Boosting the Electrocatalytic Activity of Fe−Co Dual‐Atom Catalysts for Oxygen Reduction Reaction by Ligand‐Modification Engineering

Engineering the coordination environment of Ni and Pd dual active sites for promoting the oxygen reduction reaction

Precisely regulating active sites is vital for promoting the oxygen reduction reaction (ORR) activity. Here we reported highly active Ni-Pd co-doped N-coordinated graphene towards ORR achieved by edge termination and O doping. Our first-principles calculations demonstrated that edge termination effectively boost the ORR activity, and armchair-edge termination was energetically more favorable than zigzag-edge termination. Notably, after O doping Ni-Pd active center, armchair edge-terminated Ni-Pd active site exhibited better ORR activity, and the lowest overpotential was only 0.31 V. This improvement in activity was attributed to the shift of the d-band center of Ni atom toward the Fermi-level and the shift of the d-band center of Pd atom away from the Fermi-level, thus regulating the *OH adsorption strength. This work paves the way for developing highly active graphene-based dual-atom catalysts for ORR through edge and doping engineering.

Asymmetric Microenvironment Tailoring Strategies of Atomically Dispersed Dual-Site Catalysts for Oxygen Reduction and CO2 Reduction Reactions

Dual-atom catalysts (DACs) with atomically dispersed dual-sites, as an extension of single-atom catalysts (SACs), have recently become a new hot topic in heterogeneous catalysis due to their maximized atom efficiency and dual-site diverse synergy, because the synergistic diversity of dual-sites achieved by asymmetric microenvironment tailoring can efficiently boost the catalytic activity by optimizing the electronic structure of DACs. Here, this work first summarizes the frequently-used experimental synthesis and characterization methods of DACs. Then, four synergistic catalytic mechanisms (cascade mechanism, assistance mechanism, co-adsorption mechanism and bifunction mechanism) and four key modulating methods (active site asymmetric strategy, transverse/axial-modification engineering, distance engineering and strain engineering) are elaborated comprehensively. The emphasis is placed on the effects of asymmetric microenvironment of DACs on oxygen/carbon dioxide reduction reaction. Finally, some perspectives and outlooks are also addressed. In short, the review summarizes a useful asymmetric microenvironment tailoring strategy to speed up synthesis of high-performance electrocatalysts for different reactions.

Boosting the Electrocatalytic Oxygen Reduction Activity of MnN4-Doped Graphene by Axial Halogen Ligand Modification

Exploring highly active electrocatalysts as platinum (Pt) substitutes for the oxygen reduction reaction (ORR) remains a significant challenge. In this work, single Mn embedded nitrogen-doped graphene (MnN4) with and without halogen ligands (F, Cl, Br, and I) modifying were systematically investigated by density functional theory (DFT) calculations. The calculated results indicated that these ligands can transform the dyz and dxz orbitals of Mn atom in MnN4 near the Fermi-level into dz2 orbital, and shift the d-band center away from the Fermi-level to reduce the adsorption capacity for reaction intermediates, thus enhancing the ORR catalytic activity of MnN4. Notably, Br and I modified MnN4 respectively with the lowest overpotentials of 0.41 and 0.39 V, possess superior ORR catalytic activity. This work is helpful for comprehensively understanding the ligand modification mechanism of single-atom catalysts and develops highly active ORR electrocatalysts.

Boosting the oxygen reduction reaction activity of dual-atom catalysts on N-doped graphene by regulating the N coordination environment

Development of low-cost and high-efficiency oxygen reduction reaction (ORR) catalysts is of significance for fuel cells and metal-air batteries. Here, by regulating the N environment, a series of dual-atom embedded N5-coordinated graphene catalysts, namely M1M2N5 (M1, M2 = Fe, Co, and Ni), were constructed and systematically investigated by DFT calculations. The results reveal that all M1M2N5 configurations are structurally and thermodynamically stable. The strong adsorption of *OH hinders the proceeding of ORR on the surface of M1M2N5, but M1M2N5(OH2) complexes are formed to improve their catalytic activity. In particular, FeNiN5(OH2) and CoNiN5(OH2) with the overpotentials of 0.33 and 0.41 V, respectively, possess superior ORR catalytic activity. This superiority should be attributed to the reduced occupation of d-orbitals of Fe and Co atoms in the Fermi level and the apparent shift of dyz and dz2 orbitals of Ni atoms towards the Fermi level after adsorbing *OH, thus regulating the active sites and exhibiting appropriate adsorption strength for reaction intermediates. This work provides significant insight into the ORR mechanism and theoretical guidance for the discovery and design of low-cost and high-efficiency graphene-based dual-atom ORR catalysts.

Advances in Transition-Metal-Based Dual-Atom Oxygen Electrocatalysts

Oxygen electrocatalysis has aroused considerable interest over the past years because of the new energy technologies boom in hydrogen energy and metal-air battery. However, due to the sluggish kinetic of the four-electron transfer process in oxygen reduction reaction and oxygen evolution reaction, the electro-catalysts are urgently needed to accelerate the oxygen electrocatalysis. Benefit from the high atom utilization efficiency, unprecedentedly high catalytic activity, and selectivity, single-atom catalysts (SACs) are considered the most promising candidate to replace the traditional Pt-group-metal catalysts. Compared with SACs, the dual-atom catalysts (DACs) are attracting more attraction including higher metal loading, more versatile active sites, and excellent catalytic activity. Therefore, it is essential to explore the new universal methods approaching to the preparation, characterization, and to elucidate the catalytic mechanisms of the DACs. In this review, several general synthetic strategies and structural characterization methods of DACs are introduced and the involved oxygen catalytic mechanisms are discussed. Moreover, the state-of-the-art electrocatalytic applications including fuel cells, metal-air batteries, and water splitting have been sorted out at present. The authors hope this review has given some insights and inspiration to the researches about DACs in electro-catalysis.

Co,N-doped carbon sheets prepared by a facile method as high-efficiency oxygen reduction catalysts

Transition metal and nitrogen codoped carbon materials have emerged as one of the most promising candidates to replace noble metal-based oxygen reduction reaction (ORR) catalysts. However, the development of high-efficiency, stable and low-cost metal-nitrogen-carbon catalysts still remains a challenge. In this study, cobalt and nitrogen codoped carbon sheet catalysts were successfully prepared by a simple self-injected vapor phase growth and template method. The catalysts exhibited a multilevel pore structure with a large specific surface area and resulting physical characteristics. The catalysts have excellent onset and half-wave potentials during the ORR. Notably, the onset (E ₀) and half-wave potential (E _1/2) in alkaline media for the Co-N-C-43.8 catalyst are 31 mV and 3 mV higher than those of a commercial Pt/C catalyst, respectively. Moreover, the durability of the Co-N-C-43.8 catalyst remains at a 93% current density after 10 000 s, while that of a commercial Pt/C catalyst only remains at 83%. Also, the Co-N-C-43.8 catalyst has little change in the current density after the addition of methanol. These results indicate that the Co,N-doped carbon sheet is a promising ORR catalyst.