Theoretical Study On The Effect Of Additives On The Stability Of Positive Electrolyte In All Vanadium Redox Flow Battery

All vanadium redox flow battery（VRFB）is currently one favorable choice among energy storage technologies.Its electrolyte consists of sulfuric acid with different valence vanadium ions,and is one of the most important components of all vanadium redox flow battery,which stability is crucial for improving the performance of the battery.However,the precipitate is produced in the cathode electrolyte as the temperature rises,which reduces the stability and the performance of the battery.Former researchers proposed certain additives to solve this problem,but its mechanism is not yet clear.In this paper,the formation mechanism of V₂O₅ precipitate and the potential additives to extend the lifetime of VRFB are explored further by density functional theory.The main results of this study are as follows:1.A tetrahedral structure of[VO₂（H₂O）₂]⁺surrounded by water molecules in electrolyte is discovered at the room temperature instead of[VO₂（H₂O）₃]⁺·H₂O in previous study.[VO₂（H₂O）₂]⁺forms a more stable structure-[VO（OH）₂（H₂O）]⁺in proton transfer reaction.2.In positive electrolyte without additive,proton in[VO（OH）₂（H₂O）]⁺could be captured by water molecule and intermediate of VO（OH）₃ is formed.Then VO（OH）₃ molecules combine with V-O-V bond in dehydrated process through the transition state,and the activation free energy is 29.37 kcal?mol^-1.In other words,dehydrated process cannot take place and form crystallization of V₂O₅ at room temperature.But the precipitation may happen at higher temperature,and the stability of the battery is very sensitive to its working condition.3.Since H₃PO₄ is ionized easily in electrolyte,the anions of phosphates in acidic aqueous solutions are predominantly in the form of H₂PO₄^-.Proton in reactant is rapidly extracted by H₂PO₄^-ion to form VO（OH）₃ with phosphate additive electrolytes.VO（OH）₃ and H₃PO₄ molecules react and form complexes with V-O-P bond,where the activation free energy is 7.43 kcal?mol^-1 lower than the case without additives.As a result,the phosphate additive could prevent the formation of precipitate,improve the stability of the electrolyte,and achieve longer lifetime of the battery.4.The study shows that,pyrophosphate,oxalate,acetate and citrate additives can also improve the stability of electrolyte;borate,phenol,aluminum hydroxide,urea and glycerin additives have little contribution to the stability of the electrolyte.