Current pulse method for in situ measurement of electrochemical capacitance

More than 20 years ago, CSIRO developed an instrument that measures uncompensated resistance simultaneously with constant voltage, constant current or cyclic voltammetric measurements. The instrument, known as a ‘resistometer’ applies a square current pulse (typically I = 1 A, Δt = 70 μs) and measures the resultant voltage pulse to calculate uncompensated resistance (R = V/I). In the present work, modification of this instrument has allowed in situ measurement of capacitance as well as resistance by measurement of the voltage change from before application of the pulse to during the rest period of the pulse (C = I · Δt/ΔV). Testing with a dummy cell (a Randles Equivalent circuit) with capacitances from 10 μF to 1 mF confirms this relationship and measures the capacitance to within 10% error. Aqueous cyclic voltammetric measurements on metal disk working electrodes at the millimetre scale validate the in situ electrochemical measurement. Larger electrodes, however, with capacitances in the mF range show a lower frequency-dependent value of capacitance when compared to measurements derived from AC impedance techniques. Measurements in molten salts show increasing capacitance with increasing depth of a copper rod immersed in a CaCl2 bath (950 °C) as expected.