Engineering adjacent N, P and S active sites on hierarchical porous carbon nanoshells for superior oxygen reduction reaction and rechargeable Zn-air batteries

Salt Effect Engineering Single Fe-N2P2-Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn-Air Batteries

Developing efficient metal-nitrogen-carbon (M-N-C) single-atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn-air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect-induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra-axial Cl on interlinked porous carbon nanosheets, achieving a superior single-atom Fe catalyst (denoted as Fe-NP-Cl-C) for ORR and Zn-air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe-N2P2-Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe-NP-Cl-C catalyst exhibits distinguished ORR performance with a half-wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn-air battery based on Fe-NP-Cl-C delivers an extremely high peak power density of 260 mW cm-2 and a large specific capacity of 812 mA h g-1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single-atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.

Engineering Mn-Nx sites on porous carbon via molecular assembly strategy for long-life zinc-air batteries

Nitrogen-coordinated manganese atoms on carbon materials denoted as MnNC, serve as the highly active non-precious metal electrocatalysts for oxygen reduction reaction (ORR) in zinc-air batteries (ZABs). Nonetheless, a significant challenge arises from the tendency of Mn atoms to aggregate during heat treatment, thereby compromising ORR performance in ZABs. In this work, the molecular assembly strategy based on the hydrogen bond interaction was employed to fabricate the MnNC electrocatalyst. This approach promotes the dispersion of Mn atoms, creating abundant Mn-Nx active sites. Furthermore, the resulting three-dimensional porous nanostructure, formed by molecular assembly, significantly enhances accessibility to the Mn-Nx active sites. The porous nanostructure not only shortens the diffusion path of reactants and charges but also improves mass transfer. The MnNC exhibits impressive ORR catalytic performance with a half-wave potential of 0.90 V (vs. RHE). The liquid-type ZAB based on MnNC displays a high specific capacity of 816.6 mAh/g and an extended charge-discharge cycle life of 1000 h. Quasi-solid-state ZAB based on MnNC can operate stably for 24 h. This work presents an effective strategy to synthesize transition metal-nitrogen-carbon (MNC) electrocatalysts tailored for long-life zinc-air battery.

Triethylenediamine cobalt complex encapsulated in a metal-organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Transition metal single atom catalysts (TM SACs) are the most promising oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, the low density of M-Nx active sites seriously hinders further improvement of the ORR electrocatalytic activity. Here, a strategy for encapsulating nitrogen-rich guest molecules (triethylenediamine cobalt complex, [Co(en)3]3+) was proposed to construct a high-performance cobalt single-atom catalyst (Co-encapsulated SAC/NC). With this strategy, the guest molecules are encapsulated into metal-organic framework (MOF) cages as an additional cobalt source to boost cobalt loading, while abundant nitrogen from guest molecules contributes to the formation of Co-N4 active sites. Remarkably, the resulting Co-encapsulated SAC/NC has a high cobalt loading amount of 4.03 wt%, and spherical aberration-corrected transmission electron microscopy (AC-TEM) has confirmed that most cobalt exists in a single-atom state. As a result, the Co-encapsulated SAC/NC exhibits excellent ORR catalytic performance with a half-wave potential of 0.88 V. Furthermore, Zn-air batteries employing Co-encapsulated SAC/NC as air cathode show high peak power density and excellent cycling stability. Density functional theory (DFT) calculations reveal that adjacent active sites have different rate-determining steps and lower reaction energy barriers than a single active site.

Copper nanodot-embedded nitrogen and fluorine co-doped porous carbon nanofibers as advanced electrocatalysts for rechargeable zinc-air batteries

Porous carbon-based electrocatalysts for cathodes in zinc-air batteries (ZABs) are limited by their low catalytic activity and poor electronic conductivity, making it difficult for them to be quickly commercialized. To solve these problems of ZABs, copper nanodot-embedded N, F co-doped porous carbon nanofibers (CuNDs@NFPCNFs) are prepared to enhance the electronic conductivity and catalytic activity in this study. The CuNDs@NFPCNFs exhibit excellent oxygen reduction reaction (ORR) performance based on experimental and density functional theory (DFT) simulation results. The copper nanodots (CuNDs) and N, F co-doped carbon nanofibers (NFPCNFs) synergistically enhance the electrocatalytic activity. The CuNDs in the NFPCNFs also enhance the electronic conductivity to facilitate electron transfer during the ORR. The open porous structure of the NFPCNFs promotes the fast diffusion of dissolved oxygen and the formation of abundant gas-liquid-solid interfaces, leading to enhanced ORR activity. Finally, the CuNDs@NFPCNFs show excellent ORR performance, maintaining 92.5% of the catalytic activity after a long-term ORR test of 20000 s. The CuNDs@NFPCNFs also demonstrate super stable charge-discharge cycling for over 400 h, a high specific capacity of 771.3 mAh g^-1 and an excellent power density of 204.9 mW cm^-2 as a cathode electrode in ZABs. This work is expected to provide reference and guidance for research on the mechanism of action of metal nanodot-enhanced carbon materials for ORR electrocatalyst design.

Advanced and Stable Metal-Free Electrocatalyst for Energy Storage and Conversion: The Structure-Effect Relationship of Heteroatoms in Carbon

Ever-developing energy device technologies require the exploration of advanced materials with multiple functions. Heteroatom-doped carbon has been attracting attention as an advanced electrocatalyst for zinc-air fuel cell applications. However, the efficient use of heteroatoms and the identification of active sites are still worth investigating. Herein, a tridoped carbon is designed in this work with multiple porosities and high specific surface area (980 m^-2 g^-1). The synergistic effects of nitrogen (N), phosphorus (P), and oxygen (O) in micromesoporous carbon on oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysis are first investigated comprehensively. Metal-free N-, P-, and O-codoped micromesoporous carbon (NPO-MC) exhibits attractive catalytic activity in zinc-air batteries and outperforms a number of other catalysts. Combined with a detailed study of N, P, and O dopants, four optimized doped carbon structures are employed. Meanwhile, density functional theory (DFT) calculations are made for the codoped species. The lowest free energy barrier for the ORR can be attributed to the pyridine nitrogen and N-P doping structures, which is an important reason for the remarkable performance of NPO-MC catalyst in electrocatalysis.