Functional and Smart Materials: Structural Evolution and Structure Analysis

Towards understanding the poor thermal stability of V5+ electrolyte solution in Vanadium Redox Flow Batteries

“…the number of hydroxyl groups created, is directly proportional to the charge of the vanadium cation (z+) and inversely proportional to the pH of the solution, which is explained in the charge-pH diagram reported by Jørgensen [16,22]. In the case of compound A, the higher charge of the vanadium cation (V 5+ ) in combination with the increase in temperature leads to a maximum hydrolysis ratio (h = 3) leading to the neutral compound B.…”

“…Hence, compound A is the plausible initial structure in the V 5+ electrolyte solution before the thermally induced structural transition. Since the structural transition is followed by V 2 O 5 precipitation, the most probable way this process can occur is by condensation of mononuclear vanadium species containing hydroxyl groups [16]. Therefore the new vanadium species formed at high temperature (>335 K) most likely contain some hydroxyl groups in its structure.…”

“…3 shows 17 O line width during heating cycle (only compound A) and cooling cycle (representing both compounds A and B). The relatively smaller line width during cooling cycle indicates the possible faster proton exchange in solution and if this fast proton exchange happens directly between hydroxyl groups of two compound B molecules, it will facilitate the condensation followed by a hydrolysis reaction (2H 3 VO 4 → V 2 O 5 + 3H 2 O) [16]. Such a hydrolysis reaction can lead to the formation of V-O-V bonding groups known as structural building mosaics of the solid V 2 O 5 which would ultimately lead to the crystallization of V 2 O 5 [16].…”

“…The relatively smaller line width during cooling cycle indicates the possible faster proton exchange in solution and if this fast proton exchange happens directly between hydroxyl groups of two compound B molecules, it will facilitate the condensation followed by a hydrolysis reaction (2H 3 VO 4 → V 2 O 5 + 3H 2 O) [16]. Such a hydrolysis reaction can lead to the formation of V-O-V bonding groups known as structural building mosaics of the solid V 2 O 5 which would ultimately lead to the crystallization of V 2 O 5 [16]. Further, it is interesting to note that the 17 O NMR spectra shows relatively larger line width at low temperatures (<0 • C) which indicates slower proton exchange rates in the solution at this low temperature region.…”

“…In the electrolyte medium the vanadium cations are solvated by water molecules forming hydrated cations [V(H 2 O) n ] y+ where the electron density from molecular orbital of the water molecules is transferred to the empty orbitals of the metal cation [16]. This charge transfer weakens the O-H bond in the coordinated water molecules and makes it more acidic leading to deprotonation of the hydrated vanadium cations as follows [16]:…”