Fast Fourier Transform Current Pulse method for dynamic measurements of cell ohmic resistance during electrolysis

In general, resistance measurements under non-steady-state conditions (e.g. during gas evolution) are difficult to obtain continuously. We have developed a method, based on a current pulse technique, that provides dynamic information on the cell ohmic resistance during electrolysis reactions. The technique is particularly useful for systems where there is significant gas evolution at one or both electrodes, e.g., electrowinning of metals in molten salt electrolytes. The Fast Fourier Transform Current Pulse (FFTCP) method, applies a bi-polar square current pulse (typically I = 1 A, Δt = 210 μs) and measures the resultant voltage pulse to calculate ohmic resistance (R = V/I). For systems where the interfacial capacitance is relatively large (in the order of mF), the resistance can be calculated directly using Ohm's Law. If the interfacial capacitance is small (of the order of μF), a Fast Fourier Transform (FFT) algorithm is used to treat the current pulse measurements and produce a dynamic electrochemical impedance spectrum. From this spectrum, an equivalent series resistance (ESR) can be obtained. The FFTCP method is validated using a Randles test circuit. It is also used to measure the ESR change due to gas bubble evolution in an aqueous 1 M KOH solution, using the real impedance value at the cross-over point with the real impedance axis (i.e. when Z″ = 0 Ω, at high frequency). The FFTCP technique allows continuous measurements of resistance in operating electrolysis cells.