Layer Of Pt Research Articles

Influence of the composition of isopropyl alcohol/water mixture solvents in catalyst ink solutions on proton exchange membrane fuel cell performance

We study the morphology of Nafion in the dilute IPA (isopropyl alcohol)/water mixture solutions containing 20–100 wt.% of IPA and in the Pt–C/Nafion gas diffusion electrodes (GDEs; where Pt–C is the carbon powder deposited on its surface with Pt particles), which are prepared by spraying on the carbon paper surfaces with a layer of Pt–C, Nafion and IPA/water ink solution. The fuel cell performance of the GDEs strongly depends on the Nafion morphology in the ink solutions. A lower IPA content in the Pt–C/Nafion ink solutions results in the formation of larger and higher negatively charged Nafion aggregated particles, which leads to higher steric hindrance of the deposition of Nafion ionomer on the surface of Pt–C particles and thus a thinner Nafion film in contact on the Pt–C particle surfaces. The thinner Nafion film in contact with the Pt particles in the CL increases the chances of the Pt particles in contact with the H2/O2 gas, leading to a higher fuel cell performance.

Investigation of Pd-based electrocatalysts for oxygen reduction in PEMFCs operating under automotive conditions

Composite Pd-based electrocatalysts consisting of a surface layer of Pt (5% wt.) supported on a core Pd3Co1 alloy were prepared. Two preparation approaches were investigated. One consisting of a single-step reduction procedure; in the second method, preparation of the PdCo alloy and deposition of a Pt overlayer occurred in two distinct steps. The catalyst prepared by a one-step process showed oxidised Pt species on the surface even if characterized by a smaller crystallite size with respect to the two-step Pd-based catalyst (4 nm vs. 6 nm). Moreover, the two-step process showed an enrichment of Pt on the surface and a smaller content of Co in the outermost layers. The enhanced surface characteristics of the two-step Pd catalyst resulted in a better performance. At 80 °C, the mass activity was lower than a Pt3Co1 alloy catalyst with the same crystallographic structure. Interestingly, the composite PtPdCo catalyst showed a significant increase of performance as the temperature was increased to 110 °C whereas the Pt3Co1 showed a decrease due to a prevailing effect of ionomer dry-out in the catalytic layer. The composite catalyst appeared sufficiently stable after 104 electrochemical cycles between 0.6 and 0.9 V at 110 °C and 33% R.H.

Electrocatalytic Activity of Pd, Pt, and Rh for the Electro-oxidation of Ethanol in Alkaline Electrolyte: An Online DEMS Study

The electro-oxidation of ethanol was investigated on electrodeposited layers of Pd, Pt, and Rh in alkaline electrolyte. The reaction products were monitored by experiments of online differential electrochemical mass spectrometry (DEMS). Potentiodynamic curves for the ethanol electro-oxidation catalyzed by these three different metal electrocatalysts showed similar onset potentials, but the highest Faradaic current peak was observed for the Pt electrocatalyst. Online DEMS experiments evidenced similar amounts of CO2 for the three different materials, but Pd presented the higher production of ethylacetate (acetic acid). This indicated that the electrochemical oxidation of ethanol on the Pd surface occurred to a higher extent. The formation of methane, which was observed for Pt and Rh, after potential excursions to lower potentials, was absent for Pd. On the basis of the obtained results, it was stated that, on Pt and Rh, the formation of CO2 occurs mainly via oxidation of CO and CHx,ad species formed after dissociative adsorption of ethanol or ethoxy species that takes place only at low potentials. This indicates that the dissociative adsorption of ethanol or ethoxy species is inhibited at higher potentials on Pt and Rh. On the other hand, on the Pd electrocatalyst, the reaction may occur via nondissociative adsorption of ethanol or ethoxy species at lower potentials, followed by oxidation to acetaldehyde and, after that, by a further oxidation step to acetic acid on the electrocatalyst surface. Additionally, in a parallel route, the acetaldehyde molecules adsorbed on the Pd surface can be deprotonated, yielding a reaction intermediate in which the carbon–carbon bond is less protected, and therefore, it can be dissociated on the Pd surface, producing CO2, after potential excursions to higher potentials.

Epitaxial growth of Pb(Zr0.53Ti0.47)O3 films on Pt coated magnetostrictive amorphous metallic substrates toward next generation multiferroic heterostructures

Piezoelectric films of Pb(Zr0.53Ti0.47)O3 (PZT) were deposited by pulsed laser deposition onto metallic magnetostrictive substrates. In order to optimize the growth of PZT films, a buffer layer of Pt was employed, as well as variation of deposition temperature, pressure, and laser energy. Room temperature θ-2θ x-ray diffraction measurements indicate all diffraction features correspond to reflections indexed to a single PZT phase of space group P4mm. Scanning electron microscopy images reveal pinhole-free dense films of pyramidal shaped crystal arrangements whose orientation and size were controlled by variation of oxygen pressures during deposition. The resulting PZT films had a value of d33 ∼ 46 pm/V representing a 53% increase over previous efforts to realize a piezoelectric/Metglas™ film heterostructure.

Formation of FePt Alloy Nanoparticles on Highly Oriented Pyrolytic Graphite: A Morphological and In Situ X-ray Photoelectron Spectroscopic Study

Dual layers of Pt and Fe, deposited sequentially onto highly oriented pyrolytic graphite (HOPG), were annealed under ultrahigh vacuum, from room temperature to 560 °C. The formation of FePt alloy NPs, through interdiffusion, was studied by in situ X-ray photoelectron spectroscopy (XPS), ex situ atomic force (AFM), and high-resolution transmission electron (TEM) microscopies. With increasing annealing temperature, the Pt 4f7/2 binding energy shifts positively, and the positions of the Pt 5d-6s valence band centers move away from the Fermi level and broaden. Between 300 and 400 °C, Fe and Pt atoms diffuse significantly. Simultaneously, a surface chemical reaction occurs between metal oxide and adventitious carbon on the NP surface, resulting in the disappearance of the O 1s spectrum and the formation of an amorphous hydrocarbon shell. At elevated temperatures, the shell is continually lost, through fragmentation, and replaced by a new hydrocarbon from the vacuum background, assuring that the NPs do not coalesce during the whole annealing process. Stable FePt alloy NPs are formed on the HOPG surface as the annealing temperature is increased to ∼400 °C (A1 structure) and ∼500 °C (L10 structure). A continual Pt surface enrichment occurs with increasing annealing temperature, even before the formation of stable L10 NPs, resulting in the formation of a Pt-rich layer around the NPs. On the basis of the mass balance in the system, an Fe-rich layer must lie below the Pt-rich layer, surrounding the L10 core.

Anisotropy of attenuation revealed by angular intensity distribution of elastically backscattered electrons

The structure of the first few atomic layers of Cu(111), Pt(111), and Au(111) was reflected by the directional elastic peak electron spectroscopy (DEPES). Experimental stereographic projections were compared with the theoretical data obtained by using a multiple scattering (MS) approximation. Although the same angular distribution of the maxima was observed in recorded and computed data for all samples, the relative peak intensities were found to be significantly different. The observed discrepancies suggest the anisotropic damping of electron transport in crystalline samples with respect to the low index directions. Elastic scattering of electrons penetrating the sample along the close packed atomic rows leads to an additional defocusing effect of the primary beam.

Anodic Hydrogen Oxidation at Bare and Pt-modified Ru(0001) in Flowing Electrolyte – Theory versus Experiment

ABSTRACTThis paper reports on electrochemical hydrogen oxidation at atomically smooth single crystal surfaces. These surfaces are considered as planar models for (bi)metallic nanoparticles that are commonly used as catalytically active electrode materials in low-temperature fuel cells. These samples are prepared in ultrahigh vacuum but are characterized under conditions of enhanced mass transport in hydrogen saturated electrolyte. The two examples shown in this paper are Ru(0001) with or without an atomically thin layer of Pt. The Pt thin layer turns out to be more active than pure Ru(0001) by three orders of magnitude and also more active than bulk Pt electrodes. We show that those findings agree very well with predictions based on density functional theory in combination with a simple kinetic model.

Facile preparation of carbon-supported PtNi hollow nanoparticles with high electrochemical performance

To design Pt-based materials with a hollow structure via a galvanic reaction would be one of the effective ways to prepare electro- catalysts with high activity. The galvanic reaction between Pt ions and metal template is usually conducted under limited conditions, which makes the preparation of Pt hollow nanoparticles laborious. Here, we introduce a one-step and one-pot synthetic approach for the preparation of carbon-supported PtNi alloy hollow nanoparticles with a narrow size distribution. Prepared PtNi alloys were characterized by a nonporous shell consisting of a Pt-enriched surface layer and an inner alloy layer of Pt and Ni. Due to its unique structural advantages, this material showed excellent electrocatalytic performance for oxygen reduction (3.3- and 7.8-fold enhanced mass and specific activities compared to those of a commercial carbon-supported Pt nanoparticle). A possible mechanism for the formation of PtNi hollow structure is suggested.

Fabricating microstructures on CVD diamond film

Diamond has many advanced properties which may provide potential solutions to various engineering problems. This study focuses on fabricating micro-structures on CVD diamond film. A thin layer of Au or Pt together with thermal annealing process is used to form micro-masks and reactive ion etching (RIE) method was subsequently adopted in this research to etch and fabricate micro-structures. Field emission scanning electron microscope (FESEM) and micro Raman spectroscopy were used to observe and analyse the morphology and composition of the obtained microstructures. Results show that different coated materials and thickness can produce different types of micro-masks and, as a consequence, different microstructures. Depending on the micro-mask, whisker-like or pillar-like microstructures are successfully produced and, based on the micro Raman spectrum; these microstructures still retains good diamond quality.

Detection of hydrogen using multi-walled carbon-nanotube yarns coated with nanocrystalline Pd and Pd/Pt layered structures

Robust and flexible chemiresistors fabricated from multi-walled carbon nanotube yarns decorated with nanocrystalline Pd (Pd-MWCNTs) were used to detect hydrogen from 20parts per million (ppm) and above in nitrogen at room temperature. With the chemiresistors fabricated by introducing a layer of Pt on Pd (Pt–Pd-MWCNTs), the lower limit of detection (LLD) was found to extend down to 5ppm. It is shown that the observed response to hydrogen is a resultant of two mechanisms, namely, the formation of the hydrides of Pd and nanoscopic gap closing which leads to opposing changes in resistance in the composite yarns. A deviation from the Sievert’s law is observed at concentrations above 100ppm and is attributed to the presence of these competing mechanisms. In air, the LLD was found to be 2000ppm for the Pd-MWCNT chemiresistor and 400ppm for the Pt–Pd-MWCNT chemiresistor. The Pt–Pd-MWCNT chemiresistor showed excellent response and recovery characteristics in air. For Pd-MWCNT chemiresistor, the nanoscopic gap-closing mechanism became prominent at concentrations below ∼1000ppm and allowed the detection of hydrogen down to 200ppm in air using negative changes in resistance.

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.

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb 0.93La 0.07(Zr 0.3Ti 0.7) 0.93O 3, PLZT7/30/70) thin films with and without a seeding layer of PbTiO 3 (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO 2 was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10 −9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10 −10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer ( R rms = 6.867 nm) was larger than that of PLZT film ( R rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.

The correlation between electric field emission phenomenon and Schottky contact reverse bias characteristics in nanostructured systems

Two different morphologies of nanotextured molybdenum oxide were deposited by thermal evaporation. By measuring their field emission (FE) properties, an enhancement factor was extracted. Subsequently, these films were coated with a thin layer of Pt to form Schottky contacts. The current-voltage (I-V) characteristics showed low magnitude reverse breakdown voltages, which we attributed to the localized electric field enhancement. An enhancement factor was obtained from the I-V curves. We will show that the enhancement factor extracted from the I-V curves is in good agreement with the enhancement factor extracted from the FE measurements.

Pt-decorated nanoporous gold for glucose electrooxidation in neutral and alkaline solutions

Exploiting electrocatalysts with high activity for glucose oxidation is of central importance for practical applications such as glucose fuel cell. Pt-decorated nanoporous gold (NPG-Pt), created by depositing a thin layer of Pt on NPG surface, was proposed as an active electrode for glucose electrooxidation in neutral and alkaline solutions. The structure and surface properties of NPG-Pt were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and cyclic voltammetry (CV). The electrocatalytic activity toward glucose oxidation in neutral and alkaline solutions was evaluated, which was found to depend strongly on the surface structure of NPG-Pt. A direct glucose fuel cell (DGFC) was performed based on the novel membrane electrode materials. With a low precious metal load of less than 0.3 mg cm-2 Au and 60 μg cm-2 Pt in anode and commercial Pt/C in cathode, the performance of DGFC in alkaline is much better than that in neutral condition.

Formation of continuous platinum layer on top of an organic monolayer by electrochemical deposition followed by electroless deposition

A metal–molecule–metal sandwich structure is constructed to wire the molecule to external circuits. A continuous mono-atomic thick metallic layer of platinum is formed on top of a 1,4-benzenedimethanethiol (BDMT) self-assembled monolayer (SAM) on a Au(1 1 1) substrate through electrochemical deposition followed by electroless deposition (ELD). At first, about 1/3 monolayer of Pt is formed on top of the BDMT SAM by electrochemical reduction of pre-adsorbed PtCl 4 2 - complex, then the coverage of Pt layer is increased by ELD. Two approaches for ELD of Pt are examined. In the first approach, the pre-formed sub-monolayer of Pt is used as a nucleation center for ELD of Pt by reducing Pt ions in a solution containing a reducing agent. X-ray photoelectron spectroscopy (XPS), angle resolved X-ray photoelectron spectroscopy (AR-XPS) and electrochemical measurements show that the coverage of Pt increases with the increase of plating time but the metallic Pt penetrates through organic film and deposits between SAM and Au(1 1 1) substrate if the plating time is too long. In the second approach, the electrochemically formed sub-monolayer of Pt is used not only as a nucleation center but also as a catalyst for the selective ELD of Cu. Electrochemical measurements as well as AR-XPS after a spontaneous redox replacement of Cu by Pt confirm that a full layer of Pt is formed on top of the BDMT SAM and no platinum is found underneath the SAM. The latter is a new method for a construction of a metal–molecule–metal sandwich structure, which allows wiring molecules to external circuits.

High-Throughput Characterization of Pt Supported on Thin Film Oxide Material Libraries Applied in the Oxygen Reduction Reaction

Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized.

Influence of residual stress on magnetoelectric coupling of bilayered CoFe2O4/PMN–PT thin films

Multiferroic CoFe2O4/0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 (CFO/PMN–PT) bilayered thin films with different CFO layer thicknesses were prepared on Pt/Ti/SiO2/Si substrates using the sol–gel technique. The bottom PMN–PT layer exhibited a highly (111) preferred orientation as conformed by a bidimensional (2D) planar X-ray diffractometer (XRD2). The influence of the thickness on the ferroelectric, dielectric, ferromagnetic, and magnetoelectric (ME) properties of the CFO/PMN–PT bilayered thin films was investigated. The residual stress of the top CFO layer films was characterized by means of grazing incidence X-ray diffraction. The ME signal is strongly dependent on the residual stress status of the CFO layer, which provides direct evidence for a stress-induced ME coupling mechanism in the multiferroic bilayered thin films. The maximal ME coupling coefficient of the bilayered thin films reaches up to about 65.2 mV cm−1Oe−1, which was higher than that reported previously.

Absence of morphotropic phase boundary effects in BiFeO3–PbTiO3 thin films grown via a chemical multilayer deposition method

We report an unusual behavior observed in (BiFeO3)1−x –(PbTiO3) x (BF–xPT) thin films prepared using a multilayer chemical solution deposition method. Films of different compositions were grown by depositing several bilayers of BF and PT precursors of varying BF and PT layer thicknesses followed by heat treatment in air. X-ray diffraction showed that samples of all compositions show mixing of two compounds resulting in a single-phase mixture, also confirmed by transmission electron microscopy. In contrast to bulk compositions, samples show a monoclinic (MA-type) structure suggesting disappearance of the morphotropic phase boundary (MPB) at x=0.30 as observed in the bulk. This is accompanied by the lack of any enhancement of the remanent polarization at the MPB, as shown by the ferroelectric measurements. Magnetic measurements showed an increase in the magnetization of the samples with increasing BF content. Significant magnetization in the samples indicates melting of spin spirals in the BF–xPT films, arising from a random distribution of iron atoms. Absence of Fe2+ ions was corroborated by X-ray photoelectron spectroscopy measurements. The results illustrate that thin film processing methodology significantly changes the structural evolution, in contrast to predictions from the equilibrium phase diagram, besides modifying the functional characteristics of the BP-xPT system dramatically.

Stabilization of metal counter electrodes for dye solar cells

The purpose of this study was to identify stable metal based counter electrodes (CE) for dye solar cells (DSC). Previous studies have shown that stainless steel (StS 304) suffers from corrosion when used as a counter electrode. Therefore metals which have inherently higher corrosion resistance, such as stainless steel types 321, 316 and 316L, Inconel 600 and titanium, were investigated here. When using thermal platinization for the preparation of the catalyst layer on CE, only the titanium foil based metal based DSC remained consistently stable in the 1000h light soaking test. The counter electrodes were also prepared with sputtering ∼20nm thick layer of Pt which provides a highly uniform layer on the CE which acts also as a protective coating on the metal. With sputtered Pt, DSC on all studied metals expect for Inconel remained at 80–95% of the initial efficiency after light soaking test for 1000h.

High-performance anode for Polymer Electrolyte Membrane Fuel Cells by multiple-layer Pt sputter deposition

We investigate the sputtering deposition as a tool for preparing Polymer Electrolyte Membrane Fuel Cell (PEMFC) electrodes with improved performance and catalyst utilization. Anodes of PEMFC with ultra-low loading of Pt (0.05 mg cm −2) are developed by alternate sputtering of Pt and painting layers of carbon nanotube ink with Nafion directly on the gas diffusion layer. Sputter depositing alternate layers of Pt on carbon-Nafion layer (CNL) has increased the anode activity over single-layer Pt deposited anode due to improved porosity and the presence of Pt nanoparticles in the inner CNL. Also, we investigated the influence of Nafion content in the CNL. The optimal Nafion content giving less resistance and better performance in an anode is 29 wt.%. This is significantly lower than for standard MEA anodes, indicating sufficient interfacial contact between each CNL. We studied the anodes prepared with 50 wt.% Nafion, which revealed larger ohmic resistance and also, blocks the CNL pores reducing gas permeability. Excellent mass transfer and performance is obtained with three-layer Pt sputter deposited anode with CNL containing 29 wt.% of Nafion.