Effect of gluconate ions on electroreduction phenomena during manganese deposition on glassy carbon in acidic chloride and sulfate solutions

Abstract

Cathodic processes occurring during manganese deposition from acidic baths (pH 3.5) were investigated using glassy carbon rotating disk electrode. Cyclic voltammetry, linear sweep voltammetry and anodic sweep voltammetry were applied. Speciation and buffer capacities of chloride, chloride-gluconate, sulfate and sulfate-gluconate solutions were determined. Two ranges of the potentials (vs. Ag/AgCl) were considered: from −0.8V to −1.56V and below (up to −1.8V) corresponding to hydrogen evolution and manganese deposition, respectively. In the chloride bath, strong adsorption of MnCl+ on the cathode inhibited hydrogen evolution, favored fast metal nucleation and deposition of fine grained coatings, but low buffering action of the bath enhanced self-dissolution of manganese. Addition of gluconate improved buffer capacity of the bath, inhibited metal nucleation and hindered somewhat secondary metal dissolution, but the presence of chloride ions and low ammonium ions concentration resulted in the formation of hydroxide species as a secondary product of metal oxidation. Sulfate bath prevented manganese dissolution, but buffer effect of the solution was too small to stabilize pH at the cathode surface. The best buffering action of the sulfate-gluconate system originated from the cooperative action of gluconate and ammonium species made possible to obtain metallic layer protected from the dissolution by the electrolyte. Weak adsorption of MnSO4 on the cathode in sulfate baths lead to the hydrogen evolution under limiting current in the potential range preceding manganese deposition. Independently on the bath composition, manganese started to deposit at approx. −1.56±0.01V (vs. Ag/AgCl) according to the instantaneous model of 3D nucleation. Average diffusion coefficients of the manganese species of order 10−6cm2/s were found.