In-Situ Measurement of Oxygen Particial Pressure on the Surface of a Cathode GDL with Automotive Compatible Operating Conditions

Abstract

Improvement of the area specific power density of a proton exchange membrane fuel cell (PEMFC) can reduce the total active area of the membrane electrode assembly (MEA). Reducing the total active area of the MEA is a significant opportunity to reduce the cost of a fuel cell because it can reduce the amount of the expensive materials in the MEA, including platinum based electrocatalysts, polymer electrolyte membranes, and gas diffusion media. Mass transport overpotential at high current densities is the major barrier to achieving high area specific power density. Oxygen partial pressure in the cathode is highly linked to the mass transport performance. Therefore, direct measurement of the oxygen partial pressure at the cathode is required for mass transport studies. A novel method has been developed to directly measure the oxygen partial pressure in the cathode flow field during fuel cell operation. This method uses fluorophore materials with fluorescent luminescence that is sensitive to oxygen quenching as described by the Stern-Volmer equation. The measurement range of oxygen partial pressure is dependent on the sensitivity of oxygen quenching of the fluorophore materials. Oxygen quenching rate should be modified to meet a desired range of oxygen partial pressure measurement. In the previous study, the fluorophore material with a platinum porphyrin based fluorophore, which is highly sensitive to oxygen quenching, dispersed in the polymer binder of poly-heptafluoro-n-butly, methacrylate-co-hexafluroisoporopyl, and methacrylate, a.k.a. FIB polymer, was developed. This material showed a sufficient level of intensity of fluorescent luminescence for 7 kPa to 53 kPa oxygen partial pressure. The detected oxygen partial pressure was calibrated within the acceptable temperature sensitivity, between 40°C to 90°C. This range covers the typical operating condition of an automotive PEMFC. The ink of this fluorophore material was sprayed on the surface of the cathode flow field. An in-situ measurement of the oxygen partial pressure at the cathode flow field was demonstrated to investigate the effectiveness of water management in the cathode flow field. However, this fluorophore ink material was too thick to use on the surface of the cathode GDL, because the ink materials clogged the pores of the GDL and lowered the fuel cell performance. In this study, this fluorophore ink material, platinum porphyrin based fluorophore in FIB polymer, was modified. A newly developed thinner fluorophore ink material was sprayed on the surface of the cathode GDL and it was confirmed that the fuel cell performance was not affected by the sprayed ink materials. Concurrently, it was confirmed that the oxygen quenching rate of this fluorophore materials were retained. The Stern-Volmer calibration indicates a small deviation of calibration curves between the fluorophore materials sprayed on the flat glass surface and the porous GDL surface. Numbers of bright spots were observed on the Stern-Volmer calibrated image of the GDL surface. These bright spots were considered to be corresponding to the shadow of the excitation light on the rough GDL surface. When two excitation light sources were used from different angles, the deviation was significantly mitigated. The fluorophore ink modification in this study enabled accurate in-situ oxygen partial pressure measurements on the surface of the cathode GDL to cover the typical operating conditions of an automotive PEMFC. Combined with the previously developed method of oxygen partial pressure on the flow field wall, further analysis was enabled. A significant oxygen partial pressure difference is observed between on the top ceiling of the flow field channel and on the surface of GDL. It was considered that the oxygen concentration in the cathode flow field exhibits a clear gradient from the top wall of the channel towards the GDL surface. This suggests that oxygen was drawn to the GDL surface in a laminar flow scenario in the fuel cell cathode. Detailed analysis will be discussed at the ECS meeting.