Elsevier

Electrochimica Acta

Volume 164, 10 May 2015, Pages 307-314
Electrochimica Acta

Effects of phosphate additives on the stability of positive electrolytes for vanadium flow batteries

https://doi.org/10.1016/j.electacta.2015.02.187Get rights and content

Highlights

  • A series of phosphates is investigated as additives for vanadium flow battery.

  • Superior V(V) thermal stability and improved electrochemical performance.

  • Enhanced battery efficiency and slower capacity fading.

  • Mechanism for the stabilization and performance improvement is put forward.

  • NH4H2PO4 indicates a promising candidate for additive of the positive electrolyte.

Abstract

A series of phosphates is investigated as additives to improve the stability of the electrolyte for vanadium flow battery (VFB). Two selected additives show positive effect on the stability of electrolytes under ex-situ stability tests and in situ flow cell experiments. The effects of additives on electrolyte are studied by Nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman spectroscopy, Cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS) and charge–discharge test. The results show that a VFB using the electrolyte with NH4H2PO4additive demonstrates significantly improved redox reaction reversibility and activity, and higher energy efficiency. In addition, the cell employing the electrolyte with NH4H2PO4 exhibits a charge capacity fading rate much slower than the cell without additives during the cycling at high temperature. These results indicate that the phosphate additives are highly beneficial to improving the stability and reliability of VFB.

Introduction

Large-scale energy storage has attracted increasing attention due to its urgent need in load leveling, uninterruptible power supply systems and renewable energy storage [1], [2], [3]. Vanadium flow batteries (VFBs) initiated by M. Skyllas-Kazacos from UNSW in 1980s [4], have been widely regarded as one of the most suitable options for large scale energy storage due to their significant advantages such as high energy efficiency (>75%), deep discharge ability, fast response, long cycle life and most importantly, independent power and energy ratings [5]. VFBs can realize a reversible conversion between electrical energy and chemical energy through the reactions of two redox couples of V2+/V3+in a negative half-cell and VO2+/VO2+in a positive half-cell. By using the same element (vanadium) in both half-cell electrolytes, VFBs overcome the inherent issue of cross contamination caused by diffusion of different ions across the ion-exchange membrane [3].

In a VFB, the electrolyte serves not only as an ion conductor but also as an energy storage medium to store and release energy [6]. However, the poor stability of the electrolytes especially low solubility of vanadium based electrolytes has affected the final VFB performance [7]. The precipitate of the negative electrolytes at lower temperature and positive electrolytes at higher temperature, especially when the electrolytes’ concentration exceeds 2 M, has limited the energy density of VFB (≤25 Wh kg−1), further increased the cost of the battery system [8], [9]. Therefore the solubility and stability of electrolytes are of great significance in the development of VFB systems.

Specifically, the fully charged V5+ electrolyte solution suffers from precipitation at elevated temperatures (>310 K). This poor stability is witnessed as the irreversible formation of hydrated V2O5 precipitates, which may cripple the pump circulation and lead to energy loss and the final failure of the battery [7], [8], [10].

In the past years, significant efforts have been devoted to improving the stability of the positive electrolyte, aiming at developing an electrolyte with high concentration and further improving the energy density for VFB systems [8], [9], [11]. The solubility of vanadium ions can be improved via the optimization of the supporting acid electrolyte. One of the effective methods was to enhance the solubility of the electrolyte. For example, higher concentration of sulfuric acid in electrolytes can effectively stabilize V(V) ions [8]. However, increasing the H2SO4 concentration will accelerate the precipitation of V(II), V(III) and V(IV) ions due to the common ion effect [9], [12]. Furthermore, a H2SO4 concentration of 3–4 M has been found to be more suitable, considering the cost and corrosive durability for materials. In addition, the employment of mixed acid as supporting electrolyte can considerably improve the thermal stability of V(V) ions [13], [14]. The mixed acid-based vanadium flow battery has been recently reported and displayed enhanced energy efficiency and charge/discharge capacities due to a higher vanadium concentration with excellent thermal stability, however, it also requires a high concentration of the mixed acid, which may result in the increased risk of metal corrosion [15]. Another strategy to delay the precipitation of vanadium species is to add precipitation inhibitor [16], which is one of the most economic and effective methods to stabilize the vanadium electrolytes. Normally two types of additives, e.g., inorganic/organic can be used as stabilizer for VFB electrolytes [17], [18], [19], [20], [21], [22], [23], [24].

Alcohols with ring or chain structures can increase the solubility of V(II)–V(V) ions in the solution, stabilize the electrolyte and reduce vanadium precipitates in the electrolyte [25]. However, these organic compounds suffer from low chemical stability in the strongly oxidative V(V) solution [19]. They could participate in the electrochemical reaction of the vanadium battery and subsequently result in capacity loss. Thus inorganic additives were widely investigated, e.g., phosphate based additive ((NaPO3)6, Na3PO4, Na4P2O7) [16], [17], [19], sulfate based additives (K2SO4, Na2SO4, Al2(SO4)3) [16], [19], [20], chloride based additives (BiCl3, CoCl3) [20] and metal ions (Gr3+, In3+) [26], [27], et al.

Among the reported inorganic additives, phosphate based additives are a typical kind of efficient stabilizing agents due to the interaction between vanadate and pyrophosphate or phosphate, confirming the formation of the mixed anhydrides with vanadate analogous to pyrophosphate or triphosphate by vanadium NMR spectroscopy [28]. Skyllas-Kazacos proposed sodium hexametaphosphate ((NaPO3)6) containing six phosphate groups in a ring as precipitation inhibitors for supersaturated VOSO4solutions, presumably by adsorbing on the surface of the nuclei and reducing the rate of crystal growth [16]. Zhang and co-workers also evaluated the influence of Na3PO4as stabilizing agents on both positive and negative electrolytes, indicating outstanding thermal stability but with deteriorated capacity retention through in situ flow cell test [19]. They deduced that phosphate and polyphosphate anions may have negative effects on the stability of vanadium solutions due to the formation of insoluble VOPO4 with V(V) ions. Recently, Park employed sodium pyrophosphate tetrabasic (Na4P2O7, SPT) in the positive electrolyte to improve long-term stability of a non-flow VFB single cell [17]. In spite of these reports, studies of stabilization mechanism are very limited and the systemically work on phosphate additives is not clear, leading to very few relevant strategies for improving the solubility and stability of electrolytes.

In this paper, we report our investigation of phosphate additives as an inorganic additive in positive electrolytes for VFBs and their effects on long-term stability and electrochemical performance in detail, including electrochemical properties and battery performance evaluation. We also provide an insight into the general and elementary stabilization mechanism of the phosphate additives.

Section snippets

Electrolyte preparation and NMR study

The V(IV) electrolyte solutions were prepared by dissolving VOSO4·xH2O in sulfuric acid solutions. The V(V) electrolyte solutions were prepared electrochemically by charging the V(IV) solutions in a flow cell. At the end of the electrolysis, the concentration of final solution was determined by using an Automatic Potentiometric Titration Instrument (Titrando 905, Metrohm, Switzerland). 51V and 31P NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer operating at 105.27 MHz and

51V NMR (Nuclear Magnetic Resonance) study

To validate the interaction between the phosphates with different structures (sodium or ammonium based normal salt, hydro phosphate, dihydric phosphate) and positive electrolytes, as well as to clarify the stabilization mechanism of the additives, the electrolyte with and without phosphate additives were characterized by 51V NMR to confirm the chemical environment of the vanadium ions in the electrolyte. Fig. 1(a) shows the 51V NMR spectra of V5+ solutions with different kinds of additives at

Conclusion

In this work, a series of phosphates was introduced as inorganic additives to the positive electrolyte of VFBs and their effects on long-term stability and electrochemical performance of VFB were investigated. The thermal stability test showed the additives of phosphates could effectively delay the precipitate time of V(V). The improvement of the thermal stability for the electrolyte with phosphate additives could be ascribed to the formation of the complex between the additive and vanadium

Acknowledgements

The authors greatly acknowledge financial support from the National Basic Research Program of China (973 program No. 2010CB227201) and the China Natural Science Foundation (No. 51361135701). The Raman work was carried out at the State Key Laboratory of Catalysis of Dalian Institute of Chemical Physics. We appreciate Professor Baokun Huang for the help with the Raman measurement.

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    These authors contributed equally to this work.

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