Electrocatalyst Stability under Dynamic and Stationary Operation of Fuel Cells

Kurzfassung

Durability and cost are two barriers recognized for the extensive application of proton exchange membrane (PEM) fuel cell as a promising clean energy technology. To date, considerable effort has been made to study the performance and component degradation via diverse methods and for accelerated degradation, routines have been developed. For instance, operation at open circuit voltage (OCV) is one of the most frequently employed high potential stressors for a PEM fuel cells leading to Pt electrocatalyst dissolution and membrane decomposition. Single cells as well as short stacks were aged under different operation conditions which are discussed in the literature to accelerate degradation of electrocatalyst and membranes. An example of the aging of a commercial MEA under OCV for 1600 h leads to significant performance loss and membrane thinning. The loss of electrochemical surface area is evident from the impedance and the CVs. Also a decrease of membrane resistance is observed. For further characterization of aging inner interfaces and cross section of MEAs were investigated with material sensitive PeakForce Quantitative Nanomechanical Property Mapping (QNM™). Platinum particles could be unequivocally identified at the inner interfaces and cross sections. With scanning electron microscopy (SEM) and energy dispersive element analysis (EDX) of cross sections platinum bands at the cathode, but in some cases also at the anode side, were found as well as a significant platinum content within the whole membrane. The influence of platinum inside the membrane on the membrane degradation is discussed. Also the different aging mechanisms associated to the operation conditions are elaborated. A general conclusion from these measurement is that Pt dissolution plays an important – perhaps even the crucial – role for the loss of electrochemical surface area and membrane decomposition. To demonstrate this aspect, membranes were first impregnated with Pt ions which were reduced and the fluoride emission rate was measured.