In this work EQCM was used to provide additional information about the manganese oxide deposition and dissolution from a sulfuric acid solution. The electrode materials studied were platinum and gold. The electrolyte consisted of 0.1 M H2SO4 with 5 mM Mn2+-ions and the experiments were conducted at room temperature. Cyclic voltammograms were recorded for a number of different upper reversal potentials and sweep rates. In addition, sweep-hold and step-hold experiments were conducted. In these experiments the potential was swept or stepped to various potentials at which manganese oxide deposition occurred and held for a given time. These sequences were followed by a negative-going sweep from the hold potential. To complement the EQCM experiments, manganese oxide was deposited on a platinum disc at two different potentials (1.45 V and 1.55 V) and studied by scanning electron microscopy (SEM) as a function of deposition time.
The cyclic voltammograms showed an oxidation peak commencing at about 1.42 V and peaking around 1.46 V in the positive-going scan (Fig 1), as observed elsewhere [1]. The oxidation peak coincided with an increase in mass. Clear indications of a nucleation, growth and collision mechanism were observed when the upper reversal potential was chosen from the rising part of the oxidation peak, i.e. below 1.46 V. This peak occurs below the onset of oxygen evolution and is thus related to the onset and continuous formation of manganese oxide. The shape and size of the oxidation peak were similar on gold and platinum. Subsequently, a cathodic peak (reduction of manganese oxide) was observed in the negative-going scan at potentials just positive of 1.3 V, which also appeared similarly on both electrode materials. Interestingly, a second reduction peak at around 1.2 V was observed at both electrodes, but only when the upper limit was sufficiently positive (above 1.5 V). Voltammograms recorded at different sweep rates showed no clear trend in the peak current for the oxidation peak. However, the mass deposited did show a very clear dependence and was inversely proportional to the sweep rate. The SEM images showed that the deposit formed at the higher potential had more dendrites than the deposit formed at the lower potential although almost the same amount of charge was passed in the deposition experiments.
The change from one to two reduction peaks when reversing the sweep at potentials above 1.55 V indicates a change in the reduction and deposition mechanism. The lack of sweep-rate dependence of the oxidation peak shows that it is at least not entirely governed by bulk diffusion of Mn2+. To be able to implement DSAs in zinc electrowinning it is important to understand the mechanism of manganese oxide deposition and dissolution.
- W.H. Kao and V.J. Weibel, Electrochemical oxidation of manganese(II) at a platinum electrode, J. Appl. Electrochem., 22 (1992) 21-27.