Surface Praseodymium Species on MgO: Characterization and Activity for Oxidative Coupling of Methane

On pulsing CH₄ over MgO containing various amounts of praseodymium oxide (PrO_x) at 1023 K, the CH₄ conversion decreased with increasing pulse number, and both the initial activity and selectivity to C₂ products (corresponding to the first pulse) decreased with increasing PrO_x content. Characterization by XRD, SEM-EDX and XPS showed calcined materials to contain well-dispersed PrO_x (x = 1·83–2) at low Pr concentrations, but only crystalline PrO_1·83 at high (10 wt%) Pr concentration. A PrO_1·83 phase was present at all Pr concentrations after the He treatment at 1023 K, and PrO_1·83, PrO_1·75 and PrO_1·5 after reaction. © 1997 SCI     

Oxidation of carboxylic acids at boron-doped diamond electrodes for wastewater treatment

Thin boron-doped diamond films have been prepared by HF CVD (hot filament chemical vapour deposition technique) on conductive p-Si substrate (Si/Diamond). The morphology of these Si/diamond electrodes has been investigated by SEM and Raman spectroscopy. The electrochemical behaviour of the Si/diamond electrodes in 1 M H₂SO₄ and in 1 M H₂SO₄ + carboxylic acids has been investigated by cyclic voltammetry. Finally, the electrochemical oxidation of some simple carboxylic acids (acetic, formic, oxalic) has been investigated by bulk electrolysis. These acids can be oxidized at Si/diamond anodes to CO₂, in the potential region of water and/or the supporting electrolyte decomposition, with high current efficiency.     

The underlying electrode causes the reported ‘electro-catalysis’ observed at C60-modified glassy carbon electrodes in the case of N-(4-hydroxyphenyl)ethanamide and salbutamol

The reported ‘electro-catalysis’ of C₆₀-film-modified electrodes for the electrochemical oxidation of N-(4-hydroxyphenyl)ethanamide and salbutamol has been explored at boron-doped diamond and glassy carbon electrodes. Using both C₆₀-film-modified boron-doped diamond and glassy carbon as underlying electrode substrates no electro-catalytic response is observed using the target analytes but rather the C₆₀ serves to block the electrode surface.A common experimental protocol used by researchers in this field is to electrochemically pre-treat the C₆₀-film-modified electrode. The response of employing this electrochemical pre-treatment at both bare glassy carbon and boron-doped diamond electrodes using the target analytes reveals that no effect on the electrochemical responses obtained at the boron-doped diamond electrode whereas a slight but significant effect occurs on glassy carbon which is attributed to the likely introduction of surface oxygenated species.Consequently the previously reported ‘electro-catalysis’ using C₆₀-film-modified electrode is not due to C₆₀ itself being catalytic, but rather that substrate activation through electrode pre-treatment is responsible for the observed ‘electro-catalysis’ likely through the introduction of surface oxygenated species.This work clearly shows that substrate activation is an important parameter which researchers studying C₆₀-film-modified electrodes, especially in electro-analysis needs to be considered.     

The impedance of fast charge transfer reactions on boron doped diamond electrodes

Reversible charge transfer on boron doped diamond (BDD) electrodes was studied using cyclic voltammetry and electrochemical impedance spectroscopy. Polycrystalline diamond films of 5 μm thickness with 200 and 3000 ppm boron content were prepared by chemical vapour deposition on niobium substrate. The samples were mounted in a Teflon holder and used as rotating disk electrodes (RDE) with rotation frequencies between 0 and 4000 rpm. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 0.1 M KCl + 5 mM [Ru(NH₃)₆]Cl₂/[Ru(NH₃)₆]Cl₃. The electrochemical redox behaviour of the BDD electrodes was found to differ significantly from that of an active Pt electrode. The deviations are indicated by a large peak potential difference and a shift of the peak potentials in cyclic voltammograms with increasing sweep rate. At rotating electrodes lower limiting current densities are found and the impedance diagrams exhibit an additional capacitive impedance element at high frequencies. The results are described quantitatively by an impedance model which is based on partial blocking of the diamond surface.     

MnO2-coated Ni nanorods: Enhanced high rate behavior in pseudo-capacitive supercapacitor

Ni nanorods prepared by electrochemical growth through an anodized aluminium oxide membrane were used as substrate for the electrodeposition of MnO₂ either in potentiostatic mode or by a pulsed method. Electrochemical deposition parameters were chosen for an homogeneous deposit onto Ni nanorods. Resulting Ni supported MnO₂ electrodes were tested for electrochemical performances as nanostructured negative electrodes for supercapacitors. They exhibited initial capacitances up to 190 F/g and remarkable performances at high charge/discharge rates.     

Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 m thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution at rotation frequencies 0 < f _rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.     

Electrochemical preconditioning of moderately boron doped diamond electrodes: Effect of annealing

This paper is concerned with the study of electrochemical preconditioning on moderately boron doped diamond electrodes. Samples were submitted to an isothermal annealing at 1100 °C in order to outgas the hydrogen introduced into the layer during the deposition process. Consequences of anodic and cathodic galvanostatic steps (1 C cm^− 2), in H₂SO₄ 0.5 M, have been studied on both as-deposited samples and annealed ones, by capacitance measurements, cyclic voltammetry in presence of Ce^4+/3+ redox system and XPS measurements. The results of Mott–Schottky plots and current voltage curves show that the electrochemical responses of BDD electrodes are strongly influence by annealing. After preconditioning, an enhancement of charge transfer is observed for as-deposited samples, while a more and more passivated behavior is recorded for annealed electrodes. On as-deposited samples a “new” superficial conductive layer linked to the creation of surface defects high above the valence band, is suggested after a specific electrochemical treatment which is not possible on annealed ones.     

Stability of iodine on ruthenium during copper electrodeposition and its effects on the nucleation behavior of electrodeposited copper

Cyclic voltammetry, current-time-transient measurements, and X-ray photoelectron spectroscopy (XPS) have been used to study the nucleation behavior of electrochemically deposited Cu films on Ru substrates as a function of Ru pre-treatment. Pre-treatment consisted of cathodic polarization in either 1 M H₂SO₄ or in 1 M H₂SO₄ + 1 mM KI, followed by sample emersion and placement in a 1 M H₂SO₄ + 50 mM CuSO₄ plating bath. XPS measurements confirmed the presence of adsorbed I on the Ru surface following pre-treatment in the KI/H₂SO₄ solution. Cyclic voltammogram (CV) data for electrodes either as-received or pre-reduced in H₂SO₄ and then immersed in the plating solution exhibited a broad peak in the overpotential region consistent with oxide reduction followed by Cu deposition. No underpotential deposition (UPD) feature was observed for these electrodes. In contrast, the sample pre-reduced in I-containing electrolyte exhibited a narrow Cu deposition peak in the overpotential region and a UPD Cu feature centered at 80 mV vs. Ag/AgCl. Current-time-transient (CTT) measurements of Cu deposition on as-received electrodes or electrodes pre-reduced in I-free solution exhibited potential-independent kinetics that are not well described by either progressive or instantaneous nucleation models and which at long times indicate a combination of diffusion and kinetic control. In contrast, CTT measurements of deposition kinetics for samples reduced in I-containing electrolyte exhibited complex, potential-dependent behavior and that at long times indicates diffusion control. XPS results also indicated that the iodine adlayer on Ru reduced in I-containing electrolyte is stable upon polarization to at least −200 mV vs. Ag/AgCl. These data indicate that a protective I adlayer may be deposited on an air-exposed Ru electrode as the oxide surface is electrochemically reduced, and that this layer will inhibit reformation of an oxide during the Cu electroplating process. Therefore, electrochemical pre-treatment in I-containing electrolyte may be of practical utility under industrial conditions for Cu electroplating.     

Investigation of the surface structure of dysprosium and praseodymium oxide-based polycrystalline films sensitive to ozone and vapors of ethanol and methanol

The surface of dysprosium and praseodymium oxide-based films is investigated by atomic-force microscopy and electron probe X-ray microanalysis. The electrical properties of the films are determined. It is shown that the electrical conductivity of the Dy₂O₃ films can be substantially changed by introducing dopants of zirconium and praseodymium oxides into the initial solution used for synthesizing the films (in particular, the ZrO₂-based dopant increases the electrical conductivity of the Dy₂O₃ film and the PrO_x-based dopant affects the conductivity in a complex way). The temperature dependence of the film sensitivity to ozone and vapors of ethanol and methanol is studied.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Tikhonov, Nakusov, Popova, Konyukhov.     

Improved capacitive behavior of electrochemically synthesized Mn oxide/PEDOT electrodes utilized as electrochemical capacitors

A sequential synthetic approach and a one-step method were adopted to synthesize Mn oxide/PEDOT electrodes through anodic deposition on Au coated Si substrates from aqueous solutions. In the former case, free standing Mn oxide rods (about 10 μm long and less than 1.5 μm in diameter) were first synthesized without a template through anodic deposition from a dilute solution of Mn acetate, then coated by electro-polymerization of a conducting polymer (PEDOT) giving coaxial rods. The one-step, co-electrodeposition method produced agglomerated Mn oxide/PEDOT particles. The electrochemical behavior of the deposits depended on the morphology and crystal structure of the fabricated electrodes, which were affected by the pH of electrolyte, deposition potential, current density and polymer deposition time. Structural characterization of as-deposited and cycled electrodes was conducted using XPS, SEM, TEM and AES.The Mn oxide/PEDOT coaxial core/shell electrodes prepared by the sequential method showed significantly better specific capacity and redox performance properties relative to both uncoated Mn oxide rods and co-electrodeposited Mn oxide/PEDOT electrodes. The best specific capacitance for Mn oxide/PEDOT rods produced sequentially was ∼285 F g⁻¹ with ∼92% retention after 250 cycles in 0.5 M Na₂SO₄ at 20 mV s⁻¹.     

Porous diamond with high electrochemical performance

Synthetic diamond materials are currently attracting attention for applications such as thin films supercapacitors or medical implantable electrodes where chemically stable materials featuring high double layer capacitance as well as low electrochemical impedance are sought. Those properties may be reached with high aspect ratio diamond provided that current collection is done efficiently through the diamond layer. In this paper, we introduce a very novel material, namely SPDia™, based on boron-doped diamond grown on a highly porous polypyrrole scaffold prepared by chemical vapour deposition. This composite was first characterised by SEM and Raman spectroscopy to cheque the diamond crystallinity and the structural evolution of the polypyrrole during the CVD process. Then cyclic voltammetry and electrochemical impedance spectroscopy were performed to assess its electrochemical reactivity. It was found to exhibit remarkable properties, that include a large double layer capacitance with values reaching up to 3 mF cm⁻² in aqueous LiClO₄ and a low electrochemical impedance, thus highly competitive with respect to other nanostructured diamond materials as recently reported.     

Direct electrochemistry of blue copper proteins at boron-doped diamond electrodes

Boron-doped diamond (BDD) is a promising electrode material for use in the spectro-electrochemical study of redox proteins and, in this investigation, cyclic voltammetry was used to obtain quasi-reversible electrochemical responses from two blue copper proteins, parsley plastocyanin and azurin from Pseudomonas aeruginosa. No voltammetry was observed at the virgin electrodes, but signals were observed if the electrodes were anodised, or abraded with alumina, prior to use. Plastocyanin, which has a considerable overall negative charge and a surface acidic patch which is important in forming a productive electron transfer complex with its redox partners, gave a faradaic signal at pre-treated BDD only in the presence of neomycin, a positively charged polyamine. The voltammetry of azurin, which has a small overall charge and no surface acidic patch, was obtained identically in the presence and absence of neomycin. Investigations were also carried out into the voltammetry of two site-directed mutants of azurin, M64E azurin and M44K azurin, each of which introduce a charge into the protein's surface hydrophobic patch. The oxidizing and cleaning effects of the BDD electrode pre-treatments were studied electrochemically using two inorganic probe ions, Fe(CN)₆³⁻ and Ru(NH₃)₆³⁺, and by X-ray photoelectron spectroscopy (XPS). All of the electrochemical results are discussed in relation to the electrostatic and hydrophobic contributions to the protein/diamond electrochemical interaction.     

Modeling the impedance versus voltage characteristics of LiNi0.8Co0.15Al0.05O2

The power-delivery capability of lithium-ion cells based on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes shows a significant dependence on the cell's state-of-charge. One reason for this behavior is the variation of the positive electrode's impedance with the oxide's lithium content. In this article, an electrochemical model based on concentrated solution porous electrode theory is used to model impedance data obtained on LiNi_0.8Co_0.15Al_0.05O₂-based positive electrodes charged to potentials ranging from 3.55 to 4.55 V versus Li. The parameters obtained from model fits include the exchange-current density and Li-ion diffusion coefficients in the oxide. The variations in these parameters with oxide potential are correlated with structural changes in the material observed during Li-ion intercalation-deintercalation reactions.     

A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells

Deposition processes of chromium (Cr) species were investigated for the O₂ reduction on (La,Sr)MnO₃ (LSM), Pt and (La,Sr)(Co,Fe)O₃ (LSCF) electrodes in the presence of chromia-forming alloy metallic interconnect at 900^oC under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode or electrolyte surface for the O₂ reduction reaction on Pt electrodes. The results clearly demonstrated that deposition of Cr species in solid oxide fuel cells is not an electrochemical reduction of high valent Cr vapor species to Cr₂O₃ in competition with O₂ reduction. Cr deposition at SOFC cathodes is basically a chemical dissociation reaction and is controlled by the nucleation reaction between the nucleation agent and the gaseous Cr species. The nature of the nucleation agent strongly depends on the electrode material and impurities which may be introduced during electrode and electrolyte fabrication processes.     

Microwave-electrochemical formation of colloidal zinc oxide at fluorine doped tin oxide electrodes

Colloidal ZnO is obtained during microwave-enhanced electrochemical deposition experiments from an aqueous solution containing 0.1 M Zn(NO₃)₂ and 0.02 M H₂O₂ via repetitive negative going potential cycles from 0.3 to −0.8 V vs. SCE. The effects of temperature and temperature gradients on ZnO electro-formation at fluorine doped tin oxide (FTO) electrodes are investigated with both a conventional thermostated bath system (isothermal) and an in situ microwave electrochemistry system (non-isothermal). Mainly electrodeposition of ZnO is observed in uniformly heated stagnant solution and predominantly the electro-formation of ZnO colloid is observed in the presence of microwave-induced temperature gradients in a flowing solution. For the ZnO colloid prepared via microwave activation, SAXS data suggests an average particle radius of ca. 18 nm. The increase of ZnO nanoparticle concentration during repetitive potential scans is followed by photoluminescence spectroscopy. A possible mechanism for ZnO colloid formation during electrochemical reduction of H₂O₂ is suggested.     

Effects of cobalt and cobalt oxide buffer layers on nucleation and growth of hot filament chemical vapor deposition diamond films on silicon (100)

An initial study on the nucleation and growth of diamond, using hot filament chemical vapor deposition (HFCVD) technique, was carried out on Co and CoO thin buffer layers on non-carbon substrates (Si (100)), and the results were compared with conventional scratching method. The substrate temperature during the growth was maintained at 750±50 °C. A mixture of CH₄ and H₂ (1: 100 volume %) was used for deposition. The total pressure during the two hour deposition was 30±2 Torr. X-ray photoelectron spectroscopy (XPS) study showed the diamond nucleation at different time periods on the Co and CoO seed layers. It is observed that Co helps in nucleation of diamond even though it is known to degrade the quality of diamond film on W-C substrate. The reason for improvement in our study is attributed to (i) the low content of Co (~0.01%) compared to W-C substrate (~5–6%), (ii) formation of CoSi₂ phase at elevated temperature, which might work as nucleation sites for diamond. SEM analysis reveals a change in the morphology of diamond film grown on cobalt oxide and a significant reduction in the size of densely packed crystallites. Raman spectroscopic analysis further suggests an improvement in the quality of the film grown on CoO buffer layer.     

In-situ exsolution of PrO2−x nanoparticles boost the performance of traditional Pr0.5Sr0.5MnO3-δ cathode for proton-conducting solid oxide fuel cells

By synthesizing the nominal Pr_xSr_0.5MnO_3-δ materials (x = 0.5, 0.6, 0.7, 0.8), new Pr_0.5Sr_0.5MnO_3-δ (PSM50)+PrO_2−x composite cathodes for proton-conducting solid oxide fuel cells (SOFCs) were developed. The structure analysis and morphology observations verified the exsolution of PrO_2−x particles, and the amount of exsolved PrO_2−x increased with the amount of Pr in Pr_xSr_0.5MnO_3-δ. An H-SOFC with a Pr_0.7Sr_0.5MnO_3-δ (PSM70) cathode enabled the highest reported fuel cell output for H-SOFCs with manganate cathodes. The construction of a PSM50/PrO₂ heterostructure interface can reduce the formation energy of oxygen vacancies, hence accelerating the cathode oxygen reduction reaction (ORR) kinetics, as confirmed by oxygen diffusion and surface exchange experiments. The excellent electrochemical performance was combined with its good chemical stability against CO₂ and H₂O, allowing a stable operation of the cell for over 100 h, indicating that PSM70, which was in fact PSM50 +PrO_2−x, was a highly efficient and durable cathode material for H-SOFCs.     

Ceramic nanocomposites based on oxides of transition metals for ionistors

Low-temperature synthesis methods are used to produce nanoceramic materials for electrodes of the following ionistors: (ZrO₂)_0.6(In₂O₃)_0.4, praseodymium cobaltite, as well as neodymium, lanthanum, and nickel chromites; they operate in the presence of an ion-conducting phosphorosilicate separator membrane and phosphate impregnation. Film electrodes of ionistors are fabricated that consist of nanocrystalline oxide materials deposited as a thin film on a porous electroconductive metal substrate, i.e., foamed nickel. The MnO₂-foamed nickel electrode has a specific capacity of 45.0 F g^?1, which is compared with that of industrial supercapacitors.     

TiO2 photocatalysts and diamond electrodes

Photocatalysis and electroanalysis are two seemingly disparate research areas, but they are linked by the fact that both involve the use of well-known materials, TiO₂ and diamond, respectively, in new ways in the service of both environmental and medical sciences. In the present article, recent developments in the area of TiO₂ photocatalysis and diamond electrochemistry are summarized, with emphasis on our findings at the University of Tokyo. In the photocatalysis section, we present the fundamental aspects of TiO₂ photocatalysis and its practical applications, including air purification, self-cleaning surfaces and transparent superhydrophilic coatings. The diamond electrochemistry section deals with the electrochemical characterization and applications of diamond electrodes, which exhibit high sensitivity and excellent stability for electroanalysis, in contrast to conventional electrode materials. A particularly interesting environmental application of diamond electrodes has been developed; this involves the trace analysis of lead without the use of mercury.     

Characterisation and behaviour of Ti/TiO2/noble metal anodes

The morphology, composition and the electrical and electrochemical behaviour of the anodic microporous layer, prepared by the galvanostatic anodisation of Ti after sparking, followed by galvanostatic deposition of Pt or Ir have been investigated. These electrodes are proposed to function as dimensionally stable anodes (DSAs). For Ti/TiO₂/Pt electrodes, Pt is deposited within some of the micropores of the oxide film. In contrast, for Ti/TiO₂/Ir, the metal is deposited preferentially on the top surface. This difference is thought to result from the position of the metal deposition potential with respect to the flat band potential of n-TiO₂. Optical imaging of both types of DSA suggests that only a few sites on the surface are responsible for electron exchange at the DSA-electrolyte interface. C-AFM measurements of Ti/TiO₂/Pt samples subjected to long-term anodic polarisation, suggest that the Ti-noble metal contact is progressively insulated by thickening of the TiO₂ barrier layer, promoting passivation of the DSA. For Ir coated anodes, catalytic activity is directly related to the presence of Ir and to the stability of the catalytic oxide layer. Under Cu electrowinning conditions, the electrochemically formed hydrated Ir oxide was found to be catalytically less stable, than the iridium oxide film subjected to a heat treatment.