Tunable Synthesis of Hollow Metal–Nitrogen–Carbon Capsules for Efficient Oxygen Reduction Catalysis in Proton Exchange Membrane Fuel Cells

Abstract

Atomically dispersed metal catalysts anchored on nitrogendoped (N-doped) carbons demand attention due to their superior catalytic activity relative to that of metal nanoparticle catalysts in energy storage and conversion processes. Herein, we introduce a simple and versatile strategy for the synthesis of hollow N-doped carbon capsules that contain one or more atomically dispersed metals (denoted as H−M−N x −C and H−M mix −N x −C, respectively, where M = Fe, Co, or Ni). This method utilizes the pyrolysis of nanostructured core−shell precursors produced by coating a zeolitic imidazolate framework core with a metal−tannic acid (M−TA) coordination polymer shell (containing up to three different metal cations). Pyrolysis of these core−shell precursors affords hollow N-doped carbon capsules containing monometal sites (e.g., Fe−N x , CoN x , or Ni−N x ) or multimetal sites (Fe/Co−N x , Fe/Ni−N x , Co/Ni−N x , or Fe/Co/Ni−N x ). This inventory allowed exploration of the relationship between catalyst composition and electrochemical activity for the oxygen reduction reaction (ORR) in acidic solution.and H−FeCoNi−N x −C were particularly efficient ORR catalysts in acidic solution. Furthermore, the H− Fe−N x −C catalyst exhibited outstanding initial performance when applied as a cathode material in a proton exchange membrane fuel cell. The synthetic methodology introduced here thus provides a convenient route for developing nextgeneration catalysts based on earth-abundant components.

Heterogeneous Single‐Atom Catalysts for Electrochemical CO2 Reduction Reaction

“…[ 139 ] Owning to the relative low melting point (692 K) and boiling point (1180 K), Zn tends to lost through evaporation under high temperature pyrolysis. [ 140–142 ] Considering that increasing the metal loading is believed to be a promising way to improve the activity of SACs, atomically dispersed Zn‐N‐C catalyst with high Zn loading is favorable. [ 12,128 ] Tin (Sn), together with a few main group metals like Pb, Bi and In, mainly produce formate in aqueous solution.…”

Evolution of Zn(II) single atom catalyst sites during the pyrolysis-induced transformation of ZIF-8 to N-doped carbons

“…Zeolitic imidazolate framework 8 (ZIF-8) is a porous metal organic framework (MOF) consisting of Zn 2+ nodes tetrahedrally coordinated by four nitrogen atoms from 2-methylimidazole linkers. Owing to its high surface area and porosity, along with high carbon and nitrogen content (molecular formula ZnC 8 H 10 N 4 , 42.2 wt% C, 24.6 wt% N), ZIF-8 crystals are frequently used as precursors in the synthesis of N-doped carbon materials by high temperature pyrolysis [21][22][23][24][25]. Typical pyrolysis temperatures are above 800°C, at which temperatures most of the Zn is reduced to the zerovalent state and evaporates (the boiling point of Zn is 907°C).…”

CrN-Encapsulated Hollow Cr-N-C Capsules Boosting Oxygen Reduction Catalysis in PEMFC

“…Thus, many efforts have been paid to identifying more active MN x coordination configurations. [23][24][25][26][27][28][29][30][31][32][33] However, the interactive catalytic contribution of the metal-based phases (nanoparticles and/or clusters) from aggregation has received less focus and needs further exploration. [34][35][36][37][38][39][40][41] Besides the activity, the performance of a PEMFC with M-N x -C catalysts as cathode typically degrades rapidly (∼40-80%) in the first 100 h of testing in PEMFC.…”