Composition, Structure and OER Behaviors of Highly Catalytic Manganese-Doped Bismuth Ruthenium Oxide (MBRO) for Alkaline Water Splitting

Water splitting using alkaline solutions is one of the most promising methods to produce green hydrogen and accounts for 61% of the world's hydrogen production capacity in 2020¹⁾. The anodic reaction, which is oxygen evolution reaction (OER), occurs through a 4-elecron transfer in highly corrosive solutions. While currently nickel-based materials are used for the anode because of the chemical stability of their oxides and oxyhydroxides in alkaline media, the current density during electrolysis is limited due to the low catalytic activity of those materials and cannot be as high as PEM water splitting. Our group has recently developed bismuth ruthenate-based pyrochlore oxide as a new OER catalyst and has found some excellent properties on the polarization for and durability to OER in KOH solutions. In this paper, the composition, structure, and OER behaviors of manganese-doped bismuth ruthenium oxide (MBRO) are presented with the results by instrumental analyses and electrochemical measurements.<br/><br/>MBRO was prepared by calcination at 600 ^oC of the precursor deposited in the solution which had been obtained by adding NaOH solution into the metal salt solution containing bismuth nitrate and ruthenium chloride with manganese nitrate. The composition and structure of the oxide were analyzed by AAS, RBS, EDX, XRD, and XAFS and the surface morphology and particle size were measured by SEM. The electrochemical performance was evaluated by Titanium Disk Method (TDM), in which the oxide particles loaded on a titanium disk (4 mm in diam.) was mounted in the rotating-disk electrode of a conventional three-electrode cell. The cell was also equipped with a platinum plate counter electrode and an Hg/HgO reference electrode. The electrolyte was 0.1 mol/L KOH solution and all measurements were performed at room temperature.<br/><br/>The obtained MBRO showed a single phase of pyrochlore structure and ruthenium at the B site was partly substituted with manganese. The particle size ranged from 20 to 40 nm, which was small enough to load it on a titanium disk without any binders or ionomers. The results obtained by polarization measurements revealed that the onset potential of OER was 0.45 V vs. Hg/HgO, which is the overpotential of 0.16 V, and the Tafel slope was ca. 39 mV/dec, while the other materials reported in the literature²⁾ shows the overpotential higher than 200 mV and Tafel slope of 60 mV/dec or more. The above results suggest that MBRO is suitable for the anode material of alkaline water splitting to reduce the cell voltage at high current densities. In this work, MBRO-loaded graphite electrodes were prepared and examined for OER in 6 mol/L KOH solutions, of which the results will be also presented.<br/><br/>Ref.)<br/>1) Digital Research, Current Status of Water Electrolyzer Development in the World and Trends of Makers Entering the Market, p. 11 (2022).<br/>2) T. Shinagawa,<i> et al.</i>, <i>Scientific Reports</i>, <b>5</b>, 13801 (2015).