Electrolyte Additives in Vanadium Redox Flow Batteries

Electrolyte Additives in Vanadium Redox Flow Batteries: History

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Contributor:

Wenxin Tian

Hao Du

Jianzhang Wang

Jan J. Weigand

Jian Qi

Shaona Wang

Lanjie Li

Vanadium redox flow batteries (VRFBs) are promising candidates for large-scale energy storage, and the electrolyte plays a critical role in chemical–electrical energy conversion. However, the operating temperature of VRFBs is limited to 10–40 °C because of the stability of the electrolyte. To overcome this, various chemical species are added, but the progress and mechanism have not been summarized and discussed yet.

vanadium redox flow batteries
electrolyte additives
electrochemical performance
complexation
electrostatic repulsion
growth inhibition
modifying electrode

1. Introduction

The global energy structure is gradually changing from non-renewable energy sources, such as fossil fuels, with high consumption and pollution, to green and low-carbon renewable energy sources. According to the data published by National Development and Reform Commission on 22 September 2022, the installed capacity of renewable energy in China has exceeded 1.1 billion kilowattsand is expected to exceed that of coal power and become the first major power source by 2030. However, renewable energy sources have such problems as discontinuity, instability, high abandoned wind/light rate, and difficulty in frequency/peak regulation of power grids, which may not be able to meet the growing electricity demand. Thus, large-scale and long-time energy storage technologies are urgently needed [1–5]. Among many energy storage technologies, the vanadium redox flow battery (VRFB) has high safety, long cycle life, good charging and discharging performance, rapid response, stable capacity, and low life cycle costs [1,6–11], which makes it the largest, most technologically advanced, and closest-to-industrialization liquid flow battery [12–18]. At present, the world’s largest energy storage project, a 100 MW/400 MWh vanadium battery, has been successfully connected to the grid in Dalian, China and continuous GWh-level projects have submitted bids for construction, indicating great progress in the industrialization of VRFBs.

VRFBs, first proposed by Skyllas-Kazacos in 1986 [19], consist of three key factors: electrodes, an ion exchange membrane, and electrolytes [20–23]. Among them, the electrolyte, as the core part of the vanadium battery system, greatly affects the energy density and overall performance of the battery. It can generally be divided into positive and negative electrolytes, which correspond to the sulfuric acid solutions of the V(IV)/V(V) and V(II)/V(III) redox couples in Equations (1)–(3) [24], respectively. The same elements at different oxidation states can be converted to one another at the electrodes, achieving the chemical–electrical energy conversion, as shown in Figure 1.

Positive electrode : V O^{2 +} - e^{-} + H_{2} O ⇋ V O_{2}^{+} + 2 H^{+}

Negative electrode : V^{3 +} + e^{-} ⇋ V^{2 +}

Overall reaction : V O^{2 +} + V^{3 +} + H_{2} O ⇋ V O_{2}^{+} + V^{2 +} + 2 H^{+}

Figure 1. Vanadium redox flow battery schematic.

The vanadium electrolyte is generally prepared through the methods of physical dissolution, chemical reduction, electrolysis, and chemistry–electrolysis coupling [25] Among them, the chemistry–electrolysis coupling is the dominant method, which takes high-purity V₂O₅ as the raw material and adds reducing agents such as H₂C₂O₄ [26], SO₂, [27], and elemental sulfur to the sulfuric acid to prepare the V(IV) electrolyte, and then reduces it by electrolysis to obtain the V(III) electrolyte. If the operating temperature of the vanadium electrolyte is higher than 40 °C or lower than 10 °C, both the electrolyte stability and energy density of vanadium batteries will decrease, accompanied by capacity loss and battery failure [28]. To solve this problem, additives are added to the electrolyte [29] to improve its stability and optimize the electrochemical kinetics, so as to expand the operating temperature range and raise the energy density of the VRFB. For example, the introduction of ammonium dihydrogen phosphate [30] and acidic amino acid [31] as additives can enhance the high-temperature stability of electrolytes, and ammonium and α-lactose monohydrate [32] have been used to improve the low-temperature stability of electrolytes. Additionally, additives such as polyacrylic acid (PAA) [33] can strengthen electrochemical mass transfer.

Additives in vanadium electrolytes, generally classified as inorganics, organics, and compounds, exhibit different microscopic mechanisms, including complexation, electrostatic repulsion, and growth inhibition. Specifically, while complexation improves anti-precipitation properties by changing the distribution of electron clouds, electrostatic repulsion reduces the agglomeration of V(V) ions by enhancing the dispersion effect of V(V) ions from each other. Growth inhibition, on the other hand, lowers the size of settled particles by impeding the growth kinetics of V₂O₅. Furthermore, the electrochemical mass transfer enhancers in vanadium electrolytes mainly perform hydrophilic modification to enhance electrochemical kinetics, such as MSA, which can be adsorbed on the electrodes to increase the active sites. This adsorption behavior can promote redox reaction kinetics in the interface and optimize the electrochemical performance of the VRFB. Therefore, the introduction of additives can effectively increase the operating temperature range of the vanadium electrolyte, providing effective technical support for the large-scale application of the VRFB.

2. The Function Mechanisms of Additives

The function mechanisms of additives involved in electrolyte performance improvement are still under investigation. Because additives are mainly classified into stabilizing agents, including complexing agents and electrostatic repulsion agents, and growth inhibitors and electrochemical enhancers, the stabilizing mechanisms and enhancement mechanisms will be discussed separately. Whereas the principal stabilizing mechanisms involve complexation, electrostatic repulsion, and growth inhibition, the enhancement mechanism of electrochemical mass transfer mainly involves additives’ hydrophilic modification of the electrode.

3.1.1. Complexation

Complexation is the foremost stabilizing mechanism of additives, and is applied to both inorganic and organic additives. As illustrated in Figure 2, ions (Cl⁻, H₂PO₄⁻) or functional groups (-COOH, -NH₂, -OH, -SO₃H) carrying lone-pair electrons are capable of coordinating with hydrated vanadium ions in the electrolyte to form a more stable intermediate with V–O–S, V–O–P, V–O–Cl, V–O–N, and so on, thereby effectively reducing the formation of V–O–V and inhibiting V₂O₅precipitation. Subsequently, the electron density of vanadium will increase and the local positive charge of vanadium will decrease [50]. Furthermore, the reaction barrier of forming V–O–X (X is the core element of additives) is generally lower than that of forming V–O–V from V₂O₅, which considerably reduces the generation selectivity of V₂O₅ precipitation [51,52].

Figure 2. Schematic of complexation.

In addition, a large number of studies have shown that complexation behaviors can be impacted by the geometries of hydrated vanadium ions in different valences, the synergistic effect of additives, and other supplementary factors. For one, the geometries of hydrated ions of vanadium in diverse valences are different and the complexing capacity is positively correlated with the stability of the additives to vanadium ions in all valences. Clarifying the complexation of vanadium ions with additives in each valence is necessary for both stabilization maximization and additive selection. For another, adopting synergistic effects can intensify the stabilization effect and maintain a balance between various ions, such as phosphate and ammonium. Finally, other supplementary factors can be involved, including introducing double additives to form a competing relationship, modifying the electrodes of VRFBs, integrating a thermally regenerative electrochemical cycle (TREC) into the VRFBs, and so on. Researchers should be aware of the above-mentioned considerations when selecting additives.

Taking H₃PO₄ as an example, the transformation path of V(V) in phosphates-added electrolytes is: [VO₂(H₂O)₂]⁺→ [VO(OH)₂(H₂O)]⁺→ VO(OH)₃[53]. The VO(OH)₃ intermediate can form a compound containing a V–O–P bond with H₃PO₄. Moreover, the activation energy of this reaction is generally lower than that of the formation of the V–O–V bond, which could effectively avoid V₂O₅ precipitation, as presented in Figure 3. Furthermore, the dominant form of the anions is H₂PO₄⁻after phosphate additives are added to the positive electrolyte [51]. In this case, due to the partial dimerization of V(V), the sulfate is coordinated to two oxygen atoms in a bridging or bidentate coordination. This can be explained by the coordination of H₃PO₄ or by the rearrangement of the complexation pattern due to dimerization.

Figure 3. The energy state of the V–O–V bond and V–O–P bond.

Taking another case of Cl⁻, it can form the mononuclear complex VO₂Cl(H₂O)₂ with [VO₂(H₂O)₃]⁺ at high temperatures [35]. This process, with low reaction barriers and high priority, will effectively hinder the deprotonation of [VO₂(H₂O)₃]⁺, serving as the first step during the precipitation reaction. Through DFT and NMR spectroscopy analyses, it is further revealed that V₂O₅ precipitation also could be formed by the deprotonation of di-nuclear [V₂O₃·8H₂O]⁴⁺ cations at high temperatures, as seen in Equation (6) [48]. Nevertheless, the chlorine ion can form a stable di-nuclear complex [V₂O₃Cl₂·6H₂O]²⁺ with [V₂O₃·8H₂O]⁴⁺ to prevent precipitation [48]. Figure 4 shows the geometry-optimized structures of [VO₂(H₂O)₃]⁺, VO₂Cl(H₂O)₂, [V₂O₃·8H₂O]⁴⁺, and [V₂O₃Cl₂·6H₂O]²⁺. The formation of stable structures occurs because Cl₂ has four groups of lone-pair electrons, which act as electron donors for complexation to the empty orbitals of vanadium ions. Furthermore, the nature of the V–O bond is the attraction of positive and negative charges; thus, the behavior of chlorine complexation makes the V–O bond weaker [48] and the O–H bond stronger, which can lead to a more stable H₂O molecule and impede the deprotonation of [V₂O₃·8H₂O]⁴⁺, thereby achieving high stability.

2 {[V_{2} O_{3} \cdot 8 H_{2} O]}^{4 +} \overset{{- 8 H}^{+}}{\to} {2 V}_{2} O_{5} ↓ + 12 H_{2} O

Figure 4. Geometry-optimized structures for [VO₂(H₂O)₃]⁺ compound (a), chlorine bonded VO₂Cl(H₂O)₂ compound (b), pristine di-nuclear [V₂O₃·8H₂O]⁴⁺ compound (c), and chlorine bonded [V₂O₃Cl₂·6H₂O]²⁺ compound (d).

2.1.2. Electrostatic Repulsion

In electrostatic repulsion, a common stabilizing mechanism, ions or functional groups of additives, can be adsorbed on vanadium ions by electrostatic attraction, mainly including -COOH, -OH, -SO₃H, -S-, -NH₂, and so on. This adsorption behavior promotes the formation of ionic agglomerates with vanadium ions as the core element, which can enhance the outer layer charge and generate electrostatic repulsion to varying degrees, as shown in Figure 5. Meanwhile, due to the large core–shell spatial structure, the steric hindrance of this agglomerate encapsulating vanadium ions can strengthen this repulsion effect, making vanadium ions more dispersed, to inhibit precipitation [52]. A proliferation of studies has demonstrated the dominance of electrostatic repulsion in the stabilizing mechanism of organic additives, but the application of this principle in inorganic additives is minimal. More specifically, anionic functional groups produce negatively charged groups (e.g., -COO⁻) in the electrolyte, which brings about electrostatic attraction with the positively charged vanadium ions to form agglomerates. Polar groups (e.g., -NH₂) tend to be adsorbed on vanadium ions based on the like-dissolves-like theory to generate external charge layers, boosting the repulsion effect [32,33,53].

Figure 5. Schematic of electrostatic repulsion.

2.1.3. Growth Inhibition

Growth inhibition, an uncommon stabilizing mechanism, means that some additives can inhibit the growth kinetics of V₂O₅ precipitates in the vanadium electrolyte. As shown in Figure 6, the V₂O₅ precipitates can generally grow with increasing time without additives. However, after introducing some specific additives, the surfaces of the nucleation sites of the V₂O₅ were adsorbed by various molecules to lower the growth rate of V₂O₅ precipitates, lengthening the induction time of precipitation and reducing the sizes of V₂O₅ particles [43,52,54].

Figure 6. Schematic of growth inhibition.

2.2. Electrochemical Mass Transfer Enhancement Mechanism

As Figure 7 shows, the Helmholtz model depicts how the opposite charges can form a set of polar plates in an electrolyte due to mutual attraction in the electrode–electrolyte interface, further forming a capacitor to store energy, which is called the electric double layer. The whole process of redox reaction is divided into a reaction region and a transfer region, whose rates are co-controlled by electron transfer and migrating mass transfer. The high charge gradient in the region of the electric double layer reduces the mass transfer rate of the solid–liquid interface and accelerates the reaction rate. Therefore, in the transfer region, the rate of transfer usually slows down, thus limiting the electrochemical performance, including energy efficiency, capacity retention rate, and the properties of charging and discharging. Accordingly, the mass transfer of electric double layers becomes an important speed-controlled step of electrochemical kinetics.

Figure 7. Schematic of the mass transfer of electric double layers.

The foremost mechanism of enhancing electrochemical mass transfer is that additives can be adsorbed on the surface of the electrode to promote active sites to form a “hydrophilic modification” to electrodes, mainly including hydroxyl, the sulfonic group, pyridyl, and other hydrophilic functional groups, or certain ions [32,57], as illustrated in Figure 8. This modifying behavior to electrodes can activate the interfacial activity between electrodes and the electrolyte, accelerating both the redox reaction of vanadium ions in all valences and the migration mass transfer. Alternatively, additives, such as taurine, MSA, PPS, benzoyl peroxide, and so on, can reduce the overpotential of the VRFB to facilitate the migrating mass transfer, reducing the resistance of the electric double layer and enhancing the kinetics of redox reactions [58–62].

Figure 8. Schematic of the mechanism of electrochemical mass transfer enhancers.

This entry is adapted from the peer-reviewed paper 10.3390/ma16134582

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