The ternary Fe/N/C material is a promising non-precious-metal oxygen reduction electrocatalyst for proton exchange membrane cells (PEMFCs). However, its practical application is severely hampered by poor stability under PEMFC working conditions, especially at a cell voltage higher than 0.5 V. Herein, we report a new strategy to significantly improve the stability of the Fe/N/C catalyst in PEMFCs by covalent grafting of a trifluoromethylphenyl (Ar-CF 3 ) group. The hydrophobic character of Ar-CF 3 can effectively prevent water flooding of the Fe/N/ C catalyst layer, and thus form robust mass-transport channels for gasÀliquid two-phase flow. Simultaneously, both electronwithdrawing and hydrophobic properties considerably suppress the oxidative corrosion of the carbon matrix that hosts the catalytically active sites. Therefore, fluorinated Fe/N/C could perform stably over 100 h at 0.5 V with a current density of 0.56 A cm À2 in a H 2 ÀO 2 PEMFC. Even when the cell voltage increased to 0.6 V, only 15 % performance was lost after 100 h operation.[a] Dr.
Pyrolyzed Fe/N/C catalysts have high selectivity for catalyzing the oxygen reduction reaction (ORR) and exhibit no activity toward the electrooxidation of small organic molecules (SOMs). As such, their tolerance toward SOMs has rarely been questioned. Unexpectedly, we found that SOMs can greatly suppress the ORR activity of microporous-type Fe/N/C catalysts in alkaline medium. Such a suppression effect is strengthened as molecular polarity decreases or molecular weight increases and can be attributed to the blockage of micropore transport channels for ORR-related species by SOM adsorption. Interestingly, such a suppression effect by micropore filling disappears in the acidic medium. High tolerance to SOMs in acidic medium is attributed to the protonation of pyridinic-N, which increases the polarity of the micropore surrounding and inhibits the adsorption of low-polarity SOMs. This study is helpful to guide the synthesis of porous Fe/N/C electrocatalysts, especially when they are applied in an alkaline medium.
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