Platinum supported on resorcinol–formaldehyde based carbon aerogels for PEMFC electrodes: Influence of the carbon support on electrocatalytic properties

Two carbon aerogels with different nanopore size distributions but both with high surface area, high nanoporous volume and low bulk density have been compared as platinum support. The influence of the nanostructure of the carbon aerogel on the platinum nanoparticle deposit was investigated. The platinum was deposited on the carbon by means of two different techniques, one employing an anionic platinum precursor, the other using a cationic one. The porosity of the carbon aerogels was characterized by combining N₂-sorption and mercury porosimetry. The platinum deposit was characterized by transmission electron microscopy and rotating disk electrode experimentation to measure the platinum active surface area and its activity towards oxygen reduction reaction (ORR). The structural differences between the carbon aerogels did not yield any difference in platinum deposits in terms of Pt-surface area and ORR activity. Interestingly, the ORR mass activity of the high Pt-surface area samples, obtained by the cationic insertion technique, was several times lower than that of the samples obtained by the anionic technique. This observation was attributed to the particle size effect, detrimental in the case of platinum particle size around 1 nm.     

Poisoning and inhibition of catalytic oxidations: I. The effect of silicone vapour on the gas-phase oxidations of methane,propene, carbon monoxide and hydrogen over platinum and palladium catalysts

The influence of hexamethyldisiloxane (HMDS) on the catalytic oxidations of methane, propene, carbon monoxide and hydrogen has been studied. The oxidations of methane and carbon monoxide were studied over platinum wire and platinum and palladium supported on alumina, silica and 13X zeolite. The rates of oxidation both in the absence and presence of HMDS were found to be influenced by the nature of the catalyst support. For carbon monoxide oxidation, the presence of HMDS revealed that three types of oxidation were occurring. These results, taken with those obtained for the oxidation of hydrogen and propene, show that in general the relative importance of the three types of oxidation depends on the nature of the reaction and the catalyst support.     

Fabrication and Electrocatalytic Application of Nanoporous Carbon Material with Different Pore Diameters

Here we report on the preparation of nanoporous carbons with different pore diameters loaded with different amounts of platinum and their application as anodic electrodes for the polymer electrolyte membrane fuel cells (PEMFCs). The materials were characterized by nitrogen adsorption, XRD and HRTEM. The role of the amount of the platinum and the pore diameter of the nanoporous carbon supports on the anodic performance in the PEMFC has been investigated. It has been found that the anodic activity of the platinum supported nanoporous carbon materials significantly increases with increasing the amount of platinum on the surface of the supports. Interestingly, nanoporous carbon material with a larger pore diameter shows excellent performance as an anodic electrode support. The electrode activity of the platinum loaded nanoporous carbon was also compared with that of the commercially available carbon black support for the PEMFCs. Nanoporous carbon supports are found to be the best, showing much higher performance as compared to that of carbon black.     

Improved carbon nanostructures as a novel catalyst support in the cathode side of PEMFC: a critical review

Carbon supported platinum nanoparticles are extensively used as electrocatalysts in proton exchange membrane fuel cells (PEMFCs). The stability and performance of the electrocatalyst strongly depends on the deposition method and properties of the carbon support. Carbon black is commonly used as support for platinum (Pt–C) due to its low cost and high availability, good electrical performance, and relatively high surface area. However, carbon corrosion, Ostwald ripening, and Pt dissolution have been recognized as the main cause for low durability of this electrocatalyst under high potential conditions. As a result, the necessity for promising supports with higher stability is inevitable. It has been reported that carbon nanomaterials with higher mesoporosity, surface area, electrical conductivity, stability and suitable anchoring site are promising supports under harsh oxidizing condition of PEMFCs. This review is devoted to the development of recent advances in novel carbon nanomaterials as catalyst support, such as carbon nanotubes, carbon nanofibers, graphene, mesoporous carbon, and etc., for the oxygen reduction reaction (ORR) in PEMFCs. Moreover, the main challenges such as low activity, poor durability, and high cost, are addressed and discussed. The performance and obstacles of these carbons associated with different metal catalyst deposition methods are highlighted.     

The electrocatalytic oxidation of methanol in acid electrolyte: preparation and characterization of noble metal electrocatalysts supported on pre-treated carbon-fibre papers

The electrocatalytic aspects of direct oxidation of methanol in acid electrolyte in relation to fuel cell applications is briefly reviewed. The reaction requires noble metal catalysts (in particular platinum) in large quantities (several mg cm^–2 of electrode area) to produce steady state current densities of commercial interest. Initial activities are about 10⁴ times greater than steady state values, thus the problem in methanol electro-oxidation is to stabilize the initial activity.The approaches generally taken are to develop methods of dispersing platinum more effectively, e.g. by depositing the metal on a conducting support, or to modify the properties of platinum by alloying it with a second component. The latter approach can have the effect of simply isolating platinum atoms from each other, thus limiting the size of the platinum clusters, or of electronically modifying the properties of platinum by virtue of the ligand effect of the second component in the alloy.We have developed platinum and platinum-ruthenium catalysts supported on specially treated carbon-fibre paper which have substantially higher steady state activities than conventional catalysts. The method of preparation involves an ion-exchange procedure whereby noble metal cations become chemically bound to acidic surface oxide groups on the pre-oxidized carbon-paper surface. This produces platinum with increased surface area and high specific activity. It is proposed that the highly active form of platinum is involved in an interaction with the carbon support leading to improved performance. This modified form of platinum has been shown by cyclic voltammetry to have different characteristics of hydrogen adsorption/desorption and platinum oxide reduction from those of unsupported and conventionally supported platinum.This paper was first presented at the meetingElectrochemistry: Material Recovery and the Environment held at the Thornton Centre, Chester in January 1979 and organized by the Electrochemistry Group of the Chemical Society.     

Surface-modified carbons as platinum catalyst support for PEM fuel cells

Carbon forms, such as activated carbon, carbon black, carbon nanofibers and nanotubes, can be used as support materials for precious metal catalysts used in fuel cell electrodes. This work first compares the ability of functionalized high surface area graphitic (carbon nanofibers) and amorphous (activated carbon) carbons to homogeneously support finely divided platinum catalyst particles, then contrasts the performance of platinum/carbon composite electrodes within a hydrogen fuel cell. Functionalization by concentrated acid treatment results in the creation of various oxygen carrying functionalities on the otherwise inert carbon surfaces. The degree of surface functionalization is found to be a function of the functionalization treatment strength. Chemical reduction of the platinum precursor complex using milder reducing agents in the temperature range of 75-85 °C, and using ethylene glycol at 140 °C yields the smallest platinum particle sizes observed in this study, a result confirmed by X-ray diffraction and transmission electron microscopy measurements. X-ray photoelectron spectroscopy measurements confirm the existence of platinum in primarily its metallic state on the functionalized carbon surfaces.     

Preparation of a Gas Diffusion Electrode by Ozone Gas Pretreatment and an Ion-exchange Method

Surface oxidation using ozone gas, produced by an electrolytic ozone generator, was applied for preparation of a gas-diffusion electrode (GDE) for an electrochemical energy conversion system. An uncatalyzed carbon sheet containing poly(tetrafluoroethylene) binder was first placed into contact with ozone gas to form active functional groups on the surface of the carbon; then ion-exchange between a weakly bound hydrogen of the functional groups and a platinum cation complex was performed. A GDE having highly dispersed particles of a platinum metal deposited on a porous carbon sheet ws developed by this method. The fuel cell using this GDE showed high performance.     

Tailoring and structure of PtRu nanoparticles supported on functionalized carbon for DMFC applications: New evidence of the hydrous ruthenium oxide phase

The influence of the structure and morphology of PtRu nanoparticles supported on functionalized carbon black has been investigated for CO and methanol electrooxidation in a half-cell and in a DMFC single cell. Carbon black was treated with HNO₃ to obtain an oxidized surface (Vulcan-N), and PtRu nanoparticles supported on Vulcan-N were prepared via impregnation, Bönnemann's method and the sulfito-complex route. Temperature programmed reduction (TPR) measurements evidence the presence of RuO₂·xH₂O phase in the catalyst obtained by the sulfito-complex route. This phase was stabilized by metal–support interaction, whereas alloy characteristics were estimated for PtRu catalyst obtained by impregnation and Bönnemann's method. The nature of the precursor–support interaction, induced by the nature of the functional groups on the carbon surface, affects the structure of the electrocatalyst and subsequent behavior in electroactivity. When synthesized through Bönnemann's method, the surface oxygen-containing groups of the support seem to be unable to stabilize the anhydrous precursors of platinum and ruthenium, yielding crystalline RuO₂. Methanol electrooxidation performance was clearly different in the three catalysts, whereas only a few negligible differences were observed in CO oxidation. The superior performance in DMFC of the catalysts obtained by the sulfito-complex route accounts for both the presence of RuO₂·xH₂O species and the functionalization of carbon black.     

Carbon nanofibers as potential materials for catalysts support,a mini-review on recent advances and future perspective

The element carbon has been used as an active catalyst as well as a catalyst support. This dual nature of carbon has been attributed to its characteristics such as high porosity, large surface area, excellent electron conductivity and chemical inert nature. Besides, the availability of different forms of carbon like graphene, activated carbon, carbon nanotubes and carbon nanofibers have provided carbon a versatile material to be used for different applications. Carbon has been widely used in different applications like electrical, bio-electrochemical, dry cells, electrodes and as a lubricant. However, in the last decades, the catalytic applications of carbon materials especially carbon nanotubes and carbon nanofibers have gained tremendous attention of the researchers worldwide. Carbon nanofibers, in particular due to thier excellent catalytic support profile like, high surface area, thermal stability and its 3D access to the reacting molecules, have been utilized for different chemical reactions. Metal supported on carbon nanofibers have been observed with better activities as compared to the traditional supported counterparts for the several reactions. This mini-review attempts to document the role of carbon nanofibers and their catalytic support profile for the some common chemical processes. The mini-review also suggests about the future innovations and research work for carbon nanofibers as potential future catalysts support.     

Elastomer–carbon black interaction: Influence of elastomer chemical structure and carbon black surface chemistry on bound rubber formation

The contribution of elastomer polarity and reactivity to bound rubber formation has been investigated. In this study, a number of elastomers of different chemical nature have been tested. The surface of the carbon black (N110) has also been modified by nitric acid oxidation in order to increase the concentration of surface functional groups. The experimental results have shown that the bound rubber formation is barely related to the polarity of the polymers. It is the reactive sites in both the elastomer and carbon black which are mainly responsible for bound rubber formation. It therefore appears that the elastomer/carbon black interaction leading to the formation of bound rubber is essentially a chemical process involving primary bond formation between elastomer and carbon black. The oxidized carbon black exhibits a higher surface activity which may be due to an increased concentration of oxygen-containing reactive surface sites, namely, phenolic hydroxyl, carboxyl, lactone, and quinone groups. © 1995 John Wiley & Sons, Inc.     

Further insights into the Ru nanoparticles-carbon interactions and their role in the catalytic properties

Temperature-programmed reduction (TPR) and CO adsorption microcalorimetry along with the catalytic behaviour in the n-butane/H₂ test reaction were performed in order to determine the specific interactions of Ru nanoparticles supported on different carbon materials. Aspects such as the porous structure and surface chemistry (presence and elimination of surface oxygen functional groups) of the carbon material, or the effect of the metal precursor (e.g. presence of residual chlorine) on the final metal dispersion and on the surface structure of the Ru nanoparticles have been studied.The results obtained confirm that surface oxidation of the support along with the nature of the Ru precursor affects the distribution of the metal precursor over the support (and, consequently, the final ruthenium dispersion) and also the surface site distribution. Besides, elimination of the surface oxygen functional groups of the carbon material, during the reduction treatments of the fresh catalyst samples, leads to surface reconstructions on the Ru nanoparticles that seem to expose different crystallographic planes. The presence of residual chlorine leads to electron deficient Ru sites, and this modifies the CO chemisorption heats and affects the catalytic properties in the n-butane/hydrogen test.     

Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part II: Determination of diffusion area for oxygen reduction reflects platinum accessibility

In this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon support.     

Modification of hydrophobic/hydrophilic properties of Vulcan XC72 carbon powder by grafting of trifluoromethylphenyl and phenylsulfonic acid groups

Carbon supports (glassy carbon and Vulcan XC72 powder) were modified by electrochemical and spontaneous grafting of phenylsulfonic acid (PSA) or trifluoromethylphenyl (TFMP) groups via diazonium ion reduction. The effectiveness of the grafting was confirmed electrochemically, by XPS measurements and elemental analyses. The hydrophobic or hydrophilic character of carbon surfaces was evidenced by measuring the contact angles of drops of different liquids (water, ethylene glycol and glycerol) in heptane. The surface energy was calculated and it was found, for example, that spontaneous grafting of a glassy carbon surface by PSA groups led to an increase by a factor 20 of the surface energy compared with an unmodified glassy carbon surface. The study of the grafting of such groups on XC72 carbon powder indicated that a very low grafting ratio (in wt%) led to a significant change in the macroscopic properties of the powder. Thermogravimetric analysis coupled with mass spectroscopy measurements (TGA-MS) showed that these grafted layers were thermally stable even in the presence of dispersed platinum nanoparticles. It was shown by cyclic voltammetry that the carbon substrate modification did not affect the electrochemical behavior of platinum catalyst, since the same active surface area was determined on Pt-XC72, Pt-PSA-XC72 and Pt-TFMP-XC72 catalysts.     

The influence of platinum crystallite size on the electrochemical reduction of oxygen in phosphoric acid

An experimental program was conducted to investigate the catalytic activity of platinum black and platinum supported on carbon for the electrochemical reduction of oxygen in 99 wt% phosphoric acid at 177°C as a function of platinum surface area. The activity of platinum was found to approximately double as the surface area of platinum was decreased from 80 to 10 m²/g. The Tafel slope was found to be approximately equal to 2.3 R/F on the higher surface area catalysts and greater than 2.3 RT/F on the lower surface area catalysts.     

Electrochemical characterization and reactivity of Pt nanoparticles supported on single-walled carbon nanotubes

Carbon nanotubes have been proposed as advanced metal catalyst support for electrocatalysis. In this paper, Pt nanoparticles supported on single-walled carbon nanotubes (SWCNTs)-Pt, were prepared using a solid-state reaction between the SWCNTs and two different Pt precursors, bis(dibenzylideneacetone)platinum [Pt(DBA)₂] or tri(dibenzylideneacetone)platinum [Pt(DBA)₃]. TEM images of the samples show Pt nanoparticles with a particle size around 2.5 nm with a high degree of dispersion on the SWCNTs. A detailed electrochemical characterization of the surface of the samples including irreversibly adsorbed adatoms of Bi and Ge as probe reactions has been carried out. It has been stated that SWCNTs-Pt samples subjected to the classical electrochemical activation induce a serious sintering of the Pt nanoparticles.     

Oxygen electroreduction in perfluorinated sulphonyl imides

The electroreduction of oxygen in perfluorinated sulphonyl imides has been studied with the emphasis on the identification of alternate acid electrolytes which could replace the presently used phosphoric acid as an electrolyte in H₂–O₂ fuel cells. The activity for oxygen reduction on smooth platinum and gas-fed, high surface area platinum-catalysed electrodes (10% platinum loading on XC-72 carbon support) has been examined. The polarization of the air cathode in the micro-fuel cell in bis(trifluoromethanesulphonyl)imide is 40 mV more positive compared to phosphoric acid at 100 mA cm^–2, presumably due to the increased solubility of oxygen and lower tendency of bis(trifluoromethanesulphonyl)imide to adsorb on the platinum catalyst. The related bis(fluorosulphonyl)imide is unstable in water and the hydrolysis products adsorb strongly on the catalytic sites, resulting in poor performance.     

Anchoring of Pt and PtRu to carbon nanofibers studied by density functional theory calculations

PtRu particles supported on carbon nanofibers have been reported to have higher activity as anode catalysts in proton exchange membrane fuel cells than conventional catalysts. In the present work, density functional theory calculations are used to investigate the metal–carbon interface for different crystal facets of mono-metallic Pt and the PtRu alloy. The carbon side is modeled by graphene sheets with either zigzag or armchair termination. The strongest metal–carbon interaction is predicted for a (1 1 1) facet attached to a zigzag edge. The anchoring of the PtRu metal is found to have pronounced effects on the surface composition of the alloy. Whereas the bare surface is rich in Pt, the interface with carbon favors the stoichiometric bulk composition. Core level binding energies of carbon, platinum and ruthenium are found to provide valuable signatures of the interface and give means to interpret future high resolution photoemission core level spectroscopy experiments.     

Thermal degradation of the support in carbon-supported platinum electrocatalysts for PEM fuel cells

The cathode catalyst layer in proton exchange membrane (PEM) fuel cells can contain nanometer-sized platinum particles dispersed on a high surface area carbon. In order to assess support stability, samples of carbon-supported catalysts were held at elevated temperatures under dry air conditions. The samples were weighed at regular intervals. These tests showed that the platinum particles were able to catalyze the combustion of the carbon support at moderate temperatures (125-195 °C). As the temperature increased, the rate of carbon combustion increased. The amount of carbon that was lost after extended oven exposure at a constant temperature was shown to depend on both the temperature and platinum loading. A simple first-order kinetic model was able to describe the results. With further work on a range of different carbon supports, this work is expected to help develop more stable catalyst supports for PEM fuel cells.     

Effect of Carbon‐Based Materials on the Early Hydration of Tricalcium Silicate

Two types of carbon‐based materials, i.e., mesoporous carbon and HNO₃‐oxidized carbon nanotubes, with nearly the same specific surface area and abundant in surface oxygen‐containing functional groups were selected in order to examine their effect on the hydration of tricalcium silicate (C₃S), the main portland cement component, in early stages. Different methods, including XPS and TG‐MS analyses, electrokinetic potential measurements, as well as determination of adsorption capacity for calcium ions from aqueous solutions, were used to investigate the physicochemical surface properties of the selected carbon‐based materials. It was found that the carbon‐based materials with high specific surface area and rich in oxygen‐containing functional groups on their surfaces have a catalytic effect on early C₃S hydration. It was observed that the modification of C₃S paste with the selected materials added in high concentrations (1 wt% and higher) led to an increase in the rate and degree of C₃S hydration in the early stages. The mechanism of early C₃S hydration accelerated by carbon‐based materials rich in surface functional groups was clarified by the example of the mesoporous carbon. It was found that the oxygen‐containing functional groups present on the carbon surface have both an influence on the content of calcium ions in the aqueous phase of the C₃S paste and an indirect positive effect in relation to the specific surface of C₃S.     

Preparation of hollow platinum nanospheres/carbon nanotubes nanohybrids and their improved stability for electro-oxidation of methanol

We report a facile method for the synthesis of hollow platinum nanospheres/carbon nanotubes nanohybrids (CNTs-G-PtHNs). Silver nanoparticles were used as sacrificial templates and uniformly deposited on the functionalized carbon nanotubes (CNTs). By galvanic replacement reaction between CNTs-supported silver and PtCl₆²⁻, well-dispersed hollow platinum nanospheres (PtHNs) were “grown” on CNTs. The morphology and electrochemical properties of the CNTs-G-PtHNs nanohybrids have been investigated by transmission electron microscopy and cyclic voltammetry, respectively. PtHNs in the CNTs-G-PtHNs nanohybrids have an average diameter of about 8 nm and the CNTs-G-PtHNs nanohybrids have higher electrochemical surface area and better electrocatalytic performance towards methanol oxidation than CNTs-A-PtHNs nanohybrids which were obtained by adsorbing the pre-synthesized PtHNs onto CNTs. Most importantly, the long-term stability of CNTs-G-PtHNs nanohybrids for methanol electro-oxidation has obviously improved compared with that of the CNTs-A-PtHNs nanohybrids.