Oxygen Evolution Reaction Activity of Highly Crystalline Hydrated Cobalt Oxyhydroxide Synthesized with Soft Chemistry
Authors: Nakata Haruhisa , Ikezawa Atsunori , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Introduction

Zinc-air secondary batteries are expected to have high energy density because oxygen in ambient air can be used as the active material. They are also expected to satisfy high safety standards owing to the aqueous electrolyte solutions and open structure. In addition, low cost is expected since zinc is a relatively cheap metal compared to lithium. Because of these features, Zinc-air secondary batteries are attracting attention as a next-generation large-scale energy storage device. One of the challenges for commercializing this battery system is the low energy conversion efficiency caused by the large overvoltage of the air electrode. To solve this problem, oxygen evolution reaction (OER) catalysts have been intensively researched, and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and Ca2FeCoO5 (CFCO) have been developed as OER catalysts having higher activity than conventional noble metal oxide catalysts. Since the active sites of BSCF and CFCO are reported as partially Fe-substituted CoOOH phases1,2, it is essential to understand OER characteristics of partially Fe-substituted CoOOH phases. However, the effects of the crystallinity and Fe-substitution ratio on OER activity have not been clarified. This is mainly because metastable hydrated CoOOH is conventionally synthesized at relatively low temperatures. On the other hand, we have successfully synthesized highly crystalline hydrated NixFe1–xOOH using soft chemistry and clarified the effect of crystallinity and Fe-substitution ratio on the OER activity3. In this study, we synthesized highly crystalline hydrated CoOOH using soft chemistry and evaluate its OER activity to elucidate factors controlling it.

Experimental

Hydrated CoOOH was synthesized with two-step soft chemistry following the report by Sakurai et al. 4. First, Na0.74CoO2 was obtained by calcination of Co3O4:Na2CO3 =40:60 mol% at 900 °C for 12 h. Then, synthesized Na0.74CoO2 was stirred in 7.8 mmol dm–3 Br2 / CH3CN to obtain Na0.358CoO2 by extracting Na. Finally, Na0.358CoO2 was hydrated by refluxing with ultra-pure water at 100 °C for 24 h. The synthesized samples were characterized with powder X-ray diffraction (XRD).

Results and discussion

Figure 1 shows a XRD pattern of the synthesized sample. Though relatively broad unidentified peak is observed at around 8 °, the diffraction pattern of hydrated CoOOH is observed as a main phase. Its OER performance in an aqueous solution of KOH will be presented at the site.
References
1) K. J. May, C. E. Carlton, K. A. Stoerzinger, M. Risch, J. Suntivich, Y. L. Lee, A. Grimaud, Y. S. Horn, J. Phys. Chem. Lett.,
3, 3264 (2012).
2) E. Tsuji, T. Motohashi, H. Noda, D. Kowalski, Y. Aoki, H. Tanida, J. Niikura, Y. Koyama,M. Mori,H. Arai,T. Ioroi,N. Fujiwara,Y. Uchimoto,Z. Ogumi, H. Habazaki., ChemSusChem., 10, 2864 (2017).
3) A. Ikezawa, S. Koito, H. Arai, Abs.#1I23, The 90th ECSJ Annual Meeting (2023).
4) H. Sakurai, K. Takada, T. Sasaki, F. Izumi, R. A. Dilanian, E. Takayama-Muromachi, J Phys. Soc. Jpn., 73, 2590 (2004).
This work was partially supported by JRP-LEAD with DFG (JPJSJRP20221602)

Figure 1