Effect of Co in the efficiency of the methanol electrooxidation reaction on carbon supported Pt

The effect of Co addition to carbon nanotubes supported Pt in the methanol oxidation reaction has been investigated by means of differential electrochemical mass spectrometry (DEMS). It has been observed that the CO₂ efficiency increases in carbon nanotubes supported PtCo compared to its homologous Pt catalysts, especially at potentials lower than 0.55 V. Despite of this, the Faradaic current reached by the bimetallic catalysts in the methanol electrooxidation was lower than those recorded on the monometallic samples. This is because Co addition difficult finding enough Pt vicinal sites for methanol dehydrogenation. On the other hand, it has been found that alloying Pt with Co, shifts down the d-band center of the larger element, so the strength of the interaction with adsorbates decreases. Consequently, it will be easier to oxidize CO_ad on the bimetallic surface. Furthermore, the necessary -OH_ad species for the CO_ad oxidation to CO₂ will be provided by the CNTs themselves.     

Electrooxidation of CO and methanol on well-characterized carbon supported PtxSn electrodes. Effect of crystal structure

The role of two intermetallic phases of PtSn, namely Pt₃Sn (fcc phase) and PtSn (hcp phase) for the electrooxidation of CO and methanol has been evaluated. Carbon supported Pt₃Sn and PtSn nanosized particles have been prepared by controlled surface reactions. The actual structure of the PtSn alloys has been evaluated and confirmed by means of XRD and HR-TEM studies which reveal the predominance of either the hcp or the fcc phase in each catalyst. The catalysts have been further characterized to identify the actual metal loading and Pt/Sn atomic ratio in order to eliminate particle size or metal loading effects on their electrocatalytic performance. The performance of the catalysts for the electrooxidation of CO and methanol has been evaluated by electrochemical techniques along with in situ techniques such as electrochemical coupled Infrared Reflection Absorption Spectroscopy (EC-IRAS) and differential electrochemical mass spectrometry (DEMS). Altogether, the results presented in this work reveal that Pt₃Sn fcc is more active than PtSn hcp for the electrooxidation of CO and methanol and that the contribution of the hcp phase in those electrocatalytic processes is negligible.     

Gas diffusion electrodes for the oxidation of sulphur dioxide

The catalytic activity of modified active carbon MC and organo-metallic compounds (CoTMPP and CoTAA) deposited and pyrolized upon active carbon P-33 displayed toward the electrochemical oxidation of sulphur dioxide in H₂SO₄ media is investigated. The structure of the gas diffusion electrodes (GDE) prepared with these catalysts is optimized. The relationship between electrode performance and the method by which the binding material (PTFE) is introduced is studied. Electrodes which are catalyzed with modified active carbon show the best characteristics, reaching current densities 200 mAcm^?2 at 600 mV (HE). Similar electrodes can operate 500 h continuously at a current density of 60 mAcm^?2 without a noticeable increase of polarization.     

Through-plane gas permeability of gas diffusion layers and microporous layer: Effects of carbon loading and sintering

Knowledge of the absolute permeability for the various porous layers is necessary to obtain accurate profiles for water saturation within the membrane electrode assembly (MEA) in a two-phase model of a polymer electrolyte membrane fuel cell (PEMFC). In this paper, the gas permeability of gas diffusion layers (GDLs) coated with microporous layers (MPLs) of various carbon loadings for two different carbon blacks have been experimentally measured. The permeability of the GDL was found to decrease by at least one order of magnitude after the MPL-coating. Also, the permeability of the MPLs was shown to be lower than that of the carbon substrate by 2–3 orders of magnitude. Further, it was found that the gas permeability of the MPLs changes significantly from one carbon loading to another despite the use of a single weight composition for all the MPLs coated, namely 20% PTFE and 80% carbon black. This signifies the possible inaccuracy in estimating the MPL permeability through employing the cross-section SEM images as they do not resolve the MPL penetration into the carbon substrate. Finally, the MPL sintering was found to slightly decrease the permeability of the GDL.     

Influence of anode diffusion layer on the performance of a liquid feed direct methanol fuel cell by AC impedance spectroscopy

Effect of anode diffusion layer over the performance of the liquid feed direct methanol fuel cell has been studied by AC impedance spectroscopy. The anode employed comprises of the catalyst layer and diffusion layer. The latter comprises of backing layer and catalyst‐supporting layer. The supporting layer is present in between the backing layer and the catalyst layer. The composition of the supporting layer is optimized based on the information obtained from polarization and AC impedance measurements. Among the three types of carbons (Black pearl 2000, Vulcan XC‐72, Shawinigan acetylene black), Black pearls 2000 is found to be the ideal type of carbon used in the supporting layer. The optimized loading compositions of carbon, Nafion and PTFE in the supporting layer are reported to be 3 mg cm^?2, 10 wt%, and 0 wt%, respectively. These values are rationalized on the basis of the transport of methanol and carbon dioxide and the crossover of methanol from the anode to the cathode. Copyright © 2006 John Wiley & Sons, Ltd.     

Application of hyperdispersant to the cathode diffusion layer for direct methanol fuel cell

In this study, Nafion ionomer, as a kind of hyperdispersant, was added to polytetrafluoroethylene (PTFE) water dispersion system to suppress the size of PTFE particles in the ink of microporous layer (MPL). The agglomeration behavior of PTFE in ethanol and MPL were investigated by laser diffraction, dynamic light scattering (DLS) and metallurgical microscopes. The electronic resistance, pore size distribution, gas permeability and surface hydrophobic/hydrophilic properties were also characterized for prepared gas diffusion layers (GDLs). It was shown that PTFE water dispersion system suffered flocculating when dispersed in ethanol and this agglomeration behavior was reduced by employing Nafion ionomer. With the increase in the Nafion ionomer adopted in the MPL, not only the decreased hydrophobic property was shown in the MPL, but the decreased PTFE particle size was also achieved, which results in improved crosslink of carbon and pores themselves as well as the volume loss of pores in micron scale. The increased gas permeability and electronic conductivity of the GDL made the one employing the PTFE dispersion system with 1% Nafion content own its advantages as the cathode diffusion layer for a direct methanol fuel cell (DMFC) under near-ambient conditions.     

Bucky diamond produced by annealing nanodiamond as a support of Pt electrocatalyst for methanol electrooxidation

Bucky diamond (BD) with a nanoscale diamond core surrounded by a fullerene shell was used as a support of Pt electrocatalyst for methanol electrooxidation. BDs were prepared by annealing detonation-synthesized nanodiamond (ND) powders in 10⁻³ Pa vacuum at 900–1100 °C. The electrochemical properties of the BD powders in aqueous solution were investigated. The BDs and NDs supported platinum (Pt) electrocatalysts were prepared using a microwave-assisted reduction method. Higher dispersion of Pt nanoparticles was observed on the BDs than the pristine NDs, indicating a high affinity between BDs and Pt metal. The Pt/BD catalyst had better catalytic activity and higher stability for the methanol electrooxidation in comparison to the Pt/ND prepared at the same conditions. This provides a novel nanoparticle with a high conductivity and a high stability for electrochemical applications.     

Probing the enhanced methanol electrooxidation mechanism by promoted CO tolerance on the Pd catalyst modified with TaN: A combined experimental and theoretical study

The low-palladium Pd/TaN–C catalyst is synthesized by a surfactant-free solvothermal approach and exhibits high activity (2613.18 mA mg_Pd⁻¹), durability and CO tolerance for MOR (methanol oxidation reaction) in alkaline media, 12.4 folds that of the commercial Pd/C. XPS and electrochemical results indicate that the interfacial Pd–TaNO bond is generated. This also brings the enhancement of OH_ad adsorption responsible for anti-CO poisoning ability. Density Functional Theory (DFT) calculations indicate that the reaction pathway and the rate-determining step are changed for methanol decomposition to CO on the Pd₄/TaN(001) surface compared with Pd (111). The preferred pathway can be described as: CH₃OH→CH₃O→CH₂O→CHO→CO. Furthermore, the results indicate that the adsorption of OH is enhanced and the energy barrier of COOH formation from CO + OH is reduced with the high concentration of hydroxyl on the Pd₄/TaN(001) surface, further confirming the bi-functional effect of hydroxyl on the CO tolerance.     

MWCNT-supported PtRu catalysts for the electrooxidation of methanol: Effect of the functionalized support

Functionalized carbonaceous materials have been investigated as supports of PtRu nanoparticles for the electrooxidation of methanol, using such conventional electrochemical methods as cyclic voltammetry and chronoamperommetry and by measurements in a CH₃OH/O₂-fed single cell. Further, to understand the effect of oxygen-containing groups on the supports in the methanol oxidation reaction (MOR), a kinetic study of the catalyst that has the best behavior in this process has been performed. The study at different temperatures of PtRu nanoparticles supported in multiwall carbon nanotubes (MWCNTs) with a high amounts of functional groups—PtRuCNT-ST—show that there was low CO poisoning during the MOR on this catalyst. The low apparent energy on PtRuCNT-ST in the MOR was attributed to CO diffusion or to the dissociative adsorption of methanol. Both factors had a beneficial effect on the oxygen-containing groups on MWCNTs, facilitating oxidation of the carbonaceous intermediates to CO₂ or HCOOH. These findings have been confirmed by studies in a single cell feeding with CH₃OH/O₂, demonstrating that PtRuCNT-ST is the best-performing anodic electrode.     

Gas diffusion electrodes for the oxidation of sulfur dioxide with reduced gas permeability

The effect of the thickness and structure of the active layer of gas diffusion electrodes for the oxidation of sulfur dioxide on the gas permeability and on their current-voltage characteristics were investigated. As a result of this operation electrodes ensuring 75% utilization of sulfur dioxide at i = 50 mA cm^?2 and ? = +600 mV (HE) have been developed.     

Maximized specific activity for the methanol electrooxidation by the optimized PtRu-TiO2-carbon nano-composite structure

The kinetics of the methanol electrooxidation needs to be improved to increase the power density using a lower amount of noble metal catalysts (i.e., Pt and Ru) in direct methanol fuel cells (DMFC). PtRu nanoparticles (∼5 nm) supported on TiO₂-nanoparticle (∼4 nm)-coated carbon nanofibers were proposed as an alternative active catalyst for the DMFC. The nano-sized TiO₂ can provide a short distance of electron transport from PtRu (reaction site) to the carbon (current collector). At the composite catalyst, the activity enhancement by the PtRu-TiO₂ interaction suggested the sufficient electron conductivity at the electrode. The maximized specific activity of the proposed catalyst was 3 times higher compared to that of the commercial PtRu/C. The pore structure of the catalyst was changed by the oxidation conditions due to gasification of the carbon, and the higher activity was obtained by the catalyst with the higher surface area of the micropores (>800 m² g⁻¹). However, the contribution of micropore would be a secondary effect to the activity. The maximized specific activity was obtained when the volumes of PtRu and TiO₂ were similar for the almost same size (around 5 nm) of these particles suggesting that the number of contact points between the PtRu and TiO₂ were optimized and the interaction between them was maximized. The PtRu-TiO₂-carbon nano-composite catalyst has a high potential as an alternative catalyst as the anode of DMFC.     

Performance of Pt/C catalysts prepared by microwave-assisted polyol process for methanol electrooxidation

Pt nanoparticles catalysts supported on the Vulcan XC-72 carbon black with different mean sizes have been synthesized by microwave-assisted polyol process and characterized by energy dispersive analysis of X-ray (EDAX), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results of physical examinations show that Pt nanoparticles have a narrow size distribution and are highly dispersed on the surface of carbon support, and Pt loading in Pt/C catalyst is the similar with the theoretical value. The results of cyclic voltammetry and chronoamperometry demonstrate that the Pt/C catalyst prepared by microwave-assisted polyol process at the pH value of about 12 exhibits the highest catalytic activity for methanol electrooxidation. The activity of Pt/C catalyst is also related to the microwave heating time, and the optimal heating time is 40 s in this work.     

PtRuMo/C catalysts for direct methanol fuel cells: Effect of the pretreatment on the structural characteristics and methanol electrooxidation

The influence of thermal treatment under different environments of PtRuMo/C catalyst has been investigated for CO and methanol electrooxidation in a half cell and in a DMFC single cell. The PtRuMo/C catalysts were synthesized following two step procedure while the thermal treatments consisted of heating at 300 °C in H₂ or He atmosphere for 1 h. Structural characteristics of the electrocatalysts have been studied employing a wide range of instrumental methods, including physicochemical techniques like X-ray diffraction, TEM, TPR, XPS, and electrochemical techniques like single cell studies and Fourier Transform Infrared Spectroscopy adapted to the electrochemical system for in situ studies. These electrocatalysts exhibited good dispersion and small particle size, which increased upon increasing thermal treatment. Moreover, thermal treatment, mainly under H₂ is responsible for the decrease of the lattice parameter and the increase of the spill over effect to Mo sites. These effects were also accompanied by increasing the proportion of the more reduced Ru species in this catalyst. The electrochemical characterization revealed that although all ternary catalysts were more active towards CO and methanol oxidation than the binary catalyst, the catalyst treated with H₂ improves its performance by ca. 15% higher with respect to the ternary catalysts treated either in He treatment or with no treatment. The enhancement in activity is associated with a change in the reaction path, which promotes the direct oxidation of CHO species to CO₂ without the production of the CO poisoning species. The synergistic effect of the three metals seems to be improved and the Mo–Pt and Mo–Ru interaction strengthened.     

Efficient hydrogen production from aqueous methanol in a PEM electrolyzer with porous metal flow field: Influence of PTFE treatment of the anode gas diffusion layer

In a proton exchange membrane (PEM) methanol electrolyzer, the even supply of reactant to and the smooth removal of carbon dioxide from the anode are very important in order to achieve a high hydrogen production performance. An appropriate design of flow field and gas diffusion layer (GDL) is a key factor in satisfying the above requirements. Previous research has shown that hydrogen production performance of the PEM methanol electrolyzer cell was largely improved with a porous flow field made of sintered spherical metal powder compared with a conventional groove type flow field. Based on this improvement, the current study investigated the influence of polytetrafluoroethylene (PTFE) treatment of the anode GDL on hydrogen production performance of the PEM methanol electrolyzer with porous metal flow fields. Influences of operating conditions such as methanol concentration and cell temperature with the flow field were also investigated.     

Design and preparation of highly active carbon nanotube-supported sulfated TiO2 and platinum catalysts for methanol electrooxidation

A novel electrocatalyst structure of carbon nanotube-supported sulfated TiO₂ and Pt (Pt-S-TiO₂/CNT) is reported. The Pt-S-TiO₂/CNT catalysts are prepared by a combination of improved sol-gel and ethylene glycol reduction methods. Transmission electron microscopy and X-ray diffraction show that the sulfated TiO₂ is amorphous and is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.6 nm in size are homogenously dispersed on the sulfated TiO₂ surface. Fourier transform infrared spectroscopy analysis proves that the CNT surfaces are modified with sulfated TiO₂ and a high concentration of SO_x, and adsorbed OH species exist on the surface of the sulfated TiO₂. Electrochemical studies are carried out using chronoamperometry, cyclic voltammetry, CO stripping voltammetry and impedance spectroscopy. The results indicate that Pt-S-TiO₂/CNT catalysts have much higher catalytic activity and CO tolerance for methanol electrooxidation than Pt/TiO₂/CNTs, Pt/CNTs and commercial Pt/C.     

Development of a simple and efficient method to prepare a platinum-loaded carbon electrode for methanol electrooxidation

In this study, a simple and efficient electrochemical method was developed to prepare platinum (Pt) nanoparticles modified pencil graphite electrode (PGE) to use in direct methanol fuel cells. This method is based on two successive steps including the electrochemical pre-treatment of PGE via potential sweeping in the range of −1.0 V and +2.0 V and subsequent electrochemical plating of Pt nanoparticles in a wide potential range (+2.0 V: 1.0 V). Both the electrochemical pre-treatment of PGE and to use a wide potential range in the electrochemical Pt plating tremendously increased the effective surface area, the intensity of surface functional groups (hydroxyl, carbonyl, carboxyl, etc.), the electron transfer rate and the amount of Pt loading on the electrode surface, which greatly improved methanol electrooxidation activity of the electrode. The increase in the activity of the electrode reached to 16.3 times according to the classical Pt electroplating process. The electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy measurements. The stability of the electrodes was tested via cyclic voltammetry and chronoamperometry measurements. The electrochemical surface area of the electrode prepared here was calculated as 39.46 m^2 g-1, which was 39% higher than that of commercial Pt/C catalyst (28.4 m^2 g-1). The results showed that the proposed modification process can be seen as a simple and efficient alternative for the preparation of Pt-loaded carbon electrodes which can be used in direct methanol fuel cells.     

Experimental studies of the effect of cathode diffusion layer properties on a passive direct methanol fuel cell power output

A great challenge in a passive direct methanol fuel cell (pDMFC) is how to reduce both methanol and water crossover, from the anode to the cathode side, without significant losses on its power output. Different approaches including improving the membrane and modifying the cell structure and materials have been proposed in the last years.In this work, an experimental study was carried out to evaluate the effect of the cathode diffusion layer (CDL) properties on the power output of a pDMFC. Towards a cost reduction, lower catalyst loadings were used on both anode and cathode electrodes. Since the main goal was the optimization of a pDMFC using the materials commercially available, different carbon-fibber materials were employed as CDL. The experimental results were analysed based on the polarization curves and electrochemical impedance spectroscopy measurements with innovative electric equivalent circuit allowing the identification of the different losses, including the activation resistance of the parasitic cathode methanol oxidation.A maximum power density of 3.0 mW/cm² was obtained using carbon cloth with a lower thickness as CDL and a methanol concentration of 5 M.     

Influence of PEMFC gas flow configuration on performance and water distribution studied by SANS: Evidence of the effect of gravity

A non intrusive method based on small angle neutron scattering (SANS) has been developed to determine the water concentration profile through the thickness of Nafion^® 117 membrane during fuel cell operation. This technique was used to study the effect of gas flow configuration, co- or counter-flow, on water repartition within the fuel cell both within and outside the membrane. As it has been reported previously in the literature the counter-flow configuration gives better performance than co-flow but more surprisingly we evidence a significant difference in performance between symmetric configurations either in co- or counter-flow. Indeed, for a given current density, cell voltage is higher when the cathode inlet is at the bottom of the cell. We demonstrate that the gravity retains liquid water within the cell which leads to a better membrane hydration. Moreover, we have been able to correlate the average water content within the membrane with the performance and especially with the voltage drop resulting from the membrane resistance.     

Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs

According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm⁻² and maintains a high power density of 30 mW cm⁻² (0.405 V) under constant current operation.