**1. Introduction**

The vanadium redox flow battery (VRFB), proposed by Maria Skyllas-Kazacos and dating back to 1970, is considered the most promising renewable energy storage system, with the advantages of high capacity, excellent stability, high operation security, and long cycle, and it has attracted widespread attention and been investigated worldwide [1]. The positive and negative electrolytes of VRFB are stored in two separate tanks, and they flow through a separate half-cell during operation and then return to the tank for recirculation. Each half-cell of VRFB consists of an electrode and bipolar plate, and two half-cells are separated by a membrane that allows selective ion exchange while preventing cross-contamination of the electrolyte [2]. The chemical reactions occurring at the electrodes of positive and negative half-cell, as well as the overall cell reaction, are as follows:

**Citation:** Zhang, X.; Meng, F.; Sun, L.; Zhu, Z.; Chen, D.; Wang, L. Influence of Several Phosphate-Containing Additives on the Stability and Electrochemical Behavior of Positive Electrolytes for Vanadium Redox Flow Battery. *Energies* **2022**, *15*, 7829. https://doi.org/10.3390/en15217829

Academic Editors: Luis Hernández-Callejo, Jesús Armando Aguilar Jiménez and Carlos Meza Benavides

Received: 16 September 2022 Accepted: 18 October 2022 Published: 22 October 2022

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Positive cell reaction:

$$\rm VO\_2^+ + 2H^+ + e^- \rightleftharpoons VO^{2+} + H\_2O \tag{1}$$

Negative cell reaction:

$$\mathbf{V}^{2+} \rightleftharpoons \mathbf{V}^{3+} + \mathbf{e}^- \tag{2}$$

Overall cell reaction:

$$\text{VO}\_2^+ + \text{V}^{2+} + 2\text{H}^+ \rightleftharpoons \text{VO}^{2+} + \text{V}^{3+} + \text{H}\_2\text{O} \tag{3}$$

Under the fully discharged circumstance, the positive and negative electrolytes contain only V(IV) (VO2+) and V(III), respectively. During charging, the V(III) ions in the negative electrolyte are reduced to V(II), and the VO2+ ions in the positive electrolyte are oxidized to V(V) (VO2 +). The electrons move through the bipolar plate from the positive electrode to the negative electrode, causing hydrogen ions (H+) to diffuse across the membrane to the negative electrode. The same reaction occurs in reverse when discharging [3].

Despite the rapid development of VRFB in recent years, some issues limiting its commercialization need to be addressed, one of which is the fact that the vanadium compound, as the active substance in the electrolyte, accounts for a substantial part of the capital cost (40%) [4]. In addition, the battery capacity depends on the vanadium concentration, and the pentavalent vanadium species have a low solubility in sulfuric acid (<2 M) and a narrow operating temperature window (10–40 ◦C), thus limiting the energy density of the battery (typically < 25 Wh L−1) [5]. At higher temperatures (>40 ◦C), the precipitation of V5+ in the positive electrolyte is as follows [6]:

$$2\left[\text{VO}\_2(\text{H}\_2\text{O})\_3\right]^+ \rightarrow 2\text{VO(OH)}\_3 + 2\text{H}\_2\text{O} + 2\text{H}^+\tag{4}$$

$$2\text{VO(OH)}\_{3} \rightarrow \text{V}\_{2}\text{O}\_{5} + 3\text{H}\_{2}\text{O} \tag{5}$$

The precipitation process of V2O5 is irreversible, which is mainly responsible for the loss of battery capacity. In order to improve the solubility of vanadium compounds in the sulfuric acid electrolyte, the introduction of additives is commonly performed. Owing to their being cost-effective and not interfering with electrolyte performance, they have been investigated widely nowadays [7]. Skyllas-Kazacos et al. used phosphoric acid and ammonium phosphate as additives. The results show that phosphate anions enhance the stability of V(V) compounds at high temperatures, whereas for ammonium cations, the opposite is true—ammonium cations stabilize the negative half-cell electrolyte at low temperatures. The effects of sodium triphosphate and sodium hexametaphosphate as additives were also studied; they both retarded the precipitation to a certain extent [8,9]. Roznyatovskaya et al. investigated the mechanism of precipitation retarding by phosphate in the vanadium electrolyte using nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering (DLS). It was concluded that the electrolyte stabilization mechanism by phosphoric acid at high temperatures could be attributed to the interaction between them and V(V) monomers or dimers forming two phosphate-containing substances, thus retarding the V2O5 precipitation [10]. Park et al. used 0.05 M sodium pyrophosphate as an additive in the positive electrolyte with 2.0 M V(V) and 4.0 M H2SO4, and the long-term stability of electrolyte was improved compared with the blank solution. In addition, none of the new precipitation was proved to have been generated in the electrolyte. Nonetheless, its electrochemical cycling performance was optimized [11]. Zhang et al. investigated the effect of Na3PO4 as an electrolyte additive and found that it indeed delayed the V2O5 precipitation, but the VOPO4·2H2O precipitation was detected on the positive graphite mat after several cycle tests [12]. Li et al. reported some organic additives containing hydroxyl (–OH), such as sorbitol, mannitol, glucose, and fructose, and elaborated their stabilizing mechanism, indicating that these organic additives can clad the hydrated V(V) species and thus inhibit the formation of precipitation [13]. Zhang et al. selected 1 wt% HEDP as an

electrolyte additive and confirmed that it can improve the electrolyte thermal stability and battery cycle efficiency of VRFB. Besides, the research confirmed in two ways (the Job plot and the Benesi–Hildebrand plot methods) that HEDP interacts with VO2 <sup>+</sup> in a 1:1 binding stoichiometry, which is the reason for the enhancement in the stability of the electrolyte [14]. Through the above studies, it is found that both phosphate and –OH have a good effect on stabilizing pentavalent vanadium. In summary, some research results on additives of the positive electrolyte are summarized in Table 1.


**Table 1.** Some research results on additives of the VRFB positive electrolyte.

In the present work, five additives containing both phosphate and more –OH, including 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexamethylene diamine tetramethylene phosphonic acid (HDTMPA), amino trimethylene phosphonic acid (ATMPA), sodium ethylenediamine tetramethylene phosphonate (EDTMPS), and diethyl triamine pentamethylene phosphonic acid (DTPMP), were selected and added into the V(V) electrolyte to investigate their effects on precipitation inhibition and electrochemical behavior, and the obtained results were compared with those of the original blank electrolyte. Among the five selected additives, except HEDP, other additives have not been used and discussed in such studies. The novelty of this paper is that this research has explored five kinds of phosphate containing positive electrolyte additives and their effects on stability and electrochemical performance and found two combinations that can improve the thermal stability and electrochemical performance of the electrolyte at the same time.
