Building Atomic Scale and Dense Fe─N4 Edge Sites of Highly Efficient Fe─N─C Oxygen Reduction Catalysts Using a Sacrificial Bimetallic Pyrolysis Strategy

Boosting the Performance and Durability of Fe-N-C Fuel Cell Catalysts via Integrating Mo2C Clusters

Carbon-supported nitrogen-coordinated iron single-atom (Fe-N-C) catalysts have been regarded among the most promising platinum-group-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Nevertheless, their limited intrinsic activity and unsatisfactory stability have hindered their practical applications. Here, it is reported that the integration of Mo2C clusters effectively enhances the ORR activity and stability of Fe-N-C catalysts. The composite catalyst of Fe single atoms and Mo2C clusters co-embedded on nitrogen-doped carbon (FeSA/Mo2C-NC) exhibits an excellent ORR activity with a half-wave potential of 0.82 V in acidic media and a high peak power density of 0.5 W cm-2 in an H2-air PEMFC. Moreover, improved stability is achieved with nearly no decay under H2-air conditions for 80 h at 0.4 V. Experiments with theoretical calculations elucidate that the etching effect of the phosphomolybdic acid precursor optimizes the pore size distribution of the composite catalyst, thereby exposing more active sites. The Mo2C clusters modulate the electronic configuration of the Fe-N4 sites, optimizing adsorption energy for ORR intermediates and strengthening the Fe-N bond to mitigate demetalation. This work provides valuable insights into the construction of single-atom/nanoaggregate hybrid catalysts for efficient energy-related applications.

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.

[2+1] Cycloadditions Modulate the Hydrophobicity of Ni-N4 Single-Atom Catalysts for Efficient CO2 Electroreduction

Microenvironment regulation of M-N4 single-atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M-N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni-N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni-N4 and in situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni-N4, resulting in the formation of a super-hydrophobic Ni-N4-CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni-N4-CF2 to inherit both the electronic and structural configuration of the Ni-N4 sites from Ni-N4. Enhanced electrochemical CO2-to-CO Faradaic efficiency above 98 % is achieved in a wide operating potential window from -0.7 V to -1.3 V over Ni-N4-CF2. In situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the -CF2- group repels asymmetric H-bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N-doped carbon-supported catalysts.

High quality bifunctional cathode for rechargeable zinc-air batteries using N-doped carbon nanotubes constrained CoFe alloy

Building efficient and stable bifunctional electrocatalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the advancement of rechargeable zinc-air batteries (ZABs). Here, a convenient in situ strategy is reported to controllably encapsulate CoFe alloy nanoparticles within N-doped carbon nanotubes (CoFe@NCNT). The abundant Co(Fe)-Nx active sites and the synergistic interaction between CoFe alloys and carbon nanotubes facilitate mass transfer and interfacial charge transfer, resulting in excellent dual functional electrocatalytic activity of OER/ORR with minor potential difference (ΔE = 0.73 V). Thus, the corresponding rechargeable ZAB displays high power density (194 mW cm-2), excellent specific capacity (795 mAh gZn-1), and favorable stability (900 cycles@5 mA cm-2). This work provides an approach for establishing low-cost bultifunctional electrocatalysts with excellent performance of non-noble metal nanoalloys.

Mn Single-Atom Tuning Fe-N-C Catalyst Enables Highly Efficient and Durable Oxygen Electrocatalysis and Zinc-Air Batteries

Fe-N-C catalyst is one of most promising candidates for oxygen electrocatalysis reaction in zinc-air batteries (ZABs), but achieving sustained high activity is still a challenging issue. Herein, we demonstrate that introducing Mn single atoms into Fe-N-C (Mn1@Fe-N-C/CNTs) enables the realization of highly efficient and durable oxygen electrocatalysis performance and application in ZABs. Multiple characterizations confirm that Mn1@Fe-N-C/CNTs is equipped with Mn-N2O2 and Fe-N4 sites and Fe nanoparticles. The Mn-N2O2 sites not only tune the electron structure of Fe-Nx sites to enhance intrinsic activity, but also scavenge the attack of radicals from Fe-Nx sites for improvement in ORR durability. As a result, Mn1@Fe-N-C/CNTs exhibits enhanced ORR performance to traditional Fe-N-C catalysts with high E1/2 of 0.89 V vs reversible hydrogen electrode (RHE) and maintains ORR activity after 15 000 CV. Impressively, Mn1@Fe-N-C/CNTs also presents excellent OER activity and the difference (ΔE) between E1/2 of ORR and OER potential at 10 mA cm-2 (Ej10) is only 0.59 V, outperforming most reported catalysts. In addition, the maintainable bifunctional activity of Mn1@Fe-N-C/CNTs is demonstrated in ZABs with almost unchanged cycle voltage efficiency up to 200 h. This work highlights the critical role of Mn single atoms in enhancing ORR activity and stability, promoting the development of advanced catalysts.