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The effect of high air relative humidity (RH) cycling (RHC62%/100%) on the degradation mechanisms of a single (5 × 5 cm2) proton exchange membrane fuel cells was investigated. The cell performance was compared to a cell operated at constant humidification (RHC= 62%). Runs were conducted over approximately 1,500 h at 0.3 A cm–2. The overall loss in cell performance for the high RH cycling test was 12 μV h–1whereas it was at 3 μV h–1under constant humidification. Impedance spectroscopy reveals that the ohmic and charge transfer resistances were little modified in both runs. H2crossover measurement indicated that both high RH cycling and constant RH test did not promote serious effect on gas permeability. The electroactive surface loss for anode and cathode during high air RH cycling was more significant than at constant RH operation. The water uptake determined by 1H nuclear magnetic resonance within the membrane electrode assembly (MEA) after high RH cycling was reduced by 12% in comparison with a fresh MEA. Transmission electron microscopy showed bubbles and pinholes formation in the membrane, catalyst particles agglomeration (also observed by X‐ray diffraction), catalyst particles migration in the membrane and thickness reduction of the catalytic layers. Scanning electron microscopy was conducted to observe the changes in morphology of gas diffusion layers after the runs.

The effect of low relative humidity (RH) cycling (RHC62%/0%) on the degradation mechanisms of a single proton exchange membrane fuel cell (5 × 5 cm2) was investigated and compared to a cell operated at constant humidification (RHC= 62%). The overall cell performance loss was near 33 μV h–1, which is greater than the voltage decay under constant RH condition near 3 μV h–1. The electroactive surface was reduced but to an acceptable level. Impedance spectroscopy revealed that the ohmic and charge transfer resistances were reduced by the likely improved hydration of the ionomeric layer at the catalyst due to hydrogen crossover. This was so important that H2starvation was finally responsible for the collapse of the cell after 650 h. Transmission electron microscopy showed occurrence of various phenomena, e.g., bubbles and pinholes formation in the membrane due to local overheat from hydrogen combustion at the cathode, and thickness reduction of catalytic layers. The water up take obtained by 1H NMR within the membrane electrode assembly (MEA) after low RH cycling reduced by 24% compared to a fresh MEA. Observations are also compared to those obtained at high RH cycling (RHC62%/100%) presented in Part I of this study [1].

•Fuel cell directly hybridized with supercapacitor is realistic for FC hybrid vehicle.•Direct hybridization is not detrimental to the FC durability.•Direct hybridization allows to operate with reduced minimum gas flow rate.•Reduced minimum gas flow rate is only sustainable in direct hybridization mode.•Direct hybridization allows to decrease hydrogen waste.