The sulfuric acid, vanadyl (VO 2+) and water equilibrium in Nafion membranes contacted by solutions containing these species is described. Of particular interest is the influence of composition on ionic transport behavior in membrane separators for an all-vanadium redox flow battery (VRFB). Ex-situ membrane conductivity measurements were conducted on Nafion 117 membranes equilibrated in electrolyte solutions of varying sulfuric acid and vanadyl ion concentrations. Electrolyte species imbibed in the membrane were analyzed by an experimental protocol including titration, ICP-OES and weight analysis. Sulfuric acid in the membrane can increase proton concentration but reduce proton mobility by reducing water content. In a mixed vanadyl/proton form Nafion, vanadyl has a mobility of 6.28 × 10 −5 cm 2 • V −1 • s −1 , much lower than proton mobility of 8.79 × 10 −4 cm 2 • V −1 • s −1 in H +-form Nafion. The presence of vanadyl in Nafion can also decrease the proton mobility: u H + = (8.79 − 8.04 × x VO 2+) × 10 −4 cm 2 V −1 s −1. With equilibration in a practical electrolyte containing 5 mol • dm −3 total sulfate, Nafion's conductivity is decreased due to uptake of vanadyl ions.

In situ X-ray absorption spectroscopy (XAS), using both EXAFS and the Δμ-XANES analysis procedures, is utilized to examine Ru deposition onto Pt/C cathodes at millimolar concentration of Ru in a 1 M HClO4 electrolyte. Also, electron spin resonance (ESR) spectroscopy is utilized to examine the effects of Ru3+ ion exchanged Nafion membranes. The Δμ-XANES analysis of the data allows a determination of the coverage of Ru with time (minutes to hours) and also to identify the binding site of the deposited Ru species (atop/bridged at low coverage and 3-fold at higher coverage) apparently onto the corners and edges initially and at higher coverage onto the faces of the cubooctahedral clusters when exposed to Ru n+ at OCP. The deposition of Ru appears to be inhibited at potentials where adsorbates (such as H and O(H)) usually adsorb, and Coulomb enhanced at OCP when substantial O exists on the surface. The ESR analysis of Ru3+ in the Nafion membrane indicates significant detrimental changes to the membrane in the presence of Ru ions; a decrease in the water uptake and an increase in the microviscosity of the fluid regions were noticed. Together these data indicate the critical nature of keeping the fuel cell under potential control and avoiding an uncontrolled shut down under direct methanol operating conditions.

In this contribution, we provide a synthesis of results to date describing uptake and mass transport of water, vanadium species and protons in Nafion membranes for use as separators in VRFBs. Resistance issues as well as species cross-over are important contributors to performance loss in VRFBs. After a brief discussion of our state-of-theart cell performance, we consider the uptake and transport of various species through a number of membrane materials. We draw together numerous previous studies and augment them with new data to provide a summary of our present state of understanding of the experimental facts regarding membrane behavior.

The VO2+ crossover, or permeability, through Nafion in a vanadium redox flow battery (VRFB) was monitored as a function of sulfuric acid concentration and VO2+ concentration. A vanadium rich solution was flowed on one side of the membrane through a flow field while symmetrically on the other side a blank or vanadium deficit solution was flowed. The blank solution was flowed through an electron paramagnetic resonance (EPR) cavity and the VO2+ concentration was determined from the intensity of the EPR signal. Concentration values were fit using a solution of Fick's law that allows for the effect of concentration change on the vanadium rich side. The fits resulted in permeability values of VO2+ ions across the membrane. Viscosity measurements of many VO2+ and H2SO4 solutions were made at 30–60°C. These viscosity values were then used to determine the effect of the viscosity of the flowing solution on the permeability of the ion.