Research for durability improvement of PEMFCs membrane-electrode assembly by interface engineering

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) have been getting much attention as a promising eco-friend energy device for various field such like vehicles, drones and back-up generator. Although the technology of PEMFCs has achieved great development in terms of power performance, however, durability issues still remain as a hurdle for commercialization. This thesis addresses durability of membrane-electrode assembly (MEA) in respect of mechanical and chemical degradation. First, the mechanical durability of hydrocarbon (HC) membrane was improved by external reinforcement achieved by a 3-dimensional interlocked interfacial layer (3D IIL) which forms strong adhesion between the HC membrane and gas diffusion electrode (GDE). The use of 3D IIL dramatically enhanced interfacial adhesion be-tween HC membrane and Nafion ionomer layer for 207-fold. The HC membrane tightly connected to GDE through 3D IIL showed restricted expansion by hot water owing to external reinforcement effect, and showed 1.9 times higher mechanical durability in MEA without any performance loss than that of HC membrane in conventional flat interface MEA. Second, chemical degradation of MEA by $Ce^{4+}$ ion was studied. Although Ce-based materials have been widely used as a radical scavenger to enhance chemical durability of the polymer electrolyte membrane, the sig-nificant performance drop after long term operation was often observed in the MEA containing Ce-based materials. However, the reasons of performance decay have not been studied deeply. Herein, the detrimental effect of $Ce^{4+}$ on Pt/C catalyst was proposed. Based on rotating disk electrode test, electrochemical active area (ECSA) and ORR activity of Pt/C were significantly reduced after contact with $Ce^{4+}$ ion. The oxidation of Pt and carbon by $Ce^{4+}$ ion was also confirmed by ex-situ analysis such like X-ray photoelectron spectroscopy, ultraviolet-visible spectros-copy and inductively coupled plasma-mass spectrometer. Furthermore, at the single cell level, it was also observed that $Ce^{4+}$ dissolved from $CeO_2$ causes significant performance due to ECSA loss and charge transfer resistance increasement. Finally, carbon nitride protected $CeO_2$ particle to restrict $Ce^{4+}$ ion dissolution and prevent catalyst degra-dation was suggested. By introducing carbon nitride protective layer on $CeO_2$, Ce ion dissolution rate is validly reduced to 5- to 11- fold. The MEA containing carbon nitride protected $CeO_2$ exhibited excellent open sircuit voltage (OCV) durability for 600 hr, and the performance is nearly identical even after 600 hr OCV holding, whereas the MEA containing unprotected $CeO_2$ shows significant performance decay with catalyst degradation by dissolved $Ce^{4+}$.