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.

Electrode polarization analysis is frequently used to determine free-ion diffusivity and number density in ionic conductors. In the present study, this approach is critically examined in a wide variety of electrolytes, including aqueous and nonaqueous solutions, polymer electrolytes, and ionic liquids. It is shown that the electrode polarization analysis based on the Macdonald-Trukhan model [J. Chem. Phys. 124, 144903 (2006); J. Non-Cryst. Solids 357, 3064 (2011)] progressively fails to give reasonable values of free-ion diffusivity and number density with increasing salt concentration. This should be expected because the original model of electrode polarization is designed for dilute electrolytes. An empirical correction method which yields ion diffusivities in reasonable agreement with pulsed-field gradient nuclear magnetic resonance measurements is proposed. However, the analysis of free-ion diffusivity and number density from electrode polarization should still be exercised with great caution because there is no solid theoretical justification for the proposed corrections.

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.

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.