Local Reactions in Bifunctional Air Electrodes for Aqueous Metal-Air Secondary Batteries
Authors: Ikezawa Atsunori , Miyazaki Kohei , Fukutsuka Tomokazu , Arai Hajime , Abe Takeshi 
Published: February 2020

Aqueous metal-air secondary batteries, such as zinc-air secondary batteries, are attractive power sources for large scale energy storage systems since these battery systems potentially satisfy high energy density, high safety standard and low cost. However, large overpotential in bifunctional air electrodes hinders practical applications of the systems.1 In order to reduce the overpotential, plenty of effective reaction sites is necessary as well as highly active bifunctional electrocatalysts. Though it is widely known that relatively many reaction sites are formed in porous gas diffusion electrodes composed of hydrophobic gas diffusion layer and mildly hydrophilic catalyst layer, the overpotentials in the porous gas diffusion electrodes are not low enough for the practical application. Further reduction of the overpotential in the porous gas diffusion electrodes requires not only empirical knowledge but also construction guidelines based on fundamental properties of the reaction sites. However, the microstructures of the porous gas diffusion electrodes are too complicated to be analyzed in detail.
In this work, we use a partially immersed electrode system as a model of the porous gas diffusion electrodes to investigate the fundamental properties of the reaction sites. This model system consists of a smooth planer or cylindrical electrode and an electrolyte solution covering the electrode.2-4 Owing to the simple structure of this system, mass transports of the ions and the gases can be easily simulated. In addition, we applied a platinum segmented electrode (Fig. 1(a)) and homemade multichannel current-voltage converter to the partially immersed electrode system to measure current distribution on the electrode directly.
First, we measured AC impedance between adjacent segments to characterize electrolyte solution covering the electrode. While blocking electrode behavior was observed at CHs 1-3, transmission line-type frequency dependences were observed at CHs 4-10. This indicates that CHs 1-3 were covered with relatively thick electrolyte solution, so-called meniscus, and CHs 4-10 covered with thin liquid electrolyte film. Curve fittings indicated that ionic resistance of the film was almost same on CHs 4-10, and the thickness of the film was calculated to be around 3 μm from the conductivity of the electrolyte solution. Figure 1(b) shows local current distributions on the partially immersed platinum segmented electrode with constant current (±0.400 mA) applied for oxygen reduction reaction (ORR) and oxygen evolution reaction(OER). In the case of ORR, larger currents were observed at CHs 4-6. On the other hand, relatively lower currents were observed at CHs 4-10 for OER. These results show that electrochemically active regions were different for ORR and OER. Therefore, two different regions, hydrophobic region and hydrophilic region, should be formed in the catalyst layer of the porous gas diffusion electrodes to reduce the overpotential for both ORR and OER.
Based on this finding, porous gas diffusion electrodes are prepared and the electrochemical properties will be presented at the meeting.

References

1.H. Arai, S. Müller, and O. Haas, J. Electrochem. Soc., 147, 3584 (2000).
2.F. G. Will, J. Electrochem. Soc., 110, 145(1963).
3.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, J. Electrochem. Soc.,162, A1646 (2015).
4.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, Electrochem. Commun.,84, 53 (2017).
5.A. Ikezawa, K. Miyazaki, T. Fukutsuka and T. Abe, Chem. Lett., 47, 171 (2018).

Figure 1