To clarify the contribution of these composition-related mechanisms to the overall catalyst deactivation, NNMC synthesis approaches allowing a fine composition control are urgently needed. With this motivation, we have developed a new NNMC preparation method in which polyacrylonitrile is used as the N- and C-precursor, and an inexpensive, thermally decomposable compound as the porosity-inducing agent. Initially, the precursors’ ratios and the temperature of their heat treatment are systematically modified to yield catalysts with different surface N- and (micro)porous-contents. These features are then shown to be only partially related to the catalysts’ ORR-activity (assessed in rotating disk electrode measurements in 0.1 M HClO4 – cf. Figure 1), which for the best performing materials reaches values of ≈ 1.0 A∙gNNMC−1 at 0.8 V vs. RHE. Additionally, the catalytic performance is related to the relative content of Fe-N4 sites and side phases, which is assessed through transmission electron microscopy and X-ray absorption spectroscopy. To investigate this effect in greater detail, the composition of selected catalysts is further modified by including reducing agents in the catalyst synthesis’ acid washing and / or heat treatment steps, as to favor the removal of Fe-based side-phases.6,7 Ultimately, the performance of selected catalysts with different proportions of active and inactive phases is assessed in PEFC tests which shed light on the impact of their composition on catalytic activity and stability.
In summary, this contribution will introduce a novel NNMC-synthesis approach leading to catalysts with different, well-defined compositions, allowing to draw correlations between this key property and the materials’ catalytic activity and durability in PEFC cathodes.
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Figure 1. Relation between mass-normalized ORR-activities at 0.8 V vs. RHE (A), XPS-determined surface N-contents (B) and total / microporous surface areas derived from N2-sorption porosimetry (C), for NNMCs with an initial iron content of 0.1 wt. % Fe and different initial concentrations of the porosity precursor.