Electrode kinetics in non-aqueous vanadium acetylacetonate redox flow batteries

Three electrode materials (glassy carbon, gold, and platinum) were investigated for application in a non-aqueous single-metal redox flow battery based on vanadium (III) acetylacetonate, supported by tetraethylammonium tetrafluoroborate in acetonitrile. Redox couples associated with the one-electron disproportionation of V(acac)₃ were observed in voltammetry for each metal tested. An elementary kinetic model was created and used to determine rates for oxidation or reduction of the vanadium complex. The oxidation rates for V(acac)₃ were mass-transfer limited on all electrode materials, suggesting reversible kinetics. For the V(acac)₃ reduction reaction, exchange-current densities of 1.3, 3.8, and 8.4 A m⁻² were observed on glassy carbon, platinum, and gold electrodes, respectively.     

Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of –0.060 V decade^–1 were obtained on the three electrodes in oxygen-saturated 1.0m H₂SO₄ and 1.0m H₂SO₄ + y m K₂SO₄ (0 y 1). At high over-potentials the usual Tafel slope of -0.120V decade^–1 was observed on both (111)-type and PC platinum electrodes in 1.0m H₂SO₄, whereas a slope of –0.165V decade^–1 was found on (100)-type platinum. In oxygen-saturated 1.0m H₂SO₄ the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H₂O₂ was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.     

Electrochemical studies on gold electrodes in acidic solutions of thiourea containing gold (I) thiourea complex ions

Electrochemical studies were made of the behaviour of gold electrodes in degassed acidic solutions containing between 0.00l to 0.03 M thiourea and between 10^–5 to 10^–3 M gold(I)thiourea. At anodic overpotentials of up to 0.3 V the dissolution of gold was rapid, and nearly reached the maximum diffusioncontrolled rate. The exchange current density was greater than 10^–6 A cm^–2, and dissolution proceeded at 100% efficiency. At higher anodic potentials, thiourea was oxidized to formamidine disulphide and other sulphur-containing compounds and the dissolution of gold became partly inhibited, while the current efficiency decreased markedly.The reduction of gold(I)thiourea was diffusion-controlled at cathodic overpotentials between –0.15 to –0.35 V, after which slight inhibition was observed. Thiourea itself did not contribute to the cathodic reaction, but formamidine disulphide could be reduced on a freshly deposited gold surface; in the absence of gold(I)thiourea in the solution, the reduction of formamidine disulphide caused rapid passivation of the gold surface. In 0.01 M thiourea and 0.1 M sulphuric or perchloric acid, the diffusion coefficient of the Au(CS(NH₂)₂) ₂ ⁺ ion was 1·1 x 10^–5 cm² s^–1 at 30°C.The standard reduction potential at 30° C of the redox couple Au(CS(NH₂)₂) ₂ ⁺ ¦Au on a fresh gold surface wasE ₀=0.352 V, but on a passivated gold surface this value increased to as much asE ₀=041V.     

Specificity of anodic processes in cyclic voltammetry to the type of carbon used in electrolysis of cryolite-alumina melts

Anodic processes associated with oxidation of carbon anodes used in electrolysis of cryolite-alumina melts, simulating the Hall-Héroult process, were studied by means of cyclic voltammetry in a comparative way at four graphitic carbon materials and at glassy carbon. Conditions were sought that give a current response function characteristic of diffusion-controlled oxidation of the anode by O^2– or oxyfluoride complex anions. Only at glassy carbon anodes are such conditions realized with a linear relation between response current in cyclic voltammetry and Al₂O₃ content in the melt. At the graphitic materials, complex mixed activation and diffusion controlled processes arise that are also relatively irreproducible from one experiment to the next, probably due to irreversible changes of the graphite surfaces. The effects of aluminium metal dissolved in the melt, to simulate practical smelter cell conditions, were also evaluated.     

High-performance carbon electrodes for acid methanol—air fuel cells

High-performance, Teflon-bonded carbon electrodes, catalysed with highly dispersed platinum metal, have been developed for oxygen reduction in H₂SO₄. Surface-treated Vulcan XC-72 carbon has been used as the substrate material. The electrodes can be loaded with current densities of 1.1 A cm^–2 intermittently and 900 mA cm^–2 for extended periods without serious degradation. The performance of these electrodes in the presence of methanol has also been examined.     

Aluminium alloys in sulphuric acid Part I: Electrochemical behaviour of rotating and stationary disc electrodes

Anodic oxidation of various aluminium alloys was investigated by means of rotating disc electrodes in 3 M H₂SO₄ as a function of Cl^–, F^–, Zn²⁺ and In³⁺ concentration. Al-In, Al-Zn/In and Al-Zn/Sn alloys yielded current-potential curves at the lowest overpotentials and faradaic efficiencies for anodic oxidation of up to 98% at currents 50 mA cm^–2. While these alloys were electrochemically active in the presence of chloride as the only additive in sulphuric acid, binary aluminium alloys with Ce, Ga, La, Nd, Sn, Ta, Te, Ti or Tl were only active when Cl^–, Zn²⁺ and In³⁺ species were added to the electrolyte. With the exception of Al-Ga, binary alloys displayed high faradaic efficiencies of up to 95%. Fluoride additives resulted in current-potential curves at even more negative potentials than those with chlorides. In contrast to Cl^–, fluoride ions are consumed during the aluminium oxidation process due to complexation with Al(III).     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

High-temperature chlorine corrosion of technical carbons Part II. Anodic corrosion in chloride melt

Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson graphites) have been investigated in alkali chloride melts at chlorine evolving anodes. At low temperature in chloride melts containing free Lewis acid (AlCl₃) no chlorine is evolved — even at high current densities — because chlorine, together with aluminium chloride, instantaneously form intercalation compounds with graphite and, as a consequence, the carbon desintegrates very rapidly. At 200°C carbon is consumed anodically in a C/Cl of molar ratio 70/1. With increasing temperature Scheson graphites become more stable so that at 700°C short term destruction cannot be observed in melts which contain free Lewis acid. Chlorine corrosion of carbon electrodes in purified basic alkali chloride melts, which are free of oxygen carriers and, in particular, free of water at temperatures between 600 and 800°C in basic chloride melts, is an electrochemical reaction proceeding at low current densities slower than anticipated from thermodynamic data for carbon chlorination equilibria. The anodic carbon corrosion reaction has an activation energy of only 50 kJ mol^–1 and its rate increases with increasing anode potential, or anodic current densities (rate: exp (i)). At a technical current density of 0.4 A cm^–2 at 700°C the corrosion rate is estimated to be of the order of centimeters per year, rendering carbon anodes dimensionally unstable. Most important is to note that apart from CCl₄, chlorinated carbon compounds (olefins and arenes) are generated as side-products which are noxious and ecologically dangerous and must not be released from processes which use carbon anodes for chlorine evolution from salt melts.This paper is dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.     

The bioelectrocatalytic properties of cytochrome C by direct electrochemistry on DNA film modified electrode

The direct electrochemistry of cytochrome C can be performed in weak acidic and basic aqueous solutions. Cytochrome C can be deposited as a stable and electrochemically active film on a deoxyribonucleic acid (DNA) modified glassy carbon electrode. These films can also be produced on gold, platinum, and transparent semiconducting tin oxide electrodes. Two-layer modified electrodes containing cytochrome C and a DNA film were prepared by the deposition of cytochrome C on a DNA film modified electrode. The cytochrome C/DNA film was electrocatalytically oxidation active for l-cysteine in a pH 8.3 tris(hydroxymethyl)aminomethane (TRIS)-buffered aqueous solution through both Fe^III and Fe^IV species. The electrocatalytic oxidation current developed from the anodic peak of the redox couple. The electrocatalytic oxidation properties of ascorbic acid, NH₂OH, N₂H₄, and SO₃²⁻ by a cytochrome C/DNA film were also determined, and shown to be electrocatalytically active. An electrochemical quartz crystal microbalance, cyclic voltammetry, and direct spectroelectrochemistry were used to study in situ DNA deposition on a gold disc electrode and cytochrome C deposition on DNA/Au and DNA/GC films. The direct electrochemistry of cytochrome C can also be performed, and it can be deposited as a stable and electrochemically active film on polyvinyl sulfonate, polystyrene sulfonate, TiO₂, and polyethylene glycol modified glassy carbon electrodes. The results show that cytochrome C interacts with, and deposits on, a DNA film modified electrode, and that the cytochrome C (Fe^III) oxidized form is more easily deposited on a DNA film than the cytochrome C (Fe^II) reduced form.     

Comparison of the behaviour of glassy carbon and some metals for use as nonconsumable anodes in alumina-cryolite melts

Current interest exists in development of nonconsumable anodes for the Hall-Heroult process of aluminium production and also in situ analytical probes for determination of Al₂O₃ content in the cryolite melts used in this process. A comparison of the behaviour of glassy carbon and metals such as tungsten, tungsten carbide, nickel and stainless steel (SS-316) used as anodes in alumina-cryolite melts is investigated by means of electrochemical transient techniques (cyclic voltammetry and chronoamperometry) and Tafel anodic polarization experiments. The results show that only glassy carbon could be used as a successful sensor electrode for an in situ determination of Al₂0₃ in alumina-cryolite melts and that the metals investigated are unresistant to anodic attack in such melts. Consequently, the metals investigated cannot be used as sensor electrodes for in situ electro-analytical determination of alumina in alumina-cryolite melts, nor as anodes in the production of aluminium by the Hall-Heroult process.     

Carbon electrodes with phthalocyanine catalyst in acid medium

The catalytic properties of polymeric phthalocyanines with Fe and Co as central atoms for the electroreduction of oxygen in 0.5–2.3m H₂SO₄ were studied. No noticeable dependence of the electrode potential on the concentration of H₂SO₄ was found. The electroactivity of the catalyst with a central Fe atom undergoes considerable deterioration under the given conditions, whereas the stability of the catalyst with a central Co atom is very good and the potential of an electrode containing 30% catalyst in the active mass is 100 mV more positive than that of an electrode with 13% platinum, both at 40 mA cm^–2. The electrode performance depends markedly on the sort of carbon substrate, showing a parallelism with respect to oxygen electrodes in alkaline medium. The gold mesh current collector can be replaced by the addition of carbon black to the active layer.     

Corrosion protection of steel in molten Li2CO3–K2CO3 and Na2CO3–K2CO3 mixtures in a hydrogen-containing atmosphere

The electrochemical behaviour of TiN-, TiN–AlN-, Cr- and CrN-coated 316L stainless steel in molten Li₂CO₃–K₂CO₃ and Na₂CO₃–K₂CO₃ melts in a reducing gaseous atmosphere (10% H₂–90% N₂) was studied using voltammetry and scanning electron microscopy combined with energy-dispersed X-ray analysis in the temperature range of 600–730 C. To facilitate the identification of the electrochemical reactions the voltammetric behaviour of stainless steel, titanium, nickel and gold was also investigated. Voltammetric characteristics obtained at AlN–TiN coated electrodes showed no anodic reactions at potentials more negative than that of CO^2– ₃ oxidation. Cr- and CrN-coated electrodes demonstrated a suppressed anodic dissolution after the first steady state voltammetric cycle. The voltammograms obtained for the other electrodes studied displayed the corresponding anodic metal-dissolution waves. TiN, AlN, Cr and CrN coatings seem to be the most promising as corrosion-resistant materials for the anodic compartments of molten carbonate fuel cells.     

Electrodeposition of iridium onto glassy carbon and platinum electrodes

The mechanism for the electrodeposition of iridium onto glassy carbon and platinum substrates has been investigated. Iridium coatings were characterized by X-ray photoelectron microscopy to determine their chemical compositions and the morphologies of deposits were observed by scanning electron microscopy. The deposition of iridium on glassy carbon electrodes requires, in a first stage, the formation of Ir surface sites. These sites, generated by reduction of Ir³⁺ ions for large overpotential, allow the adsorption of H atoms which act as the reducing agent for Ir³⁺ ions. In contrast, on platinum, Ir electrodeposition occurs readily due to a high H_ads surface coverage. The optimal H_ads surface coverage for the electrodeposition of Ir on Pt is close to 0.5. Electrochemical quartz crystal microbalance measurements demonstrated that the Faradaic efficiency of Ir deposition on Pt is very low (0.2–1.7%).     

Alkali and earth alkaline metal contents in the aluminium cathode in aluminium electrolysis

The contents of sodium, lithium, calcium and magnesium in aluminium in contact with NaF–AlF₃-based melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950–1030 C. The experimental data were compared with a thermodynamic model. It was found that the addition of alumina or CaF₂ to the NaF–AlF₃ melts has only a minor effect on the equilibrium content of sodium in aluminium. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a smaller effect on the concentrations of lithium, calcium and magnesium in aluminium in industrial cells.     

Characterization of oxide layers on copper by linear potential sweep voltammetry

The characteristics of the electrochemical reduction of copper oxide have been studied using linear potential sweep voltammetry. Copper(II) and copper(I) oxides, formed on copper electrodes during triangular potential cycling, reduced at potentials close to the reversible values for the CuO–Cu₂O and Cu₂O–Cu couples, respectively. The reduction of bulk Cu₂O and CuO deposited on carbon paste, glassy carbon and copper electrode surfaces required significant overpotentials. Both oxide species could be distinguished from the potentials of their cathodic reduction peaks. In each system studied, CuO was reduced before Cu₂O. We discuss the implications of these findings on the Pops and Hennessy galvanostic method for determining oxides formed on copper in the wire industry.     

High performance of electroless-plated platinum electrode for electrochemical hydrogen pumps using strontium-zirconate-based proton conductors

The performance of an electroless-plated platinum electrode for electrochemical hydrogen pumps was investigated using SrZr_0.9Y_0.1O_3−α (10Y-SZO) and SrZr_0.5Ce_0.4Y_0.1O_3−α (10Y-SZCO) as proton-conducting electrolytes. The plated electrodes using these electrolytes showed quite low overpotentials, resulting in high performance of the hydrogen pumps. The high electrode performance is interpreted from the viewpoint of the three-phase boundary (TPB) length: the plated electrode has a larger TPB length per unit area than that of the pasted electrode. The difference between 10Y-SZO and 10Y-SZCO in ability to match with platinum electrodes is also discussed, and the possibility that the TPB width of 10Y-SZCO with platinum is larger than that of 10Y-SZO is suggested.     

Kinetics and mechanism of the oxygen evolution reaction at oxide-coated Co-Ni amorphous alloy electrodes

Oxygen evolution reaction (o.e.r.) kinetics in NaOH solutions have been studied on both fresh and oxide covered Co₅₀Ni₂₅Si₁₅B₁₀ amorphous alloy (G-16) electrodes. Steady state polarization curves obtained in different aqueous xM NaOH (0.1 x 4) in the 30–80°C range fulfill Tafel relationships at low overpotentials; the Tafel slope is close to 2.3(RT/F) V dec^–1 for both G-16 and oxide coated G-16 electrodes. At high overpotentials, ohmic relationships with slopes becoming increasingly steep, regardless of the NaOH concentration, are observed. In the Tafel region, the reaction order with respect to OH^– is near 2. The apparent current density at constant potential, for oxide coated G-16 electrodes, is greater than that for uncoated G-16. The high catalytic activity of the oxide coated G-16 for the o.e.r. is attributed to its spinel-type structure. The kinetics of the o.e.r. at low overpotentials is explained through a mechanism involving a first electron transfer step followed by a rate-determining chemical step.     

Aluminium deposition and dissolution in aluminium chloride—n-butylpyridinium chloride melts

Glassy carbon and tungsten electrodes were used to investigate the mechanism of aluminium deposition and dissolution reactions in the AlCl₃—n-butylpyridinium chloride melts at ambient temperature. Chronopotentiometric results indicate that the electrodeposition of aluminium from the melt is kinetically complicated. Current reversal chronopotentiometry showed that the corrosion rate of aluminium is linearly proportional to the acidity of the melt at a given temperature. Besides impurities, the major cause of the corrosion of the deposited aluminium over a wide composition range of the melt was also found to be due to the organic butylpyridinium cation (BuPy+) serving as an electron acceptor in the corrosion process when freed from ion-pair interaction with AlCl⁻₄.     

Electrochemical studies of four organometallic redox couples as possible reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate

The electrochemical behavior of four organometallic redox couples has been studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Diffusion controlled anodic and cathodic peaks were found for the redox couples Fc/Fc⁺ (ferrocene/ferrocenium), Cc/Cc⁺ (cobaltocene/cobaltocenium) on glassy carbon, platinum and gold. Bis(biphenyl)chromium(I) tetraphenylborate (BCr/BCr⁺) yielded a well-behaved diffusion controlled pair of cathodic-anodic peaks only on glassy carbon and gold as working electrodes. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed by us. The applicability of three of the four redox couples studied as candidates for reference redox systems in this ionic liquid is discussed.     

Study of a high power density sodium polysulfide/bromine energy storage cell

Nickel catalyst supported on carbon was made by reduction of nickelous nitrate with hydrogen at high temperature. Ni/C catalyst characterization was carried out by XRD. It was found that the crystal phase of NiS and NiS₂ appeared in the impregnated catalyst. Ni/C and Pt/C catalysts gave high performance as the positive and negative electrodes of a sodium polysulfide/bromine energy storage cell, respectively. The overpotentials of the positive and negative electrodes were investigated. The effect of the electrocatalyst loading and operating temperature on the charge and discharge performance of the cell was investigated. A power density of up to 0.64 W cm^–2 (V = 1.07 V) was obtained in this energy storage cell. A cell potential efficiency of up to 88.2% was obtained when both charge and discharge current densities were 0.1 A cm^–2.