumentated the inapplicability of cyclic voltammetry at porous electrodes to evaluate the rate constant (ks) of electrode reactions. The methods of ks estimate from EIS and chronoamperometry at porous electrodes are described. Values of ks given in the literature for V(III)/V(II) and V(V)/V(IV) reactions are critically evaluated. 39. Rümmler S, Steimecke M, Schimpf S, Hartmann M, Förster S, Bron M: Highly Graphitic, Mesoporous Carbon Materials as Electrocatalysts for Vanadium Redox Reactions in AllVanadium Redox-Flow Batteries. J. Electrochem. Soc 2018, 165:A2510-A2518. 40. Mazúr P, Mrlík J, Beneš J, Pocedič J, Vrána J, Dundálek J, Kosek J: Performance evaluation of thermally treated graphite felt electrodes for vanadium redox flow battery and their fourpoint single cell characterization. Journal of Power Sources 2018, 380:105–14. 41. Choi C, Noh H, Kim S, Kim R, Lee J, Heo J, Kim H-T: Understanding the redox reaction mechanism of vanadium electrolytes in all-vanadium redox flow batteries. Journal of Energy Storage 2019, 21:321–7. 42. Gorski W, Cox JA: Voltammetry of vanadyl sulfate hydrates in the absence of a deliberately added liquid phase. Journal of Electroanalytical Chemistry 1992, 323:163–78. 43. Gao Y, Huang J, Liu Y, Chen S: Charge transport in confined concentrated solutions: A minireview. Current Opinion in Electrochemistry 2019, 13:107–11. 44. Zhao Y, Le Liu, Qiu X, Xi J: Revealing sulfuric acid concentration impact on comprehensive performance of vanadium electrolytes and flow batteries. Electrochimica Acta 2019, 303:21– 31. 45. Cecchetti M, Casalegno A, Zago M: Local potential measurement through reference electrodes in vanadium redox flow batteries: Evaluation of overpotentials and electrolytes imbalance. Journal of Power Sources 2018, 400:218–24.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: