(Invited) Electrodeposition of Manganese Oxides Utilizing Hydrothermal Electrochemical Technique

Abstract

Hydrothermal electrochemistry can conduct electrochemical experiments under high temperature and pressure conditions and is one of the useful methods to fabricate size-controllable and highly crystalline materials. Although this technique has been applied to electrodeposition of metal oxides like BaTiO3 (Yoshimura et al., Jpn. J. Appl. Phys., 28, L2007, 1989), precise control and examination of electrochemical process was hard to be achieved in previous reactors because they adopted two-electrode systems. Moreover, reaction pressure was determined by temperature, so it is difficult to discuss their specific contribution to the electrochemical process.Recently, we have developed hydrothermal electrochemical reactor in which three-electrode system was adopted in order to control and monitor the electrochemical process precisely. This system also achieves independent control of temperature and pressure so that we can discuss their contributions to the final products. Here in this work, we attempted to fabricate Manganese oxide thin film with electrodeposition technique under different temperature and pressure, and examined their electrochemical water oxidation activity. Manganese oxides are prepared in industry with electrodeposition technique, so they are good candidates as model compounds to examine the property of our home-made hydrothermal electrochemical reactor.MnO2 was deposited on Ti substrate with galvanostatic condition of 10 mA /cm2 for 20 min utilizing 0.5 M MnSO4 / 0.2 M H2SO4 aqueous solution as a precursor. Based on XRD analysis, all deposited samples were ε-MnO2. When we increase deposition temperature, XRD peaks got sharpen, indicating the crystallinity increased. SEM observation clarified their morphological difference. Namely, MnO2 deposited at 160℃ possessed needle-like structure, while spherical structure was observed for sample deposited at 100℃. To examine the effect of pressure on deposition products, we conducted electrodeposition under different pressure from 0.1 to 4 MPa while maintaining temperature at 100℃. As a result, XRD peaks were slightly shifted depending on pressure. This means lattice constant is controllable by regulating deposition pressure.Their electrochemical water oxidation activity also demonstrated the advantage of hydrothermal electrodeposition compared with conventional technique. After annealing at 200℃, 160℃-electrodeposited sample exhibited superior activity to 100℃-deposited sample. Although the elucidation of the reason for higher catalytic activity is future work, those results indicate our hydrothermal electrochemical technique is widely applicable to material synthesis such as metal oxides, metal sulfides, and so on. Figure 1