The efficiency of methanol conversion to CO2 on thin films of Pt and PtRu fuel cell catalysts

Yields were determined for the CO₂ produced upon the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ at the following four fuel cell catalyst systems: Pt black, Pt at 10 wt.% metal loading on Vulcan XC-72R carbon (C/Pt, 10%), PtRu black at 50 at.% Pt, 50 at.% Ru (PtRu (50:50) black), and PtRu at 30 wt.% Pt, 15 wt.% Ru loading on Vulcan XC-72R carbon (C/PtRu, 30 wt.% Pt, 15 wt.% Ru). Samples were electrolyzed in a small volume (50 μl) arrangement for a period of 180 s keeping the reactant depletion in the cell below 1%. The dissolved CO₂ produced was determined ex situ by infrared spectroscopy in a micro-volume transmission flow cell. For the PtRu materials, the efficiencies for CO₂ formation were near 100% at reaction potentials in the range between 0.4 V (versus the reversible hydrogen electrode (RHE), V_RHE ) and 0.9 V_RHE. At the Pt catalysts, the yields of CO₂ approached 80% between 0.8 and 1.1 V_RHE and declined rapidly below 0.8 V_RHE.     

Electrochemical removal of phenol in alkaline solution. Contribution of the anodic polymerization on different electrode materials

The removal of organic pollutants based on electropolymerization on an anode was performed in the case of phenol in alkaline solution. The polymer formed by a process involving less than two electrons per molecule of phenol, is then precipitated by decreasing the pH and finally filtered and disposed. The electrochemical polymerization of phenol (C₀ = 0.105 M) in alkaline solution (pH = 13) at 86 °C has been studied by galvanostatic electrolysis, using a range of anode materials characterized by different O₂-overpotentials (IrO₂, Pt and β-PbO₂). Measurements of total organic carbon and HPLC have been used to follow phenol oxidation; the morphology of the polymer deposited on the electrode surface has been examined by SEM. Experimental data indicate that phenol concentration decreases by oxidation according to a first order reaction suggesting a mass transport limitation process. Polymeric films formed in alkaline solution did not cause the complete deactivation of the anodes. SEM results show that the polymeric films formed on Ti/IrO₂ and Pt anodes cannot be mineralized. On the other hand, complex oxidation reactions leading to the partial incineration of polymeric materials can take place on the Ta/β-PbO₂ surface due to electrogenerated HO radicals which have an oxidizing power much higher than that of intermediaries formed respectively on IrO₂ and Pt. It is assumed that the polymer films formed on these anodes have different permeability characteristics which determine the rate of mass transfer of the phenol. The fractions of phenol converted in polymers were 25, 32 and 39% respectively with Ti/IrO₂, Pt and Ta/β-PbO₂, a series of materials in which the O₂-overvoltage increases.     

Finely dispersed Pt nanoparticles in conducting poly(o-anisidine) nanofibrillar matrix as electrocatalytic material

A new approach based on stepwise oxidation of o-anisidine is explored for generating nanoporous films of poly(o-anisidine), POA and loading of Pt nanoparticles that are subsequently used for electrocatalysis of methanol oxidation are presented and compared with bulk Pt. POA film can easily be prepared by stepwise electro-oxidation procedure with very high porosity consisting of nanofibrillar structure using without templates. Controlled sizes of Pt nanoparticles were entrapped into POA matrix by a two-step process in which first PtCl₆²⁻ ions are sorbed into the pores of polymer matrix followed by electroreduction at −0.55 V in a 0.5 M H₂SO₄ solution. Loading of Pt nanoparticles (10-200 μg/cm²) onto POA matrix were accomplished by varying the concentration (2-10 mM) of the sorbate, i.e., H₂PtCl₆. The sizes of the Pt nanoparticles were determined from TEM analysis and Pt particles were found to be in the range of 10-20 nm. The crystallite phase of Pt particles in POA was examined from XRD pattern. AFM image further supports Pt particles embedded in POA matrix.     

A novel sintering resistant and corrosion resistant Pt4ZrO2/C catalyst for high temperature PEMFCs

A novel Pt₄ZrO₂/C catalyst was prepared and compared with 20 wt.% Pt/C in terms of the sintering resistance and corrosion resistance. To evaluate their sintering resistance and corrosion resistance properties, an accelerated ageing test (AAT) was performed. The catalysts before and after AAT were characterized by cyclic voltammetry (CV), rotating disk electrode (RDE) and X-ray diffraction (XRD). After AAT, the dissolution rate of Pt and Zr in H₃PO₄ media (105 wt.%, 204 °C) was characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The electrochemical area (ECA) changes of thin film electrodes based on Pt₄ZrO₂/C and Pt/C catalysts were also evaluated using continuous CV sweep technique. All the results showed that Pt₄ZrO₂/C has higher sintering resistance and corrosion resistance than Pt/C. ‘Anchor effect’ is proposed to explain the enhanced effect of ZrO₂ in Pt₄ZrO₂/C binary catalyst compared with Pt/C that contain platinum alone.     

Novel methanol electro-oxidation catalyst assisting with functional phthalocyanine supports

Novel electro-catalyst based on phthalocyanine stabilized Pt colloids has been developed for methanol electro-oxidation. Water soluble Cu²⁺ phthalocyanine functioned with sulfonic groups were selected as catalyst supports because of the relatively high catalytic activity of Pt catalyst and nearly the same catalytic selectivity complex with Cu-phthalocyanine, compared to others that chelated with Fe, Co and Ni ions. The as-resulting Pt-CuTsPc catalysts have average particle size of 2 nm and narrow size distribution. With the assistance of CuTsPc supports, the methanol electro-oxidation activity and poison tolerance of Pt catalyst have a significant increase. I_f/I_b ratio (anodic peak current density, forward to backward) of the Pt-CuTsPc/C catalysts also has obvious increase to 2.5, from value of 0.8 for pure Pt/C catalyst. The reaction Tafel slope of Pt-CuTsPc/C catalysts is 56.6 mV dec⁻¹, much smaller than that of the Pt/C catalyst. The transient current density on Pt-CuTsPc/C at 0.60 V is enhanced to 650% of that on the Pt/C catalyst while the enhancement factor R for comparison of steady-state current obtained on Pt-CuTsPc/C and Pt/C catalyst varies between 111% and 534% in the potential region of 0.3-0.75 V.     

High performance polymer electrolyte fuel cells with ultra-low Pt loading electrodes prepared by dual ion-beam assisted deposition

Ultra-low pure Pt-based electrodes (0.04-0.12 mg_Pt/cm²) were prepared by dual ion-beam assisted deposition (dual IBAD) method on the surface of a non-catalyzed gas diffusion layer (GDL) substrate. Film thicknesses ranged between 250 and 750 Å, these are compared with a control, a conventional Pt/C (1.0 mg_Pt(MEA)/cm², E-TEK). The IBAD electrode constituted a significantly different morphology, where low density Pt deposits (largely amorphous) were formed with varying depths of penetration into the gas diffusion layer, exhibiting a gradual change towards increasing crystalline character (from 250 to 750 Å). Mass specific power density of 0.297 g_Pt/kW is reported with 250 Å IBAD deposit (0.04 mg_Pt/cm² for a total MEA loading of 0.08 mg_Pt/cm²) at 0.65 V. This is contrasted with the commercial MEA with a loading of 1 mg_Pt(MEA)/cm² where mass specific power density obtained was 1.18 g_Pt/kW (at 0.65 V), a value typical of current state of the art commercial electrodes containing Pt/C. The principal shortcoming in this effort is the area specific power density which was in the range of 0.27-0.43 W/cm² (for 250-750 Å IBAD) at 0.65 V, hence much below the automotive target value of 0.8-0.9 W/cm² (at 0.65 V). An attempt to mitigate these losses is reported with the use of patterning. In this context a series of patterns ranging from 45 to 80% Pt coverage were used in conjunction with a hexagonal hole geometry. Up to 30% lowering of mass transport losses were realized.     

Niobium-doped titanium oxide for fuel cell application

High surface area niobium-doped titanium oxide catalyst supports were synthesized through a chemical method using nano-sized polystyrene (PS) latex as a template. By varying the concentration of template with respect to titania and niobium precursors, we were able to produce niobium-doped titanium oxides with surface areas of 75-115 m²/g and sufficient electrical conductance to function as fuel cell catalyst supports. The oxides were subsequently platinized and evaluated as oxygen reduction catalysts using a rotating disk electrode. One of the samples showed a mass activity of 0.16 A/mg_Pt and a specific activity of 170 μA/cm²_Pt at 900 mV, which are comparable to our benchmark Pt/C electrocatalyst.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

Activated carbon nanotubes-supported catalyst in fuel cells

The electrochemical property of platinum loaded on activated carbon nanotubes (Pt/ACNTs) was investigated by cyclic voltammograms (CVs) recorded in H₂SO₄ and H₂SO₄/CH₃OH aqueous solutions, respectively. Compared to 0.0046 A/cm² of Pt-loaded on pristine carbon nanotubes (Pt/CNTs) with a S_BET of 164 m²/g and 0.0042 A/cm² of conventional carbon black (Pt/C, Vulcan XC-72) with a S_BET of ∼250 m²/g, a better electrochemical activity (a high current density of 0.0070 A/cm² for weak-H₂ adsorption/desorption) of the Pt/ACNTs with high specific surface area (S_BET) of 830-960 m²/g was obtained. Furthermore, the highest current density of 0.079 A/cm² at 0.65 V in anodic sweep was observed during the methanol oxidation. On the basis of Pt size, utility ratio, and electro-active specific surface area (EAS), the Pt/ACNTs with a high Pt-loading of 50 wt.% exhibited the best electrochemical activity. The present ACNTs may be an excellent support material for electrochemical catalyst in proton exchange membrane and direct methanol fuel cells.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt-carbon hybrid nanostructures

Three carbon aerogels with different meso-macropore networks were used as supports for Pt catalysts using [Pt(NH₃)₄]Cl₂ as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H₂ chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8-20 wt.% before pre-treatment showed the formation of self-assembled Pt-carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.     

Dimethoxymethane electrooxidation on low index planes of platinum single crystal in acid media

Conventional electrochemical methods have been applied to study the oxidation of a possible alternative fuel for a direct oxidation fuel cell. The electrooxidation of dimethoxymethane (DMM) was investigated on the three low index planes, (1 0 0), (1 1 0) and (1 1 1) of platinum single crystals and compared with its oxidation on a platinum polycrystalline electrode. Among platinum electrodes, electroreactivity of DMM observed is Pt(1 1 1) > Pt(1 0 0) > Pt(1 1 0) ∼ Pt poly. Hydrogen adsorption is limited by the presence of DMM, except for Pt(1 1 1) plane. In situ IR experiments show the presence of bands of CO_ads with all electrodes except Pt(1 1 1). This work shows that the mechanism of DMM electrooxidation is structure sensitive. A path takes place on Pt(1 0 0) and Pt(1 1 0) which is favourable to the formation of CO_ads. Another path proceeds on Pt(1 1 1), where CO_ads is not present and reaction does not occur at low potential. Results indicate that peak intensities are higher in perchloric acid than in sulphuric acid. So DMM adsorption is dependent on the specific adsorption of the anions. In situ IR reflectance spectroscopy identified some intermediates and reaction products of DMM adsorption and electrooxidation on Pt electrodes: CO_L (linearly bonded) and CO_B (bridge bonded), adsorbed CHO and CH₃O species, methanol and CO₂. The electrochemical and spectroelectrochemical experiments suggest a complex mechanism of DMM electrooxidation.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Oxygen reduction activity of Pt and Pt-Mn-Co electrocatalysts sputtered on nano-structured thin film support

We report on extensive measurements of oxygen reduction activity of Pt and Pt-Co-Mn electrocatalysts using the rotating ring-disk electrode (RRDE) method. The electrocatalysts were prepared by sputtering from Pt or Pt, Co and Mn targets onto 3M's nano-structured thin film support (NSTF) structures. The area specific activity of Pt/NSTF, measured in 0.1 M HClO₄ and at room temperature, is similar to that of bulk Pt. The area specific measurements show a 20 mV reduction in the Pt-Co-Mn/NSTF overpotential compared to Pt/NSTF. The corresponding kinetic gain in the area specific activity of the ternary alloy is about a factor of two. This ORR enhancement factor observed in the ternary Pt-Co-Mn/NSTF by RRDE measurements is similar to the results obtained in 50 cm² H₂/air fuel cells.     

Investigation of carbon-supported Pt nanocatalyst preparation by the polyol process for fuel cell applications

Parametric investigation of the polyol process for the preparation of carbon-supported Pt nanoparticles as catalysts for fuel cells was carried out. It was found that the concentration of glycolate anion, which is a function of pH, plays an important role in controlling Pt particle size and loading on carbon. It was observed that Pt loading decreased with increasing alkalinity of the solution. As evidenced by zeta potential measurement, this was mainly due to poor adsorption or repulsive forces between the metal colloids and the supports. In order to modify the conventional polyol process, the effect of the gas purging conditions on the characteristics of Pt/C was examined. By the optimization of the gas environment during the reaction, it was possible to obtain high loading of 39.5 wt% with a 2.8 nm size of Pt particle. From the single cell test, it was found that operating in ambient O₂ at 70 °C can deliver high performance of more than 0.6 V at 1.44 A cm⁻².     

Controlled growth and shape formation of platinum nanoparticles and their electrochemical properties

Cubic Pt nanoparticles were prepared from a solution of K₂PtCl₄ containing sodium polyacrylate as a capping reagent. The effects of the Pt/polymer molar ratio, the average molecular weight (M_w) of the polymer, and reaction temperature on the shape and size were investigated. When the polymer of M_w = 5100 was added at a molar ratio of Pt/polymer = 1/12, cubic platinum nanoparticles of an average size of 10.3 nm were predominantly formed (ca. 50% in number) at 25 °C. The electron diffraction pattern of the cubic nanoparticles revealed that they are single crystals with Pt {1 0 0} faces on the surface.The cubic nanoparticles were electrochemically active, and showed strong features of Pt {1 0 0} faces on cyclic voltammogram under argon atmosphere. After repeated potential cycling in the range 0.05-1.4 V, the features of Pt {1 0 0} were gradually lost, and changed to those of polycrystalline Pt. Rotating ring disk electrode measurements in O₂-saturated H₂SO₄ solution revealed that the cubic nanoparticles had a high catalytic activity for oxygen reduction reaction (ORR). After polycrystallization by repeated potential cycling, the activity for ORR and hydrogen peroxide formation decreased slightly, which were attributed to the surface structural change from Pt {1 0 0} to polycrystalline.     

Adsorption of Pt(cod)me2 onto organic aerogels from supercritical solutions for the synthesis of supported platinum nanoparticles

The thermodynamics and kinetics of adsorption of Pt(cod)me₂ onto resorcinol-formaldehyde aerogel (RFA) from supercritical carbon dioxide (scCO₂) was investigated by using high performance liquid chromatography (HPLC) to measure Pt(cod)me₂ concentrations in the fluid phase. It was found that the adsorption isotherms of Pt(cod)me₂ at 35 °C for different CO₂ pressures could be represented by modified Langmuir isotherms. The kinetics of adsorption was determined by following the Pt(cod)me₂ uptake of the RFA spheres; these data correspond closely to the behavior from a mass transfer model based on diffusion within the pore volume with the assumption of local equilibrium at the solid-fluid interface. The adsorbed Pt(cod)me₂ molecules were reduced at atmospheric pressure under flowing hydrogen at 200 °C. The resultant Pt nanoparticles were distributed uniformly on the surface and had narrow size distributions. The average particle size of the nanoparticles increased with platinum loading from 2.0 nm at 10 wt.% to 3.3 nm at 34 wt.%. The Pt nanoparticles in an RFA pellet had a uniform radial size distribution, even though the pellet was impregnated with Pt(cod)me₂ for too short a short period of time for the system to reach adsorption equilibrium. The high mobility of the atomic Pt evolved during the reduction process is believed to be responsible for this phenomenon. Performing the adsorption of Pt(cod)me₂ onto RFA at 80 °C led to concurrent reduction and Pt nanoparticle growth.     

Irreversible adsorption of Sn adatoms on basal planes of Pt single crystal and its impact on electrooxidation of ethanol

The behaviours of irreversible adsorption (IRA) of Sn adatoms on Pt(1 0 0), Pt(1 1 1) and Pt(1 1 0) electrodes were characterized using cyclic voltammetry. It has revealed that Sn can adsorb irreversibly on Pt(1 0 0) and Pt(1 1 1), while not significantly on Pt(1 1 0) electrode. Quantitative analysis of the relationship between 1 − θ_H and θ_Sn suggests that Sn adatoms may adsorb preferably on hollow sites of Pt(1 1 1) (threefold) and Pt(1 0 0) (fourfold) planes, which is in accordance respectively with the values 0.31 and 0.21 of coverage of IRA Sn adatoms in saturation adsorption determined on these electrodes. The IRA Sn adatoms on different basal planes of Pt single crystal yield different impact on the electrocatalytic oxidation of ethanol. It has revealed that the IRA Sn adatoms on Pt(1 0 0) electrode have declined the activity for ethanol oxidation, while IRA Sn adatoms on Pt(1 1 1) have enhanced remarkably the electrocatalytic activity with Sn coverage θ_Sn between 0.09 and 0.18. The oxidation peak potential E_p and the current density j_p of ethanol oxidation on Pt(1 1 1)/Sn were varied with θ_Sn, and the highest j_p (1258 μA cm⁻²) as well as the lowest E_p (0.20 V) were measured simultaneously at θ_Sn around 0.14. In comparison with the data obtained on a bare Pt(1 1 1), the E_p was shifted negatively by 65 mV and the j_p has been enhanced to about 1.7 times on the Pt(1 1 1)/Sn (θ_Sn = 0.14), which is ascribed to hydroxyl species adsorption at relatively low potentials on Pt(1 1 1)/Sn surfaces. The current study is of importance in revealing the fundamental aspects of modification of the basal planes of Pt single crystal using Sn adatoms, and the impact of this modification on electrocatalytic activity towards ethanol oxidation.     

Carbon monoxide oxidation on Au(1 1 1) surface decorated by spontaneously deposited Pt

Platinum is deposited spontaneously on Au(1 1 1) surface from 1 mM H₂PtCl₆ + 1 M HClO₄ solution using multiple deposition procedure. X-ray photoelectron spectroscopy (XPS) analysis has shown that after immersion into the Pt containing solution and rinsing with water, Pt(OH)₂ resides on the Au(1 1 1) substrate. Consecutive depositions as well as in situ scanning tunneling microscopy (STM) and electrochemical measurements are performed on previously electrochemically reduced Pt/Au(1 1 1) surfaces. Only homogeneous distribution of thus deposited Pt islands is observed by in situ STM. With subsequent depositions, the width of deposited Pt islands increases, but stays lower than 10 nm, while a significant increase of Pt islands height is observed, leading to moderate increase of the coverage. Cyclic voltammetry (CV) profiles of obtained Pt/Au(1 1 1) surfaces, and CO stripping curves are recorded in 0.5 M H₂SO₄ solution. CO oxidation takes place only at higher potentials shifting negatively with increasing coverage. This is discussed with respect to Pt islands width and height distributions and to the influence of the Au(1 1 1) substrate surface.     

Influence of micro-structure on corrosion behavior of a Ni-based superalloy in 3.5% NaCl

The electrochemical corrosion behavior of a Ni-based superalloy with polycrystalline (cast alloy), single-crystalline (SC (2 0 0)) and nanocrystalline (NC) micro-structures has been studied in 3.5% NaCl solution. The results indicated that among the three materials, the corrosion resistance increased in the order cast alloy < SC (2 0 0) alloy < NC coating. XPS analyses revealed that the composition of passive film on the three materials was different. In addition to Cr₂O₃ and TiO₂, some NiO was detected in the passive film on the cast alloy, little in that of the SC (2 0 0) alloy and none in that of the NC coating. The double-log plots showed the compact property of the passive film formed on the samples also varied in the order cast alloy < SC (2 0 0) alloy < NC coating, with the cast alloy displaying the worst compact property and the NC coating the best. The micro-structure influenced both the composition of passive film as well as the initial growth of passive film, which determined the compact property of the film and resulted in the observed differences in the corrosion behavior of the three materials.