The Degradation of Carbon Electrode for Rechargeable Zinc-Air Battery at an Initial State of Cycling Revealed By in-Situ Raman Measurement
Authors: Wang Tanyanyu , Mori Takanori , Kunimoto Masahiro , Homma Takayuki 
Published: July 2021

With the burgeoning energy consumption, the rechargeable zinc-air battery has attracted the worldwide attention owing to its intrinsic safety, low material cost and high theoretical specific energy density [1]. For its practical application, the degradation of air electrode made of carbon is necessary to be systematically explained for developing applicable electrode with long cycle life. Recent progresses in high-precision methods for surface analysis allow us to approach to mechanistic understanding of the electrode degradation, while normal ex-situ measurements require the rinsing and other processing which might be destructive. Hereby in this work, in-situ Raman measurements are carried out to observe the changes in chemical states of the carbon electrode in the zinc-air battery during the charging and discharging processes. Unlike other commercial air electrode with multi-component catalysts, the air electrode used for this study is the specially prepared carbon electrode with only Co3O4 particles to facilitate the Raman measurements. For the charging and discharging processes, the electrolyte of 8 M KOH solution with saturated ZnO as well as the current density of 20 mA/cm2 are applied, following the standard of zinc-air battery. Moreover, a three-electrode cell with Pt wire as the counting electrode and Hg/HgO as the reference electrode is utilized. In particular, a homemade setup is developed for the electrode so that the Raman spectra of both gas diffusion layer and catalytic layer can be easily detected from the cross-sectional view.
In the first place, distinct carbon-related peaks are clearly observed at 1348 cm-1, 1579 cm-1 and 2695 cm-1. Other shoulder peaks are also assigned to the structural defects of the carbon material. Besides, the Raman peaks at 684 cm-1, 616 cm-1, 519 cm-1, 481 cm-1 and 194 cm-1 which only shown in the near-electrolyte area are attributed to the Co3O4 particles in the catalytic layer. Then, in order to activate the carbon electrode and facilitate the electrolyte infiltrating into the catalytic layer, 6 cycles of charging and recharging with low current density of 10 mA/cm2 are conducted as a pretreatment. Consequently, new peaks at 1060 cm-1 and 440 cm-1 appear, which can be attributed to the carbonate formation and ZnO precipitation respectively [2, 3]. It is interesting that the degradation of electrode is initiated from this unexpected early state of cycling, which has not been addressed yet. We consider the carbonate might be caused by the dissolution of atmospheric CO2 and the chemical corrosion of carbon in high-concentration KOH solution.
CO2 + 2OH- →CO3
2- + H2O
C + 6OH- → CO3
2- + 3 H2O + 4e-

We also confirm that the ZnO arises from the decomposition of zincate ions in electrolyte, as the Raman peak intensity of ZnO increases while the intensity of OH- decreases during the charging process. Moreover, spectra from further cycling process suggest that the precipitation species on the electrode would long-lasting block the charging/discharging reaction sites.
Zn(OH)4
2- → ZnO + 2OH- + H2O
We believe this in-situ work would be instructive for future optimization on zinc-air battery and other metal-air battery systems that prolong the cycle life of battery. Acknowledgements This research was partially supported by the “Development of Systems and Technology for Advanced Measurement and Analysis”, and the “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING 2)” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] X. Han et al., Adv. Energy Mater., 2018, 8, 1801396 [2] N. Koura et al., J. Mole. Struct., 1996, 382, 163-169 [3] T. Otani et al., Electrochimica Acta 305, 2019, 90-100
Fig.1 The homemade cell set-up for Raman measuring and the Raman spectra showing carbonate formation as well as ZnO precipitation

Figure 1