Axial Oxygen Ligands Regulating Electronic and Geometric Structure of Zn‐N‐C Sites to Boost Oxygen Reduction Reaction

Abstract

Zn‐N‐C possesses the intrinsic inertia for Fenton‐like reaction and can retain robust durability in harsh circumstance, but it is often neglected in oxygen reduction reaction (ORR) because of its poor catalytic activity. Zn is of fully filled 3d104s2 configuration and is prone to evaporation, making it difficult to regulate the electronic and geometric structure of Zn center. Here, guided by theoretical calculations, five‐fold coordinated single‐atom Zn sites with four in‐plane N ligands is constructed and one axial O ligand (Zn‐N4‐O) by ionic liquid‐assisted molten salt template method. Additional axial O not only triggers a geometry transformation from the planar structure of Zn‐N4 to the non‐planar structure of Zn‐N4‐O, but also induces the electron transfer from Zn center to neighboring atoms and lower the d‐band center of Zn atom, which weakens the adsorption strength of *OH and decreases the energy barrier of rate determining step of ORR. Consequently, the Zn‐N4‐O sites exhibit improved ORR activity and excellent methanol tolerance with long‐term durability. The Zn‐air battery assembled by Zn‐N4‐O presents a maximum power density of 182 mW cm−2 and can operate continuously for over 160 h. This work provides new insights into the design of Zn‐based single atom catalysts through axial coordination engineering.

General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts

“…For instance, Wang and co-workers revealed that the additional axial OH ligand atop the planar Fe–N 4 can weaken the adsorption of the ORR intermediates by down-shifting the d-band centers, which consequently leads to the apparent positive shift of E onset and improves the overall ORR performance . These promotional effects are also in agreement with the studies by other groups. − Furthermore, Yang et al found that partial replacement of some N by O atoms in the planar Mn 1 –N 4 can generate asymmetric coordination geometries such as Mn 1 –N 3 O, Mn 1 –N 2 O 2 , and Mn 1 –N 1 O 3 , leading to optimization of the electron density in Mn (d-orbital). This improves the intrinsic property of Mn, which is usually the less active metal for ORR than Fe and Co, to be more active than the benchmark Pt/C catalyst.…”

Ligand-assisted formation of mesoporous Zn–N–C to realize superior catalytic activity in solvent-free CO2 cycloaddition reactions

“…S7, ESI †), NQO (404.0 eV), graphitic N (401.2 eV), pyrrolic N (400.4 eV), Zn-N (399 eV) and pyridinic N (398.2 eV), in which the proportion of the Zn-N of m-Zn-N-C sample has slightly decreased. 27 The main peaks of C1s are basically similar for Zn-N-C and m-Zn-N-C (Fig. S8, ESI †).…”

Local Geometric Distortion to Stimulate Oxygen Reduction Activity of Atomically Dispersed Zn‐Nx Sites for Zn–Air Batteries

“…Universally, unpaired electrons that gain or lose angular momentum can change the value of its g-factor. [20] Comparing with S-Zn-N-C-6, the ESR spectrum of S-Zn-N-C-950 presents a significant signal enhancement, suggesting that bending strain leads to the presence of more unpaired electrons and large magnetic moments of the Zn site in S-Zn-N-C-950, [5,21] which is favourable for improving the catalytic activity of S-Zn-N-C-950 due to the magnetic moments associated with electron movement and transfer. [22] Raman spectroscopy analysis and X-ray photoelectron spectroscopy (XPS) were carried out to reveal the chemical composition of the surface elements and carbon structure changes.…”

“…Given that the catalytic activity of M─N─C sites is directly related to their local "microenvironment", various strategies, such as coordination engineering, diatomic construction, and axial group coordination, have been employed to modulate the intrinsic activity and selectivity of the metal sites to achieve the desired applications. [4,5] Recently, materials with geometrically curved surfaces have been applied to catalytic activation reactions associated with small molecules due to local electric field enhancement effects. [6] As reported recently, via constructing bent carbon carriers loaded with Fe single atoms, the half-wave potential (E 1/2 ) of alkaline ORR can reach 0.91 V, [7] and that of acid ORR can reach 0.85 V with a durability over 30000 cycles.…”