Investigation of Triple-Phase Boundary Region in Partially Immersed Pt Segmented Electrode As a Model Electrode of Air Electrodes for Metal-Air Secondary Batteries

Abstract

Introduction There has been increasing interest in metal-air secondary batteries such as Zn-air secondary batteries and Fe-air secondary batteries. These batteries are expected to have high energy density and light weight since they can utilize ambient air as their cathode material. They are also expected to satisfy high safety standard because they can utilize non-flammable aqueous electrolyte. Therefore, metal-air secondary batteries are attractive power sources for large energy storage systems. However, large overpotential of air electrode hinders their widely use. In air electrode, the electrochemical reactions, oxygen reduction and oxygen evolution, only occur at the regions where the three different species, the electron, the electrolyte and the oxygen, can be transported, so-called triple-phase boundary (TPB) region. Therefore, observation of its property can lead to some improvement of the large overpotential. However, it is difficult to investigate the property of the TPB region using practical gas-diffusion electrodes, because their microstructure is too complicated to be analyzed in detail. Therefore, partially immersed electrode system has been applied for the investigation of the property of TPB region[1 - 3]. In these systems, mass transport of the gases and ions in the gas diffusion electrodes can be simulated. Previously, we have investigated current distributions in TPB region using partially immersed Pt segmented electrode (Fig. 1)[3]. In this research, we measure local ionic resistance of TPB regions using the electrode so as to investigate the basic properties of TPB region. Experimental Electrochemical two- or three-electrode cells were used for electrochemical measurements. In the two-electrode configuration, each platinum segment was used as working and counter electrode. In the three-electrode configuration, Pt segment, Pt wire and Hg, HgO electrode (Hg/HgO) were used as working, counter and reference electrodes, respectively. Solutions of 0.10 mol dm- 3 KOH (saturated with O2 or Ar ) was used as an electrolyte solution. We carried out AC-impedance measurement as electrochemical measurement. Before the measurement, we fully immersed Pt-supporting area of the segmented electrode (h = 0 mm) and vertically raised the micrometer head to h = 10 mm. Results and discussion First, we measured AC-impedance between 3 CH and upper segments in order to characterize thin liquid film on the electrode. Solution resistance (Rsol) between the segments showed linear behaviors. This means that cross-sectional area of the film was not changed with the heights. From the slopes and the ionic conductivities of KOH solutions, the cross-sectional area of the thin liquid film was estimated at 5.3 ± 0.1 × 10- 3 cm2 [4]. Second, we investigated local ionic resistance variations with oxygen reduction reaction (ORR). When ORR is occurred, electrode potential is decreased and OH- is transported through the film. Therefore, we investigated local ionic resistance variations with both electrode potential and ionic current in the film. For the investigations of the effects of electrode potential, we measured impedance of 5 CH at various electrode potentials using the three-electrode cell configuration under Ar atmosphere. We found that the electrode potential hardly affected the impedance. In the case of ionic current, we carried out AC impedance measurement between 5 and 6 CH under oxygen atmosphere while performing cathodic galvanostatic measurements using 7 ~ 10 CH and Pt wire as working and counter electrode respectively. Figure 2 shows variation of Rsol with cathodic galvanostatic current. We can see that the ionic resistance decreased with the decrease of the DC-current. It was proposed by Bennion et al. that OH- was concentrated in the film during ORR[1]. Therefore, we simulate the KOH concentration and the ionic resistance of the thin film using ion transport model constructed by Bennion et al. We can see in Fig. 2 that the simulated resistances show good agreement with measured ones. This result shows that we have successfully measured OH- concentration gradient predicted by Bennion et al. as ionic resistance. Reference D. N. Bennion, C. W. Tobias, J. Electrochem. Soc., 113, 593 (1966).A. Ikezawa, K. Miyazaki, T. Fukutsuka, T. Abe., J. Electrochem. Soc., 162, A1646 (2015).A. Ikezawa, K. Miyazaki, T. Fukutsuka, T. Abe., ECS meeting abstracts, MA2014-02, abstract 32 (2014).R. J. Gilliam, J. W. Graydon, D. W. Kirk, S. J. Thorpe, Int. J. Hydrogen Energy, 32, 359 (2007). Figure 1