Modulating Coordination of Iron Atom Clusters on N,P,S Triply-Doped Hollow Carbon Support towards Enhanced Electrocatalytic Oxygen Reduction

Constructing atom-clusters (ACs) with in situ modulation of coordination environment and simultaneously hollowing carbon support are critical yet challenging for improving electrocatalytic efficiency of atomically dispersed catalysts (ADCs). Herein, a general diffusion-controlled strategy based on spatial confining and Kirkendall effect is proposed to construct metallic ACs in N,P,S triply-doped hollow carbon matrix (M_ACs/NPS−HC, M=Mn, Fe, Co, Ni, Cu). Thereinto, Fe_ACs/NPS−HC with the best catalytic activity for oxygen reduction reaction (ORR) is thoroughly investigated. Unlike the benchmark sample of symmetrical N-surrounded iron single-atoms in N-doped carbon (Fe_SAs/N−C), Fe_ACs/NPS−HC comprises bi-/tri-atomic Fe centers with engineered S/N coordination. Theoretical calculation reveals that proper Fe gathering and coordination modulation could mildly delocalize the electron distribution and optimize the free energy pathways of ORR. In addition, the triple doping and hollow structure of carbon matrix could further regulate the local environment and allow sufficient exposure of active sites, resulting in more enhanced ORR kinetics on Fe_ACs/NPS−HC. The zinc-air battery assembled with Fe_ACs/NPS−HC as cathodic catalyst exhibits all-round superiority to Pt/C and most Fe-based ADCs. This work provides an exemplary method for establishing atomic-cluster catalysts with engineered S-dominated coordination and hollowed carbon matrix, which paves a new avenue for the fabrication and optimization of advanced ADCs.     

Enhanced Multiple Anchoring and Catalytic Conversion of Polysulfides by Amorphous MoS3 Nanoboxes for High-Performance Li-S Batteries

The practical implementation of lithium–sulfur batteries is obstructed by poor conductivity, sluggish redox kinetics, the shuttle effect, large volume variation, and low areal loading of sulfur electrodes. Now, amorphous N-doped carbon/MoS₃ (NC/MoS₃) nanoboxes with hollow porous architectures have been meticulously designed as an advanced sulfur host. Benefiting from the enhanced conductivity by the N-doped carbon, reduced shuttle effect by the strong chemical interaction between unsaturated Mo and lithium polysulfides, improved redox reaction kinetics by the catalytic effect of MoS₃, great tolerance of volume variation and high sulfur loading arising from flexible amorphous materials with hollow-porous structures, the amorphous NC/MoS₃ nanoboxes enabled sulfur electrodes to deliver a high areal capacity with superior rate capacity and decent cycling stability. The synthetic strategy can be generalized to fabricate other amorphous metal sulfide nanoboxes.     

Fe/Fe3C Nanoparticles Encapsulated in N-Doped Hollow Carbon Spheres as Efficient Electrocatalysts for the Oxygen Reduction Reaction over a Wide pH Range

Nonprecious-metal-based electrocatalysts with low cost, high activity, and stability are considered as one of the most promising alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR). Herein, an economical and easy-to-fabricate catalyst is developed, that is, Fe/Fe₃C embedded in N-doped hollow carbon spheres (Fe/Fe₃C/NHCS), which gave the half-wave potential of 0.84 V in 0.1 m KOH, similar to the commercial Pt/C catalyst. Surprisingly, the favorable ORR performance of the as-prepared catalyst was obtained in both acidic and neutral conditions with almost a four-electron pathway and low H₂O₂ yield, which desirable the development of the proton exchange membrane (PEM) and microbial electrolysis cell (MEC) technology. Additionally, the obtained catalyst demonstrated better long-term stability and high methanol tolerance over a wide range of pH. These features could be mainly attributed to the synergistic effect between Fe/Fe₃C and Fe-N_x sites, the hollow structure with mesopores, and the well-dispersed Fe/Fe₃C nanoparticles owing to the existence of the abundant hydrophilic groups within the HCS precursor. As such, designing an efficient and cheap ORR catalyst that can operate at alkaline, acidic, and neutral solutions is highly desirable, yet challenging.     

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts

Fe-N-C catalysts with high O₂ reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H₂O₂ reduction, a key intermediate during indirect O₂ reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeN_xC_y moieties and Fe particles encapsulated in N-doped carbon layers (0–100 %) show that both types of sites are active, although moderately, toward H₂O₂ reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeN_xC_y moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.     

Fe-ZIF8 Coating Cu Foil Derived Carbon as A pH-Universal Electrocatalyst for Efficient Oxygen Reduction Reaction

It is a great challenge to fabricate highly efficient pH-universal electrocatalysts for oxygen reduction reaction (ORR). Herein, a facile strategy, which includes coating the Fe modified ZIF8 on Cu foil and in situ pyrolysis to evaporate and dope Cu into the MOF derived carbon, is developed to fabricate Fe/Cu−N co-doped carbon material (Cu/Fe−NC). Profiting from the modulated electron distribution and textual properties, well-designed Cu/Fe−NC exhibits superior half-wave potential (E_1/2) of 0.923 V in alkaline, 0.757 V in neutral and comparable 0.801 V in acid electrolytes, respectively. Furthermore, the ultralow peroxides yield of ORR demonstrates the high selectivity of Cu/Fe−NC in full pH scale electrolytes. As expected, the self-made alkaline and neutral zinc-air batteries equipped with Cu/Fe−NC cathode display excellent discharge voltages, outstanding peak power densities and remarkable stability. This work opens a new way to fabricate highly efficient and pH-universal electrocatalysts for ORR through strategy of Fe/Cu−N co-doping, Cu foil evaporation and carbon defects capture.     

Doping effect of boron and phosphorus on nitrogen-based mesoporous carbons as electrocatalysts for oxygen reduction reaction in acid media

Doped mesoporous carbons comprising nitrogen, boron, and phosphorus (N, B, and P, respectively) were prepared as non-Pt catalysts for oxygen reduction reaction (ORR) in an acidic solution. The N-doped carbons were varied to increase their catalytic activity through by additionally doping of B and P. All the mesoporous carbons were synthesized by carbonizing polyaniline at 900 °C for the N species, while the B and P species were inserted into the carbon structure at the carbon growth step. The linear sweep voltammogram recorded in the acidic solution showed that the ORR activity of the N-doped carbon catalysts increased significantly after the addition of B. An approximately 19 % increase in the pyridinic N content at the carbon surface was observed, along with B-N-C moieties with a binding energy of 399.5 eV. The non-precious metal ORR catalysts were prepared via pyrolysis, with the insertion of an additional transition metal (iron, Fe). The deconvoluted X-ray photoelectron spectroscopy (XPS) results showed that the Fe-N peak was generated after the pyrolysis. The peak intensity of the quaternary N also increased compared with the pyridic and pyrrolic N, which indicates that Fe serves to catalyze the modification of N species. The numerical examinations showed that N- and B-doped mesoporous carbon (NBC) 1.5 % Fe had the highest limited current (4.94 mA/cm²), with the B-doped carbon still the most active mesoporous carbon catalyst for ORR. As a result, it can be said that Fe positively contributes to the formation of graphitic N, which is known to be an active site for ORR. The cyclic voltammetry results showed that the peak area of the NBC 1.5 % Fe catalyst was larger than that of the N-doped mesoporous carbon (NC) 1.5 % Fe catalyst. It was concluded that B doping enhances the ORR activity and the stability of carbon materials even after 1000 cycles under acidic conditions.     

Ice/Salt-Assisted Synthesis of Ultrathin Two-Dimensional Micro/Mesoporous Iron and Nitrogen Co-Doped Carbon as an Efficient Electrocatalyst for Oxygen Reduction

An ice/salt-assisted strategy has been developed to achieve the green and efficient synthesis of ultrathin two-dimensional (2D) micro/mesoporous carbon nanosheets (CNS) with the dominant active moieties of Fe−N₄ (Fe-N-CNS) as high-performance electrocatalysts for the oxygen reduction reaction (ORR). The strategy involves freeze-drying a mixture of iron porphyrin and KCl salt using ice as template followed by a confined pyrolysis with KCl as an independent sealed nanoreactor to facilitate the formation of 2D carbon nanosheets, N incorporation, and porosity creation. The well-defined assembly of ultrathin 2D carbon nanosheets ensures high utilization of D1 and D3 Fe−N₄ active sites, and effectively promotes the mass transport of ORR reactants by virtue of the pronounced mesoporous structure. The resulting Fe-N-CNS electrocatalyst was shown to exhibit superior ORR activity, better electrochemical durability, and methanol tolerance towards ORR in alkaline electrolyte relative to the commercial Pt/C electrocatalyst.     

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N₄ moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O₄ configuration are transformed into the ones with Fe-N₄ configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N₄. Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.     

Gelatin-Derived 1D Carbon Nanofiber Architecture with Simultaneous Decoration of Single Fe−Nx Sites and Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction

Exploring high-performance non-precious-metal electrocatalysts for the oxygen reduction reaction (ORR) is critical. Herein, a scalable and cost-effective strategy is reported for the construction of one-dimensional carbon nanofiber architectures with simultaneous decoration of single Fe−N_x sites and highly dispersed Fe/Fe₃C nanoparticles for efficient ORR, through the Fe^III-complex-assisted electrospinning of gelatin nanofibers with subsequent pre-oxidation and carbonization. Results show that the presence of a Fe^III complex enables the 1D gelatin nanofibers to be well retained during the pre-oxidation process. Owing to the distinct 1D nanofiber structure and the synergistic effect of Fe/Fe₃C and Fe−N_x sites, the resulting electrocatalyst is highly active for ORR with a half-wave potential of 0.885 V (outperforming commercial Pt/C) and a superior electrochemical stability in alkaline electrolytes. Similarly, it also shows a high power density (144.7 mW cm⁻²) and a superior stability in Zn-air batteries. This work opens a path for the design and synthesis of 1D carbon electrocatalyst for efficient ORR catalysis.     

The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions

Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn_0.5Fe_0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn_0.5Fe_0.5 exhibits high stability for both reactions.

     

Synthesis of Non-precious N-doped Mesoporous Fe3O4/CoO@NC Materials towards Efficient Oxygen Reduction Reaction for Microbial Fuel Cells

N-doped transition metal oxides are strategic materials towards the efficient oxygen reduction reaction (ORR) of microbial fuel cells (MFCs). Non-precious N-doped Fe₃O₄/CoO@NC−T (T represents carbonization temperature) catalysts are prepared by an efficient two-step strategy for ORR. Fe₃O₄/CoO@NC-750 exhibits the best performance with an efficient four-electron transfer pathway. The optimal power density of MFCs by using Fe₃O₄/CoO@NC-750 as the cathode catalyst (1243.4 mW ⋅ m⁻²) is superior to that of the MFCs with commercial Pt/C catalyst (1080 mW ⋅ m⁻²), which shows an outstanding activity towards ORR. No significant decrease in output voltage results over 70 days, which shows an excellent electrochemical stability.     

A facile nitrogen-doped carbon encapsulation of CoFe2O4 nanocrystalline for enhanced performance of lithium ion battery anodes

Nitrogen-doped (N-doped) carbon encapsulation of CoFe₂O₄ nanocrystalline is achieved by a simple pressure-assisted pyrrole pyrolysis method. The CoFe₂O₄/N-doped carbon nanocomposite (CFO/NC) delivers a capacity of 646.2 mAh g^–1 after 80 cycles at 0.1 C, exhibits stable cycling performance at various rates from 0.2 to 1.6 C and retains a capacity of 662.8 mAh g^–1 as the rate returns back to 0.1 C, showing significantly improved lithium storage reversibility compared to the bare CFO. A different lithiation mechanism of CFO/NC above and below the plateau relative to CFO in the first discharge is analyzed in detail based on the potential profiles and cyclic voltammogram curves. Morphology characterization of the cycled electrodes confirms much better integrity of CFO/NC electrode due to the buffer effect of N-doped carbon coating. Electronic conductivity and electrochemical impedance spectroscopy measurements indicate enhanced electrode reaction kinetics of CFO/NC. All the results contribute to its improved electrochemical performance.     

3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction

Three-dimensional (3D) N-doped graphene aerogel (N-GA)-supported Fe(3)O(4) nanoparticles (Fe(3)O(4)/N-GAs) as efficient cathode catalysts for the oxygen reduction reaction (ORR) are reported. The graphene hybrids exhibit an interconnected macroporous framework of graphene sheets with uniform dispersion of Fe(3)O(4) nanoparticles (NPs). In studying the effects of the carbon support on the Fe(3)O(4) NPs for the ORR, we found that Fe(3)O(4)/N-GAs show a more positive onset potential, higher cathodic density, lower H(2)O(2) yield, and higher electron transfer number for the ORR in alkaline media than Fe(3)O(4) NPs supported on N-doped carbon black or N-doped graphene sheets, highlighting the importance of the 3D macropores and high specific surface area of the GA support for improving the ORR performance. Furthermore, Fe(3)O(4)/N-GAs show better durability than the commercial Pt/C catalyst.     

Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The development of low-cost, efficient, and stable electrocatalysts for the oxygen reduction reaction (ORR) is desirable but remains a great challenge. Herein, we made a highly reactive and stable isolated single-atom Fe/N-doped porous carbon (ISA Fe/CN) catalyst with Fe loading up to 2.16 wt %. The catalyst showed excellent ORR performance with a half-wave potential (E_1/2) of 0.900 V, which outperformed commercial Pt/C and most non-precious-metal catalysts reported to date. Besides exceptionally high kinetic current density (J_k) of 37.83 mV cm⁻² at 0.85 V, it also had a good methanol tolerance and outstanding stability. Experiments demonstrated that maintaining the Fe as isolated atoms and incorporating nitrogen was essential to deliver the high performance. First principle calculations further attributed the high reactivity to the high efficiency of the single Fe atoms in transporting electrons to the adsorbed OH species.     

聚苯胺衍生Fe-N-C催化剂在碱性电解质中对氧还原反应的催化性能

经过热解聚苯胺、碳和FeCl₃的混合物制备的Fe-N-C材料在酸性电解质中对氧还原反应表现出高的催化活性;由于材料中不存在任何贵金属, 因而被认为是一类新型非贵金属氧还原催化剂. 然而这类催化剂在碱性电解质中催化氧还原反应的性能如何尚不清楚. 本文使用旋转圆盘电极技术考察了制备的两个Fe-N-C催化剂在KOH水溶液中催化氧还原反应性能, 发现这两个催化剂表现出比无金属的N掺杂碳材料更高的活性. 与商业Pt/C催化剂相比, 它们催化氧还原反应的起始电势和半波电势分别仅低60和40 mV左右, 计时电流测试表明, 它们比Pt/C催化剂显示出更好的稳定性. 此外, 在这两个Fe-N-C催化剂上的氧还原反应主要遵循四电子途径. 本工作显示, Fe-N-C材料有望用于碱性燃料电池氧还原反应催化剂.     

Construction of Co4 Atomic Clusters to Enable Fe−N4 Motifs with Highly Active and Durable Oxygen Reduction Performance

Fe−N−C catalysts with single-atom Fe−N₄ configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N₄ catalysts. The integration of Fe−N₄ configurations with highly uniform Co₄ ACs on the N-doped carbon substrate (Co₄@/Fe₁@NC) is realized through a “pre-constrained” strategy using Co₄ molecular clusters and Fe(acac)₃ implanted carbon precursors. The as-developed Co₄@/Fe₁@NC catalyst exhibits excellent ORR activity with a half-wave potential (E_1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm⁻² in a H₂−O₂ fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N₄ that modified with Co₄ ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.     

Surface site density and utilization of platinum group metal (PGM)-free Fe–NC and FeNi–NC electrocatalysts for the oxygen reduction reaction

Pyrolyzed iron-based platinum group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe–NC) are possible alternatives to platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M₁M₂–NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe–NC and FeNi–NC catalysts. Combining CO cryo chemisorption, X-ray absorption and ⁵⁷Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L_3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe–NC and bimetallic FeNi–NC; however, Ni blocks the formation of active sites during the pyrolysis and thus causes a sharp reduction in the accessible metal site density, while with only a minor direct participation as a catalytic site in the final catalyst. We also use the site density utilization factor, ϕ_{SDsurface/bulk}, as a measure of the metal site dispersion in PGM-free ORR catalysts. ϕ_{SDsurface/bulk} enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of accessible metal sites. It gives guidance for further optimization of the accessible site density of M–NC catalysts.

The gravimetric surface density and ORR catalytic turnover frequency of Fe–NC and Fe/Ni–NC catalysts were investigated. Both catalysts feature chemically identical Fe sites, but the presence of Ni lowered the gravimetric surface density of Fe sites.     

Non-planar Nest-like [Fe2S2] Cluster Sites for Efficient Oxygen Reduction Catalysis

Metal-nitrogen-carbon catalysts, as promising alternative to platinum-based catalysts for oxygen reduction reaction (ORR), are still highly expected to achieve better performance by modulating the composition and spatial structure of active site. Herein, we constructed the non-planar nest-like [Fe₂S₂] cluster sites in N-doped carbon plane. Adjacent double Fe atoms effectively weaken the O−O bond by forming a peroxide bridge-like adsorption configuration, and the introduction of S atoms breaks the planar coordination of Fe resulting in greater structural deformation tension, lower spin state, and downward shifted Fe d-band center, which together facilitate the release of OH* intermediate. Hence, the non-planar [Fe₂S₂] cluster catalyst, with a half-wave potential of 0.92 V, displays superior ORR activity than that of planar [FeN₄] or [Fe₂N₆]. This work provides insights into the co-regulation of atomic composition and spatial configuration for efficient oxygen reduction catalysis.     

Singlet Oxygen Induced Site-Specific Etching Boosts Nitrogen-Carbon Sites for High-Efficiency Oxygen Reduction

Targeted construction of carbon defects near the N dopants is an intriguing but challenging way to boost the electrocatalytic activity of N-doped carbon toward oxygen reduction reaction (ORR). Here, we report a novel site-specific etching strategy that features targeted anchoring of singlet oxygen (¹O₂) on the N-adjacent atoms to directionally construct topological carbon defects neighboring the N dopants in N-doped carbon (¹O₂−N/C). This ¹O₂−N/C exhibits the highest ORR half-wave potential of 0.915 V_RHE among all the reported metal-free carbon catalysts. Pyridinic-N bonded with a carbon pentagon of the neighboring topological carbon defects is identified as the primary active configuration, rendering enhanced adsorption of O₂, optimized adsorption energy of the ORR intermediates, and a significantly decreased total energy barrier for ORR. This ¹O₂-induced site-specific etching strategy is also applicable to different precursors, showing a tremendous potential for targeted construction of high-efficiency active sites in carbon-based materials.     

MnO/N-Doped Mesoporous Carbon as Advanced Oxygen Reduction Reaction Electrocatalyst for Zinc–Air Batteries

The development of alternative electrocatalysts exhibiting high activity in the oxygen reduction reaction (ORR) is vital for the deployment of large-scale clean energy devices, such as fuel cells and zinc–air batteries. N-doped carbon materials offer a promising platform for the design and synthesis of electrocatalysts due to their high ORR activity, high surface area, and tunable porosity. In this study, materials in which MnO nanoparticles are entrapped in N-doped mesoporous carbon (MnO/NC) were developed as electrocatalysts for the ORR, and their performances were evaluated in zinc–air batteries. The obtained carbon materials had large surface area and high electrocatalytic activity toward the ORR. The carbon compounds were fabricated by using NaCl as template in a one-pot process, which significantly simplifies the procedure for preparing mesoporous carbon materials and in turn reduces the total cost. A primary zinc–air battery based on this material exhibits an open-circuit voltage of 1.49 V, which is higher than that of conventional zinc–air batteries with Pt/C (Pt/C cell) as ORR catalyst (1.41 V). The assembled zinc–air battery delivered a peak power density of 168 mW cm⁻² at a current density of about 200 mA cm⁻², which is higher than that of an equivalent Pt/C cell (151 mW cm⁻² at a current density of ca. 200 mA cm⁻²). The electrocatalytic data revealed that MnO/NC is a promising nonprecious-metal ORR catalyst for practical applications in metal–air batteries.