Abstract
Since a few years, non-precious metal catalysts with iron or cobalt as active centers show sufficient activity to be viable candidates as electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFC). They can sustain substantial current densities when operated at low potentials. However, their stabilization at high cathode potentials, necessary for high energy efficiency, remains a daunting task. Here an Fe-N-C catalyst is stabilized over the whole potential range through functionalization with minute amounts of platinum. With the addition of 1 to 2 wt% Pt, the present Pt/Fe-N-C hybrid catalysts show a similar current density at 0.8 V than Fe-N-C but are much more stable during operation in PEMFC. Various characterization techniques, including CO stripping, demonstrate that platinum in these hybrid catalysts is ORR-inactive, not only initially but also after the PEMFC potentiostatic test. It is proposed that the present platinum species protects the Fe-based active sites from the ORR by-product H2O2, or reactive oxygen species produced from its reaction with surface Fe. This proof-of-concept paves the way for a new class of hybrid catalysts, where the activity and stability of Me-N-C catalysts can be independently addressed.
Highlights
To cite this version: Anna Mechler, Nastaran Sahraie, Vanessa Armel, Andrea Zitolo, Moulay Tahar Sougrati, et al
Extensive initial and post experiment characterization of the catalysts and polymer electrolyte membrane fuel cells (PEMFCs) electrodes before and after fuel cell operation at 0.5 V for 50 h imply that the platinum surface is initially irreversibly oxidized, not necessary as an oxide, in the hybrid Pt/Fe-N-C catalysts of the present study
The irreversibly-oxidized Pt surface is catalytically inactive toward oxygen reduction reaction (ORR) and hydrogen-peroxide reduction (HPR)
Summary
Morphology and structural characterization of the catalysts.— The porous structure in the hybrid Pt/FeNC materials and H2Fe1.0d was analyzed with N2 sorption and compared to that of Fe1.0d. The analysis of the spectra reveals a sextet and a singlet assigned to α-Fe and γ-Fe, respectively, and two quadrupole doublets D1 and D2 (Table I) The latter are assigned to FeNxCy moieties in low and medium spin states, the most active sites in Fe-N-C materials.[19,20] The slightly higher contents of α-Fe and γ-Fe in the H2-treated samples (H2Fe1.0d and all hybrid catalysts, Table I) can be explained on the basis of the partial etching of carbon by H2 during the thermal treatment in 5% H2. Figure 3. 57Fe Mossbauer spectra measured at room temperature of a) Fe1.0d, b) H2-Fe1.0d and c) Pt1.0Fe1.0d
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