Vanadium Electrolyte for All-Vanadium Redox-Flow Batteries: The Effect of the Counter Ion

Abstract

In this study, 1.6 M vanadium electrolytes in the oxidation forms V(III) and V(V) were prepared from V(IV) in sulfuric (4.7 M total sulphate), V(IV) in hydrochloric (6.1 M total chloride) acids, as well as from 1:1 mol mixture of V(III) and V(IV) (denoted as V3.5+) in hydrochloric (7.6 M total chloride) acid. These electrolyte solutions were investigated in terms of performance in vanadium redox flow battery (VRFB). The half-wave potentials of the V(III)/V(II) and V(V)/V(IV) couples, determined by cyclic voltammetry, and the electronic spectra of V(III) and V(IV) electrolyte samples, are discussed to reveal the effect of electrolyte matrix on charge-discharge behavior of a 40 cm2 cell operated with 1.6 M V3.5+ electrolytes in sulfuric and hydrochloric acids. Provided that the total vanadium concentration and the conductivity of electrolytes are comparable for both acids, respective energy efficiencies of 77% and 72–75% were attained at a current density of 50 mA∙cm−2. All electrolytes in the oxidation state V(V) were examined for chemical stability at room temperature and +45 °C by titrimetric determination of the molar ratio V(V):V(IV) and total vanadium concentration.

The Influence of Free Acid in Vanadium Redox‐Flow Battery Electrolyte on “Power Drop” Effect and Thermally Induced Degradation

“…The setup was equipped by additional pair of reference and glassy carbon electrodes, which were positioned in electrolyte tanks to monitor the cathodic, anodic redox, and half‐cell potentials (for details of cell design, see ref. [24]). The charging was conducted in galvanostatic–potentiostatic mode: the end‐of‐charge voltage was set to 1.65 V for galvanostatic step at 75 mA cm −2 and then the cutoff current was 2.5 mA cm −2 at applied voltage of 1.65 V. For discharge, the end‐of‐discharge voltage was set from 0.8 to −0.2 V depending on the electrolyte sample, i.e., appearance of “power drop” effect.…”

Redox Flow Batteries: A Literature Review Oriented to Automatic Control

“…For this reason, the extreme values of the OCV are considered experimentally. Many studies agree that the maximum value of the cell voltage during the charge is between 1.6 and 1.7 V, and drops to 1.1 V in the discharge case [82,83].…”

Redox Flow Batteries: Materials, Design and Prospects

“…The electrochemical activity and the concentration and stability of vanadium ions determine the energy density and the reliability of the VRFB. Therefore, these factors contribute to electrolyte technology improvement and are still under optimization towards a more reliable and cost-effective system [65][66][67][68]. The development of new electrode components for VRFB systems will certainly increase in the short-medium term for many industrial and residential applications.…”