Reaserches On Influences Of Dynamic Factors On The Pemfc Performance

The proton exchange membrane fuel cell（PEMFC）can directly transfer the chemical energy of fuel into electrical energy without proceeding of heat conversion,therefore,its energy conversion efficiency is high.In addition,PEMFC emits no environmental pollution.In recent years,as the fossil energy crisis and environmental degradation caused by burning fossil fuels are getting increasingly worse,the demand for sustainable energy which can replace the fossil energy becomes stronger.Fuel cells have attracted considerable attention because of their high energy conversion efficiencies and outstanding environmental friendliness.Meantime,the extensive use of green energy like PEMFC is also the futural goal of automotive power.It is critical for Fuel Cell Vehicles（FCV）to perform well in wide changeable conditions as well as bad environment,including different relative humidity（RH）,gas temperature,gas pressure,and so on.In this regard,a series of experimental reaserches as well as theorectical calculations on the influences of dynamic factors on PEMFC performance were performed in this thesis.The effects of Pt loading and operating backpressure on PEMFC Performance were analyzed.By combining theoretical calculations with experimental data,the effects of both the cathode Pt loading and operating backpressure on PEMFC performance were quantified in terms of the activation loss and ohmic loss as well as transport loss.The cathode Pt loading varies from 0.1 mg_Pt·cm^-2 to 0.4 mg_Pt·cm^-2,and the backpressure from 100kPa_abs to 200kPa_abs.Results indicate that under all conditions,the activation loss,ohmic loss and transport loss are all increased with the increase in the current density.However,when under the same backpressure condition,the transport loss of the PEMFC with a lower cathode Pt loading is more obvious than that with a higher cathode Pt loading.It is also proved that the increase in the operating backpressure will improve the cell performance,and the improvement for the PEMFC with a lower cathode Pt loading is bigger than that with a higher cathode Pt loading.In this regard,it is concluded that an appropriate increase in the operating backpressure will benefit the performance of low-Pt loading PEMFC.In addition,the mechanism of the as-obtained phenomena was discussed and it is expected that the corresponding conclusions will offer an efficient guide on the design and performance optimization of PEMFC with low or ultra-low Pt loading.The effects of the temperature and relative humidity on the PEMFC performance were investigated.By combining ohmic resistance（High Frequency Resistance,HFR）and electrochemical impedance spectroscopy（EIS）,the effects of temperature and relative humidity on the PEMFC performance were quantified.The operating temperature varies from 40oC to 85oC.Results indicate that the cell performs the best at about 80oC.The PEMFC performance will be improved obviously because both the electrochemical polarization（charge transfer）and the concentration polarization（mass transfer）decreased with the increase in the temperature from 40oC to 80oC.The effects of the cathode relative humidity and anode relative humidity（RH）on the PEMFC performance were respectively studied and the operating RH arranges from 0%to 100%.Results show that low relative humidity conditions leads to a lower ionic conductivity of the proton exchange membrane（Nafion^?membrane）in low current densities,and results in lower electrochemical active surface area.In high current densities,lower humidity conditions will lead to more serious concentration polarization and higher mass transfering resistance of O₂ in the Nafion layer.It is proved that the cell performance is more sensitive to the change of cathode RH than the change of anode RH.Furher more,appropriate RH is beneficial to weaken the ionomer membrane degradation.