Search

Abstract Electrodeposition and galvanic displacement reactions are used to prepare compact and porous Pd-Ni alloys and heterogeneous bilayer structures Pd/Ni or Pd/Pd-Ni. Pd is present in the deposits either as a pure metal or in solid solutions with Ni. Electrode activity in ethanol oxidation and Pd surface area are estimated by cyclic voltammetry. Results show that, in a first approximation, the peak current of ethanol oxidation is simply proportional to the Pd area, regardless of samples morphology, structure and composition, for the entire series of samples. Deviations from the proportionality relation indicate that Pd and Pd-rich alloys are more active than Ni-rich alloys of same Pd surface. Graphical abstract Image 104671 Highlights • Electrodeposition of compact and porous Pd-Ni alloys from simple chloride baths. • Estimation by CV of the surface area covered by Pd on the entire series of samples. • Evaluation on the same samples of the CV peak current densities j p of ethanol oxidation. • Discussion of the proportionality relation between Pd area and j p. [ABSTRACT FROM AUTHOR]

Porous Ag-Rh alloys of intermediate compositions in the metallurgic miscibility gap have been electrodeposited from mildly acid sulphate baths of the two metal ions (pH ≅ 3.3). The morphology revealed by SEM is microporous; XRD data are consistent with the formation of prevalent phases with lattice parameters depending on composition according to Vegard's law. Alloys with a predominant phase of composition Ag 47 Rh 53 – estimated from XRD peaks position – are deposited from a bath containing 0.25 M Na 2 SO 4 + 1 mM Ag 2 SO 4 + 3 mM Rh 2 (SO 4 ) 3 . Voltammograms recorded with Ag-Rh alloys in 0.1 M H 2 SO 4 show analogies with those obtained with Pd and Rh, but no clear evidence of the absorption and storage of hydrogen atoms in the alloy material, in contrast with recent literature claims of hydrogen charging of Ag-Rh nanoparticles. [ABSTRACT FROM AUTHOR]

Compact Cu-Pd alloys of compositions extending over the entire range are deposited at low overpotentials, from acidic sulphate/perchlorate baths of the two metal ions. XRD data show a lattice parameter following Vegard’s law. Porous alloy layers can be deposited from acidic chloride baths under large current densities causing vigorous hydrogen evolution. Good deposits with regular morphology are obtained only from Cu-rich baths, and for Pd contents up to ca 33 at%. XRD investigations indicate a biphasic structure, made of pure Cu and a Cu 17 Pd 83 phase, in variable ratios depending on composition. Single phase porous Cu-Pd materials are obtained by prolonged galvanic displacement of porous Cu layers. Selected electrode materials are tested in the nitrate reduction in alkaline media, comparing the effects of composition and surface area. [ABSTRACT FROM AUTHOR]

The galvanic exchange between Mn 2 + ions and electrodeposited porous PbO 2 was studied to produce a porous oxide whose lower conductivity prevented its direct oxygen bubble-templated anodic deposition. Immersion of PbO 2 layers in acid acetate solutions of Mn 2 + led to the formation of amorphous MnO x shell onto PbO 2 . Due to its amorphous nature, MnO x could not be proved to be MnO 2 by XRD. However, MnO x was cathodically stripped at the same potential as MnO 2 . The deposition of the MnO x shell onto PbO 2 enhanced the capacity of the porous electrodes. [ABSTRACT FROM AUTHOR]

Porous PbO 2 layers were produced from methanesulfonic acid or perchloric acid media, through an oxygen bubble templated anodic deposition process, i.e. by oxidizing Pb 2+ ions at large current densities, with simultaneous oxygen evolution. The effect of deposition current density, Pb 2+ concentration and nature of the acid on the deposit porosity (assessed through number of pores per unit surface area, average pore diameter, and void volume fraction) were investigated. Porous PbO 2 deposits obtained from both media consisted of the same mixture of α- and β-phases, although the corresponding compact deposits had significantly different structures (pure α-PbO 2 from methanesulfonic acid, a mixture of α- and β- PbO 2 from perchloric acid). The cycling behaviour of compact and porous PbO 2 layers in 1.0 M H 2 SO 4 was compared. During the first cycle, the reduction charge was much higher for porous than for compact layers, as a consequence of the different electrochemically active areas. For the former it depended on the layer thickness, while it was independent of it for the latter. Upon cycling, the charges exchanged by compact layers steadily increased, whereas those of porous layers, after an initial increase, slowly declined, but remained markedly higher. [ABSTRACT FROM AUTHOR]

Porous Ag layers have been prepared by hydrogen bubble templated electrodeposition from an ammonium acetate bath, under application of a large current density (–2 A cm −2 ). SEM images of the layers show macroscopic porosity and a spongy dendritic structure, analogous to those observed for other porous metals/alloys deposited with a similar procedure. A deposition charge of 30 C cm −2 produces a porous deposit with a mass of ca 1.7 mg cm −2 . The roughness factor f r is evaluated by impedance and from the charge density of Pb UPD; both methods provide estimates of f r in the order of several hundreds but values obtained from impedance analysis are more than twice larger. Mass transport to the porous electrodes is studied by linear sweep voltammetry at stationary and rotating disk electrodes. In comparison with flat Ag cathodes, porous Ag electrodes show larger sensitivity, lower limit of detection and extended range of linear response in H 2 O 2 reduction. [ABSTRACT FROM AUTHOR]

Three porous materials (Cu, a Cu-Ni alloy with 70 at.% Cu and Rh-modified Cu) have been tested as electrodes for the electroanalysis of nitrate and nitrite ions, in either neutral or basic media, using mainly a flow injection technique. Porous Cu and Cu-Ni were prepared by electrodeposition at high current density, exploiting the transient template action of hydrogen bubbles. Rh-modified Cu electrodes were obtained from porous Cu, through a galvanic displacement reaction. All materials had a linear response for both nitrates and nitrites, at concentrations up to 10 M, at least. Sensitivities, detection limits and stability were determined. Compared with Cu, used as a benchmark, (i) Rh-modified Cu had higher sensitivity for nitrates, comparable sensitivity for nitrites, lower or comparable detection limits and overall better stability; (ii) Cu-Ni had lower sensitivity, but exhibited lower detection limits and more stable performance for most analyte/medium combinations. [ABSTRACT FROM AUTHOR]

The oxygen bubble–templated deposition of porous PbO 2 was realized by anodizing Pb(II) solutions (in nitrate-acetate, sulfamate or methanesulfonate media) at large current density. This process is the anodic analogue of the more common hydrogen bubble–templated electrodeposition of porous metals. The porous PbO 2 layers consisted of a mixture of α- and β-phases, in contrast to compact layers deposited at low current density (pure α-PbO 2 ). The void volume fraction and surface roughness factor of porous PbO 2 were estimated. [ABSTRACT FROM AUTHOR]

To explore the potential of Electrochemical Impedance Spectroscopy (EIS) in monitoring a galvanic displacement reaction, the deposition of Rh by displacement of Ni has been investigated, using both Ni disc electrodes of small size and Ni foam electrodes. The Ni dissolution/Rh deposition rate has been evaluated from EIS data and compared with values obtained by an independent method. The variation of the electrode double layer capacity with the reaction duration, caused mainly by the growth of Rh deposits, has been compared with the time dependence of the Rh surface area assessed by cyclic voltammetry (through the measurement of the H desorption charge). The quality of the agreement between different methods and possible reasons for discrepancies are discussed. It is shown that EIS may be a useful tool in the study of galvanic displacement processes. [ABSTRACT FROM AUTHOR]

Porous Cu-Ni alloys with a wide range of compositions have been deposited from citrate baths containing the two metal ions, under application of large current densities (-3 A cm −2 ) leading to vigorous hydrogen evolution. The deposited alloy layers show macroscopic porosity and a spongy material made of a network of tiny dendrites. Porous RDEs reveal a significant current increase – as compared to ordinary polished electrodes – in the fast ferrocyanide reduction under pure mass transport control. Voltammograms of nitrate reduction in alkali at porous RDEs show at all potentials larger currents in comparison with experiments at compact electrodes of same composition. Electrolyses at porous sheet electrodes show large stable currents, leading to fast and selective nitrate reduction to ammonia. [ABSTRACT FROM AUTHOR]

Abstract: The performances of electrodeposited CuNi alloys in the electrochemical reduction of nitrates and nitrites in alkaline electrolyte have been studied and compared with those of the individual metals. CuNi alloys have been deposited from a citrate bath and characterized by SEM–EDS: a large range of compositions (30–95 Cuat.%) was prepared by varying the deposition potential. Application to the electrochemical reduction of nitrate (and nitrite) shows that Cu-rich alloys (Cu=70–95at.%) work better than Cu: voltammetric waves show a lower overpotential in the order of 120–150mV at fresh electrodes; chronoamperometries show higher initial currents and less severe current decay during prolonged polarization; electrolyses reduce more rapidly and with a better selectivity nitrate to ammonia. [Copyright &y& Elsevier]

Abstract: The spontaneous deposition of Pd onto Fecralloy sheets and foams was studied. Pd deposits were obtained by immersion of Fecralloy samples in mildly acid, deaerated PdCl2 solutions, at open circuit. Pd displaced all alloy elements and deposited onto the alloy which was initially coated by an oxides film and underwent transpassive dissolution. The Pd loading and the Pd surface area were determined as a function of experimental conditions, like the nature of the Fecralloy sample (sheets or foams of variable grade), the concentration of the PdCl2 solution and the duration of the deposition. The results were compared to those obtained by depositing Pd onto Ni foams, with the same procedure. The Pd deposition rates were not much different on Fecralloy and Ni, but the Pd surface area per unit Pd mass was significantly higher for Ni than for Fecralloy. The Pd deposit morphology was influenced by the presence of passive oxides on the Fecralloy surface. The catalytic activity in methanol anodic oxidation depended only on the Pd surface area, irrespective of the nature of the support (Ni or Fecralloy). [Copyright &y& Elsevier]

Abstract: A method for decorating the surface of disk electrodes with metal nanowires is presented. Cu and Ni nanowires with diameters from 1.0μm to 0.2μm are directly deposited on the electrode surface using a polycarbonate membrane filter template maintained in contact with the metal substrate by the soft homogeneous pressure of a sponge soaked with electrolyte. The morphologic and structural properties of the deposit are characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The latter shows that the head of nanowires with diameter of 0.4μm is ordinarily polycrystalline, and that of nanowires with diameter of 0.2μm is almost always monocrystalline for Cu and frequently also for Ni. Cyclic voltammetries and impedance investigations recorded in alkaline solutions at representative Ni electrodes decorated with nanowires provide consistent values of roughness factor, in the range 20–25. [Copyright &y& Elsevier]

Abstract: Spontaneous deposition of Pd onto porous electrodeposits of either Ni or Co yields active anodes for the oxidation of methanol and ethanol in basic aqueous media. These electrodes may sustain steady-state currents of the order of 0.25Acm− 2. Their peak current per unit Pd mass is about 100Ag− 1. [Copyright &y& Elsevier]

Abstract: The impedance of cylindrical electrodes and of rectangular sheet electrodes coated by passive films with either a constant or a graded thickness has been calculated using a finite element method for solving Laplace''s equation. The comparison of different electrode/cell geometries characterized by homogenous (on cylindrical electrodes) or inhomogeneous distributions of primary current (on rectangular sheet electrodes) and by homogeneous or graded thickness has allowed to clarify the origin of frequency dispersion phenomena. Calculated curves were compared with experimental impedance plots obtained with Nb and Ta sheet electrodes and with Nb cylindrical electrodes. The results of the finite element calculations for rectangular sheet electrodes are in better agreement with the experiments than those of a previous analytical calculation developed under a simplifying assumption (i.e. identical local electrolyte resistance at each point of the electrode surface) which was equivalent to neglecting current distribution induced by electrode and cell geometry. [Copyright &y& Elsevier]

Porous Ni electrodes were modified by electrodeposition of Ir nuclei from H2IrCl6 solutions at 70 °C, with the aim of activating them towards the hydrogen evolution reaction and comparing their performance with those of porous Ni electrodes activated by spontaneous deposition of noble metals (Ir and Ru). The current efficiency of Ir deposition was found to be very low (1% or lower, decreasing upon increasing deposition current density). Ir deposits characterised by SEM-EDX and XRD consisted of nanocrystals decorating the Ni dendrites forming the porous layers. Ir electrodeposition led to a strong activation of the hydrogen evolution reaction from aqueous 1 M NaOH. The electrocatalytic activity of the cathodes was independent of the Ir deposition charge above the minimum explored value of 1 C cm−2. This charge is estimated to correspond to the deposition of ca. 2.5 10−8 Ir moles cm−2. The kinetic parameters for hydrogen evolution were similar for porous Ni electrodes modified by either spontaneous deposition (studied in a previous work) or electrodeposition of Ir. [ABSTRACT FROM AUTHOR]

Abstract: Ni+IrO2 composites were deposited from suspensions of fine powders of the noble metal oxide in two Ni2+ containing electrolytes (a mildly acid NiCl2, NH4Cl solution and a basic NiSO4, (NH4)2SO4, NH4OH solution) using current densities varying from −5mAcm−2 to −2.5Acm−2. The composition, morphology and surface roughness of the deposits were studied as a function of the deposition current density (j DEP) and compared to that of Ni+RuO2 composites deposited from the chloride electrolyte. For all systems, the volume fraction of the dispersed phases ( or ) decreased as j DEP increased. At the high j DEP values usually employed to deposit porous Ni, was very low, irrespective of the deposition bath, whereas remained as high as 0.2. At the same j DEP values, the surface roughness of the Ni+RuO2 composites was ca. four times higher than that of the Ni+IrO2 composites, which was comparable to that of porous Ni devoid of dispersed particles. Since composites simultaneously possessing large surface roughness and large concentration of catalytic sites were obtained only when RuO2 was used as dispersed phase, the Ni+RuO2 cathodes had much higher catalytic activity in the hydrogen evolution reaction than Ni+IrO2 cathodes, the latter being only slightly more active than porous Ni cathodes. [Copyright &y& Elsevier]

Abstract: Porous Ni deposits, prepared by cathodic deposition, were modified by immersing them in acid deaerated solutions containing Ru(III) or Ir(IV) chloride complexes with which they readily reacted, without any activation procedure, giving rise to spontaneous deposition of either Ru or Ir. The obtained electrodes were investigated by cyclic voltammetry, impedance spectroscopy and scanning electron microscopy. All data showed that the initial large area of the Ni deposits further increased upon immersion in solutions of noble metal complexes. EDX analyses proved that the deposition of Ru reached a limiting situation in some hours, while that of Ir was slower and continued for a longer time. The persistence of intense peaks due to the Ni(II)/Ni(III) redox system showed that Ru and Ir did not form a continuous layer able to prevent the contact between Ni and electrolyte. Hydrogen evolution was studied in 1M NaOH solutions. Spontaneous deposition of both noble metals markedly improved the performance of porous Ni. The best results were achieved with Ir-modified electrodes, after immersion in Ir(IV) solution for 6h. Tafel slopes and overpotentials of Ru-modified electrodes were not as low as those of Ir-modified electrodes. [Copyright &y& Elsevier]

Abstract: RuO2 particles were co-deposited with Ni matrices, under experimental conditions known to produce porous Ni deposits, to yield Ni+RuO2 composites of large effective area, to be used as cathodes for the hydrogen evolution reaction. These conditions involved the use of NiCl2, NH4Cl solutions (pH 4.5) and large deposition current densities. The morphology and composition of these composites were studied by SEM–EDX and compared to those of either Ni+Co3O4 composites or Ni+RuO2 composites obtained from Watt''s baths. EIS was used to obtain information on the porosity of the deposited materials, and to measure the capacity of the composites and its dependence on deposition charge. The hydrogen evolution reaction was studied in both acid and basic media. Porous Ni+RuO2 cathodes performed better than analogous materials prepared from Watt''s baths. [Copyright &y& Elsevier]

Abstract: A review of the studies carried out during the last decade on the electrosynthesis of oxide–matrix composite materials for electrocatalysis is presented. The anodic deposition of oxide–matrix composites from electrolytes containing either Pb2+ or Tl+ ions and catalytic (nano)particles in suspension produces electrocatalytic materials especially suitable for anodic processes (oxygen evolution reaction) owing to the oxides stability towards oxidative degradation. The embedding of conducting catalytic particles may have a major effect on the nucleation and growth of the matrix, leading to very rough or porous deposits. The content in catalytic dispersed phase cannot be increased at will. However, dispersed phase volume fractions of the order of 0.1–0.2, attained without difficulty, are high enough to ensure marked electrocatalytic effects. In comparison with the pure matrices, composite anodes exhibit much lower overpotantials and lower Tafel slopes, comparable to those of electrocatalytic materials produced by other methods. [Copyright &y& Elsevier]

Abstract: A review of the studies carried out during the last decade on the electrosynthesis of oxide–matrix composite materials for electrocatalysis is presented. The anodic deposition of oxide–matrix composites from electrolytes containing either Pb2+ or Tl+ ions and catalytic (nano)particles in suspension produces electrocatalytic materials especially suitable for anodic processes (oxygen evolution reaction) owing to the oxides stability towards oxidative degradation. The embedding of conducting catalytic particles may have a major effect on the nucleation and growth of the matrix, leading to very rough or porous deposits. The content in catalytic dispersed phase cannot be increased at will. However, dispersed phase volume fractions of the order of 0.1–0.2, attained without difficulty, are high enough to ensure marked electrocatalytic effects. In comparison with the pure matrices, composite anodes exhibit much lower overpotantials and lower Tafel slopes, comparable to those of electrocatalytic materials produced by other methods. [Copyright &y& Elsevier]

Abstract: The anodic behaviour of p-Si electrodes is investigated in water–ethanol media by rotating disk voltammetry. The water–alcohol media, in comparison to water, lead to: (i) distorted current–potential curves, with a comparative current growth in the region of low applied potentials; (ii) a shift of the maximum of the current–pH curves from about pH 3 in water to about pH 4 in 50% ethanol v/v; (iii) a marked increase in dissolution current measured at pH>2, with an opposite effect at pH<2. The effect of ethanol on the current is discussed considering the modification of the speciation equilibria of hydrofluoric acid and possible effects on the kinetics of dissolution of the surface oxide layer. [Copyright &y& Elsevier]

The passivating effects produced on Ni49Ti51 (at.%) shape memory alloy by electropolishing in methanol–sulphuric acid electrolyte or thermal treatment in air are investigated with ion-beam techniques (secondary ion mass spectrometry (SIMS) and mass resolved ion scattering spectrometry (MARISS)) and electrochemical techniques (polarisation curves, photocurrent spectroscopy and impedance). Electropolishing produces a limited passivation due to formation of a thin layer of amorphous TiO2, which causes only a slight hindrance to the oxygen evolution reaction. Thermal treatment in air at 450°C causes a more substantial passivation due to formation of a thick oxide layer, estimated at about 80nm for a 16h treatment on the basis of weight increase, disregarding roughness. Data indicate that the film produced by thermal oxidation has probably a duplex structure, with a porous outer layer with inclusions of Ni species and a compact inner layer, in contact with the substrate, consisting of (essentially) nickel free TiO2 rutile. [Copyright &y& Elsevier]

The electrodissolution of p-Ge in aqueous NaOH solutions (pH 12–13) has been investigated with steady-state and impedance techniques. In the potential region where the process is under kinetic control, the impedance patterns are characterised by a high frequency capacitive loop, associated with the space charge, largely merged with another due to charge transfer resistance and double layer capacitance, and two low frequency loops changing from inductive to capacitive upon increasing the potential. At larger potential, under mixed control, the same high frequency features are observed, while a convection diffusion impedance dominates the low frequency response. The data relevant to pure kinetic control are interpreted with a simplified kinetic model, based on the classical scheme of Beck and Gerischer [Z. Electrochem., Ber. Bunsenges. Physik. Chem. 63 (1959) 500], neglecting transport effects and assuming only three electrochemical steps: (i) oxidation, involving water molecules, of Ge(0) to dihydroxylated Ge(II) by two holes; (ii) oxidation by one hole of Ge(II)–Ge(III) in a step requiring OH-; (iii) oxidative dissolution of Ge(III) proceeding either by hole capture or electron injection, leaving behind a surface Ge(0) species. Diagrams calculated for the limiting cases of pure depletion layer control reproduce the essential features of the experimental ones. [Copyright &y& Elsevier]

In the anodisation of Nb in acidic fluoride solutions or warm aqueous alkali, and in that of Mo in concentrated phosphoric acid, impedance spectroscopy provides evidence that 3D film growth proceeds already during the active–passive transition. Indeed, linear potential dependences of both the inverse of the high-frequency capacitance and of the product of the high-frequency resistance times the steady-state current are observed already below the peak potential. Simultaneously, a pseudo-inductive loop at intermediate frequencies is detected in the impedance spectra. In the present paper, we propose a kinetic model for the interpretation of these data. The model assumes that the metal is covered with a non-stoichiometric oxide containing at least two oxidation states of the cation. The processes of oxidative dissolution of the lower valence cations and their transformation to cations of higher valence, leading to passivation, explain the shape of the I–E curve. These processes are limited by both charge transfer at the film–solution (F/S) interface and transport of cation vacancies through the film. Thickening of the oxide film is assumed to proceed simultaneously, and is limited by transport of oxygen vacancies accelerated by an interfacial charge of cation vacancies. [Copyright &y& Elsevier]

The electrodeposition of composites consisting of a non-stoichiometric tungsten oxide matrix and either RuO2 or IrO2 dispersed particles was investigated. These materials were then tested as electrodes for both reduction (hydrogen evolution) and oxidation (oxygen evolution) reactions in acid medium. The composite deposition was carried out by cyclic voltammetry, potentiostatic or galvanostatic electrolysis of suspensions of the RuO2 or IrO2 particles in two different media described as appropriate for WO3 deposition: (i) a colloidal suspension obtained from Na2WO4 and H2SO4 or (ii) a solution obtained by dissolving W powder in aqueous H2O2. All the deposited composites were found to catalyse the hydrogen evolution reaction but none was active in oxygen evolution, presumably due to an inadequate electronic conductivity of the matrix. Such a behaviour differs from that of Pt-containing tungsten oxide electrodes which have been described as suitable anodes for the oxidation of organics. [Copyright &y& Elsevier]

Abstract: Cu–Rh alloys have been electrodeposited from sulfate solutions, pH 3.4. Although Rh is more noble than Cu, the reduction peak of Rh3+ was ca. 0.4V more negative than that of Cu2+. Therefore, the Rh/Cu ratio in the deposited alloys, determined by EDS analyses, increased as the deposition potential became more negative. XRD analyses showed that the deposited alloys were polycrystalline and suggested that metastable CuxRh1−x solid solutions were formed for 0.1≤x≤0.8, although for such compositions a miscibility gap is known to exist. The Cu–Rh alloys were tested as cathodes for the reduction of nitrates and found to have significant catalytic activity. [Copyright &y& Elsevier]

Compact Ag and Ag–Ni alloy layers have been electrodeposited from a concentrated chloride bath. The composition of the alloy can be controlled by regulation of the deposition current density j dep under conditions of accurate transport control like those ensured by the rotating disk electrode. XRD analyses suggest formation of Ag-rich crystalline phases in all samples and of Ni-rich crystalline phases only in samples with Ni fraction ≥50 at%. Test experiments of nitrate reduction in alkali show: almost exclusive reduction to nitrite on Ag; prevalent reduction to ammonia on Ni, with an important side evolution of hydrogen; predominant and more efficient reduction to ammonia on Ag–Ni, with lower hydrogen evolution and better current stability. Image 1 • Electrodeposition of compact Ag–Ni alloys from simple chloride baths. • Accurate tuning of composition by regulation of the deposition current density. • No substantial inclusion of foreign elements. • Exploration of the performances as cathode for nitrate reduction. [ABSTRACT FROM AUTHOR]

Porous oxide layers active in the oxygen evolution reaction (OER) were prepared with the following sequence of steps: (i) porous Pb was produced by cathodic hydrogen bubble templated electrodeposition; (ii) Pb was partially converted to its oxides; (iii) Co 3 O 4 shells were formed on the surface of lead/lead oxides nanowires by spontaneous galvanic displacement reactions. The effect of deposition current density and charge on the porous Pb morphology was studied. Deposits with extended surface areas were obtained and then oxidized in neutral Na 2 SO 4 solutions. Pb and PbO 2 , initially coexisting in oxidized samples, underwent a spontaneous reaction leading to the formation of Pb 3 O 4 . Both PbO 2 and Pb 3 O 4 spontaneously reacted with Co 2+ to yield Co 3 O 4 layers. Co-modified electrodes described in this work had higher activity in oxygen evolution reaction (OER) than those obtained by submitting to galvanic displacement porous PbO 2 layers prepared by oxygen bubble templated anodic deposition. [ABSTRACT FROM AUTHOR]

Zinc is recently gaining interest in the battery community as potential alternative anode material, because of its large natural abundance and potentially larger volumetric density than graphite. Nevertheless, pure Zn anodes have shown so far very poor cycling performance. Here, the electrochemical performance of Zn-rich porous Cu-Zn alloys electrodeposited by an environmentally friendly (aqueous) dynamic hydrogen bubble template method is reported. The lithiation/delithiation mechanism is studied in detail by both in situ and ex situ X-ray diffraction, indicating the reversible displacement of Zn from the Cu-Zn alloy upon reaction with Li. The influence of the alloy composition on the performance of carbon- and binder-free electrodes is also investigated. The optimal Cu:Zn atomic ratio is found to be 18:82, which provides impressive rate capability up to 10 A g−1 (≈30 C), and promising capacity retention upon more than 500 cycles. The high electronic conductivity provided by Cu, and the porous electrode morphology also enable superior lithium storage capability at low temperature. Cu18Zn82 can indeed steadily deliver ≈200 mAh g−1 at −20 °C, whereas an analogous commercial graphite electrode rapidly fades to only 12 mAh g−1. [ABSTRACT FROM AUTHOR]

Galvanic displacement reactions between porous PbO 2 , prepared by oxygen bubble templated anodic deposition, and the low-valent cations Mn 2+ , Co 2+ and Sn 2+ , have been studied using electrochemical methods, SEM-EDS and XPS. These reactions occur at open circuit by immersion of PbO 2 coated electrodes in aqueous solutions of the cations and lead to the formation of outer oxide layers onto porous PbO 2 , whose composition and chemical nature have been assessed by XPS. The growth of the outer oxide layers has been studied by combining Evans’ diagrams with SEM-EDS and XPS results, as a function of experimental variables, such as the chemical nature of the cations, their concentration and the solution temperature. The combination of oxygen bubble templated anodic deposition and galvanic displacement provides a route for the preparation of porous low-conductivity electroactive oxides not attainable by direct electrodeposition. Deposition of Co 3 O 4 thin layers by galvanic displacement results in a strong enhancement of the activity of porous PbO 2 electrodes in the oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

The optoelectronic features of 3-hydroxyflavone (3HF) self-assembled on the surface of an n-type semiconducting metal oxide (TiO2) and an insulator (Al2O3) are herein investigated. 3HF molecules use the coordinatively unsaturated metal ions present on the oxide surface to form metal complexes, which exhibit different behaviors upon light irradiation, depending on the nature of the metal ion. Specifically, we show that the photoluminescence of the surface species can be modulated according to the chemical properties of the complex (i.e. the binding metal ion), resulting in solid-state emitters in a high quantum yield (about 15%). Furthermore, photoinduced charge injection can be promoted or inhibited, providing a multifunctional hybrid system. [ABSTRACT FROM AUTHOR]

Abstract: Electrocatalysts for methanol oxidation were prepared through a two stage deposition process: porous cobalt layers were deposited, by cathodic reduction of Co2+ ions, and then modified by spontaneous deposition of Pd. A basic sulphate solution and a mildly acid chloride solution were compared as media for the electrodeposition of Co. Deposits with the highest surface roughness were obtained in the chloride solution, at large current densities. Pd was deposited onto the Co porous layers by immersing them in acid deaerated PdCl2 solutions, at open circuit. The Pd loading and the Pd surface area were estimated by UV–visible spectroscopy and by cyclic voltammetry, respectively. The Pd-modified Co electrodes were tested as anodes for methanol oxidation and compared to the similarly prepared Pd-modified Ni electrodes. The former exhibited better stability of performance and higher methanol oxidation peak currents per unit Pd mass, ca. 200Ag−1. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Anodic polarisation of Nb in warm and concentrated aqueous alkali results in sustained electrodissolution. The process is investigated in NaOH by voltammetric, steady-state and impedance techniques. The j–E steady-state curve is characterised by a current peak and a subsequent plateau extending up to the explored positive limit (8 VSCE). Upon addition of tartrate to the electrolyte the dissolution current increases markedly. In some NaOH+Na tartrate electrolytes a dissolution–precipitation mechanism sets in, with formation of a surface layer which causes a gradual current decrease; this layer is identified by XRD and elemental analysis as a mixed Na and Nb oxide hydrate. Larger currents may be sustained in KOH+K tartrate solutions without any precipitation process. Impedance diagrams recorded in the plateau region are analysed on the basis of the equivalent circuit resulting from the surface charge model; the oxide formation ratio and the main model parameters are estimated and their temperature dependence is discussed. [Copyright &y& Elsevier]

• Effect of ions released in oxide-oxide galvanic displacement reactions was studied. • Displacement between PbO 2 and Mn2+ or Co2+ were used as model reactions. • Secondary mixed oxides produced at higher [Pb2+] were richer in Pb. • Other effects of [Pb2+] on secondary oxide growth depended on divalent cation. Galvanic displacement reactions between a solid oxide and a dissolved metal cation are interesting processes for the preparation of oxide nanoparticles or electrocatalytic layers consisting of secondary mixed oxides. Their mechanism is not yet fully clarified. The composition of the secondary mixed oxides and their growth rate depend on many physical and chemical variables. In the present study, we have focused on the effects of the concentration of the ions released from the sacrificial oxide to the solution, when they are intentionally added to reaction media. We have studied the displacement of sacrificial PbO 2 by either Mn2+ or Co2+ cations in acetate solution that contained variable concentrations of Pb2+, using electrochemical methods, SEM-EDS and XPS. The evolution of the open circuit potential of the systems was monitored during the reactions. We have found that, for both divalent cations, increasing concentrations of Pb2+ ions in the acetate solutions caused the formation of mixed oxides richer in Pb. Effects on growth rate and equilibrium potential were different for Mn2+ and Co2+. [ABSTRACT FROM AUTHOR]

The galvanic displacement reaction between sacrificial PbO 2 layers and Fe(II) ions has been carried out in mildly acid acetate solutions. The effect of experimental variables, like morphology of the PbO 2 layers, solution pH, temperature and concentration of Fe(II) ions, has been investigated by combining electrochemical methods, SEM-EDS, XRD and XPS. The reaction between PbO 2 and Fe(II) ions produced secondary dual layers, consisting of an inner amorphous FeOOH film and an outer crystalline γ-FeOOH deposit, respectively. This peculiar behavior was in contrast with analogous reactions involving other transition metal ions, which yielded only continuous amorphous secondary oxide layers. PbO 2 /FeOOH layers were tested as anodes for the oxygen evolution reaction in basic media. Unlabelled Image • Porous FeOOH layers are prepared by oxide-oxide galvanic displacement reactions • Secondary deposits consist of inner amorphous and outer crystalline γ-FeOOH layers • FeOOH growth rate increases with pH and temperature, but is essentially independent of [Fe2+] • PbO 2 /FeOOH electrodes catalyze oxygen evolution reaction [ABSTRACT FROM AUTHOR]

The electrochemiluminescence (ECL) performances were comparatively investigated at flat and nanoporous gold (NPG) electrodes of different thicknesses (120 and 200 nm) and roughness factors ( f r ). The phenomena were studied using either tripropylamine (TPrA) or peroxydisulfate (S 2 O 8 2− ) as sacrificial coreactant and Ruthenium (II)-tris(2,2′-bipyridine) as emitting species. The experiments performed using TPrA showed, at first glance, a linear dependence of the ECL emission with respect to the effective surface area of the NPG electrodes. However, ECL signals were not stable in the measuring conditions, presumably due to amine absorption on the metal surface, leading to electrode corrosion and modification of the surface morphology. The experiments made using peroxydisulfate as coreactant provided conversely a stable ECL response, about proportional to the effective electrode surface area, in the considered range of thicknesses. [ABSTRACT FROM AUTHOR]

Galvanic displacement reactions between sacrificial oxide layers and metal cations, leading to the formation of secondary oxide layers, have been studied with the aim of collecting new experimental evidence on their mechanism. The study has been carried out by combining electrochemical methods, SEM-EDS and XPS depth profiling. Both porous and compact PbO 2 layers have been used as sacrificial oxides and reacted, in different experiments, with: (i) a single low-valent cation, (ii) two cations in sequential exchange reactions or (iii) two cations simultaneously. The experimental results converged to show (i) a lack of correlation between the reaction driving force and the secondary oxide growth rate, (ii) the incorporation of cations from solution at the secondary oxide/electrolyte interface, (iii) the transport of Pb species through the growing secondary oxide layer and (iv) a major effect of mass transport on the growth rate. Image 1 • The mechanism of galvanic displacement between oxides has been studied. • Secondary and tertiary oxides were grown onto compact and porous PbO 2. • Secondary oxides contained significant amounts of Pb. • The growth rate of secondary oxides was not governed by the reaction driving force. • Diffusion through the growing secondary oxide was rate determining. [ABSTRACT FROM AUTHOR]

There is a growing interest in extending plasmonics applications into the ultraviolet region of the electromagnetic spectrum. Noble metals are commonly used in plasmonic, but their intrinsic optical properties limit their use above 350 nm. Aluminum is probably the most suitable material for UV plasmonics, and in this work we fabricated substrates of nanoporous aluminum starting from an alloy of Al2Mg3. The porous metal is obtained by means of a galvanic replacement reaction. Such nanoporous metal can be exploited to achieve a plasmonic material suitable for enhanced UV Raman spectroscopy and fluorescence. Thanks to the large surface to volume ratio, this material represents a powerful platform for promoting interaction between plasmonic substrates and molecules in the UV. [ABSTRACT FROM AUTHOR]