Modeling of Triple-Phase Boundary in Air Electrodes for Metal-Air Secondary Batteries Using Partially Immersed Electrodes
There has been increasing interest in metal-air secondary batteries such as Zn-air secondary batteries, Al-air secondary batteries, etc. These batteries are expected to have high energy densities because they can utilize ambient air as their positive electrode material. They are also expected to be very safe because they can utilize aqueous electrolyte solutions. Therefore, they are very attractive power sources for electric vehicles. However, there are many problems at negative and positive electrodes, and in alkaline electrolyte solution for practical use. Among them, the low power density of air electrode is one of the most serious problems as power sources for electric vehicles.
In air electrode, the electrochemical reactions of oxygen reduction and oxygen evolution only occur at the regions where the three species, the electron, the electrolyte and the oxygen, have contact with each other, so-called triple-phase boundary. Therefore, well-fabricated triple-phase boundaries can lead to great improvement of the power densities. However, it is difficult to investigate the properties of the triple-phase boundaries by using the practical gas-diffusion electrodes, because their microstructures are very complicated. Therefore, partially immersed electrode system have been applied for the investigation of the properties of triple-phase boundaries[1-3]. In these systems, mass transport of the gases and ions will be very similar to those in the catalyst layers.
In this research, we use partially immersed Pt-supported glassy carbon electrode and partially immersed Pt interdigitated array electrode (Fig. 1) systems in alkaline condition for modeling of triple-phase boundary in air electrodes for metal-air secondary batteries.
A three-electrode electrochemical cell was used for electrochemical measurements. Preparation of partially immersed electrode for working electrode was done by fully immersing Pt-supporting area of electrode (h = 0 mm) and vertically raising electrode by micrometer head to h = 10 mm. Pt wire and Hg, HgO electrode (Hg/HgO) were used as counter and reference electrodes, respectively. Solutions of 0.10 ~ 10 mol dm-3 KOH (saturated with mixed gas of O2 and Ar ) were used as an electrolyte. Potentiostatic measurements were carried out under the mixed gas atmosphere. Steady-state oxygen reduction currents were measured. Segmental currents of Pt interdigitated array electrode were also measured simultaneously with I-V converter.
Results and discussion
Figure 2 shows the segmental currents of Pt interdigitated array electrode in 1.0 mol dm-3 KOH (saturated with mix gas of O2 and Ar (Po2= 0.2 atm) ). Relatively low segmental currents were observed from 0.0 mm to 2.9 mm. This is because intrinsic meniscus was formed at these regions. It is assumed that electrolyte thin film was formed at upper regions exceeding 3.0 mm. In these regions, current crowding at the low segments was observed. The higher the applied potential was and the lower the electrolyte conductivity was, the greater the current crowding became. This suggests that ohmic-loss should be caused at electrolyte thin film. Limiting current was also observed at low segments of electrolyte thin film. With the increase of the oxygen partial pressure, the limiting currents were increased. This indicates that oxygen diffusion should be a rate-determining step at the regions where local overpotentials were sufficiently high.
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