Showing papers in "Journal of Applied Electrochemistry in 1992"

Thin-film catalyst layers for polymer electrolyte fuel cell electrodes

Abstract: New structures for the Pt/C catalyst layer of polymer electrolyte fuel cell electrodes have been developed that substantially increase the utilization efficiency of the catalyst. Fabricating the catalyst layers and gas diffusion backings separately makes it possible to formulate each structure with the properties that are most suitable for its function. In the case of the catalyst layer, the optimal properties are hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. The catalyst layers are cast from solution as thin films that utilize the ionomer itself as a binder. The thin films are hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings are inserted when the cells are assembled. The performances of fuel cells based on the thin film catalyst layers are comparable with those of gas diffusion electrode designs that utilize several times as much platinum, thus the specific activities of the Pt catalysts in the new structures are significantly higher.

Electron conducting organic coating of mild steel by electropolymerization

Abstract: The aim was to coat mild steel with an electron conducting polymer in order to replace any possible electrochemical corrosion of the metal by another electrochemical reaction occurring on top of the film. In view of potential industrialization, electropolymerization of a water soluble monomer was studied. In most cases, this was achieved by oxidation, and the substrate had to be passivated. Polyaniline, was obtained in nitric acid solution. Films had a good structure but were brittle and powdery on the surface. Except on tin free steel, they were insulating. Polypyrrole was a better candidate. Prepared from a Na2SO4 solution, films were conductive, ensured good corrosion resistance of the substrate but they were brittle and adhesion to the substrate was not high. Furance, thiophene and iron substituted vinylpyridine complex were tried without success. Further, research should consider copolymerization of pyrrole with other monomers.

Formation of hypochlorite, chlorate and oxygen during NaCl electrolysis from alkaline solutions at an RuO2/TiO2 anode

Abstract: On-site electrolysis of a weak alkaline solution of NaCl has been applied to an increasing extent for disinfection. To optimize the electrolytic cell and the electrolysis conditions, the current efficiency for hypochlorite, chlorate and oxygen formation at a commercial RuO2/TiO2 anode were determined under various conditions. It was found that for solution with low NaCl concentrations, (lower than 200 mol m−3), and at 298 K, solution flow velocity of 0.075 ms−1 and high current density, (higher than 2 kA m−2), hypochlorite formation is determined by mass transfer of chloride. The formation of chlorate in weakly alkaline media at a chlorine and oxygen-evolving anode is ascribed to two reactions, namely, the direct oxidation of chloride to chlorate and the conversion of hypochlorite. This is suggested to split the well-known electrochemical Foerster reaction into a chemical reaction for the conversion of hypochlorite in chlorate and the electrochemical oxidation reaction of water. It is proposed that in an acidic reaction layer at the anode the mechanism of chlorate formation may be given by the following:

Development of alloy electrocatalysts for phosphoric acid fuel cells (PAFC)

Abstract: One of the great scientific achievements for electrocatalysis in the past ten years has been the ability to reproducibly form microparticles of platinum metals in conductive supports. The dimensions of these crystallites are such that up to 50% of the atom content is at the crystallite surface the are single crystals, and have metallurgical properties that are unlike, bulk metals. These have found application as electrocatalysts in hot phosphoric acid fuel cells (PAFC) for oxygen reduction and hydrogen oxidation. Much of the recent work has been involved with developing various binary and ternary alloy combinations while at the same time, maintaining the crystallite microdimensions. Pt−Co−Cr alloys are one of the most favoured combinations for oxygen reduction and the Pt−Pd alloys are favoured for hydrogen oxidation, especially in the presence of trace, poisons such as carbon monoxide and hydrogen sulphide. Operation of these electrocatalysts is the subject of much investigation, with the present conclusion that the apparent mode of catalytic enhancement with the binary and ternary platinum alloys for oxygen reduction, resides in the leaching of the non-platinum elements from the alloy crystallite surfaces to give a microroughening, and thereby an increased reaction surface area.

Kinetics of hydrogen evolution on Ni-Al alloy electrodes

Abstract: The kinetics of the hydrogen evolution reaction have been studied on Ni-Al alloy electrodes. The electrodes after leaching aluminium in alkaline solution, are very active despite their large Tafel slopes. The SEM studies show a formation of deep pores on the electrode surface. The kinetics were studied using an ac impedance technique. It was found that the impedance plots may be explained assuming a fractal model. The logarithm of the charge transfer resistance is a linear function of the overpotential. Using a nonlinear least square approximation it was found that the reaction proceeds through the Volmer-Heyrovsky mechanism and the kinetic parameters were estimated.

Transition metal-based cathodes for hydrogen evolution in alkaline solution : electrocatalysis on nickel-based ternary electrolytic codeposits

Abstract: Nickel-molybdenum-iron, nickel-molybdenum-copper, nickel-molybdenum-zinc, nickel-molybdenum-cobalt, nickel-molybdenum-tungsten and nickel-molybdenum-chromium ternary codeposits, obtained through electrodeposition on mild steel strips have been characterized with the objective of qualitatively comparing and assessing their electrocatalytic activities as hydrogen electrodes in alkaline solution. It has been concluded that their electrocatalytic effects for the hydrogen evolution reaction (h.e.r.) rank in the following order: Ni−Mo−Fe>Ni−Mo−Cu>Ni−Mo−Zn>Ni−Mo−Co∼ Ni−Mo−W>Ni−Mo−Cr>Ni-plated steel. Further investigations on these electrocatalysts have revealed that the cathodic overpotential contribution to the electrolysis voltage can be brought down by 0.3 V when compared with conventional steel cathodes. The best and most stable hydrogen evolving cathode of these, namely Ni−Mo−Fe, exhibited an overpotential of about 0.187 V for over 1500 h of continuous electrolysis in 6 M KOH at 300 mA cm−2 and 353 K. The salient features of the codeposits, such as physical characteristics, chemical composition, current-potential behaviour and the varying effects of the catalytic activation method were analysed with a view to correlating the micro-structural characteristics of the coatings with the hydrogen adsorption process. The stability under open circuit conditions, the tolerance to electrochemical corrosion and the long term stability of Ni−Mo−Fe codeposit cathodes were very encouraging. An attempt to identify the pathway for the h.e.r. on these codeposit cathodes was made, in view of the electrochemical parameters obtained experimentally.

Local mass transport effects in the FM01 laboratory electrolyser

Abstract: A number of patterns of segmented line electrodes have been manufactured using copper printed circuit board technology. These segmented electrodes have been used to investigate local mass transport effects in ICI's FM01-LC parallel plate electrolyser. It is shown that in the absence of a turbulence promoter the current distribution is uneven. Along the direction of electrolyte flow, a tertiary current distribution is observed. In addition, close to the cell entrance, an uneven current distribution occurs perpendicular to the direction of electrolyte flow; this reflects the design of the electrolyte distributor. With a turbulence promoter the current distribution is more even and the entry effects are much reduced. The turbulence promoter can, however, impose its own pattern on the current distribution perpendicular to the flow.

Synthesis and properties of polyaniline obtained using sulphamic acid

Abstract: The results of investigations of the chemical and electrochemical polymerization of aniline in sulphamic acid medium and its characterization by electrochemical and spectroscopic techniques are presented. The investigations reveal that sulphamic acid medium affects, not only the polymerization process, but also its electrochemical characteristics.

Electrochemical studies of the factors influencing the cycle stability of Prussian Blue films

Abstract: Some of the factors leading to long term cycle stability for Prussian Blue (PB) films are determined. The pH of the conversion electrolyte was found to be the overwhelming factor in film stability; cycle lifetimes in excess of 100 000 were easily achieved in solutions of pH 2–3. Concurrent with the increase in stability at lower pH was a considerable increase in switching kinetics. Surprisingly, the films grown from chloride-containing solutions were found to be slightly more stable, in terms of cycle life, compared to those grown from chloride-free solutions. In addition, we found that there occurs at least some conversion of PB form during the first cycle, and thus, form conversion kinetics cannot be evoked as a mechanism for film stability. Unfortunately, the films grown electrochemically on ITO glass exhibited a white haze which we attribute to light scattering caused by grain size. Unless it can be eliminated, this haze effect will be a serious drawback in the eventual use of PB in an electrochromic filter. Finally, the films were stable in mixed solvents that provide an expanded temperature range, and preconditioned films were stable to drying and immersion in boiling water.

Electrochemical reduction of nitrogen oxyanions in 1 M sodium hydroxide solutions at silver, copper and CuInSe2 electrodes

Abstract: The reduction of nitrate and nitrite ions was studied in 1m NaOH supporting electrolyte. Voltammetric investigations show that, on silver cathodes, nitrate reduction begins at potentials about 500 mV more positive than nitrite reduction, the latter being superimposed on hydrogen evolution. Electrolyses of nitrate solutions at −1.4V/sce give nitrite with good selectivity. On copper cathodes, nitrate and nitrite reductions occur in the same region of potentials and show similar voltammetric profiles. The dominant product of nitrite reduction is ammonia, whereas nitrate may be reduced to nitrite at −1.1 V/sce and to ammonia with high yields at −1.4 V/sce. Reduction of nitrogen oxyanions may also be performed on CuInSe2 (photo)cathodes. Photoassisted reductions of nitrate performed on p-CuInSe2 at −1.4 V/sce gave mixtures of ammonia, nitrite and hydrogen.

A polyaniline and Nafion® composite film as a rechargeable battery

Abstract: The charge-discharge characteristics of polyaniline (PAn)-Nafion®-PAn composite films used as rechargeable batteries were investigated. The batteries were formed chemically by sandwiching Nafion® between two thin layers of PAn-Nafion® composite in equilibrium with various electrolytes. It was found that cells with LiClO4 in propylene carbonate as electrolyte showed the highest capacity. The charge and discharge performance of the battery was almost independent of the choice of anions but was significantly affected by cation size. This may indicate that the insertion or removal of charge-compensating electrolyte cations from the composite layer was responsible for the redox of PAn during the charge and discharge cycles.

Nodulation of electrodeposited copper in the presence of thiourea

Abstract: This paper considers the effect of thiourea concentration, temperature and current density on electrodeposited copper nodulation. Thiourea concentration was varied from 0 to 60 mg dm−3 in the range 30 to 60°C and at current densities of 21.5, 48.4, 75.3 and 129.2 mA cm−2. Thiourea was found to initiate nodulation and this effect was always associated with a large increase in overpotential i. e., >100 mV. This phenomenon occurred only when the applied current density was greater than a critical current densityi c. The critical current density was found to be insensitive to potential over a range of about 100 mV. Also, it decreased nonlinearly but smoothly with increase in thiourea concentration at a given temperature and increased similarly with temperature at a given thiourea concentration. Galvanostatic experiments were carried out to study this phenomenon and a qualitative explanation is given based on potentiodynamic measurements and suppositions as to the possible chemical reactions taking place at the cathode surface based on evidence in the literature.

Electrodeposition of zinc-nickel alloy coatings from a chloride bath containing NH4Cl

Abstract: Zinc-nickel alloys were electrodeposited on steel from chloride baths containing NH4Cl using different plating conditions. Current density, temperature and nickel percentage in the baths were found to strongly influence the composition of the deposits and the morphology. At low current densities transition from anomalous to normal codeposition was observed. The changes in potential, current efficiency, composition and morphology which follow the transition were studied. No increase in the partial current of hydrogen reduction was observed at the potential values from which anomalous codeposition begins; this fact, plus the formation of zinc ammonium complexes, seems to exclude the precipitation of zinc hydroxide at the electrode surface. The electrodeposition of zinc-nickel alloys is discussed emphasizing the importance of kinetic parameters and cathodic potentials.

High area Ni-Zn and Ni-Co-Zn codeposits as hydrogen electrodes in alkaline solutions

Abstract: This paper describes the preparation of Ni−Zn and Ni−Co−Zn coatings by electrodeposition on a mild steel substrate. After treatment in a KOH solution to leach out part of the zinc, the materials were evaluated as cathodes for the hydrogen evolution reaction in alkaline solutions. The materials were found to have high surface areas and low Tafel slopes. Overpotentials at current densities of technological interest are significantly lower than those of the traditional materials used in water electrolysis.

Development of long-lived high-performance zinc-calcium/nickel oxide cells

Abstract: The addition of Ca(OH)2 to the zinc electrode of Zn/KOH/NiOOH cells was investigated in order to determine its effect on the rate of zinc active material redistribution (shape change) and cell cycle-life performance. Cells of equal mass and capacity, and therefore the same specific energy, containing 0, 10, 25, and 40 mol% Ca(OH)2 in their zinc electrodes were constructed and tested. The Ca(OH)2 and Zn(OH) 4 2− -supersaturated KOH solution formed a calcium-zincate complex during the discharge half-cycle. The solubility of this complex is less than that of ZnO, and the lower zinc species solubility leads to a slower rate of Zn redistribution, thereby extending the cell cycle life. The best cells tested were those with 25%-Ca(OH)2 electrodes, which lost capacity at a rate of 0.13%/cycle, compared to 0.47%/cycle in calcium-free control cells constructed in the same manner. Also, zinc active material utilization in the calcium-containing electrodes showed a dramatic improvement, compared to the calcium-free zinc electrodes.

Electroless nickel hydroxide: synthesis and characterization

Abstract: An electroless method of nickel hydroxide synthesis through the complexation-precipitation route which yields a fine particle material having a specific surface area of 178 m2 g−1 has been described. The morphology of this material as revealed by electron microscopy is distinctly different from the turbostratic nature of electrosynthesized nickel hydroxide. While the long range structure as shown by the X-ray diffraction pattern is similar to that of β-Ni(OH)2, the short range structure as revealed by infrared spectroscopy incorporates characteristics similar to that of α-Ni(OH)2. Cyclic voltammetry studies show that the electroless nickel hydroxide has a higher coulombic efficiency (>90%), a more anodic reversible potential and a higher degree of reversibility compared to the electrosynthesized nickel hydroxide and conventionally prepared nickel hydroxide.

Analysis of nonlinear Mott-Schottky plots obtained from anodically passivating amorphous and polycrystalline TiO2 films

Abstract: The present work is concerned with analysis of the nonlinear Mott-Schottky plots obtained from the measured impedances of anodically passivating amorphous and polycrystalline TiO2 films including multiple donor levels. The passivating amorphous TiO2 films were prepared on titanium in 0.5 M H2SO4 solution galvanostatically at 2 mA cm−2 at formation potentials of 2, 3 and 5 V/SCE, and the polycrystalline films at 5 mA cm−2 at potentials of 20 and 30 V/SCE. The analysis was made by obtaining a numerical solution to a modified Mott-Schottky equation by using the Euler method. Based upon the analysis, the applied potential dependence of the donor distribution across the amorphous and polycrystalline TiO2 films was determined for various film thicknesses and measuring frequencies. Ionized donor concentration increases with increasing applied potential, regardless of film thickness and measuring frequency. The thinner film and the lower measuring frequency enhance the donor concentration. It is suggested from the analysis of the nonlinear Mott-Schottky plots that the applied potential dependence of the donor distribution is attributable to the presence of multiple donor levels and that the donor concentration as a function of applied potential is definitely determined by the film thickness and measuring frequency.

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Abstract: Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.

Inhibiting and accelerating effects of some quinolines on the corrosion of zinc and some binary zinc alloys in HCl solution

Abstract: The effect of quinoline, benzo(f)quinoline and 8-hydroxyquinoline on the electrochemical and corrosion behaviour of zinc, Zn-2% Cd and Zn-2% Pb alloys in deaerated 0.1m HCl solution was studied using the potentiostatic technique. Although quinoline inhibited the corrosion of zinc at all examined concentrations, it accelerated the corrosion of the alloys. As an inhibitor, quinoline was found to have a predominant anodic effect and its adsorption conformed with the Temkin isotherm. At concentrations >10−4 m, benzo(f)quinoline inhibited the corrosion of both zinc and the alloys. Inhibition was found to be predominantly anodic without changing the mechanism of zinc dissolution. Inhibition by 8-hydroxy quinoline was found to be purely anodic occurring by surface chelation, resulting in a change of the mechanism of zinc dissolution.

Electrocatalytic oxidation of methanol on platinum microparticles in polypyrrole

Abstract: The oxidation of methanol on gold electrodes modified with polypyrrole and platinum is reported These electrodes were characterized by cyclic voltammetry and by 12h polarizations in methanol solutions They were found to give higher currents and lower rates of drift than electrodes of platinum and platinized gold The effect of varying the amount of platinum deposited is also discussed

Electrochemical oxidation of manganese(II) at a platinum electrode

Abstract: Electrochemical oxidation of Mn2+ in sulphuric acid to form MnO2 was studied using stationary and rotating platinum/platinum ring-disc electrodes. It appears that nucleation of MnO2 is governed by an equilibrium involving a Mn(III) intermediate. Growth of MnO2 involves the reduction of MnO2 surfaces by Mn2+ ions in the solution to form MnOOH intermediates. The subsequent electrochemical oxidation of MnOOH releases a hydrogen ion and results in the formation of MnO2. The rate constant of MnOOH oxidation to MnO2 was estimated to be 40 s−1. With a sufficient supply of Mn2+ ions, a layer of MnOOH is built up and the in-solid diffusion of hydrogen ions becomes the ratedetermining-step. With a low Mn2+ concentration, diffusion of Mn2+ ions from bulk electrolyte to the MnO2/electrolyte interface is a factor controlling the growth of MnO2. The activation energy and the pre-exponential term of the diffusion coefficient of Mn2+ in 0.5m sulphuric acid were determined to be 44.8 kJ mol−1 and 100 cm2 s−1, respectively.

Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane

Abstract: Oxygen reduction at a gas-fed, porous, ruthenium-pyrochlore electrode attached to a Dow Developmental Fuel Cell Membrane was measured in solutions of various pH. Electrode assemblies containing high surface area Pb2Ru2−xPbxO7−y or Bi2Ru2−xBixO7−y with different amounts of Teflon content with/without the incorporation of Dow gel in the active part of the electrode with/without a CO2-treated Vulcan XC-72 carbon substrate were tested. The oxide pyrochlores were found to be chemically stable and to show their lowest overpotential if separated from a 2.5 M H2SO4 proton reservoir by the membrane. Interesting oxygen reduction activity at room temperature was obtained with the Pb2Ru1.74Pb0.26O7−y electrode bonded with 22% by weight Teflon and incorporating 5% by weight Dow gel. The performance of the oxides against B-site Pb concentration and a measurement of the surface charge on the particles indicate that, in this configuration, the active sites for the oxygen reduction reaction are OH− species at the O-site positions of the A2B2O6′O1−y pyrochlores, especially the bridging oxygen with one Ru and one Pb near neighbour, i.e. Pb−Ob−Ru. Evidence that oxide particles precipitated on CO2-treated carbon transfer electrons to the substrate is also presented.

Analysis of radiation-grafted membranes for fuel cell electrolytes

Abstract: One of the primary obstacles to be overcome for the development of economical fuel cells is the high cost of the membrane electrolyte. The currently favoured polymer electrolytes consist of poly(tetrafluoroethylene) backbone structures and poly(perfluorosulphonic acid) side chains. In an effort to find lower cost membranes, some radiation-grafted copolymer membranes were investigated. All the membranes contained poly(styrenesulphonic acid) side chains. Three different backbone polymer structures were studied: low-density poly(styrene), poly(tetrafluoroethylene)/poly(perfluoropropylene), and poly(tetrafluoroethylene). The results indicate that the membrane consisting of a poly(tetrafluoroethylene)/poly(styrenesulphonic acid) copolymer is a promising candidate as a fuel-cell electrolyte.

Scale-up studies of an Fe/Cr redox flow battery based on shunt current analysis

Abstract: Scale-up studies of a 0.7 kW single stack redox flow battery to a 2.1 kW three-stack assembly based on shunt current analysis are presented. The influence of the shunt-dependant design parameters on the energy efficiency of the system has been analysed for a single stack battery. Scale-up studies have been carried out in order to achieve energy efficiency in the three-stack assembly similar to that obtained for the single-stack system. The influence of the length of the hydraulic interconnection of the stacks and the assembly current on energy efficiency has been analysed. A shunt current calculation model for a three-stack assembly has been developed.

Electrochemical methods in the study of localized corrosion attack

Abstract: Electrochemical methods for determining the characteristic pitting potentials of 90Cu−10Ni alloy in slightly alkaline chloride solutions are summarized and the results of measurements carried out using potentiostatic, quasi-potentiostatic, potentiodynamic and galvanostatic techniques, complemented byex situ techniques — SEM and EDXA — are discussed. In borate buffer solution a passive state is established due to the formation of the oxide film with low ionic conductivity. However, in the presence of Cl− ions, at potentials higher than a certain critical value, breakdown of the anodic passivity occurs, caused by field-stimulated chloride entry into the passive oxide film at singular point defects. The brightening of the pits formed after oxide film breakdown was established to be due to the conversion of the passivating oxide film to one of high ion conductivity “Contaminated oxide” permits the passage of the metal cation into and through it, finally leaving it at the film/solution interface where pitting can proceed. During localized attack two characteristic potentials have to be distinguished: the potential of pit nucleation,E n, above which pit nucleation starts, and the breakdown potential,E b, above which the growth of nucleated pits develops. An attempt is made to compare the values ofE n andE b obtained through different methods, and to determine the factors influencing these values in each particular method.

Comparative kinetic studies of polypyrrole electrogeneration from acetonitrile solutions

Abstract: The formation and growth of polypyrrole films on platinum electrodes from acetonitrile media has been followed by microgravimetricex situ determination of the polymer grafted on the electrode. Kinetic parameters were also obtained from the charge consumed during polymerization. The electrogenerated polymer films were checked in the background electrolyte by voltammetry and chronoamperometry. Assuming a constant oxidation or reduction charge per unit of polymer weight, these charges were used to obtain the reaction order during the polymerization processes. The kinetics were found to be dependent on [pyrrole]0.5 [LiClO4]0.5 from gravimetric determination; [pyrrole]0.4 [LiClO4]0.5 from the polymerization charge; [pyrrole]0.4 [LiClO4]0.5 from the anodic charge of the control voltammograms and [pyrrole]0.4 [LiClO4]0.4 from the anodic and cathodic charge of the control chronoamperograms. Good agreement was found between the different methods. The good agreement between order dependence was due to the low water content.

The inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel

Abstract: A study was carried out on the inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel by two organic inhibitors, namely benzotriazole and benzonitrile. Tensile testing, scanning electron microscopy, weight loss measurements and potentiodynamic polarization were the techniques used for this study. Tensile tests showed that 410 steel is highly susceptible to hydrogen stress cracking. Scanning electron microscopic observations of fracture surfaces showed a brittle quasi-cleavage type of failure when the steel was hydrogen charged from 0.5m H2SO4. Both inhibitors reduced hydrogen induced ductility loss though the fracture mode was unaltered. They showed increasing inhibition efficiencies for corrosion as well as cathodic hydrogen evolution as their concentration in H2SO4 increased from 3.9×10−5m to 8.4×10−3m. Benzonitrile was found to be a more efficient inhibitor than benzotriazole for AISI 410 stainless steel exposed to 0.5m H2SO4.

Bifunctional electrodes for an integrated water- electrolysis and hydrogen-oxygen fuel cell with a solid polymer electrolyte

Abstract: An alternative concept of an integrated water electrolysis/hydrogen-hydrogen fuel cell using metal electrocatalysts and a solid polymer electrolyte is described. Instead of operating both electrodes as hydrogen and oxygen electrodes respectively the electrodes are used as oxidation and reduction electrodes in both modes of operation. A more suitable selection of electrocatalysts and an improved cell design are possible; both can increase the efficiency of the cell considerably. New results on the electrocatalytic activity of various noble-metal containing catalysts with respect to both oxygen evolution and hydrogen oxidation in a proton exchange membrane-cell at 80°C are reported. Kinetic data derived from Tafel plots of the oxygen evolution polarization curves agree closely with those of experiments with aqueous sulphuric acid electrodes. This agreement allows the determination of kinetic parameters for electrocatalysts difficult to prepare in solid smooth electrodes but easy to be made into porous deposits. Polarization curves of the hydrogen oxidation reaction clearly indicate a relative activity rating of the studied catalysts. In cycling tests the lifetime stability of the new bifunctional oxidation electrode was determined. Polarization data obtained under these conditions agree with those obtained in earlier experiments where electrodes were exposed to only one type of oxidation reaction. During a test of 10 cycles (30 min of electrolyser and 30 min of fuel cell mode each) no changes in the electrode potential were observed. With the conventional cell design employing a hydrogen and an oxygen electrode both catalyzed with platinum and a current density of 100 mA cm−2 a storage efficiency of 50% was calculated; with the alternative concept of oxidation and reduction electrodes and selected oxidation catalysts this was improved to 57%. With further improvements these efficiencies seem possible even at current densities of 500 mA cm−2.

Rechargeable thin film batteries of polypyrrole and polyaniline

Abstract: Thin films of polypyrrole (PPy) and polyaniline (PANi) were electrochemically deposited on stainless steel substrate under potentiostatic control. Secondary batteries were assembled using these polymers as active electrode materials and propylene carbonate (PC) solutions of tetraethyl ammonium perchlorate (TEAP) as the electrolyte. In repeated charge and discharge tests, the all PANi cells performed better than the PPy-PANi cells in voltage characteristics, recyclability, charge retention and coulombic efficiency. There appears to be an optimum charging current for the polymer cells and charging the cells at rates notably different from the optimum value reduced the cell performance substantially. It is suggested that the cells operate under kinetic control and charging at extreme rates either reduces the participation of polymer redox in the overall cell operation or produces mass transfer limitations.

Physicochemical properties of electrodeposited β -lead dioxide: Effect of deposition current density

Abstract: The influence of current density on the coulometric efficiency of β-PbO2 deposition in 0.5 M cM lead nitrate, the nonstoichiometry, impurity of α-PbO2 and voltammetric double layer capacitance have been studied. While the coulometric efficiency is about 95% at current densities less than 30 mA cm−2, it decreases at higher current densities. The oxygen deficiency, δ, in β-PbO2-δ has been found to be invariant with the current density. X-ray diffraction studies provide a linear decrease in the weight percent of α-PbO2 as an impurity in the β-PbO2 with increase in current density, and the α-PbO2 is found to be absent at 100mA cm−2 or higher. The estimated double layer capacitance from the cyclic voltammograms recorded in the potential range 0.70–1.10V, increases with deposition current density, indicating enhanced surface area.