Cobalt Oxide on Boron-Doped Graphitic Carbon Nitride as Bifunctional Photocatalysts for CO2 Reduction and Hydrogen Evolution

In this work, the prepared cobalt oxide decorated boron-doped g-C3N4 (CoOx/g-C3N4) heterojunction exhibits remarkable activity in CO2 reduction (CO2RR), resulting in high yields of CH3COOH (∼383 μmol·gcatalyst-1) and CH3OH (∼371 μmol·gcatalyst-1) with 58% selectivity to C2+ under visible light. However, the same system leads to high H2 evolution (HER) by increasing the cobalt oxide content, suggesting that the selectivity and preference for the CO2RR or HER depend on oxide decoration. By comparing HER and CO2RR evolution in the same system, this work provides critical insights into the catalytic mechanism, indicating that the CoOx/g-C3N4 heterojunction formation is necessary to foster high visible light photoactivity.

Bionic Fe-N-C catalyst with abundant exposed Fe-Nx sites and enhanced mass transfer properties for efficient oxygen reduction

Transition metal and nitrogen co-doped carbon electrocatalysts are promising candidates to replace the precious metal platinum (Pt) in oxygen reduction reactions (ORR). Unfortunately, the electrochemical performance of existing electrocatalysts is restricted due to limited accessibility of active sites. Inspired by jellyfish tentacles, we design an efficient ORR micro-reactor called Fe-Nx/HC@NWs. It features abundant exposed Fe-Nx active sites dispersed on nitrogen-doped cubic carbon cages, which have a hierarchically porous and hairy structure. The accessible, atomically dispersed Fe-Nx sites and the elaborate substrate architecture synergize to provide the catalyst withremarkable ORR catalytic activity, extraordinary long-term stability, and favorable methanol tolerance in an alkaline electrolyte; overall, its performance is comparable to that of commercial carbon-supported Pt. Our synthesis is facile and controllable, paving a new avenue toward advanced non-precious metal-based electrocatalysts for energy storage and conversion.

MOF-derived multicomponent Fe2P-Co2P-Ni2P hollow architectures for efficient hydrogen evolution

In this work, we introduce Fe and Ni into Co-MOF to construct a kind of multicomponent phosphide hollow architecture with walls assembled by nanosheets. The multicomponent nature can enhance the intrinsic catalytic activity, while the sheet-like surface and inter-sheet voids provide a large active area, which is beneficial for electrolyte penetration and gas generation. As expected, the optimized product has catalytic hydrogen evolution reaction (HER) overpotentials of 105 and 161 mV at current densities of 10 and 100 mA cm^-2, respectively, and maintained long-term stability for over 100 hours at 10 mA cm^-2 current densities.

Activation of H2O Tailored by Interfacial Electronic States at a Nanoscale Interface for Enhanced Electrocatalytic Hydrogen Evolution

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level are not well-understood, especially in basic media. Here, with ZIF-67-derived Co-based carbon frameworks (Co/NCs) as model catalysts, we systematically investigated the effects of different reaction parameters on the HER kinetics and discovered that the HER activity was directly dependent not on the type of nitrogen in the carbon framework but on the relative content of surface hydroxyl and water (OH-/H2O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH-/H2O was close to 1:1, the Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., an overpotential of only 232 mV at a current density of 10 mA cm-2 in the 1 M KOH electrolyte. We unambiguously identified that the structural water molecules (SWs) in the form of hydrous hydroxyl complexes absorbed on metal centers {OHad·H2O@M+} were catalytic active sites for the enhanced HER, where M+ could be transition or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in the SWs were mainly bonded together via the spatial interaction between the p orbitals of O atoms, exhibiting features of a delocalized π-bond with a metastable state. These newly formed surface bonds or transitory states could be new weak interactions that synergistically promote both interfacial electron transfer and the activation of water (dissociation of O-H bonds) at the electrode surface, i.e., the formation of activated H adducts (H*). The capture of new surface states not only explains pH-, cation-, and transition-metal-dependent hydrogen evolution kinetics but also provides completely new insights into the understanding of other electrocatalytic reductions involving other small molecules, including CO2, CO, and N2.

Bimetallic persulfide nanoflakes assembled by dealloying and sulfurization: a versatile electro-catalyst for overall water splitting and Zn–air batteries

(CoFe)(S2)2 nanoflakes with graphene-like edges have been synthesized through dealloying and sulfurization, and exhibit multi-functional electro-catalytic performance toward the ORR, OER, HER.

Pyridinic-Type N-Doped Graphene on Cobalt Substrate as Efficient Electrocatalyst for Oxygen Reduction Reaction in Acidic Solution in Fuel Cell

In this study, we use density functional theory to investigate the catalytic activity of graphene (G), single vacancy defective graphene (G_SV), quaternary N-doped graphene (NG_Q), and pyridinic N-doped graphene (NG_py, 3NG_py, and 4NG_py) on Co(0001) substrate for an oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The results show pyridinic N-doped graphene on a Co support exhibited better performance than the NG_Q on a Co support and free-standing systems. According to the results, ORR intermediates (*OOH, *O, and *OH) become more stable due to the presence of a Co substrate. The single pyridinic (3NG_py) layer placed on Co(0001) is the most active site. The overpotential for Co/3NG_py is rather higher compared to pure Pt(111) catalyst (0.65 V). Therefore, pyridinic N-doped graphene with a cobalt support could be a promising strategy to enhance the ORR activity of N-doped graphene in PEMFCs.

Highly Efficient Synthesis of Carbon-Based Molybdenum Phosphide Nanoparticles for Electrocatalytic Hydrogen Evolution

Molybdenum phosphide in transition metal phosphides members are considered as an attractive electrocatalyst for hydrogen evolution reaction (HER). However, its unsatisfactory stability and conductivity in an alkaline environment has dragged on its development. Here, we successfully introduced N, C co-doped MoP (MoP-NC) nanoparticles by a simple and efficient two-step synthesis method using urea as a carbon source into the molybdenum phosphide system. The cheapness of urea and the excellent carbon to nitrogen ratio remove the obstacles ahead of the development of MoP-NC composites. The obtained composites have excellent HER electrocatalytic activity and stability in 1-M potassium hydroxide (KOH) solution, which requires only an overpotential of 131 mV to achieve a current density of 10 mA cm-2 and exhibits negligible performance degradation after 1000 CV cycles.

Enhanced Synergetic Catalytic Effect of Mo2C/NCNTs@Co Heterostructures in Dye-Sensitized Solar Cells: Fine-Tuned Energy Level Alignment and Efficient Charge Transfer Behavior

A highly efficient and stable electrocatalyst with the novel heterostructure of Co-embedded and N-doped carbon nanotubes supported Mo₂C nanoparticles (Mo₂C/NCNTs@Co) is creatively constructed by adopting the one-step metal catalyzed carbonization-nitridation strategy. Systematic characterizations and density functional theory (DFT) calculations reveal the advanced structural and electronic properties of Mo₂C/NCNTs@Co heterostructure, in which the Co-embedded and N-doped CNTs with tunable diameters present electron-donating effect and the work function is correspondingly regulated from 4.91 to 4.52 eV, and the size-controlled Mo₂C nanoparticles exhibit Pt-like 4d electronic structure and the well matched work function (4.85 eV) with I^-/I₃^- redox couples (4.90 eV). As a result, the conductive NCNTs@Co substrate with fine-tuned energy level alignment accelerates the electron transportation and the electron migration from NCNTs@Co to Mo₂C, and the active Mo₂C shows high affinity for I₃^- adsorption and high charge transfer ability for I₃^- reduction, which reach a decent synergetic catalytic effect in Mo₂C/NCNTs@Co heterostructure. The DSSC with Mo₂C/NCNTs@Co CE achieves a high photoelectric conversion efficiency of 8.82% and exceptional electrochemical stability with a residual efficiency of 7.95% after continuous illumination of 200 h, better than Pt-based cell. Moreover, the synergistic catalytic mechanism toward I₃^- reduction is comprehensively studied on the basis of structure-activity correlation and DFT calculations. The advanced heterostructure engineering and electronic modulation provide a new design principle to develop the efficient, stable, and economic hybrid catalysts in relevant electrocatalytic fields.

Defect-Enriched Nitrogen Doped-Graphene Quantum Dots Engineered NiCo2 S4 Nanoarray as High-Efficiency Bifunctional Catalyst for Flexible Zn-Air Battery

Flexible Zn-air batteries have recently emerged as one of the key energy storage systems of wearable/portable electronic devices, drawing enormous attention due to the high theoretical energy density, flat working voltage, low cost, and excellent safety. However, the majority of the previously reported flexible Zn-air batteries encounter problems such as sluggish oxygen reaction kinetics, inferior long-term durability, and poor flexibility induced by the rigid nature of the air cathode, all of which severely hinder their practical applications. Herein, a defect-enriched nitrogen doped-graphene quantum dots (N-GQDs) engineered 3D NiCo₂ S₄ nanoarray is developed by a facile chemical sulfuration and subsequent electrophoretic deposition process. The as-fabricated N-GQDs/NiCo₂ S₄ nanoarray grown on carbon cloth as a flexible air cathode exhibits superior electrocatalytic activities toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), outstanding cycle stability (200 h at 20 mA cm^-2 ), and excellent mechanical flexibility (without observable decay under various bending angles). These impressive enhancements in electrocatalytic performance are mainly attributed to bifunctional active sites within the N-GQDs/NiCo₂ S₄ catalyst and synergistic coupling effects between N-GQDs and NiCo₂ S₄ . Density functional theory analysis further reveals that stronger OOH* dissociation adsorption at the interface between N-GQDs and NiCo₂ S₄ lowers the overpotential of both ORR and OER.

Controlled synthesis of single cobalt atom catalysts via a facile one-pot pyrolysis for efficient oxygen reduction and hydrogen evolution reactions

Metal-nitrogen doped carbon catalysts (M-N/C) with abundantly accessible M-N_x sites, particularly single metal atom M-N/C (SAM-N/C), have been developed as a substitute for expensive Pt-based catalysts. These catalysts are used to increase the efficiency of otherwise sluggish oxygen reduction reactions (ORR) and hydrogen evolution reactions (HER). However, although the agglomerated metal nanoparticles are usually easy to form, they are very difficult to remove due to the protective surface-coating carbon layers, a factor that significantly hampers SAM-N/C fabrication. Herein, we report a one-step pyrolysis approach to successfully fabricate single cobalt atom Co-N/C (SACo-N/C) by using a Co²⁺-SCN^- coordination compound as the metal precursor. Thanks to the decomposition of Co²⁺-SCN^- compound at lower temperature than that of carbon layer deposition, Co-rich particles grow up to larger ones before carbon layers formation. Even though encapsulated by the carbon layers, it is difficult for the large Co-rich particle to be completely sealed. And thus, it makes the Co atoms possible to escape from incomplete carbon layer, to coordinate with nitrogen atoms, and to form SACo-N/C catalysts. This SACo-N/C exhibits excellent performances for both ORR (half-wave potential of 0.878 V) and HER (overpotential at 10 mA/cm² of 178 mV), and is thus a potential replacement for Pt-based catalysts. When SACo-N/C is integrated into a Zn-O₂ battery, battery with high open-circuit voltage (1.536 V) has high peak power density (266 mW/cm²) and large gravimetric energy density (755 mA h/g_Zn) at current densities of 100 mA/cm². Thus, we believe that this strategy may offer a new direction for the effective generation of SAM-N/C catalysts.

NaCl protected synthesis of 3D hierarchical metal-free porous nitrogen-doped carbon catalysts for the oxygen reduction reaction in acidic electrolyte

A high-performance 3D hierarchical porous metal-free N-doped carbon catalyst for oxygen reduction reaction in acidic medium was synthesized with ZnO as a mesoporous template and NaCl as both a macroporous template and a structure protective agent.

Bio-inspired chitosan-heme-vitamin B12-derived Fe–Co bimetallic-doped mesoporous carbons for efficiently electro-activating oxygen

Biomass-derived chitosan-heme-vitamin B₁₂ with definitive molecular structures was converted to bimetallic Fe–Co-doped mesoporous carbon for efficient oxygen electroreduction.

Boosting the Performance of Iron-Phthalocyanine as Cathode Electrocatalyst for Alkaline Polymer Fuel Cells Through Edge-Closed Conjugation

Recent progress in anion-exchange membranes has evoked increasing interests in alkaline polymer fuel cells (APFCs). A large body of recent research has demonstrated attractive activity of Fe-N macrocycle complexes as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. To be a substitute for Pt in APFCs, however, most of the macrocycle molecules remain largely unsatisfactory in both of the catalytic activity and durability. Herein, we show that a one-pot microwave conjugation results in a polymerized iron-phthalocyanine (pFePc) which exhibits extremely high ORR performance, showing activity much better than that of the FePc monomer and 20 wt % Pt/C, and similar to that of the 60 wt % Pt/C under the same catalyst loading. Furthermore, we proposed an edge-closing strategy to significantly enhance the stability of the pFePc catalyst in alkaline media by eliminating the edge anhydride groups. Using the edge-closed pFePc as the cathode catalyst in APFC, a power density as high as 452 mW·cm^-2 is achieved, which is among the best performance of non-noble metal catalyst-based APFCs so far reported.

Metallic Cobalt-Carbon Composite as Recyclable and Robust Magnetic Photocatalyst for Efficient CO2 Reduction

CO2 conversion into value-added chemical fuels driven by solar energy is an intriguing approach to address the current and future demand of energy supply. Currently, most reported surface-sensitized heterogeneous photocatalysts present poor activity and selectivity under visible light irradiation. Here, photosensitized porous metallic and magnetic 1200 CoC composites (PMMCoCC-1200) are coupled with a [Ru(bpy)3 ]Cl2 photosensitizer to efficiently reduce CO2 under visible-light irradiation in a selective and sustainable way. As a result, the CO production reaches a high yield of 1258.30 µL with selectivity of 64.21% in 6 h, superior to most reported heterogeneous photocatalysts. Systematic investigation demonstrates that the central metal cobalt is the active site for activating the adsorbed CO2 molecules and the surficial graphite carbon coating on cobalt metal is crucial for transferring the electrons from the triplet metal-to-ligand charge transfer of the photosensitizer Ru(bpy)32+ , which gives rise to significant enhancement for CO2 reduction efficiency. The fast electron injection from the excited Ru(bpy)32+ to PMMCoCC-1200 and the slow backward charge recombination result in a long-lived, charge-separated state for CO2 reduction. More impressively, the long-time stability and easy magnetic recycling ability of this metallic photocatalyst offer more benefits to the photocatalytic field.

Active Sites Engineering toward Superior Carbon-Based Oxygen Reduction Catalysts via Confinement Pyrolysis

Developing efficient and low-cost defective carbon-based catalysts for the oxygen reduction reaction (ORR) is essential to metal-air batteries and fuel cells. Active sites engineering toward these catalysts is highly desirable but challenging to realize boosted catalytic performance. Herein, a sandwich-like confinement route to achieve the controllable regulation of active sites for carbon-based catalysts is reported. In particular, three distinct catalysts including metal-free N-doped carbon (NC), single Co atoms dispersed NC (Co-N-C), and Co nanoparticles-contained Co-N-C (Co/Co-N-C) are controllably realized and clearly identified by synchrotron radiation-based X-ray spectroscopy. Electrochemical measurements suggest that the Co/Co-N-C catalyst delivers optimized ORR performance due to the rich Co-N_x active sites and their synergistic effect with metallic Co nanoparticles. This work provides deep insight for rationally designing efficient ORR catalyst based on active sites engineering.

Scalable solid-state synthesis of coralline-like nanostructured Co@CoNC electrocatalyst for Zn–air batteries

A facile and scalable solid-state synthesis strategy is developed to produce hierarchical coralline-like nanostructured electrocatalysts with cobalt nanoparticles and Co–N_X sites for efficient oxygen reduction.

Effective and Durable Co Single Atomic Cocatalysts for Photocatalytic Hydrogen Production

This research reports for the first time that single cobalt atoms anchored in nitrogen-doped graphene (Co-NG) can serve as a highly effective and durable cocatalyst for visible light photocatalytic hydrogen production from water. Results show that, under identical conditions, the hydrogen production rate (1382 μmol/h) for 0.25 wt % Co-NG-loaded CdS photocatalyst (0.25 wt % Co-NG/CdS) is 3.42 times greater than that of nitrogen-doped graphene (NG) loaded CdS photocatalyst (NG/CdS) and about 1.3 times greater than the greatest hydrogen production rate (1077 μmol/h) for 1.5 wt % Pt nanoparticle loaded CdS photocatalyst (1.5 wt % Pt-NPs/CdS). At 420 nm irradiation, the quantum efficiency of the 0.25 wt % Co-NG/CdS photocatalyst is 50.5%, the highest efficiency among those literature-reported non-noble metal cocatalysts. The Co-NG/CdS nanocomposite-based photocatalyst also has an extended durability. No activity decline was detected during three cyclic photocatalytic life span tests. The very low cocatalyst loading, along with the facile preparation technology for this non-noble metal cocatalyst, will significantly reduce the hydrogen production costs and finally lead to the commercialization of the solar catalytic hydrogen production process. Based on experimental results, we conclude that Co-NG can successfully replace noble metal cocatalysts as a highly effective and durable cocatalyst for renewable solar hydrogen production. This finding will point to a new way for the development of highly effective, long life span, non-noble metal-based cocatalysts for renewable and cost-effective hydrogen production.

The Oxygen Reduction Electrocatalytic Activity of Cobalt and Nitrogen Co-doped Carbon Nanocatalyst Synthesized by a Flat Template

The design of noble-metal-free catalysts for oxygen reduction reaction (ORR) is very important to the commercialization of fuel cells. Here, we use a Co-modified montmorillonite (Co-MMT) as a flat template to prepare Co- and N-doped nanocarbon ORR catalysts derived from carbonization of polyaniline at controlled temperatures. The use of flat template can hinder the agglomeration of polyaniline during pyrolysis process and optimize the N-rich active site density on the surface. The addition of transition metal Co in the flat MMT template can largely promote the formation of Co-N sites in prepared catalyst, facilitating the effective improvement of catalytic activity towards the ORR with a direct four-electron transfer pathway. The excellent ORR activity may be mainly attributed to high contents of graphitic N, pyridinic-N, and Co-N configurations. This study opens a new way to rationally design cheap and active ORR catalysts by using simple flat compound as a direct template.

A Novel Metal–Organic Framework Route to Embed Co Nanoparticles into Multi-Walled Carbon Nanotubes for Effective Oxygen Reduction in Alkaline Media

Metal–organic framework (MOF) materials can be used as precursors to prepare non-precious metal catalysts (NPMCs) for oxygen reduction reaction (ORR). Herein, we prepared a novel MOF material (denoted as Co-bpdc) and then combined it with multi-walled carbon nanotubes (MWCNTs) to form Co-bpdc/MWCNTs composites. After calcination, the cobalt ions from Co-bpdc were converted into Co nanoparticles, which were distributed in the graphite carbon layers and MWCNTs to form Co-bpdc/MWCNTs. The prepared catalysts were characterized by TEM (Transmission electron microscopy), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), BET (Brunauer–Emmett–Teller), and Raman spectroscopy. The electrocatalytic activity was measured by using rotating disk electrode (RDE) voltammetry. The catalysts showed higher ORR catalytic activity than the commercial Pt/C catalyst in alkaline solution. Co-bpdc/MWCNTs-100 showed the highest ORR catalytic activity, with an initial reduction potential and half-wave potential reaching 0.99 V and 0.92 V, respectively. The prepared catalysts also showed superior stability and followed the 4-electron pathway ORR process in alkaline solution.

Co Nanoparticles Encapsulated in N-Doped Carbon Nanosheets: Enhancing Oxygen Reduction Catalysis without Metal-Nitrogen Bonding

It is known that introducing metal nanoparticles (e.g., Fe and Co) into N-doped carbons can enhance the activity of N-doped carbons toward the oxygen reduction reaction (ORR). However, introducing metals into N-doped carbons inevitably causes the formation of multiple active sites. Thus, it is challenging to identify the active sites and unravel mechanisms responsible for enhanced ORR activity. Herein, by developing a new N-heterocyclic carbene (NHC)-Co complex as the nitrogen- and metal-containing precursor, we report the synthesis of N-doped carbon nanosheets embedded with Co nanoparticles as highly active ORR catalysts without direct metal-nitrogen bonding. Electrochemical measurements and X-ray absorption spectroscopy indicate that the carbon-nitrogen sites surrounding Co nanoparticles are responsible for the observed ORR activity and stability. Density functional theory calculations further reveal that Co nanoparticles could facilitate the protonation of O₂ and thus promote the ORR activity. These results provide new prospects in the rational design and synthesis of heteroatom-doped carbon materials as non-precious-metal catalysts for various electrochemical reactions.

Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective

Great endeavors are undertaken to search for low-cost, rich-reserve, and highly efficient alternatives to replace precious-metal catalysts, in order to cut costs and improve the efficiency of catalysts in industry. However, one major problem in metal catalysts, especially nonprecious-metal catalysts, is their poor stability in real catalytic processes. Recently, a novel and promising strategy to construct 2D materials encapsulating nonprecious-metal catalysts has exhibited inimitable advantages toward catalysis, especially under harsh conditions (e.g., strong acidity or alkalinity, high temperature, and high overpotential). The concept, which originates from unique electron penetration through the 2D crystal layer from the encapsulated metals to promote a catalytic reaction on the outermost surface of the 2D crystal, has been widely applied in a variety of reactions under harsh conditions. It has been vividly described as "chainmail for catalyst." Herein, recent progress concerning this chainmail catalyst is reviewed, particularly focusing on the structural design and control with the associated electronic properties of such heterostructure catalysts, and also on their extensive applications in fuel cells, water splitting, CO₂ conversion, solar cells, metal-air batteries, and heterogeneous catalysis. In addition, the current challenges that are faced in fundamental research and industrial application, and future opportunities for these fantastic catalytic materials are discussed.

From Chlorella to Nestlike Framework Constructed with Doped Carbon Nanotubes: A Biomass-Derived, High-Performance, Bifunctional Oxygen Reduction/Evolution Catalyst

The development of effective bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is significant for energy conversion systems, such as Li-air batteries, fuel cells, and water splitting technologies. Herein, a Chlorella-derived catalyst with a nestlike framework, composed of bamboolike nanotubes that encapsulate cobalt nanoparticles, has been prepared through a facile pyrolysis process. It achieves perfect bifunctional catalysis both in ORR and OER on a single catalyst. For our optimal catalyst Co/M-Chlorella-900, its ORR half-wave potential is positively shifted by 40 mV compared to that of a commercial Pt/C catalyst, and the overpotential at 10 mA cm^-2 for the OER is 23 mV lower than that of a commercial IrO₂/C catalyst in an alkaline medium. This superior bifunctional catalytic performance is benefited from the simultaneous increase of pyridinic N sites for ORR and graphitic N sites for OER. In addition, N-doped carbon-encapsulated Co nanoparticles improve both ORR and OER performance by forming new active centers. The unique nestlike carbon nanotube framework not only afforded highly dense ORR and OER active sites but also promoted the electron and mass transfer. Our catalyst also displays notable durability during the ORR and OER, making it promising for use in ORR/OER-related energy conversion systems.

Selective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes

Nitrogen modified cobalt catalysts supported on carbon were prepared by pyrolysis of the mixture generated from cobalt(ii) acetate in aqueous solution of melamine or waste melamine resins, which are widely used as industrial polymers. The obtained nanostructured materials catalyze the transfer hydrogenation of N-heteroarenes with formic acid in the absence of base. The optimal Co/Melamine-2@C-700 catalyst exhibits high activity and selectivity for the dehydrogenation of formic acid into molecular hydrogen and carbon dioxide and allows for the reduction of diverse N-heteroarenes including substrates featuring sensitive functional groups.

Surface Modification of Multi-Walled Carbon Nanotubes via Hemoglobin-Derived Iron and Nitrogen-Rich Carbon Nanolayers for the Electrocatalysis of Oxygen Reduction

The great challenge of boosting the oxygen reduction reaction (ORR) activity of non-noble-metal electrocatalysts is how to achieve effective exposure and full utilization of nitrogen-rich active sites. To realize the goals of high utilization of active sites and fast electron transport, here we report a new strategy for synthesis of an iron and nitrogen co-doped carbon nanolayers-wrapped multi-walled carbon nanotubes as ORR electrocatalyst (N-C@CNT-Fe) via using partially carbonized hemoglobin as a single-source precursor. The onset and half-wave potentials for ORR of N-C@CNT-Fe are only 45 and 54 mV lower than those on a commercial Pt/C (20 wt.% Pt) catalyst, respectively. Besides, this catalyst prepared in this work has been confirmed to follow a four-electron reaction mechanism in ORR process, and also displays ultra-high electrochemical cycling stability in both acidic and alkaline electrolytes. The enhancement of ORR activity can be not only attributed to full exposure and utilization of active site structures, but also can be resulted from the improvement of electrical conductivity owing to the introduction of CNT support. The analysis of X-ray photoelectric spectroscopy shows that both Fe–N and graphitic-N species may be the ORR active site structures of the prepared catalyst. Our study can provide a valuable idea for effective improvement of the electrocatalytic activity of non-noble-metal ORR catalysts.

Iron encapsulated in 3D N-doped carbon nanotube/porous carbon hybrid from waste biomass for enhanced oxidative activity

Novel iron encapsulated in nitrogen-doped carbon nanotubes (CNTs) supported on porous carbon (Fe@N-C) 3D structured materials for degrading organic pollutants were fabricated from a renewable, low-cost biomass, melamine, and iron salt as the precursors. SEM and TEM micrographs show that iron encapsulated bamboo shaped CNTs are vertically standing on carbon sheets, and thus, a 3D hybrid was formed. The catalytic activities of the prepared samples were thoroughly evaluated by activation of peroxymonosulfate for catalytic oxidation of Orange II solutions. The influences of some reaction conditions (pH, temperature, and concentrations of reactants, peroxymonosulfate, and dye) were extensively evaluated. It was revealed that the adsorption could enrich the pollutant which was then rapidly degraded by the catalytically generated radicals, accelerating the continuous adsorption of residual pollutant. Remarkable carbon structure, introduction of CNTs, and N/Fe doping result in promoted adsorption capability and catalytic performances. Due to the simple synthetic process and cheap carbon precursor, Fe@N-C 3D hybrid can be easily scaled up and promote the development of Fenton-like catalysts.

Synthesis and characterization of an IrO2–Fe2O3 electrocatalyst for the hydrogen evolution reaction in acidic water electrolysis

An IrO₂–Fe₂O₃ electrocatalyst was prepared for the HER in acidic water electrolysis and exhibits higher activity than IrO₂.

Construction of a porous nitrogen-doped carbon nanotube with open-ended channels to effectively utilize the active sites for excellent oxygen reduction reaction activity

We rationally designed and controllably fabricated a well-defined porous nitrogen-doped carbon nanotube with open-ended channels as a catalyst for the ORR by a twice pseudomorphic transformation of MnO₂ nanotubes.

Iron encapsulated in boron and nitrogen codoped carbon nanotubes as synergistic catalysts for Fenton-like reaction

Iron nanoparticles (NPs) encapsulated in B, N-codoped carbon nanotubes (Fe@C-BN) as heterogeneous Fenton-like catalysts were obtained by a simple and scalable pyrolysis method, and their performances were examined in the oxidative degradation of various organics in the presence of the different oxidants. The results showed that organic dyes can be effectively degraded by Fe@C-BN in the presence of peroxymonosulfate. Calcination temperature and mass of iron salt significantly affected the structures and performances of the catalysts. The effects of several reaction conditions, such as initial dye concentration, oxidant type (peroxymonosulfate, peroxydisulfate, and H2O2) and dosage, initial pH, inorganic anions, reaction temperature and dye types on oxidation as well as the stability of the composite were extensively evaluated in view of the practical applications. Through the investigation of reaction processes, HO(·) and SO4(·-) radicals were identified using quenching experiments. Owing to the synergistic effects between the iron NPs and B, N-doped carbon, Fe@C-BN catalysts intrinsically display an excellent catalytic activity for Fenton-like reaction. This study gives new insights into the design and preparation of iron NPs encapsulated in B, N-codoped carbon nanotubes as an effective strategy to enhance the overall catalytic activity.

Is Ammonium Peroxydisulate Indispensable for Preparation of Aniline-Derived Iron-Nitrogen-Carbon Electrocatalysts?

Iron and nitrogen co-doped carbon (Fe-N-C) materials are among the most active non-precious metal catalysts that could replace Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. The synthesis of the Fe-N-C catalysts often involves the use of aniline as the precursor for both N and C and ammonium peroxydisulfate (APS) as an indispensable oxidative initiator for aniline polymerization. Herein, a detailed structure and catalytic ORR performance comparison of aniline-derived Fe-N-C catalysts synthesized with and without the use of APS is reported. The APS-free preparation, which uses Fe(III) ions as the Fe source as well as the aniline polymerization initiator, results in a simple Fe-N-C catalyst with a high activity for the ORR. We show that APS is not necessary for the preparation and even detrimental to the performance of the catalyst.

Fe, Co, Ni nanocrystals encapsulated in nitrogen-doped carbon nanotubes as Fenton-like catalysts for organic pollutant removal

Magnetic metal M (M=Fe, Co, Ni) nanocrystals encapsulated in nitrogen-doped carbon nanotubes (M@N-C) were fabricated conveniently using dicyandiamide as a C/N precursor, and exhibited varying activities toward Fenton-like reaction. The surface morphology and structure of the M@N-C catalysts were characterized and an efficient catalytic degradation performance, high stability, and excellent reusability were observed. In addition, several operational factors, such as initial dye concentration, oxidant type (peroxymonosulfate, peroxydisulfate and H2O2) and dosage, reaction temperature, and dye type as well as stability of the composite were extensively evaluated in view of the practical applications. The results showed that various transition metals M significantly affected the structures and performances of the catalysts, and specially, their activity followed the order of Co>Fe>Ni in the presence of peroxymonosulfate. Moreover, HO⁡ and SO4(-) radicals participating in the process were evidenced using quenching experiments, and a rational mechanism was proposed based on a non-radical process and the free radical process. Control experiments revealed that the enhanced active sites were mainly ascribed to the synergistic effects between the metal nanocrystals and nitrogen-doped carbon. The findings of this study elucidated that encapsulation of nanocrystals in nitrogen-doped carbon nanotubes was an effective strategy to enhance the overall catalytic activity.