In situ synthesis of CoFe2O4 nanoparticles embedded in N-doped carbon nanotubes for efficient electrocatalytic oxygen reduction reaction

The development of electrocatalyst possessing superior oxygen reduction reaction (ORR) activity is highly desirable due to the low sluggish kinetics at the cathode of fuel cell. Here, CoFe₂O₄ nanoparticles embedded in N-doped carbon nanotubes electrocatalyst (denoted as CoFe₂O₄-NC) is synthesized via polymerization of pyrrole, absorbing metal ion and annealing under Ar/NH₃ atmosphere. By in situ integrating the catalytically active CoFe₂O₄ nanoparticles with the N-doped carbon nanotubes and enhancing electrical conductivity, the as-obtained electrocatalyst exhibits excellent ORR activity and long-term stability with a half-wave potential of 0.86 V and 10 h continuous cycling, outperforming the reported similar catalysts. This work opens a new path for the expansion of low cost and efficient ORR electrocatalysts to substitute Pt-based metals for energy storage and conversion devices.     

Simultaneous formation of nitrogen and sulfur-doped carbon nanotubes-mesoporous carbon and its electrocatalytic activity for oxygen reduction reaction

Nitrogen and sulfur dual doped-carbon nanotubes-mesoporous carbon (D-CNTs-MPC) composite is prepared simultaneously and is used in alkaline media as an electrocatalyst for oxygen reduction reaction (ORR). D-CNTs-MPC is synthesized by casting method using nano-CaCO₃ as a template, and binuclear cobalt phthalocyanine hexasulfonate as a carbon, nitrogen and sulfur precursor as well as the catalyst for growth of CNTs. D-CNTs-MPC possesses short CNTs adhering to loosely packed carbon with mesopores. Moreover, nitrogen and sulfur are doped into the carbon framework without addition of other heteroatom-containing precursor. The electrochemical behavior shows that D-CNTs-MPC is an active, methanol-tolerant and stable electrocatalyst for ORR.     

Metal-organic frameworks derived Co/N-doped carbon nanonecklaces as high-efficient oxygen reduction reaction electrocatalysts

Metal/carbon-based materials derived from metal-organic frameworks have recently attracted great attention for oxygen reduction reaction (ORR) electrocatalysts, but poor electrical conductivity and insufficient stability owing to granular structures, as well as low production hinder their practical applications. Herein, we design and synthesize Co nanoparticles decorated on N-doped carbon nanonecklaces (Co/N-CNNs) by way of electrospinning and following carbonization. The Co/N-CNNs hybrid exhibits excellent ORR performance with a positive half-wave potential (0.865 V) and a large limiting current density (5.35 mA cm^?2), even comparable to those of Pt/C. Such superior electrocatalytic properties are attributed to the distinctive necklace-like structure, which supplies high specific surface area, hierarchical porous structure, well-dispersed and covered Co nanoparticles, and hierarchical architecture of N-doped carbon. This work provides an effective strategy to fabricate ORR electrocatalysts with well-designed structure and mass production via electrospinning.     

Transition metal atom doped C2N as catalyst for the oxygen reduction reaction: A density functional theory study

The catalytic mechanism and activity of transition metal atom doped C₂N (M-C₂N, M = Fe, Co, Ni, and Cu) for the oxygen reduction reaction (ORR) are investigated in detail by density functional theory method. All the screened M-C₂N are thermodynamically stable based on the binding energy calculations. The adsorption energy results indicate that the adsorption strength of O₂ and ORR intermediates are decreased in the order of Fe-C₂N ˃ Co-C₂N ˃ Ni-C₂N ˃ Cu-C₂N, in which the adsorption energy values on Cu-C₂N are most close to those on the Pt(111). Based on the relative energy diagram of ORR, the energetically favorable pathway on Fe-C₂N and Co-C₂N is direct 4e⁻ mechanism, in which the O–O bond is directly dissociated after the second electron transfer. While for Ni-C₂N and Cu-C₂N, the most favorable pathway is indirect 4e⁻ mechanism, in which the H₂O₂ is formed as the intermediate product. For all studied M-C₂N, the Ni-C₂N and Cu-C₂N hold better catalytic activity, which could attribute to the contribution of metal atom and part of its activated nitrogen atoms.     

Doping oxygen triggered electrocatalytic activity of carbon interpenetrating networks in acid electrolyte

The Fe–N–C catalysts may be promising candidates for replacing platinum group metal (PGM) catalysts to solve sluggish oxygen reduction reaction (ORR) kinetics in the proton exchange membrane fuel cells. However, the activity of Fe–N–C catalysts still has a certain gap compared with commercial Pt/C. Here, we provide a way to increase the intrinsic activity of Fe–N–C catalysts by designing active sites like ketone functional groups. A self-supporting interpenetrating network catalyst, composed of carbon nanotube (CNT) and carbon nanoparticle (CNP), is synthesized via multiple carbon sources (zinc-zeolitic imidazolate frameworks, polyaniline). The interpenetrating network features abundant ketone functional groups. The density functional theory (DFT) results prove that ketone groups can promote the ORR activity of FeN₄ active sites. This offers a new idea for improving the activity of Fe–N–C catalysts co-doped by oxygen and nitrogen in acidic systems.     

Development of shape-engineered α-MnO2 materials as bi-functional catalysts for oxygen evolution reaction and oxygen reduction reaction in alkaline medium

In this work, three kinds of α-MnO₂ nano shapes, namely, nano-wires, nano-tubes and nano-particles have been prepared with a fine control over α-crystallographic form by employing hydrothermal procedure. The materials have been thoroughly characterized by X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) spectrometry, field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The MnO₂ nano shapes are used as a model system for examining the shape-influenced bi-functional electrocatalytic activity towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline medium. The bi-functional role has been investigated by cyclic voltammetry and linear sweep voltammetry with rotating ring disc electrode (RRDE) techniques. It is found that α-MnO₂ nano-wires possess enhanced electrocatalytic activity compared to other two shapes namely nano-tubes and nano-particles despite the nano-tubes having a much higher specific surface area. The insight of bi-functional electrocatalytic activity is analysed in terms of catalyst surface with the help of first principles density functional theory (DFT) calculations based on the fact of surface energies and adsorption of water on the surface for a facile reaction.     

Tuning lattice strain in biphenylene for enhanced electrocatalytic oxygen reduction reaction in proton exchange membrane fuel cells

As proton-exchange membrane fuel cell technology has grown and developed, there has been increasing demand for the design of novel catalyst architectures to achieve high power density and realize wide commercialization. Herein, based on the two-dimensional biphenylene, we compare the oxygen reduction reaction (ORR) activity on the active sites with different biaxial lattice strains using first-principles calculations. The ORR free energy diagrams of biphenylene monolayers with varying lattice strains suggest that the biaxial tensile strains are unfavorable for catalytic activity. In contrast, the biaxial compressive strains could improve the catalytic performance. The biphenylene systems with the strain of ?2% ～ ?6% (S_{-0.02～-0.06}) display overpotentials of 0.37–0.49 V. This performance is comparable to or better than the Pt (111) surface. The Bader charge transfer of adsorbed O species on various biaxial strain biphenylene catalysts could be a describer to examine the catalytic activity. The catalysts possessed the moderate transferred charge of O adsorbed species often promotes catalytic process and give the high catalysis efficiency. Overall, this work suggests that the lattice strain strategy can significantly enhance the catalytic activity of biphenylene materials and further provide guidance to design biphenylene-based catalysts in various chemical reactions.     

DFT study of the oxygen reduction reaction on iron,cobalt and manganese macrocycle active sites

The best performing non-precious metal based catalysts for polymer electrolyte membrane fuel cells are manufactured by incorporation of nitrogen into a carbon structure in the presence of iron and cobalt. Herein, density functional theory (DFT) calculations have been performed to investigate the oxygen reduction reaction on catalyst active sites modelled as transition metal macrocycles with iron, cobalt or manganese central atoms. The effects of the transition metal and macrocycle structure have been investigated. The structure of the most promising active sites has been proposed, and the detailed potential energy profiles of the oxygen reduction reaction have been obtained over the active sites, including all intermediate steps with corresponding activation barriers. The efficiency of the active sites depends primarily on the transition metal nature, and the central iron atom accounts for the higher catalytic activity than cobalt and manganese. The central manganese atom can favour the two-electron oxygen reduction pathway and thus yielding hydrogen peroxide.     

DFT-based study of single transition metal atom doped g-C3N4 as alternative oxygen reduction reaction catalysts

In this paper, the stability and the oxygen reduction reaction (ORR) catalytic activity of single transition metal atom doped g-C₃N₄ catalysts, M-C₃N₄ (M = Mn, Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, Au), were investigated in detail by performing density functional theory (DFT) calculations. The results of binding energy reveal all M-C₃N₄ are thermodynamically stable. Further dynamic calculations demonstrate they are also dynamically stable except Au-C₃N₄. Then, through comparing the value of overpotentials, we found that most of M-C₃N₄ exhibit no ORR catalytic activity except for Ag-C₃N₄ and Pd-C₃N₄, both of which have somewhat catalytic properties but still inferior to Pt(111). It may be caused by the strong adsorption between ORR intermediates (OOH, O, OH) and M-C₃N₄. We further preformed DFT calculation for the high-valent metal complexes of g-C₃N₄ (M-OH-C₃N₄) and the significant enhancement of activity is obtained. Due to the additional OH group, the overall adsorption energies of ORR intermediates on M-OH-C₃N₄ have been decreased and become more close to those on Pt(111), and ORR mechanisms have also been changed. In addition, the overpotentials of ORR on Ni-OH-C₃N₄ and Cu-OH-C₃N₄ are much close to that on the Pt(111), indicating that they possess the catalytic activity comparable to precious Pt catalyst.     

Melamine-assisted synthesis of paper mill sludge-based carbon nanotube/nanoporous carbon nanocomposite for enhanced electrocatalytic oxygen reduction activity

Developing efficient and cheap electrocatalysts as substitutes for commercial Pt/C in the oxygen reduction reaction(ORR)is extremely necessary. Herein, paper mill sludge (PMS) was utilized to produce iron, nitrogen and sulfur co-doped carbon nanotube/nanoporous carbon nanocomposite (PMS-CNT/C) by pyrolysis. PMS-CNT/C-b, one of as-prepared PMS-CNT/C exhibited excellent oxygen reduction reaction activity with an onset potential of 0.99 V vs. RHE and half-wave potential of 0.77 V vs. RHE, which was similar to the commercial Pt/C catalyst (onset potential of 0.99 V vs. RHE and half-wave potential of 0.76 V vs. RHE). It had longer-term stability and higher methanol tolerance in alkaline medium than Pt/C. Moreover, the new catalyst also exhibited excellent catalytic performance in neutral solution. The energy output of microbial fuel cells loaded with PMS-CNT/C-b catalyst was also higher than that of commercial Pt/C under neutral condition. The excellent ORR performance of PMS-CNT/C-b was due to the carbon nanotube/nanoporous structure and the synergistic effect of abundant N groups, iron nitrides and thiophene-S. The formation of CNTs in the carbon nanotube/nanoporous carbon nanocomposite was mainly attributed to melamine, which was added into PMS and was at first just considered as a nitrogen source to develop N-doped PMS-based catalysis in this work. The synthesis of paper mill sludge-based carbon nanotube/nanoporous nanocomposite and its excellent ORR activity will make the new catalyst a promising cathodic electrocatalyst alternative for fuel cells.     

Enhanced electrocatalytic performance of Cu0.02S0.01/rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature

Graphene, is a carbon allotrope, which is widely used as a substrate for various catalysts due to its interesting physicochemical properties. In the present study, graphene oxide sheets were prepared from graphite, then, the graphene oxide surface was modified by a low-temperature method using sulfur and copper atoms to obtain pseudo-enzyme Cu/S/Graphene prosthetic group. The current density passing through Cu/S/Graphene catalyst was four times higher than that passing through graphite. The novel copper-based catalyst had an extraordinary performance for oxygen reduction reaction (ORR) due to the unique bio-inspired and stoichiometric structure. The results of Raman and Dispersive X-ray spectroscopy confirmed the presence of ultra-low content of copper (2%) and sulfur (1%) atoms on the graphene surface. Thermogravimetric analysis indicated a strong interaction between nanoparticles and graphene layers. The number of electrons transferred for ORR varied from 3.98 to 4.16 in a wide range of over-potentials indicating an effective 4-electron pathway form O₂ to H₂O. The Tafel slopes indicated insignificant amount of formed copper oxide on the catalyst surface. The catalyst showed excellent electrochemical durability and its half-wave potential (E_1/2) was exhibited a negative shift only 8.2 mV after 10000 cycles.     

Investigation of electrocatalytic activity on a N-doped reduced graphene oxide surface for the oxygen reduction reaction in an alkaline medium

Today the search for new energy resources is a crucial topic for materials science. The development of new effective catalysts for the oxygen reduction reaction can significantly improve the performance of fuel cells as well as electrocatalytic hydrogen production. This study presents the scalable synthesis of nitrogen-doped graphene oxide for the oxygen reduction reaction. The combination of an ab initio theoretical investigation of the oxygen reduction reaction (ORR) mechanism and detailed electrochemical characterization allowed the identification of electrocatalytically active nitrogen functionalities. The dominant effect on electrocatalytic activity is the presence of graphitic and pyridinic nitrogen and also N-oxide functionalities. The overpotential of ORR for nitrogen-doped graphene oxide prepared by microwave-assisted synthesis outperformed the metal-doped graphene materials.     

High-yield, size-controlled synthesis of silver nanoplates and their applications as methanol-tolerant electrocatalysts in oxygen reduction reaction

A novel method for the synthesis of as-prepared Ag nanoplates in high yield and the control of their dimensions has been developed. In this method, hexadecyltrimethyl ammonium ions (CTA⁺) are used as a trace additive in a seed solution for blocking the seed surface to govern the growth direction on nanoplate in the growth pathway, leading to a high-yield production of the Ag nanoplates with mixed morphologies, mainly triangular nanoplates and nanodisks. The spectra of the obtained nanoplate solution showed a high-intensity peak attributed to the in-plane dipole resonance and a low-intensity peak at 400 nm. By decreasing the amount of CTA⁺, the mean edge length of triangular nanoplates could be changed from ∼78.7 nm-∼124.8 nm. The in-plane dipole resonance peak corresponding to change in the mean edge length shifted from 630 nm to 785 nm, respectively. The mean edge length of triangular nanoplates could also be controlled from 70 nm to 148 nm by decreasing the CTA⁺-adsorbed seed amount. To investigate the practical feasibility of application of the proposed method, the prepared nanoplates were used as a methanol-tolerant electrocatalyst in an oxygen reduction reaction (ORR). An analysis conducted using a rotating ring-disk electrode showed that these nanoplates have high activity towards the ORR and that the electron transfer numbers (n) were 3.85, 3.83, 3.81, and 2.94 for 70 nm, 124 nm, 148 nm nanoplates, and macroscopic Ag electrode, respectively. If the present of methanol, the corresponding n values of 3.82, 3.81, 3.78, and 2.30 were detected. Despite working in the methanol-tolerant solution, the prepared Ag nanoplates still exhibited high electroactivity and their ORR proceeded via an approaching 4-electron pathway.     

Facile synthesis of efficient core-shell structured iron-based carbon catalyst for oxygen reduction reaction

Controlled synthesis of efficient core-shell non-precious metal catalysts for oxygen reduction reaction (ORR) is undoubtedly crucial but challenging for the extensive application of fuel cells and metal-air batteries. Herein, we prepared a core-shell structured Fe/FeC_x nanoparticles and porous carbon composited catalyst (Fe/FeC_x@NC) via a facile two-step heat treatment strategy. The Fe/FeC_x@NC-800_?0.5 prepared with secondary anneal at 800 °C for 0.5 h exhibits superior ORR performance to the commercial Pt/C in terms of comparable onset potential, higher half-wave potential, and outstanding long-term durability in alkaline media. Through combining the physical and electrochemical characterizations of Fe/FeC_x@NC-T_?t with different anneal temperature and precursors, the outstanding ORR performance of Fe/FeC_x@NC-800_?0.5 is caused by the synergistic effect between Fe/FeC_x core and enriched pyridinic N- and graphitic N-doped carbon shell as well as porous carbon with large specific surface area. The structure-activity relationship of core-shell structured Fe–N–C catalysts for ORR provides directions for the development of advanced nonprecious metals catalysts.     

Kinetics and electrocatalytic activity of IrCo/C catalysts for oxygen reduction reaction in PEMFC

The purpose of this study is to develop a novel binary Iridium-Cobalt/C catalyst as a suitable substitute for platinum/C applied in proton exchange membrane fuel cells (PEMFCs). The carbon-supported IrCo catalysts were successfully synthesized using IrCl₃ and C₄H₆CoO₄ as the Ir and Co precursors respectively, in ethylene glycol (EG) refluxing at 120 °C. The nanostructured catalysts were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscope (TEM). Homogeneous catalyst particles supported on carbon showed a size of proximately 2 nm. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted for the characterization of the catalyst performances. With a cathodic loading of 0.4 mg_Ir cm⁻², 20%Ir-30%Co/C achieved a maximum power density of 501.6 mW cm⁻² at 0.418 V, with a 50 cm² H₂/O₂ single cell. Although such a performance is about 26% lower than commercial Pt/C catalyst, it is still helpful in terms of Pt replacement and cost reduction.     

S,N co-doped graphene quantum dots decorated TiO2 and supported with carbon for oxygen reduction reaction catalysis

Sluggish kinetics and catalyst instability in oxygen reduction reaction are the central issues in fuel cell and metal-air battery technologies. For that, highly active, stable, and low-cost non-platinum based electrocatalysts for oxygen reduction reaction are an immediate requirement in fuel cell and metal-air battery technologies. A new composite (S,N-GQD/TiO₂/C-800) is synthesized, made of sulfur (S) and nitrogen (N) co-doped graphene quantum dot (GQD) with TiO₂. This composite is supported on carbon on heating at 800 °C under N₂ atmosphere and is explored for oxygen reduction reaction (ORR) catalyst. The synthesized composite S,N-GQD/TiO₂/C-800, shows outstanding catalytic activity with an onset potential of 0.91 V and a half-wave potential of 0.82 V vs. RHE, an alkaline medium. The Tafel slope of the catalyst is 61 mV dec^?1. The catalyst is an excellent methanol tolerant and shows good stability in an alkaline medium. The excellent ORR activity of S,N-GQD/TiO₂/C-800 is ascribed to well-built interactivity between the S,N-GQD/TiO₂, and the carbon support. The unique structure offers advantages, with outstanding electrical conductivity, high surface area, and excellent charge transfer kinetics between the doped GQD and TiO₂ interface and subsequently from the carbon surface to the S,N-GQD/TiO_2.     

Synthesis of nitrogen-doped mesoporous carbon nanosheets for oxygen reduction electrocatalytic activity enhancement in acid and alkaline media

In the present work, nitrogen-doped mesoporous carbon nanosheets (NMCNs) are prepared and extensively investigated for the oxygen reduction reaction. Initially, by using dual templates, viz. graphene oxide and a cationic surfactant, silica films (thickness: 1.0 nm) are synthesized and characterized using transmission electron microscope (TEM), nitrogen adsorption-desorption (N₂ ad/des) measurements, and small-angle X-ray diffraction (SA-XRD).Morphology and structure of silica evolve along with graphene oxide concentration, while the co-operative assembly and the final structure are determined by the electrostatic interaction between the dual templates. The effect of silica template on the resultant NMCNs is investigated physicochemically by photoelectron spectroscopy (XPS), TEM, N₂ ad/des and electrochemically by cyclic voltammetry (CV), and linear sweep voltammetry (LSV).NMCNs are featured of a high content of nitrogen dopant, high specific surface area (SSA) and ultrathin thickness (1.5 nm), favoring catalysis and facilitating the mass transport of the reactive species. From the electrochemical tests, it is confirmed that NMCNs yield a high oxygen reduction reaction (ORR) electrocatalytic activity in acid and alkaline environment; this activity is similar or even better as compared with the one measured over carbon supported platinum commercial catalyst (40 wt % Pt/C).     

Influence of Fe on electrocatalytic activity of iron-nitrogen-doped carbon materials toward oxygen reduction reaction

The development of highly active nitrogen-doped carbon-based transition metal (M-N-C) compounds for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) greatly helps reduce fuel cell cost, thus rapidly promoting their commercial applications. Among different M-N-C electrocatalysts, the series of Fe-N-C materials are highly favored because of their high ORR activity. However, there remains a debate on the effect of Fe, and rare investigations focus on the influence of Fe addition in the second heat treatment usually performed after acid leaching in the catalyst synthesis. It is thus very critical to explore the influences of Fe on the ORR electrocatalytic activity, which will, in turn, guide the design of Fe-N-C materials with enhanced performance. Herein, a series of Fe-N-C electrocatalysts are synthesize and the influence of Fe on the ORR activity are speculated both experimentally and theoretically. It is deduced that the active site lies in the structure of Fe-N₄, accompanied with the addition of appropriate Fe, and the number of active sites increases without the occurrence of agglomeration particles. Moreover, it is speculated that Fe plays an important role in stabilizing N as well as constituting active sites in the second pyrolyzing process.     

Flame synthesis of nitrogen,boron co-doped carbon as efficient electrocatalyst for oxygen reduction reaction

The flame synthesis provides a simple low-cost method to produce novel carbon materials. In this study, N, B co-doped carbon (NBC) materials have been prepared by flame synthesis. Among many as-prepared samples, the NBC catalyst which prepared under carbonization temperature of 1000 °C for 3 h with acetonitrile/acetone precursor of 1:1 exhibits the best catalytic activity and stability, as well as good resistance to methanol interference for oxygen reduction reaction (ORR), with half-wave potential being almost nearly to Pt/C, and a quasi-four-electron transfer process. This study would provide an economic, environmental feasible and scalable approach for fabricating novel heteroatom co-doped carbon materials for ORR applications.     

The synthesis of highly active Cu and N co-doped electrocatalysts via strong electrostatic adsorption method for oxygen reduction reaction

The development of non-precious metal catalysts to replace scarce and costly Pt-based catalysts is crucial for oxygen reduction reaction (ORR), which involves various electrochemical energy conversions, such as fuel cells and metal-air batteries. However, it is still a challenge to devise a simpler method to obtain high-performance non-noble metal catalyst, which can be used for large-scale production. Herein, Cu and N co-doped mesocellular carbon foam (n-Cu/NC) has been developed by strong electrostatic adsorption (SEA) method, which inherits the original morphology of carbon foam, exhibiting high ORR performance with a half-wave potential of 0.825 V vs. RHE and excellent long-term durability. The outstanding performance of n-Cu/NC is attributed to the high specific surface area, hierarchical porous carbon matrix doped by multiple N atoms and uniform Cu₂O/CuO active sites induced by the SEA method. The strategy of SEA method is applied to the preparation of carbon-supported catalysts, which shows great potential in the design and large-scale production of M and N co-doped carbon materials.