The impact of ionomer type on the morphological and microstructural degradations of proton exchange membrane fuel cell electrodes under freeze-thaw cycles

摘要

Recent studies indicate that short-side-chain (SSC) ionomers in the proton exchange membrane (PEM) fuel cell electrodes substantially improve the cell performance and durability. In this study, SSC ionomer-based electrodes of different platinum (Pt) loadings are investigated when subjected to freeze-thaw (F-T) cycles between 30 degrees C and -40 degrees C, and compared with the conventional long-side-chain (LSC) ionomer-based electrodes. It is shown that the degradation patterns are similar for a given type of electrode, but different for the two types of electrodes, and independent of Pt loading. For the SSC ionomer-based electrodes, degradation occurs initially through ionomer swelling and pore expansion, and proceeds through detachment of large-scale catalyst layer (CL) flakes together with microporous layer (MPL) sheets, and ends with highly corroded morphology and microstructure. In comparison, the LSC ionomer-based electrodes degrade due to ionomer swelling and pore expansion in the initial 15 cycles, and then pore expansion becomes the main mechanism controlling the degradations. The high Pt loading LSC ionomer-based electrode degrades through simple detachment of small-scale CL flakes without damaging the MPL substantially, whereas the low Pt loading one degrades through surface corrosion, along with severe MPL degradation. Independent of the ionomer type, the Pt loading does not impact the degradation mechanism, but it does certainly affect the morphology and microstructure achieved after the same cycling period.