Pengfei Yuan

This report examines the oxygen reduction reaction (ORR) catalyzed by iron‐polyphthalocyanine (FePPc) and pyrazine‐linked iron‐coordinated tetrapyrrole (Pz‐FeTPr) conjugated organic polymer (COP) catalysts, utilizing grand‐canonical density functional theory (GC‐DFT) and microkinetic (MK) simulations. The computed half‐wave potential for AA stacking FePPc under alkaline conditions is in strong agreement with experimental findings. The ORR mechanism for AA stacking FePPc is characterized by the ^*O₂ mechanism (), with the Fe site serving as the active site. In the case of Pz‐FeTPr, the ORR mechanism is similarly governed by the ^*O₂ mechanism, with the Fe site remaining the active site at lower potentials (less than 0.5 V_RHE, vs reversible hydrogen electrode). However, at higher potentials (greater than 0.5 V_RHE), the Fe site becomes obstructed by , resulting in a shift of the active site from the Fe site to a neighboring C site (designated as type A3). The corresponding ORR mechanism at the C site is denoted as mechanism (). This mechanism yields a calculated half‐wave potential that aligns well with experimental observations. The mechanisms identified for FePPc and Pz‐FeTPr can be substantiated by the Raman signals detected in experimental studies.

This letter chooses DAE-BPy-CoPor [DAE, 1,2-bis(5'-formyl-2'-methylthien-3'-yl)cyclopentene; BPy, 2,2'-bipyridine-5,5'-dicarbaldehyde; and CoPor, two-dimensional cobalt porphyrin-based covalent organic framework] and Co-TAPP [10,15,20-tetrakis(4-aminophenyl)porphinatocobalt] as two model catalysts to investigate the effect of the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in the O₂-tolerant CO₂ electroreduction reaction (CO₂RR). Both catalysts have been proven to have CO₂RR activity at a potential range from -0.60 to -1.10 V when CO₂ and O₂ are co-feeding (Zhu, H.-J.; Si, D.-H.; Guo, H.; Chen, Z.; Cao, R.; Huang, Y.-B. Oxygen-tolerant CO₂ electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy. Nat. Commun. 2024, 15, 1479, DOI: 10.1038/s41467-024-45959-9). The calculated results by grand canonical density functional theory (GC-DFT) and microkinetic (MK) simulation show that the effect of HER could be categorized into two groups: (1) Positive *H formation energy: HER would have no effect on the turnover frequency (TOF) of the CO₂RR. (2) Negative *H formation energy: HER would also have no effect to the TOF of CO₂RR when the calculated limiting potential for HER is larger than that for CO₂RR. Otherwise, all active sites would be covered by *H and poisoned before CO₂RR can happen. The effect of ORR could be categorized into three types: (1) ORR would have no effect to the TOF of the CO₂RR when ORR happened with the CO₂RR at the same time. (2) ORR is suppressed; the active site is covered by an intermediate state (such as *OH); and the CO₂RR is also suppressed because of the poisoned active site. (3) ORR is suppressed; the active site is not covered; and the TOF of the CO₂RR is not affected.

Fe–N–C materials have emerged as highly promising non-noble metal catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells. However, they still encounter several challenges that need to be addressed. One of these challenges is establishing an atomic environment near the Fe–N₄ site, which can significantly affect catalyst activity. To investigate this, herein, we employed density functional theory (DFT). According to our computational analysis of the Gibbs free energy of the reaction based on the computational hydrogen electrode (CHE) model, we successfully determined two C–O–C structures near the Fe–N₄ site (referred to as str-11) with the highest limiting potential (0.813 V). Specifically, in the case of O-doped structures, the neighboring eight carbon (C) atoms around nitrogen (N) can be categorized into two distinct types: four C atoms (type A) exhibiting high sensitivity to the limiting potential and the remaining four C atoms (type B) displaying inert behavior. Electronic structure analysis further elucidated that a structure will have strong activity if the valence band maximum (VBM) around its gamma point is mainly contributed by d_xz, d_yz or d_z² orbitals of Fe atoms. Constant-potential calculations showed that str-11 is suitable for the ORR under both acidic and alkaline conditions with a limiting potential of 0.695 V at pH = 1 and 0.926 V at pH = 14, respectively. Additionally, microkinetic simulations indicated the possibility of str-11 as the active site for the ORR under working potential at pH = 14.

Developing stable but high active metal-nitrogen-carbon (M-N-C)-based hard carbon anode is a promising way to be the alternatives to graphene and blank hard carbon for sodium-ion batteries (SIBs), requiring the precise tailoring of the electronic structure for optimizing the Na⁺ intercalation behavior, yet is greatly challenging. Herein, Fe-N-C graphitic layer-encapsulating Fe₃C species within hard carbon nanosheets (Fe-N-C/Fe₃C@HCNs) are rationally engineered by pyrolysis of self-assembled polymer. Impressively, the Fe-N-C/Fe₃C@HCNs exhibit outstanding rate capacity (242 mAh·g⁻¹ at 2,000 mA·g⁻¹), which is 2.1 and 4.2 times higher than that of Fe-N-C and N-doped carbon (N-C), respectively, and prolonged cycling stability (176 mAh·g⁻¹ at 2,000 mA·g⁻¹ after 2,000 cycles). Theoretical calculations unveil that the Fe₃C species enhance the electronic transfer from Na to Fe-N-C, resulting in the charge redistribution between the interfaces of Fe₃C and Fe-N-C. Thus, the optimized adsorption behavior towards Na⁺ reduces the thermodynamic energy barriers. The synergistic effect of Fe₃C and Fe-N-C species maintains the structural integrity of electrode materials during the sodiation/desodiation process. The in-depth insight into the advanced Na⁺ storage mechanisms of Fe₃C@Fe-N-C offers precise guidance for the rational establishment of confinement heterostructures in SIBs.

Developing active, robust, and nonprecious electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is highly crucial and challenging. In this work, a facile strategy is developed for scalable fabrication of dicobalt phosphide (Co₂P)–cobalt nitride (CoN) core–shell nanoparticles with double active sites encapsulated in nitrogen‐doped carbon nanotubes (Co₂P/CoN‐in‐NCNTs) by straight forward pyrolysis method. Both density functional theory calculation and experimental results reveal that pyrrole nitrogen coupled with Co₂P is the most active one for HER, while Co–N–C active sites existing on the interfaces between CoN and N‐doped carbon shells are responsible for the ORR and OER activity in this catalyst. Furthermore, liquid‐state and all‐solid‐state Zn–air batteries are equipped. Co₂P/CoN‐in‐NCNTs show high power density as high as 194.6 mW cm⁻², high gravimetric energy density of 844.5 W h kg⁻¹, very low charge–discharge polarization, and excellent reversibility of 96 h at 5 mA cm⁻² in liquid system. Moreover, the Co₂P/CoN‐in‐NCNTs profiles confirm excellent activity for water splitting.

During the preparation of atomically dispersed Fe–N–C catalysts, it is difficult to avoid the formation of iron‐carbide‐containing iron clusters (“Fe_xC/Fe”), along with the desired carbon matrix containing dispersed FeN_x sites. As a result, an uncertain amount of the oxygen reduction reaction (ORR) occurs, making it difficult to maximize the catalytic efficiency. Herein, sulfuration is used to boost the activity of Fe_xC/Fe, forming an improved system, “FeNC–S–Fe_xC/Fe”, for catalysis involving oxygen. Various spectroscopic techniques are used to define the composition of the active sites, which include Fe–S bonds at the interface of the now‐S‐doped carbon matrix and the Fe_xC/Fe clusters. In addition to outstanding activity in basic media, FeNC–S–Fe_xC/Fe exhibits improved ORR activity and durability in acidic media; its half‐wave potential of 0.821 V outperforms the commercial Pt/C catalyst (20%), and its activity does not decay even after 10 000 cycles. Interestingly, the catalytic activity for the oxygen evolution reaction (OER) simultaneously improves. Thus, FeNC–S–Fe_xC/Fe can be used as a high‐performance bifunctional catalyst in Zn–air batteries. Theoretical calculations and control experiments show that the original FeN_x active centers are enhanced by the Fe_xC/Fe clusters and the Fe–S and C–S–C bonds.