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This work investigated the effect of cathode inlet relative humidity ͑RH͒ on the durability of proton exchange membrane fuel cells ͑PEMFCs͒ during startup-shutdown cycling via single-cell experiments. Electrochemical techniques, including measurements of polarization curves, electrochemical impedance spectroscopy ͑EIS͒, cyclic voltammetry ͑CV͒, and linear sweep voltammetry, were performed to examine the effect of cathode inlet RH on the degradation of PEMFCs. The performance was better for PEMFCs cycled at a lower cathode inlet RH than for those cycled at a higher cathode inlet RH on the order of 0 Ͼ 50 Ͼ 100%. The CV and EIS results showed that as the cathode inlet RH increased, the loss of electrochemically active surface area and the increase in the charge-transfer resistance ͑R ct ͒ were faster during the startup-shutdown cycling. However, changes in ohmic resistance ͑R ohm ͒ and hydrogen crossover current density were not detectable, revealing that severe membrane degradation did not occur regardless of the cathode inlet RH during startup-shutdown cycles.Polymer electrolyte membrane fuel cells ͑PEMFCs͒ continue to be considered as the most feasible power source, particularly for automotive applications, due to their high efficiency, relatively low operating temperature, fast startup, and low noxious emissions. Even with these significant advantages, which can have positive impacts in economic and environmental areas, there are several obstacles that still impede their successful commercialization. In addition to cost effectiveness, long-term durability is one key issue in fuel cell vehicles. According to the U.S. Department of Energy durability targets, an estimated 5000 h of continuous operation for fuel cell vehicle is required by 2015. 1 Therefore, optimization of the system operating conditions as well as the membrane electrode assembly ͑MEA͒ and its subcomponents must be implemented to improve performance and durability in fuel cell systems.It is commonly believed that operating conditions, including temperature, pressure, reactant stoichiometry, and gas relative humidity ͑RH͒, can influence cell performance. 2-4 Particularly, RH may play a significant role in the operation of PEMFCs, greatly influencing MEA performance. Many studies have been carried out to investigate the RH effect on PEMFCs. 3-8 Using X-ray diffraction, Borup et al. 4 showed that low RH decreased Pt particle growth but promoted corrosion of the carbon support after potential cycling. Xu et al. 3 investigated surface oxidation of the Pt cathode at different RHs via cyclic voltammetry ͑CV͒. They concluded that an increase in RH from 20 to 72% significantly facilitated the degree of Pt oxidation, which may influence the oxygen reduction kinetics. Recently, Xu et al. 6 also reported the RH effect on membrane degradation in PEMFCs at a constant voltage by showing a higher membrane degradation rate at 60% RH than at either 20 or 100% for given conditions. They further proposed the degradation mechanism of the membrane in terms of oxygen r...

The secondary electron emission coefficient γ of a MgO protective layer with various crystallinities has been successfully measured by the γ-focused ion beam system with complete elimination of the charge accumulation problem by scanning-area adjustment techniques. It is found that the (111) surface has the highest γ from 0.14 to 0.26 in comparison with the other films with (200) and (220) crystallinities for operating Ne+ ions, while ranged from 0.03 to 0.24 for Ar+ ions, under operating ion energies from 50 eV to 500 eV throughout this experiment. These observations explain why the (111) crystallinity of the MgO protective layer plays an important role in lowering the firing voltages in AC plasma display panel compared to the films with other crystallinities.

This paper uses a variety of physicochemical methods to elucidate the mechanism by which cathode inlet relative humidity (RH) degrades a proton exchange membrane fuel cell (PEMFC) during startup–shutdown cycling. The results revealed that the pronounced Pt coarsening (agglomeration)/oxidation/dissolution (detachment) and migration were observed along with corrosion of the carbon support at the majority of cathode catalyst layers when PEMFCs were exposed to a higher cathode inlet RH during 1500 startup–shutdown cycles. These changes contributed to the significant loss of Pt mass available for electrochemical reactions and an active Pt surface area at the cathode, thus decaying the performance of the fuel cell. In addition, the rate of these multiple processes was RH-dependent. However, the degradation at the anode and membrane was not as severe as those observed at the cathode and did not show any dependence on the cathode inlet RH. Based on these results, a modified mechanism for the degradation of the cathode catalyst layer during startup–shutdown cycling of PEMFCs is proposed to explain the effects of RH on fuel cell performance and durability.

Various polymer electrolyte membrane fuel cell (PEMFC) startup procedures were tested to explore possible techniques for reducing performance decay and improving durability during repeated startup–shutdown cycles. The effects of applying a dummy load, which prevents cell reversal by consuming the air at the cathode, on the degradation of a membrane electrode assembly were investigated via single-cell experiments. Electrochemical techniques, including measurement of polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry, were employed to investigate the degradation of the PEMFCs. The results showed that application of a dummy load during the startup procedure significantly reduced the performance decay, the decrease in the electrochemically active surface area, and the increase in charge-transfer resistance (Rct) , which resulted in a dramatic improvement in durability. Our results suggest that starting up PEMFCs while applying a dummy load is an effective method for mitigating performance degradation caused by cell reversal under a repetition of unprotected startup cycles.