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.

Sulfonated Diels-Alder poly(phenylene) (SDAPP) membranes were synthesized and characterized as potential electrolyte separators for vanadium redox flow batteries. The SDAPP membranes studied had ion exchange capacities of 1.4, 1.8 and 2.3 meq/g. Transmission electron microscopy imaging shows that the ionic domains in SDAPP are roughly 0.5 nm in dimension, while Nafion has a hydrophilic phase width of around 5 nm. The sulfuric acid uptake by SDAPP was higher than that for Nafion, but the materials had similar water uptake from solutions of various sulfuric acid concentrations. In equilibration with sulfuric acid concentrations ranging from 0-17.4 mol · kg −1 , SDAPP with a IEC of 2.3 meq/g had the highest conductivity, ranging from 0.21 to 0.05 S · cm −1 , while SDAPP with a IEC of 1.8 had conductivity close to Nafion 117, ranging from 0.11 to 0.02 S · cm −1 . With varying sulfuric acid concentration and temperature, vanadium permeability in SDAPP is positively correlated to the membrane's IEC. The vanadium permeability of SDAPP 2.3 is similar to that of Nafion, but permeability values for SDAPP 1.8 and SDAPP 1.4 are substantially lower. The vanadium permeation decreases with increasing electrolyte sulfuric acid concentration. Vanadium diffusion activation energy is about 20 kJ · mol −1 in both SDAPP and Nafion.The vanadium redox flow battery (VRFB) has shown technical potential for large scale electrical energy storage. 1-3 One possible role of VRFBs is their integration with the electrical grid to "level off" supply and demand mismatches and to improve overall reliability and efficiency of the grid. 1 Another scenario for VRFB use is buffering stochastic energy sources, such as solar or wind, which will improve the stability of electricity output from these renewable resources. 1,4 A VRFB is essentially a regenerative fuel cell, with a flowing operation pattern similar to proton exchange membrane fuel cells. 5,6 In a VRFB the energy is carried by vanadium redox couples V 5+ /V 4+ and V 2+ /V 3+ in electrolyte solutions. The energy is interconverted between electrical and electrochemical forms by the battery cell or stack, where the functional core is the membrane electrode assembly. The function of the membrane is to separate positive and negative electrolyte solutions and conduct ionic current, while vanadium redox reactions take place on the electrode surface in each half-cell. During battery operation, electrolyte solutions are constantly fed through the battery to support electrochemical reactions and to generate steady current output or recharge the electrolyte solution.In the VRFB cell, the electrolyte separator is a primary limiting factor in the battery's performance. 3 As has been demonstrated, resistance from the separator is the most important source of the battery's internal resistance, impeding battery performance during high current density operation. 3,7 Ideally, the electrolyte separator should have high conductivity to minimize battery efficiency losses caused by internal resistance and also hig...

Vanadium diffusion through Nafion membranes was investigated under conditions applicable to vanadium redox flow batteries (VRFBs). In this case, we examine the effects of interdiffusion of VO2+ with protons, V3+ and VO2+ by a ‘cross-diffusion’ experiment in which VO2+ and the other ion diffuse from opposite sides of the membrane. Vanadium concentrations in reservoirs designed to flow solution over both sides of the membrane were monitored using UV/vis and EPR spectroscopies. The permeability of each ion was determined and diffusion coefficients determined from permeability and measurements of ion uptake by the membrane. The VO2+ permeability is highest when H+ is the only ion on the counter side, and decreases in the following order: H+ > VO2+ > V3+.

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