Deactivation of Fe-N-C catalysts during catalyst ink preparation process
Résumé
The membrane electrode assembly (MEA) is a core component of low-temperature fuel cells. The first step of MEA manufacturing is the preparation of a catalyst ink suspension in which the catalyst powder is homogeneously dispersed in a liquid solvent through mechanical or sonic agitation. In this work, we have studied the effects of catalyst dispersion in water or alcohol solutions and subsequent drying processes on the physicochemical properties of Fe-N-C catalysts and their electrocatalytic oxygen reduction activities. We find that dispersing the model Fe-N-C catalyst comprising only FeNxCy moieties in water and subsequent drying treatment change neither its bulk structure nor surface composition, as indicated by various spectroscopic measurements before and after treatment. However, zeta potential measurements, which are very sensitive to the chemistry of functionalities present on the carbon surface, reveal that the Fe-N-C catalyst becomes slightly more acidic, and that the change in their acido-basicity is magnified with a) increasing treatment temperature and b) repetitions of a same wetting/drying treatment. This small change in the surface acido-basicity of the Fe-N-C catalyst results in a measurable and reproducible decrease in its electrocatalytic activity, which shows a positive correlation with the zeta potential changes measured at pH = 1. Observed on the Fe-N-C catalyst but not on Pt/C, it is surmised that the electrocatalytic activities of the oxygen-reducing FeNxCy moieties are influenced by the surface chemistry of the carbonaceous support. Since catalyst wetting and drying processes are essential for MEA fabrication for fuel cells, these results suggest that careful attention should be paid to the conditions employed to prepare and dry catalytic inks for the family of Fe-N-C catalysts in order to obtain their highest possible ORR activity.
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