Abstract
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.
