Revealing the role of phosphoric acid in all-vanadium redox flow batteries with DFT calculations and in situ analysis

Abstract

The present work suggests the use of a mixed water-based electrolyte containing sulfuric and phosphoric acid for both negative and positive electrolytes of a vanadium redox flow battery. Computational and experimental investigations reveal insights on the possible interactions between the vanadium ions in all oxidation states and sulphate, bisulphate, dihydrogen phosphate ions and phosphoric acid. In situ cycling experiments and ion-specific electrochemical impedance measurements confirmed a significant lowering of the charge-transfer resistance for the reduction of V(iii) ions and, consequently, an increase of the voltaic efficiency associated with the negative side of the battery. This increase of performance is attributable to the complexation of this oxidation state by phosphoric acid. So far, mixed acids have mainly been discussed with the focus on V(v) solubility. In this work we rationalize the impact of the mixed acids on the electrochemical efficiency opening new strategies on how to improve the cycling performance with ionic additives.

Tackling Capacity Fading in Vanadium Redox Flow Batteries with Amphoteric Polybenzimidazole/Nafion Bilayer Membranes

“…For bilayer membranes with 3 μm of PBI (representing a 20 % share of AEC), the diffusivities of V II , V III , V IV , and V V decreased by 70 %, 85 %, 55 %, and 17 %, respectively, compared to the pristine NR212 membrane (Figure ). The difference of the effect on V II =V 2+ and V IV =VO 2+ (which have the same nominal charge) can be explained with the presence of counterions in the first solvation shell of VO 2+ , which reduces the effective charge of the ion . Increasing the PBI layer thickness to 6 μm further decreased the transport up to a point at which vanadium crossover was not detectable anymore within the experimental time of two weeks.…”

“…The difference of the effect on V II = V 2 + and V IV = VO 2 + (whichh ave the same nominal charge) can be explained with the presence of counterions in the first solvation shell of VO 2 + ,w hich reduces the effective charge of the ion. [31,32] Increasing the PBI layer thickness to 6 mmf urtherd ecreased the transport up to a point at which vanadium crossover was not detectable anymore within the experimental time of two weeks. Thus, aP BIthickness of more than 6 mmi sn ot recommended because the resistance would further increase withouta ny benefits on vanadium transport.…”

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries

“…The inclusion of small quantities of H 3 PO 4 to the electrolyte, has been reported to provide improved thermal stabilization of the vanadium electrolyte 25,26 and reduced polarization resistance of the negative electrode. 27 To generate the appropriate vanadium oxidation states on each side of the cell, 60 mL of precursor solution was added to the positive tank and 30 mL to the negative tank. A constant voltage of 1.65 V was applied which converts the precursor in the positive tank from VO 2+ to VO 2 + and the precursor in the negative tank from VO 2+ to V 3+ and finally to V 2+ .…”

“…,26 and reduced polarization resistance of the negative electrode 27. To generate the appropriate vanadium oxidation states on each side of the cell, 60 mL of precursor solution was added to the positive tank and 30 mL to the negative tank.…”

A review of all‐vanadium redox flow battery durability: Degradation mechanisms and mitigation strategies

“…More recently, Oldenburg et al expanded on the knowledge that H 3 PO 4 was beneficial as a V(V) precipitation inhibitor and tested a 2M H 2 SO 4 /1M H 3 PO 4 mixed electrolyte. Preliminary cycling results revealed a 7% increase in voltage efficiency (VE) when compared to the equivalent purely sulfuric acid‐ supported electrolyte.…”