Preparation of LaMnO3-CNF and Its Activity for Oxygen Electrode Reaction in Alkaline Electrolyte Solution

Cathode catalysts with low overpotential for oxygen electrode reaction including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for the batteries that air and an alkaline electrolyte are used.  For this purpose, a kind of perovskite metal oxides is promised.  Although the electric conductivity is low, LaMnO₃shows relatively high activity for both ORR and OER [1, 2].  To support the electric conductivity, some kinds of carbon must be required to practical use.  Among the carbon materials, carbon nano fiber, which is partially used in lithium battery, is considered to be favorable because of adequate electric conductivity, high hydrophobicity, low surface area and quite low amount of surface functional groups.

In this study, we attempted to prepare a new composite catalyst, that LaMnO₃particles are supported on carbon nano fiber, and evaluated catalyst activity for ORR and OER.

 La(NO₃)₃ and Mn(NO₃)₂ aqueous solutions were respectively prepared at desired concentration and well mixed.  Then, the solution was poured into (CH₃)₄NOH aqueous solution.  After that, CNF (VGCF, SHOWA DENKO K.K.) was immediately added to the solution and stirred for 1 hour at room temperature.  The product was washed with pure water and dried at 120^oC for 12 hour.  Finally, it was calcined under Ar atmosphere at 600^oC for 5, 10 and 20 hour, 800 and 1000^oC for 5 hour (600-5, 600-10, 600-20, 800-5 and 1000-5) to obtain LaMnO₃ and CNF composites (LaMnO₃-CNT).

X-ray diffraction (XRD) was used to identify the product.  The product was observed by field emission scanning electron microscope (FE-SEM). ORR and OER activity were evaluated in 0.1 mol dm^-3 and 6.0 mol dm^-3KOH solution by using glassy carbon or Ti substrate.

 Figure 1 shows FE-SEM image of 800-5.  As indicated by allows in this figure, CNF is covered with particles of LaMnO₃.  In addition, the product that only edge parts were covered with particles and the product that was not covered with particles occasionally.

Figure 2 shows XRD patterns of 800-5 and 1000-5.  The patterns of 600-5, 600-10, 600-20 and 800-5 show the formation of LaMnO₃ and the different crystallite size.  In the case of 1000-5, La₂O₃and MnO are confirmed, suggesting that oxygen in the products should be consumed by CNF oxidation.

Figure 3 shows hydrodynamic voltammograms of the samples in 0.1 mol dm^-3KOH solution.  Reduction current originated from ORR is shown below 0.85 V.  From this point of view, 600-10 shows the highest on-set potential (lowest overpotential) among the samples.  The detail including OER activity of the samples will be also presented.

This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST).

[1] J Suntivich, H A. Gasteiger, N Yabuuchi, H Nakanishi, J B. Goodenough and Y Shao-Horn, DOI: 10.1038/NCHEM.1069.

[2]J O’M. Bockris and Takaaki Otagawa,J Electronchem.Soc. :ELECTROCHEMICAL SCIENCE AND TECHNOLOGY p.290-302 Vol. 131,No. 2.(1984)