Electrochemical fabrication of nanoporous gold decorated with manganese oxide nanowires from eutectic urea/choline chloride ionic liquid. Part II – Electrodeposition of Au–Mn: A study based on soft X-ray microspectroscopy

Abstract

Abstract In a previous paper (C. Mele, M. Catalano, A. Taurino, B. Bozzini. Electrochim. Acta 1 (2013) 918), we demonstrated the possibility of growing high-capacitance hybrid materials consisting of nanoporous gold (NPG)-supported MnO 2 nanowires (NW) for supercapacitors, by electrochemical etching of electrodeposited single-phase Au–Mn alloys. The present paper concentrates on the electrodeposition of Au–Mn alloys from urea/choline-chloride ionic liquid solutions: the precursors of the high-capacitance hybrid material. The electrodeposition process, giving rise to alloys with 4–26% Mn content, was followed by space-resolved soft X-ray fluorescence (XRF) and absorption (XAS) microspectroscopy, complemented with electrochemical (cyclic voltammetry), structural (X-ray diffraction) and morphological (scanning electron microscopy) characterisations. The purposely developed electrochemical cells, exhibiting a specifically designed current density distribution, have allowed the quasi-in situ mapping of the local morphology-composition changes at the electrodes. Supersaturated Au–Mn solid solutions were obtained in the whole investigated compositional range under mass transport control of Mn. Variations in the Mn oxidation state were evidenced comparing low- and high-Mn content regions. It was found that, notwithstanding the heterogeneity of the current density, the morphologically compact high-Mn regions of the particular alloys with 20–26% Mn show a notable compositional homogeneity, rendering this material ideally suited for the fabrication of the target hybrid.