Accelerated Tests Of Durability For CCM And Its Components

The durability of Catalyst Coated Membrane (CCM) is one of the most critical 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. Accelerated tests in simulated fuel cell cicumstance 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 prior of the study for the durability of the whole CCM, its components had been put into separated accelerating tests. After understanding the degradation process of Nafion membrane and Pt/C electrocatalyst, a better study could be made about CCM durability. In the study of separated components durability, we had both designed two accelerating systems, H₂O₂ and H₂O₂/Fe²⁺ for Nafion membrane , while H₂O₂ and H₂SO₄ for Pt/C electrocatalyst in terms of different conditions. In the study of the durability of the whole CCM, function of pore-forming agents and its effect for durability had been considered. In this research, conclusions had been summarized as followings:(1)30 % H₂O₂ and 30 % H₂O₂/Fe²⁺ accelerating system made a different degradation cases for Nafion membrane. 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 small holes and cracks appeared in the surface of Nafion membrane.(2) In accelerating experiments H₂O₂ produced a remarkable decrease in the agglomeration of Pt/C electrocatalyst, while as H₂SO₄ not. H₂O₂ also could make destroy to C support, but it did a little effect for Pt loss. H₂SO₄ accelerating systeminduced a comparatively large amount of Pt loss and it was proved of Pt°. In this accelerating condition, H2O2 did a negative effect to Pt [111], which was shown in the XRD patterns that the peak intensity decreased when the accelerating increased. However, its effect for the other crystal plane of Pt was not found. In addition, H2O2 could also introduce a decrease to the diffraction peak intensity of C support. The effect on the diffraction peak of Pt in H2SO4 was not distinctive.(3)Adding pore-forming agents into catalyst ink was beneficial to the increase of pore volumes, optimizing the pore structure of catalyst layers for CCM and improving the cell performance. However, it was found in the accelerating study that pore-forming agents could be harm for the durability of CCM. It was a disadvantage about the application of pore-forming agents. 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.