Effects Of Mechanical Vibration On Cold Start Performance Of Proton Exchange Membrane Fuel Cells

Fuel cell has been regarded as one of the most promising energy conversion devices in the 21 st century owing to its high-power density,fast response,low operating temperature and virtually zero emissions.The main application areas are transportation,portable uses,and stationary installations,among which,automotive and aerospace industries are the main markets for commercialization of proton exchange membrane(PEM)fuel cells.In the actual application process,extreme environmental conditions such as low-humidity,low-temperature,mechanical vibration impose severe requirements on the reliability of fuel cell.Therefore,it is essential to characterize the performance of fuel cells under low temperature environment and mechanical vibration condition.In this study,performance test and electrochemical impedance spectroscopy(EIS)are used to investigate the effect of operating condition,including temperature,humidity and mechanical vibration on the transport characteristics and performance.The work can be summarized as follows:(1)PEM fuel cell test bench is set up.At normal condition,polarization curve and EIS are carried out.The results show that increasing the humidity of inlet gas and temperature of fuel cell can effectively improve the cell performance due to low ohmic resistance and high electrochemical reaction rate.Moreover,electrochemical active surface area(ECSA)is estimated under different humidity to explain the degradation of cell performance in terms of microstructure.(2)In order to further understand the ice formation mechanism of PEM fuel cell at sub-zero temperature,various startup conditions,including initial water content,temperature,current density and voltage conditions are investigated to study the cold start behaviors of PEM fuel cell.For subzero temperature condition,lower startup temperature,worse cell performance.The lower initial water content,smaller current density and larger voltage can extend the water fill capacity to improve cold startup performance.Furthermore,it is found that the extremely low initial water content and high startup current density will aggravate the occurrence of voltage reversal causing performance degradation corresponding with decrement of ECSA.(3)At normal condition,effect of vibration is studied based on polarization curve and pressure drop.It can be seen that vibration can promote the water accumulation at the catalyst layer(CL)and micro-porous layer(MPL)interface corresponding to the rib,therefore promoting back diffusion from cathode to anode side.The higher pressure drop contributes to the fuel cell performance improvement at small current density.At large current density,vibration accelerates the process of small droplet gathering into large droplets,which is more likely to cause the formation of water film,therefore aggravating massive flooding and concentration loss.(4)At sub-zero temperature,cold start performance of PEMFC under vibration frequency and amplitude is measured.The results show that the disturbance of vibration makes the freezing position more dispersed,which is not conducive to the rapid formation of ice layer,delaying the freezing rate,especially at the natural frequency.Besides,the effect of vibration orientation on the cold start operation is carried out in this study.In the direction of gravity,that is,the direction of GDL plane,more ice may be stored under the rib area,and the water distribution is more uniform.In the direction of GDL thickness,vibration increases the probability of liquid water contacting the flow field plate,and accelerates the nucleation rate,leading to premature release of supercooled state.However,the effect of vibration of the gas flow direction is varies under different temperature conditions.At higher temperature,it is conducive to water drainage,while at lower temperature,the release of the supercooled state began in contact with the flow field.