Composite electrodes for oxygen evolution in metal electrowinning

Oxygen evolution is the most common anode reaction in the electrowinning (EW) of metals from acidic sulfate based electrolytes and is a reaction that requires high activation overpotentials. Since the the oxygen evolution reaction contributes roughly 500-800 mV to the cell voltage which is roughly 15-25% of the total cell voltage, there has been increasing interest to replace the traditionally used lead anode by alternative anodes employing better electrocatalysts that show lower oxygen evolution overpotentials.In this work an alternative anode concept is explored, the composite anode consisting of ex situ prepared MnO2 and lead metal as the composite matrix material, where a special focus has been to investigate the role of the triple phase boundary and microscopic processes that would explain why this material combination has been showing relatively low oxygen evolution overpotential (up to 250 mV lower than the traditional lead anode). After initial characterisation of different types of MnO2 as electrocatalysts for the oxygen evolution reaction (OER), it was noticed that the oxygen evolution mechanism was mass transfer dependent and that the current density measured at contant electrode potential was inversely proportionally dependent on the particle size of the MnO2 catalyst material (1/r), indicating edge effects on a microscopic level. This lead to the development of a stochastic model describing the total triple phase boundary length (Pb, MnO2 and electrolyte) proportional to 1/r. It was followed by a characterisation of the microscopic process using scanning electrochemical microscopy (SECM) and conductive atomic force microscopy (CAFM) showing that the triple phase boundary was characterised by special electrical properties and that hydrogen peroxide was generated as an intermediate. Different microscopic processes were simulated and it was shown that the conductivity of MnO2 and a newly postulated 2-step 2-material mechanism could serve as an explanation of the observed experimental results. Since the involvement of H2O2 as an intermediate in the OER was not well supported, it was attempted to measure H2O2 reactions on lead electrode, which was not successful. As a consequence a methodology involving potential step transients on a rotating disc electrode was developed and tested for one electron transfer reactions. It was furthermore shown how this method could be used to measure rate constants relating to H2O2 reactions on Pb and MnO2.