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

In a previous paper, we demonstrated the possibility of growing high-capacitance hybrid materials consisting of nanoporous gold (NPG)-supported MnO2 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.