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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

We report results of polarization measurements resolved for the negative and positive electrodes of vanadium redox batteries (VRBs) using a dynamic hydrogen electrode in an operating battery cell. Electrochemical experiments with symmetric electrolyte feeds were also performed. Greater kinetic polarization is observed at the negative (V 3/2+) electrode compared to the positive electrode (V 5/4+), in contrast with previously reported ex situ measurements. For the positive electrode, the polarization in the low-current regime was modest and was not kinetically controlled. The relative rates of reaction are a surprise since it might be expected that the V 3/2+ redox reaction is a simple outer-sphere electron transfer.

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We present an in situ electrochemical technique for the quantitative measurement and resolution of the ohmic, charge transfer and diffusion overvoltages at the negative electrode of an all-vanadium redox flow battery (VRFB) using electrochemical impedance spectroscopy (EIS). The mathematics describing the complex impedance of the V +2 /V +3 redox reaction is derived and matches the experimental data. The voltage losses contributed by each process have been resolved and quantified at various flow rates and electrode thicknesses as a function of current density during anodic and cathodic polarization. The diffusion overvoltage was affected strongly by flow rate while the charge transfer and ohmic losses were invariant. On the other hand, adopting a thicker electrode significantly changed both the charge transfer and diffusion losses due to increased surface area. Furthermore, the Tafel plot obtained from the impedance resolved charge transfer overvoltage yielded the geometric exchange current density, anodic and cathodic Tafel slopes (135 ± 5 and 121 ± 5 mV/decade respectively) and corresponding transfer coefficients α = 0.45 ± 0.02 and β = 0.50 ± 0.02 in an operating cell.