Preparation, electrochemical characterization and charge–discharge of reticulated vitreous carbon/polyaniline composite electrodes

Polyaniline was electrodeposited onto reticulated vitreous carbon – RVC – in order to obtain a tridimensional composite electrode. Three variations of these electrodes were analysed: a small-anion-doped polyaniline (RVC/Pani), a polyanion-doped polyaniline (RVC/PaniPSS) and a bi-layer type formed by an inner layer of the first electrode and an outer layer of the second one (RVC/Pani/PaniPSS). These composites were characterized by cyclic voltammetry, scanning electronic microscopy and electrochemical impedance spectroscopy. Photomicrographies, voltammetric profiles and impedance data pointed to different morphological and electrochemical characteristics for polyaniline doped with small or large anions, and a mixed behavior for the bi-layer electrodes. Charge–discharge tests for these tridimensional (3D) electrodes, employed as the cathode in lithium batteries, indicated better performance for the RVC/Pani electrode. These RVC composites presented higher specific capacities when compared with those obtained for Pani deposited onto bidimensional substrates.     

Preparation of polyaniline-poly (p-styrenesulfonic acid) composite by post-polymerization and application as positive active material for a rechargeable lithium battery

Polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) composite was prepared by thermal post-polymerization of PANI-p-styrenesulfonic acid (SSA) composite. A PANI–SSA composite was prepared by mixing PANI/N-methyl-2-pyrrolidinone solution with SSA aqueous solution. The PANI–SSA composite film was prepared by casting the composite onto an ITO glass plate. The cast film was converted to PANI-PSS film by heating at 100°C for 3h (post-polymerization process). The PANI–PSS modified ITO electrode showed electrochemical responses based on the redox reaction of PANI–PSS composite in the organic electrolyte solution, for example, propylene carbonate containing 1moldm–3 LiClO4. The PANI–PSS composite was a cation -doping polymer composite. The composite was also modified on a porous carbon material (Reticulated Vitreous CarbonTM, RVC, Energy Research and Generation, Inc.). The PANI–PSS modified RVC electrode showed similar electrochemical behaviour as the PANI–PSS modified ITO electrode. Model secondary lithium cells, Li|1moldm–3 LiClO4-propylene carbonate|PANI–PSS modified RVC electrode, were constructed and charge–discharge cycling tests were carried out. The cell showed about 60% coulombic efficiency under high current density cycling conditions (3.8Ag–1, per gram of PANI–PSS modified RVC electrode).     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

Electrochemical reduction of carbon dioxide in methanol at ambient temperature and pressure

The electrochemical reduction of carbon dioxide was studied in methanol-based supporting electrolytes on various metal electrodes at ambient temperature and pressure. The ionophore of the catholyte was benzalkonium chloride, [RN(CH3)2CH2C6H5]⁺Cl^–, where R=C8–C18, the chain length being distributed around C14. A divided H-type cell was used, the supporting electrolytes were 10–2moldm–3 benzalkonium chloride in double distilled methanol (catholyte) and a 10–1moldm–3 aqueous KHCO3 solution (anolyte). Nine different, high purity (>99.5%) metal electrodes were used: Ti, Fe, Co, Ni, Pt, Ag, Au, Zn and Sn. Carbon monoxide, methane and ethane were the main organic products. Silver, Au, Zn and Sn cathodes allowed for the best faradaic yields of CO production, the maximum amount of CO (71%, 185 mmol) being formed on the Ag electrode. Methane evolved on each of the nine tested electrodes, with current yields in the range from 0.2 to 3.0%. Ethane and ethylene were produced on the nickel electrode, with low faradaic efficiencies, 0.5 and 0.3%, respectively. No dimerization products were detected. This research can contribute to large-scale manufacturing of useful organic products from a readily available and cheap raw material: CO2-saturated methanol from industrial absorbers (the Rectisol process).     

Electrochemical studies of the nickel electrode with cobalt modification

The electrochemical behaviour of the paste-type nickel hydroxide electrode with cobalt-modified nickel foam was investigated using galvanostatic charge–discharge, electrochemical impedance spectroscopy, cyclic voltammetry and current pulse relaxation methods. Experimental results showed that the performance of the nickel electrode with substrate deposition of a thin layer of cobalt was improved markedly. This improvement could be attributed to the enhanced electrical conduction between the substrate and the active material. The enhanced electrical conduction increases the charge efficiency and the discharge depth of the nickel electrodes and therefore increases the utilization of the active material. This suggests that the electrical conduction between the substrate and the active material is essential to the practical use of paste-type nickel hydroxide electrodes.     

Amorphous nickel-valve metal-platinum group metal alloy electrodes for hydrogen-oxygen sulphuric acid fuel cells

Powder catalysts were prepared by immersion of amorphous Ni-40Zr and Ni-40Ti alloys containing a few at % of platinum group elements in HF solution. This treatment led to preferential dissolution of the valve metal and nickel with a consequent formation of microcrystalline alloy powders consisting of concentrated platinum group elements and some nickel and valve metal. Porous gas-diffusion electrodes prepared by using these alloy catalyst powders were employed for electrochemical reduction of oxygen and oxidation of hydrogen in 1 M H₂SO₄ at 25°C. The activity of the electrodes prepared from the amorphous alloys containing Pt–Ru, Pt–Rh, Pt and Pd for oxygen reduction was considerably higher than that of the platinum black electrode. Oxidation of hydrogen occurred readily close to the equilibrium potential. Amorphous alloy electrodes containing Pt–Ru, Pt–Rh and Pt were more active than the platinum black electrode for the hydrogen oxidation.     

Electrooxidation of alcohols at a nickel oxide/multi-walled carbon nanotube-modified glassy carbon electrode

Electrocatalytic oxidation of methanol and some other primary alcohols on a glassy carbon electrode modified with multi-walled carbon nanotubes and nano-sized nickel oxide (GCE/MWNT/NiO) was investigated by cyclic voltammetry and chronoamperometry in alkaline medium. The results were compared with those obtained on a nickel oxide-modified glassy carbon electrode (GCE/NiO). Both the electrodes were conditioned by potential cycling in the range of 0.1–0.6 V versus Ag/AgCl in a 0.10 M NaOH solution. The effects of various parameters such as scan rate, alcohol concentration, thickness of NiO film, and real surface area of the modified electrodes were also investigated and compared. It was found that the GCE/MWNT/NiO-modified electrode possesses an improved electrochemical behavior over the GC/NiO-modified electrode for methanol oxidation.     

Anodic behaviour of carbon materials in NaCl saturated NaAlCl4 fused electrolyte at low temperatures: A cyclic voltammetric study

The anodic behaviour of compacted graphite, graphite powder, glassy carbon and reticulated vitreous carbon electrodes in basic sodium chloroaluminate melt in the temperature range 428–573 K was studied using cyclic voltammetry. Chlorine evolution (> + 2.1 V vs Al) alone was the predominant reaction on the compact glassy carbon and fresh RVC electrodes. On compacted graphite, chlorine-assisted chloroaluminate intercalation was found to be a competitive process to the chlorine evolution. At high sweep rates, intercalation/deintercalation near the graphite lattice edges occur faster than chlorine evolution. Subsequent intercalation, however, is a slow process. Chlorine evolution predominates at higher temperatures and at higher anodic potentials. On graphite powders, a more reversible free radical chlorine adsorption/desorption process also occurs in the potential region below chlorine evolution. The process occurs at the grain boundaries, edges and defects of the graphite powder material. Intercalation/deintercalation processes are mainly responsible for the disintegration of graphitic materials in low-temperature chloroaluminate melts. Repeated intercalation/deintercalation cycles result in the irreversible transformation of the electrode surface and electrode characteristics. The surface area of the electrode is increased substantially on cycling. Electrode materials and operating conditions suitable for chlorine generation, intercalation/deintercalation and chlorine adsorption/desorption and power sources based on these processes are identified in this work.     

Introduction of ion-exchange moieties to reticulated vitreous carbon by direct chemical modification

Reticulated vitreous carbon (RVC) disks were successfully modified by covalent attachment of functional groups capable of metal ion exchange. Modification was accomplished through a three-step procedure. Oxidation of the RVC material with a mixture of H₂SO₄ and HNO₃ significantly increased the number of carboxylic acid groups on the surface. These were converted to the acid chloride using oxalyl chloride and the ion-exchange moieties were attached through amide formation. Metal-ion uptake was determined by subjecting the modified electrodes to solutions of four different metal cations. Inductively coupled plasma spectroscopy was employed to determine the metal ion concentration both before and after introduction of the modified RVC material. Uptake ranged from 20% to 99%, depending on the ion-exchange moiety and metal cation. Electrochemical analysis revealed that the modified RVC is no longer suitable for use as an electrode.     

Development of a novel metal hydride–air secondary battery

A laboratory metal hydride/air cell was evaluated. Charging was via a bifunctional air gas-diffusion electrode. Mixed nickel and cobalt oxides, supported on carbon black and activated carbon, were used as catalysts in this electrode. At 30mAcm–2 in 6m KOH, the air electrode potentials were –0.2V (oxygen reduction) and +0.65V (oxygen evolution) vs Hg/HgO. The laboratory cell was cycled for 50 cycles at the C/2 rate (10mAcm–2). The average discharge/charge voltages of the cell were 0.65 and 1.6V, respectively. The initial capacity of the metal hydride electrode decreased by about 15% after 50 cycles.     

Indirect electrooxidation of crotyl and cinnamyl alcohol using a Ni(OH)2 electrode

The heterogeneous catalytic redox behaviour of the nickel oxyhydroxide electrode surface was investigated with and without crotyl and cinnamyl alcohols using cyclic voltammetry. A comparison of the cyclic voltammograms recorded in the absence and presence of these alcohols confirmed the catalytic oxidation of the alcohols by the surface Ni³⁺/Ni²⁺ redox system at the electrode surface. Preparative scale experiments were also carried out using Ni(OH)₂ electrodes of large surface area and the redox behaviour of the electrode was confirmed by the isolation of crotonic and cinnamic acids with high yield efficiency. The influence of molecular size on the catalytic oxidation process is examined.     

Reaction of CO/CO2 gas mixtures on Ni–YSZ cermet electrodes

The reaction of carbon monoxide/carbon dioxide mixtures on Ni–YSZ cermet electrodes was investigated as a function of the electrode potential and the partial pressures of the reactants at 1273 K. Time-dependent reaction rates are observed for the CO oxidation reaction for oxygen activities corresponding to open circuit potentials in the range from –750 to –1010 mV. The electrode changes between a passive state and several active states for the CO/CO2 reaction. Periodic changes of the reaction rate for the CO oxidation are observed every 30 and 80 s. The impedance spectra recorded at the rest potential and the overpotential dependence of the CO oxidation rate indicate a change in the number of active sites in the reaction zone. In the active state, the CO oxidation reaction is more than one order of magnitude slower than the hydrogen oxidation reaction on these Ni–YSZ cermet electrodes. These results indicate clear differences in the kinetics of the CO and H2 oxidation reaction.     

Aqueous polysulphide flow-through electrodes: Effects of electrocatalyst and electrolyte composition on performance

Porous electrodes are required to achieve satisfactory performance of the aqueous sulphide/polysulphide redox couple in energy conversion and storage applications. A flow cell for testing flow-through porous electrodes was constructed and operated. The effects of electrode material, temperature, flow rate, and electrolyte composition were studied. Catalytic electrode surface layers of Co and MoS₂ demonstrated performances which were more than adequate to meet a design goal of 10–20 mA cm^–2 at less than 50mV overpotential. Flow rate variation had only a small effect on the current density-overpotential behaviour, whereas raising the temperature and/or adding dimethylformamide to the electrolyte had much larger effects. These observations are consistent with steady-state results obtained on rotating disc electrodes.     

Electrodeposition of Mesoporous Nickel onto Foamed Metals Using Surfactant and Polymer Templates

High surface area electrodes are of much interest for various applications including electrochemical detectors, batteries, and fuel cells. The development of high surface area electrodes using liquid crystal templating to enhance the surface area of three-dimensional electrodes is reported. This method uses Brij 56, a nonionic surfactant, or Pluronic P123, a triblock copolymer, to template the electrodeposition of a mesoporous nickel film onto a foamed nickel electrode. This method is found to produce a 30–35 fold increase in surface area. XRD of Pluronic P123 templated nickel on gold surfaces shows a peak consistent with a pore to pore spacing of 7.5 nanometers.     

Activated nickel anodes for average power fuel cells

The activation of nickel gas anodes has been studied using a wide range of transition metal compounds added to the nickel powder before sintering or added by impregnation to the sintered electrodes. Using a hydrogen-oxygen fuel cell for the tests, the efficiency of the impregnant for increasing the performance of the nickel/hydrogen electrode was in the range ammonium wolframate > cobalt acetate > copper basic carbonate > ammonium vanadate. The optimum performance was obtained by using a mixture of the first two compounds. The optimum conditions for the use of these electrodes are shown to be 79–82°C with a KOH electrolyte of 6.5–6.7 N and a hydrogen pressure at the electrode of 400 mm Hg. Under these conditions the cell gave 300mAcm^–2 and a maximum power density of 110mWcm^–2.     

Absorption/desorption of calcium ions on polypyrrole-loaded reticulated vitreous carbon

Polypyrrole/polystyrene sulfonate-loaded reticulated vitreous carbon electrodes RVC/PPy/PSS were studied as electrochemical cation exchangers for the absorption of Ca²⁺ ions from aqueous solutions. Two grades of RVC were used mainly with 20 and 45 PPI. The preparation and characterization of the PPy/PSS layers on RVC are described. The structure and morphology of the porous electrodes were characterized using cyclic voltammetry (CV) and scanning electron microscopy (SEM). The potential dependent absorption and desorption of Ca²⁺ on RVC/PPy/PSS electrodes was studied in 0.1 M CaCl₂ solution using chronoamperometry and atomic adsorption spectroscopy (AAS). The extent of Ca²⁺ ions absorbed into RVC/PPy/PSS was found to increase with decreasing absorption potential, the specific surface area and the extent of polymer loading. Recovery of the RVC/PPy/PSS electrode was performed by anodic polarization at potentials at which Ca²⁺ ions are desorbed. The coulomb efficiency for the Ca²⁺ desorption process was found to be higher than that for the Ca²⁺ absorption process. From these results the feasibility of using RVC/PPy/PSS for water softening was determined under the prevailing experimental conditions.     

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

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.     

Electrochemical properties of benzylmercapto and dodecylmercapto tetra substituted nickel phthalocyanine complexes: Electrocatalytic oxidation of nitrite

Nickel tetrakis(benzylmercapto)phthalocyanine (NiTBMPc) and nickel tetrakis(dodecylmercapto)phthalocyanine (NiTDMPc) complexes were synthesized and their spectral and electrochemical properties reported. The CV showed four or five redox processes for NiTBMPc and NiTDMPc, respectively. For the first time, spectroelectrochemistry gave evidence for the formation of Ni^II/Ni^I process in a NiPc complex. The rest of the processes were ring based. The NiTBMPc complex was successfully deposited on both gold and glassy carbon electrodes by electropolymerisation while NiTDMPc complex was deposited on gold electrode only. The films were electro-transformed in aqueous 0.1 M NaOH solution to the O–Ni–O oxo bridged form. The modified electrodes were characterized using electrochemical impedance spectroscopy and the results showed typical behavior for modified electrodes. Electrodes with poly-Ni(OH)Pcs films exhibited higher charge transfer resistance values, R_p than their corresponding poly-NiPcs films counterparts. All the modified electrodes showed improved catalytic activities than the unmodified electrodes towards nitrite ions electrooxidation. Better catalytic activities were observed for the modified electrodes when they were transformed to O–Ni–O oxo bridge form. All the modified electrodes exhibited high resistance to electrode surface passivation.     

Electrooxidation of benzyl alcohol at high surface area nickel (NiS x ) electrodes in alkaline solution

The heterogeneous catalytic redox behaviour of NiS _x deposited electrodes was investigated with and without benzyl alcohol in KOH solution using cyclic voltammetry and linear sweep voltammetry. The limiting current density for benzyl alcohol oxidation on a NiS _x electrode was 22 times larger than that on a polished nickel electrode. The experimental results in galvanostatic electrolysis using fractional factorial design showed that the main and interaction effects of benzyl alcohol concentration, temperature, and OH^– concentration are the key variables influencing the selectivity of benzaldehyde formation during electrolysis.     

Scanning electron microscopic studies of porous carbon electrodes used in alkaline fuel cells

Multilayer, polytetrafluoroethylene (PTFE)-bonded gas diffusion-type electrodes were prepared by the rolling method. Changing the electrode structure and manufacturing method improved alkaline fuel cell performance. Activated carbon or carbon black was used as the support material, with platinum as a catalyst and nickel screen as the backing material. Double-layer electrodes possessed both active and diffusion layers on the backing layer. However, the single-layer electrodes had only the active layer on the backing layer. The electrodes were prepared by using different PTFE contents and platinum loadings. In this study the surface photographs of the electrodes were taken with a scanning electron microscope. Elemental analyses of the surface elements were performed by energy dispersive X-ray spectroscopy (EDXS). Electrodes having activated carbon on their surfaces were observed to possess a nonuniform and porous structure. These electrodes showed better performance than electrodes having carbon black, which presented a uniform and nonporous structure.