Preparation of LaMnO3-CNF and Activity for Oxygen Electrode Reaction in KOH Aqueous Solution
Authors: Ono Kohei , Kinumoto Taro , Tsumura Tomoki , Toyoda Masahiro 
Published: February 2020

Metal-air battery and metal hydride (MH)|air battery hold promise as power sources beyond lithium-ion batteries (LIBs). In particular, we focus on metal hydride (MH)|air battery, which used MH as the anode and an alkaline medium as the electrolyte. For realization of this battery, development of highly active catalyst (i.e., with low activation overpotential) for the oxygen reduction reaction (ORR; a discharge reaction) and the oxygen evolution reaction (OER; a charge reaction) remains challenging.

We prepared a novel cathode catalyst, LaMnO3-CNF, and reported the catalytic activity for both the ORR and OER in aqueous KOH [1]. In this presentation, effect of the heat-treatment temperature on the catalytic activity is reported. In addition, for the first time, application of a rotating ring disk electrode (RRDE) technique is also reported to estimate OER efficiency without carbon oxidation reaction (COR).

Preparation of LaMnO3-CNF was reported in [1]. In brief, aqueous solutions of La(NO3)3 and Mn(NO3)2 was added to (CH3)4NOH solution  in a polyethylene beaker. CNF (VGCF-H, Showa Denko K.K.) was immediately dispersed and was stirred vigorously for 1 h. The products were collected by vacuum filtration and rinsed repeatedly with pure water and dried at 120 oC for 12 h. Finally, these were subjected to heat treatment under Ar atmosphere at 600, 800 and 1000 oC for 5 hour (denoted as 600-5, 800-5 and 1000-5).

The products were analyzed by X-ray diffraction (XRD). Product morphology was observed by field emission scanning electron microscope (FE-SEM). The electrochemical measurements for ORR and OER were carried out by RRDE technique using glassy carbon substrate and Au ring electrode. The electrolyte solution was 0.1 mol dm-3 and 1.0 mol dm-3 of KOH solutions under O2 atmosphere. Koutechky-Levich plots analysis was applied to estimate the catalytic activity for ORR.

From the XRD results, LaMnO3 was formed at 600 ºC and 800 ºC without significant oxidation of the CNF although formation of La2O3 and Mn2+(Mn3+)2O4 were confirmed at 1000 oC. In addition, FE-SEM images shows that LaMnO3 particles were loaded on the CNF.

Figure 1 a) and b) shows hydrodynamic voltammograms for ORR of CNF, 600-5 and 800-5. Disk and ring current correspond to the currents for ORR and oxidation reaction of HO2
-, respectively. The onset potential for ORR of LaMnO3-CNF was higher than that of CNF, indicating that LaMnO3 has better catalytic activity for ORR. The ring currents were also observed for LaMnO3-CNF and CNF, showing that HO2
- was formed via. two-electrons transfer reaction during the ORR.

The electron number responsible for ORR can be estimated by Koutechky-Levich equation and it was found that loading of LaMnO3 to CNF gives higher electron number. In addition, HO2
- generation rate during the ORR can be also estimated using the ring and disk currents and it was decreased by loading of LaMnO3. Therefore, it is considered that HO2
- formed during the ORR is further decomposed and/or reduced to OH- on LaMnO3. In LaMnO3-CNF series, 600-5 showed higher ORR activity than 800-5 in terms of the current per unit weight (mass activity) and per pseudo capacity.

RRDE technique is also useful to investigate OER with high accuracy. Figure 1 c) shows hydrodynamic voltammograms for OER. In these cases, the ring potential was fixed at 0.4 V vs. RHE and therefore, the ring current shown in Fig. 1 d) is assigned to oxygen reduction reaction at the ring electrode. Based on these results, the onset potential for OER can be estimated accurately and directly as 1.64 V for CNF and as 1.59 V for LaMnO3-CNF in 0.1 mol dm-3 KOH solution. Furthermore, the onset potentials were decreased with increasing in the KOH concentration. Therefore, LaMnO3-CNF also has OER activity rather than that of CNF. The current efficiency for OER can be evaluated from the disk and ring currents and the details will be also presentation.

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

Reference

[1] K. Ono, T. Kinumoto, et al., ECS Trans, in-press.

Figure 1