Pd Electrodes Research Articles

Organic Semiconductor and Transistor Electrical Characteristic Based on Carbon Nanotubes

We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0-30/μm) to the Pd electrodes. The mobility average is increased three, six and nine times for low, medium and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performance using reduced barrier height of SWCNT/pentacene interface.

A novel reverse rectifying effects and negative differential resistance in asymmetric X-biphenyl-X (X=Pt, Pd, Pb) molecular junctions

Abstract We investigate the electronic transport properties of asymmetric biphenyl systems for X-electrodes (X=Pt, Pd, Pb) using non-equilibrium Green׳s function approach combined with density functional theory. The asymmetry in such systems is constructed by coupling biphenyl molecule to Pt, Pd, Pb electrode materials through dithiocarboxylate anchoring group on the left side and thiol group on the right side. Interestingly, novel reverse rectifying behaviors are observed for Pt and Pd electrode case. This is due to stronger d-orbital characteristics of Pt and Pd electrodes which produces intense transmission peak near the Fermi level. However, negative differential resistance is realized in case of Pb electrode. These observed distinct features for different electrodes are important for next generation nanoscale devices.

Electrochemical oxidation of 2-propanol over platinum and palladium electrodes in alkaline media studied by in situ attenuated total reflection infrared spectroscopy.

The electrochemical oxidation of 2-propanol over Pt and Pd electrodes was evaluated in alkaline media. Linear sweep voltammograms (LSVs), chronoamperograms (CAs), and simultaneous time-resolved attenuated total reflection infrared (ATR-IR) spectra of both electrodes were obtained in a 0.25 M KOH solution containing 1 M 2-propanol. The onset potential of 2-propanol oxidation for Pt was lower than that for Pd in LSVs while the degree of performance degradation observed for Pd was significantly smaller than that observed for Pt in CAs. The main product of 2-propanol oxidation was acetone over both electrodes and, over Pt only, acetone produced was catalytically oxidized to the enolate ion, which was accumulated on the Pt surface, leading to significant performance degradation. Carbon dioxide and carbonate species (CO3(2-), HCO3(-)) were not observed during 2-propanol oxidation over both electrodes, indicating that the complete oxidation of 2-propanol to CO2 will be a minor reaction.

The electrochemical hydrogen storage properties of Mg67−xPdxCo33 (x=1, 3, 5, 7) electrodes with BCC phase

The ternary Mg67−xPdxCo33 (x = 1, 3, 5, 7) alloys were prepared and served as anode materials for the Ni-MH battery system. Pd facilitates the formation of a full body-centered cubic (BCC) phase in binary Mg67Co33. All Mg67−xPdxCo33 (x = 1, 3, 5, 7) alloys possess BCC structure in nano-crystalline, which were observed by XRD and TEM analyses. In addition, their lattice parameters increase with the augmentation of Pd content. The charge–discharge experiments show that Mg64Pd3Co33 owns the maximum discharge capacity of 624 mAh g−1 among Mg67−xPdxCo33 (x = 1, 3, 5, 7) electrodes, which was greatly enhanced from our previously studied binary Mg–Co and ternary Mg–Co–Pd electrodes. All electrochemical kinetics e.g. exchange current density, hydrogen atomic diffusion capability were improved by substituting Pd for Mg, which were also relevant with the increment of Pd amount in the alloys. X-ray photoelectric spectroscopy (XPS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) demonstrated that Pd relieved the severe corrosions and capacity degradations of the electrodes.

Electro-oxidation of methanol catalysed by porous nanostructured Fe/Pd-Fe electrode in alkaline medium

Nanostructured Fe/Pd-Fe catalysts are prepared first by the deposition of Fe-Zn onto the Fe electrode surface, followed by replacement of the Zn by Pd at open circuit potential in a Pd-containing alkaline solution. The surface morphology and composition of coatings are determined by scanning electron microscopy and energy dispersive X-ray techniques. The results show that the Fe/Pd-Fe coatings are porous structure and the average particle size of Pd-Fe is low, in the range of 30–80 nm. The electrocatalytic activity and stability of Fe/Pd-Fe electrodes for oxidation of methanol are examined by cyclic voltammetry and chronoamperometry techniques. The new Fe/Pd-Fe catalyst has higher electrocatalytic activity and better stability for the electro-oxidation of methanol in an alkaline media than flat Pd and smooth Fe catalysts. The onset potential and peak potential on Fe/Pd-Fe catalysts are more negative than that on flat Pd and smooth Fe electrodes for methanol electro-oxidation. All results show that the nanostructured Fe/Pd-Fe electrode is a promising catalyst towards methanol oxidation in alkaline media for fuel cell applications.

Extremely fast hydrogen absorption/desorption through platinum overlayers

The hydrogen electrosorption in thin palladium films (50–1000 nm) was investigated at palladium electrodes covered with platinum overlayers. The results for this model system show that the rates of the hydrogen sorption/desorption are orders of magnitude higher for platinized samples with respect to pure palladium. The highest absorption kinetics have been observed for Pd electrodes fully covered with 1–3 platinum monolayers. By means of electrochemical impedance spectroscopy (EIS) we have shown that the process is diffusion limited at platinized Pd layers. Diffusion coefficient, DH, determined in EIS, is two orders of magnitude higher than that previously reported for thin palladium films and approaches DH for bulk palladium. The system stability after hydrogen absorption was assessed and the sorption mechanism was discussed. Surprisingly high durability of the platinized palladium enables its use in a variety of applications where fast and selective response in the presence of hydrogen is required.

Preliminary Investigation on the Electrochemical Activity of Butanol Isomers as Potential Fuel for Direct Alcohol Fuel Cell

A preliminary investigation of electrocatalytic oxidation activity of butanol isomers has been carried out to study their potential as fuels for direct alcohol fuel cells. The electrochemical study was carried out on Pt and Pd electrodes using a three electrode cell set up in alkaline media. The primary alcohol isomers of butanol were observed to behave similarly in their electrochemical reactions whereas 2-butanol showed completely different oxidation features on both catalysts. For example, no poisoning effects were observed for 2- butanol unlike for the primary butanol isomers. In contrast, tert-butanol did not show any oxidation reaction on Pt and Pd electrodes. Furthermore, Pd was not active at all in acidic media for butanol oxidation. The reactivity of butanol isomers were found to be in the order n-butanol>iso-butanol>2-butanol>tert-butanol based on the oxidation current density values

Electrochemical Synthesis of Ammonia with Proton Conducting Solid Electrolyte Fuel Cells at Intermediate Temperatures

Electrochemical synthesis of ammonia with solid electrolyte fuel cells using proton conducting solid electrolyte such as barium cerate was investigated at intermediate temperature of around 773 K. Electrode catalysts of metal alloys and mixed conductors including metals of Pd and Ru as active sites were synthesized. Two-electrode configuration was employed with dynamic hydrogen electrode (DHE) for CE, i.e., WE(N2)|solid electrolyte|CE(H2). It was observed that ammonia formed on the working electrode (WE) through the reaction of nitrogen with proton diffusing from the counter electrode (CE). The production rate of ammonia was influenced by electrode potential and it showed the maximum at around -300~-500 mV for both of Pd and Ru-based electrodes. Ru-based catalyst that might promote nitrogen bond dissociation showed good performance on ammonia formation. The electrode optimization was also discussed in terms of conduction paths of proton and electron in the electrodes.

Experimental Demonstration of Room-Temperature Spin Transport in n-Type Germanium Epilayers.

We report an experimental demonstration of room-temperature spin transport in n-type Ge epilayers grown on a Si(001) substrate. By utilizing spin pumping under ferromagnetic resonance, which inherently endows a spin battery function for semiconductors connected with a ferromagnet, a pure spin current is generated in the n-Ge at room temperature. The pure spin current is detected by using the inverse spin-Hall effect of either a Pt or Pd electrode on n-Ge. From a theoretical model that includes a geometrical contribution, the spin diffusion length in n-Ge at room temperature is estimated to be 660 nm. Moreover, the spin relaxation time decreases with increasing temperature, in agreement with a recently proposed theory of donor-driven spin relaxation in multivalley semiconductors.

Exceptionally Fast Hydrogen Absorption and Desorption through Platinum Overlayers

Palladium alloys are widely used in heterogeneous catalysis, in this group Pd–Pt systems are of particular catalytic interest. They are active components for anodes in low temperature fuel cells and hydrogenation of aromatic compounds in fuels. Platinum monolayers deposited on palladium have been recognized for their unique catalytic properties towards oxygen reduction reaction. However, the hydrogen absorption has never been studied on platinized palladium. In this work we have demonstrated for the first time that platinum overlayers deposited on palladium surface are not only permeable to hydrogen but also enhance the kinetics of the hydrogen absorption/desorption by several orders of magnitude. In this study palladium-hydrogen was chosen as a model system forming nonstoichiometric hydrides. The hydrogen electrosorption in thin palladium films (50-1000nm) has been investigated at electrodes covered with platinum overlayers. The highest absorption kinetics have been observed for Pd electrodes fully covered with 1 – 3 platinum monolayers. By means of electrochemical impedance spectroscopy (EIS) we have shown that the process is diffusion limited at platinized samples. Diffusion coefficient, D H, determined in EIS, is two orders of magnitude higher than that previously reported for thin palladium films and approaches D Hfor bulk palladium. The approach provides insight into the mechanism of hydrogen absorption and the process of hydrogen transport in metals-hydrogen systems. Surprisingly high stability of the platinized palladium enable its use in heterogeneous catalysis, hydrogenation reactions, or hydrogen purification. It should be stressed that palladium has already been proven to serve as an efficient catalyst to facilitate hydrogen insertion into other metal hydrides. Therefore, another set of potential applications is related to hydrogen storage or sensing systems, where fast and selective reaction in the presence of hydrogen is required.

Hydrogen oxidation reaction on Pd(1 1 1) electrode in alkaline media: Ab-initio DFT study of OH effects

OH effects on the hydrogen oxidation reaction, during a metallic surface electrocatalysis, have been studied by using density functional theory (DFT) based first principles methods. The hydrogen reaction is carried out on Pd(111) surface with the help of Tafel’s hydrogen adsorption and Volmer’s alkaline reactions. Both the reactions are investigated in detail by performing Broyden–Fletcher–Goldfarb–Shanno (BFGS) type structural relaxation analysis. Relative changes in structural free energies, hydrogen adsorption, water formation parameters and electronic structure of atomic orbitals corresponding to the palladium surface and adsorbent atoms are studied during the reaction. A significant change in local density of states of s-, p-, and d-band states of the involved entities is obtained in respect to before- and after-water-formation circumstances. Spectral variations in the local density of states (LDOS) are discussed vis-à-vis the HOR, hydrogen adsorption and validation of the presence of water.

Electron transport in extended carbon-nanotube/metal contacts:Ab initiobased Green function method

We have developed a new method that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and Al contacts. We have explained why CNTFETs with Pd electrodes are p-type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-Al contacts transmission is suppressed to a significant extent, especially for holes.

Effect of Pt coverage in Pt-deposited Pd nanostructure electrodes on electrochemical properties

We have fabricated Pt-deposited Pd electrodes via a two-gun sputtering deposition system by separately operating Pd and Pt target as a function of sputtering time of Pt target. For Pt-deposited Pd electrodes (Pd/Pt-X), Pd were first deposited on the substrates at 20 W for 5min, followed by depositing Pt on the Pd-only electrodes as a function of sputtering time (X=1, 3, 5, 7, and 10min) at 20W on the Pt target. As the sputtering time of Pt target increased, the portion of Pt on the Pd electrodes increased, representing an increased coverage of Pt on the Pd electrodes. The Pd/Pt-7 electrode having an optimized Pt coverage exhibits an excellent electrocatalytic activity for methanol oxidation reaction.

Palladium Nanoparticles Supported on Highly Oriented Pyrolytic Graphite: Preparation, Reactivity and Stability

AbstractPalladium nanoparticles (Pd NPs) were deposited electrochemically on highly oriented pyrolytic graphite (HOPG) substrates by using a potentiostatic double‐pulse technique. The particle densities were in the order of 109 cm−2; the radius of the deposited Pd NPs (2–20 nm) was proportional to the 1/3 power of the growth pulse duration (tg1/3). The open‐circuit potential measured during potentiodynamic potential scans of hydrogen evolution/oxidation (HER/HOR) at Pd/HOPG electrodes (average radii of Pd NPs larger than 8 nm) was more negative than at a bulk Pd electrode; this was caused by the different phases of palladium hydride (PdHx) formed in the latter case. Pd/HOPG samples showed an increased specific current density with decreasing particle size in HER. The activity of Pd NPs for HER is primarily affected by the absorbed‐hydrogen‐atom content in the Pd lattice. The electrochemical activity of Pd NPs for the oxygen reduction reaction (ORR) in acidic media decreased for smaller particle sizes. The stability of the Pd NPs was significantly influenced by the pH of the electrolyte. Pd NPs had a higher dissolution rate in solution with lower pH. The degeneration mechanisms seen for Pd NPs on HOPG were the dissolution of metal atoms, detachment of particles, and particle agglomeration. Corrosion of the graphite substrate after the potential cycles was also observed in AFM images.

Metal-support interaction in platinum and palladium nanoparticles loaded on nitrogen-doped mesoporous carbon for oxygen reduction reaction.

Mesoporous carbons are highly porous materials, which show large surface area, chemical inertness and electrochemical performances superior to traditional carbon material. In this study, we report the preparation of nitrogen-doped and undoped mesoporous carbons by an optimized hard template procedure employing silica as template, sucrose and ammonia as carbon and nitrogen source, respectively. Surface area measurements assert a value of 900 and 600 m(2) g(-1) for the best doped and undoped samples, respectively. Such supports were then thoroughly characterized by surface science and electron microscopy tools. Afterward, they were decorated with Pt and Pd nanoparticles, and it was found that the presence of nitrogen defects plays a significant role in improving the metal particles dimension and dispersion. In fact, when doped supports are used, the resulting metal nanoparticles are smaller (2-4 nm) and less prone to aggregation. Photoemission measurements give evidence of a binding energy shift, which is consistent with the presence of an electronic interaction between nitrogen atoms and the metal nanoparticles, especially in the case of Pd. The catalytic properties of electrodes decorated with such catalyst/support systems were investigated by linear sweep voltammetry and by rotating disk electrode measurements, revealing excellent stability and good activity toward oxygen reduction reaction (ORR). In particular, although Pd nanoparticles always result in lower activity than Pt ones, both Pt and Pd electrodes based on the N-doped supports show an increased activity toward ORR with respect to the undoped ones. At the same mass loading, the Tafel slope and the stability test of the Pt@N-doped electrocatalysts indicate superior performances to that of a commercial Pt@C catalysts (30 wt % Pt on Vulcan XC-72, Johnson Matthey).

Electrical conductance of single-molecular junctions formed with palladium electrodes

We measured the conductance of a series of amine-terminated oligophenyl molecular junctions formed with palladium (Pd) electrodes by using a scanning tunneling microscope-based break-junction technique. For comparison, we also used Au electrodes to form a metal-molecular junction. We found that the molecular junctions formed with Pd electrodes had higher conductances than those formed with gold (Au) electrodes through a statistical analysis and a conductance histogram. Furthermore, the measured conductance decayed exponentially with molecular backbone length with a tunneling decay constant that was essentially the same for Pd and Au electrodes, which is attributable to the molecules that conduct through the highest occupied molecular orbital and the higher work function of Pd compared with Au. Because our work allows measuring the conductance of a molecule formed with various metal electrodes to be measured, it should be relevant to molecular junctions with various materials for future organic electronics on a molecular scale.

Chemisorption and Electrochemical Activity of Thiophenols at Well-Defined Pd(111) Surfaces: Studies by LEED, AES, HREELS, and Electrochemistry

The chemisorption and electrochemical activity of 2 ,5 -dihydroxythiophenol (DHT) and 2-(8mercaptooctyl)-1,4-benzenediol (DHOT) on well-defined Pd(111) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energyloss spectroscopy (HREELS), and electrochemistry (EC). Results confirm that DHT is chemisorbed in two discrete orientations such that at low concentrations, DHT is oxidatively bound to the surface through the diphenol and mercapto groups as quinonoid and S moieties, respectively, whereas at high concentrations, the molecule is coordinated oxidatively through the –SH group in a vertical S- η 1

Nano-composite sensors composed of single-walled carbon nanotubes and polyaniline for the detection of a nerve agent simulant gas

This paper reports the response behavior at room temperature of a composite sensor composed of single-walled nanotubes (SWCNTs) and polyaniline, to the nerve agent simulant gas dimethyl-methyl-phosphonate (DMMP), a typical Sarin simulant. The SWCNT–polyaniline composite was synthesized to obtain high-quality composites with good uniformity. The composites were drop-cast onto an oxidized Si substrate patterned with Pd electrodes. SWCNT–polyaniline composite sensors exhibited clear, sharp response curves for DMMP in air at room temperature, even at minuscule concentrations. The response and response time were 27.1% and 5.5s, respectively, at 10ppm DMMP, representing a significant improvement over the pure SWCNT network sensors previously reported. These results indicate that SWCNT–polyaniline composite sensors can be ideal DMMP sensors, operating at room temperature with high response, fast response time, and excellent reproducibility.

Effect of Gold Deposition on the Activity of Palladium Electrode for Propane-1,3-diol Oxidation

Abstract The Au deposition on polycrystalline Pd electrode and the electrocatalytic oxidation of propane-1,3-diol on the Au-deposited Pd electrodes are investigated. Structure analyses show that the surface of Pd is partially covered by Au nanoparticles. Electrochemical measurements reveal a significant improvement in the catalytic activity of Pd electrode by Au deposition for propane-1,3-diol oxidation in alkaline solution. The Au-modified Pd electrodes show oxidation potential similar to Pd electrode but considerably higher peak current density than Pd and Au electrodes.

Glycerol Oxidation on Pd Electrodes Modified with Pt

Glycerol Oxidation on Pd Electrodes Modified with Pt