J.F.C. Boodts

Oxygen evolution in acid solution on IrO2 + TiO2 ceramic films. A study by impedance, voltammetry and SEM

Thermally prepared oxide films (2 μm) of nominal composition Ir0.3Ti0.7O2 were prepared at 350, 400, 450 and 500 °C on a Ti support. Electrochemical impedance spectroscopy (EIS) at constant potential was used in the range 0.4 to 1.6 V (rhe). The results were fitted by the equivalent circuit RΩ (RfCf)(Rct,Cdl)L. The RΩ values obtained by EIS agree with the data derived from kinetic studies. The high Rct value of the 350 °C oxide layer is attributed to the incomplete decomposition of the precursor so that some of the underlying insulating TiO2 layer remains uncovered. This view is corroborated by SEM and CV studies. Oxide layers prepared at higher temperatures are stable, the Rct values increasing with calcination temperature. This is attributed to crystallite size increasing with temperature, resulting in a lower number of active sites being exposed. For all electrodes, Rct decreases with increasing potential. Cdl depends on both surface area and applied potential. At T > 350 °C, Cdl diminishes as temperature increases, which is due to decrease in the active surface area. Analogously, Cdl is observed to increase with increasing potential, which is associated with the heterogeneity of the electrode surface.

Electrocatalytic properties of ternary oxide mixtures of composition Ru0.3Ti(0.7−x)CexO2: oxygen evolution from acidic solution

Mixed oxide electrodes of general composition Ru0.3Ti(0.7−x)CexO2 were prepared by thermal decomposition of the respective chlorides using Ti as a support. x was varied between 0 and 70 mol %. Oxygen evolution was used as a model reaction to investigate the dependence of the electrocatalytic properties on oxide composition. Kinetics was studied by quasistationary current-potential curves and reaction order determination. A minimum Tafel slope of about 30 mV has been found in the 10–30% CeO2 composition range. On the basis of a zero reaction order at constant overpotential, a reaction mechanism has been proposed accounting for the composition dependence of the Tafel slope. It has been concluded that the replacement of TiO2 with CeO2 brings about an increase in the electrocatalytic activity for oxygen evolution, while the layer becomes more prone to mechanical erosion.

Kinetics and mechanism of oxygen evolution on IrO2-based electrodes containing Ti and Ce acidic solutions

A systematic investigation of the mechanistic features, the electrocatalytic activity, and the stability of thermally prepared ternary oxides of general formula Ir0.3Ti(0.7−xCexO2 (0 ⩽ x ⩽ 0.7) has been carried out using O2 evolution from 1.0 mol dm−3 aqueous HClO4 as a model reaction. The surface state of the electrodes was monitored in situ by recording voltammetric curves before and after each experiment. Stability was tested by cycling the electrodes between 0.4 and 1.4 V (rhe). For all compositions the anodic-to-cathodic charge ratio, qa/qc, was close to one. Variation of the negative potential limit did not reveal the occurrence of the cathodic dissolution phenomena previously observed with a similar RuO2-based system. Tafel slopes were independent of the CeO2 content, with a value around 30 mV. The reaction order with respect to H+ was zero at constant overpotential and ionic strength. Some inhibition of the oxygen evolution reaction is observed as Ti is replaced by Ce.

Physico-chemical and electrochemical characterization of Ru-based ternary oxides containing Ti and Ce

Abstract The effect of mixing Ru and Ce has been investigated by systematically substituting Ce for Ti in 30 mol% RuO2 + 70 mol% TiO2. A set of ternary oxide electrodes with the general formula Ru0.3 Ti(0.7−xCexO2 has been prepared from x = 0 to x = 0.7 at 10 mol% intervals by thermal decomposition of aqueous acid solutions of the chlorides as precursors using Ti as a support. Ex situ characterization included TGA, XRD and SEM. Electrodes were characterized in acid solution as a function of composition by determining the open-circuit potential, the voltammetric curve and the voltammetric charge. Variables were temperature, time and atmosphere of calcination. The electrodes showed variation of the voltammetric charge and of the voltammetric features dependent on the potential window and the number of cycles. The main reason for instability appears to be the cathodic dissolution of the Ce component, which has been confirmed by analysing the solution by ICPES. Ways of minimizing this inconvenience have been explored. Preliminary results in alkali indicate excellent stability.

Surface and electrocatalytic properties of ternary oxides Ir0.3Ti(0.7−x)PtxO2. Oxygen evolution from acidic solution

Mixed oxides of nominal composition Ir0.3Ti(0.7−x)PtxO2 (0 ≤ × ≤0.7) were prepared by thermal decomposition at 400°C of suitable precursors on Ti supports. The surface properties were investigated ex situ by SEM and EDX, and in situ by cyclic voltammetry in 1 mol dm−3 HClO4 solutions. The morphology changes at about 30 mol% Pt; at lower Pt contents the surface is enriched with Ir; at higher contents it is enriched with Pt. Cyclic voltammetry showed that the surface concentration of active sites decreases with increasing Pt content. O2 evolution was studied by means of current-potential curves and reaction order determinations. The mechanism was found to change going from Ir to Pt surface sites. The electrocatalytic properties were separated from surface area effects by normalizing the current to unit surface charge. The specific activity turned out to decrease by a factor of 10 as Ti is replaced by Pt, which in turn displaces Ir from the surface of the active mixed oxides.

Electrocatalytic properties of Ru + Ti + Ce mixed oxide electrodes for the Cl2 evolution reaction

A series of ternary oxides of nominal composition Ru0.3Ti(0.7 − x)CexO2 was prepared by thermal decomposition on Ti supports and used as electrodes to study the effect of substituting Ce for Ti on the kinetics and electrocatalysis of the Cl2 evolution reaction. The surface properties were evaluated by cyclic voltammetry. Integration of the voltammetric curves provided the voltammetric charge, q∗, which is proportional to the active surface area. Kinetic data showed that Cl2 evolution occurs with a single mechanism over the whole explored potential range and at all electrode compositions. Fractional reaction orders with respect to both Cl− and H+ revealed double-layer effects attributed to the occurrence of Cl− specific adsorption. The apparent activity increased with CeO2 content, whereas the true electrocatalytic properties, estimated by normalizing the current to unit surface charge, decreased in the presence of CeO2 with a minimum of 10% CeO2. The presence of CeO2 also makes the oxide layers more prone to mechanical erosion by the evolving gas bubbles.

Solvent and support electrolyte effects on the catalytic activity of Ti/RuO2 and Ti/IrO2 electrodes: oxidation of isosafrole as a probe model

Abstract The electrocatalytic behavior of Ti/RuO 2 and Ti/IrO 2 electrodes was investigated as function of the supporting electrolyte and solvent. A decrease was observed in the electrochemically active area of the electrode as the cation size increases. The oxidation potential of a series of organic solvents tested showed a straight relationship with Gutmanns donor number: the higher the GDN the more easily is the solvent oxidized. Oxidation of isosafrole, ISF, was used as a probe reaction to examine the influence of the supporting electrolyte cation size and the composition of the organic solvents on the electrocatalytic activity of the electrodes. The global catalytic activity for ISF oxidation is independent of the size of the supporting electrolyte cation and of the electrode material. For both electrode materials investigated it was verified that the following sequence holds for the influence of the solvent on the overall catalytic efficiency of ISF oxidation: AN>PC>DMSO≈DMF.

Ethanol Electro‐oxidation in Ruthenium‐Oxide‐Coated Titanium Electrodes

A systematic investigation of ethanol oxidation on Ti/RuO 2 electrodes has been carried out in 1.0 mol dm -3 HClO 4 acid solution. The experimental results show high selectivity toward acetaldehyde formation. One of the secondary oxidation products of ethanol forms a blocking film at the electrode surface which can be removed during electrosynthesis using pulse control of the working potential. The influence of the electrolysis conditions, the electrode mechanism of ethanol oxidation, and R Ω values measured under pseudo-Tafel plots are discussed.

Ir-based oxide electrodes: oxygen evolution reaction from mixed solvents

The influence of 30% (v/v) organic cosolvent in 1.0 mol dm−3 HClO4 on the OER electrode kinetics, surface properties and electrode stability of IrO2-based electrodes was investigated by cyclic voltammetry and polarization curves. The Tafel coefficients in the presence of cosolvent are explained in terms of the change of the rate-determining step (r.d.s.) of the OER electrode mechanism, coating dissolution and/or cosolvent oxidation. Of the several cosolvents investigated t-BuOH and PC show less effects on the OER and electrode properties making them the best choice for organic eletrosynthesis applications, in contrast to AN, which causes coating dissolution, and DMF and DMSO which show an anticipation of the voltammetric current.

Electrocatalytic activity of the ternary oxide Ru0.3PtxTi(0.7-x)O2 for chlorine evolution

Abstract The electrocatalytic activity of ternary oxides of nominal composition Ru 0.3 Pt x Ti (0.7 — x ) O 2 was investigated over the whole composition range using chlorine evolution as a model reaction. Composition was changed in 10 mol.% steps and the surface was characterized in situ by cyclic voltammetry ( cv ). Surface properties are governed by the RuO x features at low PtO x content ( x content they are controlled by PtO x . A single Tafel line of 31 ± 1 mV slope was observed at all compositions. The reaction order is zero with respect to H + and 1.5 with respect to Cl − . At high PtO x content and low Cl − concentration ( −3 ) O 2 formation appears to control the gas evolution process. An inhibition of Cl 2 evolution is observed for PtO x ⩾ 30 mol.%. On the whole, morphological effects predominate over electronic effects.