Kinetic studies of hydrogen electrode reaction on hetero-atomic junctions of bi-component electrodes

Fil: Brites Helú, Mariela Alicia. Universidad Nacional del Litoral. Facultad de Ingeniería Química; ArgentinaThis work studies the synergetic interactions between PGM catalysts (Pt, Rh) and Au over both branches of the Hydrogen Electrode Reaction (HER): the Hydrogen Oxidation Reaction (hor) and the Hydrogen Evolution Reaction (her) in acid and alkaline media. The electrodes analyzed have the particular feature of maximized lengths of the junctions between their two components under the following electrode configurations: supported nanoparticles ensembles and bi-metallic ordered arrays. The first systems were based on short-pulse sputtering of Pt and Au NPs onto HOPG, glassy carbon and metals supports. The second configuration was achieved by SECM micro-writing and mask-assisted electron beam lithography methods. Different and complementary electrochemical measurements were carried out to determinate the fundamental aspects of the junction contributions to the global electrocatalytic activity. Nanoparticles ensembles were analyzed by supporting them on microelectrodes and by scanning electrochemical microscopy. The ordered arrays were studied by mounting on rotating disc electrodes. The obtained results were analyzed using kinetic models based on rigorous treatments of the whole reaction mechanism. On the one hand, through the correlation of the experimental curves was possible to detect variations on the values of the kinetic parameters which could be caused by the interfaces. On the other hand, through the concept of excess electrocatalytic activity these changes were observed more directly. Pt and Rh modifie the performance of gold in the her and hor. In acid media the changes are associated with the electro-adsorption of H from H2 while in alkaline media the H-chemisorption from H2 is modified.La presente tesis doctoral aborda diferentes aspectos relacionados con la cinética de la reacción del electrodo de hidrógeno (HER) en un amplio rango de sobrepotenciales, incluyendo las ramas catódica (her) y anódica (hor). El análisis se focaliza en electrodos mixtos o bi-componentes con el fin de determinar la existencia de posibles procesos de interacción entre diferentes electrocatalizadores cuando forman parte de un mismo electrodo. Las configuraciones electródicas bi-componentes evaluadas (arreglos ordenados y aleatorios de nano- y micro-electrodos) presentan la característica única de maximizar la juntura o interfase entre ambos metales. Para obtener estos electrodos se emplearon los métodos de sputtering de pulsos cortos, de microscopía electroquímica de barrido (SECM) y de litografía con haz de electrones asistida por máscaras (MAEBL). Se combinaron metales nobles tales como Pt, Rh y Au y materiales carbonosos como grafito pirolítico altamente orientado (HOPG) y carbón vítreo. La correlación de las curvas experimentales corriente vs. sobrepotencial con modelos cinéticos rigurosos permitió detectar variaciones “aparentes” de los parámetros elementales causadas por el efecto de las junturas sobre estas respuestas. En segundo lugar, el uso del concepto de “actividad electrocatalítica de exceso” permitió observar de manera directa estos cambios. Se encontraron evidencias de que la juntura bimetálica Au/Pt u Au/Rh promueve la generación de una región de la superficie electródica, circundante a las mismas, que presenta una actividad electrocatalítica mayor que en su estado puro y viéndose afectados principalmente los procesos de electro-adsorción de H desde H2 y de quimisorción de H desde H2.Agencia Nacional de Promoción Científica y TecnológicaUniversidad Nacional del LitoralConsejo Nacional de Investigaciones Científicas y Técnica

Strategies for fabrication of highly-dense ordered arrays of metal microdisks by the scanning electrochemical microscopy microwriting approach

The microwriting approach of the scanning electrochemical microscopy (SECM) was adapted and optimized for the fabrication of extensive ordered arrays of gold microdisks deposited on conductive supports. This technique uses a SECM tip as a source for the localized delivery of metal ions by well-controlled electro-dissolution of the metal microelectrode. In this work, novel configurations of recessed-microelectrode tips (radii ≤ 12.5 μm) were developed to make possible the deposition of a large number of well-defined disks from the same probe by a semi-automatic procedure. A sequence of steps that coordinates tip potential, tip position, and substrate potential values was designed and implemented to improve the repeatability and confinement of the deposited microdisks. The effects of potential and time conditions for gold electro-dissolution and electrodeposition on the morphological properties of gold disks deposited on Pt and glassy carbon were explored. The selective modification of glassy carbon-supported gold arrays with an electrocatalytic material (Pt) for a specific reaction (hydrogen evolution) and their further SECM activity screening were described to illustrate a way to use them as multielectrode platforms.Fil: Brites Helú, Mariela Alicia. Universidad Nacional del Litoral; ArgentinaFil: Fernández, José L.. Universidad Nacional del Litoral; Argentin

Analysis of the hydrogen electrode reaction mechanism in thin-layer cells. 3. Study of hydrogen electro-oxidation by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) in the feedback mode was used to analyze the complete mechanism of the hydrogen oxidation reaction (hor). A general SECM equation was proposed to carry out the kinetic analysis of the dependences of the tip current (iT) on the substrate potential (ES) and on the tip-substrate distance (d). This SECM equation involves the contribution of a thin-layer cell (TLC) with no aprioristic restrictions about the reaction mechanism. A dependence of the TLC current on ES previously developed for the hor operating through the Tafel-Heyrovsky-Volmer mechanism was included in the proposed SECM model. The domain of conditions where the equation properly reproduces the SECM responses was determined by contrasting the calculated iT(ES, d) dependences with simulated curves obtained by a numerical method using varied kinetic and geometric parameters. The experimental conditions where the model can be used were explored by analyzing the hor on Pt. The present treatment provides a versatile tool for the application of SECM to the kinetic and mechanistic analysis of the hor, which can be easily adapted for the analysis of any multi-step electrocatalyzed reaction on infinite substrates.Fil: Brites Helú, Mariela Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; ArgentinaFil: Fernandez, Jose Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentin

Análisis de procesos sinérgicos en la reacción de oxidación de hidrógeno sobre electrodos bimetálicos mediante microscopía electroquímica de barrido (SECM)

Se analizó mediante SECM el mecanismo de la reacción de oxidación de hidrógeno (hor: H₂ 2H+ + 2e-) en los alrededores de las junturas bimetálicas Pt/Au de electrodos heterogéneos, a fin de detectar cambios mecanísticos causados por efectos sinergéticos locales. Para ello se incrementó la resolución mediante el uso de tips nanométricos que se obtuvieron cubriendo tips de punta cónica con polimetilmetacrilato, que durante el polimerizado se contrae dejando expuesto sólo el ápice metálico de unas pocos nanómetros. Los sustratos empleados fueron arreglos ordenados de microelectrodos de Pt sobre Au fabricados mediante litografía de electrones.Los experimentos SECM se realizaron en el modo feedback usando la cupla H+/H₂ como mediador. Las junturas fueron localizadas obteniendo imágenes de actividad del arreglo, y una vez ubicado el tip se analizó el mecanismo de la hor mediante curvas de polarización medidas a lo largo de la sección transversal de la juntura. Se detectaron cambios de las respuestas en la región intermetálica, siendo el más llamativo el corrimiento del hombro característico de la transición de rutas hacia menores sobrepotenciales, indicando que ocurre un aumento de la velocidad de la etapa de Heyrovsky en esta zona.Fil: Fernandez, Wanda Valeria. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Brites Helú, Mariela Alicia. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Fernandez, Jose Luis. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaXXIII Congreso de la Sociedad Iberoamericana de ElectroquímicaCuscoPerúSociedad Iberoamericana de Electroquímic

Electro-adsorption reactions and surface mobility of electro-adsorbed species sensed by scanning electrochemical induced desorption

A theoretical analysis of electro-adsorption reactions and of the surface diffusion of electro-adsorbed species based on scanning electrochemical microscopy (SECM) in the feedback mode, usually known as scanning electrochemical induced desorption (SECMID), is presented. Numerical simulations of the classical feedback process were carried out by including in the model a potential-dependent electro-adsorption reaction from the mediator at the substrate and allowing the adsorbed species (Aad) to diffuse over the substrate surface affecting the mediator loop. As in classical SECMID, the local variation of the mediator concentration underneath the tip causes a potential-dependent gradient of the Aad surface coverage at the substrate over the tip-affected region, which drives the Aad surface diffusion toward this area and the consequent positive feedback of mediator, reaching a steady state. The simulated steady-state dependences of the tip current (iT) on the substrate potential (ES) show the presence of a peak over the potential range affected by the electro-adsorption reaction, whose amplitude at a given tip-substrate distance is mostly influenced by the surface diffusion coefficient of Aad and the density of adsorption sites at the substrate. When this surface process is parallel to an electrode reaction of the mediator that proceeds over the same potential range, the adsorption/diffusion peak is overlapped with the typical Butler-Volmer type response of the electrode reaction, affecting the iT(ES) shape and interfering with the determination of kinetic parameters from this dependence. These phenomena were experimentally observed when using the H+/H2 mediator loop on Au and Pt, where Had is electro-adsorbed from H+, a process that in the case of Pt is parallel to the oxidation of the tip generated H2.Fil: Brites Helú, Mariela Alicia. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada E Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bonazza, Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico Para la Industria Química; ArgentinaFil: Fernandez, Jose Luis. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada E Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Ordered Array Electrodes Fabricated by a Mask-Assisted Electron-Beam Method as Platforms for Studying Kinetic and Mass-Transport Phenomena on Electrocatalysts

This work describes a method for fabrication of extensive ordered arrays of microelectrodes with varied geometries, surrounded either by an insulating surface of poly(methyl methacrylate) (PMMA) or by a conductive material such as gold or glassy carbon (GC). The method is based on procedures from electron beam lithography (EBL) but, in contrast to classic EBL, it can be applied by using widely available conventional SEM instruments that are not specifically tailored for EBL operation. The electron gun of the SEM is used to irradiate and modify a PMMA film that is covered by a micro- or nano-structured mask (i.e., a TEM grid), which is further selectively revealed. Each array can be evaluated in two configurations, when it is surrounded by the PMMA film, and when it is in contact with the exposed support after PMMA removal. The first configuration is useful to evaluate the electrochemical behavior of pure microelectrode arrays for correlating it with model equations. The second configuration is particularly useful when the substrate material by itself is inactive for the studied reaction. In the latter case, any detected differences between the electrochemical behavior of the PMMA-coated array and that of the bi-component array should come from the contributions of the microelectrode boundaries. These arrays were employed for studying the hydrogen oxidation reaction in alkaline medium on Au/Rh and on GC/Rh in order to detect possible kinetic interactions of both components at the heterojunctions.Fil: Brites Helú, Mariela Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Fernandez, Wanda Valeria. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Fernandez, Jose Luis. Universidad Nacional del Litoral; Argentin

Combining IR Spectroscopy and Monte Carlo Simulations to Identify CO Adsorption Sites on Bimetallic Alloys

The atomic distribution on the surface of alloys dictates the nature of the ensembles available as possible active sites during catalytic reactions. In the present work, an infrared spectroscopic study of carbon monoxide adsorption on the surface of AuPd/Pd(111) alloys, combined with Monte Carlo simulations of the surface and bulk atomic distribution, identifies the correct distribution of available surface adsorption sites. For gold coverages >0.9 monolayers (ML), CO adsorbs weakly on top of Au atoms and with higher adsorption energy on top of Pd atoms (CO top ), distributed mostly as monomers on the surface. For ? Au = 0.8-0.4 ML, Pd-CO top is the predominant species, even though several other sites with multiple coordination are available. The simulations show no perfect ordering of the surface but a slight tendency to form lines of Pd atoms, thus favoring the appearance of bridge but not 3-fold hollow sites. Using 13 CO: 12 CO isotopic mixtures, the frequency shifts due to chemical and intermolecular coupling effects has been determined for the CO top IR signal. These effects mostly cancel each other out, so that only small frequency shifts are seen, implying the presence of significant electronic/ligand effects. At ? Au < 0.5 ML, hollow sites are experimentally observed in agreement to the simulated model surfaces. Their IR absorption bands are tentatively distinguished as fcc and hcp hollow sites by correlating with the simulated distribution of Au and Pd atoms on subsurface sites, where for ? Au < 0.5 ML an enrichment by Au atoms is seen in the near-surface region.Fil: Manzi, Sergio Javier. Universidad Nacional de San Luis; ArgentinaFil: Brites Helú, Mariela Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Tysoe, Wilfred T.. University Of Wisconsin Milwaukee;Fil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentin

Nanoparticle ensemble electrodes: fabrication by short-pulse sputtering and characterization by scanning probe microscopy and voltammetry

Nanoparticle ensembles (NPEs) are useful platforms to study the electrocatalytic properties of nano-sized materials. A fabrication method that guarantees a good control over the morphological features, as well as a reliable characterization, are required for a proper analysis of electrochemical data measured on these electrodes. This work describes a simple, fast, and clean procedure for the fabrication of random arrays of Pt nanoparticles (NPs) supported on flat electrodes by sputtering of metal targets for short times. Besides, a strategy for morphological characterization of the ensembles based on a combination of scanning tunneling microscopy (STM) and voltammetric information is reported. The method permits to attain good control on the size and coverage of NPs in the ensemble through the sputtering parameters. Pt NPs sputtered on highly ordered pyrolytic graphite (HOPG) grow bidimensionally with heights in the range 0.8–1.2 nm, while the lateral diameters of NPs and the surface coverage are adjusted in the range 2–15 nm by controlling both the sputtering current and the sputtering time. The stability and electrochemical behavior for proton reduction under mixed control in acid media were evaluated on HOPG-supported Pt ensembles fabricated by the described method.Fil: Brites Helú, Mariela Alicia. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Gennero, Maria Rosa. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Chialvo, Abel Cesar. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Fernandez, Jose Luis. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentin