• Andersen, Shuang Ma
• Sieborg, Bertil
• Larsen, Mikkel Juul
• Morgen, Per
• Grahl-Madsen, Laila
• Chiriaev, Serguei

Abstract

Here, we report a study on the structural characteristics of the membrane electrode assembly (MEA) samples obtained from a low-temperature polymer electrolyte membrane fuel cell (LT-PEMFC) stack subjected to a long-term durability study by operating for ~18000 h under real-life conditions. The chemical and physical state of the degraded MEAs obtained from industrial partner were investigated through structural characterizations aiming to probe their different components, namely the cathode and anode electrocatalysts, the Nafion® ionomer in the catalyst layer, the gas diffusion layers (GDLs) and the PEM electrolyte. X-ray diffraction and electron microscopy studies suggested no significant degradation of the electrocatalysts. Similarly, the cathode and anode GDLs exhibited no significant change in structure and porosity under Helium Ion Microscopy (HIM) observation. Size exclusion chromatography (SEC) and thermogravimetry (TG) confirmed macrostructure integrity of the polymer electrolyte.<br/>Nevertheless, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectroscopy (XRF) and elemental analysts through CHN analyzer suggested significant degradation of the sulfonic group, especially in terms of sulfur content, which indicates a considerable reduction of proton conductivity. As shown in Figure 1, a sample taken from the electrode catalyst layer after MEA delamination, the relative intensities of the peak corresponding to the –SO3- group w.r.t. the –O–CF2– group are considerably lower for spent electrodes compared to the pristine ones.<br/>Hence, degradation of the Nafion®, especially in form of ionomer contributing the triple-phase-boundary (TPB) in the catalyst layer was found to be the principal cause for the performance degradation, while the Pt/C catalyst degradation in terms of particle size enlargement, dissolution or change of crystallinity was minimal. The study suggests that under real-life operating conditions, the ionomer degradation plays a significant role over the electrocatalyst degradation in low temperature PEMFCs. Mitigation of the ionomer degradation must be emphasized as a strategy to strengthen the PEMFC durability.

Topics

• polymer
• phase
• x-ray diffraction
• x-ray photoelectron spectroscopy
• thermogravimetry
• electron microscopy
• porosity
• durability
• size-exclusion chromatography
• crystallinity
• fluorescence spectroscopy
• X-ray fluorescence spectroscopy