Effects of Carbon Supports on Air Electrode Properties of Lanthanum Manganite-Based Catalysts for Li-Air Batteries

In recent years, Li-air secondary batteries (LABs) have been attracted much attention as next-generation energy strange because of the possibility of greatly higher energy density (> 500 Wh kg -1 ) beyond the conventional Li-ion batteries [1]. However, the LAB systems have some serious problems to be solved before the commercialization; e.g. suppression of Li metal dendrite at the anode, improvement of durability for the electrolyte solutions during the charge/discharge cycling, development of high performance air electrode catalysts at the cathode, etc. Especially for the air electrode catalysts, high catalytic activities for both O 2 reduction reaction (ORR) and O 2 evolution reaction (OER) and good durability are demanded at the cathode. In this study, we synthesized tree types of La 0.6 Sr 0.4 MnO 3 (LSM)-based catalysts loaded on different carbon supports (Ketjen black (KB, EC600JD, Lion Co.), carbon nanotube (CNT, Φ= 20-30 nm, Wako) and Graphene (GNP, Strem Chemicals Inc.)) and investigated the effects of carbon supports on the catalytic activities and durability. The obtained LSM/KB catalysts were synthesized by the modified reversed method [2]. The products were characterized by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and thermo-gravimetry-differential thermal analysis (TG-DTA). The LS x M/KB catalysts were dispersed in 1-hexanol, casted on the glassy carbon disk of an RRDE, and then covered with an alkaline ionomer binder (AS-4, Tokuyama Co.). The ORR and OER activity[1] of the LSM/KB catalyst was evaluated by hydrodynamic voltammetry (HV) using a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50 o C. Fig.1 shows the HV curves of 20 wt% LSM/KB, LSM/CNT and LSM/GNP catalysts in O 2 saturated 0.1 M KOH at 50 o C. The magnitude of onset potentials at 100 μA for the ORR was in the order of LSM/KB ≈ LSM/CNT > LSM/GNP. This trend agreed well with that of only carbon supports and the ORR currents were enhanced by loading the LSM nanoparticles. This indicates that the ORR activity of LSM nanoparticle drastically influenced by the various properties of carbon supports such as the surface area, electronic conductivity, functional group of the carbons, degree of graphitization, etc. Fig. 2 shows the change in the normalized ECSAs of LSM/C catalysts during the cycling test. The magnitude of oxidation currents were in the order of LSM/GNP > LSM/CNT > LSM/KB. This was the opposite trend against that for the ORR activity.