390531 Solvation of Vanadium Cations: Effects of Sulfuric and Triflic Acids
The desire, in recent decades, to replace fossil fuel with renewable green energy sources such as solar and wind has significantly increasing as a result of environmental issues. One of the limiting factors for the utilization of renewable energy sources in the electrical supply grid is the intermittent nature of these sources which requires the employment of megawatt 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. RFBs provide 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. In addition, RFBs have the unique advantage of having power and energy decoupled due to the separation of the electrolyte reservoirs and the battery stacks.
Of the various RFBs currently under investigation, the vanadium redox flow battery (VRFB) demonstrates design flexibility, good electrochemical activity, reversibility, and low maintenance cost. 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. Despite the numerous advantages of VRFBs, there still exist a number of issues that hinder the commercialization of these storage devices. For example, the poor stability of the VO2+ solution at high temperatures and high concentrations, results in precipitation of hydrated V2O5 and energy loss . Although, the stability of the VO2+ ion may be increased by the addition of hydrochloric acid, eventually it will form a gel-type precipitate involving the V3+ cation. It is therefore important to understand the chemistry and thermodynamics of solvation of vanadium cations in the electrolyte for design improvements. The other challenging area arises from the membranes used in these devices. The most widely used PEM in VRFBs is Nafion which degrades as a result of exposure to the highly acidic electrolytes. It has also been found that the low ion selectivity of PEMs allows permeation of vanadium cations and cross contamination of the electrolytes. Hence, 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.
Recently, we determined the structure and hydration thermodynamics of the four cations in bulk water with first principles based electronic structure calculations and a quasi-chemical theory. Here we examine the effects of the highly acidic environment in a VRFB, on the hydration of the four vanadium cations in an effort to understand the mechanism of migration of the species in the ion exchange membrane. The effects of both sulfuric and triflic (CF3SO3H) acid are examined as they mimic the electrolyte and membrane environment of a VRFB. Hybrid density functional theory in conjunction with a continuum solvation model was utilized to obtain the local structures of the hydrated vanadium cations in proximity of sulfuric acid, triflic acid and their conjugate anions.
The results indicate that the acids/anions do not form strong covalent bonds with the vanadium cations. It is found that the acid molecules interact directly to the V2+ cation while their conjugate anions (HSO4- and CF3SO3-) interact with cation from outside of the V2+ hydration shell. Also, the direct interaction of acid with the V2+ cation results in the proton dissociation of only the stronger acid, triflic, and not the sulfuric acid. However, in the complexes of V3+ both the triflic and sulfuric acid protons fully dissociate. For VO2+ cation, the axial water in the first hydration shell is replaced by acids or anions. Also, the results indicate that there are no interactions between the oxo-oxygen of VO2+ and acidic groups, regardless of the negative atomic charge on the oxo-oxygen. In contrast, the oxo-oxygen of VO2+ interacts with the acidic hydrogen and protonates by triflic acid or sulfuric acid presence.