Towards Understanding the Stabilities of Hydrated Vanadium (V) Complex Ions and the Pathway of V2O5 Precipitation in Catholyte Solution of Vanadium Redox Flow Battery

Abstract

The renewable energy storage systems are being fascinated worldwide due to the rapid increase in global energy consumption rate and a wide-ranging scarcity of the nonrenewable natural resources. Being a vanadium redox flow battery (VRFB) a larger capacity electrochemical energy storage system, it is widely recognized as a promising technology more especially for compensating the fluctuated renewable energies in the electrical grids. Despite its unique uses of "all vanadium" species (V²⁺/V³⁺ and V⁵⁺/V⁴⁺ redox couples) as potential electrolytes for storing/delivering electric energy, it has been facing a major limitation related to poor thermal stability of the V⁵⁺ (catholyte) solution. This insight is mainly aimed at investigating the most stable hydrated V⁵⁺ complex ion exists in the VRFB catholyte solution among [VO₂(H₂O)₄]⁺ and [VO₂(H₂O)₃]⁺.H₂O, and understanding its most probable role to initiate the V₂O₅ precipitation process at high temperature ranges. By employing density functional theory (DFT) as a robust computational quantum mechanical model, the latter complex ion is revealed as an energetically low electronic structure with a stabilization energy DE = 25.73 kJ/mol. lower than that of the former complex ion, verifying theoretically the existence of [VO₂(H₂O)₃]⁺ unit as a foremost V⁵⁺ species alike in experimental observations. The same DFT predictions are believed to be a root cause for making the VRFB catholyte solution less stable, as the [VO₂(H₂O)₃]⁺  ions preferentially undergo thermal transition into VO(OH)₃ compound followed by the condensation and hydrolysis reactions, leading to the development of V−O−V structural unit that would ultimately results the V₂O₅ crystallization.