Effect of carbon precursor and initial pH on cobalt-doped carbon xerogel for oxygen reduction

In this study, cobalt-doped carbon xerogel (Co-CX) was synthesised via sol-gel polymerisation of phenolic compounds (i.e., resorcinol, phenol and m-cresol) and formaldehyde, and this polymerisation was catalysed by cobalt nitrate and followed by a carbonisation process. The effect of the initial pH value (5.5, 6.5 and 7.5) as well as the type of carbon precursors on the structural properties of Co-CX was investigated via field emission scanning electron microscope (FESEM), Brunauer-Emmett-Teller (BET) and X-ray diffractometry (XRD). The catalytic activity of Co-CX for the oxygen reduction reaction (ORR) in 0.1 M KOH was studied using a rotating ring-disk electrode (RRDE) technique. The structural properties and ORR activities were affected by different initial pH values as well as the type of carbon precursor. A carbon precursor consisting of resorcinol-formaldehyde with an initial pH value of 7.5 exhibited the best catalytic activity. The initial pH plays an important role in promoting micro/mesopores. The FESEM and BET results revealed that Co doping promotes the formation of additional pores. The RRDE result indicated that Co-CX exhibited good catalytic activity that tends to favour a four-electron pathway.     

Ultra-low Pt loading catalytic layer based on buckypaper for oxygen reduction reaction

Buckypaper is a freestanding, self-supported and well defined membrane-like black thin film. We developed a buckypaper film containing a MWNTs-rich surface and a CNFs-rich surface. Pt nanoparticles supported on this film with an ultra-low Pt content, 0.05 mg/cm², as a catalytic layer for oxygen reduction reaction (ORR) were synthesized by a sputtering process (Pt/BP-SP). The physical and chemical properties as well as electrochemical performance of this nanocomposite were studied. For comparison, Pt nanoparticles supported on buckypaper with a Pt content of 0.10 mg/cm² were prepared by an electrodeposition process (Pt/BP-ED). The results proved that Pt/BP-SP possessed a smaller particle size and a more uniform distribution on buckypaper than Pt/BP-ED. Cyclic voltammetric investigation showed that Pt/BP-SP had a higher electrochemical surface area although its Pt content was lower than that of Pt/BP-ED. Tafel studies proved that Pt/BP-SP had a move positive equilibrium potential and a higher apparent exchange current density. Moreover, the results of linear sweep voltammetric (LSV) analysis exhibited that Pt/BP-SP had a higher ORR activity compared with Pt/BP-ED, and LSV at different rotation rates revealed that ORR on Pt/BP-SP electrode was a 4e⁻ process.     

Strategies to improve the catalytic activity of Fe-based catalysts for nitrogen reduction reaction

Electrochemical ammonia synthesis from N₂ under mild condition is considered a promising strategy to store energy produced by renewable sources, but it is affected by the lack of efficient catalysts for nitrogen reduction. In this work Fe-based nanoparticles with different morphology are deposited on carbon cloth via drop-casting and chemical reduction. The catalyst activity has been evaluated by cyclic voltammetry and chronoamperometry, using a 0.01 M phosphate buffered electrolyte (PBS). The produced ammonia has been determined through the indophenol method. As effective strategy to improve the catalytic activity, the morphology and particle size have been optimized and an electrochemical activation procedure has been implemented. Activation increases the available active sites and is related to higher amount of oxygen vacancies and Fe⁺²/Fe⁺³ ratio. Catalysts with optimized morphology produce ammonia at −0.35 V vs RHE with yield of 26.44 μg mg_cat⁻¹h⁻¹ and Faradaic efficiency of 20.4%, more than five times higher than without activation.     

A glassy carbon electrode modified with tailored nanostructures of cobalt oxide for oxygen reduction reaction

Herein we report on various surface morphological characteristics of the synthesized cobalt oxide (Co₃O₄) nanostructures obtained by means of facile one-step hydrothermal method for oxygen reduction reaction (ORR). The synthesized nanostructures of Co₃O₄ were adequately characterized by field emission scanning electron microscopy (FESEM) fitted with Energy-dispersive X-ray spectroscopy (EDX) elemental mapping, X-ray diffraction (XRD) and Raman techniques. The electrochemical studies were carried out to analyse the performance of as-synthesized catalysts for ORR by cyclic voltammetry (CV), and chronoamperometric (CA) techniques. A higher electrocatalytic response was observed for Co₃O₄ nanocubes compared with all the other controlled electrodes by CV with a current density of 0.69 mA/cm² at a potential value of −0.46 V. The as-synthesized material showed adequate tolerance against methanol observed by CV in the presence of 0.5 M methanol, and good stability when compared with commercial Pt/C catalyst using the CA technique.     

Enhanced catalytic activity of nanoshell carbon co-doped with boron and nitrogen in the oxygen reduction reaction

The use of carbon cathode catalysts in polymer electrolyte fuel cells instead of the current platinum catalysts is attracting increasing attention. We claim that two factors are important for enhancing the activity of carbon cathode catalysts in the oxygen reduction reaction (ORR): the formation of a nanoshell structure and co-doping with boron and nitrogen. Herein, we investigate the preparation and characterization of active ORR carbon catalysts that combine the above factors. Boron and nitrogen (BN)-doped nanoshell-containing carbon (BN-NSCC) was prepared by carbonizing a mixture of poly(furfuryl alcohol), cobalt phthalocyanine, melamine, and a trifluoroborane–methanol complex at 1000 °C. Transmission electron microscopy and X-ray photoelectron spectroscopy revealed the formation of nanoshell structures with distorted graphitic layers and the introduction of boron and nitrogen atoms, respectively. The ORR activity was evaluated in oxygen-saturated 0.5 mol dm^?3 H₂SO₄ using Koutecky–Levich analysis. The BN-NSCC showed an eight to ten times higher ORR activity than undoped NSCC, with an increased number of electrons participating in the reaction. Tafel analysis revealed a change in the rate-determining step caused by BN-doping. Thus, the combination of a nanoshell structure and co-doping with boron and nitrogen was found to improve the ORR activity of carbon catalysts.     

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.     

Cattail leaf-derived nitrogen-doped carbons via hydrothermal ammonia treatment for electrocatalytic oxygen reduction in an alkaline electrolyte

Cattail leaf-derived nitrogen-doped carbons (CL-NCs) were prepared by hydrothermal treatment in ammonia solution and subsequent pyrolysis for application as catalysts for the oxygen reduction reaction (ORR). The ammonia concentration was varied at 1.0, 1.5, and 2.0 M to alter the nitrogen doping content. The characterization results revealed that CL-NCs exhibited an amorphous structure, while the density of structural defects increased as the ammonia concentration increased. The CL-NC prepared without hydrothermal ammonia treatment had a nonporous structure with a low specific surface area (5 m² g^?1). With hydrothermal ammonia treatment, CL-NCs exhibited a micro–mesoporous structure with a higher surface area (113–496 m² g^?1); however, the surface area was significantly diminished at higher ammonia concentrations due to the deterioration of the pore structure. The nitrogen-doping content in CL-NCs varied from 0.65 to 1.55 atom% with the predominant ratios of pyridinic-N and graphitic-N. For electrochemical evaluation in an alkaline electrolyte (0.1 M KOH), CL-NC prepared at an ammonia concentration of 1.0 M showed the highest ORR activity among all samples, as indicated by the most positive onset potential (?0.05 V vs. Ag/AgCl) and half-wave potential (?0.22 V vs. Ag/AgCl) as well as the highest diffusion-limiting current density with a more favorable reduction via a direct four-electron pathway (n = 3.23–3.52). The ORR activity of CL-NCs had a similar trend to their specific surface area rather than nitrogen doping content, indicating the important role of surface area and porosity in enhancing the ORR activity. Moreover, it possessed excellent stability under long-term operation and exposure to methanol. The results obtained in this work could be helpful information for the further development and utilization of biomass-derived NCs for ORR catalysts.     

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.     

Atomically thin titanium carbide used as high-efficient,low-cost and stable catalyst for oxygen reduction reaction

Addressed herein, a highly effective, low-cost and stable TiC nanocatalyst was successfully synthesized through ultrasonic exfoliation of commercial TiC in deionized water and its application as a catalyst in the oxygen reduction reaction is outlined. A facile ultrasonic exfoliation was applied to fabricate atomically thin TiC. The structure, morphology, composition and catalytic properties of atomically thin TiC were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. Atomically thin TiC possesses superior ORR activity in terms of catalytic performance, methanol tolerance, and long-term durability. This work has been provided a low-cost, efficient and stable substitute catalyst for Pt/C to promote the industrialization for energy storage and conversion devices.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

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.     

Synergistic improvement of oxygen reduction reaction on gold/cerium-phosphate catalysts

The main challenge in fuel cells lies in improving slow oxygen reduction reaction (ORR) kinetics causing low conversion efficiencies. Here, we introduce the Au/CePO₄-binary nanocomposites as effective oxygen reduction catalysts in alkaline media. The ORR activity comparable with Pt is achieved through the serial 4-electron reduction pathway. The bi-functionality of CePO₄ is suggested to explain the remarkably enhanced activity on the Au/CePO₄ nanocomposites. Significantly, the own catalytic activity of CePO₄ for hydrogen peroxide is demonstrated, validating synergistic effects with Au for complete ORR.     

Synthesis of hollow Fe,Co, and N-doped carbon catalysts from conducting polymer-metal-organic-frameworks core-shell particles for their application in an oxygen reduction reaction

Oxygen reduction reaction (ORR), one of the key reactions for fuel cells and zinc-air batteries, should be improved for higher performance. Herein, we fabricated hollow Fe, Co, and nitrogen co-doped carbon (H-FeCo-NC) catalyst, which was prepared by carbonization of core-shell particles made of polypyrrole (PPy)-coated polystyrene (PS) spheres as cores and (Zn, Co) bimetallic-zeolitic imidazolate frameworks (ZnCoBZIFs) as shells. PPy was used as a nitrogen and a carbon source. The H-FeCo-NC catalyst had a high surface area of 324.08 m² g^?1 with uniformly distributed Fe and Co species, and excellent ORR performance with the half-wave potential of 0.888 V vs. reversible hydrogen electrode in alkaline media. Furthermore, the H-FeCo-NC catalyst demonstrated exceptional stability, durability, and tolerance to methanol crossover.     

Influence of pre-treatment on the catalytic activity of carbon and its Co-based catalyst for oxygen reduction reaction

Impact of carbon pre-treatment on the catalytic activity and selectivity of its own and its relevant non-precious metal Co-based catalyst (carbon-supported cobalt diethylenetriamine, CoDETA/C) for oxygen reduction reaction is investigated. Three pre-treatment methods involving thermal treatment, H₂O₂-oxidation and KOH-activation are used in this paper. Electrochemical activity demonstrated by cyclic voltammograms and rotating ring disk electrode technique in O₂-saturated electrolyte shows that pre-treatment step has a significant effect on the catalytic activity and selectivity of carbon and its Co-based catalyst: (1) for carbon sample, a KOH-activation gives the highest activity in acid medium, while a H₂O₂-oxidation in alkaline solution; and (2) for its Co-based catalyst, the as-ground gives the highest activity and selectivity in acid solution, while a KOH-activation in alkaline medium. Raman spectra indicate that pre-treatment can decrease the disorder of carbon matrix. X-ray diffraction shows that face-centered cubic α-Co phases are present and pre-treatment of carbon can decrease the size of metal Co dispersed on the catalyst surface.     

Exploring the active sites of nitrogen-doped graphene as catalysts for the oxygen reduction reaction

Nitrogen-doped graphene is studied as a kind of non-noble metal catalyst for the oxygen reduction reaction in the cathode of fuel cells. Graphene is synthesized by pyrolyzing ion exchange resin and nitrogen doping is realized by a second pyrolysis step with nitrogen precursor. High resolution transmission electron microscopy proves the graphene is composed by 8–10 graphitic layers. The defect of graphene caused by nitrogen doping is detected by Raman spectra. The nitrogen group of the doped graphene is studied in detail with X-ray photoelectron spectroscopy spectra and a special type of nitrogen: valley-N is distinguished. The valley-N is proved to play an important role in the oxygen reduction reaction. Nitrogen content is found not directly related with the activity of the oxygen reduction reaction.     

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.     

Bimetallic ZnCo zeolitic imidazolate framework/polypyrrole-polyaniline derived Co/N-doped carbon for oxygen reduction reaction

Non-noble-metal based materials with high activity for oxygen reduction reaction (ORR) are urgently required to substitute Pt-based materials. Herein, metallic Co/N-doped carbon electrocatalysts were synthesized via a facile pyrolysis method of bimetal ZIF(ZnCo)/polypyrrole-polyaniline (ppy-pani) precursors. The evaporation of Zn and the introduced ppy-pani can lead to the porous structure and effectively hinder the aggregation of Co species, which results in the small nanoparticles uniformly distributed on carbon matrices. A high ECSA and a high content of Co species are obtained after the introduction of ppy-pani, thus resulting in the abundant Co²⁺ species to enhance ORR. Therefore, the optimized ZIF/ppy-pani-750 exhibits a high E_1/2 (~0.86 V) and a low Tafel slope (~43.6 mV dec⁻¹).     

A review of heat-treatment effects on activity and stability of PEM fuel cell catalysts for oxygen reduction reaction

This paper reviews over 120 papers regarding the effect of heat treatment on the catalytic activity and stability of proton exchange membrane (PEM) fuel cell catalysts. These catalysts include primarily unsupported and carbon-supported platinum (Pt), Pt alloys, non-Pt alloys, and transition metal macrocycles. The heat treatment can induce changes in catalyst properties such as particle size, morphology, dispersion of the metal on the support, alloying degree, active site formation, catalytic activity, and catalytic stability. The optimum heat-treatment temperature and time period are strongly dependent on the individual catalyst. With respect to Pt-based catalysts, heat treatment can induce particle-size growth, better alloying degree, and changes in the catalyst surface morphology from amorphous to more ordered states, all of which have a remarkable effect on oxygen reduction reaction (ORR) activity and stability. However, heat treatment of the catalyst carbon supports can also significantly affect the ORR catalytic activity of the supported catalyst. Regarding non-noble catalysts, in particular transition metal macrocycles, heat treatment is also important in ORR activity and stability improvement. In fact, heat treatment is a necessary step for introducing more active catalytic sites. For metal chalcogenide catalysts, it seems that heat treatment may not be necessary for catalytic activity and stability improvement. More research is necessary to improve our fundamental understanding and to develop a new strategy that includes innovative heat-treatment processes for enhancing fuel cell catalyst activity and stability.     

The influence of the structural properties of carbon on the oxygen reduction reaction of nitrogen modified carbon based catalysts

Nitrogen modified carbon based catalysts for the oxygen reduction reaction (ORR) were synthesized using three different types of carbon, carbon black (CB), carbon nanotube (CNT) and platelet carbon nanofiber (P-CNF) with nitrogen containing organic precursors. The relationship between the ORR activity and the carbon nanostructure was explored using various electrochemical and physical characterization methods. It was found that the ORR activity was affected by the type and content of the nitrogen functional group instead of the carbon surface area. The formation of nitrogen functional group, in turn, strongly depends on the carbon nanostructure. Unlike the basal plane, the edge plane exposure provides the appropriate geometry for the nitrogen incorporation into carbon structure, resulting in high nitrogen content and high pyridinic-N and graphitic-N content, providing an active site for ORR. Therefore, the P-CNF based catalyst with the highest edge plane exposure has the highest ORR activity despite having the smallest surface area.     

Effects of composition on electrochemical properties of a non-precious metal catalyst towards oxygen reduction reaction

A family of non-precious metal catalysts, Co-PPy-TsOH/C, has been synthesized with different amount of pyrrole and p-toluenesulfonic acid (TsOH). Elemental contents of Co, N, C, S, H and O in the obtained catalysts have been measured with physicochemical techniques and the performance of these catalysts towards oxygen reduction reaction (ORR) have been evaluated with electrochemical techniques. Then, the results obtained have been discussed with principal component analysis and linear correlation analysis to find the correlation/anticorrelation between the composition and electrochemical properties. It is revealed that the used amount of pyrrole has much more apparent effect than TsOH on elemental contents in the Co-PPy-TsOH/C catalysts, while both of them influence the ORR activity and mechanism of the catalysts. Besides, the effects of the contents of each element on the electrochemical performance have also been analyzed to guide the future development of similar catalysts.