Xianbo Jin

Metallic Cavity Electrodes for Investigation of Powders Electrochemical Reduction of NiO and Cr 2 O 3 Powders in Molten CaCl 2

Metallic cavity electrodes (MCEs) for quick electrochemical investigations of powders in high-temperature molten salts are described. Molybdenum foils (thickness ∼0.5 mm; width 1.0-2.0 mm; length 100-150 mm) were tested with one or two cavities of through circular holes (diameter 0.3-1.0 mm) filled with submilligrams of the fine powder of NiO or Cr 2 O 3 in molten CaCl 2 at 1173 K. Cyclic voltammograms exhibited features of thin-layer or chemically modified electrodes with irreversible reactions. Charge-transfer coefficients for the electroreduction of both NiO and Cr 2 O 3 were estimated to be much smaller than 0.5. Chronoamperometry demonstrated fast cathodic conversion, in a time of the order of magnitude of 10 2 s, of the oxide powders to the respective metals inside the cavity, as confirmed by scanning electron microscopy and energy-dispersive X-ray. The results also suggested electrochemically driven interactions between the oxide and the Mg 2 + and Ca 2 + ions in the molten salt before oxygen removal.

New Indole-Based Metal-Free Organic Dyes for Dye-Sensitized Solar Cells

Two indole-based organic dyes were conveniently prepared and well characterized. The triphenylamine or carbazole moieties were bonded to the indole group acting as a potential electron donor, which can tune the HOMO and LUMO levels of the resultant dye, and another triphenylamine or carbazole group was linked to the pyrrole ring on the nitrogen atom, which was expected to suppress the aggregation of the dye in the solid state to some degree. These two dyes were utilized as dye sensitizers in dye-sensitized solar cells and demonstrated efficient photon-to-electron conversion properties.

Metal‐to‐Oxide Molar Volume Ratio: The Overlooked Barrier to Solid‐State Electroreduction and a “Green” Bypass through Recyclable NH4HCO3

As early as 1923 Pilling and Bedworth reported the dependence of metal oxidation behavior on the metal-to-oxide molar volume ratio. In 1910 and 1940, respectively, Hunter and Kroll succeeded in carbochlorination of TiO2 (rutile) to TiCl4, and sodioand magnesiothermic reduction of the chloride to titanium. They did not consider at all the metal-to-oxide molar volume ratio. Unfortunately, even after 60 years of research and industrial development, the Kroll process is still highly energy and carbon intensive (45–55 kWh and > 2 kg CO2 per kg Ti sponge; see Supporting Information). [1d] This makes titanium too costly to use widely, although it has very rich resources, and is ideal for making energy-saving vehicles, durable medical implants, and lightweight and corrosionresistant off-shore wind turbines. Alternatives to the Kroll process have long been sought. In particular, the solid-state electroreduction (or electrodeoxidation) of metal oxides to the respective metals or alloys in molten salts has emerged with the merits of, for example, simple and fast operation and low energy consumption and emission. In the past decade, worldwide research on this electrolytic process has shown acceptable energy consumption (e.g., 33 kWh/kg Ti), but the current efficiency is still too low (e.g., 15%) to achieve a low O content in the produced Ti ( 0.3 wt % O). The irony is that, when the method is applied to produce Cr (< 0.2 wt % O), the current efficiency can exceed 75%. ZrO2 was also recently electroreduced to Zr (0.18 wt% O) at 45% current efficiency, although Zr and Ti have many comparable properties, for example, high solubility for oxygen. Apparently, an unseen barrier remains in the electroreduction of TiO2 to Ti. This communication identifies the metal-to-oxide molar volume ratio as an intrinsic barrier to the solid-state reduction of TiO2 to Ti. More importantly, an effective green bypass is demonstrated through recyclable use of NH4HCO3 and two-voltage electrolysis. Electroreduction of a solid oxide has been confirmed to proceed through the propagation of the metal j oxide j electrolyte three-phase interlines (3PIs), starting from the surface and then entering the oxide precursor. c,4c,5] According to the 3PI models, the initially formed metal layer on the oxide surface must be sufficiently porous to allow molten salt to access the underlying oxide to form new 3PIs. Thus, the reduction-generated O ions can diffuse through the electrolyte in the pores of the metal layer before entering the bulk electrolyte and being discharged at the anode. Formation of the porous metal layer may be attributed to one or both of two factors. First, removal of oxygen from the solid oxide is expected to leave vacancies, and hence a porous metal. Second, when a porous oxide precursor is used, it may also benefit formation of porous metal during solid-state reduction. The latter is experimentally controllable to a certain degree, but the former could only be true if oxygen removal did not cause a decrease in atom packing density. In other words, the molar volume of the metal, Vm = Mm/1m, should be smaller than the equivalent molar volume of its oxide, Vo = Mo/n1o, where the subscripts m and o represent the metal and oxide, respectively, V is the molar volume, M the molar mass, 1 the density, and n the number of metal atoms in the oxide formula (e.g. n = 1 for TiO2 and MgO, n = 2 for Cr2O3 and Ta2O5, and n = 3 for Fe3O4). Table 1 lists the Vm/Vo ratios for some typical metals. For most metals listed (and many more unlisted), Vm/Vo< 1, which accounts for the success in using the electroreduction

Chloride ion enhanced thermal stability of carbon dioxide captured by monoethanolamine in hydroxyl imidazolium based ionic liquids

Hydrogen bonding between protonated monoethanolamine and chloride ion can benefit the capture and thermal stabilisation of carbon dioxide in hydroxyl imidazolium based ionic liquids for potential reclamation of the captured carbon by, for example, electrolysis and catalytic synthesis.

Thermo-solvatochromism of chloro-nickel complexes in 1-hydroxyalkyl-3-methyl-imidazolium cation based ionic liquids

Sunlight can be directly absorbed by many coloured solids or liquids to re-generate heat but the temperature achievable is usually below 100 °C. Consequently, thermally responsive physical and/or chemical processes that can effectively utilise this almost free but low temperature solar heat are becoming increasingly important, considering the inevitable change in energy supply from fossil fuels to renewable sources in the near future. In this work, the thermochromic and solvatochromic behaviour of chloro-Ni(II) complexes was investigated by visual observation and vis-spectroscopy in 1-hydroxyalkyl-3-methylimidazolium (CnOHmim+, n = 2 or 3) based ionic liquids between room temperature and 85 °C. The thermochromism was a result of the tetrahedral complex, NiCl42− (blue, hot) being solvolysed into various octahedral complexes, e.g. [NiClx(CnOHmim+–ClO4−)y]2−x (x + y = 6) or [NiClx(CnOHmim)y]z+–(ClO4−)z (z = 2 + y − x) (yellow or green, cold) in the ionic liquids. The capability of the CnOHmim+ ligand to encourage the formation of octahedral chloro-Ni(II) complexes with a high number of chloride ligands could be attributed to the electrostatic attraction in the octahedral configurations. These new systems were found to be sensitive to water, but the lost thermo-solvatochromism was thermally recoverable. The enthalpy change, ΔH, of the tetrahedral–octahedral configuration conversion of the Ni(II) complexes in these ionic liquids was estimated to be in the range of 30–40 kJ mol−1 and the entropy change, ΔS (298K), 140–160 J mol−1 K−1. These thermodynamic properties promise low energy thermochromic applications.

Up-scalable and controllable electrolytic production of photo-responsive nanostructured silicon

The electrochemical reduction of solid silica has been investigated in molten CaCl2 at 900 °C for the one-step, up-scalable, controllable and affordable production of nanostructured silicon with promising photo-responsive properties. Cyclic voltammetry of the metallic cavity electrode loaded with fine silica powder was performed to elaborate the electrochemical reduction mechanism. Potentiostatic electrolysis of porous and dense silica pellets was carried out at different potentials, focusing on the influences of the electrolysis potential and the microstructure of the precursory silica on the product purity and microstructure. The findings suggest a potential range between −0.60 and −0.95 V (vs. Ag/AgCl) for the production of nanostructured silicon with high purity (>99 wt%). According to the elucidated mechanism on the electro-growth of the silicon nanostructures, optimal process parameters for the controllable preparation of high-purity silicon nanoparticles and nanowires were identified. Scaling-up the optimal electrolysis was successful at the gram-scale for the preparation of high-purity silicon nanowires which exhibited promising photo-responsive properties.

Direct and low energy electrolytic co-reduction of mixed oxides to zirconium-based multi-phase hydrogen storage alloys in molten salts

Direct synthesis of Zr-based AB2-type hydrogen storage alloys (HSAs) from mixed oxide precursors has been achieved by electrolysis in molten CaCl2 at 900 °C and a cell voltage below 3.2 V. The process resembled direct oxide-to-metal conversion in solid state, and the target alloys, namely ZrCr2, ZrCr0.7Ni1.3 and Zr0.5Ti0.5V0.5Cr0.2Ni1.3, were formed in situ during electrolysis without going through any melting step. Electrolysis energy consumption could be as low as 9.59 kWh (kg-HSA)−1 and the metal recovery yield was generally higher than 90%. The electrolytic products were readily obtained as powders with the designated compositions and crystal structures (e.g. the C14 and C15 Laves phases). More importantly, these Zr-based electrolytic HSA powders were composed of nodular micro-particles which are very desirable for fabrication of electrodes with micro-porosity to facilitate electrolyte ex- and ingression. Galvanostatic discharge-charge tests of the as-prepared electrolytic HSA powders resulted in similar or higher hydrogen storage capacities (up to 280 mAh g−1) in comparison with the same HSAs prepared by e.g. arc-melting the individual metals as reported in literature. Particularly, the electrolytic Zr-based HSAs were unique for their high initial capacities without any pre-treatment for activation, and they also exhibited highly satisfactory discharge rate capability with less than 20% capacity loss when the discharge current increased from 50 to 600 mA g−1.

Cyclic Voltammetry of ZrO2 Powder in the Metallic Cavity Electrode in Molten CaCl2

The electrochemical reduction of the insulative ZrO 2 powder in molten CaCl 2 was investigated using the metallic cavity electrode (MCE) in molten CaCl 2 at 850°C. Cyclic voltammograms (CVs) revealed two consecutive reduction peaks corresponding to (i) ZrO 2 to Zr x O (x ≥ 1) and (ii) Zr x O to Zr. The intermediate, Zr x O, was metastable and underwent disproportionation to ZrO 2 and Zr, which was responsible for the detection of Zr metal in the potentiostatic reduction at less negative potentials. In the anodic scan, four main oxidation processes were observed. The relevant reactions were rationalized as the reoxidation of (iii) Zr x O to ZrO 2 , (iv) Zr to ZrO 2 , (v) Zr to ZrCl 2 , and (vi) Zr to ZrCl 4 . The metastable intermediate also contributed to the unique current variations in the anodic potential scans under different conditions. Potentiostatic electrolysis of the ZrO 2 powder in the MCE at the feature potentials of the CVs and analyses of the electrolysis products by scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed the electroreduction mechanism and revealed the localized conversion of the dense aggregates of the submicrometer particles of ZrO 2 to cauliflower-like aggregates of the nanoparticulates of Zr in the early stage of the electroreduction process.

Solid state reactions: an electrochemical approach in molten salts

Solid state reactions play crucial roles in highly efficient electrochemical energy conversion and storage devices, typically lithium ion batteries and ruthenium oxide supercapacitors. Electrolytic processes may also involve solid-to-solid changes such as the anodisation of metals for designated surface nanostructures and functionalities. More recently, a novel and green route for the production of metals and alloys in molten salts has drawn worldwide attention. It is based on the electro-reduction of metal compounds, particularly oxides, to the respective metals or alloys in the solid-state. This article reviews recent advancement of this new process in terms of potential applications and fundamental understanding.

Synthesis of cambered nano-walls of SnO2/rGO composites using a recyclable melamine template for lithium-ion batteries

Graphene and graphene/metal oxide composite materials have attracted considerable interest for use as energy materials due to their excellent electrochemical performances. Here, we propose using melamine as a template for the synthesis of cambered nano-walls of SnO2/rGO materials. Melamine powder can effectively absorb SnO2/GO from the solution to form a core–shell structure of melamine@SnO2/GO. After thermal reduction of GO at 200 °C to form the melamine@SnO2/rGO, melamine was dissolved in hot water at 80 °C, leaving behind the cambered SnO2/rGO nano-walls. Melamine is recyclable since it precipitates when its solution cools to room temperature. The thickness of the SnO2/rGO nano-walls can be easily controlled by adjusting the mass ratio of melamine to SnO2/GO. When the mass ratio was set to ten, cambered walls of SnO2/rGO with a thickness of about 100–200 nm were achieved. The resulting SnO2/rGO delivered an initial reversible capacity of 998 mA h g−1 at a current density of 100 mA g−1 and a capacity of 855 mA h g−1 after 100 discharge–charge cycles in a potential range between 0.02 and 3.0 V vs. Li/Li+. It also showed good rate performance with a reversible capacity of 460 mA h g−1 at 1 A g−1. These high capacities can be linked to the special cambered nano-walls which ensure fast solid diffusion in addition to providing an effective liquid-channel and buffer-volume in the electrode. The proposed synthesis method is easily scalable and should be applicable to many other graphene based energy materials.