Voltammetric and Impedance Investigation of Vanadium Oxidation States in Sulfuric Acid
Data obtained from impedance measurements enjoy lot of attention in electrochemical studies, because they provide wealth of information relevant to static, as well as dynamic, parameters of the system. The nature of proper impedance data collection (and successful interpretation) requires that the data be collected while the system under observation is stable, at equilibrium, although the equilibrium can be static, as well as dynamic. With this requirement in mind, and also due to some complexity, most impedance analyzers (or their software) will not perform other task while impedance data are taken. However, with fast instrumentation, slow change in bias polarization, deemed sufficiently benign to operate at a (pseudo)equilibrium of the system, should allow impedance data collection as a function of the bias potential.
We present here data taken with Solartron ModuLab setup, which we programmed to perform extremely slow potential scans, and to collect simultaneously impedance data. A control study was performed on potassium hexacyanoferrate (II)in potassium nitrate solution; a classical redox couple. The electro-chemical experiment performed was cyclic voltammetry on a rotated disk electrode and voltammetry of vanadium(IV) in sulfuric acid. This is a system relevant to the redox flow battery energy storage system. A rather slow voltammetric scan (1 mV/s), coupled with rapid data collection using the multi-sine impedance function, was successfully used to collect impedance data without distortion caused by the changing voltage bias during the voltammetric scan.
While the instrument facilitates collection of the data series as described, it does not have a built in data processing that would allow sequential evaluation of all the impedance data series collected. We have developed such an evaluation routine and we are presenting, for the first time, detailed impedance data results running in parallel with the data for voltammetric measurements.
Impedance evaluation usually relies on fitting the data to a suitable equivalent circuit. Attached file is an example shoving variation of one model resistor in the circuit in response to the applied potential.