Lithium-air battery attracts great attention because of its high energy density. In this study, we focus on the aqueous lithium–air secondary battery. The aqueous electrolyte realizes cost reduction and ensures the safe use of a battery. On the other hand, low solubility of oxygen in aqueous electrolyte limits oxygen transport to the reaction site. To realize a more powerful aqueous lithium–air battery, it is necessary to clarify and improve the oxygen transport phenomena in the porous cathode. In this study, we developed the measurement technique of oxygen concentration in the porous cathode under operating condition by inserting a fine optical fiber oxygen sensor.
In this study, platinum tetrakis pentrafluoropheny porphine (PtTFPP) was employed as oxygen indicator . When the PtTFPP is exposed to excitation light (λ = 405 nm), phosphorescence emission (λ= 650 nm) is produced, and its intensity depends on oxygen partial pressure. Thus, quantitative oxygen concentration can be obtained by measuring phosphorescence intensity and using calibration data. The PtTFPP was painted at the edge of optical fiber of which diameter was 110 μm. In order to measure the phosphorescence intensity, bifurcated optical fibers were connected to excitation light source and spectrometer, respectively. As shown in Figure 1, phosphorescence intensity changed at 650 nm depending on oxygen concentration.
The experimental setup is shown in Figure 2. The porous cathode (SIGRACET® 10AA, SGL Group, USA) was fixed near electrolyte interface horizontally. The tiny optical fiber sensor was moved up and inserted in the cathode. The temporal oxygen concentration change from the start of discharge was measured at arbitrary positions in the porous cathode. The composite anode was water-stable and had a multilayer structure for the aqueous lithium air battery .
Measurement of spatial and temporal distribution of oxygen concentration has been conducted under the assumption that discharge experiment had reproducibility. Initial position (z = 0 μm) was defined arbitrary, and the sensor was moved up each 70 μm. Time variation of oxygen concentration was measured at each point. Finally, spatiotemporal distribution of oxygen concentration for each current density was obtained.
As shown in Figure 3, the spatial and temporal oxygen concentration distribution at 0.10 mA/cm2 was successfully observed. When the discharge was started, the oxygen concentration gradually decreased in the cathode, and finally, oxygen concentration reached considerably low value even in the condition of 0.10 mA/cm2. In this experiment, oxygen concentration gradients are present at both ends of the atmosphere side (0-140 μm) and electrolyte side (490-910 μm) of the cathode.
Flux for both side of cathode, upper side and lower side, was discussed and comprehensible interpretation was obtained. Although dissolved oxygen that forms flux from the lower side of cathode has an effective supply to the porous electrode, electrolyte layer formed at between the atmosphere and the cathode causes oxygen transport resistance.
This work was supported by JSPS KAKENHI Grant Number 15K13881, 15H02347.
 CS Chu., CA Lin, Sensors and Actuators B: Chemica, 2014, 195, 259-265.