Stabilization And Durability Improvement Of Proton Exchange Membrane Fuel Cell Catalyst And Membrane Electrode Assembly

Proton exchange membrane fuel cell(PEMFC)has many advantages,such as high efficiency of energy conversion,zero pollution and quick start-up in low temperature.It can be broadly applied in the military and civilian fields,including transportation,portable power supplies,power stations,aerospace and underwater submarines.Thus,it gains more and more attentions from governments and research teams.In recent years,PEMFC has been greatly developed,but it still needs to overcome the challenges of cost and durability,in order to realize the large-scale commercialization.The main reason for the high cost of PEMFC is the use of expensive precious metal catalysts.In addition,the working conditions of PEMFC are harsh,involving high acidity,high temperature,high humidity and high voltage,etc.These will lead to the corrosion and aging of catalyst,which brings in severe problems to the performance and durability of PEMFC.Improving the durability of fuel cell has become a critical research topic in the field of fuel cell.A large number of studies show that the degradation of the catalyst in the membrane electrode assembly(MEA)and the deterioration of the catalyst layer are the main reasons leading to the poor durability of PEMFCs.In order to solve these problems,this article adopts the method of the oxide decoration to stabilize the Pt/C catalyst.We have mixed graphene or nitrogen-doped graphene with the oxide,to increase the electrical conductivity of oxide.By integrating the as-prepared catalysts into the catalyst layer and optimizing the composition of catalyst layer,we succeeded in improving the performance and durability of the MEA.The main research works and achievements are as following:First of all,the research work of using silica oxide coated commercial Pt/C catalyst to improve its stability was carried out.Based on this,we added graphene into the precursor solution of silicon oxide.The results show that:Silicon oxide and graphene coating can reduce the degradation of catalyst performance due to the stabilization effect of silica-graphene mixture against the aging of Pt/C catalysts.When the coating amount of silicon oxide is 10 wt.%and the mass ratio of graphene to oxide was 0.02,the MEA performance of catalyst Pt/C@10Si-0.02G was the best,and the current density was 809 m A/cm~2at 0.7 V.The current density at 0.6 V is 1302m A/cm~2,and the maximum power density was 875W/cm~2.Electrochemical impedance tests on the MEA showed that the addition of graphene could slightly improve the conductivity of the catalyst.According to the forced aging method recommended by the Department of Energy of the United States,after the accelerated aging of the MEA with 10000 and 30000 voltage cycles,the voltage of the commercial Pt/C membrane electrode at 0.8 A/cm~2was reduced by 46 m V and 93 m V,respectively.However,after accelerated aging of Pt/C@10Si-0.02G,the voltage attenuation of single cell at 0.8 A/cm~2is obviously less than Pt/C,and decreased by18 m V and 34 m V after 10000 and 30000 cycles,respectively.However,the attenuation of Pt/C only coated by oxide was 26 m V and 50 m V,respectively.These results indicated that the Pt/C catalyst coated with the mixture can effectively improve the stability of the MEA,and the addition of graphene can further improve the stabilization effect of the catalyst.According to TEM and single-cell in-situ CV test results,the stability improvement might be mainly due to the oxide-graphene coating alleviating the migration and aggregation of platinum nanoparticles in the catalyst and reducing the loss of ECSA.Additionally,we carried out the research work on the mixed coating of commercial Pt/C catalyst with silicon oxide-nitrogen doped graphene,and found that the addition of a small amount of nitrogen doped graphene could effectively compensate the oxygen reduction reaction(ORR)performance of the catalyst due to silicon oxide coating.On this basis,we investigated the effect of adding appropriate amount of fluorocarbons(FC)to the MEA performance and durability of the cathode catalyst layer using silicon oxide-nitrogen doped graphene coated catalyst.Electrochemical characterization of the catalyst showed that when the coating amount of silicon oxide was 10%and the mass ratio of nitrogen-doped graphene to oxide was0.04,the ORR performance of catalyst Pt/C@10Si-0.04NG was the best.Single cell test showed that the MEA performance was the best when Pt/C@10Si-0.04NG was used as the cathode catalyst,and the adding amount of FC is 10%of the amount of cathode catalyst layer,the MEA current density was about 962 m A/cm~2and 1474m A/cm~2at 0.7 V and 0.6 V,respectively,the maximum power density was923m W/cm~2.The results of contact angle measurement and high-current electrochemical impedance spectroscopy showed that the addition of FC improves the hydrophobicity of the cathode catalyst layer,which might contribute to the water management in cathode catalyst layer,then eventually improved the MEA performance.After 30000 voltage cycles,voltage of Pt/C@10Si-0.04NG-10FC reduced 46m V when the current density is 0.8A/cm~2,while the voltage of Pt/C reduced 93m V.This indicated that the durability of Pt/C@10Si-0.04NG-10FC had been greatly improved compared with that of Pt/C.