2-, and 3-Dimensional Monitoring of the Oxygen Partial Pressure within a Polymer Electrolyte Fuel Cell during Power Generation at Temperatures Higher Than 80 o C
Authors: Schreiber Christopher Leon , Kapulwa Anna , Inukai Junji 
Published: December 2024

Introduction:

Despite the high potential and expected major impact of PEFCs on the economy of the future, there are still challenges to overcome, before commercialized. PEFCs operated at temperatures higher than 100°C are expected to be used especially in the heavy-duty vehicle department due to their use of smaller and lighter radiators and lower susceptibility to catalyst poisoning. To achieve this goal however, PEFCs operated above 100°C need higher performance, stability, and durability, while reducing the cost at the same time. The performance, durability and stability of a fuel cell are related to the distribution of physical and chemical parameters, e.g. oxygen partial pressure (p(O2)). During the operation of the fuel cell, the distribution of those parameters is inhomogeneous.[1] Therefore, we used an in-house developed 2-dimensional non-destructive real-time/space visualization system to achieve an understanding of the inside the fuel cell during operation at higher temperatures.

Experimental:

The PEFCs used for this experiment have an active area of 4 cm2 with 10 straight gas flow channels. To visualize the oxygen partial pressure, an oxygen sensitive dye (PtTFPP) was used. PtTFPP has absorption peaks at 407, 530 and 540 nm and an emission peak at 650 nm. The emission peak is quenched by oxygen partial pressure, and the emission intensity decreases monotonically with increasing oxygen partial pressure (Fig.1). The oxygen sensitive dye was applied on the surface of the cathode side GDL. For the excitation, a laser with a wavelength of 532 nm was used and the emitted light was captured by a CCD camera.[2] The laser light was first diffused and then guided by mirrors into the cell. In order for the laser light to enter the fuel cell the cathode side endplate, which was usually made of metal, was exchanged with a transparent quartz glass endplate. Visualizations were carried out at 90, 100, and 110 oC at a constant water vapor pressure of 37.97 kPa, which equaled 53.6, 36.8, and 25.9% RH, respectively. Prior to the visualization, calibrations at 15, 18, 21, and 24% O2 concentration, and performance tests in form of IV and CV were carried out for each temperature. The gas flow rate during the experiments was set to 100 mL min-1 Air/H2 (parallel flow) at cathode and anode, respectively. For the oxygen concentration of 24% diluted (with N2) oxygen with a total flowrate of 100 mL min-1 was used.

Results and Discussion:

The IV performance tests showed decreasing cell performance with increasing temperatures, which was expected because of the increasing resistance due to drying out of the membrane. Furthermore, a decrease of the ECSA was observed with increasing temperatures. The 2D-Visualization showed that oxygen partial pressure on the surface of the GDL beneath the gas flow channels was much higher than theoretically calculated. Furthermore, at lower current densities the oxygen partial pressure was higher in the outlet than at the inlet of the cell, which was unexpected (Fig. 2). It was previously shown (Kakizawa et al.) that liquid water accumulates near the outlet of the cell. The accumulated liquid water as well as the water vapor at elevated temperatures is expected to hinder the diffusion of O2´to the catalyst layer which could explain the increased oxygen partial pressure near the outlet and the decreased oxygen partial pressure near the inlet. However, power is generated, and oxygen was consumed elsewhere in the cell, namely inside the GDL close to the catalyst layer. This model will be researched using a 3-dimensional visualization system in the future.

[1] Y. Kakizawa, C. L. Schreiber, S. Takamuku, M. Uchida, A. Iiyama, J. Inukai, Visualization of the oxygen partial pressure in a proton exchange membrane fuel cell during cell operation with low oxygen concentrations, J. Power Sources, 483, 229193 (2021).

[2] Y. Kakizawa, T. Kobayashi, M. Uchida, T. Ohno, T. Suga, M. Teranishi, M. Yoneda, T. Saiki, H. Nishide, M. Watanabe, A. Iiyama, J. Inukai, Oscillation mechanism in polymer electrolyte membrane fuel cell studied by operando monitoring of oxygen partial pressure using optical probes, J. Surf. Finish. Soc. Jpn, 72, 230 (2021).

Figure 1