Equilibrium and Dynamic Absorption of Electrolyte Species in Cation/Anion Exchange Membranes of Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow batteries (VRFBs) rely on ion exchange membranes (IEMs) to separate the positive and negative compartments while maintaining electrical neutrality of the cell, by allowing the transport of ionic charge carriers. Cation exchange membranes (CEMs) and anion exchange membranes (AEMs), the two principal types of IEM, have both been employed in VRFBs. The performance of these IEMs can be influenced by the absorption of species from the electrolyte. In this study, a typical commercial CEM (Nafion 117) and AEM (FAP 450), were examined with respect to vanadium uptake, after exposure to electrolyte at different states of charge. The two types of membrane were found to behave very differently, with the AEM showing very high selectivity for VV, which resulted in a significant increase in area‐specific resistivity. In contrast, the CEM absorbed VII more strongly than vanadium in other oxidation states. These findings are essential for the development of an effective membrane for VRFB applications.

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

“…Another parameter to choose the optimal molar ratio of VOSO 4 to sulfuric acid is the conductivity of the resulting V(IV) solutions, which is primarily provided by H 2 SO 4 and should be high to minimize the ohmic polarization. [11] Generally, the conductivity depends on the total concentrations of vanadium, sulfate/bisulfate, and free protons at each given SoC. As shown in Figure 1, the maximal conductivity of V(IV) electrolyte solutions is attained at H 2 SO 4 concentrations higher than 4 M. This corresponds to the vanadium concentrations lower than 1 M. As the vanadium concentration of VRFB commercial electrolytes is commonly in the range from 1.6 to 2 M, the solution of 1.6 M VOSO 4 in 3 M H 2 SO 4 (total sulfate concentration of 4.5 M) was chosen to compose the V(IV)-n series.…”

“…One of a reason for it may be the loss of vanadium species as vanadium pentoxide precipitates on the anion exchange membrane, which is supposed to happen during the electrolysis to high SoC. [3,11] During the electrolysis of V(IV) solutions, sulfate anions are expected to move through membrane from anolyte to catholyte according to the commonly suggested mass balance equations (Equation (4)-(5)). [19] ðanolyteÞ…”

“…[3] The accumulation of insoluble V(V) species from catholyte was previously confirmed for cell with anion exchange separator operated at SoC from 10% to 80%. [11] However, anion exchange membranes are often used for VRFB cell construction due to lower permeability of V(II), V(III), and V(IV) components of electrolyte than cation exchanger. [11] To verify, if the membrane in the cell becomes blocked by V(V) species, the cell resistance was measured by electrochemical impedance before the charging step and directly before the discharge, i.e., at the high SoC of the cell (Figure 6).…”

“…[11] However, anion exchange membranes are often used for VRFB cell construction due to lower permeability of V(II), V(III), and V(IV) components of electrolyte than cation exchanger. [11] To verify, if the membrane in the cell becomes blocked by V(V) species, the cell resistance was measured by electrochemical impedance before the charging step and directly before the discharge, i.e., at the high SoC of the cell (Figure 6). Indeed, apart from the membrane resistance other components such as electrolyte conductivity, resistance of electrodes, and contact resistance contribute to the common cell resistance.…”

Reversible Electrochemical Ion Intercalation at an Electrified Liquid|liquid Interface Functionalised with Porphyrin Nanostructures

“…5 Ion exchange membranes (IEMs) are used in combination with electrochemical potential gradients to drive intercalation and de-intercalation, typically through an ion exchange mechanism, for the selective transport of certain ionic components between phases with different chemical compositions. 6,7 Such IEMs are critical to the operation of fuel cells, 8 electrolysers, 9 redox flow batteries, 10 reverse electrodialysers, 11 and microbial fuel cells. 12 Electrochemical methods are particularly well suited to provide key information on the thermodynamics and kinetics of these ion transfer processes.…”