Ordered mesoporous carbons with controlled particle sizes as catalyst supports for direct methanol fuel cell cathodes

The ordered mesoporous carbons (OMCs) with various primary particle sizes were synthesized and the effect of the particle size of the OMC supports on their performance for the oxygen reduction reaction (ORR) in direct methanol fuel cells was investigated. The ordered mesoporous silica (OMS) templates with particle sizes of 100, 300, and 700 nm (OMS-100, -300, and -700) were synthesized by changing the synthesis pH and Na content in the silica source, sodium silicate. The OMCs with similar particle sizes and morphologies (OMC-100, -300, and -700) were faithfully replicated by using the corresponding OMSs as templates and phenanthrene as a carbon source. Structural characterizations revealed that three OMCs exhibit uniform mesopores of 4–5 nm and BET surface areas of 600–800 m² g⁻¹. The Pt nanoparticles of ca. 3 nm were supported onto these OMCs and the resulting Pt/OMC catalysts were tested for the ORR. The three OMC supported catalysts exhibited the catalyst utilization efficiencies and ORR activities of similar range, with the values of Pt/OMC-300 catalyst being slightly higher than the other two catalysts.     

Impact of framework structure of ordered mesoporous carbons on the performance of supported Pt catalysts for oxygen reduction reaction

Ordered mesoporous carbons (OMCs) are investigated as support materials for Pt catalysts for oxygen reduction reaction (ORR). Three types of OMCs (CMK-3, CMK-3G, and CMK-5) are prepared by a nanocasting method using ordered mesoporous silica, SBA-15, as a template. These OMCs with the same hexagonal mesostructure have different carbon frameworks and graphiticity, which can affect their surface area and microporosity. Pt nanoparticles with an average size of 1 nm are uniformly supported on the three OMCs and Ketjenblack® and their electrochemical performance and durability are evaluated. Pt/CMK-3G exhibits the highest electrochemically active surface area, kinetic current density, mass activity, and half-wave potential, whereas Pt/CMK-3 shows the lowest values. Pt/CMK-3G also shows the highest ORR activity after an accelerated durability test, with a minimal shift in half-wave potential. The higher ORR activity of Pt/CMK-3G is attributed to the formation of highly crystalline Pt particles as well as its highly graphitic, crystalline carbon structure, which causes the weak adsorption of surface oxide and a strong interaction between the Pt particles and the support. Moreover, we can establish that the mass activity of the catalysts is nearly inversely proportional to the micropore volume of the carbon supports.     

有序介孔炭的合成及其磺化制备固体酸

对于高酸值油脂原料,在酯交换反应制备生物柴油前需对其进行降酸处理,然而常用的H₂SO₄催化降酸法存在设备腐蚀和环境污染等问题。采用软模板法合成有序介孔炭(OMCs),利用TEM、SAXRD及N₂吸附/脱附等对其进行结构性能表征,探讨了模板剂与前驱体的质量比、热聚合时间、煅烧温度等工艺条件对OMC介孔结构的影响,获得了具有高度有序二维六方孔道结构的介孔炭,孔径3~4 nm,比表面积>600 m²/g。进而通过自由基加成法对OMC磺化制得磺化介孔炭,其介孔有序性能够得到很好的保持。合成的磺化介孔炭将作为固体酸催化剂用于后续高酸值油脂制备生物柴油的降酸过程。     

The Active Site Structure of Transition Metal Ion‐Chelating Ordered Mesoporous Carbon Fuel Cell Catalysts

Transition metal ion‐chelating ordered mesoporous carbons (TM‐OMCs) were studied as polymer electrolyte membrane fuel cell cathode catalysts. The active site structure of the TM‐OMCs was studied by X‐ray absorption spectroscopy in combination with variations of a range of synthesis variables of the TM‐OMCs. The variations were found to have significant influence on the catalyst structure both in the mesoscale and on the atomic local structure allowing for detailed conclusions on the nature of the active sites. The main active site was found to be FeN_x chelates. An additional highly active site was found and proposed to be a FeN_x‐dioxygen site. It was further found that the catalytic activity could be increased threefold by acid washing and subsequent heat treatment of the as‐synthesized TM‐OMC materials.     

Synthesis of nanocast ordered mesoporous carbons and their application as electrode materials for supercapacitor

Various nanocast ordered mesoporous carbons (OMCs) were synthesized using mesoporous silicas such as SBA-15, SBA-16, KIT-6, SBA-3 and MCM-48 as templates via nanocasting pathway. The structures of OMCs were analyzed by X-ray diffraction, transmission electron microscope and nitrogen sorption technique. These OMCs with well-defined pore structure were used as model electrode materials for investigating the influence of pore structure on their double layer capacitances. The cyclic voltammetry and galvanostatic charge/discharge measurements were conducted to estimate the capacitive behaviour of OMCs. The results show that the mesopore structures of OMCs play an important role in improving surface utilization for the formation of electrical double layer. OMCs synthesized from SBA-15 and SBA-16 show great advantage over others because their micropores are being easy accessible through the mesopores, thus allowing rapid electrolyte ion diffusion. To achieve a higher specific capacitance (μF cm⁻²), the optimized amount ratio between micropore and mesopore needs to be controlled. In addition, great impact of the electrode disc thickness on the capacitive performance was demonstrated by a series of careful measurements.     

Preparation and oxygen reduction activity of BN-doped carbons

B-, N- and BN-doped carbons were prepared by the carbonization of poly(furfuryl alcohol) containing either BF₃-MeOH complex, melamine or both. Various characterization techniques were conducted in order to understand the controlling factors for the electrocatalytic activity of carbons for oxygen reduction reaction (ORR). Analytical inspections of the obtained XRD profiles, such as the Diamond and the Hirsch methods, revealed that the doping of boron and nitrogen broadened the graphene size distribution; while it did not influence the stacking manner of graphenes. All doped carbons showed reductions in electrical conductivity by the factors of a quarter to a hundredth of the pristine carbons. Development of porosity was observed for the doped carbons. B1s and N1s XPS analyses revealed the presences of B-C bonds and B-N-C moieties in the BN-doped carbons. The oxygen reductive activity, which was assessed by the ORR current density normalized by BET surface area, was increased by the factors of 4-5 for B-doped and N-doped carbons, while a factor of 20 was attained for the BN-doped carbons. The ORR activity of the prepared carbons was found to be controlled by the presence of both the edge-N and B-N-C moieties, which were distinguished by XPS.     

Enhanced electrocatalytic activity due to additional phosphorous doping in nitrogen and sulfur-doped graphene: A comprehensive study

Effect on oxygen reduction reaction (ORR) of ternary-doped reduced graphene oxide (RGO) as an electrocatalyst is evaluated by employing thiourea as a single source of nitrogen (N) and sulfur (S), and triphenylphosphine for phosphorous (P) as precursors for heteroatom doping. The topographical studies show that by doping the RGO, disruption in surface charge and spin asymmetry is introduced into the carbon matrix due to the difference in the bond length and electronegativity between carbon and heteroatoms, which makes carbon lattice ORR active. Ternary (N, S and P)-doped RGO shows excellent ORR activity, which is ∼2 times better than that of binary (N and S)-doped RGO, and ∼5 times better than that of single (P)-doped RGO. The catalytic activity of the ternary-doped carbon even exceeds the commercial Pt in alkaline medium. Additional P doping causes remarkable synergistic effect on binary N and S-doped RGO by generating active P–N species, improving graphitic order and increasing surface area as well as mesopore volume, which in turn enhances the ORR activity.     

Ordered mesoporous carbons (OMC) as supports of electrocatalysts for direct methanol fuel cells (DMFC): Effect of carbon precursors of OMC on DMFC performances

This paper presents the effect of graphitic character of ordered mesoporous carbons (OMCs) on the performances of OMC supported catalysts for direct methanol fuel cells (DMFC). Two OMC samples with hexagonal mesostructure were prepared from phenanthrene and sucrose by nano-replication method using mesoporous silica as a template. Structural characterizations revealed that both OMCs exhibited large BET surface area and uniform mesopores, while the OMC synthesized from phenanthrene exhibited lower sheet resistance than the OMC derived from sucrose. The Pt nanoparticles were supported on both OMCs with very high dispersion, as the particle size was estimated under 3 nm despite high metal loading of 60 wt.%. In DMFC single cell test, the OMC supported Pt catalysts exhibited much higher performance than the commercial catalyst, which may be attributed to the high surface area and uniform mesopore networks of OMC. In particular, it was found that the performance of OMC supported catalysts can be significantly enhanced by lowering the resistance of OMC.     

Ordered mesoporous carbons supported wacker‐type catalyst for catalytic oxidative carbonylation

Ordered mesoporous carbons (OMCs) were used as supports to prepare Wacker‐type catalysts for diethyl carbonate (DEC) synthesis by oxidative carbonylation of ethanol in a gas‐phase reaction. The effect of support structure on the dispersion of the active species and catalytic properties were investigated. Nitrogen sorption, X‐ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed that the active components have encapsulated in pore channels of OMCs. Characterizations of the catalysts, such as TEM, scanning electron microscope (SEM) and XRD, indicated that active components supported on OMCs have better dispersion compared to activated carbon (AC). The ethanol conversion of the catalysts was improved by ～65% using OMCs as the catalyst support than AC. The stability of the catalytic activity can also be enhanced through surface modification of OMCs. Surface oxygen‐containing groups (OCGs) on OMCs before and after surface modification were characterized by transmission IR spectra and the Beohm titration. The relationship between surface OCGs and anchor ability of OMCs was studied. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3797–3805, 2013     

Carbon as catalyst and support for electrochemical energy conversion

Carbon has unique characteristics that make it an ideal material for use in a wide variety of electrochemical applications ranging from metal refining to electrocatalysis and fuel cells. In polymer electrolyte fuel cells (PEFCs), carbon is used as a gas diffusion layer, electrocatalyst support and oxygen reduction reaction (ORR) electrocatalyst. When used as electrocatalyst support, amorphous carbonaceous materials suffer from enhanced oxidation rates at high potentials over time. This drawback has prompted an extensive effort to improve the properties of amorphous carbon and to identify alternate carbon-based materials to replace carbon blacks. Alternate support materials are classified in carbon nanotubes and fibers, mesoporous carbon, multi-layer graphene (undoped and doped with metal nanoparticles) and reduced graphene oxide. A comparative review of all these supports is provided. Work on catalytically active carbon hybrids is focused on the development of non-precious metal electrocatalysts that will significantly reduce the cost without sacrificing catalytic activity. Of the newer electrocatalysts, nitrogen/metal-functionalized carbons and composites are emerging as possible contenders for commercial PEFCs. Nitrogen-doped carbon hybrids with transition metals and their polymer composites exhibit high ORR activity and selectivity and these catalytic properties are presented in detail in this review.     

Comparative study on the electrocatalytic activities of ordered mesoporous carbons and graphene

In this work, a comparative study on the electrocatalytic activities of ordered mesoporous carbons (OMCs) and graphene (GR) is presented. Using voltammetry and amperometry as detection methods, four DNA bases, double-stranded DNA (dsDNA), six important electroactive compounds and various biomolecules were employed to investigate their electrochemical responses on OMC and GR modified glassy carbon electrodes (OMC/GCE and GR/GCE). The results show that OMC/GCE enhances the electron transfer kinetics of these compounds compared to GR/GCE. The discrepancy in electrochemical activities can be attributed to the different microstructures of OMC and GR, which were examined by transmission electron microscopy, X-ray photoelectron spectra, X-ray diffraction, Raman spectra and nitrogen adsorption–desorption.     

Low-temperature synthesis of nitrogen/sulfur co-doped three-dimensional graphene frameworks as efficient metal-free electrocatalyst for oxygen reduction reaction

The development of metal-free catalyst for oxygen reduction reaction (ORR) is one of the most challenging tasks in fuel cells. Heteroatom doped graphenes have been recognized as the promising candidate. In this work, we have developed a one-pot hydrothermal approach towards three-dimensional nitrogen and sulfur co-doped graphene frameworks (N/S-GFs) employing graphene oxide and ammonium thiocyanate as the precursors. N/S-GFs manifest excellent catalytic behavior with mainly four electron transfer pathway in ORR in alkaline condition.     

Nitrogen doped carbon nanotubes and their impact on the oxygen reduction reaction in fuel cells

Nitrogen doped carbon nanotubes (N-CNTs) with different nitrogen contents were synthesized as non-precious catalysts for oxygen reduction reaction (ORR) through varying the relative amount of pyridine and ethanol in the stock solution. The structural and chemical properties of the N-CNTs were investigated using scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The characterizations indicated that an increase in the pyridine to ethanol ratio of the stock solution produced N-CNTs with higher nitrogen contents. The electrocatalytic activity of N-CNTs towards ORR in alkaline conditions was evaluated using rotating ring disc electrode voltammetry which showed a positive correlation between nitrogen content and ORR activities. By combining the results of ORR activity and material characterization, it is concluded that an increase in the nitrogen content of N-CNTs can effectively improve the ORR activities.     

Pore structure and graphitic surface nature of ordered mesoporous carbons probed by low-pressure nitrogen adsorption

Ordered mesoporous carbons (OMCs) were produced by pyrolysis of sucrose adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in hydrofluoric acid. Subsequently, some of these OMCs were heat-treated at temperatures of up to 1600 °C. The OMC pore structure was studied by low-pressure nitrogen adsorption. Information on the graphitic order of the surface of the mesopore walls was also obtained from the nitrogen adsorption data. These results were correlated to the order of the graphene layers at the outer surface, which was studied by X-ray photoelectron spectroscopy (XPS).

The OMCs were predominantly mesoporous, but they also contained micropores. For OMCs produced in an SBA-15 matrix, the micropore volume decreased upon heating. After heating to 1600 °C, nearly all micropores had disappeared. Furthermore, upon heating the width of the mesopores increased from 35 to 50 Å. All these changes can be explained by a shrinking of the OMC framework upon heating. A different behavior was found for OMCs derived from MCM-48. Upon heating these materials at increasingly high temperatures, the width of the mesopores first decreased, and for temperatures above 1100 °C it increased again. For all OMCs studied the graphitic order of the mesopores and the order of the graphene layers at the outer surface increased upon heating. For a given temperature, the graphitic surface order of OMCs derived from SBA-15 and MCM-48 was similar.     

Nitrogen‐Doped Carbon Materials Prepared by Ammoxidation as Solid Base Catalysts for Knoevenagel Condensation and Transesterification Reactions

Nitrogen‐doped carbon materials were prepared by ammoxidation of commercial carbon sources (carbon black and activated carbon) and applied as base catalysts for Knoevenagel and transesterification reactions. It was shown that these carbon materials were active and the activities were different depending on the ammoxidation conditions (temperature and ammonia concentration in air) and carbon sources used. The bulk, textural, and surface properties of the nitrogen‐doped carbon materials were examined by several methods to clarify possible factors determining their final catalytic activities. The activated carbon‐derived catalysts were more active than the carbon black‐derived ones. The surface area and porosity were not responsible for this difference between the two carbon sources but the difference in the reactivity with oxygen was important. The reactivity of carbon sources with oxygen should influence the doping of nitrogen onto their surfaces by ammoxidation with ammonia and air and the resulting activities as base catalysts. The catalytic activity increases with the amount of nitrogen doped and, therefore, the nitrogen doped should be responsible for the catalytic activities. In addition, the activities are maximal at a ratio of nitrogen to oxygen of around 1, suggesting the importance of cooperative functions of nitrogen and oxygen on the surface of carbons.     

Ordered mesoporous carbons with various pore sizes: Preparation and naphthalene adsorption performance

Ordered mesoporous carbons (OMCs) with tunable pores were synthesized by a soft-template method with F127 as a template and boric acid as a pore regulator agent. The prepared samples were characterized by small-angle X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and N₂ adsorption–desorption. The results show that the OMCs had well ordered, two-dimensional (2D) hexagonal structures and the pore sizes were finely tunable in the range 3.4–4.7 nm when boric acid was used as the pore regulator agent. The adsorption experiments showed that the OMCs had a strong adsorption affinity to naphthalene and the maximum adsorption amount was shown to reach up to 303.2 mg/g. Furthermore, the mesopore volume between 2 and 3.5 nm of OMCs was crucial to the adsorption capacity, and OMCs with pore sizes of 2–3.5 nm were much more favorable for the naphthalene adsorption process. The adsorption isotherms of naphthalene on OMCs matched well with the Langmuir adsorption isotherm. Theoretical studies showed that the adsorption kinetics of naphthalene on OMCs accounted well for the use of the Langmuir adsorption kinetics equation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Nitrogen/sulfur dual-doped mesoporous carbon with controllable morphology as a catalyst support for the methanol oxidation reaction

Ordered mesoporous carbon (OMC) was synthesized by nano-casting method using novel fluidic precursor – acrylonitrile telomer (ANT). By the penetration of mesoporous silica template with pure ANT, followed by the stabilization, carbonization and removal of the template, we obtained highly ordered mesoporous carbon rods (specific area 408 m² g⁻¹). When an acetone solution of ANT (66 and 33 wt.%) was used instead of pure ANT, carbon materials with mesopore ranging from 2 to 7 nm were obtained (specific area 843 and 1012 m² g⁻¹ respectively). Both nitrogen and sulfur atoms were doped into mesoporous carbon with 4 and 0.6 at.% using nitrogen containing monomer and sulfur containing chain transfer agent, without involving complicated synthetic technique and poisonous gaseous compounds. This method was proved to be a facile way to synthesize nitrogen and sulfur containing OMC with partially controllable pore distribution and morphology. More importantly, due to unique mesopore structure and heteroatom doping, Pt nano-particles deposited on the OMCs showed electrocatalytic activity as high as 508 mA mg⁻¹ Pt in methanol oxidation which is 1.7-fold of activity of Pt deposited on commercial Vulcan carbon black.     

有序介孔碳对低浓度气相萘的吸附特性分析

对3种常见有序介孔碳（OMCs）吸附脱除典型气相多环芳烃--萘进行了研究。分别采用吸附等温线模型（Langmuir、Freundlich、Sips）和恒定浓度波动力学模型对吸附等温线和穿透曲进行拟合分析。采用程序升温脱附法通过失重曲线分析了吸附剂的再生性能。结果表明：Langmuir模型和Sips模型能很好地描述低浓度气相萘在OMCs上的静态吸附行为（R² > 99%），吸附量呈CMK-5 > CMK-3 > FDU-15排列。恒定浓度波动力学模型具有较高的拟合度，介孔促使3种吸附剂对萘分子具有较高的吸附扩散系数。具有微通孔结构的CMK-5和FDU-15表现出更好的再生性能。综合吸脱附动力学分析，CMK-5在3种吸附剂中表现出更好的应用潜能。     

PEFC Cathode Alloy Catalysts Prepared by a Novel Process Using a Chelating Agent

A new Fe-Pt alloy catalyst for the oxygen reduction reaction of PEMFCs was prepared using a chelating agent, EDTA-Fe(III), as its non-precious metal source. Characterization showed that while the conventional catalyst showed particle aggregation over 5 nm, the new catalyst had a uniform particle size and a larger surface area. This new catalyst showed a higher catalytic mass activity than a conventional alloy catalyst although the specific activities are in the same range.     

PtCo supported on ordered mesoporous carbon as an electrode catalyst for methanol oxidation

A catalyst for methanol oxidation, PtCo supported on graphitized mesoporous carbon, has been synthesized and its electrochemical activity for methanol oxidation has been investigated. The graphitized mesoporous carbon support with ordered pore structure and high surface area of 585 m² g⁻¹ was prepared by one-step melt casting method using Al doped hexagonal mesoporous silica as hard templates and mineral pitches as carbon precursors followed by carbonization at 800 °C. The materials were characterized by X-ray diffraction, Raman spectra, field emission scanning electron microscopy, transmission electron microscopy and nitrogen sorption techniques. Cyclic voltammetry and amperometric i-t tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that the graphitized mesoporous carbon exhibits large electrochemical capacitance and good electric property. After supported with 20 wt%Pt or 20 wt%PtCo nanoparticles, the resultant mesostructured composites show 26-97% higher electrochemical catalytic activity for methanol oxidation than commercial catalyst 20 wt%Pt/C in mass activity (mA mg Pt⁻¹).