Molten salt electro-preparation of graphitic carbons

Graphite has been used in a wide range of applications since the discovery due to its great chemical stability, excellent electrical conductivity, availability, and ease of processing. However, the synthesis of graphite materials still remains energy-intensive as they are usually produced through a high-temperature treatment (>3000°C). Herein, we introduce a molten salt electrochemical approach utilizing carbon dioxide (CO₂) or amorphous carbons as raw precursors for graphite synthesis. With the assistance of molten salts, the processes can be conducted at moderate temperatures (700-850°C). The mechanisms of the electrochemical conversion of CO₂ and amorphous carbons into graphitic materials are presented. Furthermore, the factors that affect the graphitization degree of the prepared graphitic products, such as molten salt composition, working temperature, cell voltage, additives, and electrodes, are discussed. The energy storage applications of these graphitic carbons in batteries and supercapacitors are also summarized. Moreover, the energy consumption and cost estimation of the processes are reviewed, which provides perspectives on the large-scale synthesis of graphitic carbons using this molten salt electrochemical strategy.

Capture and electrochemical conversion of CO2 in molten alkali metal borate-carbonate blends

A family of blended compositions of molten mixed lithium and sodium borate (Li_1.5Na_1.5BO₃) and eutectic lithium-potassium carbonate (Li_1.24K_0.76CO₃) salts has been introduced as reversible carbon dioxide absorbents and as media for CO₂ electrolysis for carbon conversion. Material properties, temperature effects and kinetics of CO₂ uptake were examined. Li, Na borate can absorb up to 7.3 mmol g^-1 CO₂ at 600 °C. The blended borate-carbonate compositions are molten in the 550-600 °C temperature range, with viscosity adjustable to within a 10-1000 Pa s window depending on the borate/carbonate ratio. The blends can withstand cyclic temperature and CO₂ pressure swings without significant deterioration of their CO₂ uptake capabilities. Addition of eutectic carbonate into mixed Li, Na borate salts lowers overall CO₂ uptake due to the lower solubility of CO₂ in carbonate. However, addition of the eutectic lowers the temperature of the pressure swing operation and dramatically accelerates the CO₂ uptake during the initial stage of the absorption, potentially enabling a faster cycling. Electroreduction of CO₂ and carbon deposition on a galvanized steel cathode was more effective with increasing carbonate fraction in the molten alkali borate/carbonate blend. Blended borate/carbonate compositions with 50-60% borate content possessed sufficiently high loading capacity for CO₂ and simultaneously enabled maximum carbon product yield and Coulombic efficiency. Most of the recovered carbon product was shown to be in the form of multiwalled carbon nanotube.

Liquid-Metal-Enabled Mechanical-Energy-Induced CO2 Conversion

A green carbon capture and conversion technology offering scalability and economic viability for mitigating CO₂ emissions is reported. The technology uses suspensions of gallium liquid metal to reduce CO₂ into carbonaceous solid products and O₂ at near room temperature. The nonpolar nature of the liquid gallium interface allows the solid products to instantaneously exfoliate, hence keeping active sites accessible. The solid co-contributor of silver-gallium rods ensures a cyclic sustainable process. The overall process relies on mechanical energy as the input, which drives nano-dimensional triboelectrochemical reactions. When a gallium/silver fluoride mix at 7:1 mass ratio is employed to create the reaction material, 92% efficiency is obtained at a remarkably low input energy of 230 kWh (excluding the energy used for dissolving CO₂ ) for the capture and conversion of a tonne of CO₂ . This green technology presents an economical solution for CO₂ emissions.

Diamond formation in an electric field under deep Earth conditions

Most natural diamonds are formed in Earth's lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth's mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral-forming processes, carbon isotope fractionation, and the global carbon cycle.

Recent Advances in Solar Thermal Electrochemical Process (STEP) for Carbon Neutral Products and High Value Nanocarbons

Climate change represents one of the most important environmental issues of our time. Due to high levels of anthropogenic CO₂ emissions, atmospheric CO₂ has for the first time ever exceeded 415 ppm and has increased from 315 ppm in 1950. An annual increase in atmospheric CO₂ of ∼2 ppm is equal to a net increase of ∼15.6 billion tons of CO₂. The combustion of fossil fuels for electricity and transportation is still the main reason accounting for the CO₂ accumulation. On the top of that, fossil fuels are widely used in our modern industry for the productions of indispensable social staples. For instance, the millennia old thermal reduction of iron ore by charcoal or baked coal (3C + 2Fe₂O₃ → 4Fe + 3CO₂) continues as the main method for the production of iron. The artificial fertilizer ammonia boosts the global population and is mainly produced from the Haber-Bosch process, in which hydrogen is generated via steam reforming of methane (CH₄ + 2H₂O → 4H₂ + CO₂). Sequestration and diminution of CO₂ require the development of a portfolio of technologies on (1) efficient and long-term harvesting of renewable energy, that is, solar, not only for electricity but also directly as the energy force in vital chemical processes, wherever possible, (2) carbon-neutral processes to replace current industrial processes that emit vast amounts of CO₂, such as iron and ammonia production, and (3) new, low-cost technologies for CO₂ capture and conversion with particular interests in the exploration of CO₂ as the feedstock for fuels or other valuable chemicals and materials. To this end, we conducted some studies on the sustainable synthesis of ammonia and iron with net-zero CO₂ emissions and large-scale CO₂ capture and conversion into fuels and high value nanocarbon products via electrolysis in molten salt(s) with the introduction of the Solar Thermal Electrochemical Process (STEP). In STEP, solar UV-visible energy is focused on a photovoltaic device that generates the electricity to drive the electrolysis, while concurrently the solar thermal energy is focused on a second system to generate heat for the electrolysis cell. The utilization of the full spectrum of sunlight in STEP results in a higher solar energy efficiency than other solar conversion processes. STEP has been applied to conduct (1) CO₂-free ammonia synthesis from nitrogen and water with the aid of nano-Fe₂O₃ in a molten hydroxide electrolyte, (2) CO₂-free production of iron via electrochemical reduction of iron ore in molten carbonate, (3) CO₂ capture and conversion into nanostructured carbon products as well as fuels in molten or mixed molten electrolytes, and (4) organic electrosynthesis of benzoic acid from benzene without overoxidizing into CO₂. In this Account, we highlight some recent achievements in these topics and propose that using STEP is a highly efficient strategy for saving energy and, consequently, the environment. STEP is an ideal tool that can theoretically be applied to all endothermic reactions.

Physical characteristics of capacitive carbons derived from the electrolytic reduction of alkali metal carbonate molten salts

Carbons have been synthesized through the reduction of molten carbonate systems under varied conditions. The mechanism and kinetics of carbon electrodeposition has been investigated. Carbon morphologies include amorphous, graphite-like, and spherical aggregate phases. Increased graphitic character is observed in carbons electrodeposited at more cathodic potentials, particularly at higher temperatures. Bonding has been investigated and oxygen functionalised sp2 and sp3 structures have been identified. The level of functionalization decreases in carbons with reduced amorphous and increased graphitic character. Thermal decomposition of electrodepositied carbons has been investigated and zero order kinetics have been identified. A relationship has been identified between elevated oxygen functionalization and increased pseudo-capacitance, with carbons deposited at 0.15 A cm-2 showing capacitances of 400 F g-1 in 0.5 M H2SO4 at sweep rates of 10 mV s-1.

Fabrication of graphite via electrochemical conversion of CO2 in a CaCl2 based molten salt at a relatively low temperature

Fabrication of graphite by electrochemical splitting of CO₂ in a CaCl₂ molten salt is a promising approach for the efficient and economical utilization of CO₂. Systematically understanding the graphitization mechanism is of great significance to optimize the process. In this work, how pulse parameter and type of anode affect morphologies and crystallinity of graphite nanostructures were both investigated. The results indicate that the optimum current efficiency, energy consumption and highest degree of graphitization can be achieved by employing an appropriate pulse current parameter (T_on : T_off = 120 : 5), and with the utilization of a RuO₂–TiO₂ inert anode. The microstructure and morphologies show noticeable change by varying electrolytic conditions. In addition, the present study provides an insight into facile ways to improve the graphitization degree by electrochemical conversion of CO₂ at a relatively low temperature.

Fabrication of graphite by electrochemical splitting of CO₂ in a CaCl₂ molten salt is a promising approach for the efficient and economical utilization of CO₂.

A novel route to synthesize carbon spheres and carbon nanotubes from carbon dioxide in a molten carbonate electrolyzer

Carbon dioxide is readily converted into carbon spheres (CSs) and carbon nanotubes (CNTs) in a molten carbonate electrolyzer.

Carbon dioxide electrolysis and carbon deposition in alkaline-earth-carbonate-included molten salts electrolyzer

Alkaline-earth-carbonate-included molten salts sustain continuous CO₂ capture and electrochemical conversion.

Effect of molten carbonate composition on the generation of carbon material

Ambient CO₂ is readily split into zerovalent carbon at diverse electrolytes.

Syngas production: diverse H2/CO range by regulating carbonates electrolyte composition from CO2/H2O via co-electrolysis in eutectic molten salts

This work reports the simultaneous production of CO and H₂ with a broadened H₂/CO ratio range.

Molten-salt treatment of waste biomass for preparation of carbon with enhanced capacitive properties and electrocatalytic activity towards oxygen reduction

Carbon powders are building blocks for electrochemical energy storage/conversion devices. Green, cost-affordable and facile preparation of carbon with applicable electrochemical properties is therefore essential for effective utilization of fluctuating renewable energy. Herein, the preparation of carbon nanoflakes via impregnation of waste biomass i.e. boiled coffee beans in molten Na₂CO₃–K₂CO₃ (with equal mass) at 800 °C and molten CaCl₂ at 850 °C is reported. The microstructure and surface chemistry of the obtained carbons are specified. The correlations between synthetic conditions and microstructure/surface chemistry of the obtained carbons are rationalized. The derived carbon nanosheets are tested and compared as active materials for supercapacitors in a configuration of symmetric full cells in 1 M MeEt₃NBF₄ in acetonitrile and electrocatalysts towards the oxygen reduction reaction (ORR) in O₂-saturated 0.1 M aqueous KOH. Despite the lower surface area, the carbon nanosheets derived in molten Na₂CO₃–K₂CO₃ exhibit enhanced capacitive properties and electrocatalytic ORR activity. The present study highlights the importance of thermal media on the microstructure, surface chemistry and electrochemistry of carbon from biomass.

Molten salt CO2capture and electro-transformation (MSCC-ET) into capacitive carbon at medium temperature: effect of the electrolyte composition

Electrochemical transformation of CO₂into functional materials or fuels (i.e., carbon, CO) in high temperature molten salts has been demonstrated as a promising way of carbon capture, utilisation and storage (CCUS) in recent years. In a view of continuous operation, the electrolysis process should match very well with the CO₂absorption kinetics. At the same time, in consideration of the energy efficiency, a molten salt electrochemical cell running at lower temperature is more beneficial to a process powered by the fluctuating renewable electricity from solar/wind farms. Ternary carbonates (Li : Na : K = 43.5 : 31.5 : 25.0) and binary chlorides (Li : K = 58.5 : 41.5), two typical kinds of eutectic melt with low melting points and a wide electrochemical potential window, could be the ideal supporting electrolyte for the molten salt CO₂capture and electro-transformation (MSCC-ET) process. In this work, the CO₂absorption behaviour in Li₂O/CaO containing carbonates and chlorides were investigated on a home-made gas absorption testing system. The electrode processes as well as the morphology and properties of carbon obtained in different salts are compared to each other. It was found that the composition of molten salts significantly affects the absorption of CO₂, electrode processes and performance of the product. Furthermore, the relationship between the absorption and electro-transformation kinetics are discussed based on the findings.