Operando 3D-Monitoring of the Oxygen Partial Pressure (p(O2)) within a Polymer Electrolyte Membrane Fuel Cell (PEMFC) during Cell Operations at Higher Temperatures

Abstract

Introduction:Hydrogen technologies, especially the Proton Exchange Membrane Fuel Cell (PEMFC), are promising alternative power sources, due to their low to non-carbon emission, high power density, high efficiency, and fast start-up. Despite their high potential and expected major impact on the economy of the future, there are still challenges to overcome, before PEMFCs can be globally commercialized. Soon, PEMFCs operated above 100°C are expected to be used for automobiles, due to their use of smaller and lighter radiators, higher reaction rates and lower susceptibility to catalyst poisoning. To achieve this goal however, high-temperature PEMFCs need a 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, like 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 3-dimensional non-destructive real-time/space visualization system to achieve an understanding of the p(O2) inside the fuel cell during operation at higher temperatures.Experimental:The PEMFC used for this experiment has an active area of 4 cm2 and uses 10 straight gas flow channels. Prior to the assembling, 5 pinholes with a diameter of 90 µm were manufactured into the GDL on the cathode side both underneath the rib and the gas flow channel (Fig. 1). Those pinholes are also manufactured into the current collector and insulator of the PEMFC on cathode side. The holes are used to insert optical fibers from outside the cell to approximately 20 µm (adjustable with an accuracy of 1 µm) above the catalyst layer of the CCM. Prior to the use, the fibers with a core diameter of 10 µm are etched in HF solution to reduce their clad diameter from 125 to 50 µm. Furthermore, the apex of the fiber is cleaved perpendicular to the axis of the fiber. Before insertion an oxygen sensitive dye (PtTFPP, (Fig. 2)) is applied to the apex of the fibers. 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. For the excitation, a laser with a wavelength of 532 nm is used and the emitted light is captured by a CCD camera.[2] Visualizations were carried out at 80, 90, 100, 110 and 120 oC at a constant water vapor pressure of 37.97 kPa, which equals 80, 53.6, 36.8, 25,9 and 18.5% RH respectively, at increasing current densities. Prior to the visualization, calibrations, 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 Air/H2 (parallel flow) at cathode and anode respectively. 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, which could also be due to the drying out of the membrane as well as the binding material around the catalyst nanoparticles which reduces gas diffusion and the effective catalyst surface area. In this first try, the monitoring was done with 3 fibers under one rib located near the inlet, the middle of the cell and near the outlet of the cell. With increasing current density, the oxygen partial pressure was decreasing at every temperature. Interestingly, the oxygen partial pressure distribution changed with increasing temperature. The reason for the changing oxygen partial pressure distribution could be due to accumulation of heat (Fig. 3) and increasing water vapor pressure at higher temperatures. Further experiments with 5 fibers including measurements in the gas flow channel as well as measurements with back pressure to increase the cell performance will be done shortly to confirm this finding.[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