The Electrochemistry of Ammonium Dihydrogen Phosphate, Disodium Phosphate, Ammonium Chloride on Mg-based and Polycrystalline Pt Electrodes

In this study, the electrochemical mechanisms of struvite formation were investigated by studying the behaviour of Mg-based and (poly)crystalline-Pt electrodes in the presence of ammonium dihydrogen phosphate, disodium phosphate, and ammonium chloride solutions. It was found that a precipitate was formed on the Mg-based electrodes, while the poly-Pt electrode showed typical voltammetric features in aqueous electrolytes. The structure, morphology, and composition of the electrochemically obtained precipitates collected from the surface of the Mg-based electrodes were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results showed that the precipitate morphology was impacted by the composition of the electrolytes and the purity of the magnesium. Moreover, the Tafel kinetics and corrosion rates (vcorr) directly correlated with the electrolyte and the electrode material compositions, whereby both showed a general increase when the Mg-based electrodes were employed in the presence of chloride anions. The Tafel slopes (β) obtained for the oxygen reduction reaction (ORR) with the poly-Pt electrode showed a significant difference between the forward and reverse potential scans in the various electrolytes used, a finding which was mainly attributed to the adsorption of hydroxide species, alkali cations, and phosphate anions to the surface whereby these species blocked the ORR active sites.