Luis A. Avaca

Characterisation of surfaces modified by sol-gel derived RuxIr1−xO2 coatings for oxygen evolution in acid medium

The preparation of sol–gel derived RuxIr1−xO2 catalytic coatings (x=1, 0.9, 0.7, 0.5, 0.3, 0.1 and 0) for oxygen evolution in acid media is reported. The films were deposited on titanium plates by painting the substrate with a sonocatalysed sol containing varying amounts of ruthenium and iridium acetylacetonates. This was followed by a heat-treatment at 400°C for 1 h. The redox processes occurring on those films were characterised by cyclic voltammetry at 20 mV s−1 in 0.5 M aqueous H2SO4. The measured voltammetric charges are related to the active areas of the electrodes and increase with the iridium content going through a maximum at x=0.5. The oxygen evolution reaction (OER) was studied through polarisation curves recorded under galvanostatic control from 10 μA cm−2 up to 100 μA cm−2. Tafel plots normalised by the voltammetric charges revealed the true catalytic effect of the RuO2 for the OER. The combination of electrocatalytic and roughness effects made the Ru0.7Ir0.3O2 coating the most effective for the OER. The Tafel slope values changed from 40 mV dec−1 for pure RuO2 coatings to 60 mV dec−1 for pure IrO2. These variations indicate a strong dependence of the reaction pathway with the nature of the coating. Accelerated tests performed at 700 mA cm−2 in 30% aqueous H2SO4 and 80°C showed that the limiting factor in the operational life of these electrodes is the dissolution of the active layer and that the Ru0.1Ir0.9O2 coating has the longer life-time under those conditions.

The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption

The hydrogen evolution reaction (her) was studied on bright nickel electrodes in alkaline solutions at several temperatures. The formation of nickel hydrides in the potential region of the her was previously analyzed by cyclic voltammetry and the electrode surface was pre-conditioned to minimize the effect of the hydrides in the experimental response of the system. Data from polarization curves was conveniently analyzed by non-linear regression fitting resulting in a Volmer-Heyrovsky mechanism in which the second reaction must be the rds to explain the characteristics of the hydride formation. The calculated value of the rate constant for the electrochemical desorption step was 1.65 × 10−11 mol cm2 s−1 at 25°C. The apparent energy of activation was 58 kJ mol−1, a value somewhat higher than those in the literature and that was justified in terms of the specific pre-treatment of the surface. A voltammetric method for determining the real electrochemical active area of Ni electrodes is also described.

Influence of the preparation method on the morphological and electrochemical properties of Ti/IrO2-coated electrodes

Abstract The IrO 2 layer deposited on a Ti substrate for electrocatalysis of the oxygen evolution reaction (OER) in acidic media was obtained from one of three different deposition methods consisting of painting the substrate with the corresponding solutions followed by heat treatment at a temperature range 400–600°C for 1 h. The coating solutions were prepared by dissolving IrCl 3  · 3H 2 O in 15% HCl (traditional method), by dissolving iridium acetylacetonate in isopropanol and acetic acid (inorganic sol–gel method) and by dissolving IrCl 3  · 3H 2 O in ethyleneglycol and citric acid (Pechini method). The redox processes occurring on these films were characterized by cyclic voltammetry at 20 mV s −1 in 0.5 M aqueous H 2 SO 4 at room temperature. The measured anodic voltammetric charge is related to the active area of the electrode; the electrode derived from traditional method had the highest value. The Tafel plots revealed that the electrocatalytic activities are not affected by the surface morphology, indicating that the preparation procedures produce electrocatalytic coatings with comparable activity. For an applied current density of 100 mA cm −2 the combination of the conductivity and roughness effects caused the electrode derived from inorganic gel and heat-treated at 400°C to exhibit the lowest overpotential for the OER. On the other hand, the accelerated life tests performed at 80°C in 30% aqueous H 2 SO 4 with an anodic current density of 750 mA cm −2 showed that the stability of the coatings increases with the temperature of the heat-treatment and the Pechini-derived electrodes had a longer-life time under this condition in all the cases. The accelerated life test showed that the factor limiting the operational life of these electrodes is the dissolution of the active layer.

Active surface area determination of Pd-Si alloys by H-adsorption

Abstract The active surface area of a PdSi alloy (80-20 at.%) was determined by means of the anodic charge associated to the desorption peaks of underpotential deposited hydrogen atoms. The value found was very close to that expected for a complete monolayer of PdH, considering the percentage of Pd on the surface. The results are comparable to those obtained by the classical cyclic voltammetric method of reduction of oxides grown at different inversion potentials. The agreement suggests that one full monolayer of adsorbed hydrogen is formed on the Pd portion of the alloy surface.

Electrochemical and morphological studies of electrodeposited Ni–Fe–Mo–Zn alloys tailored for water electrolysis

Abstract The co-deposition of nickel, iron and molybdenum onto mild steel substrates with or without the addition and subsequent leaching of zinc rendered cathode materials very active for hydrogen generation in alkaline solutions. SEM analysis showed that adherence to the substrate can be achieved by initial deposition of the other components prior to Zn incorporation into the solution. Coatings of Ni–Fe–Mo (51.4-5.5-43.1 at%) and Ni–Fe–Mo–(Zn) (70.9-15.3-12.7-1.1 at% after leaching) were very active for hydrogen evolution at temperatures ranging from 25 to 80°C in 6 M KOH solutions. Steady-state polarisation curves furnished Tafel plots with slopes almost invariant with temperature and revealed an electrochemical activity comparable to the best materials reported so far. For the Ni–Fe–Mo–(Zn) coating the overpotential for hydrogen evolution at 80°C was as low as 83.1 mV . Long-time electrolyses at a constant current density of 135 mA cm −2 showed a considerable degradation of the surface and a decay in performance but the overpotential value after 440 h stabilised in 157 mV , a satisfactory value for industrial applications.

Voltametria de onda quadrada. Primeira parte: aspectos teóricos

The theoretical aspects of square wave voltammetry were discussed. Reversible, irreversible and quase-reversible electrode reactions were analyzed and the correlations between parameters like frequency, period, square wave potential and amplitude were showed. In this way, diagnostic relationships allow to characterize the electrode process. The analytical applications were discussed in base of the increment in the analytical response (current) due to the characteristics of the developed equations and the unique mode of collecting the electrode response, i.e., the direct and reverse signals. Finally, recent advances in the basic theory, as the applications to the hydrodynamic electrode and the ultramicroelectrode were also analyzed, and the multiple pulses square wave voltammetry was also introduced.

Electroanalytical determination of 4-nitrophenol by square wave voltammetry on diamond electrodes

The anodic voltammetric behavior of 4-Nitrophenol on a Boron-doped diamond electrode in aqueous solution has been studied using square wave voltammetry. After optimization of the experimental conditions, that model molecule was analyzed in pure water solutions using a Britton-Robinson buffer with pH 6.0 as the supporting electrolyte. Oxidation occurs at 1.0 V vs. Ag/AgCl in an irreversible two-electron process controlled by adsorption of the species. The detection limit (DL) obtained was 2.8 mg L-1. This result was comparable to that obtained from reduction of the molecule at -0.8 V vs. Ag/AgCl under the same experimental conditions (DL = 4.2 mg L-1). Both DL values are within the limits required by the legislation for drinking water (30 mg L-1). The combination of square wave voltammetry and diamond electrodes has proved to be an interesting and desirable alternative for the analytical determination of organic molecules.

Electroanalytical determination of the herbicide picloram in natural waters by square wave voltammetry

O uso da voltametria de onda quadrada para a determinacao eletroanalitica do picloram em meio aquoso e discutido neste trabalho. Encontrou-se que a reacao ocorre por um mecanismo irreversivel, envolvendo a transferencia de quatro eletrons, com o reagente adsorvido na superficie do eletrodo. Uma aplicacao pratica foi levada a efeito, com amostras de aguas contaminadas artificialmente com o herbicida. Estas amostras consistiam de agua pura e de tres diferentes amostras, coletadas em tres diferentes riachos da regiao, dois dos quais eram altamente poluidos com efluentes domesticos e industriais. Os limites de deteccao calculados variaram entre 11 (para a agua pura) e 39 ppb para a amostra mais contaminada. Estes valores sao suficientemente baixos para indicar a metodologia em aplicacoes de analise de tracos. This paper describes the use of square wave voltammetry for the electroanalytical determination of picloram in aqueous media. The reaction was found to occur via an irreversible mechanism involving the transference of four electrons, with the reagent adsorbed in the electrode surface. The practical application was carried out with spiked water samples originated from purified water and from three different local creeks, two of which are highly polluted with industrial and domestic effluents. The calculated detection limits ranged from 11 (for pure and unpolluted waters) to 39 ppb for the most contaminated sample. These low values indicate that such methodology can be conveniently applied in trace analysis.

Sol-gel ZrO2 coatings for chemical protection of stainless steel

ZrO2 coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC3H7)4 as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm3 of ZrO2. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35–0.75 μm. The influence of the ZrO2 coatings on the corrosion behavior of stainless steel substrates in aqueous NaCl was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments.

Protection of 316L stainless steel against corrosion by SiO2 coatings

Oxide films prepared by sol-gel methods and presenting high resistance to heat, corrosion, friction and wear, as well as excellent mechanical properties, have recently been developed and put into practical use as structuraI materials [1-5]. The process of preparation offers potential advantages for modifying the properties of surfaces by low-temperature treatment without altering the original properties of strength and toughness of the substrates. A number of reports on sol-gel coatings concerning the prevention of chemical corrosion and oxidation have been published [6-10]. All of these films increase the protection of metal substrates from air corrosion (tested up to 800 °C) and acid attack (tested up to 80 °C). The most promising corrosion prevention for stainless steel has been studied by our group using sol-gel films of ZrO2, SiO2, SiO2-TiO2 and SiO 2A1203 prepared by dip-coating using sonocatalysed sols [11-14]. The. properties of these coatings have been studied by electrochemical techniques in NaC1 and H2SO 4 solutions. Although preliminary measurements have shown that SiO2 films are not the best protective coatings [13], they provide a very adequate model system to correlate corrosion protection with the physical structure of the sol-gel films. In this work, amorphous coatings of SiO 2 were deposited on 316L stainless steel by the dip-coating technique using a sol preparation involving sonocatalysis. The films were prepared through hydrolysis polymerization of metal alkoxide solutions and conversion to an oxide layer by heating at relatively low temperatures. The effect of the time of heat treatment of the SiO2 films on the corrosion resistance of stainless steel was studied in 15% H2SO 4 through potentiodynamic polarization curves at 25 °C. The substrate used in the experiments was 316L stainless steel (SS 316L, Caseurop, France) with chemical composition (wt %): 67.25 Fe, 18.55 Cr, 11.16 Ni, 2.01 Mo, 0.026 Cu, 1.71 Mn and 0.028 C. The specimens were machined into dimensions of 30 mm × 15 mm x I mm, degreased ultrasonically in acetone, cleansed by distilled water then dried in air. For silica films, tetraethylorthosilicate Si(OC2H5) 4 (TEOS) was used as the source of silica, absolute ethanol (C2HsOH) as solvent and glacial acetic acid CH3COOH as catalyst. The silica sonosol was prepared by dissolving Si(OCzHs) 4 in absolute ethanol to which a small amount of acetic acid CH3COOH was added. The volume ratios of Si(OC2Hs)4/C2HsOH and Si(OC2Hs)4/CH3COOH were, respectively, 1 and 5. The mixture was submitted to intense ultrasonic irradiation (20 kHz) produced by a transducer (Heat Systems Ultrasonics W385). After 25 min the resulting sol was homogenized and remained stable for about five weeks at room temperature when kept in a closed vessel. Coating films were formed on the substrates by dipping into the clear sonosol and withdrawing at a speed of 10 cmmin -1. The resulting gel films were dried at 60 °C for 15 min and densified in a furnace with air atmosphere by increasing the temperature at a rate of 5 °C min -1 up to 450 °C when an isothermal holding of 1 h was applied in order to remove the organic residues. The temperature was then increased again at the same rate up to either 600 or 800 °C and maintained at that value for variable lengths of time to complete the densification and obtain adherent coatings. The average thickness of the heat-treated films at 800 °C was around 0.4/zm. X-ray diffraction (XRD) analysis of the substrate and coatings was done with a Philips diffractometer using CuKo, The diffractogram of SS 316L shows the existence of a crystalline phase which corresponds to the cubic phase of the alloy containing Cr, Fe and Ni [4]. When the steel was heated at 800 °C for 2 h in air, additional XRD peaks appear corresponding to a mixture of cubic and hexagonal CrzO 3 [4]. In contrast, samples coated with SiO2 analysed after oxidation tests in air at 800 °C for 2 h showed only the peaks of the original substrate, indicating that the silica coating remains amorphous and inhibits any oxidation of the base material. A Bomem Fourier transformation infrared (F-FIR) spectrometer was used to obtain high resolution spectra of the coatings in the 400-4000 cm -1 range; the measurements were carried out at room temperature by reflection at an incident angle of 30 ° . The spectrum of a coating deposited on SS 316L and