Abstract
Hydrogen bleeding into the cathode inlet of a proton exchange membrane (PEM) fuel cell could be a simple approach to reduce the H2 concentration in the fuel cell exhaust during transient operating conditions (e.g., start-up or fast transients) of a PEM fuel cell system; it could also serve as an additional heating source during cold start-up. In this experimental study, we address the question whether the chemical stability of the polymer electrolyte membrane is affected negatively by a hydrogen bleed into the cathode inlet of a PEM fuel cell. First, rotating ring disc electrode (RRDE) experiments were carried out to detect whether any additional H2O2 is produced during the oxygen reduction reaction in O2 saturated electrolytes in the absence and presence of H2. Dry open circuit voltage (OCV) experiments were then performed for more than 250 hours in 50 cm2 single cells at 120°C and 18% relative humidity (RH) in order to investigate the effect of a 4 vol. % H2-bleed into the cathode inlet on membrane stability. Finally, the distribution of membrane pin-holes was determined on membrane electrode assemblies (MEAs) after the dry OCV tests conducted with or without H2-bleed using an infrared (IR) camera setup. In addition, the diffusion-limited hydrogen oxidation current on the cathode side was modeled in order to estimate the maximum areal heat flux near the cathode inlet, which would be caused if the H2 oxidation rate were to be diffusion-limited.
Highlights
With the introduction of PEM based hydrogen fuel cell cars into the market, several government and state regulations have to be addressed, e.g., the global technical regulation on hydrogen and fuel cell vehicles which limits the exhaust gases of a fuel cell car.[1]
While H2-bleeding into the cathode feed clearly offers advantages for fuel cell system operation, it poses several questions regarding the long-term chemical and thermal stability of the membrane and the membrane electrode assembly (MEA), even though short-term degradation tests at normal operating conditions (≤ 100 h at 80◦C) did not indicate any problems:[3,4] Does H2-bleed into the cathode feed lead to enhanced H2O2 production, which could accelerate polymer membrane degradation, resulting in membrane pin-holes and premature MEA failure? Does the injected H2 react homogeneously across the entire active area or does it react locally near the inlet region, in which case local hot-spots could lead to accelerated polymer membrane degradation?
Hydrogen peroxide formation in hydrogen/oxygen atmosphere.—To detect whether any additional hydrogen peroxide is produced during the oxygen reduction reaction (ORR) due to H2-bleed into oxygen, rotating ring disk electrode (RRDE) experiments were performed
Summary
With the introduction of PEM (proton exchange membrane) based hydrogen fuel cell cars into the market, several government and state regulations have to be addressed, e.g., the global technical regulation on hydrogen and fuel cell vehicles which limits the exhaust gases ( non-consumed hydrogen) of a fuel cell car.[1]. Let us first examine MEA-6 (reference case without H2-bleed into the cathode inlet), for which the ex-situ leak test was conducted both before (at ≈170 h) and after (at ≈250 h) the accelerated increase in the OCV drop was noted: a leak rate above the permeation rate through a pin-hole free membrane was only observed once an acceleration of the OCV drop was detected, confirming the validity of the in-situ measurement.
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