Electrodeposition on a porous electrode with low initial conductivity: Effect of the oxidant on the dynamics of the cathode deposit mass

The dynamics of filling the non-equipotential porous matrix with metal mi(t) during its deposition from a solution with an oxidant was determined with the help of numerical calculations according to the previously developed dynamic model of a porous electrode (PE) and investigated experimentally. It was established that a characteristic feature of the dependence mi(t) in this case is the occurrence of an initial induction period tind with the low metal deposition rate. The reasons of tind occurrence and the effect of various factors on its duration were investigated. It was shown that two different versions are possible for the further development of the deposition process, depending on conditions: (1) At the low initial, or rapidly decreasing with time, concentration of the oxidant, PE gradually becomes equipotential; this is followed by preferred metal deposition near the front end of the PE. The final distribution of the deposit is monotonous, and its mass is larger than that for the equipotential porous matrix. (2) At the high concentration of the oxidant, the PE does not reach the equipotential state during the whole deposition time. Quite contrary, the anode zone appears in its middle part as time goes on; complete dissolution of the deposited metal occurs. As a result, the cathode deposit appears as two separate fragments, while its final mass decreases substantially. The experimental data on the dynamics of the deposition of copper from the sulphate solution and silver from thiosulphate one in the presence of an oxidant on VINN-250 carbon felt are in good agreement with the first version; in both cases these data exhibit the initial induction period followed by a monotonous increase in the mass of the cathode deposit.