Study On Carbon Reduction And Carbon Fixation Based On High Temperature Molten Salt Chemistry

The Global warming and environmental deterioration call for exploring clean and renewable energy, reducing greenhouse gases emissions by improving energy efficiency and developing much greener industrial processes, searching for new methods for capture, fixture and ultilization of CO2, as well as protecting and remedying the environment and so on. High temperature molten salt, a kind of hot ionic liquid, has wide liquid temperature range, huge heat capacity, good ionic conductance, wide electrochemical window and fast kinetics for reactions. It is a good media for chemical and electrochemical reactions which has been used for synthesizing materials, electrochemical metallurgy, recycling of nuclear wastes, processing wastes and so on. In recent decades, the short-process metallurgy (e.g. solid oxide electrolysis) and novel energy technologies such as concentrated solar powder (CSP) and liquid metal batteries (LMB) have drawn increasing attentions, which are all based on high temperature molten salt. This thesis focuses on developing inert anode for low-carbon or zero-carbon metallurgy (inert anode), new technologies for transformation and utilization of CO2and waste biomass, and energy efficient electrochemical process for preparing energy storage nanomaterials in molten salt. The purpose of these researches is exploring new pathways for cutting and fixing carbon using molten salt as a reaction media. The main work and results are summarized as follows:(1) Direct electrochemical reduction of oxides in CaCl2based molten salt to produce metals and alloys has been proved as a short-process and energy efficient metallurgical process in recent15years. However, the applied carbon anode still generates CO2under anodic polarization. An inert anode is crucial for realizing a practically green process. The anodic behaviors of some pure metals and Ni based alloys in the melt and the possibility of Ni based alloys using as inert anode were investigated in Chaper2. Key fundamental data about stability of typical metals, nickel based alloys and oxides in molten calcium based melt were studied through thermodynamic calculations, measurements of anodic polarization curves of typical metals, investigation of solubilities and dissolving mechanism of typical oxides in the melt. The order of the stability of typical metals and oxides in molten salt according to the thermodynamic data is summarized in the E-metal diagram. It is found that the electrochemical stability order follows La, Ce, Hf, Al, Zr, Ti, Zn, Si, Nb, V, Cr, Ta, Fe, Cu, Co, Sn, Ag, Ni, W, Mo, Ir, Pt, Ru, Au which agrees well with the experimental results follows Fe、Cu、Ag、Co、Ni、Mo、Pt. In addition, a method on measuring the solubility was established and the effects of temperature and oxygen anion concentration of the melt on the solubilitiy of oxides were investigated. The results reveal that the solubility of NiO decreases with the increasing concentration of CaO in the melts but the solubility of Fe2O3increases, which indicates the solubility process may have two different dissolving mechanisms. According to the above results, the Ni based alloys were casted and their electrochemical stability in the melt was tested. Accounting for the highly corrosive chloride based melts, the "nickel alloy-molten carbonate-YSZ (yttria stabilized zirconia)" composite anode was proposed and used as an inert anode to electrochemically split Ta2O5to produce Ta and O2.(2) More than1billions tons of greenhouse gases are emitted annually from iron and steel metallurgical industry. In fact, the iron ores, such as Fe2O3and Fe3O4have relatively positive reduction potentials. Therefore, a less aggressive melt with enough wide potential window such as molten carbonate might be an ideal melt for electrochemically splitting Fe2O3to iron and oxygen working with a cost-affordable inert anode. This idea was proposed and realized in Chapter3. In Na2CO3-K2CO3melt at750℃, the NilOCu11Fe is an ideal inert anode for electrochemically splitting Fe2O3to Fe and O2. The results demonstrate that the Fe2O3is reduced to iron through two steps and the oxygen is generated on the Ni alloy inert anode which is protected by an in-situ formed conductive NiFe2O4coating. The energy consumption for producing1kg Fe can be as low as2.87kWh with a current efficiency of95%.(3) Following the green electrochemical method for extracting iron from Fe2O3in molten carbonate, Chapter4focuses on capturing and electrochemically transforming CO2to carbon and oxygen in molten carbonate. In molten Li2CO3-Na2CO3-K2CO3at500℃, the SnO2can serve as an ideal inert anode for electrochemically splitting CO2/carbonate to carbon and oxygen. In order to ensure an energy consumption lower than30kWh/kg-carbon and a higher current efficiency above75%, the cell voltage should be between3V and4V. Moreover, the obtained carbon has good electrochemical property for energy storage in1mol.L-1H2SO4and all of the carbon obtained at different cell voltages (from2.8V to5V) has a capacity more than200F/g at a charge-diacharge rate of2A/g. Moreover, the carbon obtained at5.0V has a high adsorption capacity for Cr(VI) of167mg/g. Additionally, the carbon deposition mechanism including the direct electrochemical reduction of CO2and the deposition of dissolved CO2in terms of carbon anions are preliminary discussed. All of the results demonstrate using molten carbonate to capture and electrochemically transform CO2into value-added materials is an effective way to cutting CO2emission and using it as a cheap and clean carbon feedstock.(4) CO2can be naturally fixed and utilized by plants through their photosynthesis function, while the combustion of waste biomass will release it into atmosphere, especially in developing countries. As we know, most molten salts have very big thermal capacity so that they can be used as heat absorbent for concentrated solar heat and the inside of the melt is a high temperature environment free of oxygen and water. Chapter5proposes a method using the solar-heated molten salt to harvest value-added carbon by pyrolysis of waste biomass which is a totally clean process in terms of transforming and storing carbon fixed by photosynthesis of plants. In this chapter, CaCl2, CaCl2-NaCl, Na2CO3-K2CO3, Li2CO3-Na2CO3-K2CO3are selected as target melt for carbonization of waste biomass. The obtained carbon has large electrochemical capacitance and good absorption ability for Cr (Ⅵ) in low pH solution ranges from2to5. The results indicate the Na2CO3-K2CO3is a suitable media for carbonization of waste biomass for its innocuity, low cost and cleaner products. During the carbonization process, the molten salt plays an important role in making highly capacitive and absorptive carbon. The carbon derived from rice husk in Na2CO3-K2CO3at850℃has a capacity of186F/g in lmol.L-1H2SO4and the carbon derived from the rice straw has an absorption capacity for Cr(VI) of35.4mg/g with a removal efficiency larger than99.5%at13℃in an original Cr (VI) concentration of34.5mg/L and the pH of the solution is2.1.(5) Developing new energy technologies is one of important ways for reducing CO2and the exploring new energy storage materials (ESMs) is a key issue. However, the energy consumption and carbon emission for preparing the ESMs are always ignored. In Chapter6, nano-CaB6with highly gravimetric energy was prepared by direct electrolysis of calciborite (CaB2O4) in molten CaCl2-NaCl at600℃. It was realized for the first time that1g nao-CaB6can deliver2400mAh at a current density of250mA/g in30wt%KOH solution, which is about3times of Zn. It is found the CaB6with smaller particle size has much higher electrochemical activity and bigger capacity. The energy consumption for production of1kg CaB6is34.9kWh with a current efficiency of44%. In addition, the germanium nanowire and porous germanium were directly fabricated by electrochemical reduction of GeO2in the same melt at600℃with an energy consumption as low as3.26kWh/kg-Ge and a high current efficiency of86%. The formation of Ge nanowire was supposed to grow via a solid-liquid-solid (SLS) mechanism and the generation of porous Ge consisted of electrochemical reduction of GeO2, electrochemical intercalation of calcium and chemical dealloying of calcium. Besides, the produced Ge has a good capacity and stable cycle ability for calcium storage which is a promising energy storage material for thermobattery. In CaCl2-NaCl melt at600℃, the obtained Ge has a capacity of570mAh/g under a charging current density of5A/g.