Non-Destructive Optical Measurement of Oxygen Concentration on PEFC Catalyst Layer
Authors: Uemura Suguru , Yoshida Toshihiko , Jao Ting-Chu , Sasabe Takashi , Hirai Shuichiro 
Published: February 2020

Tiny optical fiber sensor was applied to realize non-destructive measurement of oxygen concentration on PEFC catalyst layer.
Insufficient supply of oxygen to the cathode catalyst layer caused by transport resistance is critical issue. Fundamental study in order to achieve efficient oxygen transport is required, however; non-destructive and in-situ measurement of oxygen concentration on the catalyst layer was not achieved. In this study, oxygen concentration on the catalyst layer under operating condition was measured by using a tiny optical fiber and oxygen indicator.
Figure 1 shows experimental setup. Platinum tetrakis pentrafluoropheny porphine (PtTFPP) was employed as oxygen indicator[1]. When the PtTFPP is exposed to excitation light (l = 405 nm), phosphorescence emission (l = 650 nm) is produced. Phosphorescence intensity is determined by oxygen partial pressure. Thus, quantitative oxygen concentration can be obtained by measuring phosphorescence intensity and using calibration data. In this study, PtTFPP was painted at the edge of fused silica optical fiber of which diameter was 110 mm. In order to measure the phosphorescence intensity, bifurcated optical fibers were connected to excitation light source and spectrometer, respectively.
The cell has an active area of 4 cm2 (2 × 2 cm) with straight channels. The channel width and depth were 1.0 and 1.0 mm, respectively, and the rib-to-channel ratio was 1. The carbon paper GDL without MPL (SIGRACET®24BA, SGL Group, USA) was used for both the anode and the cathode side. In order to measure the oxygen concentration on the catalyst layer with reducing insertion effect, optical fiber was inserted from the intrinsic large pore of GDL without boring a hole, and the edge of the fiber was touched on the catalyst layer surface. Flow rates of hydrogen and air were 50NmL/min and 125 NmL/min, respectively. Cell was operated under 33 °C because phosphorescence intensity becomes low under high temperature. Bubbler temperature was 22 °C (relative humidity was 50%)
In the experiment, cell voltage was varied from 0.9 V to 0.2 V, and change of current density and oxygen concentration were measured. Figure 2 shows current-voltage (IV) characteristics. In the higher current density condition, variation of plots suggests emergence of flooding in the cathode. Since the flooding decreased gas diffusivity in the cathode catalyst layer, IV characteristics fluctuate at high current density and low cell voltage condition. As shown in Figure 3, change of oxygen concentration on the catalyst layer was measured successfully. Oxygen concentration decreases monotonically with decreasing cell voltage. Minimum value of the concentration was below 2 % at 0.2 V cell voltage.
Figure 2 indicated that total cell performance was declined by flooding. On the other hand, local oxygen concentration of the catalyst layer was able to be measured by using a tiny optical fiber. Figure 3 shows large amount of oxygen was still consumed under high current density and it means that power generation concentrated on local surface of the catalyst layer where surface not covered with the liquid water.
As discussed above, oxygen concentration on cathode catalyst layer was measured non-destructively. This technique holds further potential to measure oxygen transport phenomena on the catalyst layer under 60-80 °C cell operation by improving optical oxygen sensor sensitivity.

Acknowledgement
This study is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Experimental setup in present study was assisted by Yoshihiko Aoki (Tokyo Institute of Technology).

Reference
[1] CS Chu., CA Lin, Sensors and Actuators B: Chemica, 2014, 195, 259-265.

Figure 1