Electrocatalytic reduction of dioxygen at glassy carbon electrodes modified with polypyrrole/anthraquinonedisulphonate composite film in various pH solutions

Functionalized polypyrrole (PPy) film with anthraquinonedisulphonate (AQDS) incorporated as dopant was prepared by anodic polymerization of pyrrole (Py) at a glassy carbon electrode from aqueous solution. The electrochemical behavior of AQDS in PPy matrix and the electrocatalytic reduction of dioxygen on the resulting composite film were investigated in various pH solutions. The formal potential of AQDS and the reduction potential of dioxygen both exhibit pH dependence. In all pH solutions employed, the electrocatalytic reduction of dioxygen at the PPy/AQDS composite film establishes a pathway of irreversible two-electron reduction to form hydrogen peroxide. The pH 6.0 buffer solution is a more suitable medium for the reduction of dioxygen, where the PPy/AQDS composite film showed a more efficient electrocatalytic performance. It was found that AQDS is an effective mediator for the reduction of dioxygen and the reduced AQ is responsible for the enhanced catalytic activity. The catalytic current is under mixed kinetic-diffusion control. The number of electrons transferred and kinetic parameters of dioxygen reduction were determined using cyclic voltammetry, rotating disk voltammetry and Tafel polarization technique.     

Electrocatalytic reduction of oxygen at glassy carbon electrode modified by polypyrrole/anthraquinones composite film in various pH media

The electrocatalytic reduction of dioxygen by one mono and four dihydroxy derivatives of 9,10-anthraquinone (AQ) incorporated in polypyrrole (PPy) matrix on glassy carbon electrode has been investigated. The electrochemical behaviour of the modified electrodes was examined in various pH media and both the formal potential of anthraquinones and reduction potential of dioxygen exhibited pH dependence. AQ and PPy composite film showed excellent electrocatalytic performance for the reduction of O₂ to H₂O₂. pH 6.0 was chosen as the most suitable medium to study the electrocatalysis by comparing the peak potential of oxygen reduction and enhancement in peak current for oxygen reduction. The diffusion coefficient values of AQ at the modified electrodes and the number of electrons involved in AQ reduction were evaluated by chronoamperometric and chronocoulometric techniques, respectively. In addition, hydrodynamic voltammetric studies showed the involvement of two electrons in O₂ reduction. The mass specific activity of AQ used, the diffusion coefficient of oxygen and the heterogeneous rate constants for the oxygen reduction at the surface of modified electrodes were also determined by rotating disk voltammetry.     

Redox chemistry of H2S oxidation by the British gas Stretford process part V: Aspects of the process chemistry

Stretford processes use air to oxidize H₂S in process and natural gases to elemental sulphur, by absorption in aqueous solution at about pH 9 and reaction of the resulting HS^– ions with dissolved oxygen, in the presence of anthraquinone disulphonates (AQDS) and vanadium (v) species, which act as catalysts. Kinetic measurements of the reactions (AQ27DS + HS^– ions), (V(v) + HS^– ions) and (AQ27DSH^– + O₂), primarily used stopped flow spectrophotometry, as reported here, following papers on the electrochemical behaviour of the individual redox couples in Stretford Processes. The course of reaction (AQ27DS + HS^– ions) was also followed with a gold bead indicator electrode, the potential of which was determined essentially by the AQ27DS/AQ27DSH^– couple as the former species were reduced to the latter. Attempts to use⁵¹V NMR to characterize aqueous vanadium-sulphur complexes were inconclusive. A possible mechanism for Stretford Processes is postulated, involving polysulphide (S _n ^2–) ions as intermediates, which are oxidized to elemental sulphur by another intermediate, H₂O₂, formed by reaction of AQ27DSH^– ions and dissolved oxygen.     

Electroreduction of oxygen on glassy carbon electrodes modified with in situ generated anthraquinone diazonium cations

The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with in situ generated diazonium cations of anthraquinone (AQ) has been studied using the rotating disk electrode (RDE) technique. The electrografting of the GC electrodes was carried out in two different media: in acetonitrile and in an aqueous acidic solution (0.5 M HCl). 1- and 2-Aminoanthraquinone were used as starting compounds for the formation of the corresponding diazonium derivatives. The anthraquinone diazonium cations were generated by reaction of the aminoanthraquinones with tert-butyl nitrite and sodium nitrite in acetonitrile and in 0.5 M HCl, respectively. For comparison purposes, the previously synthesised and crystallised diazonium tetrafluoroborates of anthraquinone were used for the GC surface modification. Cyclic voltammetry was employed to determine the surface concentration of AQ in O₂ free 0.1 M KOH. The electrocatalytic behaviour towards O₂ reduction was similar for all the AQ-modified electrodes studied. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model. The rate constant of the reaction between the semiquinone radical anion of AQ and molecular oxygen was virtually independent of the point of attachment of the quinone to the electrode surface.     

A dual-chamber microbial fuel cell with conductive film-modified anode and cathode and its application for the neutral electro-Fenton process

This study reports on the modification of the anode and the cathode in a dual-chamber microbial fuel cell (MFC) with a polypyrrole (PPy)/anthraquinone-2,6-disulfonate (AQDS) conductive film to boost its performance and the application of the MFC to drive neutral electron-Fenton reactions occurring in the cathode chamber. The MFC equipped with the conductive film-coated anode and cathode delivered the maximum power density of 823 mW cm⁻² that was one order of magnitude larger than that obtained in the MFC with the unmodified electrodes. This was resulted from the enhanced activities of microbial metabolism in the anode and oxygen reduction in the cathode owing to the decoration of both electrodes with the PPy/AQDS composite. The MFC with the modified electrodes resulted in the largest rate of H₂O₂ generation in the cathode chamber by the two-electron reduction of O₂. The increase in the concentration of H₂O₂ was beneficial for the enhancement in the amount of hydroxyl radicals produced by the reaction of H₂O₂ with Fe²⁺, thus allowing an increased oxidative ability of the electro-Fenton process towards the decolorization and mineralization of an azo dye (i.e., Orange II) at pH 7.0.     

The pH-dependence of oxygen reduction on quinone-modified glassy carbon electrodes

The electrochemical reduction of oxygen has been studied on quinone-modified glassy carbon (GC) electrodes as a function of solution pH using the rotating disk electrode (RDE) technique. The surface of GC was grafted with anthraquinone (AQ) and phenanthrenequinone (PQ) by electrochemical reduction of their diazonium derivatives and the oxygen reduction measurements were carried out at different pHs (pH 7-14). The redox-potentials of surface-bound quinones were determined using cyclic voltammetry (CV). The kinetic parameters of oxygen reduction on GC/AQ and GC/PQ electrodes were determined considering a surface redox catalytic cycle model for quinone-modified electrodes.     

Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM)

The redox competition mode of scanning electrochemical microscopy (RC-SECM) has been utilized to visualize the local electrocatalytic activity of metalloporphyrin spots towards oxygen reduction in 0.1 M phosphate buffer as electrolyte solution. The metalloporphyrin spots were obtained by electrochemically induced deposition using a droplet cell. Tetratolyl porphyrins (TTPs) of Mn, Fe and Co have been investigated, with that containing Mn as central metal atom showing highest catalytic activity. The multiple stable oxidation states of Mn were seen as a key factor in the influence of the metal ion on the catalytic activity. From the RC-SECM results, it is shown that oxygen reduction at a manganese TTP (MnTTP) modified electrode surface yielded the least amount of H₂O₂ when compared to iron TTP (FeTTP) and cobalt TTP (CoTTP). As further confirmed by means of rotating disc electrode (RDE) measurements this was attributed to the high activity of MnTTP for H₂O₂ reduction.     

Studies of electrochemical reduction of dioxygen with RRDE

The electrochemical behaviors of dioxygen (O₂) were studied by using rotate ring-disk electrode (RRDE) and other electrochemical methods at bare glassy carbon electrode (GCE) and single-walled carbon nanotubes (SWNTs)-dihexadecyl hydrogen phosphate (DHP) film modified GCE. The results showed that the electrochemical reduction of dioxygen was considered to proceed by a two-step two-electron reduction pathway at both bare GCE and SWNTs-DHP film modified GCE in 0.1 mol/L air-saturated sodium hydroxide (NaOH). Maybe because each reaction rate for two cases was different the cyclic voltammograms measurements exhibited different behaviors. The detection of ring current confirmed the presence of middle product hydrogen peroxide (H₂O₂). Furthermore, larger current and more positive reduction potential indicated that SWNTs showed a catalytic effect towards the electrochemical reduction of dioxygen.     

A modified composite film electrode of polyoxometalate/carbon nanotubes and its electrocatalytic reduction

Keggin-type polyoxometalate (H₄SiMo₁₂O₄₀) and carbon nanotubes (CNTs) coated by poly(allylamine hydrochloride) (PAH) were alternately deposited on glassy carbon (GC) electrodes by an electrochemical growth method in acidic aqueous solution. The preparation of the film electrode was simple and convenient. Thus-prepared multilayer films and the electrochemical behavior of the composite film modified electrode were characterized by UV–vis spectroscopy and cyclic voltammetry. It was shown that the multilayer films are uniform and stable. The resulting multilayer film modified electrode behaves as an electrochemical sensor because of its low overpotential for the catalytic reduction of S₂O₈ ²⁻ and NO₂ ⁻ in acidic aqueous solution.     

Electro-Fenton degradation of azo dye using polypyrrole/anthraquinonedisulphonate composite film modified graphite cathode in acidic aqueous solutions

Amaranth azo dye has been degraded by electro-Fenton method using an undivided cell containing the polypyrrole (PPy)/anthraquinonedisulphonate (AQDS) composite film modified graphite cathode and Pt anode. In acidic media, the PPy/AQDS composite film exhibits the characteristic of gas diffusion cathode and is highly efficient for hydrogen peroxide electrogeneration with high generation rate and current efficiency. This new electro-Fenton system can degrade amaranth azo dye efficiently in various acidic solutions. The amaranth decay and total organic carbon (TOC) removal were determined as a function of pH, cathode potential, Fe²⁺ and doping AQDS concentrations. Total dye decay and 80.3% mineralization were achieved at the optimum conditions (pH 3.0, E_cath = −0.65 V vs. SCE, 2.0 mM Fe²⁺ concentration). The electrochemical stability and electrocatalytic activity of the composite film after use in electro-Fenton process were also investigated using cyclic voltammetry (CV) and Fourier transfer infrared (FTIR) spectroscopy technologies.     

Effects of poly-1,5-diaminoanthraquinone morphology on oxygen reduction in acidic solution

The poly-1,5-diaminoanthraquinone (P15DAAQ) modified Pt electrodes show electrocatalytic activity for oxygen reduction reaction (ORR) with oxygen reduction peak at about 0.39 V in 0.1 M H₂SO₄. The P15DAAQ with different thickness has different morphology. The effects of morphologies on the electrocatalytic behaviors of P15DAAQ for oxygen reduction reaction are investigated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) measurements. We propose two different O₂-transport processes on electrodes modified with thin P15DAAQ and thick P15DAAQ. Together with the quantitative analysis with O₂-transport dynamics, electron-transfer resistance, and catalytic reaction rate during ORR, thin P15DAAQ electrode performs better electrocatalysis for ORR, although thick P15DAAQ provides higher real surface area and more reactive sites which is beneficial for ORR within a short time.     

Advances in the Electro‐Fenton Process for Remediation of Recalcitrant Organic Compounds

The electro‐Fenton (EF) process is a promising method combining electrochemical reactions and Fenton's reagent. In this hybrid process, the electrical current induces the in situ generation of H₂O₂ via reduction of oxygen, and the catalytic reaction is propagated by Fe²⁺ regeneration, which can take place by reduction of Fe³⁺ with H₂O₂, hydroperoxyl radical, organic radical intermediates, or directly at the cathode. Recent advances in the EF process are discussed and several key variables analyzed, including electrode material, initial pH, and Fenton's reagents, in order to extend the applicability of this technology.     

Preparation of Ag–Cu bimetallic dendritic nanostructures and their hydrogen peroxide electroreduction property

In this work, dendritic silver–copper (Ag–Cu) nanostructures were synthesised on a copper foil by electrodeposition and subsequently galvanic displacement reaction without any surfactant. The crystalline nature of the nanostructures was examined by X-ray diffraction, and the morphology of the material was investigated by field-emission scanning electron microscopy. The applied potential, displacement reaction time, and silver nitrate solution concentration exerted different effects on the nanoparticle shape. And a possible growth mechanism of the Ag–Cu dendrites was proposed based on the experimental results. The electrochemical properties of the Ag–Cu dendrite-modified electrode were characterised by linear sweep voltammetry. The reduction peak potential of hydrogen peroxide (H₂O₂) was about ?0.25 V (vs. a saturated calomel electrode), which indicated that the as-synthesised Ag–Cu dendrites had favourable electroreduction activity towards hydrogen peroxide. At the same time, we found that the solution pH also affected the electrocatalytic ability of the dendrites for H₂O₂ reduction, which was important for the design of a NaBH₄–H₂O₂ battery.     

Approaches for multicopper oxidases in the design of electrochemical sensors for analytical applications

We report an effective approach for the construction of a biomimetic sensor of multicopper oxidases by immobilizing a cyclic-tetrameric copper(II) species, containing the ligand (4-imidazolyl)ethylene-2-amino-1-ethylpyridine (apyhist), in the Nafion^® membrane on a vitreous carbon electrode surface. This complex provides a tetranuclear arrangement of copper ions that allows an effective reduction of oxygen to water, in a catalytic cycle involving four electrons. The electrochemical reduction of oxygen was studied at pH 9.0 buffer solution by using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry and scanning electrochemical microscopy techniques. The mediator shows good electrocatalytic ability for the reduction of O₂ at pH 9.0, with reduction of overpotential (350 mV) and increased current response in comparison with results obtained with a bare glassy carbon electrode. The heterogeneous rate constant (k^′ME) for the reduction of O₂ at the modified electrode was determined by using a Koutecky-Levich plot. In addition, the charge transport rate through the coating and the apparent diffusion coefficient of O₂ into the modifier film were also evaluated. The overall process was found to be governed by the charge transport through the coating, occurring at the interface or at a finite layer at the electrode/coating interface. The proposed study opens up the way for the development of bioelectronic devices based on molecular recognition and self-organization.     

Electro-reduction of oxygen and electro-oxidation of methanol at Pd monolayer-modified macroporous Pt electrode

With polystyrene latex spheres self-assembled on indium tin oxide-coated glass electrode as templates, highly ordered macroporous Pt was prepared by electrochemical deposition. Then, the macroporous Pt was modified by Pd monolayer involving the galvanic displacement of Cu monolayer formed by under-potential deposition on macroporous Pt. Electrocatalytic properties of the Pd-modified macroporous Pt electrode for oxygen reduction were investigated by cyclic voltammetry and chronoamperometry in O₂-saturated solution containing 0.1 M HClO₄. Methanol electro-oxidation on the Pd-modified macroporous Pt surfaces in 0.5 M H₂SO₄ containing 1 M CH₃OH was studied by cyclic voltammetry. The corresponding results showed that Pd-modified macroporous Pt electrode had negative catalytic activity for methanol oxidation in compared with macroporous Pt. However, Pd-modified macroporous Pt electrode had positive electrocatalytic activity to O₂ reduction.     

Electrocatalytic oxygen reduction and hydrogen evolution reactions on phthalocyanine modified electrodes: Electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring

This study describes electrochemical, in situ spectroelectrochemical, and in situ electrocolorimetric monitoring of the electrocatalytic reduction of molecular oxygen and hydronium ion on the phthalocyanine-modified electrodes. For this purpose, electrochemical and in situ spectroelectrochemical characterizations of the metallophthalocyanines (MPc) bearing tetrakis-[4-((4′-trifluoromethyl)phenoxy)phenoxy] groups were performed. While CoPc gives both metal-based and ring-based redox processes, H₂Pc, ZnPc and CuPc show only ring-based electron transfer processes. In situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes. The presence of O₂ in the electrolyte system influences both oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate electrocatalytic activity of the complexes for the oxygen reduction reaction. Perchloric acid titrations monitored by voltammetry represent possible electrocatalytic activities of the complexes for hydrogen evolution reaction. CoPc and CuPc coated on a glassy carbon electrode decrease the overpotential of the working electrode for H⁺ reduction. The nature of the metal center changes the electrocatalytic activities for hydrogen evolution reaction in aqueous solution. Although CuPc has an inactive metal center, its electrocatalytic activity is recorded more than CoPc for H⁺ reduction in aqueous solution.     

Electrochemical behavior and its electrocatalytic properties of chemically modified electrode with Keggin-type [SiNi(H2O)W11O39]

A monolayer of Keggin-type heteropolyanion [SiNi(H₂O)W₁₁O₃₉]⁶⁻ was fabricated by electrodepositing [SiNi(H₂O)W₁₁O₃₉]⁶⁻ on cysteamine modified gold electrode. The monolayer of [SiNi(H₂O)W₁₁O₃₉]⁶⁻ modified gold electrode was characterized by atomic force microscopy (AFM) and electrochemical method. AFM results showed the [SiNi(H₂O)W₁₁O₃₉]⁶⁻ uniformly deposited on the electrode surface and formed a porous monolayer. Cyclic voltammetry exhibited one oxidation peak and two reduction peaks in 1.0 M H₂SO₄ in the potential range of −0.2 to 0.7 V. The constructed electrode could exist in a large pH (0-7.6) range and showed good catalytic activity towards the reduction of bromate anion (BrO₃⁻) and nitrite (NO₂⁻), and oxidation of ascorbic acid (AA) in acidic solution. The well catalytic active of the electrode was ascribed to the porous structure of the [SiNi(H₂O)W₁₁O₃₉]6⁻ monolayer.     

Die H2S-oxidation an aktiver kohle—ein elektrochemischer prozess?

Proceeding from the observation that a galvanic cell can be formed with activated carbon as electrode material and aqueous solutions of H₂S and oxygen, a mechanism is submitted for discussion, according to which the oxidation of H₂S catalyzed by activated carbon takes place by this very formation of cells on the microscopic surfaces of the carbon. The known effects of H₂O, H⁺ ions, as well as Fe and I content of the activated carbon on catalysis may be explained by this model.The catalytic inefficiency of dry activated carbon with respect to the oxidation of H₂S with oxygen-containing gases at ambient temperature is due to the fact that electrolyte is indispensable for electrochemical processes. Carbon hexagon layers with reversibly chemisorbed O₂ behave as oxygen electrode while layers with adsorbed H₂S act as fuel electrode. Only after addition of electrolyte elemental sulphur can separate on the fuel electrode.The pH value can drop far below pH 2 due to H₂SO₄ formed in a side reaction. With such acid concentrations, H₃S⁺ ions are increasingly formed. Since H₃S⁺ is no longer accessible to electrochemical oxidation, catalysis is retarded within these pH ranges.Iron bonded in complex form to the layers behaves as an electron acceptor. By this, the layers and the adsorbed H₂S are positively polarized which allows oxidation of the H₂S to sulphuric acid. The negatively polarized iron assumes the function of the oxygen electrode.Iodine blocks the direct access of H₂S to the layer or fuel electrode respectively. By direct chemical reaction, also within the strongly acidic range, elemental iodine reacts with H₂S forming elemental sulphur. The iodide ions formed are subject to electrochemical oxidation to I₂ on the carbon layer.The special advantage of the electrochemical oxidation mechanism is that the O₂ and H₂S molecules must not necessarily be in direct contact.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

Electrochemical reduction of oxygen on gold and boron-doped diamond electrodes in ambient temperature, molten acetamide-urea-ammonium nitrate eutectic melt

The electrochemical reduction of oxygen has been studied on gold, boron-doped diamond (BDD) and glassy carbon (GC) electrodes in a ternary eutectic mixture of acetamide (CH₃CONH₂), urea (NH₂CONH₂) and ammonium nitrate (NH₄NO₃). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and rotating disk electrode (RDE) voltammetry techniques have been employed to follow oxygen reduction reaction (ORR). The mechanism for the electrochemical reduction of oxygen on polycrystalline gold involves 2-step, 2-electron pathways of O₂ to H₂O₂ and further reduction of H₂O₂ to H₂O. The first 2-electron reduction of O₂ to H₂O₂ passes through superoxide intermediate by 1-electron reduction of oxygen. Kinetic results suggest that the initial 1-electron reduction of oxygen to HO₂ is the rate-determining step of ORR on gold surfaces. The chronoamperometric and RDE studies show a potential dependent change in the number of electrons on gold electrode. The oxygen reduction reaction on boron-doped diamond (BDD) seems to proceed via a direct 4-electron process. The reduction of oxygen on the glassy carbon (GC) electrode is a single step, irreversible, diffusion limited 2-electron reduction process to peroxide.