Operando X-Ray Imaging of Gas-Electrolyte Behavior in the Zinc-Air Batteries
Authors: Kang Wooseok , Kodama Manabu , Ikezawa Atsunori , Arai Hajime , Hirai Shuichiro 
Published: September 2023

Zinc-air batteries have the potential to be widely used due to their high energy density, low cost, and safety. However, some problems with the electrolyte in the batteries have prevented their widespread use. For example, hydrogen evolution can occur in the zinc anode, reducing the battery's usable capacity. The oxygen reduction performance of the air electrodes is also a challenge, as the electrolyte, catalyst, and gas all interact with each other. Understanding the behavior of the electrolyte and gas is crucial to addressing these problems. Previous analyses, such as operando gas analysis, have provided some insights, but they have not been able to pinpoint the specific locations where these problems occur. More detailed, real-time observation of the cell, known as operando visualization, is needed to better understand these issues.
In this study, x-ray imaging is conducted to visualize the electrolyte behavior inside a zinc-air battery. X-rays are highly permeable and nondestructive, allowing for real-time, operando observation of electrolytes in the battery. This allows for the simultaneous observation and comparison of the electrochemical performance of the cell and the behavior of the electrolyte during open circuit voltage (OCV), charging, and discharging.
To obtain clear x-ray images, the x-ray permeable cell shown in fig.1 is designed with a small electrode size (2 x 10 mm) and an ABS polymer cell casing. This allowed for sufficient contrast in the acquired images, enabling the observation of the liquid electrolyte between the electrodes and inside the catalyst layer. The anode of the cell was made of zinc, and the cathode was a mixture of carbon black, PTFE binder, and LCCO (La0.6Ca0.4CoO3) catalyst that was coated onto carbon paper as gas diffusion layer (GDL) using a hot pressing process. A 6 mol/L KOH aqueous solution is used as the electrolyte. X-ray images were obtained using a Rigaku nano3DX and a molybdenum target. During the observation, the charge/discharge condition was controlled using a potentio-galvanostat. The hydrogen gas (H2) concentration is also monitored using an electrochemical sensor.
Figure 2 shows x-ray images of the cell near the air electrode in four different conditions: (a) without electrolyte, (b) with electrolyte, (c) during the discharge phase, and (d) during the charging phase. Because liquid electrolyte absorbs more x-rays than air, the areas with liquid electrolyte appear darker in the images than the areas without electrolyte (Figure 2 (a) and (b)). During the discharge phase, some bright circular shapes appear, which are thought to be bubbles (Figure 2 (c)). These bubbles are also generated during the charging phase (Figure 2 (d)), and it appears that the formation of the bubbles is not dependent on the cell voltage or current density. The experimental data from the H2 sensor shows that H2 concentration increased during the discharge phase, indicating that the bubbles contain hydrogen gas.
The hydrogen evolution reaction (HER) in zinc-air batteries is a natural reaction given the electrochemical potential of hydrogen and zinc. Until now, the HER problem has only been discussed in relation to the open circuit voltage (OCV) and the charging phase. It was previously thought that HER hardly occurs during the discharging phase because the cell voltage typically decreases at this time. However, the experimental data shows that HER does not stop during the discharging phase. In addition, the researchers observed that the bubbles formed during this phase disrupt ion transfer efficiency, causing a drop in cell voltage.
This study demonstrates the successful visualization of electrolytes inside a zinc-air battery using x-ray imaging. In addition, the researchers observed unexpected hydrogen evolution during the discharging phase. Since HER has a significant impact on the lifetime and safety of the battery, the mechanism behind HER during the discharging phase is an important problem for zinc-air batteries to address.

Figure 1