Composite gas-diffusion anodes for hydrogen generation by the sulfate-acid procedure

New composite catalytically-active coatings of gas-diffusion graphite electrodes were offered for the electrochemical SO₂ oxidation in the sulfate-acid cycle of hydrogen generation. Kinetics of SO₂ oxidation on porous graphite electrodes activated by platinum and metal-oxide catalysts was studied.     

Graphitized carbon materials for electrosynthesis of Н<Subscript>2</Subscript>О<Subscript>2</Subscript> from О<Subscript>2</Subscript> in gas-diffusion electrodes

New graphitized carbon materials: technical carbon N220, С140, and СН85 (Omsktekhuglerod) were studied as catalysts of electrosynthesis of alkaline solutions of hydrogen peroxide from oxygen in gasdiffusion electrodes (GDEs). The kinetic parameters of oxygen reduction in alkaline solution and the capacity of gas-diffusion electrodes based on technical carbon N220, С140, and СН85 were determined. Data on the kinetics of hydrogen peroxide accumulation were obtained at different current densities. The fraction of current γ spent on the reduction of oxygen to hydrogen peroxide was determined. The rate constants of hydrogen peroxide decomposition under the given conditions were calculated.     

The Mass Transfer of Proton on Meniscus Nafion/Pt/HOPG Electrode

Proton-exchange membrane fuel cells (PEMFCs) recently have been studied extensively because of their high performance^[1-3]. Since a small contacting area between the platinum catalyst and polymer electrolyte in PEMFCs, the platinum utilization is very important for this kind of cells. In order to improve the platinum utilization, Nafion solution is often impregnated into the gas-diffusion electrodes of PEMFCs. We introduced a partially immersed Nafion-coated electrode, Nafion/Pt/HOPG(highly oriented pyrolytic graphite) as a model electrode in PEMFC, to examine the effects of Nafion coating on the mass transport of hydrogen and proton at the meniscus and supermeniscus formed on the electrodes.     

Estimation of the Relative Thickness of Gas-Diffusion Electrodes of Carbon-Black Blends in the Course of Electrosynthesis of the Peroxide Ion from Oxygen

The results of a study of electrosynthesis of hydrogen peroxide from oxygen in an alkaline electrolyte on gas-diffusion electrodes of acetylene carbon black and blends of acetylene carbon black and furnace black are compared. The relative thickness of gas-diffusion hydrophobic electrodes operating under inner-kinetic conditions is estimated from polarization curves. Improving the hydrogen peroxide removal from pores of the gas-diffusion electrodes made of carbon black blends with various hydrophilic and hydrophobic properties stabilizes the relative thickness of the electrodes in comparison with the electrodes of acetylene carbon black.     

Pt-H_xWO_3复合电极的制备及其对氧还原的电催化

应用电位扫描法制备3种铂与氢钨青铜(HxWO3)的复合电极,即铂上沉积氢钨青铜(HxWO3/Pt),氢钨青铜上沉积铂(Pt/HxWO3)及铂和氢钨青铜共沉积(Pt-HxWO3).测试结果表明,氢钨青铜能显著提高铂电催化对氧还原的活性,其中以复合电极Pt/HxWO3的电催化活性最高,这可能是因为在玻碳基底上先电沉积氢钨青铜再电沉积铂增加了铂的有效比表面,该电极制备为减少铂用量提供一条有效途径.     

Thin-film rotating disk electrode as a tool for comparing the activity of catalysts in the hydrogen oxidation reaction

A thin-film disperse rotating disk electrode is used to study the hydrogen oxidation reaction on platinum catalysts E-TEK with different purification degrees and on disperse palladium catalysts obtained from colloid solutions of organometallic complex precursors with subsequent thermal decomposition in an inert atmosphere or in hydrogen at diverse temperatures. Kinetic currents of the hydrogen oxidation reaction on these catalysts are determined at a potential of 0.025 V. The obtained values of currents may be utilized for performing a comparative estimation of the activity of various catalysts and the degree of their purification from the precursors or accidental impurities.     

New accelerated method of impregnation by aqueous electrolyte of carbon black gas-diffusion electrodes to study their structural and electrochemical characteristics

A new method of fast impregnation of carbon black gas-diffusion hydrophobized electrodes is suggested under their cathodic polarization in an alkaline aqueous solution of tetrabutylammonium bromide (TBAB) at the potentials of hydrogen evolution. As dependent on the quantitative content of polytetrafluoroethylene (PTFE) in the electrode, current density, time, TBAB concentration, various degree of electrode wetting is observed, up to nearly complete electrode flooding in just several hours. When electrodes are stored, their original electrolyte porosity is not recovered. the electrode with 8 wt % of PTFE was used to show the effect of the electrode flooding degree on the double layer capacity, average diameter of electrolyte pores, their surface area and activity in the case of oxygen reduction. This method may be used for simulation of the process of flooding of gas-diffusion electrodes by electrolyte in the course of their prolonged operation.     

Ultradisperse catalytic layers supported by nanotubes and poly(diallyldimethylammonium)chloride polymer

A catalytic system consisting of carbon nanotubes, poly(diallyldimethylammonium)chloride, and a very thin layer of platinum or platinum-ruthenium is assembled layer-by-layer (LbL) on a glassy carbon (GC) electrode. Deposits of platinum metals are studied by electrochemical methods, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Such catalyst layers are shown to exhibit much higher activity in the methanol oxidation reaction as compared with commercial and electroplated catalysts. The currents compared are calculated per the surface area of deposited metals determined with respect to hydrogen adsorption.     

Preparation, characterization, and reactivity of Pt/SDBC catalysts for the hydrogen-water isotopic exchange reaction

Pt/SDBC catalysts, which are used for the hydrogen-water isotopic exchange reaction, were prepared. TGA experiments showed that the treatment temperature of Pt/SDBC catalysts in inert gas is limited to 400 °C and the maximum allowable heat treatment temperature in oxygen is 200 °C. From nitrogen adsorption and hydrogen chemisorption measurements, it was shown that the dispersion of platinum particles depended on the physical properties, i.e., specific surface area and pore structure of SDBC. It was found that the heat treatment could not impact the structure of SDBC and the oxygen treatment at 150 °C improved the platinum dispersion. It was shown by XPS analysis that the oxygen treatment of impregnated Pt/SDBC increased the fraction of platinum metal state and platinum dispersion. As the supported platinum area increases, the catalytic activity of Pt/SDBC for the hydrogen-water vapor isotopic exchange reaction increases. It indicates that the hydrogen chemisorption measurement can be used to estimate the catalytic activities of Pt/SDBC catalysts. It was not observed that the particle size of supported platinum affected the specific reaction rate at 60 °C. It implies that this reaction is structure insensitive.     

Nanostructured catalysts for cathodes of oxygen-hydrogen fuel cells

Bimetallic catalysts platinum-cobalt, platinum-chromium, and platinum-tungsten, deposited onto highly dispersed carbon black from complex cluster-type compounds of corresponding metals with a 1: 1 atomic ratio of metals are developed. The catalysts are characterized by methods of x-ray diffraction analysis and energy dispersive analysis of x-rays. The procedure involving use of a thin-film rotating disk electrode is employed to probe kinetic parameters of the oxygen reduction reaction on the catalysts developed. The investigated binary catalysts exhibit specific electrochemical characteristics that are not inferior and, in some cases, are superior to the characteristics intrinsic to the commercial platinum catalyst E-TEK, when tested in the composition of a gas-diffusion electrode under conditions that are close to real conditions in which cathodes of oxygen-hydrogen fuel cells operate.     

Gas-diffusion separation and flow injection potentiometry

Summary The similarities and differences of the operation principle of gas-sensing electrodes and potentiometric detection coupled to gas-diffusion separation in flow injection analysis are discussed with special emphasis on selectivity and sensitivity aspects. Several examples of application are presented highlighting the improvements in detectability obtained by gas-diffusion flow injection potentiometry. High sensitivity determination of ammonium is achieved through accumulation of ammonia released from the sample stream in the small recipient volume of the gas-diffusion unit. A method for almost specific determination of cyanide is presented making use of gas-diffusion separation of hydrogen cyanide and potentiometric detection with a selective AgI membrane electrode. The interference of sulfide is totally prevented by its oxidation in the donor line. If applied to potentiometric measurement following gas diffusion separation an intrinsically non-selective metallic silver wire electrode turns out to enable the selective detection of sulfide with high sensitivity and fast response. A new approach for diffusive sampling and on-line detection of gas-phase contaminants is exemplified by the determination of NO_x.     

Direct and electrically wired bioelectrocatalysis by hydrogenase from Thiocapsa roseopersicina.

Hydrogen enzyme electrodes based on direct and mediated bioelectrocatalysis were developed. Direct bioelectrocatalysis of hydrogen oxidation/evolution was observed for hydrogenase adsorbed on carbon filament material. The equilibrium hydrogen potential was achieved on mediatorless hydrogen enzyme electrodes in hydrogen atmosphere. The electrocatalytic activity of hydrogenase in direct bioelectrocatalysis of hydrogen oxidation was two orders of magnitude higher compared to platinum. The reported electrode remained 50% activity after 6 months of storage with periodical testing. Wired bioelectrocatalysis was achieved by adsorption of hydrogenase onto electropolymerized redox mediator N-methyl-N'-(12-pyrrol-1-yl-dodecyl)-4,4'-bipyridinium ditetrafluoroborate.     

Electrocatalytic Properties of Binary Systems Based on Platinum and Palladium in the Reaction of Oxidation of Hydrogen Poisoned by Carbon Monoxide

Investigation of a number of binary systems based on platinum and palladium and synthesized on carbon black XC72 as catalysts in the reaction of electrooxidation of hydrogen and hydrogen poisoned by carbon monoxide is carried out with use made of a complex of electrochemical methods. Exchange currents of the hydrogen reaction are determined as calculated per metal weight and its surface area. The developed methods of synthesis of binary systems are shown to make it possible to produce catalysts whose activity in the reaction of electrooxidation of hydrogen is commensurate with the activity of a platinum catalyst. The maximum tolerance with respect to carbon monoxide impurities among the binary systems investigated is found to be exhibited by PtMo and PdAu. The activity of an anode based on platinum in a model fuel cell with a solid polymer electrolyte drops by 2 and 7 times in the presence of 30 and 150 ppm CO, while the observed decrease in the current for system PtMo is equal to 10 and 50%, respectively.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 840–851.Original Russian Text Copyright © 2005 by Tarasevich, Bogdanovskaya, Grafov, Zagudaeva, Rybalka, Kapustin, Kolbanovskii.     

Enzyme Electrodes Using Bioelectrocatalytic Reduction of Hydrogen Peroxide

Abstract

Amperometric electrodes have been constructed using the direct electron transfer between electrode and peroxidase for mediatorless hydrogen peroxide detection at a potential of ?0.010 V. For this purpose peroxidase was either adsorbed on pyrographite or immobilized in electrochemically synthesized polypyrrole layers on pyrographite or platinum electrodes.     

Enhancement of sensors selectivity by gas-diffusion separation : Part 1. Flow-injection potentiometric determination of cyanide with a metallic silver-wire electrode

The potentiometric response of a metallic silver-wire electrode in the presence of silver ion complexing agents is theoretically derived on the basis of the Nernst equation. The cyanide response is shown to be in good agreement with the theory. The analytical utility of this inherently non-selective sensor is demonstrated by its application in gas-diffusion flow-injection analysis. By making use of a membrane barrier that prevents other than gaseous species from passing through, the almost specific determination of cyanide becomes feasible. Gaseous interferents (i.e., hydrogen sulphide, sulphur dioxide and nitrogen oxides) are chemically converted prior to entering the gas-diffusion unit. The apparent selectivity coefficients thus obtained are significantly better than those reported for common cyanide-selective electrodes.     

Electrochemical characteristics of platinum-based binary catalysts for middle-temperature hydrogen-air fuel cells with phosphoric acid electrolyte

The high-temperature synthesis based on commercial catalyst E-TEK (40% Pt) using cobalt, chromium, and iron organic precursors as well as d-metal salts yielded PtM (1:1) catalysts (PtCo, PtCr, PtMn, PtNi, PtFe, and PtV) for electroreduction of molecular oxygen in concentrated H₃PO₄ at the temperature of 160°C. The phase composition of the synthesized catalysts was studied by powder diffraction. The electrochemical measurements were carried out in 15 M H₃PO₄ at 20 and 160°C using a model gas diffusion electrode. An assumption was made that close charging curves recorded for synthesized PtM catalysts in both hydrogen and oxygen adsorption ranges were due to formation of the core-shell structure: alloy core and surface layers enriched with platinum. The Tafel curves of molecular oxygen reduction in 15 M H₃PO₄ at 160°C were characterized with the sole slope of 0.10 to 0.11 V. The catalytic activity in the range of potentials from 0.8 to 0.9 V (RHE) was shown approximately twice as that of pure platinum catalyst. The highest activity was recorded for PtCo and PtCr binary catalysts. Their use in middle-temperature hydrogen-air fuel cells with solid polymeric electrolyte based on polybenzimidazole doped with phosphoric acid enabled 2- to 3-fold decrease of the platinum share in the cathode.     

Fuel cell with polymer electrolyte: calculation of overall characteristics of oxygen cathode with account for processes of gas,vapor, and heat exchange

Computer simulation was performed for the processes occurring in the basic elements of the cathode (active layer, gas-diffusion layer) and bipolar plate of a fuel cell with Nafion as electrolyte and a platinum catalyst. Current generation in the active layer was considered together with the heat exchange processes (release of the heat formed in the active layer through the gas-diffusion layer into the bipolar plate), gas and vapor exchange in the gas-diffusion layer and process of the gas reagent (oxygen) saturation by water vapor in the bipolar plate channels. Voltammetric curves and dependences on the cathode potential of the power density, vapor flow dissipated from the active layer to the bipolar plate, actual active layer temperature and reduced partial pressures of oxygen and water vapors near the interface between the active and gas-diffusion layers were calculated. Analysis is performed of the way the heating of the cathode active layer intensifies the process of current generation in it, significantly increasing the value of overall characteristics of the cathode (current and power density).     

Effect of trialkylammonium salts and current density on the electrosynthesis of hydrogen peroxide from oxygen in a gas-diffusion electrode in acid solutions

The influence exerted by the nature of cation of a supporting electrolyte and by the current density on the electroreduction of oxygen to hydrogen peroxide in acid K₂SO₄ solutions (pH 0.9–1.4) in gas-diffusion hydrophobized carbon black electrodes with varied electrolyte porosity was studied.     

Effect of Carbonaceous Carrier and Promoter on Electrocatalytical Properties of Composite Cathodes in Fuel Cells with Alkaline Electrolyte

The activity of composite catalysts, Pt and Co-porphyrin- or Fe-phthalocyanine-based pyropolymers on low-disperse carbonaceous carriers (graphite, carbon black), in the oxygen and H₂O₂electroreduction in 1 M KOH is studied. Kinetic parameters of oxygen electroreduction are determined from experiments with rotating disk and model floating electrodes. Possible mechanism of the oxygen electroreduction reaction is discussed; it includes a slow stage of attachment of the second electron on the pyropolymer/carbonaceous carrier or joining the first electron (under the conditions of Temkin adsorption) on the platinum/graphite catalysts.     

Study of <Emphasis Type="Italic">n</Emphasis>-hexane isomerization on Pt/SO<Subscript>4</Subscript>/ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts: Effect of the state of platinum on catalytic and adsorption properties

The state of surface Pt atoms in the Pt/SO₄/ZrO₂/Al₂O₃ catalyst and the effect of the state of platinum on its adsorption and catalytic properties in the reaction of n-hexane isomerization were studied. The Pt-X/Al₂O₃ alumina-platinum catalysts modified with various halogens (X = Br, Cl, and F) and their mechanical mixtures with the SO₄/ZrO₂/Al₂O₃ superacid catalyst were used in this study. With the use of IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration, it was found that ionic platinum species were present on the reduced form of the catalysts. These species can adsorb to three hydrogen atoms per each surface platinum atom. The specific properties of ionic platinum manifested themselves in the formation of a hydride form of adsorbed hydrogen. It is believed that the catalytic activity and operational stability of the superacid system based on sulfated zirconium dioxide were due to the participation of ionic and metallic platinum in the activation of hydrogen for the reaction of n-hexane isomerization.