Electrochemical studies of sodium borohydride in alkaline aqueous solutions using a gold electrode

Cyclic voltammetry using a gold disk working electrode has been employed for quantifying sodium borohydride in alkaline aqueous media. A 6 mm diameter disk working electrode is shown to offer higher sensitivity compared to a 1 mm diameter electrode, demonstrated by the slope of the calibration curve being 38 times greater when the 6 mm electrode is used. The dynamic range is also greater with the 6 mm electrode, evidenced by the square of the correlation coefficient for regression over the same concentration range. The influence of the matrix on the calibration curves was evaluated; 2 M NaOH, 2 M NaOH + 10% NaBO₂, 2 M NaOH + 20% NaBO₂, 2 M NaOH + 25% NaBO₂, and 25% NaBO₂ in solution at pH 12 were the matrices investigated. The matrix was shown to affect the calibration curves; generally, increasing content of NaBO₂ caused the slope to decrease. Cyclic voltammetry was then used to assess the stability of sodium borohydride in aqueous solutions of different compositions.     

Fabrication of seven cells in single tubular solid oxide fuel cell using multi-pass extrusion process

A solid oxide fuel cell (SOFC) with high specific electrolyte surface area was fabricated using the multi-pass extrusion process. The cell configuration was NiO-YSZ/YSZ/LSM. In this design, one tube contained 7 individual fuel cells. The outer diameter of the tube was 5 mm and 4.1–4.2 mm after extrusion and sintering, respectively. The length of the cell was optimized to 3–5 cm with continuous channels of 400–450 μm in diameter. The porous microstructures of the anode and the cathode and the dense microstructure of the electrolyte were observed by scanning electron microscopy. The relative density of the electrolyte was >96% and the thickness was 15–20 μm. The XRD profiles indicated no undesirable phases after co-sintering at 1300 °C.     

Direct electrodeposition of metal nanowires on electrode surface

A method for decorating the surface of disk electrodes with metal nanowires is presented. Cu and Ni nanowires with diameters from 1.0 μm to 0.2 μm are directly deposited on the electrode surface using a polycarbonate membrane filter template maintained in contact with the metal substrate by the soft homogeneous pressure of a sponge soaked with electrolyte. The morphologic and structural properties of the deposit are characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The latter shows that the head of nanowires with diameter of 0.4 μm is ordinarily polycrystalline, and that of nanowires with diameter of 0.2 μm is almost always monocrystalline for Cu and frequently also for Ni. Cyclic voltammetries and impedance investigations recorded in alkaline solutions at representative Ni electrodes decorated with nanowires provide consistent values of roughness factor, in the range 20–25.     

Capacitive performance of an ultralong aligned carbon nanotube electrode in an ionic liquid at 60 °C

An ultralong (1.0 mm) aligned carbon nanotube (ACNT) electrode was fabricated by a cut/paste method. The electrode retains the intrinsic properties, including robust mechanical property, high surface area, and regular pore structure, of individual nanotubes. Electrochemical properties of the ACNT electrode in an ionic liquid (IL) electrolyte were studied by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance spectroscopy. The ACNT electrode achieved a specific capacitance of 27 F/g, had excellent rate capability, and a long cycle life at 60 °C, indicating that an ACNT electrode/IL electrolyte electrochemical double layer capacitor is promising for high temperature (60 °C) applications. The capacitive performance of ACNT electrode is excellent, because it possesses large pores and regular pore structures, which is revealed by N₂ adsorption and scanning electron microscopy.     

Characterization of gas diffusion layers for PEMFC

A carbon-filled gas diffusion layer (CFGDL), which is in the configuration similar to conventional carbon cloth gas diffusion layer (GDL) coated with carbon layer on both faces, was investigated and compared with conventional carbon paper-based single-layer and dual-layer GDLs. Like the carbon cloth GDL, CFGDL has presented superior performances over the single-layer or dual-layer GDL in all three polarization (activation, ohmic and concentration) controlled regions under electrochemical characterizations (steady-state polarization and electrochemical impedance spectra). The results from SEM showed that CFGDL has the same thickness of 0.11 mm as that of single-layer GDL, while dual-layer GDL has a thickness of 0.18 mm. The fully filled carbon paper with carbon/PTFE filler, as seen in the SEM image, displayed good support for the catalyst layer and electrolyte phase, allowing good electrical contact between the GDL/catalyst/membrane and GDL/flow field plate to be achieved. From porosimetry analysis, CFGDL presented a lower porosity of 67% and a much smaller average pore diameter of 4.7 μm compared to the single-layer GDL (porosity of 77% and pore diameter of 35.8 μm) and dual-layer GDL (porosity of 73% and pore diameter of 25.5 μm); however, it also gave the largest limiting current density, which reflects the improvement in mass transportation. This phenomenon is likely attributed to the fast removal of micro-water droplets formed in the CFGDL structure of the electrode.     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Enhanced cyclability and surface characteristics of lithium batteries by Li-Mg co-deposition and addition of HF acid in electrolyte

The effect of Mg(ClO₄)₂ and HF acid in 1 M LiPF₆/EC + DEC + DME electrolyte on the cycle behaviors and the surface characteristics of the lithium electrode were investigated. The morphology of the electrode is greatly improved using Li-Mg co-deposition and HF acid, and results in the enhancement of the cyclability observed by electrochemical measurement. The scanning electron microscopy images of the electrode surface show densely deposited lithium-magnesium alloy particles with a hemispherical shape on the entire surface of the electrode. The improved coulombic efficiency was attributed to the synergistic effect due to the formation of Li-Mg alloy and LiF caused by Li-Mg co-deposition and HF to the electrolyte, respectively.     

Simple synthesis of surface-modified hierarchical copper oxide spheres with needle-like morphology as anode for lithium ion batteries

Hierarchical, nanostructured copper oxide spheres were synthesized in a stirred solution of cupric acetate and ammonium hydroxide. Cetyltrimethylammonium bromide (CTAB) was used as a surfactant to modify the surface morphology of CuO spheres. Ordered nano-needle arrays can be formed on the surface of the CuO spheres (instead of disordered nano-leaves) in the presence of CTAB. Each CuO sphere is about 2 μm in diameter and possesses a large number of nano-needles that are about 20-40 nm in width and more than 300 nm in length. The needle-like hierarchical structure can greatly increase the contact area between CuO and electrolyte, which provides more sites for Li⁺ accommodation, shortens the diffusion length of Li⁺ and enhances the reactivity of electrode reaction, especially at high rates. After 50 cycles, the reversible capacity of the prepared needle-like CuO can sustain 62.4% and 56.4% of the 2nd cycle at a rate of 0.1C and 1C, respectively.     

Determination of open-circuit potentials at gas/electrode/YSZ boundary versus molten carbonate reference electrode at medium temperatures: I. Potentials of Au and Pt in O2 and H2 + H2O atmospheres

A double gas concentration cell as combination of the cell with the yttria stabilized zirconia (YSZ) electrolyte and the cell with molten Li₂CO₃ + Na₂CO₃ eutectics is proposed as an alternative cell system with a standard reference electrode for measurements of the open-circuit potential (OCP) values of electrodes in oxygen concentration cell with the yttria stabilized zirconia (YSZ) electrolyte. In this double-cell one electrode is common for the two cells and the reference electrode is the standard molten carbonate half-cell with 0.33O₂ + 0.67CO₂ atmosphere. This reference electrode should enable the monitoring of OCP and overpotential values in polarization studies in the three-electrodes configuration. If the possible reaction between the solid YSZ and liquid molten carbonates electrolyte is very slow, the measured values of the open-circuit-voltage (OCV) of this cell may be considered equal to the respective reversible electromotive forces (EMF). Very good resistance of the smooth YSZ products to the corrosion in highly dehydrated Li/Na molten carbonates has been shown in experiments lasting few 1000 h. Hence, the consistency of OCV values with the respective EMF values have been tested at various partial pressures of CO₂ and O₂ in the gas mixtures above the molten carbonate electrolyte and at various partial pressures of O₂ + Ar or H₂ + H₂O gas mixtures at the Au or Pt electrodes/YSZ interface. The results have shown the reliability of the double-cell in determination of the open-circuit potentials (OCP) of gas electrodes at the YSZ surface as measured versus the reference electrode with molten carbonate electrolyte. The consistency of OCP and EMF values has been shown satisfying and enhances to use the proposed double-cell in further investigations of OCP and overpotential values at TPB of electrode/YSZ/mixture of reacting gases. At high differences of O₂ partial pressures on both sides of the YSZ membrane some permeation of this gas through the YSZ membrane has been observed. Probably, this effect has an electrochemical character.     

Thermal stability of the interface between discharged non-graphitizable carbon and electrolyte for lithium-ion batteries

The thermal stability of a non-graphitizable carbon electrode was studied quantitatively by differential scanning calorimetry (DSC). Charged non-graphitizable carbon electrode powder gave exothermic peaks at around 300 °C and the heat values varied depending on the ratio of the electrode powder to coexisting electrolyte solution. Based on the similarities in the exothermic behaviors of charged graphite and non-graphitizable carbon electrodes, the exothermic reactions at around 300 °C should be assigned to the reductive decomposition of a surface film by charged non-graphitizable carbon. On the other hand, non-graphitizable carbon electrode powder showed exothermic reactions at around 290 °C even at a discharged state, while almost no exothermic heat was seen for a discharged graphite electrode powder at temperatures above 250 °C. The heat values decreased as Li-ions in the non-graphitizable carbon electrode were extracted. Based on the present results and a consideration of the slow diffusion and irreversible trapping of Li-ions in non-graphitizable carbon, Li-ions remaining in non-graphitizable carbon could induce exothermic reactions at around 290 °C, even at a discharged state.     

Electric double layer capacitors with gelled polymer electrolytes based on poly(ethylene oxide) cured with poly(propylene oxide) diamines

Using a gel electrolyte for electric double layer capacitors usually encountered a drawback of poor contact between the electrolyte and the electrode surface. A gel electrolyte consisting of poly(ethylene oxide) crosslinked with poly(propylene oxide) as a host, propylene carbonate (PC) as a plasticizer, and LiClO₄ as a electrolytic salt was synthesized for double layer capacitors. Diglycidyl ether of bisphenol-A was blended with the polymer precursors to enhance the mechanical properties and increase the internal free volume. This gel electrolyte showed an ionic conductivity as high as 2 × 10⁻³ S cm⁻¹ at 25 °C and was electrochemically stable over a wide potential range (ca. 5 V). By sandwiching this gel-electrolyte film with two activated carbon cloth electrodes (1100 m² g⁻¹ in surface area), we obtained a capacitor with a specific capacitance of 86 F g⁻¹ discharged at 0.5 mA cm⁻², while the capacitance was 82 F g⁻¹ for a capacitor equipped with a liquid electrolyte of 1 M LiClO₄/PC. The capacitance decrease with the current density was less significant for the gel-electrolyte capacitor. We found that the less restricted ion diffusion near the electrolyte/electrode interface led to the smaller overall resistance of the gel-electrolyte capacitor. The high performance of the gel-electrolyte capacitor has demonstrated that the developed polymer network not only facilitated ion motion in the electrolyte bulk phase but also gave an intimate contact with the carbon surface. The side chains of the polymer in the amorphous phase could stretch across the boundary layer at the electrolyte/electrode interface to come into contact with the carbon surface, thus improving transport of Li⁺ ions by the segmental mobility in polymer.     

Dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte Part I: Spectral analysis of potential fluctuations

The dynamic behaviour of a simplified electrolyser with a two phase solid-liquid electrolyte was investigated. The current-voltage curve and the impedance of the working electrode (current collector) was measured. In addition the fluctuations of the working electrode potential and of the electrolyte resistance close to the electrode were analysed, especially by spectral analysis. Conductive zinc-coated polystyrene beads and insulating glass beads were compared. For glass beads the kinetics were controlled by ohmic drop fluctuations due to the collisions of the beads onto the collector. With zinc beads the charge exchanges between the beads and the current collector were also observed and contributed to the potential fluctuations. A complementary analysis of the elementary potential and electrolyte resistance transients will be reported in Part II; the electrochemical behaviour of the collector will then be modelled.     

The electrochemiluminescent behavior of luminol on an electrically heating controlled microelectrode at cathodic potential

A new ECL detection system equipped with an electrically heating controlled microelectrode as the working electrode was established in this paper. The electrically heated microelectrode was made of a platinum wire (25 μm in diameter, 6 mm in long). Linear scan voltammetry has been applied to investigate the ECL behavior of luminol on the heated microelectrode during cathotic scanning. Good sensitivity and reproducibility for measurement of the ECL of luminol could be obtained at the heated microelectrode.     

Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode

Borohydride oxidation has been investigated using a rotating disk electrode technique. The parameters, such as apparent rate constant, Tafel slope, Levich slope, number of electrons exchanged and reaction order, have been determined. The borohydride ion is oxidised on the gold electrode with an electrochemical rate constant of around 1 cm s⁻¹ at intermediate potentials where side reactions had less effect. Influences of temperature, concentrations of borohydride and supporting electrolyte (NaOH) on the parameters were evaluated.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon-steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.     

The electrochemical synthesis of polyaniline/polysulfone composite films and electrocatalytic activity for ascorbic acid oxidation

Polyaniline (PANI)/polysulfone (PSF) composite films with asymmetric porous structure were successfully prepared by electropolymerization. The back face (in contact with the electrode) of the freestanding composite film is green while the outer face is white. The chemical component and the morphology of the surfaces were characterized by FTIR spectra and scanning electron microscopy, respectively. It was shown that replicate films gave reproducible voltammetry in 0.5 M H₂SO₄. The influence of the electrolyte and the acidic concentration on the redox peak currents of polyaniline were investigated in detail. The composite film electrode showed good electrocatalytic activity for ascorbic acid, which the anodic overpotential was evidently reduced compared with that obtained at bare Pt electrode. The diffusion coefficient of ascorbic acid was 1.38 × 10⁻⁶ cm² s⁻¹.     

Enhanced mass transport to a reticulated vitreous carbon rotating cylinder electrode using jet flow

The mass transport characteristics of a porous, rotating cylinder electrode (RCE, 1.0 cm diameter; 0.5, 0.9 or 1.2 cm long; 1.25, 2.25, 3.00 cm³ overall volume; 250-2000 rpm speed) fabricated from reticulated vitreous carbon (RVC, 60 ppi or 100 ppi) were investigated. The deposition of copper from an acid sulfate electrolyte (typically, deoxygenated 1 mM CuSO₄ in pH 2, 0.5 M Na₂SO₄ at 298 K) was used as a test reaction. The effect of a jet flow of electrolyte towards the electrode and the introduction of polypropylene baffles in the electrochemical cell were studied at controlled rotation rates of the RCE. The product of mass transport coefficient and volumetric electrode area (k_mA_e) is related to the rotation speed of the electrode. For the 60 ppi RVC RCE, the jet electrolyte flow (3.5 cm³ s⁻¹) enhanced the mass transport rates by a factor of 1.46 at low rotation speeds; this factor was reduced to 1.08 at high rotation speeds. For a 100 ppi electrode, the enhanced mass transport decreased from 1.26 to 1.03 at low and high rotation rates, respectively. Under the experimental conditions, baffles showed little effect on the mass transport rates to the RVC RCE. Mass transport to jet flow at an RVC RCE is compared to other RCEs using dimensionless group correlation.     

Anodic, cathodic and cyclic voltammetric deposition of ruthenium oxides from aqueous RuCl3 solutions

Anodic, cathodic and cyclic voltammetric (CV) deposition of ruthenium oxides from aqueous RuCl₃ solutions have been investigated using stationary and rotating disk electrodes (RDE) in this work. The CV deposition behavior was examined using a RDE to differentiate the contribution of current from the reactions of ruthenium ions in the electrolyte and ruthenium oxides already adsorbed on the electrode. The results indicate that the CV growth of ruthenium oxides within the potential range of aqueous electrolyte decomposition is attributed to the anodic oxidation of ruthenium ions in the electrolyte. Cathodic deposition occurs only at potential negative than −0.30 V versus saturated calomel electrode (SCE) when H₂ evolves on the electrodes. Anodic deposition of ruthenium oxides can be obtained effectively in the potential range of ca. 0.9-1.1 V versus SCE, depending on the pH value of the electrolyte. The optimum anodic and cathodic deposition potential for maximum deposition efficiency is 1.0 and −0.9 V versus SCE, respectively, in the electrolyte solution of pH 2.     

Preparation of micro-dot electrodes of LiCoO2 and Li4Ti5O12 for lithium micro-batteries

Fabrications of micro-dot electrodes of LiCoO₂ and Li₄Ti₅O₁₂ on Au substrates were demonstrated using a sol-gel process combined with a micro-injection technology. A typical size of prepared dots was about 100 μm in diameter, and the dot population on the substrate was 2400 dots cm⁻². The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were characterized with scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy, and cyclic voltammetry. The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were evaluated in an organic electrolyte as cathode and anode for lithium micro-battery, respectively. The LiCoO₂ micro-dot electrode exhibited reversible electrochemical behavior in a potential range from 3.8 to 4.2 V versus Li/Li⁺, and the Li₄Ti₅O₁₂ micro-dot electrode showed sharp redox peaks at 1.5 V.     

Investigation of polymer electrolyte hybrid supercapacitor based on manganese oxide-carbon electrodes

A hybrid supercapacitor based on manganese oxide, activated carbon and polymer electrolyte was developed and electrochemically investigated. The capacitive performance obtained from the polymer electrolyte based supercapacitor was similar to that of an aqueous electrolyte based supercapacitor, tested for comparison in the same operative conditions. A durability test carried out for 2500 cycles showed stable and slowly increasing performance. The specific capacitance of hybrid supercapacitor was 48 F g⁻¹ (192 F g⁻¹ as a mean one electrode capacitance), in which that of the positive electrode was 384 F g⁻¹ of MnO₂ and that of negative electrode 117 F g⁻¹ of carbon. The impedance analysis evidenced that although the polymer electrolyte based hybrid supercapacitor showed higher resistance compared to that of the liquid electrolyte based supercapacitor, this drawback was counterbalanced by better ion transport features, which were evident at lower frequencies, where similar values of capacitances were obtained from the different supercapacitors.