Aspects of electron transfer processes in vanadium redox flow batteries

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ScienceDirect Review Article

Aspects of electron transfer processes in vanadium redox-flow batteries Nataliya Roznyatovskaya1,2, Jens Noack1,2, Karsten Pinkwart1,2 and Jens Tübke1,2 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.

Addresses 1 Fraunhofer Institute for Chemical Technology, Applied Electrochemistry, Joseph-von-Fraunhofer-Str. 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: Roznyatovskaya, Nataliya (nataliya.roznyatovskaya@ict.fraunhofer.de)

Current Opinion in Electrochemistry 2020, 19:42–48

vanadium couples (V(III)/V(II), V(IV)/V(III), and V(V)/ V(IV)1) at the electrodeeelectrolyte 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, that is, 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:

This review comes from a themed issue on Fundamental and Theoretical Electrochemistry Edited by Galina Tsirlina For a complete overview see the Issue and the Editorial

þ VOþ þ e 2 þ 2H

discharge # VO2þ þ H2 O charge

Available online 15 October 2019

ðpositive half cellÞ

https://doi.org/10.1016/j.coelec.2019.10.003

(1)

E0 ¼ 1:0 V vs: SHE

2451-9103/© 2019 Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

discharge Keywords Vanadium redox-flow battery, Vanadium electrode reactions, Kinetics.

V

3þ

þe

# V 2þ charge

(2)

ðnegative half cellÞ E0 ¼ 0:26 V vs: SHE

Introduction The electrode reactions of vanadium species attract increasing attention since the last two decades. This is because of the fact that the redox reactions of three Current Opinion in Electrochemistry 2020, 19:42–48

1 For brevity, vanadium species in different oxidation states are denoted here as V(II), V(III), V(IV), and V(V), unless the structure of vanadium species in solution is considered.

2 The actual vanadium speciation in VRFB electrolytes and therefore reversible potentials can deviate from that ones, which are related to Eqs. (1) and (2).

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Vanadium redox-flow battery reactions Roznyatovskaya et al.

Because 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 (Eq. (3)) are known to have high activation energy, that is, slow kinetics [1] and are not well characterized.

VO

2þ

þ 2H

þ

þe

chargeðanolyteÞ # V 3þ þ H2 O chargeðcatholyteÞ

(3)

E0 ¼ 0:337 V vs: SHE

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, that is, precharging proceeds by electrochemical catalytic (EC’) 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

43

precharging 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 noncomplexing or weakly complexing acids (like HClO4) present as hexa-aqua cations of common structure [V(H2O)6]nþ. 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 are 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 that the reduction of V(III) to V(II) in millimolar solutions is reversible at the dropping mercury electrode. The

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, CP-carbon 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 [16–20,22]. www.sciencedirect.com

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44 Fundamental and Theoretical Electrochemistry

charge transfer process in H2SO4, HClO4, and HCl solutions of V(III) at this electrode was supposed to proceed through the following scheme [5]: V 3þ #VX 2þ #VX þ 2 þe

2þ VX þ /V 3þ / V 2þ 2 / VX

(4)

(5)

where X is complexing anion (counterion like Cl , or HSO-4).

The complexation equilibrium Eq. (4) was assumed to occur infinitely fast so that the reduction of noncomplexed V(III) cation was the rate-determining step Eq. (5) in these electrolytes. Thereby, no significant complexation of V(II) was expected in sulfate 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(III)/V(II) reaction 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 outersphere mechanism was based on the evaluation of ks in the context of Marcus theory for outer-sphere reactions.3 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 current densities (j0) at glassy carbon electrode in 0.2 M vanadium electrolyte was recently reported for HCl matrix (1.6 M total chloride) compared with H2SO4 matrix (0.7 M total sulfate) [11]. Because 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) electro-oxidation 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 electro-oxidation 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].

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.

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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 in 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, ionpairing) at given concentration are the main obstacles for comparison of the reported data and their application to the development of battery materials or modeling 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 dioxycations (VOþ 2 ) and oxocations (VO2þ), respectively. The water molecules from the first coordination shell are labile and can be replaced by sulfate or bisulfate as bidentate 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 VO3þ ionic form in highly concentrated sulfuric acid and its reaction 4þ with VOþ 2 to form V2O3 moiety was stated in the past [28]. Therefore in diluted acidic solutions, the vanadium dioxycation and oxocation core are considered commonly in connection with protonation equilibria to propose the mechanistic pathways for V(V)/V(IV) redox reactions [29] (Figure 2a). Gattrell et al. [29,30] examined extensively the kinetic mechanism of the V(IV)/V(V) couple in H2SO4 electrolyte solutions with 50 mM total vanadium concentration at graphite electrode (pretreated by polishing or slightly oxidized). The reaction of V(IV) oxidation was expected to consist of three elementary steps: chemicaleelectrochemicale chemical (CEC) mechanism at low overpotentials, 4 which changes to a electrochemicalechemicalechemical (ECC) mechanism at higher anodic or cathodic 4 Overpotentials are referred in this case to the formal potential of the redox couple, that is, reversible potential under nonstandard conditions.

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Vanadium redox-flow battery reactions Roznyatovskaya et al.

45

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 sulfate or bisulfate are omitted here.

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 (a < 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.

[29,30]. 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*]. www.sciencedirect.com

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 V2O3þ 3 ions resulted in the change of the reduction mechanism of V(V) ions, from a VOþ 2 - centered reduction at low overpotentials (CEC mechanism) to a V2O3þ 3 - 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 chemical-electrochemical-chemical-chemical (CECC) mechanism at the electrode was including adsorption of V2O3þ 3 proposed [31**]. The formation of vanadium concatenated, that is, dimeric species was recently confirmed by Current Opinion in Electrochemistry 2020, 19:42–48

46 Fundamental and Theoretical Electrochemistry

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*].

The presence of carbon surface o-benzoquinoneetype 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 catechol-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 CeO groups as inner-sphere monodentate complexes during reduction of V(V) was supported by Car Parrinello molecular dynamicsebased 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 electrochemical impedance spectroscopy (EIS) and chronoamperometry. Rümmler et al. [39] 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. Kinetics of the V(V)/V(IV) reaction, which was investigated in-situ of a VRFB by Mazúr et al. [40], was found to be unaffected by oxygen functionalization.

The passivation of the carbon electrode by strong chemisorption of V2O3þ 3 dimeric structures at surface CeO groups was also presumed by Wang et al. [31**].

Choi et al. [41] assumes on the basis of molecular dynamics 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 because of less-organized nature of their hydration shell, enabling electron transfer without the participation of OH groups. The comparison of these Current Opinion in Electrochemistry 2020, 19:42–48

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 being discussed.

It is interesting to note that for electro-oxidation of V(IV) in solid samples of VOSO4∙xH2O (x = 3 or 5) at carbon disk, ultramicroelectrode reversible cyclic voltammograms were obtained in contrast to solutionphase systems [42]. Considering the solid hydrates of vanadyl sulfate as a limiting case of a very concentrated solution, 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 that is, 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, that is, ultimate 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 that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Acknowledgements This work is funded by the Fraunhofer TALENTA Programm for support of female scientists.

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