Catalytic Activity of Double-Perovskite Type BalnMn2O5 for Oxygen Reduction Reaction Using in Rechargeable Zinc-Air Batteries

    There have been increased demands for the development of high energy efficient and affordable energy storage devices as the alternatives to lithium-ion secondary batteries for electrical vehicles, portable electronic devices and so on. Recently, rechargeable zinc-air batteries have attracted much attention from the point of view of high-capacity and safety batteries. To realize the rechargeable zinc-air batteries, active cathode catalysts for oxygen reduction reaction and evolution reaction (ORR and OER) play important roles in the overall battery performance because of the majority of energy losses occurring at the cathode reactions. It is well known that the transition-metal oxide family with a perovskite-type ABO₃ structure is one of the promising catalysts for ORR and OER. Especially, manganese oxides such as LaMnO_3+δ and La_1-xCa_xMnO₃ showed high ORR activities. Some researchers reported that the ORR activity for oxide catalysts primarily correlates to the electronic states and valence of Mn. However, a clear strategy has not been revealed to prepare active catalysts for ORR. On the other hand, more recently, in the case of high-temperature oxygen intake/release reactions of the double-perovskite type BaLnMn₂O_5+δ (Ln = Y, Gd, Nd and La; Fig. 1a), Motohashi et al. reported that the redox characteristics could be controlled by the radius of Ln ions, even though the valence of Mn remained unchanged [1]. Therefore, in this study, we focused on the ORR activity of the double-perovskite type BaLnMn₂O_5+δ (Ln: Y, Gd, Nd and La).

    BaLnMn₂O_5+δ (Ln: Y, Gd, Nd, and La) powder catalysts were synthesized via a citrate precursor route combined with the oxygen-pressure-controlled encapsulation technique. Ln₂O₃ (fired at 1000°C overnight prior to use), Ba(NO₃)₂ and Mn(NO₃)₂·6H₂O  were used as starting materials. Appropriate amounts of these reagents were dissolved in diluted HNO₃ (for Ln₂O₃) or Milli-Q water [for Ba(NO₃)₂ and Mn(NO₃)₂·6H₂O] to prepare Ln, Ba, and Mn nitrate solutions. These solutions were mixed in a crucible in which equimolar citric acid was subsequently added as a complexing agent. The citrate solution was stirred and heated at 60-70°C to promote polymerization. The gelatinous product was prefired at 450°C in air for 1 h and then at 1000°C in flowing N₂ gas for 24 h. The resulting precursor powder was pressed into pellets and placed in an evacuated silica ampule together with an equal amount of FeO powder, which acts as a getter for excess oxygen. The silica ampule was heated at 1100°C for 24 h, followed by quenching into ice water.

    The crystallinity, morphology and particle size of the resultant samples were characterized with XRD and SEM. Electrochemical measurements were carried out using a rotating ring disk electrode (RRDE, Pt ring) with a conventional three electrode system. The catalyst ethanol ink (BaLnMn₂O₅: acetylene black: Nafion = 5: 1: 1) was dropped and dried on a glassy carbon electrode as a working electrode. A Pt plate and a Hg/HgO electrode were used as a counter electrode and a reference electrode, respectively. Argon- or oxygen-saturated 4 mol dm^-3 potassium hydroxide aqueous solution was used as an electrolyte. The scan speed and rotating speed of the RRDE were 1 mV sec^-1 and 1600 rpm, respectively.

    The XRD data indicate that all the as-synthesized BaLnMn₂O_5+δ products are essentially phase-pure of the fully reduced δ ≈ 0 form with a tetragonal unit cell. Both the a- and c-axis lengths are in agreement with those in the previous literature. As expected, these values systematically increase with the increasing ionic radius of Ln³⁺ in the order of Y, Gd, Nd, and La. The oxygen content (5+δ) values determined by iodometry are close to 5.00 for all the Ln-products. The particle sizes of the samples were almost the same as 1~2 μm.

    Fig 1b shows onset potentials for ORR as a function of the radius of Ln ions of the samples. It can be seen that the onset potential is shifted toward positive side as the radius of the Ln ions increases, in spite of the fixed Mn valence for all the samples. The most active catalysts for ORR were BaNdMn₂O₅ and BaLaMn₂O₅, which contain larger Ln ions. The result implied that the size of A site ion played an important role in the catalytic activity for ORR.

Fig. 1 (a) the crystal structure and (b) the onset potentials for ORR as a function of the radius of Ln ions of BaLnMn₂O_5+δ (Ln: Y, Gd, Nd, and La).

[1] T. Motohashi, M. Kimura, T. Inayoshi, T. Ueda, Y. Masubuchi and S. Kikkawa, Dalton Trans. in press.