General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities

Abstract

E fficient and cost-effective electrocatalysts play critical roles in energy conversion and storage [1][2][3] . Homogeneous and heterogeneous catalysts represent two parallel frontiers of electrocatalysts, each with their own merits and drawbacks 4,5 . Homogeneous catalysts are attractive for their highly uniform active sites, tunable coordination environment and maximized atom utilization efficiency, but are limited by their relatively poor stability and recyclability. Heterogeneous catalysts are appealing for their high durability, excellent recyclability, and easy immobilization and integration with electrodes, but usually have rather low atom utilization efficiency due to the limited surface sites accessible to reactants. To this end, considerable efforts have been devoted to developing nanoscale heterogeneous catalysts that can increase the exposed surface atoms 3 . However, the inhomogeneity in the distribution of particle sizes and facets poses a serious challenge for controlling active sites and fundamental mechanistic studies 6,7 . In contrast, homogeneous catalysts typically exhibit the well-defined atomic structure with tunable coordination environment that is essential for deciphering the catalytic reaction pathway and rational design of targeted catalysts with tailored catalytic properties 8 . Single-atom catalysts (SACs) with monodispersed single atoms supported on solid substrates are recently emerging as an exciting class of catalysts that combine the merits of both homogeneous and heterogeneous catalysts [9][10][11][12][13][14] . However, most SACs studied to date employ metal oxides (for example, TiO 2 , CeO 2 and FeO x ) as supporting substrates to prevent atom aggregation [15][16][17][18] , which cannot be readily applied in electrocatalytic applications due to their low electrical conductivity and/or poor stability in harsh liquid-phase electrolytes (for example, strong acid or base). Atomic transitionmetal-nitrogen moieties supported in carbon (M-N-Cs) represent a unique class of SACs with high electrical conductivity and superior (electro)chemical stability for electrocatalytic applications 19 . In particular, Fe-based M-N-Cs have been extensively studied as electrocatalysts towards the oxygen reduction reaction (ORR) with demonstrated activity and stability approaching those of commercial Pt/C catalysts 20,21 . In addition, as suggested by numerous theoretical studies, M-N-Cs are promising candidates for catalysing a wide range of electrochemical processes, such as hydrogen reduction/oxidation 22 , CO 2 /CO reduction 23 and N 2 reduction 24 . A significant advantage of SACs is that the well-defined single atomic site could allow precise understanding of the catalytic reaction pathway, and rational design of targeted catalysts with tailored activity (in a manner similar to homogeneous catalyst design). However, this perceived advantage has been investigated theoretically

Rational Design of Graphene‐Supported Single Atom Catalysts for Hydrogen Evolution Reaction

“…HER is a combination of proton adsorption on catalyst surface via Volmer reaction (H + + e − + * → H*, where * refers to catalysts surface) followed by desorption of H 2 through either Tafel reaction (2H* → H 2 + 2*) or Heyrovsky reaction (H + + e − + H* → H 2 + *). [13][14][15]18] Based on these results we constructed a molecular model of SACs for hydrogen adsorption energy calculation including all possible active sites, namely, graphitic-N, graphitic-C, pyridinic-N, terminal pyridinic-N, pyrrolic-N, and N-oxide species. The free energy change for hydrogen adsorption on the catalyst surface (ΔG H *) determines the kinetics of the HER.…”

“…[13,22] In addition, the bonding and coordination environment around metal in the SACs were further studied with the EXAFS Fourier transform (FT) for all three SACs shown in Figure 4d-f. Figure 4d shows that the EXAFS FT spectra of Co-SAC exhibits a major peak at around 1.43 Å, which is different from CoO peak at 1.63 Å and CoCo peaks at 2.1 and 2.7 Å, respectively, corresponding to CoN bonding. Both 1s→3d and 1s→4pz peak intensities are higher for Ni-SAC than for NiO, similar things (2p→5d and 2p→6pz) happened for W-SAC.…”

“…A few of them are unsaturated coordination environment of metal active centers and quantum size effects, where electron confinement produces distinctive highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and discrete energy level, and finally strong metal-support interaction facilitates charge transfer between them. However, among all transition metals, a few have been widely investigated as a SAC on nitrogen doped (N-doped) graphene substrate toward ORR, [12] OER, [13] HER, [14] and CO 2 reduction, [15] but their origin of activity still remains unclear. [7d,9] However, the low conductivity and instability of these substrates in a harsh electrolytic environment (strong acid and base) force us to search for alternatives.…”

Metallic Porous Iron Nitride and Tantalum Nitride Single Crystals with Enhanced Electrocatalysis Performance

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Previous studies show that the metalnitrogen moieties with unsaturated nitrogen coordination numbers possess higher catalytic activities and the reported smallest nitrogen coordination number is confirmed to be ≈2 for Co-N 2 and Fe-N 2 moieties using extended X-ray absorption fine structure spectra analysis. [7][8][9][10] The short length scale of nanostructured catalysts makes it extremely challenging to quantify metal-nitrogen moiety structure and distributions and to correspondingly elucidate the electronic structure features and unusual activity. However, the active metalnitrogen moieties confined at the surface layer of materials that host catalysis reactions still remain unclear.

Building metal-nitrogen moieties with unsaturated nitrogen coordination numbers to enhance material's catalytic activity remains a fundamental challenge.

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“…[7][8][9][10] The short length scale of nanostructured catalysts makes it extremely challenging to quantify metal-nitrogen moiety structure and distributions and to correspondingly elucidate the electronic structure features and unusual activity. The state-of-the-art pyrolysis technique of specific precursors is commonly used to construct metal-nitrogen moieties.…”