Effects of Carbon Supports on Air Electrode Properties of Lanthanum Manganite-Based Catalysts for Li-Air Batteries
Authors: Tachikawa Yusuke , Suzuki Hiroshi , Kosaka Shinpei , Shiroishi Hidenobu , Tanaka Yumi , Seki Shiro , Saito Morihiro 
Published: February 2020

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 O2 reduction reaction (ORR) and O2 evolution reaction (OER) and good durability are demanded at the cathode. In this study, we synthesized tree types of La0.6Sr0.4MnO3 (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 50oC.

Fig.1 shows the HV curves of 20 wt% LSM/KB, LSM/CNT and LSM/GNP catalysts in O2saturated 0.1 M KOH at 50oC. 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. The order was in good agreement with the degree of graphitization of carbon supports from the data of XRD patterns, which is also the opposite order of the surface area of carbon supports (KB (1270 m2 g-1) > LSM/CNT (200 m2 g-1) > graphene (100 m2 g-1). Therefore, the relationship between the ORR activity and durability of the LSM/C catalysts was found to be trade-off for the carbon supports. As a result, the LSM/CNT catalyst exhibited the best performances. The OER activity of LSM/C catalysts will be presented in the meeting. 

We gratefully thank to Tokuyama Co. for providing alkaline ionomer binder sample. This work was supported by the KAKENHI (25870899) and JST "A Tenure-track Program" from MEXT, Japan. 

[1] J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn, Science, 334(12) 1383 (2011).

[2] M. Saito, T. Takakuwa, T. Kenko, H. Daimon, A. Tasaka,, M. Inaba, H. Shiroishi, T. Hatai, J. Kuwano, ECS Trans., 58(1) 1335 (2013).

Figure 1