Low temperature preparation of carbon-supported PdCo alloy electrocatalysts for methanol-tolerant oxygen reduction reaction

Carbon-supported PdCo alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH₄F as a complexing agent and H₃BO₃ as a buffer, followed by NaBH₄ reduction. As-prepared PdCo bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared PdCo alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd₂Co₁/C catalyst is ca. 3.0 nm, which is much smaller than that of the PdCo/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared PdCo/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on PdCo/C catalysts follows a four-electron process leading to water. Moreover, the PdCo/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC).     

Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound

A clean and simple wet chemical process using dilute aqueous ozone (O₃) solution with or without ultrasound (US) was used to functionalize single-walled carbon nanotubes (SWCNTs). Both O₃ and O₃/US treatments greatly increased the stability of SWCNTs in water. Results of X-ray photoelectron spectroscopy (XPS) showed that the surface oxygen to carbon atomic ratio increased by more than 600% after 72 h of O₃ treatment. Moreover, the effective particle size of SWCNTs was reduced from the initial 4400 to ∼300 and ∼150 nm, after 24 h of O₃ and O₃/US treatment, respectively. The zeta potential of treated SWCNTs decreased from 3.0 to −35.0 mV (at pH 4) after 2 h of treatment with both O₃ and O₃/US. Based on the XPS results, the oxidation pathway was proposed: at the onset of the oxidation reaction, the CC double bond was first converted to COH which was then oxidized to CO and OCOH concurrently. Oxidation reactions could be described well with first order expressions. Treatment time controlled the extent of surface oxidation and subsequently the stability and dispersion of SWCNTs in water.     

Oxidization of carbon nanotubes through hydroxyl radical induced by pulsed O2 plasma and its application for O2 reduction in electro-Fenton

The oxidation of carbon nanotubes (CNTs) by hydroxyl radical produced by pulsed O₂ plasma in a gas-liquid hybrid discharge reactor was conducted with the goal of enhancing their solubility and improving the yield of H₂O₂ in electro-Fenton. Data from the characterization experiments showed that oxygen bearing groups (COH, COO, COOH, CO) were formed on the surface of CNTs. The possible mechanism indicated that introduction of oxygen bearing groups onto CNTs could be attributed to the attacks by hydroxyl radical. The oxidized CNTs were easily dispersed in ethanol. The H₂O₂ yield on the original CNTs was 102 mg/L at −0.85 V after 90 min; in contrast, H₂O₂ yield on CNTs-15 reached 146 mg/L under the same conditions, resulting from the enhancement of the accessibility of O₂ on CNTs. In the electro-Fenton, the removal of methyl orange on the original CNTs was around 40%, and it increased to 95% on CNTs-15.     

Electrochemical anticorrosion performance evaluation of Al2O3 coatings deposited by MOCVD on an industrial brass substrate

Alumina (Al₂O₃) coatings of different thickness were deposited on OT59 brass substrate (BS) using the metal organic chemical vapour deposition (MOCVD) technique to evaluate the corrosion performance by EIS measurements. The used precursor was dimethyl-aluminium-isopropoxide. Electrochemical characterizations of the deposited films were performed in a standard very aggressive acidic solution (aerated 1N H₂SO₄ at 25 °C up to 168 h of immersion time) by means of direct current method (Tafel curves) and electrochemical impedance spectroscopy (EIS). The Rutherford backscattering spectroscopy (RBS) indicated that the films are very pure with the correct Al₂O₃ stoichiometry, while the IR absorption spectra showed that the films did not contain any OH groups. The surface film morphology was investigated by atomic force microscopy (AFM) and displayed a globular texture. The films were very smooth, with a maximum root mean square roughness, for example, of 14 nm for a 0.96 μm thick coating. The EIS data confirmed, as expected, that a 2.40 μm Al₂O₃ layer ensures the best corrosion protection after 168 h of immersion in the very acidic environment used.     

Electrochemical sensors for detection of hydrogen in air: model of the non-Nernstian potentiometric response of platinum gas diffusion electrodes

The potentiometric response of three different platinum gas diffusion electrodes deposited on H₃PO₄ doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H₂ or mixtures of H₂ and O₂. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H₂H₂ON₂ mixtures can be explained by a difference of water activity on both sides of the membrane. Under H₂O₂N₂ mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

An electrochemical study of H2O2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H₂O₂ concentration range of 10⁻⁴ to 10⁻² mol dm⁻³. The Tafel slopes for H₂O₂ reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec⁻¹, respectively, independent of pH and the concentration of H₂O₂. Both the reduction and oxidation of H₂O₂ on γ-FeOOH have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH⁻. For both cases the electroactive species is the molecular H₂O₂, not its base form, HO₂⁻. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HOOH⁻ and HO for the reduction, and HO₂H⁺, HO₂, and O₂⁻ for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH⁻ and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the γ-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H₂O₂ decomposition.     

Cathodic deposition of molybdenum and vanadium mixed oxyhydroxide films from V-substituted polymolybdophosphate

We report a new electrochemical route for fabricating molybdenum and vanadium mixed oxyhydroxide films on Au electrode from Keggin-type vanadium-substituted polymolybdophosphate. The process involves a potentiodynamic reduction of aqueous 9-molybdo(VI)-3-vanadophosphosphate(V) ([PMo₉V₃O₄₀]⁶⁻) in the potential region between 0 and −0.7 V versus Ag/AgCl. The resulting MoV oxyhydroxide film electrode gave a stable redox behavior in Na₂SO₄ electrolyte of pH 3. X-ray photoelectron spectroscopy revealed that this results from oxidation/reduction of Mo⁵⁺/Mo⁶⁺ in the film which accompanies extraction/insertion of protons for charge compensation. The deposited V ions remained in the film upon repetitive potential cycling without affecting their oxidation state. Voltammetric data in the presence of sodium nitrite showed electrocatalytic activity of the MoV oxyhydroxide toward the electroreduction of nitrite.     

Synthesis and high electrochemical activity of poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid)

Poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was synthesized using chemical oxidative copolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid at 50 °C. The conductivity of the PAAHB copolymer synthesized at the optimum conditions is 0.47 S cm⁻¹ that is lower than that of polyaniline, but is slightly affected by water. The cyclic voltammograms demonstrate that the PAAHB copolymer has excellent redox activity from highly acidic solution to pH 12.0 in a wider potential range. This is attributed to the synergistic effect of the SO₃⁻ and OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the pH dependence of the redox activity and conductivity of the PAAHB copolymer prepared chemically is much better than that of polyaniline, and is further improved, compared to the PAAHB copolymer prepared electrochemically. The proton NMR spectrum of the PAAHB copolymer demonstrates that the SO₃⁻ group exists in the copolymer chain instead of the SO₃H group. The ESR spectra show that the ESR signal intensity is a function of the monomer concentration ratio of AHB to aniline in the mixture. The morphology of the PAAHB copolymer is also dependent on the monomer concentration ratio in the mixture.     

Spectroelectrochemical study of perchloroethylene reduction at copper electrodes in neutral aqueous medium

The potential-dependent chemical reaction of perchloroethylene (PCE) on copper in neutral noncomplexing aqueous media is explored by means of surface-enhanced Raman spectroscopy (SERS), linear sweep voltammetry and preparative electrolysis at controlled potential. Voltammetric peaks associated with copper oxide reduction in Na₂SO₄ solution in the presence and the absence of Cl⁻ are correlated with simultaneously acquired SER spectra. Perchloroethylene undergoes a dechlorination process at potentials at E ≤ −0.3 V vs. Ag/AgCl/KCl (3 M), as shown by the emergence of an intense CuCl stretching band at 290 cm⁻¹ and a CH stretching band together with the presence of Cl⁻ in the catholyte. In the potential region between 0 and −0.9 V vs. Ag/AgCl/KCl (3 M) a broad band assigned to CC structures is observed in the triple-bond region (∼1900 cm⁻¹, FWHM = 180 cm⁻¹). In addition, dichloroethylene (DCE) is detected (but not trichloroethylene (TCE)) in this potential region during preparative electrolysis. At potentials lower than −1 V vs. Ag/AgCl/KCl (3 M) carbon residues are the main product, detected on the copper surface by SERS (and confirmed by XPS), whereas in solution higher levels of dichloroethylene and trichloroethylene are detected with a DCE/TCE ratio below 1.     

Enhancement of ionic conductivity by mixing lithium borate with lithium aluminate

Several lithium borates (Salt A and Salt B) and a lithium aluminate (Salt C) with electron-withdrawing groups, OC₆F₅, OCOCF₃ or N(SO₂CF₃)₂, and oligoether chains (O(CH₂CH₂O)_nCH₃) directly bonded to the ate complex center, B or Al, were prepared. Lithium borate and lithium aluminate were mixed to get mix-salt electrolytes. Higher ionic conductivity was observed for the mix-salt than for the pure-salt. Conductivity as high as 1.1 × 10⁻⁴ S/cm at ambient temperature (25 °C) was achieved for the electrolyte in the optimized composition. The reason for such mixing effect on enhancement of ionic conductivity was discussed. Other electrochemical properties including electrochemical stability, compatibility with lithium anode and cyclic performance were also investigated for the mix-salt electrolytes.