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We outline the analysis of performance of redox flow batteries (RFBs) using polarization curves. This method allows the researcher immediate access to sources of performance losses in flow batteries operating at steady state. We provide guidance on 'best practices' for use of this tool, illustrated using examples from single cells operating as vanadium redox batteries.

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.

We report on single-electrode electrochemical impedance spectroscopy studies of an all-vanadium redox battery using a dynamic hydrogen reference electrode. The negative electrode, comprising the V 2+ /V 3+ couple, contributes approximately 80% of the total cell overpotential during discharge. The impedance spectra measured at the negative electrode exhibit high-frequency, semicircular arcs which correspond to the double layer capacitance in parallel with a faradaic resistance. The faradaic resistance decreases in magnitude with increasing polarization. Integration of the current-dependent faradaic resistance quantifies the fraction of the overvoltage that is attributed to the kinetic limitations of the charge transfer reaction.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 160.36.178.25 Downloaded on 2015-01-08 to IP