Carbon-supported PtRu Catalysts Research Articles

Vapor-fed Methanol Oxidation Reaction Kinetics on High Surface Area Carbon Supported PtRu Catalysts in a Direct Methanol Fuel Cell

Direct methanol fuel cells (DMFC) have several advantages for a broad spectrum of energy density demanding applications, ranging from portable devices to stationary backup power. DMFCs deliver power at high efficiencies compared to internal combustion technologies and uses high energy density methanol as the fuel. However, DMFCs face several challenges, including methanol crossover at the cathode and slow methanol oxidation reaction (MOR) kinetics at the anode that impede its DMFC performance relative to hydrogen polymer electrolyte fuel cells (PEFCs). Since the the anode is a much bigger source of overpotentials than the H2 PEFC, we must carefully consider it’s characteristics in designing the membrane electrode assembly (MEA) and it requires extensive experimental and modeling studies to investigate these hindrances and mitigate their impacts on performance. It is widely known that vapor-fed DMFCs suffer less from methanol crossover. However, a complete understanding of the vapor-fed MOR kinetics in porous electrodes is lacking. When modeling DMFCs it is a common practice to assume that the reaction order of the MOR is zero. However previous studies have shown that the liquid-fed MOR on PtRu/C catalysts has areaction order of 0.5 [1, 2]. However, we found that the reaction order of the vapor-fed MOR can differ from that value. We also investigated the effect of different catalyst loading and the effect of methanol crossover on reaction order.This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008440. References S.Lj. Gojkovic´ et al., Electrochimica Acta 48 (2003) 3607/3614D. Chu, S. Gilman, J. Electrochem. Soc. 143 (1996) 1685.

Improved Catalysts for Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) are attractive among advanced energy conversion technologies due to the high energy density of methanol, the overall simplicity of system and easy refueling.[1] Although they made an early market entry but did not achieve large-scale commercialization mainly because of the cost, performance and durability issues. The state-of-art DMFC systems are based on the use of Pt-Ru catalyst at the anode, Pt catalyst at the cathode, and a polymer electrolyte membrane separating the two electrodes.[2] Our present study is focused on the anode catalyst, specifically to improve its performance and durability and reduce its cost. To this end, we have examined the use of titanium oxide-ruthenium oxide solid solutions as co-catalyst supports. We have prepared Pt/Ti0.7Ru0.3O2 for use as an anode catalyst. Ti0.7Ru0.3O2 as a co-catalyst support was synthesized in a one-step hydrothermal process and then platinum was deposited using polyol method. The structure of the synthesized catalyst was investigated by XRD, SEM and EDX. Results of steady-state voltammetry suggest that the electrocatalytic activity and stability of the newly synthesized catalysts compare well with the commercially available carbon-supported Pt-Ru catalyst. The Tafel slopes suggested that the well-known bifunctional mechanism for methanol oxidation was operative on the new catalyst as well. With further optimization of this type of catalyst we expect to be able to lower the noble metal content further to enable practical use of these materials in fuel cells.

An ethanol electrooxidation study on carbon-supported Pt-Ru nanoparticles for direct ethanol fuel cells

ABSTRACTCarbon supported Pt-Ru catalysts were prepared at varying Pt:Ru ratios by polyol method. The crystallite sizes of these catalysts were determined by X-ray diffraction technique. The specific surface areas of these catalysts were also defined by Brunauer–Emmett–Teller technique. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy measurements were conducted on these carbon supported Pt-Ru catalysts to investigate the effect of ruthenium on the ethanol electrooxidation kinetics. Results indicated that Pt-Ru (25:1) catalyst showed the best ethanol electrooxidation activity. In conclusion, ethanol electrooxidation mechanism was proposed.

Ordered mesoporous carbon supported bifunctional PtM (M = Ru, Fe, Mo) electrocatalysts for a fuel cell anode

The deposition onto an ordered mesoporous carbon (OMC) support of well dispersed PtM (M = Ru, Fe, Mo) alloy nanoparticles (NPs) were synthesized by a direct replication method using SBA-15 as the hard template, furfuryl alcohol and trimethylbeneze as the primary carbon sources, and metal acetylacetonate as the alloying metal precursor and secondary carbon source. The physicochemical properties of the PtM-OMC catalysts were characterized by N2 adsorption-desorption, X-ray diffraction, transmission electron microscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. The alloy PtM NPs have an average size of 2−3 nm and were well dispersed in the pore channels of the OMC support. The second metal (M) in the PtM NPs was mostly in the reduced state, and formed a typical core (Pt)-shell (M) structure. Cyclic voltammetry measurements showed that these PtM-OMC electrodes had excellent electrocatalytic activities and tolerance to CO poisoning during the methanol oxidation reaction, which surpassed those of typical activated carbon-supported PtRu catalysts. In particular, the PtFe-OMC catalyst, which exhibited the best performance, can be a practical anodic electrocatalyst in direct methanol fuel cells due to its superior stability, excellent CO tolerance, and low production cost.

Platinum–Ruthenium Nanotubes and Platinum–Ruthenium Coated Copper Nanowires As Efficient Catalysts for Electro-Oxidation of Methanol

The sluggish kinetics of methanol oxidation reaction (MOR) is a major barrier to the commercialization of direct methanol fuel cells (DMFCs). In this work, we report a facile synthesis of platinum–ruthenium nanotubes (PtRuNTs) and platinum–ruthenium-coated copper nanowires (PtRu/CuNWs) by galvanic displacement reaction using copper nanowires as a template. The PtRu compositional effect on MOR is investigated; the optimum Pt/Ru bulk atomic ratio is about 4 and surface atomic ratio about 1 for both PtRuNTs and PtRu/CuNWs. Enhanced specific MOR activities are observed on both PtRuNTs and PtRu/CuNWs compared with the benchmark commercial carbon-supported PtRu catalyst (PtRu/C, Hispec 12100). X-ray photoelectron spectroscopy (XPS) reveals a larger extent of electron transfer from Ru to Pt on PtRu/CuNWs, which may lead to a modification of the d-band center of Pt and consequently a weaker bonding of CO (the poisoning intermediate) on Pt and a higher MOR activity on PtRu/CuNWs.

Highly alloyed PtRu nanoparticles confined in porous carbon structure as a durable electrocatalyst for methanol oxidation.

The state-of-the-art carbon-supported PtRu catalysts are widely used as the anode catalysts in polymer electrolyte fuel cells (PEMFCs) but suffer from instability issues. Severe ruthenium dissolution occurring at potentials higher than 0.5 V vs NHE would result in a loss of catalytic activity of PtRu hence a worse performance of the fuel cell. In this work, we report an ultrastable PtRu electrocatalyst for methanol oxidation by confining highly alloyed PtRu nanoparticles in a hierarchical porous carbon structure. The structural characteristics, e.g., the surface composition and the morphology evolution, of the catalyst during the accelerated degradation test were characterized by the Cu-stripping voltammetry and the TEM/SEM observations. From the various characterization results, it is revealed that both the high alloying degree and the pore confinement of PtRu nanoalloys play significant roles in suppressing the degradation processes, including Ru dissolution and particle agglomeration/migration. This report provides an opportunity for efficient design and fabrication of highly stable bimetallic or trimetallic electrocatalysts in a large variety of applications.

Effect of nitrogen post-doping on a commercial platinum–ruthenium/carbon anode catalyst

This work investigates the effects of after-the-fact chemical modification of a state-of-the-art commercial carbon-supported PtRu catalyst for direct methanol fuel cells (DMFCs). A commercial PtRu/C (JM HiSPEC-10000) catalyst is post-doped with nitrogen by ion-implantation, where “post-doped” denotes nitrogen doping after metal is carbon-supported. Composition and performance of the PtRu/C catalyst post-modified with nitrogen at several dosages are evaluated using X-ray photoelectron spectroscopy (XPS), rotating disk electrode (RDE), and membrane electrode assemblies (MEAs) for DMFC. Overall, implantation at high dosage results in 16% higher electrochemical surface area and enhances performance, specifically in the mass transfer region. Rotating disk electrode (RDE) results show that after 5000 cycles of accelerated durability testing to high potential, the modified catalyst retains 34% more electrochemical surface area (ECSA) than the unmodified catalyst. The benefits of nitrogen post-doping are further substantiated by DMFC durability studies (carried out for 425 h), where the MEA with the modified catalyst exhibits higher surface area and performance stability in comparison to the MEA with unmodified catalyst. These results demonstrate that post-doping of nitrogen in a commercial PtRu/C catalyst is an effective approach, capable of improving the performance of available best-in-class commercial catalysts.

Enhanced Methanol Electro-Oxidation on Pt-Ru Decorated Self-Assembled TiO2-Carbon Hybrid Nanostructure

Porous titanium oxide-carbon hybrid nanostructure (TiO2-C) is directly synthesized via supramolecular self-assembly with in situ crystallization process. TiO2-C supported Pt-Ru electrocatalyst (Pt-Ru/TiO2-C) is synthesized and investigated as an alternative anode catalyst for direct methanol fuel cells (DMFCs). TiO2-C with a specific surface area of 350m2/g and an average pore radius of 21.8Aå has been synthesized. X-ray diffraction, Raman spectroscopy and Transmission electron microscopy (TEM) have been employed to evaluate the crystalline nature and the structural properties of TiO2-C. TEM images reveal uniform distribution of Pt-Ru nanoparticles (dPt-Ru =1.5-3.5 nm) on TiO2-C. The methanol oxidation and accelerated durability (ADT) studies for Pt-Ru/TiO2-C show enhanced catalytic activity and durability, respectively compared to that for carbon supported Pt-Ru. DMFC employing Pt-Ru/TiO2-C as an anode catalyst delivers a peak-power density of 91mW/cm2 at 65ºC as compared to the peak-power density of 60mW/cm2 obtained with carbon supported Pt-Ru catalyst under similar conditions.

New-Concept CO-Tolerant Anode Catalysts Using a Rh Porphyrin-Deposited PtRu/C

CO poisoning of the Pt-based anodes (PtRu) of stationary proton exchange membrane fuel cells remains a severe problem for their realization. We tried to counteract this problem using a CO oxidation electrocatalyst. Rh porphyrins, which can catalyze electrochemical CO oxidation in low potential regions, were adsorbed on a carbon-supported PtRu catalyst (PtRu/C) to increase its CO tolerance. The combined electrocatalysts (Rh(porphyrin)−PtRu/C) exhibited significantly higher activity toward CO(2%)-contaminated H2 than the PtRu/C alone; the onset potential for H2 oxidation reaches 0.2 V (vs a reversible hydrogen electrode) on the combined catalyst. In contrast, the composite of PtRu black and Rh(porphyrin) scarcely exhibited CO tolerance. This suggests that the enhancement of CO tolerance in Rh(porphyrin)−PtRu/C is mainly caused by Rh porphyrins on carbon. The significance of Rh porphyrins on carbon was shown by the findings that a mixed catalyst composed of carbon-supported Rh(porphyrin) (Rh(porphyrin)/C) and PtRu/C gave high oxidation activity toward CO(2%)/H2. Rh porphyrin on carbon would decrease the CO concentration around PtRu particles and thus reduce the CO poisoning of PtRu catalysts.

Impacts of Ru Dissolution and Crossover on Polymer Electrolyte Membrane Fuel Cell Performance and Anode Functionality

Carbon-supported PtRu catalysts are commonly used as anode catalysts in polymer electrolyte membrane fuel cells (PEMFCs) to provide CO tolerance for reformate fuel applications. However, Ru dissolution and subsequent crossover from the anode to the cathode have been identified as critical durability issues. In this investigation, half-cell electrochemical studies were carried out to characterize the changes and impacts imposed by Ru contamination on the cathode. In addition, an anode accelerated stress test (AST) was employed in a fuel cell to simulate potential spikes that occur during fuel cell start-ups and shutdowns to induce Ru dissolution and crossover. The electrochemical experiments showed that the oxygen reduction reaction (ORR) activity and the changes in cyclic voltammetry (CV) characteristics correlated well with the amount of Ru deposited on the cathode. From the single-cell AST results, it was evident that both the performance and CO tolerance were severely impacted by Ru dissolution and crossover.

Preparation of Bimodal Porous Carbon Supported PtRu Catalysts for Fuel Cells

AbstractIn this work, we synthesised the bimodal porous carbon supported electro‐catalyst as a novel catalyst for fuel cells. For this purpose, the bimodal porous carbons with different pore sizes were prepared by imprinting method. Three kinds of bimodal porous carbons were fabricated by using the silica spheres with diameters of ca. 50, 100 and 300 nm and SBA‐15 particle having 200–250 nm diameter and 700–900 nm length as templates. The BET surface areas of the bimoal porous carbons were determined to be 235–294 m2 g–1, which are similar to that of Vulcan XC72. To evaluate the electro‐catalytic activity, the PtRu nanoparticles of ca. 1.9–2.6 nm were loaded on the bimodal porous carbon supports. The PtRu/C100, synthesised by using SBA‐15 and 100 nm silica sphere, had the higher electrochemical surface area (ECSA) and the lower on‐set potential and showed comparable performance with the commercial catalyst.

A highly efficient synthesis approach of supported Pt-Ru catalyst for direct methanol fuel cell

Abstract A highly efficient synthesis approach of urea-assisted homogeneous deposition (HD) coupled with H 2 reduction has been employed to synthesize carbon-supported Pt-Ru catalyst with high metal loading of 60 wt%. The urea-assisted HD method permits in situ gradual and homogeneous generation of hydroxide ions, resulting in smaller Pt-Ru nanaoparticles deposited on the carbon support along with better particle size distribution compared with the catalyst prepared by the conventional impregnation–NaBH 4 reduction. The Pt-Ru catalyst prepared by the HD-H 2 method has demonstrated greatly enhanced catalytic activity towards methanol oxidation and much improved fuel cell performance compared with the one prepared by impregnation–NaBH 4 reduction. HD-H 2 method is simple with mild synthesis condition, but provides very efficient approach for the preparation of Pt-based catalysts for fuel cells.

Electrocatalytic properties of carbon-supported Pt-Ru catalysts with the high alloying degree for formic acid electrooxidation

A series of carbon-supported bimetallic Pt-Ru catalysts with high alloying degree and different Pt/Ru atomic ratio have been prepared by a chemical reduction method in the H 2O/ethanol/tetrahydrofuran (THF) mixture solvent. The structural and electronic properties of catalysts are characterized using X-ray reflection (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM). The electrooxidation of formic acid on these Pt-Ru nanoparticles are investigated by using cyclic voltammetry, chronoamperometry and CO-stripping measurements. The results of electrochemical measurements illustrate that the alloying degree and Pt/Ru atomic ratio of Pt-Ru catalyst play an important role in the electrocatalytic activity of the Pt-Ru/C catalyst for formic acid electrooxidation due to the bifunctional mechanism and the electronic effect. Since formic acid is an intermediate in the methanol electrooxidation on Pt electrode in acidic electrolyte, the observation provides an additional fundamental understanding of the structure–activity relationship of Pt-Ru catalyst for methanol electrooxidation.

High activity PtRu/C catalysts synthesized by a modified impregnation method for methanol electro-oxidation

A modified impregnation method was used to prepare highly dispersive carbon-supported PtRu catalyst (PtRu/C). Two modifications to the conventional impregnation method were performed: one was to precipitate the precursors ((NH 4) 2PtCl 6 and Ru(OH) 3) on the carbon support before metal reduction; the other was to add a buffer into the synthetic solution to stabilize the pH. The prepared catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, even higher than that of current commercially available, state-of-the-art catalysts. The morphology of the prepared catalyst was characterized using TEM and XRD measurements to determine particle sizes, alloying degree, and lattice parameters. Electrochemical methods were also used to ascertain the electrochemical active surface area and the specific activity of the catalyst. Based on XPS measurements, the high activity of this catalyst was found to originate from both metallic Ru (Ru 0) and hydrous ruthenium oxides (RuO x H y ) species on the catalyst surface. However, RuO x H y was found to be more active than metallic Ru. In addition, the anhydrous ruthenium oxide (RuO 2) species on the catalyst surface was found to be less active.

Methanol Electrooxidation on PtRu Bulk Alloys and Carbon-Supported PtRu Nanoparticle Catalysts: A Quantitative DEMS Study

Methanol electrooxidation on smooth Pt and PtRu bulk alloys and carbon-supported Pt and PtRu nanoparticle catalysts has been studied using cyclic voltammetry and potential step chronoamperometry combined with differential electrochemical mass spectrometry (DEMS). The current efficiencies for generated CO2 and methyl formate were calculated from Faradaic current (coulometric charge) and mass spectrometric currents (charges) at m/z=44 and m/z=60. The effects of Ru content in PtRu catalysts, catalyst loading/roughness, and the concentration of sulfuric acid as supporting electrolyte on the reaction kinetics and product distribution during methanol electrooxidation have been investigated. The results indicate that Pt-rich PtRu alloys and carbon-supported PtRu catalysts with ca. 20 atom % Ru content exhibit the highest catalytic activity for methanol electrooxidation, that is, the highest Faradaic current and the highest current efficiency for CO2 generation at low applied potentials. As the catalyst loading/roughness increases, the current efficiency for CO2 formation increases due to the further oxidation of soluble intermediates (formaldehyde and formic acid). At high concentrations of sulfuric acid, the electrooxidation of methanol was suppressed; both the oxidative current and the current efficiency of CO2 decreased, likely due to sulfate/bisulfate adsorption.

Impact of PtRu Anode Catalyst Degradation on DMFC MEA Performance

This paper reports an 'MEA based' accelerated ageing test designed primarily to investigate stability of anode catalysts for DMFCs. The effect of potential cycling on the anode with increasing number of cycles is investigated. Methanol stripping voltammetry is used to characterize changes at both the anode and cathode catalysts and the changes observed are related to quantitative changes in performance of the MEA polarization data. Changes in catalyst morphology and surface composition are characterized by TEM. In particular, comparisons between unsupported PtRu ('black') and carbon supported PtRu catalysts are made.

Characterization of Carbon Supported PtRu Catalyst Layers for Direct Methanol Fuel Cells

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Characterization of composite carbon supported PtRu catalyst and its catalytic performance for methanol oxidation

A composite carbon prepared by blending Vulcan XC-72 carbon with multi-walled carbon nanotubes (MWCNTs) had been used as a support of PtRu alloy catalyst (PtRu/C + MWCNTs) for methanol oxidation reaction. The obtained electrocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of the composite carbon support on the alloy content and PtRu surface state was discussed. The electrocatalytic property of the PtRu/C + MWCNTs catalyst was investigated by cyclic voltammetry (CV), chromoamperometry, CO stripping voltammetry and DMFC single cell test. The results demonstrated that, compared to the PtRu/C and PtRu/MWCNTs catalysts, the PtRu/C + MWCNTs catalyst exhibited superior electrocatalytic activity for methanol oxidation.

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation

PtRu (1:1) catalysts supported on low surface area carbon of the Sibunit family (S(BET) = 72 m(2) g(-1)) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H(2)SO(4) electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

On the Role of Reactant Transport and (Surface) Alloy Formation for the CO Tolerance of Carbon Supported PtRu Polymer Electrolyte Fuel Cell Catalysts

AbstractThe role of atomic scale intermixing for the electrocatalytic activity of bimetallic PtRu anode catalysts in reformate operated polymer electrolyte fuel cells (PEFC) was investigated, exploiting the specific properties of colloid based catalyst synthesis for the selective preparation of alloyed and non‐alloyed bimetallic catalysts. Three different carbon supported PtRu catalysts with different degrees of Pt and Ru intermixing, consisting of (i) carbon supported PtRu alloy particles (PtRu/C), (ii) Pt and Ru particles co‐deposited on the same carbon support (Pt+Ru/C), and (iii) a mixture of carbon supported Pt and carbon supported Ru (Pt/C+Ru/C) as well as the respective monometallic Pt/C and Ru/C catalysts were prepared and characterized by electron microscopy (TEM), X‐ray absorption spectroscopy, and CO stripping. Their performance as PEFC anode catalysts was evaluated by oxidation of a H2/2%CO gas mixture (simulated reformate) under fuel cell relevant conditions (elevated temperature, continuous reaction and controlled reactant transport) in a rotating disk electrode (RDE) set‐up. The CO tolerance and H2 oxidation activity of the three catalysts is comparable and distinctly different from that of the monometallic catalysts. The results indicate significant transport of the reactants, COad and/or OHad, between Pt and Ru surface areas and particles for all three catalysts, with only subtle differences from the alloy catalyst to the physical mixture. The high activity and CO tolerance of the bimetallic catalysts, through the formation of bimetallic surfaces, is explained, e.g., by contact formation in nanoparticle agglomerates or by material transport and subsequent surface decoration/surface alloy formation during catalyst preparation, conditioning, and operation. The instability and mobility of the catalysts under these conditions closely resembles concepts in gas phase catalysis.