George Z. Chen

Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride

Many reactive metals are difficult to prepare in pure form without complicated and expensive procedures. Although titanium has many desirable properties (it is light, strong and corrosion-resistant), its use has been restricted because of its high processing cost. In the current pyrometallurgical process—the Kroll process—the titanium minerals rutile and ilmenite are carbo-chlorinated to remove oxygen, iron and other impurities, producing a TiCl4 vapour. This is then reduced to titanium metal by magnesium metal; the by-product MgCl2 is removed by vacuum distillation. The prediction that this process would be replaced by an electrochemical route has not been fulfilled; attempts involving the electro-deposition of titanium from ionic solutions have been hampered by difficulties in eliminating the redox cycling of multivalent titanium ions and in handling very reactive dendritic products. Here we report an electrochemical method for the direct reduction of solid TiO2, in which the oxygen is ionized, dissolved in a molten salt and discharged at the anode, leaving pure titanium at the cathode. The simplicity and rapidity of this process compared to conventional routes should result in reduced production costs and the approach should be applicable to a wide range of metal oxides.

Mechanisms of electrochemical recognition of cations, anions and neutral guest species by redox-active receptor molecules

Abstract This short review highlights the mechanisms involved in electrochemically sensing cationic, anionic and neutral guest species by redox-active receptors and is an update to a previously published review article (P.D. Beer, P.A. Gale, Z. Chen, Adv. Phys. Org. Chem. 31 (1998) 1). Mechanisms of redox–complexation coupling are discussed together with recent examples of redox-responsive molecular receptors from the literature that illustrate them.

Electrochemical fabrication and capacitance of composite films of carbon nanotubes and polyaniline

Nanoporous composite films of multi-walled carbon nanotubes (MWNTs) and polyaniline (PAn) were grown electrochemically from acidic aqueous solutions, such that the constituents were deposited simultaneously onto graphite electrodes. Scanning electron microscopy (SEM) revealed that the composite films consisted of nanoporous networks of MWNTs coated with PAn. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrated that these composite films had similar electrochemical response rates to pure PAn films, but a lower resistance and much improved mechanical integrity. The specific electrochemical capacitance of the composite films, per unit area of the original electrode, reached as high as 3.5 F cm−2, a significantly greater value than that of 2.3 F cm−2 for pure PAn films prepared similarly.

Voltammetric Studies of the Oxygen-Titanium Binary System in Molten Calcium Chloride

Cyclic voltammetry was applied to study the O-Ti binary system in pre-electrolyzed (2.7-3.0 V) molten CaCl 2 at 900°C. Before melting, the salt was thermally dried in air, following a four-stage heating program (60°C/h, 1 h at 90°C, 10°C/h, 4 h at 300°C) to minimize hydrolysis. The O-Ti system was represented by oxide-scale-coated titanium that was prepared by heating commercially pure titanium in air at 700°C. The voltammograms exhibited three reduction processes at electrode potentials more positive than that of the reduction of calcium cation. Two of these, at less negative potentials, could be attributed to the reductions of TiO 2 to Ti 2 O 3 , and TiO to Ti metal, respectively. The Ti 2 O 3 was found to disproportionate into TiO 2 and TiO. The third process occurred at a more negative potential, but still more positive than that of calcium cation reduction, and was proven to be the cathodic ionization of dissolved oxygen from the metal phase. It was also found that for partially reduced oxide scale, various calcium titanates could form under conditions that promote reaction of CaO with, or calcium cation intercalation into, the oxide scale.

Polymer-carbon nanotube composites

This invention relates to a composite comprising carbon nanotubes coated with a polymer, wherein the polymer comprises at least one hydrophobic monomer unit. This invention also relates to a process for the production of a composite comprising a polymer and carbon nanotubes.

Unequalisation of electrode capacitances for enhanced energy capacity in asymmetrical supercapacitors

An increase beyond 70% in specific energy of an asymmetrical supercapacitor of carbon (−) and composite of carbon nanotubes and polyaniline (+) was achieved by firstly identifying the ‘cell voltage limiting electrode’, and then adjusting the capacitance ratio of the ±electrodes from 1.0 to 1.3.

Individual and Bipolarly Stacked Asymmetrical Aqueous Supercapacitors of CNTs / SnO2 and CNTs / MnO2 Nanocomposites

Asymmetrical supercapacitors with aqueous electrolytes were fabricated from carbon nanotubes (CNTs) individually coated with SnO 2 (CNTs/SnO 2 ) and MnO 2 (CNTs/MnO 2 ) as the negative and positive electrodes, respectively. The CNTs/SnO 2 nanocomposite is used as the negative electrode material in an asymmetrical supercapacitor. The physicochemical properties of the CNTs/SnO 2 and CNTs/MnO 2 nanocomposites were examined by X-ray diffraction, scanning and transmission electron microscopy, cyclic voltammetry, and galvanostatic charge―discharge. Individually, the supercapacitors were tested for charge and discharge to a cell voltage of 1.70 V in 2.0 M KCl without noticeable water decomposition. The asymmetrical cell could reach the specific energy of 20.3 Wh/kg, which is comparable to that obtained from electric double-layer supercapacitors using organic electrolytes (17―18 Wh/kg). The maximum specific power of the cell, 143.7 kW/kg, is perhaps the highest among all reported aqueous asymmetrical supercapacitors. It also shows an exceptional stability of over 1000 cycles, with the capacity loss being less than 8%. A 10 V stack was also constructed with nine individual supercapacitors connected through bipolar electrodes of the nanocomposites and porous separators containing 1.0 M Na 2 SO 4 . The stack exhibited remarkable capacitive behavior resulting from the individual cells.

Theoretical specific capacitance based on charge storage mechanisms of conducting polymers: Comment on ‘Vertically oriented arrays of polyaniline nanorods and their super electrochemical properties’

A recently claimed ultra high specific capacitance of 3407 F g(-1) for aligned polyaniline nanorods by the titled communication is shown to be contradictory to both the mechanism of charge storage in conducting polymers, and the experimental findings in other nanofibrils of polyaniline, and may thus stimulate debate.

Manganese oxide based materials for supercapacitors

Abstract Manganese oxides, as an environmentally friendly material with various oxidation states, have a long history as an electrode material for batteries. Recently, a new energy storage system, supercapacitor, with its unique power density and energy density range, has been put in the focus. With a vast number of ongoing researches in exploring the MnOx as the electrode materials for supercapacitors, this article provides a comprehensive review on the recent findings in this area. Different approaches to synthesise and the electrochemical behaviours of various forms of pure and composite MnOx were presented. Important parameters that can influence the electrochemical behaviour of manganese oxides based materials are summarised. The state of the art of engineering of MnOx into composites or specific nanostructures with improved electrochemical performance is reviewed. A brief survey on the performance of symmetric and hybrid supercapacitors with MnOx as electrode material is given and appropriate cell configuration has been proven to be necessary for the optimisation of the capacitive performance. Moreover, the essential difference of charge storage mechanism between battery and supercapacitor for MnOx electrode is stated. Based on these literature findings, MnOx is believed to be a type of promising and competitive electrode material for applications in supercapacitors.

Reduction of titanium and other metal oxides using electrodeoxidation

Abstract Many reactive and refractory metals are currently produced industrially by reducing their compounds, including oxides, using a more reactive metal. In some cases, where there is substantial oxygen solubility in the metal, the oxygen is first removed by carbochlorination followed by reduction. Titanium and zirconium are made by reduction of the volatile tetrachlorides by magnesium. The processes consist essentially of two reduction steps: reducing magnesium chloride to magnesium metal and then reduction of the metal compound; this makes the overall reduction process relatively expensive. Electrodeoxidation is very simple in that the oxide to be reduced is rendered cathodic in molten alkaline earth chloride. By applying a voltage below the decomposition potential of the salt, it has been found that ionisation of oxygen is the dominant cathode reaction, rather than alkaline earth metal deposition. In the laboratory, this technique has been applied to reduce a large number of metal oxides to the metals, including titanium, zirconium, chromium, niobium, tantalum, uranium and nickel. Furthermore, when mixed oxides are used as the cathode, alloys or intermetallic compounds of uniform composition are obtained. This may offer advantages over conventional technology for those alloys that are difficult to prepare at present, owing to differences in either density or vapour pressure.