Methanol electrooxidation on platinum mesh electrodes bonded to solid polymer electrolytes

Methanol electrooxidation on smooth platinum electrodes bonded to solid polymer electrolytes was studied in water and acid solution by voltammetric measurements with different scanning rates. An enhancement of the oxidation rates was observed in these systems as compared to identical platinum electrodes in contact with liquid electrolytes. This electrocatalytic effect strongly depends on the measuring conditions and on the electrode potential. The reasons for the catalytic effects at different potentials are discussed. Received: 8 January 1997 / Accepted: 1 December 1997     

Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheres in alkaline media

Noble metal (Pt, Pd) electrocatalysts supported on carbon microspheres (CMS) are used for methanol and ethanol oxidation in alkaline media. The results show that noble metal electrocatalysts supported on carbon microspheres give better performance than that supported on carbon black. It is well known that palladium is not a good electrocatalyst for methanol oxidation, but it shows excellently higher activity and better steady-state electrolysis than Pt for ethanol electrooxidation in alkaline media. The results show a synergistic effect by the interaction between Pd and carbon microspheres. The Pd supported on carbon microspheres in this paper possesses excellent electrocatalytic properties and may be of great potential in direct ethanol fuel cells.     

Methanol dehydrogenation and oxidation on Pt(111) in alkaline solutions

Adsorption, dehydrogenation, and oxidation of methanol on Pt(111) in alkaline solutions has been examined from a fundamental mechanistic perspective, focusing on the role of adsorbate-adsorbate interactions and the effect of defects on reactivity. CO has been confirmed as the main poisoning species, affecting the rate of methanol dehydrogenation primarily through repulsive interactions with methanol dehydrogenation intermediates. At direct methanol fuel cell (DMFC)-relevant potentials, methanol oxidation occurs almost entirely through a CO intermediate, and the rate of CO oxidation is the main limiting factor in methanol oxidation. Small Pt island defects greatly enhance CO oxidation, though they are effective only when the CO coverage is 0.20 ML or higher. Large Pt islands enhance CO oxidation as well, but unlike small Pt islands, they also promote methanol dehydrogenation. Perturbations in electronic structure are responsible for the CO oxidation effect of defects, but the role of large Pt islands in promoting methanol dehydrogenation is primarily explained by surface geometric structure.     

Oxygen evolution on aged IrO x /Ti electrodes in alkaline solutions

O₂ evolution from 1 mol dm⁻¹ NaOH aqueous solution was studied on IrO _x /Ti electrodes already used for more than 3 years (aged). IrO _x was prepared by thermal decomposition of the chloride in the temperature range from 330 to 500 °C. Half of the electrodes were stored in air between experiments, the other half in water. The state of the electrode surface was monitored by recording voltammetric curves in a potential region prior to O₂ evolution before and after each group of experiments. O₂ evolution was studied by measuring quasistationary current–potential curves. Tafel slopes were derived using two different approaches. The reaction order with respect to OH⁻ was also determined and found to be fractional. Results show that the reaction mechanism does not depend on either the calcination temperature or the storage conditions. However, stability appears to be higher for electrodes calcined at higher temperatures and stored in air. Dedicated to Professor Algirdas Vaškelis on the occasion of his 70th birthday.     

Electrocatalytic evolution of hydrogen on amorphous Fe-Nb-B-Rare-Earth-Metal electrodes from alkaline solutions

Cathodic processes for electrochemical generation of hydrogen in aqueous KOH solutions in the presence of active electrodes made of Fe-Nb-B-based amorphous metallic alloys alloyed with rare-earth metals (Y, Gd, Dy) were studied. All the amorphous alloys exhibit a catalytic effect in the electrochemical evolution of hydrogen from 5 M aqueous KOH solutions. The electrocatalytic system with Fe₈₂Nb₂B₁₄Gd₂ and Fe₈₂Nb₂B₁₄Dy₂ electrodes exhibiting a prolonged autocatalytic effect can be recommended for practical application.     

Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst

The electrooxidation of methanol on a novel carbon supported PtRu electrocatalyst produced via colloidal PtRu precursors was investigated by thin-film-electrode (TFE) measurements and compared with commercially available Pt and PtRu catalysts. The PtRu-colloid-based catalyst shows similiar activity towards methanol oxidation as other conventional PtRu catalysts. A comparison with literature data from half-cell measurements at similiar mass-specific current densities clearly demonstrates the high potential of the colloid-based PtRu catalyst for fuel-cell applications.     

Hydrogel-based flexible micro-reference electrodes for use in alkaline and neutral pH solutions

Two types of nonbreakable, flexible micro-reference electrodes filled with gel-electrolytes were prepared for use in solutions with alkaline and neutral pH. The electrodes are intended for electrochemical measurements, in which chloride-free conditions are important. Due to the flexible, bendable construction of the electrodes, electrochemical experiments at locations difficult to access with common reference electrodes are enabled. Hg|HgO-type electrodes were prepared from amalgamated Au wires, followed by oxidation of the amalgam, which is mounted in a PTFE tube filled with 0.1M NaOH solution immobilized in a PAA-g-PEO gel. The potential of this type of electrode was found to be 0.162?±?0.002 V (SHE) at room temperature. Cu|CuSO₄ electrodes, consisting of a Cu wire immersed in a saturated CuSO₄ solution jellied with gelatin, showed a stable open-circuit potential of 0.312?±?0.001 V (SHE). Further characterization of the electrodes was performed in terms of electrochemical impedance spectroscopy and micro-polarization measurements. As an alternative to the flexible electrodes, rigid electrodes in glass enclosure were fabricated in analogy to the flexible-type electrodes.     

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

The electrooxidation of glucose on platinum electrodes in buffered media

Kinetic studies of glucose electrooxidation are of special interest in fundamental and applied electrochemistry. The electrochemical reaction on bright Pt electrodes is investigated through the application of combined potential/time perturbation programs and electrolytes of different compositions at temperatures ranging from 30 to 64°C. The electrooxidation of glucose should occur through the formation of several intermediate species and the overall reaction pathway should consist of different stages considering electroadsorption, chemical and electrochemical processes. The influence of potential perturbation conditions, anion adsorption, inhibition reactions of products and temperature are analysed through changes in U/I profiles.     

Ethanol electrooxidation on Pd/C nanoparticles in alkaline media

Carbon-supported Pd nanoparticles were prepared by microwave heating-glycol reduction method, and characterized by a wide array of experimental techniques including X-ray diffraction spectroscopy(XRD) and transmission electron microscopy(TEM). The electrooxidation behaviors of ethanol on the Pd/C electrode in alkaline media were investigated using cyclic voltammetry(CV), chronoamperometry(CA), electrochemical impedance spectroscopy(EIS) and single cell performance methods. Pd/C electrode for ethanol oxidation showed high electro-catalytic activity and long term stability. However, it is observed that the current density decreases with the increasing of the potential and negative impedance presents in the potential from-0.1 to0.1 V. The decreasing current density and the negative impedance could be due to the adsorbed intermediates species that inhibited the further oxidation of ethanol. Based on the chemical reaction analysis and EIS spectra, equivalent circuits relating to various potential zones have been obtained. These results reveal the dynamic adsorption of intermediates species on Pd surfaces. Significantly, it is clarified that the adsorption behavior begins from the maximum catalysis of electro-catalysis and ends in the formation of the palladium(II) oxide layer on the electrode surface.     

Recent progress in electrochemical oxidation of saccharides at gold and copper electrodes in alkaline solutions

This article reviews the progress made in the past 10 years, on electrochemical oxidation of saccharides in alkaline media for gold and copper electrodes. The mechanism and processes associated with the electrochemical oxidation of saccharides at native and surface coated electrodes continues to be of great interest. Despite the effort and various mechanisms proposed, still the need for an electrochemically active material that understands the complexity associated with saccharides continues to increase as their detection poses a challenge for bioanalytical chemistry and liquid chromatography.     

Voltammetric and in situ infrared spectroscopy studies of hydroxyurea electrooxidation at Au(111) electrodes in HClO4 solutions

The oxidation of hydroxyurea (H₂NCONHOH, HU) at Au(111) single crystal and Au(111)-25 nm thin film electrodes is studied spectroelectrochemically in HClO₄ solutions using external reflection infrared and Surface Enhanced Infrared Reflection Absorption Spectroscopy under Attenuated Total Reflection conditions (ATR-SEIRAS). The in situ external reflection spectra prove the formation of dissolved carbon dioxide and adsorbed cyanate during the electrooxidation of HU. The enhanced surface sensitivity of the ATR-SEIRAS experiments facilitates the detection of adsorbed cyanate and allows the observation of other adsorbate bands tentatively ascribed to reaction intermediates involving partial oxidation of the HU molecules, some of them corresponding to nitrosyl groups.     

Methanol electrooxidation on novel modified carbon paste electrodes with supported poly(isonicotinic acid) (sodium dodecyl sulfate)/Ni-Co electrocatalysts

Poly(isonicotinic acid) (PINA) was formed by successive cyclic voltammetry in monomer solution in the presence of sodium dodecyl sulfate (SDS) on the surface of a carbon paste electrode (CPE). Ni(II) and Co(II) ions were incorporated into the electrode by immersion of the polymer-modified electrodes in Ni(II) and Co(II) ion solutions in different proportions. After the preparation of modified electrodes, their electrochemical behavior was studied by cyclic voltammetric experiments. Electrocatalytic oxidation of methanol at the surface of the modified electrodes was studied in 1?M NaOH solution. These modified electrodes exhibit high electrocatalytic activity and stability in alkaline solution, showing oxidation peaks at low potentials with high current densities. The electrooxidation of methanol was found to be more efficient on CPE/PINA(SDS)/Ni₈₀Co₂₀ than on CPE/PINA(SDS)/Ni and CPE/PINA(SDS)/Ni₅₀Co₅₀. The effects of various parameters such as scan rates and methanol concentration on the electrooxidation of methanol are also investigated.     

Electrochemical reduction of oxygen on double-walled carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions

The oxygen reduction reaction has been investigated on double-walled carbon nanotube (DWCNT) modified glassy carbon (GC) electrodes in acid and alkaline media using the rotating disk electrode (RDE) method. The surface morphology and composition of DWCNT samples was examined by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Aqueous suspensions of DWCNTs were prepared using Nafion and non-ionic surfactant Triton X-100 as dispersing agents. The RDE results indicated that the DWCNT modified GC electrodes are active catalysts for oxygen reduction in alkaline solution. In acid media DWCNT/GC electrodes possess poor electrocatalytic properties for O₂ reduction which indicates lack of metal catalyst impurities in the DWCNT material studied. The oxygen reduction behaviour of DWCNTs was similar to that of multi-walled carbon nanotubes (MWCNTs) observed in our previous studies.     

Phenomenological aspects related to the electrochemical behaviour of smooth palladium electrodes in alkaline solutions

The voltammetric behaviour of smooth palladium electrodes in 1 M NaOH is studied in the potential range related to the thermodynamic stability of water. The electrosorption of H atoms on bulk Pd appears as a reversible reaction coupled to a diffusion process which occurs within bulk Pd. The voltammetric electrodesorption of H from bulk Pd is a process under mixed control, i.e. the diffusion from the bulk and the surface oxidation of H atoms. Fast pseudocapacitive reactions are detected in the range 0.2–0.4 V associated with the adsorption of H atoms at the submonolayer level. The initial stages of Pd oxide layer formation, at ca. 0.68 V, involves two reversible stages. The Pd oxide monolayer formation is achieved at 1.25 V/RHE and is followed by the formation of a third reversible system. This system is enhanced by an excursion in the potential range of the oxygen evolution reaction. This reversible system is probably a redox system involving Pd(II)/Pd(IV) species. The voltammetric electroreduction of the Pd oxide film shows rather irreversible behaviour. Inhibition effects on the reversible adsorption of H atoms due to residual oxide species were observed as well as inhibition on loading the Pd electrode with hydrogen to form the (α + β)-PdH phase. Rotating ring-disc experiments demonstrate that Pd electrodissolution in basic solutions is much smaller than in acid solutions. However, soluble palladium species are detected, especially during the formation of the fast redox systems, in the potential range related to Pd oxide layer growth.     

Cuboctahedral Pd nanoparticles on WC for enhanced methanol electrooxidation in alkaline solution

We report cuboctahedral Pd nanoparticles on WC synthesized by the polyol process using ethylene glycol with NO(3)(-) and Fe(3+)/Fe(2+) ions. The cuboctahedral Pd/WC shows much improved electrocatalytic activity for methanol electrooxidation in alkaline solution.     

Hydride formation at Ni-containing glassy-metal electrodes during the H2 evolution reaction in alkaline solutions

A group of Ni-containing glassy metal alloys that are of interest as electrocatalysts for cathodic hydrogen production in water electrolysis have been examined with regard to the role of hydride formation and the participation of sorbed H during cathodic H₂ evolution in alkaline solutions. Hydride formation is important as it may determine, in part, the electrocatalytic properties of the alloys at their surfaces, for cathodic H₂ evolution. Characteristic time-dependences of currents observed near, but negative to the H₂ reversible potential in response to a stepwise change of potential, together with potential-relaxation measurements on open-circuit following current interruption, lead to the conclusion that a three-dimensional hydride is formed in the near-surface region of the metals during the H₂ evolution process. Complementary cyclic-voltammetry experiments show that hydrogen deposition-absorption, or H oxidation processes, can be distinguished from possible spontaneous oxidation of the glassy metals, over somewhat overlapping potential regions.     

Pulsed amperometric detection of sulfur compounds: Part I. Initial studies of platinum electrodes in alkaline solutions

Conventional voltammetric and amperometric techniques at Pt or Au electrodes have not been considered applicable for quantitative detection of most sulfur compounds. This is the result of the observed loss of electrode activity caused by accumulation of sulfurous adsorbates and surface oxides. Pulsed Amperometric Detection (PAD) at Pt and Au is highly sensitive for the anodic detection of sulfur compounds, organic and inorganic, which adsorb on the electrode surface. The detection of thiourea by PAD at Pt is described here, and is concluded to correspond to the surface-oxide catalyzed oxidation of the sulfur moiety of adsorbed thiourea to sulfate. Results reported here for detection in a flow-injection system (FI/PAD) produce linear calibration plots of I_p vs. c^b at low concentration and linear plots of 1/I_p vs 1/c^b at high concentrations. The detection limit for thiourea is 0.38 ppm (i.e., ca 17 ng/45 μl sample) for the FI system used.     

Uncovering the electrooxidation behavior of 5-hydroxymethylfurfural on Ni/Co electrodes

Biomass, derived from plant photosynthesis that captures carbon dioxide to form carbohydrates, offers vast renewable reserves. The electrooxidation of biomass, coupled with the hydrogen evolution reaction,enables the simultaneous production of biomass-based plastic monomers and green hydrogen, attracting significant scholarly interest. However, ambiguity remains regarding the adsorption mechanism at the catalyst surface(Langmuir-Hinshelwood or Eley-Rideal) and the adsorbed substrate groups. To a...