Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochem. reactivity of B doped diamond (BDD) electrodes. A comparative study was carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown B doped diamond electrodes. The effect of doping a pure Csp3 diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. To establish the effect of doping on a pure Csp3 diamond electrode, the amt. of graphitic domains was increased by HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp2 contribution of the electrode. The presence of graphitic domains can be correlatively assocd. with the presence of kinetically active redox sites. The electrochem. reactivity of B doped diamond electrodes shows a distribution of kinetic consts. on the whole surface of the electrode corresponding to different active sites. The authors have studied by cyclic voltammetry and electrochem. impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochem. pulse cleaning procedure is proposed to remove them. [on SciFinder (R)]

Electrochemical promotion of Pt catalysts for gas phase reactions

Arnaud Jaccoud

The subject of this work is the study of the electrochemical promotion of catalysis (EPOC), also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). In this phenomenon, application of small currents or potentials on catalysts in contact with solid electrolytes leads to very pronounced strongly non-Faradaic and reversible changes in catalytic activity and selectivity. This work concerns the system composed of platinum film electrodes, deposited on the YSZ electrolyte (ZrO2 doped with 8% Y2O3), in contact with an oxygen containing atmosphere (the O2(g),Pt/YSZ system). The aim of this work was to focus on two points in the phenomenon of EPOC: 1) The nature of the species present on the surface of the catalyst and their impact on the catalytic activity. 2) The phenomenon of permanent promotion of catalysis (P-EPOC), for which the catalytic rate enhancement is not totally reversible, leading to a long lasting activated state. The Pt electrodes were prepared via three different methods (sputtering, thermal decomposition and screen-printing) and, as expected, the resulting microstructure, observed by scanning electron microscopy (SEM), was found to depend greatly on the preparation technique. The electrochemical response of each electrode was characterized by the technique of cyclic voltammetry. Due to the occurrence of different site-related electrochemical reactions, the voltammogram shape was highly influenced by the microstructure of the Pt/YSZ contact. Based on the preliminary studies, the cermet electrode was chosen for additional electrochemical experiments, using chronoamperometry, chronopotentiometry, cyclic voltammetry, steady state polarization and impedance spectroscopy. The results of all these techniques suggest that under anodic polarization, two parallel reactions take place. The first reaction is the oxygen evolution, giving rise to a steady state current under a constant applied potential. The other reaction is the oxygen storage (oxygen injection) in the form of Pt-O species. This second process gives rise to a time dependant current under a constant applied potential, and is therefore responsible for the pseudocapacitive behavior of the electrode. Moreover, linear sweep voltammetry measurements indicated that, by application of an anodic potential, at least three types of Pt-O species were stored, following distinct kinetics. Based on the amount of stored Pt-O and on the corresponding storage kinetics, they were attributed to three different locations on the electrode: 1) at the Pt/YSZ interface, 2) diffusing from the tpb toward the Pt/gas interface, 3) diffusing from the Pt/YSZ interface toward the bulk of the platinum electrode. It has been shown that the Pt film exhibited various activity states toward the catalytic oxidation of ethylene, depending on the oxygen species present at its surface. As evidenced by XPS measurements and in agreement with the thermodynamic data for the given system, the inactive (deactivated) state has been attributed to the formation of surface platinum oxide at T > 550°C. Decreasing the reaction temperature to 525°C caused the slow spontaneous reactivation of the catalyst and several days were necessary to reach the active state, where most of the catalyst was present in its metallic state. The catalyst activity recovery could be accelerated by the application of anodic potential pulses. In agreement with the mechanism of EPOC, this was explained by the oxide destabilizing effect of the anodically produced Oδ- species. Anodic polarization also caused reversible electrochemical promotion of catalysis at unusual high temperature (600°C). In addition, very intriguing catalytic relaxation transients were observed after prolonged anodic polarization times. Indeed, it was found that enhanced catalytic activity could be maintained for up to 10 hours after current interruption. Combination of these latter results, the related literature, the state of the art model of EPOC, and the results obtained by electrochemical techniques led to the proposition of an original model based on the processes of Pt-O storage/release at various locations of the O2(g),Pt/YSZ system. According to this model, anodic polarization produces Oδ- back-spillover species, promoting the catalytic activity, in agreement with the mechanism of EPOC. In parallel, hidden oxygen species are stored at the Pt/YSZ interface and diffuse toward the Pt phase. When the polarization is switched off, these hidden oxygen species diffuse to the gas-exposed surface and cause non-Faradaic promotion, as back-spillover Oδ- do. The large amount of stored charge and its slow diffusion controlled emergence causes the rate enhancement to last for hours.

EPFL2007

Electron Transfer Kinetics on Boron-Doped Diamond Part I: Influence of Anodic Treatment

Christos Comninellis, Ilaria Duo

Boron-doped diamond electrodes, both as-grown and polarized anodically under different conditions, were prepd. in order to study the chem. and electrochem. changes of diamond and clarify the role played by the surface-state d. Many different treatments were employed: as-grown (BDDag), mildly polarized (BDDmild), strongly polarized in perchloric acid (BDDsevere), and strongly polarized in a sulfuric acid-acetic acid mixt. (BDDAcOH). Charge transfer processes at the electrode surface were studied by cyclic voltammetry. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (FeIII/II;(CN)6) and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H2Q) redox reaction were chosen to test the electrochem. properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The active surface area and the reaction rate consts. decreased significantly upon anodic polarization. Important drops in the charge carrier concn. on the surface and in true surface area led to hindrance of electron transfer at the electrode. [on SciFinder (R)]

2004

Control of electron transfer kinetics at boron-doped diamond electrodes by specific surface modification

Ilaria Duo

Diamond films with different levels of doping have been the subject of applications and fundamental research in electrochemistry, opening up a new branch known as the electrochemistry of diamond. The electrochemical properties of diamond mostly depend on the operating conditions during the deposition of the film and on the treatment of the surface. The absence so far of a standard procedure in the production and treatment of diamond films has created a wide range of diamond quality and properties. This work would try to clarify some aspects of the electrochemical activity of boron-doped diamond electrodes in light of the results obtained in our and other laboratories. Boron-doped diamond (BDD) electrodes, both as-grown and polarised anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the rule played by the surface-state density. Many different treatments were employed, but only the most relevant results will be presented in this work, viz., those obtained with four kinds of surface treatment: as-grown (BDDag), mildly polarised (BDDmild), strongly polarised in perchloric acid (BDDsevere), and strongly polarised in a sulphuric acid-acetic acid mixture (BDDAcOH). Charge transfer processes at the electrode surface were studied by steady-state (polarisation) and dynamic (cyclic voltammetry) potential step methods. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe(CN)6III/II) and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H2Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The reaction rate constants decreased significantly upon anodic polarisation. Important drops in the carrier surface concentration and in true surface area led to hindrance of electron transfer at the electrode surface. The concept of non-diamond (sp2) impurities as charge transfer mediators on a diamond surface was suggested as an explanation for the electrochemical behaviour of diamond electrodes after surface oxidation treatment. To verify this concept, graphite particles were deposited on mildly polarised diamond electrodes (BDDmild) so as to prepare diamond-graphite composite electrodes (BDD-g), and their properties were compared with those of as-grown (BDDag) diamond and carbon electrodes. The sp2 coverage on BDD was estimated by cyclic voltammetry. A strong analogy existed between as-grown diamond electrodes and diamond-graphite composite electrodes. In fact, by depositing graphite particles on diamond after its deactivation by anodic polarisation is was possible to restore the initial properties of the as-grown diamond electrode. In the potential region of electrolyte decomposition, sp2 was eliminated from the BDD-graphite composite surface by oxidation to CO2. BDD itself behaved as an "non-active" electrode. In this case the oxidation of organic compounds was mediated by hydroxyl radicals formed during water discharge, without any participation of the electrode surface. The behaviour of "non-active" diamond in organic oxidation processes occurring in the potential region of water discharge was described in greater detail in the chapter about electrochemical oxidation of ethylenediaminetetraacetic acid (EDTA). The theoretical model developed for organic oxidation at "non-active" anodes was used to predict results for the oxidation of EDTA at boron-doped diamond electrodes. The very good fit between experimental results and theoretical values confirmed that EDTA oxidation at diamond in the potential region of electrolyte decomposition was a fast reaction involving free electrogenerated hydroxyl radical intermediates formed at the anode during water discharge. Nanoparticles of IrO2 were deposited by thermal decomposition on boron-doped diamond electrodes (BDD-IrO2) in order to improve and better understand their electrocatalytic activity toward redox processes and the oxygen evolution reaction (OER). Electrodes with different IrO2 loadings were prepared and their electrochemical properties tested. The reaction rate constants of redox processes increased by several orders of magnitude, the overpotential of the oxygen evolution reaction decreased by about one volt. The IrO2 deposition completely altered the electrochemical properties of diamond from an "non-active" to an "active" material. The high electrocatalytic activity of BDD-IrO2 electrodes toward the OER was also confirmed in an indirect way by the observation that the oxidation of organic substances and the production of peroxodisulphate are inhibited. The high degree of dispersion of the IrO2 particles led to a high electrocatalytic activity, even at extremely low IrO2 loadings. On the basis of experimental results a phenomenological model was proposed. Boron-doped diamond was defined as a degenerated semiconductor material because of the quite high charge density on its surface due to the boron doping (> 1021 boron atoms cm-3), to the non diamond (sp2) carbon and in general to surface species. This distribution of charges on the surface promoted processes of electron transfer across the interface diamond/electrolyte. Deposition of particles on the diamond surface (graphite or IrO2) increased further this surface charge density until reaching a perfectly metallic behaviour. Since charge transfer processes occurred on the deposited particles at lower overpotential than on diamond sites, surface modified diamond electrodes showed the electrochemical properties of the deposited material.