Investigation of the surface properties and the hydrogen evolution reaction, HER, at thermal rhodium oxide electrodes

A systematic investigation was conducted of the surface properties and the HER at electrodes of nominal composition Ti/RhxTi((1-x))O-y prepared by thermal decomposition (T-cal: 500 degrees C; t(cal): 2h; O-2 flux: 5 dm(-1) min(-1)) from salt precursor solutions dissolved in 6.0 mol dm(-3) HNO3. Films were characterized ex situ by SEM, EDX, XPS and XRD and in situ by open circuit potential measurements and CV. The electrochemical behaviour was investigated by CV as function of the anodic, E-lambda,E-a, and cathodic, E-lambda,E-c, switching potentials showing the Rh surface oxidation states strongly depend on these experimental variables. Surface Rh-sites are reduced to metallic rhodium in the cathodic potential region while higher oxidation states (I-III) are formed at more positive potentials (E >= 0.5 V/RHE). Hydrogen adsorption and desorption peaks as well as a short double layer charging region are observed at intermediate potential values. The HER was investigated by Tafel coefficients and reaction order with respect to H+ as function of nominal Rh-content. (c) 2006 Elsevier Ltd. All rights reserved.5272359236

Structural, morphological and surface properties as a function of composition of Ru+Ti+Pt mixed-oxide electrodes

A systematic investigation of the structural, morphological and surface properties of thermally prepared Ru0.3Ti(0.7 - x)PtxO2 (0 less than or equal to x less than or equal to 0 7) electrodes was conducted by ex situ (SEM, EDX, XPS) and in situ (voltammetry and open circuit potential) techniques. Significant changes were observed on substituting TiO2 for PtOx. The electrochemical surface area increased dramatically (similar to 10 times) reaching a maximum at 40% PtOx (nominal). Ru surface enrichment was observed while Pt + Ru segregates as rectangular crystals from the coatings. At the higher Pt-contents, surface electrochemistry is mainly dominated by Pt solid-state transitions, especially after continuous potential cycling (0.4-1.4 V vs r.h.e.). Pt-containing coatings were revealed to be unstable (erosion) when the cathodic switching potential, E-lambda,E-c,, was fixed at 0.4 V. Limiting En, to 0.9 V vs r.h.e, eliminates coating instability. Preferential Ru dissolution, promoted by cathodic Pt dissolution, was revealed by ICPES. With the exception of the 0-20% PtOx containing coatings, where evidence for the presence of Ru-VI was found, Ti and Ru are present in the +4 oxidation while the +2 oxidation state predominates for Pt. (C) 1998 Elsevier Science Ltd. All rights reserved.4316-172515252

Surface characterization by UHV techniques and cyclic voltammetry of thermal IrO2-based electrocatalysts containing TiO2 and CeO2

Electrodes of nominal composition Ir0.3Ti0.7-xCexO2 (0 less than or equal to x less than or equal to 0.7) have been prepared by thermal decomposition of mixtures of the chloride precursors (450 degrees C). A systematic study of the morphology, chemical composition and surface properties of these electrodes was performed using both ex situ (SEM, EDX, XPS) and in situ (cyclic voltammetry) techniques. A spectrophotometric study of the precursor salt mixtures was done. SEM analysis showed the introduction of CeO2 into the binary system to provoke a significant increase in the active surface area of these electrode materials, which is corroborated by the CV data (anodic voltammetric charge, q(a)*, double layer capacitance, C-dl, and roughness factor). Apparent Ti-enrichment of the more external region of the coatings, as evidenced by EDX, is related to the fact this technique analyses a thicker part of the oxide bulk (estimated as ca. 1/3 of the layer thickness). No real surface Ti-enrichment occurs as supported by close to nominal Ir/Ti ratios obtained by XPS (a real surface technique). XPS measurements also support Ir, Ti and Ce are present in freshly prepared oxide layers in two oxidation states, which is in agreement with the spectrophotometric results.94571171

Surface, kinetics and electrocatalytic properties of the Ti/Ti + Ru+Ce)O-2-system for the oxygen evolution reaction in alkaline medium

Ti-supported (Ti + Ru + Ce)O-2 electrodes, prepared at 450 degrees C, were characterised by XRD, open-circuit potential (E-oc), capacity data (C) and morphology factor (phi) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. E-oc-data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO2-content. phi-Values remained in the 0.18-0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH-), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH- was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, alpha(ap). Analysis of the true and apparent electrocatalytic activities revealed the O-2-evolution reaction rate is affected by oxide composition due to morphologic effects. (c) 2005 Elsevier Ltd. All rights reserved.51142809281

XPS study of the state of iridium, platinum, titanium and oxygen in thermally formed IrO2+TiO2+PtOx films

Electrodes of nominal composition Ir0.3Ti(0.7-x)PtxO2 (x = 0; 0.4 and 0.7) have been prepared by thermal decomposition of mixtures of the chloride precursors. H2PtCl6 was used as Pt-precursor. A systematic study of the chemical composition and oxidation states of the elements of these electrodes was performed by XPS. XPS analysis showed that the surface of the PtOx-containing coatings are Pt enriched. Additionally, XPS revealed that the Ir signals are almost absent from the spectra of PtOx-containing electrodes, while the Ti signal is completely absent. On the basis of the XPS results it is possible to propose a model of the surface structure of these electrodes: the grains, composed almost exclusively of Ir Jr Pt, are Pt enriched and distributed in a matrix, which although having Ti in its composition, besides Ir and Pt, is also Pt enriched. This finding corroborates the electrochemical behaviour of these electrodes, which is characteristic of polycrystalline Pt. (C) 2000 Elsevier Science B.V. All rights reserved.1704170011912