Giant multilayer electrocatalytic effect investigation on Pt/Bi/Pt nanostructured electrodes towards CO and methanol electrooxidation

This work describes CO and methanol electrooxidation over Bi/Pt 1.2:1.2 or 10:10 ML electrodeposited on a bulk platinum substrate. It could be observed in a blank solution that the same features for bulk Pt and Pt/Bi/Pt MM and the hydrogen region were overlaid. The electroactive areas, calculated using the hydrogen desorption method, are the same for both bulk Pt and Pt/Bi/Pt MM electrodes. This is in agreement with AFM images and RMS values, which were the same for bulk Pt and Pt/Bi/Pt MM electrodes. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 138 and 197 mV for Pt/Bi/Pt 10:10 and 1.2:1.2 ML, respectively, in comparison to bulk Pt. Moreover, for methanol electrooxidation the Pt/Bi/Pt 1.2:1.2 ML electrodes presented an enhancement of 315% and 22 times in the peak current density compared to bulk Pt using cyclic voltammetry and chronoamperometry techniques respectively. Using electrochemical impedance spectroscopy, it was possible to observe the lowest resistance charge transfer for Pt/Bi/Pt 1.2:1.2 ML compared to Pt/Bi/Pt 10:10 ML and bulk Pt respectively. We designate the Pt/Bi/Pt MM electrodes as a giant multilayer electrocatalytic (GME) system, due to their enhanced electrocatalytical properties.     

Electrooxidation of methanol on polycrystalline and single crystal gold electrodes

Oxidation of methanol has been investigated on polycrystalline and single crystal gold electrodes: Au(poly), Au(1 1 1) and Au(2 1 0), in acidic, neutral and alkaline solutions. As expected, catalytic activity of gold towards methanol oxidation increases with increasing pH of the solution. It has been found that in all studied solutions methanol is oxidised in two potential regions, prior to gold surface oxide monolayer formation and in more positive potentials, on gold surface oxide after so called “turn over”. Surface structure of the electrode has little influence on the oxidation current, however potentials at which oxidation is observed depends on the crystallographic orientation. The mechanism of electro-oxidation of methanol on gold is discussed.     

Dependence of low-potential CO electrooxidation on the number of Pt monolayers on gold

The Pt-CO system is the only one in which a mere change of the potential at which the electrode is held while the compound to be oxidized is introduced in the cell dramatically affects the activity of the electrode. So, if the Pt electrode is held at a potential in the hydrogen region while CO is bubbled in the electrolyte, electrooxidation of dissolved CO occurs at a potential of 0.6 V vs the relative hydrogen electrode, which is considerably lower than the usual one of 0.9 V at which CO is oxidized when bubbled at open circuit. We have studied the dependence of this unique phenomenon on the number of Pt monolayers (MLs) deposited on Au. The peak potential in CVs of CO-stripping decreased with increasing number of Pt MLs, showing an increasing activity of the Pt film for CO oxidation. This is in agreement with the d-band model of Nørskov, which predicts that CO binds more strongly to a Pt ML on Au than to bulk Pt. Other contributing factor to the increasing activity of the Pt film with increasing number of Pt MLs is the increasing number of Pt atoms in step and kink positions, known to be active centres for the oxidation of both adsorbed and dissolved CO. This second factor is inferred from the monotonic increase of the hydrogen charge, which indicates that the second and successive MLs become increasingly rougher. As for the electrooxidation at low potentials of dissolved CO, it was completely absent with one Pt ML, and progressively emerged with increasing number of Pt MLs, a well-defined peak appearing with four MLs. The parallel evolution of the peak of dissolved CO oxidation and of the pre-peak in CO-stripping CVs confirms that dissolved CO oxidation takes place only on those Pt atoms that have become free of adsorbed CO by its oxidation in the pre-peak.     

Polythiophene, polyaniline and polypyrrole electrodes modified by electrodeposition of Pt and Pt+Pb for formic acid electrooxidation

The electrosynthesis of polythiophene (PTh), polyaniline (PANI) and polypyrrole (PPy) films modified by dispersion of Pt or Pt+Pb and its employment in the electrocatalytic oxidation of HCOOH are studied and compared. The influence of parameters such as polymer film thickness, the number of dispersed Pt particles, the amount of Pb deposited and the presence of Pb2+ in the electrolyte on the electrooxidation of HCOOH is investigated. Electrode systems including the polymer and a mixture of Pt and Pb particles show a better electrocatalytic activity than electrodes having a polymer–Pt combination or bulk Pt electrodes. Furthermore, during the electrooxidation of HCOOH using polymer–(Pt+Pb) electrodes the presence of fewer poisoning species is observed, indicating that the role of Pb in these electrode systems is in agreement with the Pb adatom effect observed when bulk Pt electrodes are used. However, the presence of Pb(ii) in the electrolyte is not required for the PTh–(Pt+Pb) electrode system and, in addition, a better electrocatalytic effect is obtained in this case. With application of an appropriate E/t program the activity is unchanged over a long time.     

Porous-microelectrode study on Pt/C catalysts for methanol electrooxidation

We have developed a porous-microelectrode (PME) to investigate the electroactivity of catalyst particles for proton exchange membrane fuel cells. The cavity at the tip of the PME was filled with Pt/C catalysts prepared by impregnation method. Cyclic voltammograms (CVs) recorded in 1 N H₂SO₄ aqueous solution revealed that the active area of the stacked catalysts exist not only at the surface but also inside of the stack. For methanol electrooxidation, 30 wt.% Pt/C exhibited the highest electroactivity, whereas the 50 wt.% Pt/C showed extremely small current. The small current is considered as a result of a small active-surface area. Methanol oxidation peak potential shifted toward cathodic direction as Pt-loading decreased, which agrees well with the Pt-oxide formation potential. The activation energy for methanol oxidation was assessed to be 44±3 kJ mol⁻¹ for all Pt/C catalysts and Pt-disc electrode.     

Electrochemical characterization of the electrooxidation of methanol,ethanol and formic acid on Pt/C and PtRu/C electrodes

Methanol, ethanol and formic acid electrooxidations in acid medium on Pt/C and PtRu/C catalysts were investigated. The catalysts were prepared by a microwave-assisted polyol process. Cyclic voltammetry and chronoamperometry were employed to provide quantitative and qualitative information on the kinetics of methanol, ethanol and formic acid oxidations. The PtRu/C catalyst showed higher anodic current densities than the Pt/C catalyst and the addition of Ru reduced the poisoning effect.     

Laser irradiation of ZnO:Al/Ag/ZnO:Al multilayers for electrical isolation in thin film photovoltaics

Laser irradiation of ZnO:Al/Ag/ZnO:Al transparent contacts is investigated for segmentation purposes. The quality of the irradiated areas has been experimentally evaluated by separation resistance measurements, and the results are complemented with a thermal model used for numerical simulations of the laser process. The presence of the Ag interlayer plays two key effects on the laser scribing process by increasing the maximum temperature reached in the structure and accelerating the cool down process. These evidences can promote the use of ultra-thin ZnO:Al/Ag/ZnO:Al electrode in large-area products, such as for solar modules.     

Pt supported on highly graphitized lace-like carbon for methanol electrooxidation

A simple solvothermal method has been used to synthesize highly graphitized lace-like carbon (GLC) using ethanol as the carbon source and Mg as reducing agent. The GLC is characterized by transmission electron microscopy, X-ray diffraction, N₂ adsorption, Raman spectroscopy and electrochemical techniques. The GLC synthesized at optimized conditions shows interlaced structure with an average thickness of 3 nm. Platinum on GLC electrocatalysts were prepared for methanol oxidation in acidic media for the first time. They show extremely higher activity for methanol oxidation compared to Pt/C electrocatalyst for the same Pt loadings. GLCs act as structural units to form mesopores and channels in the catalyst layers, which lead to the increase of the electrochemical active surface area and improvement in the mass transport by reducing the liquid sealing effect.     

Au/Pt nanoparticle systems in methanol and carbon monoxide electroxidation

Different Au/Pt bimetallic systems have been synthesised by following the approach suggested by Brust. The nanoparticles have been anchored to glassy carbon surface through a place-exchange reaction involving dithiol molecules. The resulting modified electrode consists of heterogeneous nanostructured Au and Pt patchwork. The different nanoparticles systems developed have been employed for the electroxidation of methanol and carbon monoxide in alkaline aqueous media. The results show that the electrocatalytic activity of the bimetallic systems is enhanced with respect to the single monometallic NP systems.     

Electrocatalytic properties of NDGA and NDGA/FAD hybrid film modified electrodes for NADH/NAD redox reaction

The fabrication of monolayers composed of nordihydroguaiaretic acid (NDGA), and hybrid films composed of NDGA-flavin adenine dinucleotide (FAD) adsorbed films was performed in neutral aqueous solutions to produce electrochemically active thin films exhibiting one and two redox couples, respectively. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA and hybrid NDGA/FAD film monolayers. The NDGA modified film electrocatalytically oxidized NADH, ascorbic acid, dopamine, and N₂H₄ in neutral aqueous solutions. Well-separated voltammetric peaks were observed for dopamine and uric acid mixtures, and also for ascorbic acid and uric acid mixtures using the NDGA/GC modified electrode. When transferred to various aqueous buffered solutions, the two redox couples of the NDGA/FAD hybrid film and their formal potentials were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD⁺ by a NDGA/FAD hybrid film in neutral aqueous solutions was carried out, and the electrocatalytic oxidation of NADH was performed using a NDGA/FAD hybrid film.     

Nitrate reduction on single-crystal platinum electrodes

The structure sensitivity of the reduction of nitrate has been studied on a series of single-crystal platinum electrodes by cyclic voltammmetry and in situ FTIRAS in sulfuric and perchloric acid solutions. The nitrate reduction is a structure-sensitive reaction on single-crystal platinum electrodes. However, this structure sensitivity is essentially controlled by other species (hydrogen, sulfate) that interact strongly with the electrode surface rather than by a structure-sensitive nitrate adsorption, dissociation or reduction.Voltammetric and spectroscopic data point to adsorbed nitric oxide (NO) as the main stable intermediate of the nitrate reduction to ammonia. No evidence for the formation of N₂O was found. On surfaces with sufficiently wide (1 1 1) terraces in the absence of specifically adsorbed sulfate, an oxidizable nitrate reduction product is detected voltammetrically, which may tentatively be attributed to the formation of a small amount of hydroxylamine earlier during the voltammetric scan.     

Electrochemical characterization of SnO2 electrodes doped with Ru and Pt

Antimony-platinum doped tin dioxide electrodes supported on titanium have been prepared by thermal decomposition. The effect of the progressive replacement of Sb with Ru (x = 0.00; 3.25; 6.50; 13.00 at.%) on their electrochemical response in acid medium has been analysed by cyclic voltammetry. The morphology of the coatings was observed by scanning electron microscopy. Ti/SnO₂-Sb-Pt electrodes without Ru presented a cracked-mud structure, typical of oxide electrodes prepared by thermal decomposition. The introduction of Ru in the oxide layer modified the coating morphology. The roughness increased and passed through a maximum with the increase of Ru content. A relation between the surface morphology, the roughness factor, voltammetric charge and the electrochemical activity has been established. The mechanism and electrocatalytic activity towards the oxygen evolution reaction has been studied from Tafel measurements. The progressive introduction of Ru in the electrodes increased their electrocatalytic activity for the oxygen evolution reaction with a change on the mechanism from non-active to active electrodes. The electrocatalytic activity mainly depends on electronic factors.     

Electrochemical preparation of epinephrine/Nafion chemically modified electrodes and their electrocatalytic oxidation of ascorbic acid and dopamine

Two types of epinephrine and cyclized epinephrine quinone films have been prepared using cyclic voltammetry from the epinephrine in the strong acidic solutions and neutral aqueous solutions over different scanning potential ranges. The cyclic voltammogram of the epinephrine film is characterized by one redox couple at about +0.5 V (versus Ag|AgCl) and cyclized epinephrine quinone film exhibits one redox couples at about −0.15 V (versus Ag|AgCl) .In addition to cyclic voltammetry and an electrochemical quartz crystal microbalance (EQCM) were used to study the growth mechanism of the epinephrine and cyclized epinephrine quinone molecules. The electrocatalytic oxidation of catecholamines (dopamine and norepinephrine) and also ascorbic acid were investigated in acidic aqueous solutions using epinephrine films. The rotating ring-disk electrode technique was used to investigate the mechanism of electrochemical oxidation of dopamine and ascorbic acid.     

Electrochemical behaviour and physical properties of Cu/Co multilayers

Cobalt/copper multilayer formation was analysed over different substrates in order to control the bilayer thickness and quality of coatings. Using a sulphate-citrate bath, at pH 4.7, electrodeposition conditions were optimised to prepare good alternate Cu/Co layers with high efficiency, by minimising the oxidation of cobalt layers during copper deposition. For multilayers with cobalt and copper layers of several nanometers, direct observation by scanning electron microscopy (SEM) or tapping mode-atomic force microscopy (TMAFM) gives layer thickness in agreement with the value calculated from deposition charges. Therefore, this calculation has been used to estimate the thickness of the thinner layers for which direct observation was difficult or impossible. A relation between the response of different characterisation methods was found. When the thickness of each layer in Cu/Co multilayers is >1.8-2 nm, potentiodynamic stripping experiments show separate peaks related to cobalt and copper oxidation. In these conditions, magnetoresistance of the coating is low. For thinner layers, the stripping response shows overlapping peaks or even a single oxidation peak. In these conditions the magnetoresistance begin to increase.     

Characterizing electrocatalytic surfaces: Electrochemical and NMR studies of methanol and carbon monoxide on Pt/C

We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a ¹³CO-adlayer prepared by electrochemical adsorption from low concentration ¹³CH₃OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed ¹³CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface.     

Carbon monoxide electrooxidation on porous Pt–Ru electrodes in sulphuric acid

CO electrooxidation on a Pt–Ru/C catalyst was investigated in sulphuric acid electrolyte. The physico-chemical properties of the Pt–Ru/C catalyst were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of temperature, CO partial pressure and proton concentration on the electrochemical oxidation rate was investigated by steady-state galvanostatic polarization measurements. The apparent activation energy decreased from 70 to 30kJmol–1 as the overpotential increased from 0.5 to 0.9V vs NHE. The reaction order with respect to carbon monoxide increased, passing from 0.4 to 1, with the increase of the overpotential from 0.5 to 0.7V vs NHE; a reaction order close to –1 with respect to the protonic concentration was observed, irrespective of the potential. Tafel slopes of about 136mVdec–1 were determined for oxidation of CO and CO/N2 mixtures.     

Electrocatalytic activity of Pt/C catalysts for methanol electrooxidation promoted by molybdovanadophosphoric acid

A Pt/C catalyst modified by the Keggin-structure molybdovanadophosphoric acid (PMV) is prepared by cyclic voltammetry and the modified Pt/C catalyst is studied for methanol electrooxidation. The results show that the PMV modified Pt/C catalyst has increased the electron transfer coefficient of the rate-determining step and diminished the adsorption of CO on Pt/C catalysts. Significant improvements in the catalytic activity and stability for methanol electrooxidation are observed, and it indicates that the PMV combined with Pt/C catalyst can be considered as a good electrocatalyst material for potential application in direct methanol fuel cells.     

The effect of CeO2 on Pt/CeO2/CNT catalyst for CO electrooxidation

Pt/CeO₂/CNT catalysts were prepared by adsorbing Pt nanoparticles on the supports of CNTs coated with CeO₂. The electrocatalytic performances in respect to the electrooxidation of chemisorbed CO were tested using potential step and stripping voltammetry methods under variable sweep rate and temperature conditions. At 10 mV s^–1, the CO stripping voltammogram exhibited the peak splitting phenomenon. The oxidation charge and the peak potential of the two voltammetric peaks changed regularly with the number of Pt and CeO₂ neighbours, the sweep rate, and the temperature. We considered that the low potential peak originated from the reaction of CO_ads with hydroxyl groups on CeO₂ adjacent to Pt sites, while the high potential peak came from the reaction of CO_ads with hydroxyl groups produced on pure Pt. Furthermore, the experimental results of the peak potential against the logarithm of the sweep rate and the logarithm of the current maximum time against the step potential were plotted and intersecting lines with different slopes in high and low potential regions in the plot were observed. The lines intersected at lower potentials on the Pt/CeO₂/CNT electrode than on the Pt/CNT electrode, which was attributed to the contribution of hydroxyl groups on CeO₂.     

Electrodeposition of Co-Cu-Zn/Cu multilayers: influence of anomalous codeposition on the formation of ternary multilayers

Electrodeposited Co-Cu-Zn/Cu multilayers have been prepared with various control modes. It was found in all cases that the Zn content of the magnetic layer is higher than that in the corresponding dc-plated sample. The data of the composition analysis were elucidated by the assumption of anomalous codeposition of Zn and Co. When the less noble layer is deposited, a Zn-rich zone is produced first, then it is covered with a Co-rich layer. If exchange reaction was allowed during the pulse-plating, Co dissolved selectively while the Zn content of the Co-rich layer remained unchanged. The relative resistance of Zn against the exchange reaction is due to the limited accessibility of the Zn-rich zone of the sample rather than electrochemical reasons. The structure of the deposit was analyzed by X-ray diffraction (XRD). It was revealed that Zn incorporation leads to strongly textured deposits with (1 1 1) orientation. The Zn atoms also enlarge the lattice distances of all phases formed. The incorporation of Zn into the magnetic layer of the multilayers decreases the magnetoresistance of the samples.     

Electrochemical sensors based on electrodes modified with synthetic hydrotalcites

The behaviour of electrodes modified with conductive hydrotalcites is reported as far as their electrochemical and electrocatalytic properties in basic solutions are concerned. In particular, cobalt and nickel based materials are compared. Pt electrodes, modified with a film of Ni/Al-NO₃ or Co/Al-NO₃ hydrotalcite, obtained by direct electrosynthesis, have been evaluated as amperometric sensors for oxidizable substrates in flow systems. Mono and polyhydric compounds have been analysed to compare the performances of the two sensors. The experimental conditions to obtain the best current signal in terms of reproducibility and sensitivity have been established as to the working and activation potential of the electrode, concentration and flow rate of the carrier solution, volume of the solution to be injected. The Co based HT is not responsive to monohydric compounds, and shows a sensitivity worse than the Ni based HT. The modified electrodes are simple and rapid to prepare, and can operate continuously in flowing solutions for 1-2 days at least.