Indicators for Optical Analysis of Local Conditions in Bifunctional Air Electrodes
Authors: Nonaka Nanako , Ikezawa Atsunori , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Introduction

Bifunctional air electrodes using aqueous alkaline solutions have attracted increasing attention as positive electrodes for metal-air secondary batteries and alkaline regenerative fuel cells, which have the advantages of high capacity and high safety. However, the large overpotential for the reaction in bifunctional air electrodes impedes their practical application. In order to reduce the overpotential, it is necessary to design electrodes based on an understanding of the reaction field in the electrodes since the favorable reaction fields for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are different [1]. Our research group has developed an operando measurement system using confocal optical microscope to visualize the local conditions of electrodes during the battery operation [2, 3] and has succeeded in visualizing the local state of charge in a LiFePO4 composite electrode from the brightness [2]. Though operando measurement system using a confocal microscope have the advantage of high versatility, high accessibility and sufficient time or spatial resolution, its application to bifunctional air electrode has not been reported. One of the challenges in visualizing the local conditions in bifunctional air electrodes is the absence of clear color changes corresponding to the local conditions. In this study, we searched for indicators to optically visualize local conditions in bifunctional air electrodes. The optical properties of the candidate indicators were evaluated by using a confocal microscope.

Experimental

Ag and Ni were selected as candidate indicators for visualizing local potential since they undergo redox reactions at around the potential for OER in alkaline electrolytes. A three-electrode cell for operando optical observation was constructed using metal foils of the candidate materials as the working electrode, Ni mesh as the counter electrode, Hg/HgO as the reference electrode, and 8 mol dm–3 KOH solution as electrolyte. Constant potential measurements were conducted at around the potential range of OER. At the same time, the optical changes of the working electrode were observed with a confocal microscope (ECCS, Lasertec) to evaluate their optical properties as potential indicators.

Results and discussion

Fig. 1 shows the applied potentials and the normalized red and blue signal outputs of the Ag and Ni foil electrodes. The potentials are written in terms of RHE. As for the Ag foil, the red signal output decreased significantly with a potential step from 1.18 V to 1.23 V during oxidation, and the red signal output increased significantly with a potential step from 1.18 V to 1.13 V during reduction. This significant output change corresponds to the redox reaction of Ag/Ag2O. Regarding the Ni foil, the blue signal output decreased with a potential step from 1.38 V to 1.43 V during oxidation, and the blue signal output increased in multiple steps from 1.43 V to 1.23 V during reduction. This signal output change corresponds to the redox reaction of Ni(OH)2/NiOOH. These results indicate that Ag and Ni foils can be used as the local potential indicators for OER. The pH and oxygen partial pressure dependencies will also be reported at the site.

Acknowledgments

This work was financially supported by JST PRESTO and JRP-LEAD with DFG (JPJSJRP20221602).

References

[1] A. Ikezawa et al, Electrochem. Commun., 84, 53 (2017).
[2] H. Arai et al, J. Phys. Chem. C, 125, 3776 (2021).
[3] A. Ikezawa, M. Horiuchi, H. Arai, J
. Power Sources, 507, 230291 (2021).

Figure 1