Studies On The Design Of Components And Durability Of CCM For PEMFC

Catalyst Coated Membrane (CCM), as the key part of the Proton Exchange Membrane Fuel Cell (PEMFC), is mainly consisted of proton exchange membrane and two catalyst layers. Preparation and optimization of CCM are the pivotal techniques for PEMFC. The CCM's structure and fabrication technology not only determine the performance of PEMFC, but also affect on reducing cost, improving power density and energy density, and accelerating their commercialization. And the durability of Catalyst Coated Membrane (CCM) is another most critical that issues in the process of commercialization for Proton Exchange Membrane Fuel Cell (PEMFC), which is also a highly challenging work in current fuel cell research. Changing components of the catalyst lays are used in this thesis, which is propitious to improve the structure of CCM. And accelerated tests in simulated fuel cell circumstance are used in this study. The advantage of the acceleration test is not necessarily confined in the operated cell work conditions, costing less money and made it possible to study one special factor for the durability. The complexity lies in that one should have a rich knowledge about the reaction mechanism in CCM so that accelerating experiments could be devised and provide the possibility for exploring the degradability mechanism of CCM properly and effectively.In the experiments of changing components of catalyst layer, the content of Nafion is variety, which the catalysts are 40% and 60% Pt/C in the first place. Then adding hydrophobic component in the hydrophilic electrode could prevent the flooding. At last, in the study of the durability of the whole CCM, we had designed four accelerating systems, H₂O, 10%H₂O₂30%H₂O₂ and H₂O₂/Fe²⁺ . In this research, conclusions had been summarized as followings:(1)For the 40% Pt/C catalyst, when the content of Nafion is 25%, the performance of CCM is the better, and the utilization of catalyst is the most superior. At the same time, the cell performance is also stable. On the another hand, for 60% Pt/C, when the content of Nafion is betweem 20%²5%, the performance of single cell are nearly approximations. But in the electrochemistry test process, the CCM that the content of Nafion is 20% is the betterone.(2)Adding hydrophobe into catalyst ink was beneficial to improve the hydrophobic nature, optimize the structure of catalyst layers for CCM and enhance stability of the cell performance. In thesis, the CCM adding 5% hydrophobe (PTFE) had the best performance while testing single cell. Adding more hydrophobe would depress the performance. However, it was found in the electrochemistry study that hydrophobe could be harm for the resistance of CCM. It was a disadvantage about the application of hydrophobe.(3)The accelerated test results show that the catalyst layer was more easily debonded from CCM surface, caused by the degradation of Nafion that was used as bonging agent as well as proton conductor. Then the catalyst layer failed off and the performance of CCM decreased. H₂O and 10%H₂O₂ accelerating systems were destroyed inapparently. But30%H₂O₂ and 30%H₂O₂/Fe²⁺ accelerating system made a different degradation cases for catalyst layers of CCM. The former made a degradation state in the similarity form of layer falling off, and the latter a crack form. In both of the accelerating systems, the attack occured on the CF₂ end group, however, the difference may lie with that H₂O₂ had no selectivity for the oxygen-contain CF₂ end group, while as H₂O₂/Fe²⁺ had a fierce selectivity for it. So it resulted in the difference of the degradation mechanism. The degradation of Nafion in H₂O₂ was uniform and gradual, but its degradation in H₂O₂/Fe²⁺ was nonuniform that due to the nonuniform distribution of oxygen-contain CF₂ end group. The degradation in H₂O₂/Fe²⁺ had made cracks appeared in the catalyst layer, then resulted in the falling the catalyst layer off and destroying the Nafion membrane inside.