Geir Martin Haarberg

Review of New Carbon Materials as Catalyst Supports in Direct Alcohol Fuel Cells

Abstract Electrocatalytic reactions in direct alcohol fuel cells involve solid, liquid, and gas phase transport and electron and proton transfer. Better supports for the electrocatalysts are needed to carry out the reactions successfully and give a longer lifetime for the electrocatalysts. An ideal carbon support should have a high specific surface area, good electric conductivity, suitable pore size, favorable surface functional groups, good corrosion resistance, and low cost. Much work has been done on developing new carbon materials and modifying the carbon materials by pretreatment with acid, alkali, oxidant, or polymer to meet these requirements. In this work, commercial carbon supports that include the widely used carbon black Vulcan XC-72R, acetylene black, black pearls 2000, Printex XE-2, and Ketjen Black EC were briefly reviewed. New carbon materials such as carbon nanofibers, carbon nanotubes, ordered porous carbon, mesocarbon microbeads, carbon nanohorns, carbon nanocoils, and carbon aerogels were reviewed in detail. These new carbon materials generally give better performance due to their special structure, better crystallinity, and faster mass transfer when compared to the commercial materials, and carbon nanotubes demonstrated the best performance up to the present time.

Electrodeposition of magnesium from halide melts—charge transfer and diffusion kinetics

Electrodeposition of magnesium from molten MgCl2 and MgCl2-MgF2 (78-22 mol%) mixtures was studied by electrochemical techniques. The process was found to be quasi-reversible with a cathodic electrochemical rate constant for the total charge transfer reaction of about 10−3 cm/s at 780 °C. Underpotential deposition of an adsorbed layer of reduced Mg(II)-species was observed during cyclic voltammetry and potential step measurements. The electrode capacitance was determined from galvanostatic pulses and found to depend on the applied current density. This has been explained by a pseudo capacitance due to the formation of an adsorbed layer. At potentials positive to nucleation a diffusion controlled process was identified. The determined concentration of the diffusing species, as well as the fact that Mg(II) species are the only cations in the system, indicate that the process is diffusion of dissolved metal (the product) from the electrode interface to the electrolyte.

A chemical and electrochemical study of titanium ions in the molten equimolar CaCl2+NaCl mixture at 550°C.

Abstract The chemical and electrochemical properties of solutions of titanium chlorides in the fused CaCl 2 +NaCl equimolar mixture were studied at 550°C using different working electrodes. We determined the stability range of the various oxidation states of titanium and calculated the standard potentials of the Ti(III)/Ti(II) and Ti(II)/Ti(0) redox couples, the solubility products of the titanium oxides, the kinetic parameters of the electrochemical systems at the different electrodes and the diffusion coefficients of the electroactive species. We have also studied the titanium electrodeposition process by potential step measurements which indicated instantaneous nucleation of titanium at tungsten substrates. All these data allowed us to build-up the potential-pO 2− diagram which summarizes the properties of Ti-O compounds in the melt studied and can be used (together with the E-pO 2− diagram of chlorinating gaseous mixtures) to predict operating conditions for the process of industrial production of metallic titanium from molten salt systems.

Oxoacidity reactions in equimolar molten CaCl2-NaCl mixture at 575 °C

Abstract The behaviour of an yttria-stabilized zirconia electrode in the CaCl2-NaCl melt has been investigated at 575 °C. The response of the electrode is Nernstian and permits its use in the titration of Mg(II) with oxide ions. The solubility product of MgO was also determined. We have calculated the equilibrium constant of the carbonate ion dissociation as well as those corresponding to the systems HCl H 2 O and Cl 2 O 2 , finding that molten CaCl2-NaCl is more acidic than the LiCl-KCl eutectic melt and the BaCl2-CaCl2-NaCl (23.5:24.5:52 mol%) melt. On the other hand, the mixture can be purified with respect to oxide ions by gaseous reagents such as HC1 and Cl2.

Kinetics and mechanism of the magnesium electrode reaction in molten magnesium chloride

Using electrochemical impedance spectroscopy (EIS) and relaxation method with galvanostatic perturbation (RM) the kinetics and mechanism of the magnesium electrode reaction in pure molten M9Cl2 have been determined at several temperatures. A three-step electrode process has been found, the high frequency process being pure charge transfer with the low frequency process showing mixed charge transfer-diffusion character. The low frequency step has also been treated as a preceding chemical reaction followed by charge transfer. On the basis of the corresponding exchange current densities and Warburg diffusion impedance, a mechanism of the overall electrode reaction in this melt is proposed.

Electrodeposition of Magnesium from CaCl2 ­ NaCl ­ KCl ­ MgCl2 Melts

Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic (tungsten and molybdenum) and glassy carbon electrodes. Pure CaCl 2 melt and different alkali chloride solvents including binary CaCl 2 -NaCl, equimolar composition, and ternary CaCl 2 -NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl 2 concentrations, were used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic magnesium (923 K) so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process in melts containing CaCl 2 due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Three-dimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of the diffusion coefficient of Mg(II) ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was a stable process in the ternary mixture containing MgCl 2 , which was related to the presence of 10 mol % KCl. Moreover, in this melt a remarkable pseudopassivation process was found at high overpotentials.

Anodic behavior of carbon electrodes in CaO-CaCl2 melts at 1123 K

The anodic process on glassy carbon electrodes in molten CaCl 2 containing CaO was investigated at 1123 K by cyclic voltammetry and convolution voltammetry. It was found that the oxidation sequence occurs through a consecutive two-step electrochemical reaction: O 2- (diss) + x C → C x O(ads) + 2e and O 2- (diss) + C x O(ads) → CO 2 (gas) + (x - 1)C + 2e. Both steps were found to be irreversible. Experimental data indicate that the first electrochemical step is preceded by the dissociation of a complex. The diffusion coefficient of the oxygen-containing species, the charge-transfer coefficient, and the charge-transfer rate constant were found equal to 2.0 × 10 -6 cm 2 s -1 , 0.43, and 1.8 × 10 -5 cm s - respectively

Electrochemical Behavior of Dissolved Fe2O3 in Molten CaCl2-KF

The electrochemical behavior of dissolved Fe2O3 in 82. 5CaCl2-17.5KF (mole percent, %) was studied using cyclic voltammetry, chronoamperometry, and galvanostatic electrolysis at 827 °C, and the deposits were characterized by XRD and SEM. Pure iron was deposited on a rotating cylinder (210 r/min) with a cell voltage less than −1.0 V. Deposition rate was controlled by diffusion on a molybdenum electrode. The diffusion coefficient of iron species Fe(III) in the melt at 827 °C was found to be 9.7×10−5 cm2/s.

Cyclic and linear voltammetry on Ti/IrO2–Ta2O5–MnOx electrodes in sulfuric acid containing Mn2+ ions

Abstract The anodic current density for oxygen evolution was studied on Ti/IrO 2 –Ta 2 O 5 electrodes where some of the Ta 2 O 5 was replaced by MnO x ( x ≤2) in sulfuric acid and in sulfuric acid containing Mn 2+ ions. Deposition of MnO 2 on the anode surface reduced the anodic current density for oxygen evolution for all coatings. Scanning electron microscopy images of the Ti/IrO 2 –Ta 2 O 5 electrode during voltammetric experiments showed that the MnO 2 deposit was reduced completely during the cathodic scan.

MaterialElectrochemical Behavior of Dissolved Fe2O3 in Molten CaCl2-KF

The electrochemical behavior of dissolved Fe2O3 in 82. 5CaCl2-17.5KF (mole percent, %) was studied using cyclic voltammetry, chronoamperometry, and galvanostatic electrolysis at 827 °C, and the deposits were characterized by XRD and SEM. Pure iron was deposited on a rotating cylinder (210 r/min) with a cell voltage less than −1.0 V. Deposition rate was controlled by diffusion on a molybdenum electrode. The diffusion coefficient of iron species Fe(III) in the melt at 827 °C was found to be 9.7×10−5 cm2/s.