Dihua Wang

Capture and electrochemical conversion of CO2 to value-added carbon and oxygen by molten salt electrolysis

A molten salt electrochemical system comprising a eutectic mixture of Li–Na–K carbonates, a Ni cathode, and a SnO2 inert anode is proposed for the capture and electrochemical conversion of CO2. It is demonstrated that CO2 can be effectively captured by molten carbonates, and subsequently electrochemically split into amorphous carbon on the cathode, and oxygen gas at the anode. The carbon materials generated at the cathode exhibit high BET surface areas of more than 400 m2 g−1 and as such, represent value-added products for a variety of applications such as energy storage and pollutant adsorption. In the carbonate eutectic (500 °C), the presence of Li2CO3 is shown to be required for the deposition of carbon from the melt, wherein O2− or Li2O serves as the intermediate for CO2 capture and electrochemical conversion. SnO2 proved to be an effective anode for the electrochemical evolution of oxygen. Electrochemical reactions were found to proceed at relatively high current efficiencies, even though the current densities exceed 50 mA cm−2. The intrinsic nature of alkaline oxides for CO2 capture, the conversion of CO2 to value-added products, and the ability to drive the process with renewable energy sources such as solar power, enables the technology to be engineered for high flux capture and utilization of CO2.

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.

Production of Oxygen Gas and Liquid Metal by Electrochemical Decomposition of Molten Iron Oxide

Molten oxide electrolysis (MOE) is the electrolytic decomposition of a metal oxide, most preferably into liquid metal and oxygen gas. The successful deployment of MOE hinges upon the existence of an inert anode capable of sustained oxygen evolution. Herein we report the results of a program of materials design, selection, and testing of candidate anode materials and demonstrate the utility of iridium in this application. An electrolysis cell fitted with an iridium anode operating at 0.55 A cm ―2 produced liquid metal and oxygen gas by the decomposition of iron oxide dissolved in a solvent electrolyte of molten MgO―CaO―SiO 2 ―Al 2 O 3 . The erosion rate of iridium was measured to be less than 8 mm y ―1 . The stability of iridium is attributed to a mix of mechanisms including the electrochemical formation and simultaneous thermal decomposition of a surface film of iridium oxide.

Na2SO4-assisted synthesis of hexagonal-phase WO3 nanosheet assemblies with applicable electrochromic and adsorption properties

For the first time, a mesoporous hexagonal-phase Na0.17WO3.085·0.29H2O nanosheet/microflower hierarchical structure has been synthesized employing a one-pot hydrothermal process with the assistance of Na2SO4. It is shown that Na2SO4 not only acts as a stabilizer to facilitate the generation of a metastable hexagonal phase, but also function as a structure directing agent to assist the construction of nanosheet assemblies. The formation mechanisms have been rationalized. The materials have been thoroughly characterized by XRD/BET/FESEM/EDX/TEM/TGA. This hexagonal-phase Na0.17WO3.085·0.29H2O nanosheet/microflower hierarchical structure exhibits applicable electrochromic and adsorptive properties due to its unique crystallographic configuration and microstructures, promising its application in energy-saving smart windows and wastewater treatment.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium-Storage Properties

Crystalline silicon(Si)/germanium(Ge) alloy nanotubes and hollow particles are synthesized for the first time through a one-pot electrolytic process. The morphology of these alloy structures can be easily tailored from nanotubes to hollow particles by varying the overpotential during the electro-reduction reaction. The continuous solid diffusion governed by the nanoscale Kirkendall effect results in the formation of inner void in the alloy particles. Benefitting from the compositional and structural advantages, these SiGe alloy nanotubes exhibit much enhanced lithium-storage performance compared with the individual solid Si and Ge nanowires as the anode material for lithium-ion batteries.

Verification and implications of the dissolution–electrodeposition process during the electro-reduction of solid silica in molten CaCl2

With the verification of the existence of the dissolution-electrodeposition mechanism during the electro-reduction of solid silica in molten CaCl2, the present study not only provides direct scientific support for the controllable electrolytic extraction of nanostructured silicon in molten salts but it also opens an avenue to a continuous silicon extraction process via the electro-deposition of dissolved silicates in molten CaCl2. In addition, the present study increases the general understanding of the versatile material extraction route via the electro-deoxidization process of solid oxides in molten salts, which also provokes reconsiderations on the electrochemistry of insulating compounds.

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.

Harvesting capacitive carbon by carbonization of waste biomass in molten salts.

Conversion of waste biomass to value-added carbon is an environmentally benign utilization of waste biomass to reduce greenhouse gas emissions and air pollution caused by open burning. In this study, various waste biomasses are converted to capacitive carbon by a single-step molten salt carbonization (MSC) process. The as-prepared carbon materials are amorphous with oxygen-containing functional groups on the surface. For the same type of waste biomass, the carbon materials obtained in Na2CO3-K2CO3 melt have the highest Brunauer-Emmett-Teller (BET) surface area and specific capacitance. The carbon yield decreases with increasing reaction temperature, while the surface area increases with increasing carbonization temperature. A working temperature above 700 °C is required for producing capacitive carbon. The good dissolving ability of alkaline carbonate molten decreases the yield of carbon from waste biomasses, but helps to produce high surface area carbon. The specific capacitance data confirm that Na2CO3-K2CO3 melt is the best for producing capacitive carbon. The specific capacitance of carbon derived from peanut shell is as high as 160 F g(-1) and 40 μF cm(-2), and retains 95% after 10,000 cycles at a rate of 1 A g(-1). MSC offers a simple and environmentally sound way for transforming waste biomass to highly capacitive carbon as well as an effective carbon sequestration method.

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.