The role of phosphate additive in stabilization of sulphuric-acid-based vanadium(V) electrolyte for all-vanadium redox-flow batteries

Journal of Power Sources 363 (2017) 234e243

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The role of phosphate additive in stabilization of sulphuric-acid-based vanadium(V) electrolyte for all-vanadium redox-flow batteries € hne c, Nataliya V. Roznyatovskaya a, *, Vitaly A. Roznyatovsky b, Carl-Christoph Ho Matthias Fühl a, Tobias Gerber a, Michael Küttinger a, Jens Noack a, Peter Fischer a, Karsten Pinkwart a, Jens Tübke a a b c

Fraunhofer Institute for Chemical Technology, Applied Electrochemistry, Joseph-von-Fraunhofer-Str. 7, Pfinztal, 76327, Germany M.V. Lomonosov Moscow State University, Chemistry Department, Leninskiye Gory 1-3 GSP-1, 119991, Moscow, Russian Federation Fraunhofer Institute for Chemical Technology, Environmental Engineering, Joseph-von-Fraunhofer-str. 7, 76327, Pfinztal, Germany

h i g h l i g h t s
Temperature variable NMR study of thermally induced VRFB electrolyte ageing.
Ex-situ monitoring of the VRFB catholyte by dynamic light scattering.
Practical limit of total phosphate concentration in VRFB electrolyte.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 June 2017 Received in revised form 24 July 2017 Accepted 26 July 2017

Catholyte in all-vanadium redox-flow battery (VRFB) which consists of vanadium salts dissolved in sulphuric acid is known to be stabilized by phosphoric acid to slow down the thermal aging at temperatures higher than 40  C. To reveal the role of phosphoric acid, the thermally-induced aggregation is investigated using variable-temperature 51V, 31P, 17O, 1H nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering (DLS). The results indicate that the thermal stabilization of vanadium(V) electrolyte is attained by the involvement of monomeric and dimeric vanadium(V) species in the reaction with phosphoric acid which is concurrent to the formation of neutral hydroxo-aqua vanadium(V) precipitation precursor. The dimers are stabilized by counter ions due to association reaction or if such stabilization is not possible, precipitation of vanadium pentoxide is favored. The evolution of particles size distributions at 50  C in electrolyte samples containing 1.6 M vanadium and 4.0 M total sulphate and the pathways of precipitate formation are discussed. The optimal total phosphate concentration is found to be of 0.15 M. However, the induction time is assumed to be dependent not only on the total phosphate concentrations, but also on the ratio of total vanadium(V) to sulphate concentrations. © 2017 Elsevier B.V. All rights reserved.

Keywords: Vanadium redox-flow battery Phosphate additives Thermally-induced ageing Electrolyte

1. Introduction Redox-flow batteries are developed primarily for stationary applications, such as a long-term storage of the energy obtained from regenerative sources, for example wind and solar energy [1]. Among various types of redox-flow batteries the all-vanadium

* Corresponding author. E-mail address: (N.V. Roznyatovskaya).

[email protected]

http://dx.doi.org/10.1016/j.jpowsour.2017.07.100 0378-7753/© 2017 Elsevier B.V. All rights reserved.

redox-flow battery in sulphuric acid media has received extensive attention and is the most developed in the past decade [2]. However, the chemistry of VRFB is disadvantaged by the fact that the catholyte at high battery state-of-charge (SoC) at elevated temperatures is prone to form a precipitate, which cannot be dissolved anymore and hinders battery performance [3]. In practice the VRFB is charged in such a way, that SoC does not exceed 80% till 85% and is equipped with a temperature management system, which increases the battery cost. At the chemical level this problem can be solved by using mixed acid supporting electrolytes [4] or additives [3,5]. The additives were usually chosen based on

N.V. Roznyatovskaya et al. / Journal of Power Sources 363 (2017) 234e243

phenomenological observations. The common screening method for additives involves the combination of redox titration, spectrophotometric control of chemical stability of the additive against vanadium(V) (VV), which is a strong oxidizing agent, and estimation of induction time, i.e. the time until any sign of aggregation visible by the naked eye occur after the sample being hold at a defined temperature [6]. These procedures, however, are unsuitable to obtain more exact information about underlying processes of precipitate formation or to estimate accurately the starting point of aggregation. However, for a search for additives a thorough understanding of solution chemistry under battery relevant conditions is essential. The identification of VV species in VRFB electrolyte is complicated by the fact that the majority of speciation studies were performed in solutions of very low total VV concentrations (in mM range) or containing other counter ions such as perchloric acid, or chloride. Moreover, precise equilibrium study requires an adjustment of the ionic strength which is, however, in fact limiting the highest possible values of VV concentrations. It is commonly accepted that VV exists in diluted solutions as a dioxo-aqua cation in a distorted octahedral coordination ((1) in Fig. 1) [7,8]. Recently, it was shown by computational quantum mechanical modelling (DFT) [9e11], that [VO2(H2O)4]þ has a loosely bound water and undergoes the conversion to a more stable [VO2(H2O)3]þ ((2) in Fig. 1) having a penta-coordinated bipyramidal coordination sphere. The average coordination number of VV in aquated dioxyvanadate cation [VO2(H2O)n]þ is between 5 and 6. With an increase in the total sulphate concentration, the sulphate or bisulfate ions presenting in electrolyte can form an outersphere complex with VV cation [VO2(H2O)n]A(1h$m)þ (reaction (1)), h in which the coordinated water of the VV cation and/or other solute species is retained. With further increase of sulphate concentration the complexation reaction takes place by the consequent substitution of water molecules from the inner coordination sphere of VV for ligands without distortion of VOþ 2 group ([VO2(H2O)n (1h$m)þ ). VV speciation is reported to be VOþ 2 , VO2SO4 and hAh] V VO2(HSO4)2 in solutions of 0.02 M V and 0.5e1.5 M sulphuric acid [12,13]. 4 [VO2(H2O)n[VO2(H2O)n]þ þ Am 4 [VO2(H2O)n]A(1h$m)þ h (1h$m)þ (1) hAh] 2 [VO2(H2O)n]þ þ Am 4 [Vdim(H2O)x]A(z-h$m)þ 4 [Vdim(h H2O)yAh](z-h$m)þ

(2)

where n ¼ 3e4, m ¼ 1e2, x ¼ 6e8, y ¼ 0e8, z ¼ 2e4, Am ¼ HSO 4, SO2 4 , h ethe number of monodentate ligands. The available data on thermodynamics of the system VVesulfurewater were calculated for the activities of VV and dissolved

Fig. 1. Structures of monomeric VV species (1) [VO2(H2O)4]þ and (2) [VO2(H2O)3]þ in sulphuric acid media suggested in Refs. [9e11] by using DFT simulation.

235

sulfur equal to 0.01 and 1, respectively [14] and suggest that the sulphate and bisulfate enlarge the regions of stability of VOþ 2 V forming VO2SO 4 . In contrast to it, recent calculations of V electronic local structure in proximity of hydronium cation, sulphuric acid, and their conjugate anions based on hybrid DFT in conjunction with continuum solvation model indicate that (i) none of these species covalently bond to VV cation, (ii) the protons favorably interact and protonate the oxo-group of the [VO2(H2O)3]þ cation, (iii) hydrated [VO2(H2O)3]þ cation is fairly stable in the presence of HSO 4 , whereas it forms a highly reactive hydroxyl group in proximity to SO2 4 [9]. Thus forming [VO2(OH)(H2O)] (or written as VO(OH)3) intermediate is supposed to be a precursor in the VV precipitation reaction. In concentrated sulphuric acid the VV monomer is reported to 2þ undergo dimerization reaction (2) to form V2O4þ 3 or V2O4 core zþ moiety [15] (denoted as [Vdim(H2O)x] ). It should be noted that the subdivision into low and high VV concentrations is approximate and is primarily related to the solubility limit of V2O5 in sulphuric acid solutions. The VRFB relevant VV total concentrations exceed this limit and from this point of view the VV electrolyte can be considered as supersaturated solution. The complexation of VV with counter ions at VV concentrations higher than 0.4 M was suggested using extended X-ray absorption fine structure and large-angle X-ray scattering [8] and Raman spectroscopy [16]. Due to the complexation the VV species are stabilized in solutions at high VV concentrations and the major species under highly acidic 3 conditions are reported to be VO2SO 4 , VO2(SO4)2 , VO2(HSO4)2, 4þ 2þ VO , V O and V O [16]. The formation of inner-sphere com3 2 3 2 4 plexes of VV dimer with sulphate ([Vdim(H2O)yAh](z-h$m)þ) is suggested in Refs. [15e17], where A is assumed to act as a bidentate ligand. In solutions of above 2 M VV and 5 M total sulphate copolymeric VV species may be formed involving extensive V-O-V, V-O-S bonding and closer interactions [16]. Therefore, the thermal stability of catholyte depends on the VV speciation, which in turn is a function of not only solution acidity and VV concentration, but also of VV to sulphate ratio. Among the phosphate additives for VRFB electrolyte orthophosphoric acid, various phosphate salts of alkali metals and ammonium [18e20], tripoly- and hexametaphosphates, pyrophosphate [19] have been evaluated. Sodium hexametaphosphate is found not only to efficiently stabilize the VV catholyte at elevated temperatures, but also to inhibit the precipitation of VIV salt from supersaturated solution at low temperatures presumably by adsorbing on the surface of the nuclei and reducing the rate of crystal growth [21]. The VV electrolyte solutions with addition of sodium phosphate salts were investigated by 51V and 31P NMR spectroscopy [20]. The changes in 51V NMR spectra with temperature were attributed to the changes of VV structure in electrolyte solution, however, no temperature-variable 31P NMR study was performed. In contrast, the precipitation of VV as VOPO4 in the cell was observed after flow cell test, leading to the conclusion that the phosphate additives should be avoided in the VRFB electrolyte composition [18]. In this work VRFB catholyte at SoC z100% was investigated by using variable-temperature 51V, 31P, 17O, 1H NMR and DLS techniques to gain more information about the VV speciation and more insight into the role of phosphate for precipitate formation. Lightscattering was recently applied to investigate stability of catholyte in terms of determination of induction time and the Arrhenius plot construction [22]. However, no any particles size distributions in thermally aged electrolyte have been discussed. The ex-situ thermal stability test for VV catholyte in dependence on total phosphate concentration was performed to determine the optimal concentration of phosphate additive.

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an error. Kinematic viscosity was determined at temperatures from 0  C to 38  C using Ubbelohde type viscometer (Type 501, K-value 0.03, Schott, Germany). The dynamic viscosity at 50  C was estimated using a linear regression function based on the determined dynamic viscosity values (Tables 1 and 2 in Appendix A). These values were used as set-up parameters for DLS measurements. All NMR spectra were recorded on a Agilent MR-400 NMR spectrometer with 5 mm OneNMR probe operated at 400.0, 161.9, 105.1 and 54.2 MHz for 1H, 31P, 51V and 17O, respectively. All measurements were done without stabilization of resonance conditions, i.e. without internal lock. A sample with commercial D2O was used as external reference. Chemical shifts were measured with respect to VOCl3, H2O, H3PO4 and tetramethylsilane for 51V, 17O, 31P and 1H, respectively. Single scan was used to acquire the 1H spectrum using 6.4 kHz spectral window. The parameters to record the 51 V, 17O, 31P NMR spectra were as follows: 512 scans within 89.0 kHz, 15000 scans within 73.5 kHz and 512 scans within 6.9 kHz window.

2. Experimental 2.1. Chemicals Orthophosphoric acid (85%, Ph. Eur p.a.), sulphuric acid (95%, supra) were VWR and Carl-Roth reagents respectively, mono- and dibasic sodium, ammonium and potassium phosphates (purum 99%) were purchased from Sigma-Aldrich and used without purification. Vanadyl sulphate hydrate (technical grade) and a commercial 1.6 M vanadium electrolyte solution (denoted as V3.5þ) with a 50%: 50% mole fraction of VIII and VIV, approx. 2 M sulphuric acid and approx. 0.05 M phosphoric acid were from GfE (Gesellschaft für Elektrometallurgie mbH, Germany). 2.2. Electrolyte preparation Vanadium(V) electrolyte probes have been prepared by electrolysis of either a commercial vanadium electrolyte V3.5þ having already phosphoric acid additive (V5-SP series, Table 1) or VIV solution in 2 M sulphuric acid to obtain the additive free VV probes (V5-S-series, Table 1). The electrolysis was carried out in a cell assembled with Fumasep FAP-450 membrane (FuMa-Tech GmbH, Germany), GFD 4.6 EA or GFA 6 graphite felts (SGL Group, Germany) and TF6 (SGL Group, Germany) or FU 4369 (Schunk Kohlenstofftechnik GmbH, Germany) graphite bipolar plates. The felts were pretreated and were compressed to 76% of their original thickness in the cell.

2.4. Variable temperature measurements and thermal stability test To determine the induction time for the precipitation of VV species in electrolyte, electrolyte samples of 1 ml in closed vials were kept in a test chamber conditioned at 50  C (Weiss WKL 64, Germany) and were daily visually inspected until the precipitation started. The thermally induced formation of particles in the VV electrolyte sample at 50  C was examined by means of DLS technique (Zetasizer Nano ZS, Malvern Instruments Ltd, England) with a noninvasive backscatter optic (173 scattering optic). The light source was a He-Ne laser with 633 nm. The preconditioning at defined temperature (room temperature or 50  C) was performed directly in the DLS cell (glass cuvette) and then the size distribution curves were registered periodically within the pre-set time intervals. The used dispersant refractive index for the DLS is 1.4. Variable-temperature measurements were performed in 7  C (7 K) steps from 0 to 63  C (273 Ke336 K). The parameters for NMR spectra registration were the same as for the 1H, 31P, 51V and 17O spectra taken at room temperature. The NMR spectra were recorded in 10e12 min after the electrolyte sample was tempered at preset temperature in the following sequence: 1H, 17O, 51V and 31P. The integral intensities and frequencies of resonance lines in 17O and 31P spectra were determined using the home-made software Intspect [23].

2.3. Electrolyte characterization The vanadium concentrations in the VV electrolyte samples were determined by potentiometric cerimetric titration. The total sulphate concentration ([S]) was determined gravimetrically (thereby the amount of phosphoric acid [P] was neglected). The total vanadium concentration was 1.58 M for phosphate containing sample (denoted as V5-SP-1.6 in Table 1) and 2.3 M for additive free VV sample (V5-SP-2.3 in Table 1). Samples for NMR concentration dependence study were prepared by dilution of V5SP-1.6 and V5-S-1.2 probes by matrix solution, i.e. by 2 M sulphuric and 0.05 M phosphoric acid or by 2 M sulphuric acid, respectively, to maintain the same acidity. Unfortunately, it was not easy to obtain the VV samples with pre-defined VV to [S] ratio having different vanadium stock solutions for electrolysis. Since the electrolysis of commercial V3.5þ solution appears to be more practice relevant, VIV solution was not used a common stock for additivecontaining and additive-free samples. Dynamic viscosity was derived from kinematic viscosity and density values. The density of the electrolyte was determined using a hygrometer (Brand GmbH & Co KG, Germany). To ensure a constant temperature during the measurement the samples were conditioned at pre-defined temperature by a refrigeration bath €ltemaschinenbau AG, Germany). circulator (CC 75, Peter Huber Ka Four measurements were taken to calculate an average to minimize

3. Results and discussion 3.1. Electrolyte study at room temperature As stated above, the changes in V-O or V-O-S core of dioxo vanadium cation are expected to occur in the VV electrolyte under consideration. Therefore, 51V, 17O, 31P and 1H NMR spectra were recorded in VV solution of two series: V5-SP and V5-S, i.e. with and

Table 1 Abbreviations and analysis data for VV electrolyte samples used in the work. Set

V5-SP series V5-SP-1.6 V5-S series V5-S-2.3a V5-S-1.2 a

VV mol. (%)

Total concentrations (M)

Mol. ratio

Preparation procedure

VV

[S]

[P]

[S]: VV

[P]: VV

98.8

1.58

4.0

z0.05

2.5

0.03

97.8

2.3 1.2

4.4 3.2

0

1.9 2.6

The solution was not sufficiently stable and was not used for NMR and DLS measurements.

Electrolysis of 1.6 M V3.5þ Electrolysis of 2.3 M VIV Dilution of V5-S-2.3

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237

without H3PO4 additive, respectively. An example set of results is given in Fig. 2. Both of the 51V spectra (Fig. 3a) for the sample of 1.6 M VV (V5-SP-1.6) and of 1.2 M VV (V5-S-1.2) displayed a single resonance line, which was centred at 556 ppm and 550 ppm for V5-SP-1.6 and V5-S-1.2, respectively. The d(51V) and line width for V5-SP-1.6 (1.9 kHz) were in good agreement with the values of 557 ppm and 1.6 kHz, reported for 1.6 M VV electrolyte solution [20]. However, these resonance lines were broader and downfield shifted compared to the common signal of vanadium(V) dioxocation [VO2(H2O)n]þ in diluted acid solutions, which is known to be at 545 ppm with line width of ca. 1 kHz [24,25]. 17 O NMR spectra (Fig. 2b) are characterized by two peaks for both of the electrolyte probes: the signal O(1) at 80 ppm (line width 1.1 kHz) for V5-SP-1.6 and at 61 ppm (1.3 kHz) for V5-S-1.2, another signal O(2) had the same chemical shift of 163 ppm for both of the probes and was narrower than the peak O(1). It is known that the oxo-oxygens of [VO2(H2O)n]þ in diluted solutions are not observable by NMR undergoing rapid exchange reaction. Such exchange might simply involve proton transfer from ligand H2O, followed by solvate-solvent exchange [24]. In fact both of the 17O NMR spectra

Fig. 3. Chemical shift (solid curves) and line width (dashed curves) in (a) 51V, (b) 17O, (c) 31P and (d) 1H NMR spectra in dependence on V(V) concentration evaluated for the samples with (V5-SP series) and without (V-S series) phosphate additive.

Fig. 2. 51V (a), 17O (b), 31P (c) and 1H (d) NMR spectra in VV electrolyte with (V5-SP-1.6) and without (V5-S-1.2) 0.05 M phosphoric acid.

in Fig. 2b did not show any signal at 0 ppm, which would correspond to the resonance shift of oxygen from neat or bulk water [26]. It can be a confirmation for the fact that reactions of ligand exchange with water or protonation tend to be fast on the NMR time scale, giving time-averaged signals. Therefore, the broad signal O(1) was presumably a superposition of 17O resonances from hydroxy-, water or eP¼O groups of the dissolved species in the presence of VV. The O(2) was attributed to the eS¼O groups of the sulphuric acid [26]. It is important to note that 17O NMR spectra indicated the absence of polyoxoanions which would yield well-resolved multiple resonance lines [26]. 31 P NMR spectrum of V5-SP-1.6 probe (Fig. 2c) displays two signals P(1) at 0.6 ppm and less intensive P(2) at 2.9 ppm. For comparison d(31P) value in VV e free reference solution was 0 ppm. Both of the P(1) and P(2) were downfield shifted and broadened compared to the reference spectrum. It indicated the interaction of phosphate with VV species resulting in the presence of two different types of 31P containing species. Since pKa1 of H3PO4 is 2.15, the free acid was expected to be fully protonated in 2 M H2SO4 electrolyte and large variation of d(31P) values (ca. 3e7 ppm) could be expected in case of changing of the ester O-P-O bond angle [26]. 1 H NMR (Fig. 2d) spectra displayed a single line with d(1H) values of 6.7 and 6.8 ppm for V5-S-1.2 and V5-SP-1.6 respectively. Due to the proton exchange reaction, mentioned above, 1H NMR lines could be the average signal from various proton containing groups and 1H NMR spectra were recorded primarily in order to follow the changes of the spectra with temperature. The differences

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in 1H NMR line width for V5-S-1.2 and V5-SP-1.6 resulted presumably from the differences in VV to total sulphate ratios or in free acid concentration. 51V,17O and 1H NMR spectra suggest that even though the vanadium nuclei of VV species present in V5-SP-1.6 and V5-S-1.2 probes were in chemically almost equivalent environments, the dioxocation [VO2(H2O)n]þ was unlikely to be the predominating species. Since equilibria of VV association with counter ions, dimerization (reactions (1), (2)) and consequently the Vv speciation was expected to be dependent of VV concentration, we investigated the concentration dependence of 51V, 17O, 31P and 1H NMR spectra for both of the series: V5-S and V5-SP. For this series of measurements, the proton concentration was kept constant in all samples by using matrix solution for dilution, i.e. either 2 M H2SO4 or its mixture with 0.05 M H3PO4. The sets of data are presented in Fig. 3. The 51V NMR spectra for both of the V5-SP and V5-S series (Fig. 3a) are characterized by the presence of a single resonance line which shifts downfield and narrows with dilution of probes. Finally, if VV concentration decreased to the 0.3e0.4 M range, a d(51V) value of 545 ppm was attained for both of series V5-S and V5-SP, thereby the line width was less than 1 kHz. This d(51V) value is reported for VV solutions of 0.1 M or VV in a non-complexing medium such as perchloric acid [15,25] and is common for a [VO2(H2O)n]þ cation, which adopts either octahedral ((1) in Fig. 1) or bypiramidal ((2) in Fig. 1) configuration. The d(51V) values for both of configurations are expected to be similar [10]. The evolution of 51V NMR spectra with VV concentration is consistent with the participation of VV species in the reaction of dimerization [15e17], which is accompanied by changes in coordination geometry. The difference in d(51V) value between the concentrated VV sample (556 ppm) and the most diluted one (545 ppm) was of about 10 ppm. The same difference can be derived from DFT-calculated 4þ d(51V) parameters for the compounds of VOþ 2 and V3O4 validated 51 V by V NMR measurements for V of 1e2.3 M in HCl [27] Moreover, it is known that hard substituents (O2, OH, i.e. for example V-O-V bonds) give rise to high-field shifts with respect to the oxo groups 51 of VOþ 2 core [28]. Therefore, it was suggested that the V resonance line for VV species at the concentrations in the range from 0.4 to 1.6 M can be assigned to the mixture of both monomer and dinuclear VV cations at equilibrium, which broadens and up-field shifted in concentrated VV solutions due to dimerization. 17 O NMR spectra within the VV concentration range under consideration displayed two signals O(1) and O(2) (Fig. 3b). d(17O) of the O(1) line was strongly dependent on VV concentration and varied from 6 ppm in VV-free reference solutions to 80 ppm in highly concentrated V5-S-1.6. In the same range could be the signals of eO- or eOH groups (-50e90 ppm) and of eP-O-, P¼O groups (5e25 ppm and 25e75 ppm) [26]. Therefore, O(1) can be assigned to a common signal of 17O nuclei of water molecules bound to VV ions, -OH groups of sulphuric acid or P¼O group undergoing a rapid exchange reactions, i.e. interaction of V-O core with counter ions and water. This exchange reaction becomes less significant at low VV concentrations. Another resonance signal O(2) at 163 ppm remained the same within the VV concentration range for both the V5-SP and V5-S series as well as for VV-free reference solutions. This peak was attributed to S¼O group of sulphuric acid. However, the line width of O(2) increased from ca. 200 Hz to 1 kHz with the increase of VV concentration. In contrast to 51V and 17O NMR spectra, not only chemical shift, but also the number of resonance lines in 31P NMR spectra of V5-SP series depends on VV concentration (Fig. 3c). The 31P signal P(2) at 2.5 ppm could be observed only at VV of 1.6 M and disappeared if the VV electrolyte probes were diluted. The signal P(1), which was broad for V5-SP-1.6 sample (160 Hz) became narrow (10 Hz) at VV concentrations below 0.8 M and slightly up-field shifted from 0.5 to

0.05 ppm. The d(31P) value of 0 ppm was observed in a VV-free reference solution and was related to H3PO4. Similar 31P peak broadening at room temperature was reported in Ref. [20] for VV electrolyte in sulphuric acid with addition of Na2HPO4 and NH4H2PO4 compared to a VV free reference solution. In contrast to the results of this work, the VV electrolyte with addition of phosphate in form of Na2HPO4 and NH4H2PO4 [20] displayed a single broad peak at d(31P) between 0 and 3.2 ppm centred at 1.5 or 1.2 ppm for Na2HPO4 and NH4H2PO4, respectively. The broadening of 31P and 51V resonance signals and their concentration dependence confirmed the assumed interaction of VV with phosphate. The chemistry of VV is featured by formation of vanadatophosphate anhydrides [29] or even more complex heteropolyacids represented by formula [HxPV13O40](12x)- [7], which exists in solutions of VV and phosphoric acid at pH from 1.3 to 4.0 [30]. However, heteropolyacids are characterized by multiple peaks in 51V NMR spectrum and decompose at pH less than 1.3 [30], therefore the species forming in V5-SP-1.6 solution were likely to be vanadatophosphate anhydrides. 1 H NMR resonance signal (Fig. 3d) shifts downfield from 5.7 to 6.6 ppm and broadens for both of V5-S and V5-SP series, when the VV concentration is increased, 1H. The line width for V5-S series remained several times larger than for V5-SP series in the VV concentration range (340 Hz and 120 Hz respectively). The results of NMR characterization of V5-S and V5-SP series are summarized in Table 2. The common characteristics of all the 51V, 17 O, 31P and 1H NMR spectra is that resonance lines become broader and shifted as the VV concentration is increased. Since the chemical shift is an indication of (chemical) environment of nuclei and the line widths in NMR spectra (in case of dynamic systems) is related to exchange equilibria between two or more species on the submillisecond scale [28]. The assumed equilibrium reaction of VV dimerization and complexation of VV with sulphate or phosphate displaces into direction of VV monomers or free, non-bounded into complex VV species in diluted VV solutions. Thereby the species pointed out in Table 2 were supposed to be involved in the equilibrium, so that the average of the chemical shift from free VV and mixed anhydride of VV and phosphate or associates with sulphuric acid was observed. 3.2. Variable temperature measurements Variable temperature NMR measurements were performed for V5-SP-1.6 samples, as the composition of this one is more practice relevant. Fig. 4 shows the evolution of NMR spectral parameters with temperature. It can be seen that 51V resonance line (Fig. 4a) was strongly dependent on temperature and d(51V) varied from 557 to 552 ppm as the temperature increased. The temperature dependence of 51V line width was characterized by a minimum at 35  C. It is known that the heating of electrolyte sample over 30  C results in initiation of a precipitation process in VV catholytes of certain compositions [22]. The temperature dependent evolution of d(51V) is very similar to the results of variable-temperature 51V NMR study reported for 2 M VV electrolyte without phosphoric acid [31] and for 1.7 M VV with 0.15 M Na2HPO4 and 0.15 M Na3PO4 as additives [20]. In contrast to the results of this work the minimum was observed at 17  C for the probe with Na3PO4 and a slightly expressed minimum at approx. 40  C for Na2HPO4 [20]. However, the comparison is complicated by the fact that exact total sulphate or proton concentrations are usually not given or unknown. The analysis of 17O NMR spectra is presented in Fig. 4b. The resonance peak O(1) underwent only a slight shift from 78 to 83 ppm. The peak O(2) related to ¼O group of sulphuric acid remained constant, indicating that this group was not involved in

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239

Table 2 Assignment of chemical shift values from NMR data. CV(V), M

Chemical shift, ppm 51

<0.4 0.4e1.6 0e1.6 0e1.6 0.3e1.6 0e1.6 1.6

V

Species 17

O

1

H

31

P

546 546e557 163 O(2) 6 - 80 O(1) 5.7e6.2 0.0e0.5 P(1) 2.75 P(2)

2þ core VOþ 2 , VO(OH) 2þ a VOþ , Vzþ 2 , VO(OH) dim ¼O from H2SO4 -OH, -O- from acids, water, VV or Vzþ dim core, ¼O from H3PO4, Hþ from H3Oþ, acids, -OH groups 2 3[P] ¼ H2PO 4 , HPO4 , PO4 [P] free or as outer-sphere ligand a [P] as inner-sphere ligand of VV or Vzþ dim

a Outer-, inner-sphere complexes with sulphate ions are not differentiated here. The total charge of the species depends on the amount of anionic units linked either as monodentate or bidendate ligand.

Fig. 4. Temperature dependence of chemical shift (solid curves) and line width (dashed curves) in (a) 51V, (b) 17O, (c) 31P and (d) 1H NMR spectra evaluated for the sample with phosphate additive V5-SP-1.6.

the thermally induced VV conversions. Both peaks O(1) and O(2) became narrower with increased temperature and the relative integral intensities for O(1) and O(2) resonance peaks were independent of the temperature (Fig. 2 in Appendix A). Since the initiation of VV precipitation is assumed to proceed through the 17 protonation of oxo-groups at VOþ 2 moiety [28], O NMR line width should be directly influenced by temperature. Actually, 17O NMR spectra show relatively large lines width at temperatures lower than 20  C which indicated slower proton exchange rates in the solution. Slower proton exchange rate means in turn slow kinetics of VV precipitation. Indeed, 1H resonance line became upfield-

shifted (Fig. 4d) with the increase of temperature similarly with the electrolyte dilution effect (Fig. 3d), and the temperature dependence of line width of 1H NMR spectra exhibited the maximum centred at 25  C. It can confirm that the proton not only participates in the processes of VV precipitate formation but initiates the precipitation. The evolution of 51V, 17O and 1H NMR spectra with temperature can be seen in the Supplementary Materials section (Appendix A, Fig. 1). The d(31P) for both of P(1) and P(2) were found to be temperature independent up to 35  C (Fig. 4c). When the temperature exceeded this value the peak P(2) disappeared. The line width of P(2) only slightly varied with temperature in contrast to the line width of P(1), which increased with temperature similar to the effect of increasing the VV concentration (Fig. 3c). It is important to note that the minimum of 51V NMR line width and the coalescence of the peaks P(1) and P(2) were found to be at the same temperature of 35  C at the variable-temperature NMR spectra. The evolution of 31P NMR spectra is to be considered in more details (Fig. 5). The peaks P(1) and P(2) were very good resolved at low temperatures having the integral intensities at 0  C of 90% and 10%, respectively (Fig. 2 in ESI). In contrast to 17O and 51V resonance lines, the 31P lines were narrow at low temperature. They broadened and coalesced with increased temperature. The temperature variable 31P NMR registered in 0.05 M H3PO4 and 2 M H2SO4 mixture and taken as VV-free reference displayed only one peak at 0 ppm within the same temperature range. This peak was considerably narrower compared to the signal for V5-SP-1.6 sample. This indicates the presence of phosphor containing species other than phosphoric acid featured by the resonance line P(2). The thermally induced disappearance of P(2) is caused presumably by decomposition of this species at the temperatures over 35  C. Therefore, the addition of phosphoric acid leads to changes in VV speciation and thereby to stabilization of VV electrolyte. To reveal the thermally induced formation of particles in VV solution or to observe the changes in particle size distribution derived from DLS measurements (as the kinetics of temperature induced precipitation) and thereby to estimate more precisely the induction time, DLS study was performed using the following common protocol. The parameters necessary for DLS measurement such as dynamic viscosity were calculated from the experimentally determined values of kinematic viscosity and density (in the range of thermal stability of the probe) and are shown in Table 1 (Appendix A). To measure the values at 50  C the temperature dependence of viscosity has been extrapolated (Table 2 in Appendix A). Though the DLS technique is originally developed for the study of colloidal solutions, it can be applied under the following assumptions [32]: (i) the particles are spherically shaped, (ii) the sample solution is homogeneous, so that no deposition occurs at

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Fig. 5. Evolution of

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31

P NMR spectrum of the sample V5-SP-1.6 with temperature.

the cuvette walls, (iii) absorption of visible light does not affect the measurement. These assumptions can be related to VV catholyte only to some extent. The hydrated vanadium pentoxide, which is commonly the final precipitate, is known to have a flat ribbon structure, consisting of entangled polymeric fibres [33]. In spite of the fact that the changes in 51V, 1H and 31P NMR spectra have been observed starting from 35  C, holding of the sample at this temperature was insufficient to trace the particles formation by DLS procedure. Therefore, the time elapsed particle size distributions based on intensity of measured light-scattering in the V5-SP-1.6 probe at room temperature and at 50  C were compared (Fig. 6). It can be seen that the particle size distribution at room temperature was monomodal with a maximum centred at 0.83 ± 0.06 nm. This fraction (denoted as V1 nm) remained stable within the time of experiment at room temperature, i.e. 1.5 h. This time was selected based on a preliminary experiment. We observed no any changes in particle size distribution at 40  C during 50 h and only the same fraction of particles (V1 nm) could be seen at the size distribution plot. Therefore, the duration of the measurement at room temperature was limited to 1.5 h, as the induction time was known to be reciprocal to the temperature and it would not be reasonable to extend the duration of the measurement at 25  C. When this sample was subjected to 50  C, a broader particles size distribution for the V1 nm particles was detected. The measured maximum was centred at 1.12 ± 0.08 nm. The size distribution became bimodal after about 14 h at 50  C, i.e. another fraction of species with the mean size of about 279 nm and low intensity

appeared (Fig. 6b). Simultaneously the size distribution of the fraction V1 nm became narrower. After 23 h the peak corresponding to the fraction of larger particles shifted towards values of more than 1 mm (V1 mm) and then after 37 h at 50  C the detected-mean particles size was 4.05 ± 0.11 mm. The fraction of small species as V1 nm disappeared completely. In contrast to it, the visual inspection of this probe V5-SP-1.6 allowed to notice the first signs of agglomeration only after approximately 2.5 days, i.e. 60 h. To consider the size of particles or to relate it to vanadium species, care should be taken because of assumptions discussed above. For a more precise measurement a modified Stokes-Einstein equation, which is valid for nanotubes or non-spherical particles, can be used for DLS data acquisition and treatment [32]. If the obtained size of particles V1 nm is taken as indicative, it is consistent with the size of pristine dinuclear [V2O3$8H2O]4þ unit of 0.76 nm using an assumption of linear configuration of V-O-V core. This value is evaluated from the bond lengths, calculated for DFT geometry-optimized structure [11,27]. Another rough estimation of the size of VV dimeric unit can be derived from the crystallographic parameters of V2O3(SO4)2 [34]. Approximate size of VV dimers calculated from the volume of crystallographic cell, which is given to be 0.8 nm3, is 1.15 nm. Therefore, it can be suggested, that at room temperature there is no any VV species larger than dimeric in VV electrolyte. This is in good agreement with the results of NMR investigation in this work. This dimeric unit represented by formula [V2O3(H2O)8]4þ in Fig. 7 is supposed to be in equilibrium with monomeric [VO2(H2O)3]þ. The dimer V2O2þ 4 is not considered in Fig. 7, because the V2O2þ 3 may predominate at high sulphate concentrations [16]. Since it was impossible to differentiate between the inner- and the outer-sphere complexes of VV species with sulphate or bisulfate using the NMR measurements, the counter ions are omitted in Fig. 7. The pathways of VV formation proposed in Fig. 7 are based on combination of the results of this work and literature data on DFT simulations of VV interactions with counter ions of catholyte solution [9,31]. According to DFT simulation data [9] the interaction of [VO2(H2O)3]þ monomeric unit with bisulphate does not result in V distortion of VOþ 2 core and V species are predicted to be stable. In contrast to it the interaction of VV with sulphuric acid leads to the protonation of oxo-oxygen group of VOþ to form 2 [VO(OH)(H2O)3]2þ. The interaction with sulphate gives a neutral intermediate [VO2(OH)(H2O)], thereby sulphate ion is likely to assist the deprotonation of water linked to VV centre. Both of these hydroxo-oxo VV species can be precursors for VV precipitation at elevated temperatures [35]. It can be suggested, that [VO(OH)(H2O)3]2þ similarly to [VO2(H2O)3]þ can undergo dimerization or is in equilibrium with VV dimer (Fig. 8). This suggestion is supported by the fact that the cleavage of V-O-V bridge at high total sulphate concentration has been stated as a result of Raman study of VV catholyte [16]. Moreover, no any dimers formed in highly concentrated sulphuric acid, i.e. oleum, because of dissociation of the dimer into VO(HSO4)3 species [36]. All these assumptions are in good agreement with the 17O and 51V NMR results of this work and can explain the broadening of peak lines with the increase of VV concentration or temperature. In the presence of phosphoric acid the formation of mixed anhydride of VV monomer is supposed, which is responsible for the broadening of 31P NMR peaks. The appearance of the resonance line P(2) (Fig. 2c) can be explained by the association of VV dimer with phosphoric acid, to form the outer-sphere or inner-sphere complexes. These species as well as vanadato-phosphate anhydride can be precursors of VOPO4 precipitate, which was observed under the excess of phosphoric acid or phosphates additives in VV electrolyte. The thermal stabilization of VV electrolyte is attained by the involvement of monomeric and dimeric VV species in the reaction

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Fig. 6. Evolution of size distribution curves derived from DLS measurements in the catholyte sample V5-SP-1.6 at 25  C (a) and during storage at 50  C (b).

Fig. 7. Proposed pathways for VV precipitate formation in sulphuric acid electrolyte.

Fig. 8. Induction time determined in electrolyte sample V5-SP-1.6 after addition of (a) orthophosphoric acid, dibasic orthophosphates salts, (b) monobasic orthophosphates salts. The total concentration of phosphate in these trials implies the additive concentration and 0.05 M pristine phosphate of V5-SP-1.6.

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with phosphoric acid, which delays the formation of neutral hydroxo-aqua VV precursor of precipitation. The dissociation of [V2O3(H2O)(8-x)(H2PO4)x](4x) or [V2O3(H2O)(8-2x)(HPO4)x](42x) at elevated temperatures whereby phosphate ion is liberated, is in good agreement with coalescence of the peaks P(1) and P(2) in variable-temperature 31P NMR spectra. It is interesting to note, that the coexistence of smaller (V1 nm) and larger VV (V279 nm) species is prolonged before irreversible precipitation occurs (Fig. 5b). Unfortunately, the combination of NMR and DLS techniques does not enable to get to more details about further VV polymerization steps or vanadium pentoxide phase formation. 3.3. Effect of phosphate concentration If the VV electrolyte is stabilized at elevated temperature in terms of delayed precipitation due to formation of species with phosphate, the increase in phosphate concentration is expected to extend the induction time. In order to prove this assumption, the phosphate concentration in the sample V5-SP-1.6 was increased by addition of either phosphoric acid or salts of phosphoric acid and the induction time was determined at 50  C. The appearance of cloudiness, formation of flakes or deposits at 50  C was taken as an indication for the start of thermal ageing and the corresponding induction time is displayed in Fig. 8. It can be seen that induction time increased from 2.5 days for pristine V5-SP-1.6 sample to approximately 10 days, if 0.1 M of additive was added. Considerable standard deviation in determination of induction time, which is seen in Fig. 8b can be explained by the formation of VOPO4 in several experiments or difficulties to detect the signs of VOPO4 formation by naked eye. Actually, if the additive concentration exceeded 0.1 M the precipitation of VOPO4 was observed, which was reported earlier for electrolyte of 2 M VV and 5 M total sulphate [18]. It is essential to note, that the induction time for V5-SP-1.6 (Fig. 8) depended merely on the total concentration of phosphate containing additives and less on the nature of the additive. The most efficient additive in case of V5-SP-1.6 electrolyte sample was found to be Na2HPO4 at the concentration of 0.1 M. Comparable results are known from the investigation of phosphate additives for VV electrolyte of 1.6 M VV and 3 M sulphuric acid [20]. In fact, inconsistency in the induction times, which could be found in the literature even for the catholytes of the same VV concentrations, is presumably due to the differences in the total sulphate and proton concentrations, which were often not given or could not be determined. The practical concentration of the phosphate additive should be related to the ratio of VV to total sulphate concentrations, which as well has an impact onto thermal stability. 4. Conclusions The 51V, 17O, 31P and 1H NMR spectra were found to be very dependent on total VV concentration and temperature. Such process as dimerization and reaction with phosphate were assumed to result in the broadening of 51V, 17O and 1H resonance lines and changes in d(51V), d(17O) and d(1H). The role of phosphoric acid in stabilizing of catholyte during increase of temperature is likely to be in the interaction with VV monomers or dimers to form two types of phosphate containing species so that the precipitation of vanadium pentoxide is delayed. One type of these species disappears, if the temperature exceeds 35  C. DLS technique may be useful for ex-situ monitoring of the VRFB catholyte and to evaluate the beginning of thermally induced ageing. The results obtained by DLS study of catholyte are in a good correlation with the available information about thermal ageing phenomena in catholyte.

It was found by the DLS study, that the aggregation or ageing process starts considerably earlier than it can be evaluated by common visual inspection of pre-conditioned samples. The equilibrium between smaller (average size of 1 nm) and larger (>279 nm) particles is assumed to precede the aggregation process and to affect the induction time. The practical limit of total phosphate concentration to be used as additive is below 0.15 M. If the phosphate additive exceeds this limit, the precipitation of VOPO4 occurs. Acknowledgements This work is financially supported by the German Federal Ministry for Economic Affairs and Energy (BMWi) in the context of the project “VRFB for private households” (0325755B). The authors are grateful to Dr. Melanie Schroeder and Dr. Hannes Barsch (SCHMID Energy Systems GmbH, Germany) for fruitful discussions and technical assistance in this work. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.100. References [1] J. Noack, N. Roznyatovskaya, T. Herr, P. Fischer, Angew. Chem. Int. Ed. 54 (2015) 9776e9809. [2] M. Ulaganathan, V. Aravindan, Q. Yan, S. Madhavi, M. Skyllas-Kazacos, T.M. Lim, Adv. Mater. Interfaces 3 (2016) 1500309e1500331. [3] M. Skyllas-Kazacos, L. Cao, M. Kazacos, N. Kausar, A. Mousa, ChemSusChem 9 (2016) 1521e1543. [4] L. Li, S. Kim, W. Wang, M. Vijayakumar, Z. Nie, B. Chen, J. Zhang, G. Xia, J. Hu, G. Graff, J. Liu, Z. Yang, Adv. Energy Mater. 1 (2011) 394e400. [5] C. Choi, S. Kim, R. Kim, Y. Choi, S. Kim, H.-y. Jung, J.H. Yang, H.-T. Kim, Renew. Sustain. Energy Rev. 69 (2017) 263e274. [6] T.D. Nguyen, A. Whitehead, G.G. Scherer, N. Wai, M.O. Oo, A. Bhattarai, G.P. Chandra, Z.J. Xu, J. Power Sources 334 (2016) 94e103. [7] D.C. Crans, A.S. Tracey, The chemistry of vanadium in aqueous and nonaqueous solution, in: A.S. Tracey, D.C. Crans (Eds.), Vanadium Compounds, Chemistry, Biochemistry, and Therapeutic Applications, vol. 711, American Chemical Society, Washington, USA, 1998, pp. 2e29. [8] J. Krakowiak, D. Lundberg, I. Persson, Inorg. Chem. 51 (2012) 9598e9609. [9] F. Sepehr, S.J. Paddison, J. Phys. Chem. A 119 (2015) 5749e5761. [10] M. Bühl, M. Parrinello, Chem. Eur. J. 7 (2001) 4487e4494. [11] M. Vijayakumar, W. Wang, Z. Nie, V. Sprenkle, J. Hu, J. Power Sources 241 (2013) 173e177. [12] A.P. Filippov, I.V. Kyarsing, Zh Neorg. Khim 23 (1978) 1523e1528. [13] M. Rakib, G. Durand, Hydrometallurgy 43 (1996) 355e366. [14] X. Zhou, C. Wei, M. Li, S. Qiu, X. Li, Hydrometallurgy 106 (2011) 104e112. [15] C. Madic, G.M. Begun, R.L. Hahn, J.P. Launay, W.E. Thiessen, Inorg. Chem. 23 (1984) 469e476. [16] N. Kausar, R. Howe, M. Skyllas-Kazacos, J. Appl. Electrochem. 31 (2001) 1327e1332. [17] L.D. Kurbatova, D.I. Kurbatov, Russ. J. Inorg. Chem. 51 (2006) 841e843. [18] J. Zhang, L. Li, Z. Nie, B. Chen, M. Vijayakumar, S. Kim, W. Wang, B. Schwenzer, J. Liu, Z. Yang, Journal of, Appl. Electrochem. 41 (2011) 1215e1221. [19] N. Kausar, A. Mousa, M. Skyllas-Kazacos, ChemElectroChem 3 (2016) 276e282. [20] C. Ding, X. Ni, X. Li, X. Xi, X. Han, X. Bao, H. Zhang, Electrochimica Acta 164 (2015) 307e314. [21] M. Skyllas-Kazacos, C. Peng, M. Cheng, Electrochem. Solid-State Lett. 2 (1999) 121e122. [22] D. Oboroceanu, N. Quill, C. Lenihan, D. Ní Eidhin, S.P. Albu, R.P. Lynch, D. Noel Buckley, ECS Trans. 75 (2017) 49e63. [23] V.A. Roznyatovsky, S.M. Gerdov, Y.K. Grishin, D.N. Laikov, Y.A. Ustynyuk, Russ. Chem. Bull. 52 (2003) 552e556. [24] O.W. Howarth, Prog. Nucl. Magnetic Reson. Spectrosc. 22 (1990) 453e485. [25] X. Lu, Electrochimica Acta 46 (2001) 4281e4287. [26] J.C. Lindon, G.E. Tranter, D. Koppenaal, Encyclopedia of Spectroscopy and Spectrometry, second ed., Academic Press, 2016. [27] S. Kim, M. Vijayakumar, W. Wang, J. Zhang, B. Chen, Z. Nie, F. Chen, J. Hu, L. Li, Z. Yang, Phys. Chem. Chem. Phys. 13 (2011) 18186e18193. [28] G.A. Webb, Annual Reports on NMR Spectroscopy, first ed., vol. 62, Academic Press, 2007. [29] M.J. Gresser, A.S. Tracey, K.M. Parkinson, J. Am. Chem. Soc. 108 (1986) 6229e6234.

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