Key Single‐Atom Electrocatalysis in Metal—Organic Framework (MOF)‐Derived Bifunctional Catalysts

Abstract

Metal-organic framework (MOF)-derived materials have attracted increasing interest and show promising catalytic performances in many fields. Intensive efforts have been focused on the structure design and metal-site integration in MOF-derived catalysts. However, the key catalytic processes related with the metal sites in MOF-derived catalysts that dominate the electrocatalytic performance still remain obscure. Herein, we show a neglected but critical issue in the pyrolytic synthesis of MOF-derived catalysts: the coupled evolution of dual sites, that is, metallic sites and single-atom metal sites. The identification of active sites of single-atom sites from the visible particles has been elucidated through the combined X-ray spectroscopic, electron microscopic, and electrochemical studies. Interestingly, after a total removal of metallic cobalt sites, catalysts with purified single-atom metal sites show no faltering activity for either the oxygen reduction reaction (ORR) or hydrogen evolution reaction (HER), while significantly enhanced ORR selectivity is achieved; this reveals the dominant activity and selectivity contribution from single-atom electrocatalysis. The insight of the coupled evolution of dual sites and the as-demonstrated dual-site decoupling strategies open up a new routine for the design and synthesis of MOF-derived catalysts with the optimized single-atom electrocatalysis towards various electrochemical reactions.

Oxygen Reduction Reactions on Single‐ or Few‐Atom Discrete Active Sites for Heterogeneous Catalysis

“…In acidic conditions, Zhao et al also reported strong performance of Co–N 4 moieties embedded in a carbon matrix. Interestingly and in line with other reports, the authors showed that when going from Co nanoparticles toward homogeneously dispersed Co–N 4 sites the catalytic performance improved and the catalytic mechanism approached a full 4e − process with very low yield of H 2 O 2 . Theoretical calculations show that both Co–N 4 , and Co–N 2 moieties embedded in a carbon matrix are thermodynamically stable .…”

“…Interestingly and in line with other reports, the authors showed that when going from Co nanoparticles toward homogeneously dispersed Co-N 4 sites the catalytic performance improved and the catalytic mechanism approached a full 4e− process with very low yield of H 2 O 2 . [155] Theoretical calculations show that both Co-N 4 , and Co-N 2 moieties embedded in a carbon matrix are thermodynamically stable. [156,157] From the reports available at present date, only few experimental support for Co-N 2 moieties are however reported [60,158] in all other cases EXAFS is shown to give a Co-N coordination close to four, although some reports identify their active centers as Co-N x .…”

Graphene-supported metal single-atom catalysts: a concise review

“…The noble metal atomic radii of SACs are in the range of 1-1.5 Å; meanwhile, the minimum size and low loading determine the isolated catalytic active sites. Zhao et al [100] prepared metal-organic framework-derived catalysts with improved ORR selectivity. Wang et al [87] predicted that single Si atoms would exhibit superior activity and ultra-high selectivity based on DFT calculations.…”

From Supramolecular Species to Self‐Templated Porous Carbon and Metal‐Doped Carbon for Oxygen Reduction Reaction Catalysts

“…[14] Compared with bulk metal catalysts,a tomically dispersed metal or metal-doped carbon possesses distinct electronic structure and fully exposed active sites,r endering catalytic reactions better reactivity and selectivity. [16] Nevertheless,t he fabrication of M-N-C is complicated and challenging as less active bulk metal and metal carbide impurities are usually presented in the product and need to be removed to fully exert high activity of M-N-C. [17] Considering the superior framework stability during pyrolysis,m etal-coordinated supramolecules are expected to be an ideal precursor toward simple yet effective synthesis of high active metal-doped carbon catalysts without additional extensive purification. [16] Nevertheless,t he fabrication of M-N-C is complicated and challenging as less active bulk metal and metal carbide impurities are usually presented in the product and need to be removed to fully exert high activity of M-N-C. [17] Considering the superior framework stability during pyrolysis,m etal-coordinated supramolecules are expected to be an ideal precursor toward simple yet effective synthesis of high active metal-doped carbon catalysts without additional extensive purification.…”

“…[15] Specifically,M -N-C catalysts exhibited comparable activity to commercial Pt/C electrocatalysts in oxygen reduction reaction (ORR) that is the cornerstone for fuel cell and metal-air battery applications. [16] Nevertheless,t he fabrication of M-N-C is complicated and challenging as less active bulk metal and metal carbide impurities are usually presented in the product and need to be removed to fully exert high activity of M-N-C. [17] Considering the superior framework stability during pyrolysis,m etal-coordinated supramolecules are expected to be an ideal precursor toward simple yet effective synthesis of high active metal-doped carbon catalysts without additional extensive purification.…”