Oxygen Reduction Reaction In Alkaline Electrolyte Research Articles

Modulating Spin of Atomic Manganese Center for High-Performance Oxygen Reduction Reaction.

Single atom catalysts (SACs) are promising non-precious catalysts for oxygen reduction reaction (ORR). Unfortunately, the ORR SACs usually suffer from unsatisfactory activity and in particular poor stability. Herein, we report atomically dispersed manganese (Mn) embedded on nitrogen and sulfur co-doped graphene as an efficient and robust electrocatalyst for ORR in alkaline electrolyte, realizing a half-wave potential (E1/2) of 0.883 V vs. reversible hydrogen electrode (RHE) with negligible activity degradation after 40,000 cyclic voltammetry (CV) cycles in 0.1 M KOH. Introducing sulfur (S) to form Mn-S coordination changes the spin state of single Mn atom from high-spin to low-spin, which effectively optimizes the oxygen intermediates adsorption over the single Mn atomic sites and thus greatly improves the ORR activity.

Modulating Spin of Atomic Manganese Center for High‐Performance Oxygen Reduction Reaction

Single atom catalysts (SACs) are promising non‐precious catalysts for oxygen reduction reaction (ORR). Unfortunately, the ORR SACs usually suffer from unsatisfactory activity and in particular poor stability. Herein, we report atomically dispersed manganese (Mn) embedded on nitrogen and sulfur co‐doped graphene as an efficient and robust electrocatalyst for ORR in alkaline electrolyte, realizing a half‐wave potential (E1/2) of 0.883 V vs. reversible hydrogen electrode (RHE) with negligible activity degradation after 40,000 cyclic voltammetry (CV) cycles in 0.1 M KOH. Introducing sulfur (S) to form Mn‐S coordination changes the spin state of single Mn atom from high‐spin to low‐spin, which effectively optimizes the oxygen intermediates adsorption over the single Mn atomic sites and thus greatly improves the ORR activity.

Advanced Hollow Cubic FeCo-N-C Cathode Electrocatalyst for Ultrahigh-Power Aluminum-Air Battery.

The exploration of electrocatalysts toward oxygen reduction reaction (ORR) is pivotal in the development of diverse batteries and fuel cells that rely on ORR. Here, a FeCo-N-C electrocatalyst (FeCo-HNC) featuring with atomically dispersed dual metal sites (Fe-Co) and hollow cubic structure is reported, which exhibits high activity for electrocatalysis of ORR in alkaline electrolyte, as evidenced by a half-wave potential of 0.907V, outperforming that of the commercial Pt/C catalyst. The practicality of such FeCo-HNC catalyst is demonstrated by integrating it as the cathode catalyst into an alkaline aluminum-air battery (AAB) paring with an aluminum plate serving as the anode. This AAB demonstrates an unprecedented power density of 804mWcm-2 in ambient air and an impressive 1200mWcm-2 in an oxygen-rich environment. These results not only establish a new benchmark but also set a groundbreaking record for the highest power density among all AABs reported to date. Moreover, they stand shoulder to shoulder with state-of-the-art H2-O2 fuel cells. This AAB exhibits robust stability with continuous operation for an impressive 200 h. This groundbreaking achievement underscores the immense potential and forward strides that the present work brings to the field.

Synthesis of novel hollow carbon nanotubes @ Co-Fe alloy/iron phthalocyanine electrocatalyst by self-assembly method for OER and ORR study

The exploration of an inexpensive catalyst with effective oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities to replace precious metal electrocatalysts has been of great interest in recent years. In this paper, CNT@Co-Fe/FePc composites were synthesized with high efficiency using a self-assembly strategy. In the synthesis process, carbon nanotubes (CNTs) were synthesized as the base material using crab shells as templates and honey as the carbon and nitrogen source. CoFe alloy nanoparticles derived from CoFe-based prussian blue analogs were self-assembled and anchored on CNTs, and then iron phthalocyanine with Fe-N4 portion was coupled with CNTs by π-π stacking. Finally formed a homogeneous and stable composite material. The self-assembly method is simple and efficient to achieve good metal loading. The prepared CNT@Co2-Fe1/FePc exhibits excellent catalytic performance and stability for OER and ORR in alkaline electrolyte. The overpotential at 10 mA cm−2 of CNT@Co2-Fe1/FePc for OER is 343 mV with a very small Tafel slope of 43.59 mV dec−1. CNT@Co2-Fe1/FePc has high methanol tolerance and persistence for ORR. This work provides a strategy for the construction of transition metal-carbon composite electrocatalytic materials with excellent catalytic performance and stability for OER and ORR reactions.

Toward quantification of active site density and size-dependent ORR activity of ZIF-derived carbons in alkaline electrolyte

ZIF-derived carbons (ZDCs) represent a novel category of non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR). Nevertheless, establishing the correlation between their structure and performance is challenging due to their diverse structural properties and ill-defined active site moieties. Here we investigate the effect of ZIF-67 derived carbon’s particle size and corresponding distribution of assorted nitrogen functionalities on the ORR. Our findings reveal a substantial increase in ORR activity with decreasing ZDC particle size, while the exact influence of nitrogen functionalities remains uncertain. Most importantly, the present work unveils a correlation between the ORR activity and the site density (SD) of Co-Nx within the ZDC materials, which can be attributed to the concurrently increasing SD and accessibility of Co-Nx on the ZDC particles with small sizes. The present work provides novel insights into the optimization of ZDCs as high-performance NPMCs for ORR in alkaline electrolytes.

Tailoring hierarchically porous nanoarchitectured N-doped carbon decorated with FeIIN4 moiety and encapsulated Fe/Fe3C nanoparticles as a synergistic catalyst for ORR in Zn-air battery

Rational designing and exploiting non-noble metal electrocatalysts with desirable nanoarchitecture and abundant active sites are crucial but challenging requirements for the development of fuel cells and Zn-air batteries (ZABs). Herein, a highly efficient carbonous hybrid is developed using a facile and effective synthesis strategy via template-assisted and optimized post-pyrolysis processes. The optimized electrocatalyst (denoted as Fe@NC-700) has extremely large specific surface area (1262.8 m2 g–1), and presents a desirable hierarchically porous nanoarchitecture, including plenty micro-/meso-pores, which endows the catalyst with the enhanced mass transfer for the reaction species. More importantly, Fe@NC-700 possesses not only numerous FeIIN4 moiety uniformly dispersed in N-doped carbon matrix, but also encapsulated Fe/Fe3C nanoparticles, synergistically boosting the electrocatalytic activity towards oxygen reduction reaction (ORR) verified by density functional theory (DFT) calculation. Due to the beneficial microstructure and rich active sites, the catalyst has a positive half-wave potential (E1/2) of 0.865 V for ORR in alkaline electrolyte, intrinsic 4e– reaction path and robust stability (only 14 mV negative shift of E1/2 after 10,000 potential cycles), outperforming the Pt/C. Impressively, the aqueous and quasi-solid-state primary ZABs assembled with Fe@NC-700 as cathode catalyst demonstrate superior discharge performance and excellent durability, holding promising potential in practical application of energy conversion devices.

Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90V versus RHE and a half-wave potential of 0.72V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

High‐Temperature Air Synthesis: A Facile Approach to Nitrogen‐Doped, Metal‐Free Carbon Electrocatalysts

AbstractThis study presents a novel, straightforward method for synthesizing hierarchical nitrogen‐doped carbon structures, positioning metal‐free, carbon‐based materials as potential substitutes in electrochemical reactions such as oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). The unique method involves a single‐step pyrolysis process in an air atmosphere, eliminating the need for an inert atmosphere and pre‐treatment procedures. It enables simultaneous self‐templating and heteroatom doping, resulting in oxygen‐rich functional groups embedded in the nitrogen‐doped carbon structure. We also crafted a carbon structure without heteroatom doping, comparing its electrochemical performance in ORR and HER. Our findings indicate that carbon catalysts pyrolyzed at higher temperatures have more pyridinic N, functional groups, and active sites‐ factors conducive to electrochemical reactions. We tested the air‐synthesized electrocatalysts for ORR in alkaline electrolyte and employed the optimized nitrogen‐doped carbon catalyst, pyrolyzed at 700 °C in an air atmosphere, as cathode material in a zinc‐air battery. This catalyst demonstrated ORR performance comparable to the commercial Pt/C catalyst and showed minimal overpotential in acidic HER. Our research establishes a pioneering technique for synthesizing porous, metal‐free, nitrogen‐doped carbon materials, paving the way for potential energy applications.

FeNC Catalysts with High Catalytic Activity and Stability for Oxygen Reduction Reaction

Although Pt-based catalysts have been considered as the most effective electrocatalyst for the cathodic oxygen reduction reaction (ORR) of direct methanol fuel cells (DMFCs), they still suffer from the drawbacks of high cost, poor long-term stability and methanol/ CO poisoning effects. Thus, developing low-cost ORR catalysts with high efficiency, durability and antipoisoning ability is of paramount importance. Herein, a series of non-noble metal FeNC materials are prepared through a facile pyrolysis process and used as the electrocatalysts toward ORR in alkaline electrolyte. Results show that the Fe0.50NC-800-1h catalyst pyrolyzed at 800 ºC for 1 h with the mass ratio of Fe(NO3)3•6H2O to melamine being 0.50 exhibits the highest catalytic performance among the as-prepared FeNC catalysts. The half-wave potential of ORR is ca. 0.81 V, which is only 38 mV lower than that on the noble metal Pt/C catalyst. Besides, it also displays higher stability and methanol tolerance than Pt/C. There is almost no change in the current during the chronoamperometric test when methanol is added in the electrolyte whereas significant decrease is found on Pt/C catalyst. This study of FeNC catalysts provides new insights on understanding the ORR mechanism and suggests a promising strategy to develop low-cost and highly efficient non-noble metal electrocatalysts for ORR.

Self-catalyzed Co, N-doped carbon nanotubes-grafted hollow carbon polyhedrons as efficient trifunctional electrocatalysts for zinc-air batteries and self-powered overall water splitting

It is still essential and challenging to explore inexpensive and versatile electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), for the development of rechargeable zinc-air batteries (ZABs) and overall water splitting. Herein, a rambutan-like trifunctional electrocatalyst is fabricated by re-growth of secondary zeolitic imidazole frameworks (ZIFs) on ZIF-8-derived ZnO and the following carbonization treatment. Co nanoparticles (NPs) are encapsulated into N-doped carbon nanotubes (NCNT) grafted N-enriched hollow carbon (NHC) polyhedrons to form the Co-NCNT@NHC catalyst. The strong synergy between the N-doped carbon matrix and Co NPs endows Co-NCNT@NHC with trifunctional catalytic activity. The Co-NCNT@NHC displays a half-wave potential of 0.88 V versus RHE for ORR in alkaline electrolyte, an overpotential of 300 mV at 20 mA cm−2 for OER, and an overpotential of 180 mV at 10 mA cm−2 for HER. Impressively, a water electrolyzer is successfully powered by two rechargeable ZABs in series, with Co-NCNT@NHC as the 'all-in-one' electrocatalyst. These findings are inspiring for the rational fabrication of high-performance and multifunctional electrocatalysts intended for the practical application of integrated energy-related systems.

Sb2S3-templated synthesis of sulfur-doped Sb-N-C with hierarchical architecture and high metal loading for H2O2 electrosynthesis

Selective two-electron (2e−) oxygen reduction reaction (ORR) offers great opportunities for hydrogen peroxide (H2O2) electrosynthesis and its widespread employment depends on identifying cost-effective catalysts with high activity and selectivity. Main-group metal and nitrogen coordinated carbons (M-N-Cs) are promising but remain largely underexplored due to the low metal-atom density and the lack of understanding in the structure-property correlation. Here, we report using a nanoarchitectured Sb2S3 template to synthesize high-density (10.32 wt%) antimony (Sb) single atoms on nitrogen- and sulfur-codoped carbon nanofibers (Sb-NSCF), which exhibits both high selectivity (97.2%) and mass activity (114.9 A g−1 at 0.65 V) toward the 2e− ORR in alkaline electrolyte. Further, when evaluated with a practical flow cell, Sb-NSCF shows a high production rate of 7.46 mol gcatalyst−1 h−1 with negligible loss in activity and selectivity in a 75-h continuous electrolysis. Density functional theory calculations demonstrate that the coordination configuration and the S dopants synergistically contribute to the enhanced 2e− ORR activity and selectivity of the Sb-N4 moieties.

N-doped carbon networks as bifunctional electrocatalyst toward integrated electrochemical devices for Zn-air batteries driving microbial CO2 electrolysis cell

A simple pyrolysis route has been developed for fabrication of nitrogen-doped carbon networks (N-Cnet) that not only features a large number of mesoporous structures, but also holds high specific surface area and excellent conductivity. The as-developed N-Cnet performs impressively toward electrocatalysis of oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR). For ORR in alkaline electrolyte, the N-Cnet manifests an initial potential of 0.95 V and half-wave potential of 0.86 V that are comparable to the benchmarked Pt/C catalysts. While for CO2RR, the N-Cnet shows a high selectivity for CO production with Faraday efficiency (FE) of as high as 95% and long-term stability. We further demonstrate the potential application of N-Cnet in electrochemical devices of zinc-air batteries (ZABs) and microbial CO2 electrolysis cell (MEC), the former is capable of releasing a maximum power density of 141 mW cm-2, while the later shows capability for efficiently converting CO2 into CO with the assistance of external voltage. We thus propose and set up an integrated proof-of-concept electrochemical device with ZABs driving MEC that can implement the feasibility of self-powered conversion of CO2 to value-added CO.

Strong anion exchange for improved NiCo2S4 oxygen reduction reaction via interlayer spacing manipulation

Transition metal sulfides as high-performance, cost-effective, and stable electrocatalysts for oxygen reduction reaction (ORR) have attracted much attention. The replacement of sulfur atom by oxygen atom in metal sulfides was considered to be the fundamental reason for its decreased activity. Adequate anion exchange can greatly reduce the generation of oxygen species in transition metal sulfide, which further enhances the intrinsic electrical conductivity and stability of transition metal sulfides. Herein, a novel and simple approach to larger interlayer spacing was provided by introducing interlayer water in layered hydroxides precursor. The NCS-O 1 with treated layered hydroxides precursor exhibits few oxygen impurities and excellent activity for the ORR in alkaline electrolyte, presenting the close limiting diffusion current density and lower Tafel slope (reaches 56 mV dec −1 , vs. Pt/C 70) in comparison to 20% commercial Pt/C. Hence, our results provide an interesting design path for such catalysts. • A novel approach to synthesize NiCo 2 S 4 with few oxygen impurities. • The larger interlayer spacing in Ni–Co hydroxide favors the anion exchange. • The oxygen incorporation in NiCo 2 S 4 causes lower d-band center of metal sites. • The decreased oxygen impurities in NiCo 2 S 4 are conducive to oxygen reduction.

Au(001) Thin Films: Impact of Structure and Mosaicity on the Oxygen Reduction Reaction in Alkaline Medium

Au(001) thin films with thickness varying from ca. 25 to 83 nm were deposited on MgO(001) substrates using pulsed laser deposition. Epitaxial growth of (001) Au on the (001) MgO single-crystal substrate was observed with a Au(001)[010]//[010](001)MgO cube-on-cube epitaxial relationship for substrate temperature ≤200 °C. Detailed X-ray reciprocal space mapping (RSM) analysis and atomic force microscopy (AFM) imaging of films with different thicknesses were conducted. There is a nonmonotonic dependence of both the in-plane and out-of-plane lattice parameters with the films’ thickness, while the lateral coherence length increases with the thickness. The maximum tensile strain measured was 0.3% for 65 nm thick Au(001) film. Cyclic voltammetry (CV) in acidic and alkaline media and Pb underpotential deposition (Pb UPD) measurements were used as fingerprints to assess the structure of the surface. All films exhibited the characteristic response of Au(001) single-crystal surfaces, although with some differences in the potential region between 0.7 and 1.2 V vs reversible hydrogen electrode (RHE), which corresponds to the adsorption and partial discharge of OH– anions at the electrode surface. Since epitaxial Au(001) thin films do not lend themselves to steady-state voltammetry with rotating disks or microelectrodes due to their square shape and the insulating nature of MgO substrates, the electrocatalytic activity of the films for the oxygen reduction reaction (ORR) in the alkaline electrolyte was assessed by sampled current voltammetry. It was shown that both the onset potential (Eonset) and the half-wave potential (E1/2) for the ORR do not show any dependence on strain but rather become less negative (more active) with an increase of the lateral coherence length. These data are rationalized in the context of recent density functional theory (DFT) calculations and the mechanisms governing the ORR in alkaline electrolytes.

Influence of Co doping on physiochemical properties of MnFe2O4/C nano compounds toward oxygen reduction reaction

Recently, several researchers have found that the manganese ferrite (MnFe2O4) nanoparticle can be a promising candidate for the oxygen reduction reaction (ORR). However, the MnFe2O4 nanoparticles still face challenges on their ORR activity and stability for practical application. In this work. The Co-doped MnFe2O4 electrocatalysts are explored to improve their intrinsic activity for the ORR in alkaline electrolytes. The MnFe2−xCoxO4/C (x = 0.25–0.75) electrocatalysts show significant improvements on the ORR performance, particularly the MnFe1.5Co0.5O4/C electrocatalyst exhibits the best ORR performance with 80 mV positive shift in half-wave potential compared to the pristine MnFe2O4. Meanwhile, the MnFe1.5Co0.5O4/C nanoparticles display a uniform spherical shape with a size of ~11–15 nm, compared to the irregular shapes of the MnFe2O4/C nanoparticles with sizes in the range of ~8-~70 nm. The Co doping has redistributed the cations between the tetrahedral and octahedral sites in the MnFe2O4 structure from the XRD, TEM, FTIR, and XPS analysis. Furthermore, the particles sizes of the doped MnFe2O4/C nanoparticles are smaller and uniform, with more oxygen vacancies. In addition, the Co doping has enriched the surface Mn3+ ions and improved the conductivity of the MnFe2O4/C nanoparticles. The above results are closely related to the enhanced ORR activity of the Co-doped MnFe2O4/C compared to the pristine of the MnFe2O4/C.

Metal-Organic Framework and Carbon Black supported MOFs as dynamic electrocatalyst for oxygen reduction reaction in an alkaline electrolyte

The Pt-based expensive catalysts and sluggish kinetics at cathode in oxygen reduction reaction (ORR) hinder the rapid commercialization of fuel cells. The quest for cheap, non-noble metal catalysts to replace Pt-based catalysts has thus become a critical issue in the field of fuel cells. The carbon black (CB) and CB supported catalyst have been explored with the ultimate goal of finding a substitute for Pt-based catalysts in fuel cells. In the present work, we synthesized Zn-based MOF (1), 1 selectivity gives H2O2 followed by two-electron pathways. However, sample 1 modification might be needed to enhance its selectivity for the generation of H2O. Two composites of MOFs with carbon black and 1 were prepared to increase the H2O yield, called 1⊃CB and 1⊃SCB. The electrochemical generation of H2O2 was analyzed by the rotating ring disk electrode (RRDE) using catalyst 1. Following the addition of CB, H2O2 yields decreased from above 93% (1) to 59% and 75% for 1⊃CB and 1⊃SCB, respectively. CB modified catalysts moved towards four-electron pathways due to the conductive nature of CB. Electrochemical Impedance Spectroscopy (EIS) has also been performed to study in detail the conductivity effect of CB and kinetic behavior of ORR in alkaline electrolyte. This research opens up a new path for ORR to advance non-precious metal catalysts based on MOFs. Synopsis: This paper describes how we synthesized carbon Black (CB) supported MOF: Zn-based MOF (1), 1 selectivity gives H2O2 (two-electron pathways). Two composites of MOFs (1⊃CB and 1⊃SCB) were prepared to increase the H2O yield. After addition of CB, H2O2 yields decreased from above 93% (1) to 75% (1⊃SCB) and 59% (1⊃CB).

Reduced graphene oxide-carbon black composite supported iron phthalocyanine as a high-efficiency and stable catalyst for oxygen reduction reaction

It has been found that only catalytic sites are situated on the surfaces or sub-surfaces of supports to accelerate oxygen reduction reaction (ORR) rates, whereas those situated within the interior regions of supports do not actually participate in catalysis ORR. Therefore, catalyst support technology has been regarded as another effective tactic for improving the catalytic activities of catalysts. In the present study, ORR catalysts supported by composite materials were created by uniformly mixing super-conducting carbon black with tetra-β-(8-quinolinoxy) iron phthalocyanine-loaded reduced graphene oxides (rGO). The morphologic structures of the prepared material were investigated using AFM, SEM, FTIR, UV, XPS, and Raman spectroscopic methods. The catalytic activities and stabilities toward the ORR in alkaline electrolyte were assessed using CV and LSV processes. The experimental results demonstrated that insertions of the carbon black (CB) nanospheres between the rGO sheets not only impeded the stacking of the rGO nanosheets, but also achieved the full utilization of the synergistic effects between them. The improved electrochemical catalytic activities of the tetra-β-(8-quinolinoxy) iron phthalocyanine (TQFePc) via CB/rGO hybrid support were mainly attributed to the strong π-π interaction between the TQFePc molecules and the rGO nanosheets, as well as the novel 3D sandwiched graphene/carbon nanosphere/graphene structures.

Amino-1H-tetrazole-regulated high-density nitrogen-doped hollow carbon nanospheres for long-life Zn-air batteries.

High-density nitrogen-doped porous carbon catalysts have been regarded as promising alternatives to precious metals in proton-exchange membrane fuel cells (PEMFC) and metal–air batteries based on the oxygen reduction reaction (ORR). We herein synthesized high-density pyridinic and graphitic N-doped hollow carbon nanospheres (G&P N-HCS) using a high-yield amino-1H-tetrazole (ATTZ) via a self-sacrificial-template method. The synthesized G&P N-HCS shows a high N content (15.2 at%), in which pyridinic (Pr) and graphitic (Gr) N are highly reactive for the ORR catalysis. We found that the half-wave potential and limiting current density of G&P N-HCS are comparable to the state-of-the-art Pt/C, whereas its cyclic durability is much superior to that of Pt/C. Experimental results indicate that an optimal ratio (1 : 1) between Gr N and Pr N in G&P N-HCS exhibits the highest ORR performances, rather than Gr N-dominated N-HCS or Pr N-dominated N-HCS. Notably, N-HCS containing only Gr N and Pr N has poor catalytic performance for ORR in alkaline electrolytes. Density functional theory (DFT) simulations untangle the catalytic nature of Pr and Gr N and decipher the relations between the N type(s) and total N content required for the ORR catalysis. This study provides a new way to design efficient N-doped porous carbon-enriched active sites, and solves the cathode catalyst in the commercialization of PEMFC and metal–air batteries.

Fe atoms anchored on defective nitrogen doped hollow carbon spheres as efficient electrocatalysts for oxygen reduction reaction

Defective electrocatalysts, especially for intrinsic defective carbon, have aroused a wide concern owing to high spin and charge densities. However, the designated nitrogen species favorable for creating defects by the removal of nitrogen, and the influence of defects for the coordination structure of active site and oxygen reduction reaction (ORR) activity have not been elucidated. Herein, we designed and synthesized a pair of electrocatalysts, denoted as Fe-N/C and Fe-ND/C for coordination sites of atomic iron-nitrogen and iron-nitrogen/defect configuration embedded in hollow carbon spheres, respectively, through direct pyrolysis of their corresponding hollow carbon spheres adsorbed with Fe(acac)3. The nitrogen defects were fabricated via the evaporation of pyrrolic-N on nitrogen doped hollow carbon spheres. Results of comparative experiments between Fe-N/C and Fe-ND/C reveal that Fe-ND/C shows superior ORR activity with an onset potential of 30 mV higher than that of Fe-N/C. Fe-ND sites are more favorable for the enhancement of ORR activity. Density functional theory (DFT) calculation demonstrates that Fe-ND/C with proposed coordination structure of FeN4−x (0<x<4) anchored by OH as axial ligand during ORR, weakens the strong binding of OH* intermediate and promotes the desorption of OH* as rate-determining step for ORR in alkaline electrolyte. Thus, Fe-ND/C electrocatalysts present much better ORR activity compared with that of Fe-N/C with proposed coordination structure of FeN4.

3D N-doped ordered mesoporous carbon supported single-atom Fe-N-C catalysts with superior performance for oxygen reduction reaction and zinc-air battery

Single-atom Fe-N-C electrocatalysts have emerged as the most promising oxygen reduction reaction (ORR) catalyst. However, the low Fe loading and inaccessibility of Fe-N-C sites limit the overall ORR activity. Here, we report an efficient single-atom electrocatalyst (Fe-N-C/N-OMC) with Fe-N-C sites embedded in three-dimensional (3D) N-doped ordered mesoporous carbon framework. Fe-N-C/N-OMC shows high half-wave potential, kinetic current density, turnover frequency and mass activity towards ORR in alkaline electrolyte. Experiments and theoretical calculations suggest that the ultra-high ORR activity stems from the boosted intrinsic activity of FeN4 sites by graphitic N dopants, high density of accessible active site generated by high Fe and N loadings and ordered mesoporous carbon structure as well as facilitated mass and electron transport in 3D interconnected pores. Fe-N-C/N-OMC also shows comparable ORR activity to Pt/C in acidic electrolyte. As the cathode for zinc-air battery, Fe-N-C/N-OMC exhibits high open-circuit voltage, high power density and remarkable durability.