Concentration- and Temperature-Driven Evolution of V5+ Solution Structure in Redox-Flow Battery Electrolytes Studied By Multinuclear NMR

The ongoing implementation of renewable energy generation systems into worldwide electrical grids increasingly demands additional energy storage platforms to maintain stability through their associated intermittency, with vanadium redox-flow batteries being a forefront solution owing to the decoupling of energy- and power-density that they afford.1 However, a lingering performance issue associated with this chemistry in its conventional format, with the vanadium ions dissolved in concentrated aqueous sulfuric acid, is the poor thermal stability of the V5+ cation in solution. This leads to precipitation as the hydrated penta-coordinated vanadate complex, stable at room temperature, is deprotonated and converted to insoluble V2O5 by a condensation reaction induced at elevated temperature.2 This process ultimately limits the concentration of the electrolyte – adversely impacting the available energy density – and its operation at temperatures above ~310 K. While alternatives to this relatively simple electrolyte composition have shown great promise in rectifying this thermal- and concentration-induced instability of the solvated V5+, via the introduction of mixed acid solvent systems3 or stabilizing additives,4 a complete understanding in the original formulation at a range of length- and time-scales and across a multitude of conditions will aid the further development of alternative systems, and provide a means to benchmark their performance at a very fundamental level. This study therefore aims to greatly expand on previous multinuclear NMR work2,5 on the elucidation of this precipitation mechanism by combining a thorough exploration of concentration effects on V5+ solvation structure and dynamics in conjunction with an investigation of their thermal behavior. Ten samples spanning the concentration range of 0 M to 1.7 M V5+ (all with 5 M H2SO4) were examined over temperatures from 298 K to 383 K ascending, and subsequently likewise descending. Combining 1H, 17O, 33S, and 51V linewidth and shift measurements, the evolving solution structure with increasing temperature – and the associated hysteresis on again decreasing it – is mapped as a function of the V5+ concentration. In particular, the extent of the thermal hysteresis previously established with 17O and 51V linewidth and shift measurements at high V5+ concentration2 is quantitatively examined as the concentration is lowered. This spectroscopic analysis is complemented by a similar variable-concentration and variable-temperature study of the 1H self-diffusivity via pulsed-field gradient (PFG) NMR and by 17O longitudinal (T1) relaxation measurements, to determine both the effect of this evolving solution structure on the dynamics and transport, and how they are impacted by increasing V5+ concentration via the associated activation energy trends. The 1H PFG-NMR measurements, coupled with high-accuracy dynamic viscosity measurements via the falling-ball method, are also used to examine the evolution of the effective proton hydrodynamic radius as the V5+ aggregation reaction occurs. The totality of the data, collected across such a wide range of concentration and temperature conditions, yields an unprecedented map of the V5+ solvation phenomena and solution dynamics which spans molecular to bulk length- and time-scales, and which will serve as a useful platform of comparison for analyzing the effectiveness of alternative solvent systems and stabilizing agents presently undergoing development. References M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011) https://iopscience.iop.org/article/10.1149/1.3599565. M. Vijayakumar et al., J. Power Sources, 196, 3669–3672 (2011) http://dx.doi.org/10.1016/j.jpowsour.2010.11.126. L. Li et al., Adv. Energy Mater., 1, 394–400 (2011) http://doi.wiley.com/10.1002/aenm.201100008. N. Kausar, A. Mousa, and M. Skyllas-Kazacos, ChemElectroChem, 3, 276–282 (2016) http://doi.wiley.com/10.1002/celc.201500453. M. Vijayakumar et al., Chempluschem, 80, 428–437 (2015) http://doi.wiley.com/10.1002/cplu.201402139.