Oxide layer formation behavior on a Pb-Ag@MnO2 composite anode in Mn2+-containing H2SO4 solution

Abstract

During zinc electrowinning, the oxygen evolution activity and oxide layer stability of lead-based anodes are critically influenced by the presence of Mn2+ ions in the electrolyte and their subsequent oxidative deposition behavior. In this work, three types of composite anodes (Pb-Ag, Pb-MnO2, and Pb-Ag@MnO2) were fabricated via powder metallurgy. Their electrochemical performance and surface evolution were systematically compared with that of a conventional industrial PbAg alloy anode in a simulated zinc electrowinning electrolyte containing 160 g l–1 H2SO4 and 4 g l–1 Mn2+. The investigation focused on the temporal evolution of anodic potential, the phase composition and microstructure of the oxide layer, as well as the variation of Mn3+ concentration in the electrolyte. Particular emphasis was placed on elucidating the mechanistic role of Ag@MnO2 active particles in modulating the Mn2+ oxidation pathway and the subsequent formation of the oxide layer. The results indicate that during the initial stage of electrowinning, the electrolytes for both the PbAg alloy and the Pb-Ag composite anode exhibit relatively high Mn3+ concentrations, reaching 66.00 and 71.50 mg l–1 at 12 h, respectively. These values subsequently decrease as loosely packed particulate MnO2 deposits rapidly on the anode surface. Under these conditions, the deposited MnO2 is primarily generated via the disproportionation of the Mn3+ intermediate. In contrast, for the Pb-MnO2 and Pb-Ag@MnO2 composite anodes, no Mn3+ is detectable in the electrolyte throughout the entire polarization period, indicating that MnO2 formation proceeds predominantly through a direct electrochemical oxidation pathway. Notably, on the Pb-Ag@MnO2 composite anode, a smooth and compact MnO2 underlayer preferentially forms, followed by the slow deposition of particulate MnO2 on top of this dense inner layer. Even after 72 h of polarization, this inner layer remains continuously compact and exhibits robust interfacial adhesion to the substrate. The average anodic potential of the Pb-Ag@MnO2 composite anode is comparable to that of the industrial PbAg alloy and is significantly lower than that of the Pb-MnO2 composite anode. Overall, the Ag@MnO2 particles effectively retard the rapid accumulation of loose MnO2 deposits by suppressing the formation of the Mn3+ intermediate while simultaneously promoting the direct electrochemical oxidation of Mn2+.