Effect of Mn precursors on the morphology and electrocatalytic activity toward water oxidation of micro-nanostructured MnO_x films prepared by voltammetric deposition

MnO_x is a promising nonprecious metal-based water oxidation catalyst that is low in cost, abundant, and has low toxicity. The electrocatalytic properties of micro/nanostructured materials depend partially on the material’s morphology type. Here, we describe the morphology-dependent electrocatalytic activities toward water electrooxidation, i.e., the oxygen evolution reaction, of micro-nanostructured MnO_x films prepared by voltammetric deposition. Films were prepared under fixed deposition conditions using one of three water soluble Mn salt precursors: manganese nitrate (Mn(NO₃)), manganese acetate (Mn(CH₃COO)₂), and manganese chloride (MnCl₂). Each of the as-prepared MnO_x films was heated at 400 °C to tune the morphology, oxidation state of Mn and water electrooxidation efficiency. The as-prepared films lost significant amounts of their activity over several voltammetric water oxidation cycles. Heat treatment of the as-prepared films significantly improved the signal and stability of the water electrooxidation reaction. The water oxidation signal at 1.5 V, obtained using 400 °C-heated micro-nanostructured MnO_x films prepared from each of the three precursors, followed the order Mn(NO₃)₂ > Mn(CH₃COO)₂ > MnCl₂. The reason of these different activities was due to their different morphologies as the same phase, ɑ-Mn₂O₃, was found in the entire 400 °C-treated MnO_x films. The nanostructured MnO_x film prepared from Mn(NO₃)₂ was heated at different temperatures, and a temperature of 300 °C was found to be optimal for water electrooxidation efficiency. The high resolution transmission electron microscopic image and X-ray photoelectron spectra revealed that the optimal MnO_x was α-Mn₃O₄ phase, which might be a factor along with its special morphology for improving water oxidation reaction.