Tungsten oxide in polymer electrolyte fuel cell electrodes—A thin-film model electrode study

Thin films of WO_x and Pt on WO_x were evaporated onto the microporous layer of a gas diffusion layer (GDL) and served as model electrodes in the polymer electrolyte fuel cell (PEFC) as well as in liquid electrolyte measurements. In order to study the effects of introducing WO_x in PEFC electrodes, precise amounts of WO_x (films ranging from 0 to 40 nm) with or without a top layer of Pt (3 nm) were prepared. The structure of the thin-film model electrodes was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy prior to the electrochemical investigations. The electrodes were analyzed by cyclic voltammetry and the electrocatalytic activity for hydrogen oxidation reaction (HOR) and CO oxidation was examined. The impact of Nafion in the electrode structure was examined by comparing samples with and without Nafion solution sprayed onto the electrode. Fuel cell measurements showed an increased amount of hydrogen tungsten bronzes formed for increasing WO_x thicknesses and that Pt affected the intercalation/deintercalation process, but not the total amount of bronzes. The oxidation of pre-adsorbed CO was shifted to lower potentials for WO_x containing electrodes, suggesting that Pt-WO_x is a more CO-tolerant catalyst than Pt. For the HOR, Pt on thicker films of WO_x showed an increased limiting current, most likely originating from the increased electrochemically active surface area due to proton conductivity and hydrogen permeability in the WO_x film. From measurements in liquid electrolyte it was seen that the system behaved very differently compared to the fuel cell measurements. This exemplifies the large differences between the liquid electrolyte and fuel cell systems. The thin-film model electrodes are shown to be a very useful tool to study the effects of introducing new materials in the PEFC catalysts. The fact that a variety of different measurements can be performed with the same electrode structure is a particular strength.     

The oxygen evolution reaction on platinum,iridium, ruthenium and their alloys at 80°C in acid solutions

Kinetic parametes were determined for the oxygen evolution reaction on 50–50 atom percent alloys of RuIr, RuPt, and IrPt and compared with results obtained using ruthenium, iridium, platinum, and RuO₂/TiO₂ electrodes. The potentiostatic studies were made on oxide covered electrodes at 80°C in both 1.0 M H₂SO₄ and 1.0 M CF₃SO₃H. Cyclic voltammetric studies showed that while these noble metals and alloys are about equally effective as electrocatalysts for the hydrogen evolution reaction, striking differences in activity are found for the oxygen evolution reaction. The order of electrocatalytic activity towards oxygen evolution in H₂SO₄ is Ru > RuIr alloy ～ RuO₂/TiO₂ ～ Ir > IrPt alloy > RuPt alloy > Pt. The type of acid used had very little effect on the kinetic parameters. The lower electrocatalytic activities when platinum is present is probably due to the formation of a platinum oxide film. The dual barrier model is used to interpret the results for the electrodes containing platinum. The best electrocatalysts for oxygen evolution in acid solutions consist of noble metals which form oxide films (RuO₂, IrO₂) possessing metallic characteristics.     

The electrochemical oxidation of organics using tungsten oxide based electrodes

High surface area tungsten oxide (WO_x) based electrodes containing centers of Pt, Sn or Ru were synthesized. The WO_x electrodes were found to display good capacitive behavior and relatively high specific capacitance values of up to 180 F g⁻¹. The oxidation behavior of particularly HCOOH and (COOH)₂, using the WO_x electrodes containing Pt and Sn centers (Pt/WO_x and Sn/WO_x, respectively), was studied in detail in aqueous solutions at high potentials, i.e. at which O₂ is evolved. Both HCOOH and (COOH)₂ appear to be oxidized following 1st order kinetics. The (COOH)₂ oxidation reaction is faster than the HCOOH reaction using otherwise the same experimental conditions. The reaction mechanism of both the HCOOH and (COOH)₂ oxidation was found to most likely involve the adsorptive interaction of the two organics with the anode surface. The WO_x based anodes appear to be promising catalysts for the anodic oxidation of both (COOH)₂ and HCOOH.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

The role of the WO x ad-component to Pt and PtRu catalysts in the electrochemical CH3OH oxidation reaction

High surface area carbon-supported platinum-based catalysts, Pt/C, PtWO _x /C, PtRu/C and PtRuWO _x /C, were prepared via a chemical reduction route using single metal precursor salts. The catalyst particles were found to be in the nanoscale range, and the addition of Ru clearly decreased the particle size. The Ru was found to be partially incorporated into the face centered cubic lattice of Pt and to form a single Ru catalyst component. X-ray diffraction and X-ray photon spectroscopy did not provide evidence for electronic interactions between WO _x and Pt as well as WO _x and Ru. However, the addition of tungsten to the PtRuWO _x /C catalyst resulted in a high degree of catalyst particle agglomeration. Both Ru containing catalysts showed significantly higher activities for the CH₃OH oxidation reaction in terms of Pt + Ru mass as well as electroactive Pt + Ru surface area than the Pt/C and PtWO _x /C catalysts. The addition of tungsten appeared to mainly result in some ‘physical’ modification of the catalytically active Pt and Ru surface components such as differences in electroactive surface area rather than promotion of the CH₃OH oxidation reaction via a true catalytic mechanism.     

WO3 Nanoparticles Synthesized Through Ion Exchange Route as Pt Electrocatalyst Support for Alcohol Oxidation

Tungsten oxide (WO₃) nanocrystals with the diameter <5 nm supported on porous carbonized resin (denoted as C‐WO₃) are synthesized. The WO₃ precursors are dispersed at ion level through ion exchange route, then reduced to WO₃ nanocrystals. Pt nanoparticles are loaded on the porous C‐WO₃ matrix (denoted as Pt/C‐WO₃) and used as electrocatalyst in fuel cell for alcohol oxidation, in which WO₃ is found efficient promotion effect on Pt electrocatalyst in the electrochemical activity and stability. Thereinto, Pt/C‐WO₃ gives 1.63 times higher current densities than the commercial Pt/C (TKK) for methanol oxidation at the same Pt loadings. Moreover, Pt/C‐WO₃ electrocatalyst shows excellent properties in mass transfer than Pt/C (TKK). The present method can be readily scaled up for the production of other nanomaterials as well as WO₃.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Platinum—tin/alumina catalyst: Modification of the metallic phase after successive oxidation—reduction cycles

PtSn/Al₂O₃ catalysts, prepared by different deposition techniques and submitted to successive oxidation—reduction cycles at 500°C, were characterized by TPR and test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), and compared with fresh samples. Interpretation of the results is based on the fact that in catalysts obtained by both coimpregnation and successive impregnation, a higher quantity of PtSn alloys was produced as a consequence of the oxidation—reduction cycles. In catalysts prepared by impregnating the support with the [PtCl₂(SnCl₃)₂]²⁻ complex, the cycles produced changes in the metallic phase probably due to either a surface enrichment of tin in alloy particles or a modification of the alloy phase composition.     

Binary and ternary anode catalyst formulations including the elements W, Sn and Mo for PEMFCs operated on methanol or reformate gas

In an exploratory approach to find improved electrocatalyst formulations binary and ternary carbon supported catalysts with the elements Pt and Ru, W, Mo or Sn, respectively, amending the choice of Pt and Pt/Ru catalysts by addition of non-Pt metal cocatalysts were manufactured by impregnation and a colloid method and tested towards their activity for anodic oxidation of H₂ containing 150 ppm CO and of methanol. Membrane-electrode-assemblies with noble metal loadings of 0.4 mg cm⁻² were manufactured and tested in fuel cell operation at 75°C with H₂ fuel contaminated by CO and at 95°C for operation on methanol. Cocatalytic activities were found for the elements W and Mo for oxidation of H₂/CO and methanol while in the case of Sn a cocatalytic activity was only found for H₂/CO-oxidation. Both for oxidation of methanol and H₂/CO the system Pt/Ru/W was superior to the other systems tested. The colloid method was found to be highly suitable for synthesizing polymetallic PEM catalysts.     

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO₃/ZrO₂, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO₄/ZrO₂) is compared with a Pt/WO₃/ZrO₂ catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO₃/ZrO₂ catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.     

The effect of platinum on the performance of WO3 nanocrystal photocatalysts for the oxidation of Methyl Orange and iso‐propanol

BACKGROUND: This research investigated the effect of platinum (Pt) on the reactivity of tungsten oxide (WO₃) for the visible light photocatalytic oxidation of dyes. RESULTS: Nanocrystalline tungsten oxide (WO₃) photocatalysts were synthesised by a sol‐gel process and employed for the photocatalytic degradation of Methyl Orange under visible light. For comparison commercial bulk WO₃ materials were also studied for the same reaction. These materials were fully characterised using X‐ray diffraction (XRD), UV‐visible diffuse reflection spectroscopy and transmission electron microscopy (TEM). The photocatalytic oxidation of iso‐propanol was used as a model reaction to follow the concomitant reduction of molecular oxygen. No reactions occured in the absence of platinum, which is an essential co‐catalyst for the multi‐electron reduction of oxygen. The platinised WO₃ catalysts were stable for multiple oxidation–reduction cycles. The results from the catalytic activity measurements showed that platinised nanocrystalline WO₃ is a superior oxidation photocatalyst when compared with bulk WO₃. Methyl Orange was completely decolourised in 4 h. CONCLUSIONS: The enhanced performance of nanocrystalline Pt‐WO₃ is attributed to improved charge separation in the nanosized photocatalyst. Platinum is an essential co‐catalyst to reduce oxygen. This photocatalyst could be applied to the treatment of organic pollutants in wastewater, with the advantage of using visible light compared with the widely studied TiO₂, which requires UV light. Copyright © 2011 Society of Chemical Industry     

Reaction of dinaphthyl and diphenyl ethers at liquefaction conditions

The reactions of 2,2′-dinaphthyl ether and diphenyl ether were studied at 375–425°C using 6.9 MPa (cold) hydrogen or nitrogen, 9,10-dihydrophenanthrene (DHP) and decalin as solvents, and a molybdenum sulfide catalyst. We chose to examine these compounds as models for the cleavage of diaryl ether bridges during coal liquefaction. The molybdenum sulfide was added to the reaction as MoS₃, which should transform to the active MoS₂ catalyst. Cleavage of the C_arO in 2,2′-dinaphthyl ether, at reaction temperatures of 375 and 400°C, proceeded in the sequence H₂ < DHPN₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < MoS₃H₂ < Dec.MoS₃H₂. At 425°C, the MoS₃H₂ and Dec.MoS₃H₂ systems exchange places in this order. Diphenyl ether is less reactive than dinaphthyl ether toward hydrogenolysis reactions under these conditions. The conversion rate of diphenyl ether increases in the order H₂ < DHPH₂ < DHPMoS₃N₂ < DHPMoS₃H₂ < Dec.MoS₃H₂ < MoS₃H₂. Although the rates of conversion of the two ethers are different, the relative effects of using a reactive gaseous atmosphere, donor solvent, catalyst - or some combination of these factors - are the same for both compounds. In liquefaction experiments, hydrogen donor solvent or hydrogen shuttling solvent seems necessary to reduce retrogressive reactions. However, a solvent interacting strongly with catalyst and scavenging hydrogen atoms can reduce the activity of catalysts in hydrocracking reactions.     

Hydrogen spillover phenomenon: Enhanced reversible hydrogen adsorption/desorption at Ta2O5-coated Pt electrode in acidic media

The current study is concerned with the preparation and characterization of tantalum oxide-loaded Pt (TaO_x/Pt) electrodes for hydrogen spillover application. XPS, SEM, EDX and XRD techniques are used to characterize the TaO_x/Pt surfaces. TaO_x/Pt electrodes were prepared by galvanostatic electrodeposition of Ta on Pt from LiF-NaF (60:40 mol%) molten salts containing K₂TaF₇ (20 wt%) at 800 °C and then by annealing in air at various temperatures (200, 400 and 600 °C). The thus-fabricated TaO_x/Pt electrodes were compared with the non-annealed Ta/Pt and the unmodified Pt electrodes for the hydrogen adsorption/desorption (H_ads/H_des) reaction. The oxidation of Ta to the stoichiometric oxide (Ta₂O₅) increases with increasing the annealing temperature as revealed from XPS and X-ray diffraction (XRD) measurements. The higher the annealing temperature the larger is the enhancement in the H_ads/H_des reaction at TaO_x/Pt electrode. The extraordinary increase in the hydrogen adsorption/desorption at the electrode annealed at 600 °C is explained on the basis of a hydrogen spillover-reverse spillover mechanism. The hydrogen adsorption at the TaO_x/Pt electrode is a diffusion-controlled process.     

Residual hydrogen in supported platinum catalysts and its influence on their catalytic properties

When Al₂O₃-, SiO₂-, or TiO₂-supported Pt catalysts were reduced above 400°C, there was a sharp decrease in their hydrogenolysis activity for n-pentane at 400°C. The chemisorption of H₂ on the catalyst at 20°C also decreased as if a part of the surface Pt atoms was no longer accessible to chemisorption. A few air-H₂ cycles at 20°C restored part of the hydrogenolysis activity and H₂ chemisorption capacity lost on reduction of the catalyst at higher temperatures. An air oxidation at 500°C followed by reduction at 400°C fully restored the original properties of the catalyst. The uptake of hydrogen by oxidized catalysts above 400°C was more than would correspond to the topmost layer of the Pt surface only. In the case of Pt-TiO₂, a reduction of the carrier was shown to occur at about 500°C by means of temperature-programmed desorption and reduction studies. The attenuation of the catalytic properties of the three types of catalysts studied seems to be caused mainly by a stronger chemisorption of H₂ on Pt since such effects are observed on Pt-b1ack also. Self-inhibition by strongly chemisorbed H₂ seems to be a widespread phenomenon in catalysis.     

Calorimetric study of the reversibility of CO pollutant adsorption on high loaded Pt/carbon catalysts used in PEM fuel cells

A major obstacle to the broader use of fuel cells is the poisoning of supported Pt catalysts by the CO present in virtually all feeds. In this paper, the microcalorimetry technique was employed to study and compare the CO adsorption properties of different commercial carbon-supported platinum catalysts with high Pt loading, aimed to be used in proton exchange membrane fuel cells (PEMFCs) applications. Combined with other techniques of characterization, such as BET, XRD, TPD-MS and TPR, adsorption microcalorimetry has permitted a better understanding of the studied systems. The pore architecture of Pt/C catalysts was found to influence the kinetics of heat release during CO adsorption. The accessibility of CO molecules to the adsorption sites increased with the mesoporosity of the catalyst. The degree of catalyst poisoning by CO upon successive air/H₂/CO cycles varied between 2 and 30% for the different studied samples. These results confirm that the surface chemistry of the catalyst, and in particular the Pt deposition method, affects the surface site energy distribution and consequently the adsorptive properties towards H₂ and CO. It was found that both H₂ and CO are chemisorbed on the investigated samples. Pt/C powders exhibit higher differential heats of carbon monoxide adsorption in comparison with hydrogen adsorption. A reaction between pre-adsorbed H₂ and CO from the gas phase takes place on Pt/C catalysts as a result of competitive adsorption.     

Amorphous nickel-valve metal-platinum group metal alloy electrodes for hydrogen-oxygen sulphuric acid fuel cells

Powder catalysts were prepared by immersion of amorphous Ni-40Zr and Ni-40Ti alloys containing a few at % of platinum group elements in HF solution. This treatment led to preferential dissolution of the valve metal and nickel with a consequent formation of microcrystalline alloy powders consisting of concentrated platinum group elements and some nickel and valve metal. Porous gas-diffusion electrodes prepared by using these alloy catalyst powders were employed for electrochemical reduction of oxygen and oxidation of hydrogen in 1 M H₂SO₄ at 25°C. The activity of the electrodes prepared from the amorphous alloys containing Pt–Ru, Pt–Rh, Pt and Pd for oxygen reduction was considerably higher than that of the platinum black electrode. Oxidation of hydrogen occurred readily close to the equilibrium potential. Amorphous alloy electrodes containing Pt–Ru, Pt–Rh and Pt were more active than the platinum black electrode for the hydrogen oxidation.     

Development of composite anode electrocatalyst for direct methanol fuel cells

Different effects of support hydrophilicity and metal-oxide on the performance of Pt-based catalysts were investigated with the aim of improving the mass activities toward methanol electrooxidation. Both potentiodynamic and potentiostatic measurements revealed that improved surface hydrophilicity of multi-wall carbon nanotubes (MWCNTs) could promote the dispersion of Pt nanoparticles and, consequently, promote the Pt utilization and reduce the polarization in methanol electrooxidation. In addition, WO₃ was shown to play a supportive role in enhancing catalytic activity. The interaction between Pt and WO₃ was examined by CO-stripping and CO oxidation transient experiments. The results suggested that the activity and the kinetics of monolayer CO_ads electrooxidation of Pt nanoparticles are enhanced by the adjacent WO₃ via a bifunctional mechanism, which accounts for improved activity in methanol electrooxidation.     

Radical formation in polymer electrolyte fuel cell components as studied by ESR spectroscopy

To clarify the deterioration mechanism for polymer electrolyte fuel cells, OH radical formation at the catalyst electrodes was investigated by ESR (electron spin resonance) spectroscopy using a flow cell with the catalyst electrodes. OH radicals produced from H₂O₂ were detected by a DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trapping method for a Nafion-coated Pt/Carbon catalyst electrode under a high potential (0.85 V versus RHE) on supplying H₂ and under low potentials (lower than 0.40 V). When Pt–Ru catalysts were employed instead of Pt catalysts, the formation of OH radicals was barely detected. The results suggest the possibility of the formation of OH radicals by the oxidation of H₂O₂ at the oxidized Pt surface under a positive potential as well as the reduction of H₂O₂ at the clean Pt surface under a low potential.     

Effect of preparation conditions on the structure and catalytic activity of carbon-supported platinum for the electrooxidation of methanol

Carbon-supported platinum catalysts (Pt/C) were prepared by treatment of PtO₂/C under different conditions: (a) heating at 380°C in air, argon and hydrogen; (b) electrochemical reduction in H₂SO₄; and (c) reduction with NaBH₄. The effect of the preparation conditions on the structure and the catalytic activity of the catalysts for the electrooxidation of CH₃OH in acid media was studied. The highest activity was achieved for the catalyst treated in air. The activity is determined by the crystal faces exposed at the particle surface as well as particle size and the partial oxidation of the carbon support.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.