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.     

Porous carbonized egg white as efficient electrocatalyst for oxygen reduction reaction

Hierarchical porous carbonized egg white (EW) is synthesized and used as oxygen reduction reaction (ORR) catalyst. The typical EW that is carbonized at 650 °C (EW-650) possesses ultrahigh specific surface area of 1904 m² g^?1, large average pore diameter of 9.8 nm, high contents of doped heteroatoms (N, O and S), fairly high graphitization degree and dense defects, corresponding to dense active sites and excellent transportation of both mass and electrons. Therefore, the EW-650 shows higher activity than commercial Pt/C for ORR in both alkaline and acidic media even at higher mass loadings, with excellent cyclic stability. The effects of reaction temperature and electrolyte concentration on ORR activity are also studied. It is found that appropriate temperature and electrolyte concentration speed up ORR kinetics, ensure higher oxygen solubility and favor mass transportation.     

Co3O4@g-C3N4 supported on N-doped graphene as effective electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) is a key component of numerous energy conversion equipment, including metal-air batteries and fuel cells. Reasonably designing high-efficiency non-noble materials as ORR electrocatalysts is crucial for large-scale practical applications. In this work, a calcination-hydrothermal method is used to prepare Co₃O₄@g-C₃N₄ (g-C₃N₄ wrapped Co₃O₄ nanoparticle) supported on nitrogen doped graphene (NG). The electrochemical activity of composites is estimated by cyclic voltammograms and linear sweep voltammetry in 0.1 M KOH medium. Owing to the positive synergistic role stemming from the Co₃O_4, g-C₃N₄, Co-N_x effective sites and N modified graphene in the composite material, the Co₃O₄@g-C₃N₄/NG owns positive onset potential of 0.920 V (vs. RHE) and half-wave potential of 0.846 V (vs. RHE), which are superior to onset potential of 0.917 V and half-wave potential of 0.824 V for commercial Pt/C, respectively. Additionally, it also exhibits longer-term stability and stronger methanol resistance comparing with Pt/C. The nonprecious metal catalyst could be used as a hopeful catalyst to substitute commercial Pt/C for ORR.     

Stability of hemin/C electrocatalyst for oxygen reduction reaction

Hemin has been reported to be an effective electrocatalyst for mediating the oxygen reduction reaction. In this work, the stability of hemin/C is extensively investigated in both acid and alkaline media by the electrochemical methods. It is found that the pristine hemin/C yields significant change in the composition and the electrochemical features when it undergoes the potential cycling in acid media. In comparison, the catalyst shows superior stability in alkaline media. The pyrolysis can improve the stability of the hemin/C catalyst by removing the organic groups in hemin; however, the heat treatment cannot prevent the metal ion loss in acid media. Finally, the acid-leaching experiment reveals that the active center for the 4-electron reaction tends to get lost in acid, indicating that the iron metal ion should be involved in catalyzing the 4-electron reduction reaction. Furthermore, the XPS result indicates that the element N is also involved in the active center. Therefore, it can be concluded that the Fe–N contributes to the active center for the complete reduction of oxygen in alkaline media.     

Microbial synthesis of highly dispersed nano-Pd electrocatalyst for oxygen reduction reaction

Microbial fabrication is eco-friendly for nobel-metal catalysts typically used in proton exchange membrane fuel cell (PEMFC). In our study, nano-Pd electrocatalysts were successfully prepared by using three Shewanellas as precursors through hydrogen reduction (200 °C) and carbonization (800 °C). The analysis revealed that the catalysts showed outstanding ORR electrocatalytic performance via a predominant four-electron oxygen reduction pathway in alkaline medium. The best performance was obtained for Pd/HNC-32, which showed a mass activity at 0.526 A mg^?1, 3.78 times higher than that of commercial Pd/C. Shewanella putrefaciens CN-32 was a more effective Pd-adsorbent. The enhanced performance can be ascribed to the small Pd-particle size and uniform dispersion on microbial support, which results from stronger hydrophilicity of Shewanella putrefaciens CN-32. The content of nitrogen is another key to the performance of Pd/HNC-32. This study developed a promising strategy for screening microbial strains for electrocatalyst fabrication.     

The synthesis of phenanthroline and bipyridine based ligand for the preparation of Fe-Nx/C type electrocatalyst for oxygen reduction

In this study, we synthesized a nitrogen rich compound (FPPHA) as the precursor of oxygen reduction reaction (ORR) catalysts, which was prepared based on 1,10-phenanthroline-5-amine and 2,2’-bipyridyl-5,5’-dialdehyde. The FPPHA-Fe complex was formed and be loaded on the carbon powder to form the FPPHA-Fe/C composite catalyst. The pyrolysis of the FPPHA-Fe/C composite was conducted at different temperatures, including 700 °C, 800 °C, and 900 °C, and the resultant pyrolyzed materials were designated to FPPHA-Fe/C-700, FPPHA-Fe/C-800, FPPHA-Fe/C-900, respectively. The physical characteristics of the catalysts were examined by powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller analysis, X-ray photoelectron spectrometer (XPS) and scanning electron microscopy (SEM) etc. The ORR performance of the composite catalysts were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and rotating ring disk electrodes (RRDE) in an alkaline solution. The results suggested that the pyrolysis process has a positive effect on the ORR activity of the catalysts, and among the catalysts the Fe-FPPHA/C-800 had the most preferable performance, which reduces oxygen through a predominantly four electron pathway with an average electron transfer number of 3.86 and an average hydrogen peroxide yield of 6.7%.     

Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction

In order to develop cheap electrochemical oxygen reduction reaction (ORR) catalysts, N-doped CNTs grafted on Vulcan carbon were synthesized via pyrolysis of dicyandiamide on Fe₂O₃/C. Various contents of iron in Fe₂O₃/C (0, 20, 40 and 60 wt. %) were used as supports to investigate the effects and roles of iron content on ORR. It was shown that the iron acted as a promoter for doping nitrogen into carbon; as the iron content increased, the amount of nitrogen doping also increased. TEM and element analysis results indicated that iron induced growth of CNTs and facilitated N-doping in carbon. However, further increase in iron content higher than 20 wt. % showed negative effects on the ORR activity due to a decrease of the surface area of the prepared catalysts. Hence, the catalyst with the highest performance was observed when dicyandiamide was pyrolyzed with Fe₂O₃/C 20 wt. % (Fe-N-C-20) and the order of activity towards ORR was Fe-N-C-20 > Fe-N-C-40 > Fe-N-C-60 > Fe-N-C-0 > Vulcan XC-72R.     

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

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.     

Facile synthesis of boron and nitrogen-doped graphene as efficient electrocatalyst for the oxygen reduction reaction in alkaline media

Boron-doped graphene and nitrogen-doped graphene have been respectively synthesized by a facile thermal solid-state reaction of graphene oxide with boric acid and urea. The morphology and structure of the doped graphene have been characterized by the scanning electron microscopy, infrared spectroscopy, ultraviolet visible spectroscopy and X-ray photoelectron spectroscopy, while the electrocatalytic activity toward oxygen reduction reaction has been evaluated by the cyclic voltammetry. It has been shown that the morphology, structure, doping level and fashions of graphene could be finely tuned by the thermal treatment conditions, and which have substantial effects on the activity of oxygen reduction reaction. The boron-doped graphene and nitrogen-doped graphene calcined at 700 °C demonstrate excellent electrocatalytic oxygen reduction activities as the appropriate introduction of boron and nitrogen functional groups in graphene, which might be promising for low temperature fuel cell applications.     

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.     

Nitrogen doped graphene quantum dots (N-GQDs)/Co3O4 composite material as an efficient bi-functional electrocatalyst for oxygen evolution and oxygen reduction reactions

In this work, a facile development of a bi-functional electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. A composite material comprising of tiny particles of nitrogen doped graphene quantum dots (N-GQDs) embedded into cobalt oxide (Co₃O₄) flakes is prepared by sodium borohydride reduction method and followed by annealing at 600 °C under inert atmosphere. Structural, morphological and crystalline features are analyzed using FESEM, TEM, HRTEM, XRD and XPS studies. Moreover, optical and fluorescence properties of N-GQDs are studied using UV–visible and fluorescence spectroscopic techniques. These studies clearly reveal and confirm the formation of a composite material. Further electrochemical characteristics toward OER and ORR are investigated by using linear sweep voltammetry (LSV) and cyclic voltammetry (CV) techniques. Compared to the individual entities of pure Co₃O₄ and N-GQDs alone, the electrocatalytic activity of N-GQDs/Co₃O₄ composite material is significantly higher towards ORR. Similarly, the same composite material is also used as an electrocatalyst for OER in 0.1 M KOH aqueous electrolyte and it exhibits a lower overpotential of 330 mV to obtain a current density of 10 mA/cm² along with higher electrocatalytic activity and the reason is mainly attributed to the synergistic effect between N-GQDs and Co₃O₄. Thus, N-GQDs/Co₃O₄ composite material is demonstrated to be a high performance bi-functional electrocatalyst for ORR and OER.     

Hybrid dual-template induced nitrogen-doped hierarchically porous carbon as highly efficient oxygen reduction electrocatalyst

Fuel cell, as a promising future energy device, is one of the potential electric generators maintaining the sustainable development of our society and alleviating the major problems related to the energy shortage and environment pollution. However, the high cost and the limited resource of Pt remain the key obstacles for the commercialization of fuel cells. A hybrid dual-template strategy is developed to synthesize the nitrogen-doped ordered hierarchically porous carbon (NHMC) via a surfactant-templating organic resol self-assembly with F127 as soft template and SiO₂ nanosphere as the hard-template. The NHMC catalyst presents three-dimensional hierarchically porous structure, composed of small ordered mesopore (∼3.8 nm), large 3-D interconnected mesopore (∼12 nm) as well as micropore. The catalyst exhibits tailored pore structures and a well-tuned surface chemical environment. It displays high onset potential of 0.91 V and half-wave potential of 0.76 V as well as high limiting current via four-electron pathway for oxygen reduction reaction (ORR). The NHMC also shows high stability and excellent methanol tolerance. This work brings inspiration for the synthesis of low-cos Pt-free catalyst with high activity, stability as well as high methanol tolerance.     

CeO2 nanoparticle-decorated reduced graphene oxide as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions

Highly efficient bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential for the development of renewable energy technologies such as fuel cells and metal-air batteries. Herein, a ceria (CeO₂) – modified reduced graphene oxide (CeO₂/rGO) nanocomposite was fabricated via a facile yet cost-effective method under a mild condition. The prepared CeO₂/rGO nanocomposite showed remarkable catalytic activity, high tolerance to methanol and durability toward ORR in alkaline media. Meanwhile, the catalyst also displayed remarkable activity for the OER with more negative onset potential and higher current compared with commercial Pt/C catalyst. The high oxygen reaction activity of the catalyst could contribute to synergistic effect of the combination of the oxygen vacancies of CeO₂ and excellent electronic conductivity of rGO. The results suggested that the CeO₂/rGO nanocomposite has potential advantages as a bifunctional electrocatalyst in the practical applications.     

Cobalt selenide electrocatalyst supported by nitrogen-doped carbon and its stable activity toward oxygen reduction reaction

We have prepared durable catalysts of CoSe₂/N-carbon using low-cost raw materials, measured their activities, peroxide yields, stabilities in reducing molecular oxygen, and characterized their crystalline phases and morphology. CoSe₂/N-carbon is featured with an active support, N-carbon, which by itself shows high stability as evidenced in its small activity decay. After 1000 CV cycles, the half-wave potential (E_1/2) of N-carbon decreases from 0.667 V to 0.636 V in 0.5 M H₂SO₄. Loading of CoSe₂ enhances the activity of N-carbon, when the samples were synthesized above 385 °C and formulated with the Se/Co ratio higher than 10. The higher activity is attributed to the pyrite phase of CoSe₂. But the stability of pyrite CoSe₂ is less than that of N-carbon. Corrosion during the stability test exposes the active sites of underlying N-carbon, which sustains the catalyst activity. Consequently the E_1/2 value of the active CoSe₂/N-carbon decreases moderately, from 0.711 V to 0.644 V after 1000 CV cycles. In contrast, the E_1/2 value of CoSe₂/C descends much more, from 0.681 V to 0.475 V.     

Fe3C nanoparticles-loaded 3D nanoporous N-doped carbon: A highly efficient electrocatalyst for oxygen reduction in alkaline media

High-performance, non-precious metal electrocatalysts have been widely considered among the most prospective candidates to replace Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Herein, we report a synthetic method, involving templating, polymerization and pyrolysis, that produces catalytically active iron carbide (Fe₃C) nanoparticles-loaded porous N-doped carbon materials from polyaniline- and Fe(III)-modified mesoporous graphitic carbon nitride (g-C₃N₄). We also show that the resulting noble metal-free materials exhibit good electrocatalytic activity for ORR, with good onset and half-wave potentials, in O₂-saturated alkaline solution. The structure, composition, crystallinity, and electrocatalytic activity of these materials are found to depend on the pyrolysis temperature and the specific components in the precursor. In particular, the material obtained by pyrolysis at 1000 °C, named Fe₃C/NC-1000, shows excellent electrocatalytic activity and better performance, in terms of both onset and half-wave potentials, than Pt/C (20 wt% Pt). The material also tolerates the methanol crossover reaction better than Pt/C and shows negligible shift in onset and half-wave potentials to negative values even after use in 3000 cycles of electrocatalysis. This robust, non-noble metal-based carbon material can potentially become a viable alternative to precious metal electrocatalysts for ORR.     

Nitrogen-doped carbon xerogels catalyst for oxygen reduction reaction: Improved structural and catalytic activity by enhancing nitrogen species and cobalt insertion

Highly active nitrogen-doped carbon xerogel (N-CX) electrocatalysts for the oxygen reduction reaction (ORR) were synthesized through a simple sol-gel method. The N-CX samples are prepared using resorcinol – formaldehyde resin as the carbon precursor and dicyandiamide as the nitrogen precursor. The N-CX samples carbonized at different temperatures are inspected to interpret the effect of the high-temperature conditions towards the structures and ORR activity of the final products. As-prepared N-CX samples with different carbonized temperatures are characterized via X-ray photoelectron spectroscopy (XPS), X-ray diffractometry and Raman spectroscopy. The N-CX sample carbonized at 800 °C demonstrated the greatest ORR activity, and the structural properties and catalytic activities of the catalyst are then further improved by insertion of cobalt metal under an ammonia atmosphere. Metal doping evidently promotes the catalytic activity of the N-CX catalyst. Raman and XPS studies show that cobalt increases the creation of pyridinic-N and quaternary-N groups through the formation of more graphitic structures. The ammonia atmosphere is demonstrated to act as an additional N source by increasing the total N content in the carbon structure after high temperature treatment of the N-CX catalyst. Metal-N-like and metal carbide configurations generated play a role in catalyst production with high catalytic activity.     

Boosting electrocatalysis of oxygen reduction and evolution reactions with cost-effective cobalt and nitrogen-doped carbons prepared by simple carbonization of ionic liquids

Development of cost-effective, bi-functional carbon electrocatalysts via direct carbonization of ionic liquids (bis(cholinium) tetrachlorocobaltate(II) ([Ch]₂[CoCl₄]) and bis(1-butyl-3-methylimidazolium) tetrachlorocobaltate(II) ([Bmim]₂[CoCl₄])) is reported herein for the first time. Carbon electrocatalysts, dual-doped with cobalt and nitrogen, were tested for oxygen reduction (ORR) and oxygen evolution (OER) reactions. Both materials show high bi-functional catalytic activity and excellent stability due to synergistic effects arising from the presence of nitrogen and cobalt. Electrocatalyst prepared by carbonization of [Ch]₂[CoCl₄] show exceptional activity and selectivity toward ORR. Conversely, electrocatalyst prepared from [Bmim]₂[CoCl₄] showed a slightly better OER performance indicating that different catalytic sites are responsible for O₂ reduction and H₂O oxidation. Parent CoO particles with graphitic nitrogen boost activity for ORR, while elemental Co supported by nitrogen atoms is responsible for OER activity. Finally, energy consumption during electrolytic oxygen production was calculated revealing energy saving when using two materials as anode electrocatalysts.     

Rationale approach of nitrogen doping at defect sites of multiwalled carbon nanotubes: A strategy for oxygen reduction electrocatalysis

A development of electrocatalyst for oxygen reduction reaction (ORR) is one of the crucial reactions for low temperature fuel cell applications. The state-of-the-art catalyst (platinum based) have various limitations such as low abundance, extortionate price and sensitive towards impurities. Therefore, design of high performance non-platinum electrocatalyst is the most challenging issue for low temperature fuel cell. In this work, we discuss the nitrogen doping at defect sites of multiwalled carbon nanotubes (MWCNTs) using melamine foam as a template for efficient CNT assembly with subsequent role of different nitrogen containing agents such as melamine and hexamine. Templated assembly of functionalized CNT through melamine foam provides easy approach towards nitrogen doping that followed effective exposure of active sites (like pyridinic-N and oxidic-N) for electroreduction of oxygen. As-prepared N-CNT catalysts (prepared using both the precursors) show better ORR activity than Pt/C in alkaline medium. A sharp reduction peak in their cyclic voltammogram under O₂-saturated 0.1 M KOH solution proves their activity towards ORR electrocatalysis. More interestingly, the onset potentials of ~0.92 V and ~1.1 V versus RHE for N-CNT obtained by hexamine and melamine respectively indicate superior onsets than that of Pt/C (~1.04 V vs RHE). Furthermore, the best N-CNT catalyst (obtained by melamine) reveals better stability up to 15,000 cycles than Pt/C with zero response towards methanol, exhibiting an excellent methanol tolerance.     

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.