Study of the Mechanism of the Vanadium 4+/5+ Redox Reaction in Acidic Solutions

Abstract

Study of the mechanism of the vanadium 4+/5+ redox reaction in acidic solutionsThe mechanism of the vanadium VO 2ϩ /VO 2 ϩ redox couple has been examined in acidic aqueous solutions. A detailed understanding of this chemistry is of interest for improving and optimizing the performance of vanadium redox-flow batteries, a promising electrochemical electricity storage technology. The vanadium 4ϩ/5ϩ redox reactions were studied at a rotating disk graphite electrode and polarization curves were obtained in sulfuric acid and perchloric acid, with varying pH and vanadium concentrations. The results were compared to model predictions for different mechanisms. The data were consistent with a model with a multistep chemical-electrochemical-chemical mechanism at low overpotentials, which changes to a multistep electrochemicalchemical-chemical mechanism at higher anodic or cathodic overpotentials. Unusually high Tafel slopes ͑350-450 mV/decade͒ were observed for the reduction of VO 2 ϩ at higher overpotentials. While this could not be directly explained by the model, insights gained through the use of the model can provide the basis for some suggestions.

Modelling the effects of oxygen evolution in the all-vanadium redox flow battery

“…The actual kinetics are highly complex [14,15] but the work of Shah and coworkers [12,13] has demonstrated that the above mechanism provides a good approximation.…”

“…In this paper, the evolution of oxygen in the positive electrode on charge, a known side reaction [15,16] with the effects of operating temperature are also incorporated into the model:…”

“…The rate constant for the reaction at the positive electrode was determined by Gattrel et al [15], while the rate constant for the negative electrode reaction was derived through a fitting procedure as in [10,13]; the value was adjusted until the desired fit to the experimental charge-discharge curve in the base case (described below) The model was solved using the COMSOL Multiphysics ® package, which is based on the finite-element method. A quadratic basis was used in all of the simulations, together with a minimum of 2548 elements and a maximum of 7256 elements, based on grid dependency checks that compared the times to SOC for successively more refined meshes.…”

Modeling and Quantification of Electrochemical Reactions in RDE (Rotating Disk Electrode) and IRDE (Inverted Rotating Disk Electrode) Based Reactors

“…In the linear region of this curve, the transfer coefficient can be deduced from the slope and the rate constants from the intercept. The Tafel method is well established for simple reaction mechanisms (Bamford & Compton, 1986;Diard et al, 1996), but it becomes much more complicated for complex mechanisms (Gattrell et al, 2004;Wang et al, 2004). When there are significant diffusional or ohmic effects in the electrolyte, or additional electrode reactions, the Tafel plot deviates from linearity (Yeum & Devereux, 1989) and the charge transfer parameters cannot be determined.…”

Characterization of the reaction environment in a filter-press redox flow reactor

“…The redox flow batteries consist of two electrolytes recycled through bipolar cells from large reservoirs; chemical and electrical energy can be interchanged by reversible electrochemical reactions that take place on the electrodes when the electrolytes are charged or discharged [30][31][32][33][34]. The electrolytes are charged when electrical current is applied between the two electrodes and they are pumped through the reactor, if the electrical energy is needed the reactions are reversed and the energy can be withdrawn from the cell.…”