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Summary: The mechanical degradation of the proton exchange membrane (PEM) is one of the main aspects affecting the lifetime of proton exchange membrane fuel cells (PEMFCs). It was observed in our previous study that the stress/strain distribution in the PEM of fuel cells with metallic bipolar plates (BPPs) is more complex, owing to manufacturing and assembly errors of the BPPs. The present study further concentrates on the stress/strain evolution in the membrane of fuel cells throughout the assembly and running processes by a finite element model. In membranes at the joint area between the gasket and gas diffusion layers, a serious stress concentration aggravated as the misalignment displacement increases. As for the membrane in reaction area, the plastic strain reaches highest level at the center of the groove after hygrothermal loading. The maximum stress is mainly relevant to the temperature and humidity and has little concern with the misalignment. The model and results of this study offer guidance regarding the design of PEMFC. Owing to the stress concentration, an additional protection should be set in the joint area, and the assembly error should be limited within 0.05 mm. [ABSTRACT FROM AUTHOR]

Abstract It is necessary to obtain internal heat and water distribution of proton exchange membrane fuel cells in order to perform water-heat management. Most of the existing works focus on the measurement in small experimental fuel cells, which cannot effectively guide the development of large-area fuel cells. In this study, an in-situ measurement method using micro-sensors is developed to observe the temperature and relative humidity in a commercial-size fuel cell with parallel flow fields and an active area of 250 cm2. A sensor array is incorporated into the cathode flow field, and well-designed waterproof and sealing structures protect sensors from liquid water. The sensors are verified with accuracy and response speed. The fuel cell performance and electrochemical impedance spectroscopy under co-flow and counter-flow configurations considering anode gas humidification are investigated. The results show that the temperature distribution is more uneven in the cases with lower output voltage, and the fuel cell performance is significantly affected by the humidity near the air inlet area. As current density increases, the relative humidity drops while dew point temperature keeps almost unchanged. The experimental method and analysis are beneficial to the understanding of the water-heat state and optimization of fuel cell stack design. Highlights • Measurement with micro-sensors for the commercial-size fuel cell is effective. • Uneven temperature and humidity distribution in the cathode flow field is obtained. • Performance and resistance are greatly related to temperature-humidity distribution. • Counter-flow without humidification gets good performance and water-heat balance. [ABSTRACT FROM AUTHOR]

Abstract Mechanical degradation, caused by local stress concentration and variation, significantly affects the lifetime of proton exchange membrane fuel cells. This study constitutes the first numerical investigation of stress evolution in the membrane between the frame of the membrane exchange assembly (MEA) and gas diffusion layer (GDL) throughout the processes of assembly, operation and gas filling in fuel cells. A finite element model is outlined to determine mechanical deformation of the membrane by exerting assembly displacement, hygrothermal conditions and gas pressure in turn. It is observed that severe stress concentration and bending deformations occur in the joint-area membrane. The results show that a plastic deformation occurs after the temperature and water content are increased, and would be substantially enhanced by the gas pressure difference between the anode and cathode. The in-plane stress may throw some light on the rapid degradation of the membrane between the frame and GDL. The gas pressure difference, which exceeds 10 kPa, leads to a rapid increase in the in-plane stress and plastic deformation. Decreasing the joint width may not be a good approach for reducing the stress/strain concentration. It is suggested that additional gasket seals or adhesive protection layers are helpful in joining frame and GDL. Highlights • Numerical model of fuel cells is developed to investigate membrane degradation. • Stress concentration and plastic deformation are caused in the joint-area membrane. • As much as 70% increase is found in the in-plane stress after gas filling. • Gas pressure difference exceeding 10 kPa leads to a rapid increase in deformation. • The maximum stress and plastic strain are obtained if joint width is 0.05–0.15 mm. [ABSTRACT FROM AUTHOR]

Air-cooled proton exchange membrane (PEM) fuel cells simplify fuel cell design by combining oxygen supply and air cooling in open cathode channels. Their performance is sensitive to the structure of cathode channels, which significantly affects distribution of temperature, relative humidity and mass transfer in the cells. This study offers a three-dimensional air-cooled fuel cell model with consideration of electrochemistry simulation to investigate the effect of cathode channel design. Experiments are conducted to validate the model. It is observed that obvious gradient in the distributions of temperature, humidity and oxygen concentration lies in the membrane exchange assembly (MEA) between the channel and rib owing to air dual functions in distributing oxygen and cooling the stack. For models with fixed rib-channel ratio of 1.0, the performance is better when channel width is smaller. Considering the effect of contact resistance when the ratio is small, rib-channel ratio within a reasonable range of around 3.0 is preferred in order to enhance the performance. Channels with curved features improve the mass transfer from channel to catalyst layer, thus increasing the cell performance. This study is helpful for enhancing our understanding of the relationship between cell performance and cathode channel design in the air-cooled fuel cell. • A validated three-dimensional model is built to investigate air-cooled fuel cells. • Larger channel width induces uneven distribution of RH and oxygen concentration. • Rib-channel ratio around 3.0 is preferred in order to enhance the performance. • RH rises with regular fluctuation along the direction of cathode channel. • Channels with curved features improve mass transfer and cell performance. [ABSTRACT FROM AUTHOR]