A Novel Bi-Functional Catalyst, Na-Bi-Ru-O, for Oxygen Reactions at Two or Three Phase Boundaries with Alkaline Media

Abstract

Low carbon technology requires highly efficient energy conversion materials and systems which include the catalyst, the electrode, the separator, and etc. for uses in electrolytic hydrogen production (EHP) and air rechargeable batteries (ARB) linked to solar and wind power generation. One of the important materials is the oxygen catalyst working for OER and/or ORR at the anode of EHP and the positive electrode of ARB, and it has been reported a variety of materials including metals, alloys, oxides, sulfides, doped and un-doped carbon, and others [1-3]. We have been also developing pyrochlore oxides, general formula A2B2O7, containing bismuth at the A-site and ruthenium at the B-site especially for aqueous ARBs, e.g., metal hydride/air and zinc/air batteries, and recently have found that a novel oxide catalyst similar to bismuth ruthenium oxide (BRO) except that sodium is also the component and is highly active for OER and ORR with low Tafel slopes. This paper introduces the pyrochlore-like oxide containing sodium, bismuth, and ruthenium with the preparation process and the results on characterization and electrochemical properties for oxygen reactions of the obtained NBRO catalyst.NBRO was prepared by co-precipitation method, in which the calcination of the precipitates obtained by adding excess NaOH solutions into the metal salt solution containing bismuth and ruthenium. The preparation with LiOH solutions replacing NaOH solutions was also carried out for comparison. The obtained oxides were characterized by XRD, RBS, EXAFS, AES, AAS, and SEM, and electrochemical measurements were performed using a three-electrode cell. The working electrode was the titanium disk covered with the oxides, which was mounted in a rotating-disk electrode equipment (Titanium disk method, TDM), and the electrolyte was 0.1 mol/L KOH solutions. Linear sweep voltammetry was performed for anodic and cathodic polarization, and oxygen reduction current was obtained with the difference in current measured in the solutions under N2 or O2 bubbling. Tafel plots of polarization curves gave Tafel slope and exchange current density, i 0. The air electrode, 23mm x 23mm x 0.2 mm, comprising the oxide catalyst, graphite, and PTFE was also prepared and tested with a half-open cell using 6 mol/L KOH solutions, in which the air electrode contacted the solution at one side and faced to air at the other side.The XRD pattern of NBRO was close to that of BRO prepared by solid mixing of Bi2O3 and RuO2 [4], while the EXAFS results indicated that sodium occupied near the A-site of pyrochlore structure as well as bismuth, and the elemental analysis revealed that sodium was about 4 to 6 at%. SEM observation also showed that the primary particle size of NBRO ranged from 20 nm to 40 nm and the secondary one was under 500 nm. Polarization curves by TDM gave that the overpotentials for OER and ORR were 200 mV and 300 mV, respectively, and Tafel slopes for both the reactions were about 40 mV/dec, which are much lower than the other materials such as platinum, cobalt, and other types of oxides. The air electrode using NBRO further demonstrated a high catalytic activity at two phase or three phase boundaries with 6 mol/L KOH solutions, which are, for example, a very high current density, 800 mA/cm2, for both OER and ORR, and a good cyclability up to 3000 cycles. Therefore, NBRO is expected to reduce the polarization of ARB and alkaline water electrolysis for EHP.