Nitrogen doped amorphous carbon as metal free electrocatalyst for oxygen reduction reaction

A non metal catalyst for the oxygen reduction reaction is prepared by simply pyrolyzing ion exchange resin D113 in NH₃. The product is nitrogen doped amorphous carbon. The pyrolysis of D113 exchanged with iron ion results in nitrogen doped graphitic carbon. The amorphous carbon is easier to be doped by NH₃ with higher nitrogen content. The nitrogen doped amorphous carbon is more active than graphitized carbon, together with much improved stability. The higher activity is explained by the higher total nitrogen content and higher pyridinic/graphitic nitrogen percentage. The higher stability is because there is no loss or dissolution of the active sites. The results of this work prove metal element and graphitization of carbon are not necessary factors for nitrogen doped carbon as non noble metal catalyst for the oxygen reduction reaction.     

Metal-free electrocatalysts obtained from agave waste by solar pyrolysis for oxygen reduction reaction

Characterization and electrochemical evaluation of novel metal-free electrocatalysts obtained by solar pyrolysis is reported. Carbon-based electrocatalysts were prepared from agave bagasse waste, using a sustainable process based on concentrated solar energy as heat source. Agave was processed in a spherical borosilicate glass solar reactor using a heating rate of 30 °C min^?1 to a target of 500, 700, or 900 °C, and maintaining temperature for 1 h under inert atmosphere. The structure and composition of the prepared electrocatalysts were influenced by pyrolysis temperature. In addition, electrocatalytic activity towards the oxygen reduction reaction in 0.1 M KOH solution was explored. The electrocatalyst obtained at 500 °C showed the highest activity among all pyrolyzed samples due to its moderate surface area, but mostly due to its higher oxygen content. The metal-free electrocatalysts reported in this work are promising eco-friendly alternative as cathode materials for anion-exchange membrane fuel cells. This study provides a sustainable approach to use agricultural biomass waste to produce valuable materials for electrochemical energy devices.     

Self-doped Sargassum spp. derived biocarbon as electrocatalysts for ORR in alkaline media

In this work, high surface area N-doped carbon synthesis from Sargassum spp. is reported as a low cost alternative for electrocatalysts production for the oxygen reduction reaction (ORR). First, Sargassum spp. was activated with potassium hydroxide (SKPH) and then doped with hydrazine (SKPHD). As a result of the activation process, SKPH obtained a high surface area (2289 m² g⁻¹), 0.16% nitrogen and 2.63% sulfur content; it also showed four-electron-transfer mechanism when tested as electrocatalyst in alkaline medium. Besides, SKPHD presented 3.60% nitrogen content in the bulk and higher ORR activity (0.838 V onset potential vs. RHE and 4.59 mA cm⁻² current density) very close to 20% Pt/C (5.25 mA cm⁻²). Results indicate that using seaweeds as a synthesis precursor is an alternative for the Sargassum spp. disposal in the Caribbean due to its high availability and efficiency towards ORR.     

Heteroatoms doped C3N4 as high performance catalysts for the oxygen reduction reaction

Different kinds of carbon nitride were successfully synthesized through pyrolyzing the precursors. Their physical properties were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. While the electrochemical properties were measured by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The results showed that the sulfur-doped carbon nitride exhibited better electrochemical catalytic properties towards oxygen reduction reaction. Most importantly, the onset potential of sulfur-doped carbon nitride was 0.77 V (vs. RHE), which positively shifted 40 mV than that of the carbon nitride. The calculation of kinetics parameters showed that it occurred through an approximately four electron pathway with a lower Tafel slope (115 mV/decade). Furthermore, the sulfur-doped carbon nitride also presented excellent stability and methanol tolerance.     

Interrelations between sulfur,iron, nitrogen,pore and graphite matrix for oxygen reduction reaction

Nitrogen doped lotus-stem carbon and that being promoted by sulfur or iron (N-LC, NS-LC, NFe-LG and NSFe-LG) are synthesized. The interrelations between pore size, graphitization degree, N content, Fe–N content, S doping, and their effects on oxygen reduction reaction (ORR) activity and stability in alkaline media are studied. The NSFe-LG shows the most excellent performances for ORR. The Fe-free NS-LC shows close ORR activity and comparable stability to the NSFe-LG, which may make metal-free catalyst attractive.     

A novel CuFe-based catalyst for the oxygen reduction reaction in alkaline media

The primary objective of this work is to develop alternative electrocatalysts to Pt-based materials for the oxygen reduction reaction (ORR) in alkaline fuel cells. We synthesized a bicore CuFe/C composite electrocatalyst by impregnation of iron and copper phthalocyanine-based complexes into a carbon support, followed by pyrolysis at 800-900 °C in an Ar atmosphere. This novel composite catalyst exhibits electrochemical performance for ORR in 0.1 M KOH similar to a commercial Pt/C (BASF Fuel Cell, 30%) catalyst at 6-fold lower CuFe loading. High resolution X-ray photoelectron spectroscopy (HR-XPS) results indicate that coordination bonding between Fe and N atoms still remains and show that a mixed Cu(I)/Cu(II) valency exists in the CuFe/C catalyst after high temperature heat treatment. The Cu(I)/Cu(II) redox mediator adjacent to Fe atoms is crucial to provide electrons to the N_xFe-O₂ adduct and maximize the overall rate of the reduction reaction. The results of this study may offer a new approach to development of efficient catalysts for oxygen reduction to water in alkaline media.     

Nitrogen doped porous carbon with iron promotion for oxygen reduction reaction in alkaline and acidic media

Nitrogen doped water-hyacinth graphite with little iron (NFe-WHG) is synthesized by using water hyacinth as carbon source, dopamine hydrochloride as N source and Fe(NO₃)₃ as Fe source. The water hyacinth is carbonized to porous carbon; the addition of Fe increases pore diameter, graphitization degree, total N and pyridinic N content. The characterizations indicate that the doping N contributes great on ORR activity, yet the residual Fe species themselves show inconspicuous catalytic effect on ORR. The NFe-WHG with the above features displays superior ORR activity in alkaline media and comparable ORR activity to commercial Pt/C in acidic media. Due to the graphite matrix and that most of the Fe species have been removed, the NFe-WHG shows excellent stability in both alkaline and acidic media with excellent anti-methanol and anti-CO performances.     

Nitrogen doped lotus stem carbon as electrocatalyst comparable to Pt/C for oxygen reduction reaction in alkaline media

The nitrogen doped carbon with high content of pyridine N and porous structure indicates high activity for oxygen reduction reaction (ORR). In this paper, nitrogen doped lotus stem carbon (N-LSC) with 6.3 at% of N (containing 52 at% of pyridine N) and porous structure is developed by using lotus stem as carbon source and dopamine hydrochloride as nitrogen source. The ORR activity, stability and methanol tolerance are characterized. The results show that the N-LSC has comparable activity to Pt/C, and much better methanol tolerance and stability than Pt/C. The porous structure and high content of pyridine N are believed to lead to the high ORR performances of the N-LSC.     

Ni nanoparticles embedded in N doped carbon nanotubes derived from a metal organic framework with improved performance for oxygen evolution reaction

Recently, to improve the catalytic activity of oxygen evolution reaction (OER) electrocatalysts, some design strategies, such as the decrease of the catalyst particle size, the formation of the porous structure and the couple of carbon-based materials, are receiving increased attention in energy-related systems. Herein, based on metal organic framework (MOF), we develop an effective strategy to synthesize Ni nanoparticles embedded in N doped carbon nanotubes (Ni NPs@N-CNTs) catalyst. In consequence, the Ni NPs@N-CNTs integrates the advantageous features of NPs and N-CNTs towards OER, such as more catalytic sites, large surface area, pore-rich structure and good electrical conductivity. Benefiting from the favorable features, the Ni NPs@N-CNTs exhibits a better OER performance than commercial RuO₂ in alkaline medium, which includes a lower onset potential (1.49 V), a smaller Tafel slope (106 mV dec^?1). The present work opens a new window for the construction of the coupling materials between NPs and carbon-based materials to increase the electrocatalytic activity of transition metal catalysts.     

Iron-based sulfur and nitrogen dual doped porous carbon as durable electrocatalysts for oxygen reduction reaction

The widespread use of fuel cell technology is hampered by the use of expensive and scarce platinum metal in electrodes which is required to facilitate the sluggish oxygen reduction reaction (ORR). In this work, a viable synthetic approach was developed to prepare iron-based sulfur and nitrogen dual doped porous carbon (Fe@SNDC) for use in ORR. Benzimidazole, a commercially available monomer, was used as a precursor for N doped carbon and calcined with potassium thiocyanate at different temperatures to tune the pore size, nitrogen content and different types of nitrogen functionality such as pyridinic, pyrrolic and graphitic. The Fe@SNDC–950 with high surface area, optimum N content of about 5 at% and high amount of pyridinic and graphitic N displayed an onset potential and half-wave potential of 0.98 and 0.83 V vs RHE, respectively, in 0.1 M KOH solution. The catalyst also exhibits similar oxygen reduction reaction performance compared to Pt/C (20 wt%) in acidic media. Furthermore, when compared to commercially available Pt/C (20 wt%), Fe@SNDC–950 showed enhanced durability over 6 h and poison tolerance in case of methanol crossover with the concentration up to 3.0 M in oxygen saturated alkaline electrolyte. Our study demonstrates that the presence of N and S along with Fe-N moieties synergistically served as ORR active sites while the high surface area with accessible pores allowed for efficient mass transfer and interaction of oxygen molecules to the active sites contributing to the ORR activity of the catalyst.     

Influence of preparation process on non-noble metal-based composite electrocatalysts for oxygen reduction reaction

Composite electrocatalysts based on the transition metal cobalt are synthesized for the oxygen reduction reaction (ORR) by mechanically mixing three basic precursors containing carbon black, cobalt acetate and tetramethoxy-phenylporphyrin (TMPP). The influence of various mixing processes, solvents, pyrolysis temperature and precursor ratios on the ORR activity of the electrocatalysts is investigated by means of their electrochemical characteristics. Levich–Koutecky plots show that the number of transferred electrons during ORR on these catalysts varies between 2 and 4. A pyrolyzed mixture synthesized by ultrasonication exhibits better activity for ORR than that prepared by ball milling. The solvent is found to have a significant effect on the performance of the catalysts in acidic media. The catalyst synthesized in water, a poor solvent of TMPP, has better activity than that synthesized in a good solvent of TMPP, such as N,N-dimethylformamide and acetic acid. The optimum pyrolysis temperature is 600 °C. Various mole ratios and Co/carbon weight ratios are examined. Maximum activity is found at a 1:1 TMPP/Co mole ratio and a 5% Co/carbon weight ratio.     

Carbon nanotubes supported Fe-based compounds as the electrode catalyst for oxygen reduction reaction

An amorphous Fe-based catalyst supported on polypyrrole-modified carbon nanotubes is synthesized by a chemical method. The microstructure, surface composition and morphology are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The synthesized amorphous Fe-based catalyst is composed of amorphous FeOOH and microcrystalline Fe₂O₃. Compared with a crystalline FeOOH catalyst, the amorphous Fe-based catalyst demonstrates higher electrocatalytic activity toward the oxygen reduction reaction (ORR), due to its amorphous structure and large specific surface area. It is considered that amorphization of transition metal compounds could be one of the methods used to improve their catalytic activity toward the ORR.     

Magnesium oxide as synergistic catalyst for oxygen reduction reaction on strontium doped lanthanum cobalt ferrite

MgO has been demonstrated to have positive effects on the oxygen reduction reaction (ORR) at the strontium doped lanthanum cobalt ferrite (LSCF) cathode for solid oxide fuel cells. MgO particles, which were prepared by the infiltration technique using magnesium nitrate as the precursor, are compatible with LSCF under the experimental conditions for electrode fabrication. Electrical conductivity relaxation investigation demonstrated that the chemical oxygen surface exchange coefficient at 750 °C can be improved by a factor up to 2.4 through coating LSCF surface with MgO particles. Electrochemical impedance analysis showed that the area specific interfacial polarization resistance can be obviously reduced, such as from 0.49 Ω cm² to 0.31 Ω cm² at 650 °C when a porous LSCF electrode supported on samaria-doped ceria electrolyte was infiltrated with 1.01 wt% MgO particles. Distribution of relaxation time (DRT) analysis suggested that the resistance reduction is associated with the charge transfer process. Thus, MgO can enhance ORR on LSCF electrocatalyst, like the previously reported alkaline earth metal compounds of BaCO₃, SrCO₃ and CaO.     

Defect locating: One-step to monodispersed CoFe2O4/rGO nanoparticles for oxygen reduction and oxygen evolution

In this work, monodispersed CoFe₂O₄/reduced graphene oxide (rGO) nanoparticles have been successfully synthetized in one step from Co(Ⅱ) acetylacetonate, Fe(Ⅲ) acetylacetonate, benzylamine and graphene oxide (GO). A facile solvent method was designed to skillfully integrate the crystal growth process of CoFe₂O₄, the reduction process of GO and their compound process. In synthesis process, large numbers of defects on GO thin layers were smartly used to disperse CoFe₂O₄ nanoparticles. The micromorphology and the distribution of as-prepared samples were identified via X-ray diffraction (XRD), transmission electron microscope (TEM) and element mapping spectra. Results showed that the monodispersed CoFe₂O₄ nanoparticles were uniformly coupled with rGO thin layers. Good performance for both oxygen reduction and oxygen evolution of as-prepared CoFe₂O₄/rGO (0.92 V onset potential for oxygen reduction and 1.59 V overpotential at 10 mA cm⁻² for oxygen evolution, vs. RHE) were found during a series of electrochemical tests, which make it a promising bi-functional catalyst in the field of fuel cells and metal-air batteries.     

Polyoxomolybdate anchored graphite oxide: Noble metal-free electrocatalyst for oxygen reduction reaction

We present the synthesis of a noble metal-free electrocatalyst, polyoxomolybdate/reduced graphite oxide (PMA/rGO) composite, which showed enhancement in the kinetics for oxygen reduction reaction (ORR). The composite material was prepared by simple and cost effective method. Mere heating of the precursors at low temperature (200 °C) resulted in molecular assembly of PMA on GO in the form of clusters which behaved as active centers for efficient ORR. The electrochemical study of PMA/rGO-2 (PMA to GO weight ratio of 1:2) catalyst carried out by rotating disk electrode (RDE) method, showed considerable electrocatalytic activity with E_onset of 1.0 V vs. RHE and current density of 4.0 mA/cm² at 1600 rpm in alkaline condition. Additionally, as-prepared PMA/rGO-2 catalyst showed a single step ~ 4 electron transfer pathway similar to commercial Pt/C catalyst; confirmed through rotating ring disk electrode (RRDE) study. Interestingly, PMA/rGO-2 electrocatalyst exhibited substantially higher stability than Pt/C catalyst even after 20K potential cycles (though the current density of former catalyst is inferior to later). Further, in a methanol cross-over test, PMA/rGO-2 was found to be inactive towards methanol oxidation reactions, which could nullify the issues due to the fuel cross-over effect, if employed as cathode in direct methanol fuel cells. The enhanced ORR activity and significant stability is attributable to the anchoring and homogenous distribution of polyoxomolybdate clusters on graphite oxide.     

Medulla stachyuri-derived iron and nitrogen co- doped 2D porous carbon-flakes for highly efficient oxygen reduction electrocatalysis and supercapacitors

Iron and nitrogen co-doped two-dimensional (2D) porous carbon-flakes have been fabricated by using foam-like Medulla stachyuri (MS, the stem pith of tetrapanax papyrifer) as both carbon precursor and template and ammonium ferric citrate as iron and nitrogen precursor. The ammonium ferric citrate-impregnated foams are subsequently converted into iron and nitrogen co-doped 2D porous carbon-flakes by pyrolysis at high temperature in an inert atmosphere. The porous carbon-flakes fabricated at 900 °C (MS-Fe-900) possess high surface area (1140.9 m² g⁻¹) and effective Fe/N co-doping (0.22 at.% Fe and 2.02 at.% N). In comparison with Pt/C, MS-Fe-900 exhibits superior ORR activity (E₀ = 968 mV; E_1/2 = 830 mV vs RHE), preferable methanol/CO tolerance and better stability. Furthermore, the MS-Fe-900-based electrode presents high-rate performance (80.1% capacitance retention from 1 to 100 A g⁻¹), and good cycling stability for over 10000 cycles in 6 M KOH electrolyte. This work takes full advantage of the unique structure of biomass and provides a feasible approach to develop cost-efficient and high performance activated carbon materials for ORR electrocatalysis and supercapacitors.     

Trace sulfur promoted Fe,N-codoped carbon black as electrocatalyst for oxygen reduction reaction

Doping carbon materials with Fe and N attracts great attention due to its promising application in preparing ORR electrode with high performance and low cost. Previously, Fe, N-codoped catalyst (Fe/N/C) had been synthesized via a simple one-pot method using carbon materials, dopamine and FeCl₃ by our group. However, the unstable activity and low selectivity (electron transfer number of ∼3.5) are key problems that should be solved. Herein, trace sulfur has been introduced into Fe, N-codoped carbon black by using 2-mercaptoethanol as an adhesive sulfur precursor. By the doping of trace S atoms (∼0.25 at%) into Fe, N-codoped carbon frameworks, the ORR performance has been obviously improved simply without any re-treatment process, such as acid-etching or nitrogen supplement. The mechanism of this process has been systematically investigated by changing the amount of initial sulfur precursor. A moderate amount of trace sulfur can effectively enhance the ORR performance of Fe, N-codoped carbon black due to suitable interactions among Fe, N, S and C elements. Both the content and the state of Fe and N species on the surface of carbon black can be changed and controlled by trace sulfur. The as-synthesized 1.0 SFe/N/C catalyst exhibits a good ORR activity (E_1/2 = 0.749 V, J_k = 54.56 mA cm⁻²) and a total 4-electron selectivity. 1.0 SFe/N/C also shows better catalytic stability and methanol tolerance than 20 wt% Pt/C.     

Metallic iron doped vitamin B12/C as efficient nonprecious metal catalysts for oxygen reduction reaction

The development of efficient nonprecious metal catalysts for oxygen reduction reaction (ORR) is crucial but challenging. Herein, one simple and effective strategy is developed to synthesize bimetallic nitrogen-doped carbon catalysts by pyrolyzing Fe-doped Vitamin B12 (VB12) supported carbon black (Fe-VB12/C). A typical Fe₂₀-VB12/C catalyst with a nominal iron content of 20 wt% pyrolyzed at 700 °C exhibits remarkably ORR activity in alkaline medium (half-wave potential of 0.88 V, 10 mV positive than that of commercial Pt/C), high selectivity (electron transfer number > 3.93), excellent stability (only 6 mV negative shift of half-wave potential after 5000 potential cycles) and good methanol-tolerance. The superior ORR activity of the composite is mainly attributed to the improved mesoporous structure and co-existence of abundant Fe-N_x and Co-N_x active sites. Meanwhile, the metallic Fe are necessary for the improved ORR activity by means of the interaction of metallic Fe with neighboring active sites.     

Recent progress in heteroatom doped carbon based electrocatalysts for oxygen reduction reaction in anion exchange membrane fuel cells

Oxygen reduction reaction (ORR) is a key step in many electrochemical devices such as fuel cells and metal-air batteries. However, the reaction proceeds at a significant overpotential requiring Pt-based catalysts. The scarcity and economical challenges associated with Pt is one of the major limitations for the commercialization of the devices. In this context, the electrochemical research community is constantly exploring other low-cost and earth-abundant materials as ORR catalysts. Carbon nanomaterials are identified as promising electrocatalysts due to their superb electronic conductivity together with high specific surface area. However, the low reactivity of carbon is the major limiting factor in the fabrication of ORR catalysts. Recent studies have proved that chemical modification of the carbon network (substitution of foreign atoms, Ex: N, S, B, F, P) could alter the reactivity of carbon nanomaterials for ORR. Many doping strategies have been proposed including single atom doping, co-doping and multi-atom doping. The heteroatom doped carbons have delivered promising results towards ORR in alkaline media. This review presents a rational approach of doping methods and the electrochemical properties of heteroatom doped carbons, and we believe that this review could be a guiding material to design advanced non-noble catalysts for ORR in the coming future.     

Design of Fe and Cu bimetallic integration on N and F co-doped porous carbon material for oxygen reduction reaction

At present, fuel cell is considered to be one of the most ideal application technologies of hydrogen energy. In order to develop fuel cells on a large scale and in a sustainable way, low platinum or non-platinum oxygen reduction catalyst has become a research hotspot. In this work, a kind of doped carbon-based catalyst MPFe₁Cu₁-850 is prepared by high temperature synthesis. The catalyst has an ultra-thin lamellar porous structure. In an acidic medium, the MPFe₁Cu₁-850 displays good oxygen reduction reaction (ORR) activity (ΔE_1/2 = 0.725 V). In addition, it shows better stability (91%) and higher methanol tolerance than that of commercial Pt/C catalyst. In our catalyst MPFe₁Cu₁-850, the contents of nitrogen, iron and copper are 10.31 at.%, 0.51 at.%, and 0.50 at.%, respectively. This work shows that high N content, and the proper ratio of iron to copper (Fe:Cu = 1:1), are conducive to the enhancement of ORR activity.