Fe-Nx Active Sites Research Articles

Densely accessible Fe-Nx active sites decorated mesoporous-carbon-spheres for oxygen reduction towards high performance aluminum-air flow batteries

Single atomic iron modified nitrogen-doped carbons (SA Fe-N-C) are promising alternatives to Pt-based counterparts for facilitating the sluggish oxygen reduction reaction (ORR). However, both the low density of Fe-Nx active sites and their less utilization render these catalysts inefficient. To address these issues, a synergistic polymer-silicon dual templates strategy is reported to fabricate dense SA Fe-Nx sites on mesoporous carbon spheres (SA-Fe-Nx-MPCS). Thanks to the dense and accessible SA Fe-Nx sites, the SA-Fe-Nx-MPCS exhibits better ORR catalytic activity and stability than the commercial Pt/C catalyst. Moreover, the SA-Fe-Nx-MPCS result in a 3D porous framework with abundant channels in air cathode, facilitating mass transfer and enhancing accessibility of Fe-Nx moieties. The assembled aluminum-air flow battery exhibits a large peak power density of 130 mW cm−2 and a high gravimetric energy density of ca. 2905 Wh kgAl−1, as well as a superior stable discharging voltage over 300 h.

Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks

The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper, chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in terms of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity due to both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

Thermally Controlled Construction of Fe–Nx Active Sites on the Edge of a Graphene Nanoribbon for an Electrocatalytic Oxygen Reduction Reaction

Pyrolytically prepared iron and nitrogen codoped carbon (Fe/N/C) catalysts are promising nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell applications. Fabrication of the Fe/N/C catalysts with Fe-Nx active sites having precise structures is now required. We developed a strategy for thermally controlled construction of the Fe-Nx structure in Fe/N/C catalysts by applying a bottom-up synthetic methodology based on a N-doped graphene nanoribbon (N-GNR). The preorganized aromatic rings within the precursors assist graphitization during generation of the N-GNR structure with iron-coordinating sites. The Fe/N/C catalyst prepared from the N-GNR precursor, iron ion, and the carbon support Vulcan XC-72R provides a high onset potential of 0.88 V (vs reversible hydrogen electrode (RHE)) and promotes efficient four-electron ORR. X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) studies reveal that the N-GNR precursor induces the formation of iron-coordinating nitrogen species during pyrolysis. The details of the graphitization process of the precursor were further investigated by analyzing the precursors pyrolyzed at various temperatures using MgO particles as a sacrificial template, with the results indicating that the graphitized structure was obtained at 700 °C. The preorganized N-GNR precursors and its pyrolysis conditions for graphitization are found to be important factors for generation of the Fe-Nx active sites along with the N-GNR structure in high-performance Fe/N/C catalysts for the ORR.

Synthesis of Fe3C@C core-shell catalysts with controlled shell composition for robust oxygen evolution reaction

In this study, iron carbide (Fe3C) particles were encapsulated in graphitic carbon shells by a facile carbonation process. These core-shell carbides, in which iron carbide is encapsulated in graphene-like layer (Fe3C@C) and nitrogen doped graphene-like layer (Fe3C@C–N) were investigated as oxygen evolution reaction (OER) catalysts to investigate the effect of the carbon shell on catalyst activity and stability. Due to the protective effect of the graphitic carbon shells, the performance of Fe3C@C and Fe3C@C–N were considerably better than the bare Fe3C nanoparticles. The OER activity of Fe3C@C–N is comparable with that of carbon-supported ruthenium oxide (RuO2/C), and this core-shell carbide has a remarkable stability and high turnover frequency under alkaline conditions. To elucidate the real active sites of these core-shell carbides, the role of the Fe3C core, Fe–Nx active sites, and effect of nitrogen doping in the shell were investigated in detail.

Highly accessible sites of Fe-N on biomass-derived N, P co-doped hierarchical porous carbon for oxygen reduction reaction

Metal nitrogen-carbon catalysts have become a promising alternative to platinum-based catalysts in fuel cells due to their high stability and platinum-like activity. However, the corrosion and deactivation of active sites in the solution still restrict the inherent reaction kinetic rate. For this reason, it is important to stabilize the catalyst through a controllable doping strategy to obtain high activity catalysts for oxygen reduction reactions (ORR). Herein, the pyrolysis strategy is demonstrated in the synthesis of iron-based catalysts co-doped with nitrogen and biomass-derived phosphorus (denoted as N, P-Fe/C), and the pore size of the catalyst is mostly distributed at 1 nm or 50 nm, respectively. The half-wave potential (0.893 V) and the current density (4.05 mA cm−2) at 0.85 V of the catalyst exceed those of the commercial Pt/C. The remarkable ORR performance can be attributed to its distinct hierarchical pore structure, the modulation effect of nitrogen and phosphorus co-doping on the carbon matrix, and the combined effect of the FeNx active sites, which improves the accessibility of reactants and accelerating the absorption/desorption of the reaction intermediate, thereby increasing reaction rates. And N, P-Fe/C has great potential as a promising substitute for platinum-based catalysts.

Fabrication of Bioresource-Derived Porous Carbon-Supported Iron as an Efficient Oxidase Mimic for Dual-Channel Biosensing.

Herein, we designed a new strategy for fabricating a renewable bioresource-derived N-doped hierarchical porous carbon-supported iron (Fe/NPC)-based oxidase mimic. The obtained results suggested that Fe/NPC possessed a large specific surface area (1144 m2/g) and pore volume (0.62 cm3/g) to afford extensive Fe-Nx active sites. Taking advantages of the remarkable oxidase-mimicking activity, outstanding stability, and reusability of Fe/NPC, a novel dual-channel biosensing system was strategically fabricated for sensitively determining acetylcholinesterase (AChE) through the integration of Fe/NPC and fluorescent silver nanoclusters (AgNCs) for the first time. The limits of detection for AChE can achieve as low as 0.0032 and 0.0073 U/L by the outputting fluorometric and colorimetric dual signals, respectively. Additionally, this dual-signal system was applied to analyze human erythrocyte AChE and its inhibitor with robust analytical performance. This work provides one sustainable and effective avenue to apply a bioresource for fabricating an Fe/NPC-based oxidase mimic with high catalytic performance and also gives new impetuses for developing novel biosensors by applying Fe/NPC-based enzyme mimics as substitutes for the natural enzyme.

Highly dispersed Fe-Nx active sites on Graphitic-N dominated porous carbon for synergetic catalysis of oxygen reduction reaction

Sluggish oxygen reduction reaction (ORR) kinetics is the biggest obstacle in the development of metal air batteries. High-efficiency ORR catalysts with low cost are in urgent needs. Herein, we propose a robust Fe–N–C ORR catalyst with highly dispersed Fe-Nx active sites on graphitic-N dominated porous carbon. The synchrotron radiation analyses reveal the discrete Fe-Nx coordination derived from the Prussian blue (PB) while the graphitic-N doped porous carbon formed by pyrolysis of the polyaniline (PANI). In virtue of their synergistic catalysis, the tailored Fe–N–C catalyst shows a comparable catalytic activity and a better methanol-tolerance than the commercial Pt/C catalyst. The assembled Zn-Air battery demonstrates a specific capacity of 783 mAh g−1 as well as energy density of 917 Wh kg−1 at 10 mA cm−2, and a maximum power density of 112 mW cm−2 at 191 mA cm−2.

Ultrafine Fe/Fe3C decorated on Fe-N -C as bifunctional oxygen electrocatalysts for efficient Zn-air batteries

Abstract Efficient bifunctional oxygen electrocatalysts for ORR and OER are fundamental to the development of high performance metal-air batteries. Herein, a facile cost-efficient two-step pyrolysis strategy for the fabrication of a bifunctional oxygen electrocatalyst has been proposed. The efficient non-precious-metal-based electrocatalyst, Fe/Fe3C@Fe-Nx-C consists of highly curved onion-like carbon shells that encapsulate Fe/Fe3C nanoparticles, distributed on an extensively porous graphitic carbon aerogel. The obtained Fe/Fe3C@Fe-Nx-C aerogel exhibited superb electrochemical activity, excellent durability, and high methanol tolerance. The experimental results indicated that the assembly of onion-like carbon shells with encapsulated Fe/Fe3C yielded highly curved carbon surfaces with abundant Fe-Nx active sites, a porous structure, and enhanced electrocatalytic activity towards ORR and OER, hence displaying promising potential for application as an air cathode in rechargeable Zn-air batteries. The constructed Zn-air battery possessed an exceptional peak power density of ~147 mW cm−2, outstanding cycling stability (200 cycles, 1 h per cycle), and a small voltage gap of 0.87 V. This study offers valuable insights regarding the construction of low-cost and highly active bifunctional oxygen electrocatalysts for efficient air batteries.

Using a Fe-doping MOFs strategy to effectively improve the electrochemical activity of N-doped C materials for oxygen reduction reaction in alkaline medium

For the rational design of heteroatom-doping carbon catalysts toward the oxygen reduction reaction (ORR), various structure-wise and porous metal-organic-frameworks (MOFs) were preferred especially the transition metal imidazoles frameworks (ZIFs) with abundant N and C elements. Herein, taking advantage of the volatile nature of Zn in ZIF-Zn at high temperatures, we synthesized the bimetallic ZIFs with Zn/Fe molar ratio of 10:1 firstly, and then obtained a structure-define and porous Fe- and N-doped C electrocatalyst by calcining at 900 °C in Ar flow. With the well-defined polyhedrons morphology, high surface area, and evenly distributed C-Nx and Fe-Nx active sites, the as-synthesized Fe-N-C catalysts including the acid-leached Fe-N-C gave superior ORR capability, selectivity, and stability in 0.1 M KOH, and can be also applied in acidic electrolyte. Inspiringly, Fe-N-C exhibited excellent ORR activity exceeding Pt/C benchmark in 0.1 M KOH with a half-wave potential (E1/2) of 0.878 V and the kinetic current density (jk at 0.85 V) of 13.2 mA cm−2, and the single N-C material. And after acid leaching, the obtained Fe-N-C(L) catalyst still maintained the excellent ORR activity (E1/2 = 0.855 V, jk = 6.8 mA cm−2). Additionally, though their ORR performance in acid was inferior to that of Pt/C, the gap was also significantly narrowed by Fe-introduction into N-C structures. Experiments demonstrated that Fe-Nx structure in N-doped C matrix and cooperated with C-Nx centers were very essential to deliver the significantly enhanced ORR activity, in addition to good structural characteristics, i.e., the high pore volume, abundant defects, and good conductivity. This synthesis method based on M-doped MOFs was expected to be transplanted to obtain other energy-relative non-precious metal catalysts.

The Fe-N-C Nanozyme with Both Accelerated and Inhibited Biocatalytic Activities Capable of Accessing Drug-Drug Interactions.

Emerging as a cost-effective and robust enzyme mimic, nanozymes have drawn increasing attention with broad applications ranging from cancer therapy to biosensing. Developing nanozymes with both accelerated and inhibited biocatalytic properties in a biological context is intriguing to peruse more advanced functions of natural enzymes, but remains challenging, because most nanozymes are lack of enzyme-like molecular structures. By re-visiting and engineering the well-known Fe-N-C electrocatalyst that has a heme-like Fe-Nx active sites, herein, it is reported that Fe-N-C could not only catalyze drug metabolization but also had inhibition behaviors similar to cytochrome P450 (CYP), endowing it a potential replacement of CYP for preliminary evaluation of massive potential chemicals, drug dosing guide, and outcome prediction. In addition, in contrast to electrocatalysts, the highly graphitic framework of Fe-N-C may not be obligatory for a competitive CYP-like activity.

Nano-Fe0 embedded in N-doped carbon architectures for enhanced oxidation of aqueous contaminants

Nano-Fe0 embedded in N-doped carbon architectures (Fe0@N-C) were used as a peroxymonosulfate (PMS) activator for the mineralization of organic pollutants. Fe0@N-C was synthesized through a MOF-mediated recrystallization approach. Fe0@N-C exhibited high activity and reusability, better than that of most conventional metal catalysts. Further, the influences of some representative factors for the PMS activation were comprehensively analyzed and optimized. The enhanced performance of Fe0@N-C was primarily ascribed to abundant reactive oxygen species, especially 1O2, released in the oxidation process at the solid/aqueous interface. Nano-Fe0 wrapped with N-doped graphitic layers could preferentially generate effective Fe0 and Fe-Nx active sites with the electronic structure reconstruction, endowing the outstanding catalytic activity. The unique porous carbon structure not only protected the embedded nano-Fe0 from poisoning or leaching, but also provided suitable mass transfer channels and also promoted electron tunneling efficiency. These outstanding characteristics manifested Fe0@N-C had promising potential for organic pollutant degradation.

Ordered mesoporous carbon with atomically dispersed Fe-Nx as oxygen reduction reaction electrocatalyst in air-cathode microbial fuel cells

As a renewable energy source, microbial fuel cell (MFC) exhibits a great potential in solution of energy crisis and environmental pollution. The cathode catalyst for oxygen reduction reaction (ORR) is one of the most important factors that can affect the performance of fuel cell. Herein, MOF-derived ordered mesoporous carbon (Fe–N/C) with atomically dispersed Fe-Nx active sites was successfully prepared. It can significantly improve the ORR activity in alkaline, acidic or neutral electrolyte, inspiring us to establish a MFC using it as cathode catalyst. The Fe–N/C-MFC performs great maximum power density (1232.9 mW m−2), high Open Circuit Voltage (0.644 V) and large constant output voltage (0.46 V) which are all superior to that of 20% Pt–C. The ohmic resistance (Rohm) and charge transfer resistance (Rct) of Fe–N/C decrease by 36.2% and 88.7% from 25.4 Ω to 110.6 Ω (N/C) to 16.2 Ω and 12.5 Ω (Fe–N/C), respectively. Note that the Fe–N/C holds an ordered mesoporous structure with the characteristic of Fe-Nx active sites doping, and content of pyridine-like N and graphite-like N are both significantly increased compared with N/C sample. These results make the atomically dispersed Fe-Nx ordered mesoporous Fe–N/C promising for commercial application of MFC.

Fe-Nx Sites enriched microporous carbon nanoflower planted with tangled bamboo-like carbon nanotube as a strong polysulfides anchor for lithium–sulfur batteries

Serious shuttle effect and sluggish conversion kinetics of lithium polysulfides (LiPSs) have a massive impact on obstructing the practical application of lithium-sulfur (Li–S) batteries. To address such issues, Fe-Nx sites enriched microporous nanoflowers planted with tangled bamboo-like carbon nanotubes (Fe-Nx-C/Fe3C-CNTs NFs) are found to be effective catalytic mediators with strong anchoring capabilities for LiPSs. The bamboo-like carbon nanotubes catalyzed by Fe3C/Fe entangled each other to form a conductive network, which encloses a flower-like microporous carbon core with embedded well-dispersed Fe-Nx active sites. As expected, electrons smoothly transfer along the dense conductive bamboo-like carbon network while the flower-like carbon core consisting of micropores induces the homogeneous distribution of tiny sulfur and favors the lithium ions migration with all directions. Meanwhile, Fe-Nx sites strongly trap long-chain LiPSs with chemical anchoring, and catalyze the redox conversion of LiPSs. Due to the aforementioned synergistic effects, the S@Fe-Nx-C/Fe3C-CNTs NFs cathode exhibited a remarkable specific capacity (635 mAh gs−1) at 3 C and a favorable capacity decay with 0.04% per cycle even after 400 cycles at 1 C.

Iron-Catalyzed Selective Denitrification over N-Doped Mesoporous Carbon.

Electrocatalytic denitrification has been considered as one of the most promising technologies to selectively reduce excessive nitrogen oxyanions to benign dinitrogen in polluted water. The development of nonprecious metal catalysts with high performance for the denitrification is imperative but still a great challenge. Herein, we report an iron-based electrocatalyst for high-efficiency and selective denitrification. This nanocatalyst is prepared by template assembly of iron-based metal organic frameworks on surface-functionalized bimodal mesoporous carbon and subsequent pyrolysis under argon atmosphere. It contains well-dispersed FeNx sites and Fe nanoparticles (most of them are wrapped by N-doped graphitic carbon), with high surface area (1080 m2 g-1) and uniform mesopore and micropore (5.0 and 1.7 nm) for the construction of electrodes. Experimental results confirm that the catalyst can achieve an excellent nitrate removal capacity of 3410 mg N/g Fe, along with a reduction efficiency up to 87% and N2 selectivity of 81% in neutral electrolyte (100 mg/L NO3--N and 0.1 M Na2SO4) within 24 h. It is proposed that the active FeNx sites cooperated with sufficient Fe nanoparticles to be conducive to adsorption of O in NO3- on the catalyst for the improved performance. The material also presents long-term durability and good adaptability in the range of pH 5-9 and simulated neutral wastewater, mainly contributed by the protection of the mesostructure and graphitic N-doped carbon. These findings open the door to rational design of nonprecious metal iron-based functional electrocatalysts for water purification and environmental remediation.

Atomically Dispersed Fe on Nanosheet-linked, Defect-rich, Highly N-Doped 3D Porous Carbon for Efficient Oxygen Reduction

Exploring cost-effective and high-performance oxygen reduction reaction(ORR) electrocatalysts to replace precious platinum-based materials is crucial for developing electrochemical energy conversion devices but remains a great challenge. Herein, Fe single atoms anchored on nanosheet-linked, defect-rich, highly N-doped 3D porous carbon(Fe-SAs/NLPC) electrocatalysts were obtained by pyrolyzing salt-sealed Fe-doped zeolitic imidazolate frameworks(ZIFs). NaCl functions both as pore-forming agent and closed nanoreactor, which can not only lead to the formation of defects-rich three-dimensional interconnected structures with high N-doping content to expose abundant active sites, promote mass transfer and electron transfer, but also facilitate the effective incorporation of Fe to form Fe-Nx active sites without aggregation. These unique characteristics render Fe-SAs/NLPC outstanding electrocatalytic activity for ORR, with one-set potential of 0.96 V and high kinetic current density(jK) of 33.32 mA/cm2 in alkaline medium, which surpass the values of most nonprecious-metal catalysts and even commercial Pt/C.

(Invited) Self-Template Synthesis of Atomically Dispersed Fe/N-Codoped Nanocarbon As Efficient Bifunctional Oxygen Electrocatalyst

High performance and durable bifunctional oxygen electrocatalysts are of great importance for the commercial application of environmentally friendly and sustainable energy through electrochemical devices such as rechargeable Zn–air batteries and fuel cells. Highly distributed catalysts with maximum atomic utilization are considered to be a promising alternative for current noble-metal-based catalysts. Herein, a high performance and durable bifunctional oxygen electrocatalyst for both ORR and OER is synthesized by a facile self-template and polypyrrole encapsulation method. The super performance atomically dispersed Fe/N-codoped double shelled hollow carbon nanospheres (AD-Fe/N-C-900) catalyst has a special open porous double shell hollow structure which is beneficial to exposure of more Fe-Nx active sites for ORR and OER, and improves mass transfer, resulting in a fabulous bifunctional ORR/OER electrocatalytic activity (△E =0.655 V), which is better than the commercial Pt/C (△E =1.035 V) and the commercial IrO2 (△E =0.93 V), along with remark durability and long-term stability for the ORR in alkaline medium. Based on our results and analysis, the remarkable electrocatalytic activity of AD-Fe/N-C-900 might be attributed to the high content of mesopore, high density of Fe-Nx active sites, and robust double shell hollow structure. The novel design of the atomically dispersed Fe/N-codoped porous double shell hollow nanocarbon enable boosted oxygen catalyst by a facile method has certain enlightening effect on the design and development of other bifunctional electrocatalysts in the next generation reversible energy conversion systems. Our work provides a design idea for the synthesis of high activity and stability bifunctional atom-level distributed catalysts. Figure 1

Highly exposed atomic Fe–N active sites within carbon nanorods towards electrocatalytic reduction of CO2 to CO

Single-atom transition-metal-anchored nitrogen-doped carbon (M-N-C) materials show great potential in electrochemical reduction of CO2 to CO. However, the development of catalysts with high exposure density of M-Nx active sites remains key challenge. Herein, we report an atomically dispersed and highly exposed Fe coordinated to nitrogen (Fe-Nx) active sites doped within carbon nanorods (Fe–N–C) by pyrolysis of sea urchin-like FeOOH-polyaniline (FeOOH-PANI) composite. Results from X-ray photoelectron spectra (XPS) and operando X-ray absorption fine structure (XAFS) spectra confirmed the atomically dispersed Fe species and indicated the Fe coordinated with four N atoms to form the highly exposed Fe-Nx active sites. Detailed examination of the Fe–N–C electrocatalyst reveals a high selectivity for CO2 reduction, presenting CO Faradaic efficiency (FECO) of 95% with CO partial current density (jCO) of 1.9 mA cm−2 at a relatively low overpotential of 530 mV. The high-performance is a result of the porous structure of catalyst with highly exposed Fe-Nx active sites, as well as the larger specific surface area and electrochemical active surface area. Our work proposes an effective and feasible way to designing M-N-C catalysts for efficient electrochemical reduction of CO2.

Seeded growth of branched iron–nitrogen-doped carbon nanotubes as a high performance and durable non-precious fuel cell cathode

Achieving a high-density Fe-Nx moieties in a highly graphitic and porous carbon matrix is critical for developing iron/nitrogen-doped carbon (Fe–N/C) electrocatalysts with both high activity and high stability on oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Herein, we report the growth of Fe/N-doped carbon nanotubes (CNTs) by pyrolysis of Fe-doped zeolitic imidazolate framework-8 (Fe-ZIF-8) precursors on a primary Fe/N-doped CNT seed. Such seeded growth enabled the successful conversion of the Fe-ZIF-8 precursor, with inherent Fe-Nx complex, into a large number of secondary CNTs doped with high-density Fe-Nx sites while keeping a high degree of graphitization. The resulted catalyst possesses high-density Fe-Nx active sites, porous network, enhanced electron conductivity and improved anti-corrosion capability which lead to both improved performance and durability in PEMFCs compared to the pyrolyzed Fe-ZIF-8 catalyst (Fe-ZIF′). In addition, the highly graphitic structure combined with the hierarchically branched CNT network enables a high hydrophobicity for efficient water removal and thus much slower performance decay than the benchmark Fe-ZIF’ catalyst during accelerated stress tests. The successful introduction of active Fe-Nx sites into highly graphitic and branched CNTs provides a new way for the development of durably active noble-metal-free ORR electrocatalysts.

Hierarchically open-porous carbon networks enriched with exclusive Fe–Nx active sites as efficient oxygen reduction catalysts towards acidic H2–O2 PEM fuel cell and alkaline Zn–air battery

The development of both H2–O2 proton exchange membrane fuel cells (PEMFCs) and Zn-air batteries are challenged by high-efficiency air cathodes with a high oxygen reduction reaction (ORR) electrocatalytic-activity. The increase of density and accessibility of active sites are widely accepted effective approach to boost the oxygen reduction reaction. In this work, we develop a template-free g-C3N4-assisted pyrolysis strategy to transform ZIF-8 polyhedrons to hierarchically open-porous carbon networks incorporated exclusive Fe–Nx active sites. Due to the integration of high specific surface area, highly open porous architecture with meso/micro-pores, and high-density Fe-Nx active species, the electro-catalysis performance of optimal catalysts exceed commercial Pt/C in alkaline solution, and is comparable to that of commercial Pt/C in acidic solution, which is one of the best recently reported carbon-based electrocatalysts. Impressively, when it is used as an air cathode catalyst, it achieves a high power density of 481 mW·cm−2 in acidic H2–O2 proton exchange membrane fuel cell (PEMFC) and an excellent performance in the alkaline Zn–air battery (a high power density of 121 mW·cm−2).

Metal–organic framework-derived mesoporous carbon nanoframes embedded with atomically dispersed Fe–Nx active sites for efficient bifunctional oxygen and carbon dioxide electroreduction

Metal–organic frameworks (MOFs) can be utilized as superior precursors to pyrolytically achieve the single-atom catalysts. However, most of the atomic active sites are buried inside the microporous carbon matrix, which severely impedes the accessibility of active sites and limits mass transport. Herein, the mesoporous carbon nanoframes with hierarchical pore size distribution and atomically dispersed Fe–Nx active sites were synthesized from Fe-doped MOF precursors. The dense Fe atoms within the nanostructures are dispersed in single-atom level with metalloporphyrin-like Fe–N4 configuration. The introduction of plentiful large mesopores into the nanoframes would not only enhance the accessibility of abundant single-atom active sites, but also boost mass and charge transports. Such distinctive nanostructure led to exceptional bifunctional electrocatalytic performances for oxygen reduction reaction, with more positive onset potential (1.01 V vs. 0.97 V) and half-wave potential (0.89 V vs. 0.82 V) compared with the commercial Pt/C, and electrochemical carbon dioxide reduction.