Study On The Influence Of Super-cooled Water On PEFC Sub-zero Start-up Capability And Characteristics

Polymer electrolyte fuel cells(PEFCs),as vehicle power source,must withstand the test of complex operation conditions and harsh environment.Among these,the sub-zero start-up capability constitutes one of the main obstacles to the diffusion of fuel cell vehicles in cold regions.Utilizing the super-cooled water is expected to further enhance the PEFCs sub-zero start-up capability.In this thesis,focused on the super-cooled water,the necessity of utilizing the super-cooled water are analyzed through an estimation model.Furthermore,the neutrons imaging and electrochemical testing methods are introduced to characterize the icing process of super-cooled water and reveal the effects of ice formation on the fuel cell sub-zero start-up performance.In the study of estimating the sub-zero start-up capability,the relationship between the water storage capacity,the heat production,the fuel cell thermal mass and the temperature rise during sub-zero start-up process is analyzed through the estimation model.In the model,the maximum temperature rise of two types of fuel cell stacks,one with metallic bipolar plate and one with graphite bipolar plate,are calculated in the cases of with and without the presence of the super-cooled water.The calculation results point out the necessity of utilizing the super-cooled water to reach the target of starting-up successfully from-30°C for the fuel cell stacks with graphite bipolar plate.In the study of utilizing neutrons imaging to visualize the icing process of supercooled water,a specially designed neutrons speed selector is used and the neutrons with specific wavelength are obtained.Adopting the dual spectrum neutrons radiography,the super-cooled water and ice can be distinguished and the contrast between the super-cooled and ice can reach 6% which is three times that of the previous research result 1.6%.The improvement of the contrast makes this method more reliable and increases the application prospects of neutrons imaging on distinguishing the super-cooled water and ice in fuel cells.The visualization results point out that much super-cooled water can move into flow channels during the sub-zero start-up process and the icing process of super-cooled water directly causes the failure of start-up of PEFCs.In the study of using electrochemical methods to explore the influence of ice formation on the fuel cell sub-zero start-up performance,firstly the PEFCs output performance testing is carried out under different operation conditions and the cumulative water production and the water storage capacity are compared to confirm the effectiveness of super-cooled water movement in expanding the water storage capacity.Moreover,based on the output performance results,the random characteristics of subzero start-up process caused by super-cooled water are also analyzed.Secondly,the influence of icing process on the fuel cell resistances and electrochemical active surface area is explored by dynamic electrochemical impedance spectroscopy(DEIS)and cyclic voltammetry(CV)testing experiments.It is found that ice formation causes the sharp increase of mass transport resistance but has no obvious influence on the electrochemical active surface area.The research work in this thesis will provide the guidance for utilizing the supercooled water and further improving the PEFCs sub-zero start-up capability.