Solvation of Vanadium Cations in Sulfuric and Perfluorosulfonic Acids and Their Effect on the Morphology of PFSA Membranes

The desire, in recent decades, to replace fossil fuel with renewable green energy sources has led to the increasing demand for large scale electrical energy storage devices. Redox flow batteries (RFBs) are among the promising large-scale electrical energy storage systems and have received significant attention due to their superior properties such as long life-span, simple installation, the possibility of instant charging by replacing the electrolytes, and the ability to repeatedly store and convert electrical energy into chemical energy and vice versa. Of the various RFBs currently under investigation, the vanadium redox flow battery (VRFB) demonstrates design flexibility, good electrochemical activity, reversibility, and low maintenance cost [1,2]. A VRFB consists of a V²⁺/V³⁺ sulfate solution at the negative electrolyte and a VO²⁺/VO₂⁺ sulfate solution at the positive electrolyte separated by a proton exchange membrane (PEM). The wide span between the standard reduction potentials of the two half-cells in these batteries produces a standard voltage of 1.25 V, which is comparable to other types of RFBs [3].

Of the many issues that hinder the commercialization of these storage devices, the poor stability of VO₂⁺ solution at high temperatures and high concentrations, results in precipitation of hydrated V₂O₅ and energy loss [4]. Although, the stability of VO₂⁺ ion may be increased by the addition of hydrochloric acid, eventually it will form a gel type precipitate involving the V³⁺ cation [5]. It has also been found that the low ion selectivity of PEMs (mostly perfluorosulfonic acid (PFSA) membranes) allows permeation of vanadium cations and cross contamination of the electrolytes [6,7]. Hence, it is important to understand the solvation of vanadium cations in both the electrolyte solution and PFSA membrane. Knowing the nature of the solvated cation in electrolyte can help to understand the stability of the electrolyte solution as well as the formation of other ionic complexes. Also, the study of the diffused cations within PFSA can help in designing membranes with higher ion selectivity, distinguishing the proton from vanadium cations.

In this work the hydration structure of all four vanadium cations in the sulfuric acid environment is examined with ab initio electronic structure calculations. The introduction of a sulfuric acid molecule to each of the hydrated vanadium ions was performed beginning from different initial positions for the acid molecule with respect to the aqueous complex, and determining a global minimum energy conformation for each system. Since the dissociation of the acid molecule depends on its concentration and pH of the solution, the hydration structures were examined in the presence of SO₄²ˉ, HSO₄ˉ, and H₂SO₄. The results indicate that the sulfuric acid in both dissociated and un-dissociated form can affect the first hydration of vanadium cations compared to their bulk water structure [8]. Figure 1 shows the global minimum structures obtained for the hydrated V³⁺ in presence of sulfuric acid, and the bisulfate and sulfate ions.

To study the solvation of vanadium cations in perfluorosulfonic acid systems, electronic structure calculations were performed in the presence of trifluoromethanesulfonic (triflic) acid and it’s dissociated (triflate) anion. The introduction of triflic acid or triflate to each system was performed using the same protocol used for the sulfuric acid. It has been found that similar to sulfuric acid case the triflic acid can also affect the first hydration of vanadium cations. However, none of the resulting global minimums had shown a strong covalent bonding indicating the presence of weak interaction between the diffused vanadium ions and sulfonic acid.

The above results were used in the morphology prediction of a PFSA membrane used in VRFBs, assisting in the definition of the coarse-grained species. The coarse-grained simulations performed using dissipative particle dynamics simulations to study the effect of the diffusion of different vanadium cations through hydrated Nafion. The morphology of the hydrated Nafion, two dimensional contour plots of species densities, the average radius of the water domains and average diffusivity of the different species were determined.

References

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[2]   X.F. Li, H.M. Zhang, Z.S. Mai, H.Z. Zhang, I. Vankelecom, Energy Environ. Sci. 4, 1147 (2011).

[3]   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, 306 (2011).

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