Effects of B-Site Modification of Bi-Ru Based Oxide on Catalytic Activity for Oxygen Reactions

Abstract

An aqueous air secondary battery uses oxygen in air as the active mass of the air electrode, at which the reactions are oxygen reduction during discharge and oxygen evolution during charge. The polarization of the air electrode is known to be large and a highly active bi-functional oxygen catalyst is needed to reduce the polarization. We have been developing a metal hydride/air secondary battery using KOH solutions and have found that bismuth and ruthenium based oxide (BRO) can suppress the polarization of oxygen reactions [1]. Although BRO has a good bi-functionality, there is still an issue that the polarization of oxygen reduction is ca. 100 mV larger than that of oxygen evolution. In this work, we aimed to improve the catalytic activity of BRO with modification of B-site element by substituting ruthenium with manganese. Then, the modified BRO (MBRO) was examined by titanium disk method (TDM) developed by our group [2], which is very useful to know the activity of oxygen catalyst, because no oxygen reactions on titanium occur in the examined potential range so that no oxygen current from the substrate is excluded. The electrochemical polarization data was analyzed to obtain some kinetic parameters such as Tafel slope and exchange current density.MBRO 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, ruthenium, and manganese. BRO was also synthesized for comparison. The obtained oxides were characterized by XRD, SEM, and EDX, and electrochemical measurements were carried out using a three-electrode cell. The working electrode was the titanium disk covered with MBRO or BRO, which was mounted in a rotating-disk electrode equipment, 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. The specific activity was evaluated with the normalized current by the catalyst’s weight, i w, or by the double layer charge, i c, and Tafel plots of polarization curves gave Tafel slope and exchange current density, i 0, for MBRO and BRO.The XRD patterns of MBRO were close to that of BRO except that some peaks of MBRO in the high 2θ region showed the peak sift to a higher angle with increasing manganese. EDX measurements of MBRO indicated that the metal ratio of the obtained oxide was almost the same as that of the precursor solution. SEM observation also revealed that the particle size of MBRO ranged from 20 nm to 60 nm with no clear dependence with the metal ratio. The polarization of MBRO for oxygen reduction was smaller than that of BRO and was more reduced with increasing manganese up to 30 at%, while a little difference in polarization for oxygen evolution was seen between MBRO and BRO. The analysis of Tafel slope and exchange current density from LSV data gave some interesting relationship between these parameters and the polarization for ORR and OER that the exchange current density of MBRO was 6 to 8 times larger than that of BRO at high manganese ratios, which is the main reason why MBRO showed a reduction in ORR polarization, while Tafel slope of MBRO for ORR was almost the same as or slightly higher than that of BRO, and Tafel slope for OER was hardly changed between MBRO and BRO. From the results, we found that the B-site modification of BRO with manganese can enhance the kinetics of electron transfer for the oxygen reactions.