Paolo Guerriero

Electrodeposition of PbO2 + CoOx composites by simultaneous oxidation of pb2+ and Co2+ and their use as anodes for O2 evolution

Abstract PbO2+CoOx composites have been deposited by anodisation of sulphamate solutions containing Pb2+ and Co2+ ions. The effect of experimental variables, like total Co2++Pb2+ concentration, Co2+/(Co2++Pb2+) ratio, potential, pH and angular speed of the anode, on the deposit composition has been investigated. The results are discussed according to the theories developed for the cathodic deposition of alloys, concluding that the system conforms to the trends typical of irregular deposition. XRD and XPS analyses have shown that the deposits are amorphous, but have not allowed to fully elucidate the nature of the Co containing compound(s) codeposited with PbO2. When used as anodes for oxygen evolution in aqueous NaOH, the PbO2+CoOx composites exhibit Tafel slopes of ca. 60 mV decade−1 and a reaction order of 1 with respect to OH−. Their overpotential is similar to that of PbO2+Co3O4 composites prepared by anodisation of suspensions. A marked activation of the PbO2+CoOx electrodes, during ca. 3 days of continuous oxygen evolution, is explained as the result of an increase of the Co surface concentration, as shown by XPS and impedance data.

Preparation of anodes for oxygen evolution by electrodeposition of composite Pb and Co oxides

Abstract The simultaneous anodic oxidation of Pb 2+ and Co 2+ in a sulphamate bath containing suspended Co 3 O 4 (or NiCo 2 O 4 ) particles has been studied as a method for preparing composite oxides in which the particles are dispersed in a matrix consisting of PbO 2 and hydrous Co oxides, CoO x with x 2 +CoO x )+Co 3 O 4 composites thus prepared are very rough and their Co concentration, expressed as X Co = N Co /( N Co + N Pb ), where N represents the number of atoms, approaches 0.5. As a result, they are more active catalysts in the oxygen evolution reaction than either PbO 2 +CoO x or PbO 2 +Co 3 O 4 prepared in the past by our group. For samples prepared in the presence of particles, the potential required to evolve oxygen at a fixed current density varies roughly linearly with X Co , irrespective of the fact that Co is present as Co 3 O 4 or CoO x .

Electrochemical deposition and properties of PbO2 + Co3O4 composites

Abstract Electrolysis of suspensions of Co 3 O 4 particles in Pb 2+ -containing electrolytes has been used for depositing PbO 2 + Co 3 O 4 composite layers on Ni rotating dise anodes. A sufficiently high angular speed of the electrode is necessary to obtain layers of homogeneous thickness and Co 3 O 4 concentration. The volume fraction of Co 3 O 4 particles in the deposit α reaches a limiting value of ca. 0.1 when the volume fraction of particles in suspension C exceeds 0.008. The current density j has little effect on α as long as it is in the range 1 to 20 mA cm −2 ; if j increases further, α decreases. PbO 2 + Co 3 O 4 composite layers have been studied as electrode materials for the oxygen evolution reaction (mainly in NaOH solution). The overpotential and Tafel slope decrease upon increasing α. At a fixed potential, j is roughly proportional to OH − concentration. The PbO 2 + Co 3 O 4 electrode performance is fairly stable at 25°C but declines with time at higher temperature.

Electrodeposited composite electrode materials: effect of the concentration of the electrocatalytic dispersed phase on the electrode activity

Abstract PbO 2 +Co 3 O 4 , Tl 2 O 3 +Co 3 O 4 and Ni+Co 3 O 4 composite layers have been obtained by either anodic (oxide matrices) or cathodic (Ni-matrix) electrodeposition. Their electrocatalytic activity in the oxygen evolution reaction has been investigated with the aim of assessing the effect of concentration of Co 3 O 4 and surface roughness. Different composites showed a different dependence of oxygen evolution current density on Co 3 O 4 content. Only in the case of PbO 2 +Co 3 O 4 , the electrode effective area appeared to increase with the dispersed phase concentration, resulting in a non-linear relationship between oxygen evolution rate and Co 3 O 4 content.

Electrodissolution and corrosion of CuInS2 photoanodes with lamellar morphology

Abstract Crystals of n -CuInS 2 grown in a steep temperature gradient show lamellar morphology, prone to cleavage, and inclusions of copper-poor phases like CuIn 5 S 8 . Photoelectrooxidation of n -CuInS 2 in 1 M HClO 4 , a non-protective electrolyte, results in massive dissolution of the metal ions and deposition of sulfur, with marked photocurrent quenching. Operation of n -CuInS 2 (photo)anodes in acidic polyiodide medium causes significant corrosion and decay of photocurrent output. Micrographs of corroded areas show progressive and irregular etching of the CuInS 2 layers, which appear strongly polycristalline. EDX microanalyses show very similar composition before and after corrosion, suggesting that the latter causes stoichiometric removal and dissolution of the components and does not form new phases of significant thickness.

Electrodeposited Tl2 O 3‐Matrix Composites I. Effect of the Dispersed Phase on Nucleation and Growth of the Matrix

The electrodeposition of Tl 2 O 3 /Co 3 O 4 composites has been performed by anodic oxidation of Tl + ions in the presence of suspended Co 3 O 4 particles. This study, aimed at obtaining and characterizing new electrocatalytic materials for oxygen evolution reaction, has shown that Co 3 O 4 particles enhance the rate of nucleation and growth of the Tl 2 O 3 matrix. At each potential, induction times are shorter and kinetic constants are higher in the presence of Co 3 O 4 than in its absence. The chronoamperometric transients measured during composite deposition are compatible with instantaneous nucleation and 3D growth. Scanning electron microscopy and energy dispersive X-ray analyses data show that Co O4 particles become effectively embedded in the deposit from the early stages of its formation and act as centers of preferential growth for the matrix, so that the Co 3 O 4 content is higher near the electrode/composite interface than in the bulk. Tl 2 O 3 /Co 3 O 4 composites are compared with other systems for which nucleation and growth of the matrix is unaffected by the dispersed phase.

Electrodeposited Tl2 O 3‐Matrix Composites II. Electrocatalysis of Oxygen Evolution Reaction on Composites

The dependence of composition and morphology of Tl 2 O 3 /Co 2 O 4 composites on the experimental variables chosen for their anodic deposition (i.e., concentration of Co 3 O 4 particles in suspension, electrode potential, electrolysis charge, and angular speed of the electrode) has been investigated. The composite layers have been tested as electrode materials for oxygen evolution from basic media, and found to be rather effective electrocatalysts characterized by the following features: overpotential (η) at j = 100 mA cm -2 ca. 475 mV, Tafel slope close to 60 mV decade, linear dependence of j on [OH - ]. Significant electrocatalytic activity could be achieved with a Co 3 O 4 loading as low as 2.10 -5 g cm -2 . The electrode performance was maintained during a 14 day electrolysis carried out at room temperature with j = 100 mA cm -2 . Partial reduction of the composite layer was found to activate the composite electrodes, by decreasing η to ca. 415 mV (at j = 100 mA cm -2 ). However this activation was not long lasting.

Electrodeposition of AsSb alloys

Abstract AsSb alloys containing 30–95 at.% Sb were deposited onto nickel cathodes from citric acid aqueous solutions. The composition of the electrodeposits, determined by scanning electron microscopy—energy-dispersive X-ray analysis approached that of the solution at E ⩽ −0.7 V with respect to a saturated calomel electrode; at less negative potentials the antimony-to-arsenic ratio was higher in the alloy than in solution. The current efficiency approached 100% under the most favourable conditions and was higher at higher temperatures. As codeposition made the antimony deposition kinetics markedly more complicated: a negative slope region was visible in the steady state I - E curves and, correspondingly, the a.c. impedance diagrams showed negative polarization resistance. These phenomena appear to arise because the arsenic-covered areas are less active than the antimony-covered areas.

Electrodeposition of In + As thin film alloys and their conversion to InAsxP1−x by PH3 treatment

Abstract The codeposition of In + As alloys has been investigated as a preliminary step for the preparation of thin polycrystalline InAs and InAs x P 1− x films. Alloys of composition ranging from large excess In to excess As could be obtained by a suitable choice of bath and deposition conditions. Stoichiometric 1:1 deposits have been converted to InAs by simple thermal treatment. In + As alloys with excess In annealed under PH 3 flow resulted in deposits consisting of two phases, an InAs x P 1− x phase with x in the range 0.25 to 1 and almost pure InP. Photoelectrochemical investigations showed that only the latter species was photoactive.

Nb electrodissolution in aqueous alkali: dependence on the alkali metal

Nb electrodissolution is studied in solutions of alkali metal hydroxides MOH (M � /Li, Na, K, Rb and Cs). The j � /E curves show in all cases a dissolution/passivation peak followed by a current plateau. The current shows a marked dependence on the cation M: the peak current density jpk goes through a maximum for M � /K and the plateau current density jpl increases monotonically with the atomic number of M. Electrochemical impedance spectroscopy indicates that in the plateau region, a similar barrier oxide is present over the metal substrate in all solutions. XPS analyses show that polarisation causes, in certain cases, formation of a porous film of alkali metal niobates or mixed oxides, probably by a mechanism of dissolution/precipitation. It is proposed that the peak current is controlled by both the solution basicity and the solubility of this layer, whereas the dependence of the plateau current on M reflects the variations in solution basicity and in the specific ability of the metal cations to promote dissolution of the barrier oxide. # 2003 Elsevier Science B.V. All rights reserved.