Core–Shell Structured Fe–N–C Catalysts with Enriched Iron Sites in Surface Layers for Proton-Exchange Membrane Fuel Cells

Abstract

Carbon-supported and nitrogen-coordinated single iron site materials (denoted as Fe−N−C) are the most promising platinum group metal (PGM)-free cathode catalysts for the oxygen reduction reaction (ORR) because of their encouraging activity and continuously improved stability. However, current Fe−N−C catalysts derived from zeolitic imidazolate framework-8 (ZIF-8) nanocrystal precursors via thermal activation at high temperatures often suffer from low accessible Fe sites because the most active sites are buried within bulk carbon nanoparticles. The morphology limitation significantly mitigates the critical three-phase interfaces for creating effective active sites, which requires sufficient ionomer coverage for conducting protons therefore inhibiting the mass transfer of reactants (i.e., O 2 ) within electrodes in proton-exchange membrane fuel cells. Herein, we report an effective strategy for designing a core−shell composite precursor consisting of a polyhedron N-doped porous carbon core from ZIF-8 and a shell from an Fe(III) tetraphenylporphyrin chloride-based conjugated microporous polymer. The resulting core−shell structured Fe−N−C catalyst contains most of the atomic Fe sites at the shell layer with increased density. The unique catalyst design can shorten the diffusion distance of H + and O 2 and facilitate H 2 O product removal, promoting the promoted ORR in thick PGM-free cathodes. Hence, the membrane electrode assembly with optimal Fe−N−C catalysts achieved encouraging current densities of 32 mA cm −2 at 0.9 V iR-free (1.0 bar O 2 ) and 102 mA cm −2 at 0.8 V (1.0 bar air) and a peak power density of 0.43 W cm −2 (1.0 bar air). This work provides an approach to constructing critical M−N−C catalysts with easily accessible single metal active sites in surface layers for the ORR and other critical electrocatalytic reactions.

Engineering Co and Ru dual-metal atoms on nitrogen-doped carbon as highly efficient bifunctional oxygen electrocatalysts

“…25 As shown in Fig. S6, † the results demonstrated that the Co/Ru SAs-N-C possesses a larger BET surface area and higher pore volume than Co SAs-N-C and Ru SAs-N-C, 26 which could provide more paths to transfer gas and electrons. Moreover, EDS revealed that most Zn elements in the three catalysts were evaporated, and the carbon element could be accounted for in all samples, i.e., all samples were adequately carbonised (Fig.…”

Ultrahigh-Loaded Fe Single Atoms and Fe3C Nanoparticle Catalysts as Air Cathodes for High-Performance Zn–Air Batteries

“…The calcined ZIF-90 successively showed that the specific surface area had increased, which was also caused by the volatilization of zinc and the removal of NaCl after pyrolysis (Figure S2d–i). The combined HRTEM and STEM-EDS analysis showed that the atomic percentage of Fe in Fe 3 C-FeSA@3DCN was as high as 4.36%, and the loading was larger than almost all reported Fe-based catalysts. ,, The aberration-corrected-HAADF–STEM (AC-HAADF-STEM) images in Figure a–c show high-resolution images of Fe 3 C-FeSA@3DCN. Figure a clearly shows the lattice fringes of the graphitic carbon layer (002).…”

“…The combined HRTEM and STEM-EDS analysis showed that the atomic percentage of Fe in Fe 3 C-FeSA@3DCN was as high as 4.36%, and the loading was larger than almost all reported Fe-based catalysts. 17,20,36 The aberration-corrected-HAADF− STEM (AC-HAADF-STEM) images in Figure 2a−c show high-resolution images of Fe 3 C-FeSA@3DCN. Figure 2a clearly shows the lattice fringes of the graphitic carbon layer (002).…”

High specific surface area Fe–N–C electrocatalysts for the oxygen reduction reaction synthesized by a hard-template-assisted ball milling strategy

“…The 2%Fe-ZIF@NaCl and 2%Fe-ZIF samples involved five different N groups, which included oxidized N at 403.5 eV, graphite N at 401.9 eV, pyrrole N at 400.9 eV, M–N x at 399.8 eV, and pyridine N at 398.5 eV. 34 The position of M–N x was determined by both Fe–N x and Zn–N x structures and the location of the envelope peak of M–N x further indicated the existence of Zn–N x and Fe–N x structures. Pyrrole N and pyridine N were considered as the key to bind to Fe species, facilitating the formation of Fe–N x sites and accelerating 4-electron transfer in the ORR process.…”