Effects of applied voltage and temperature on the electrochemical production of carbon powders from CO2 in molten salt with an inert anode

Unraveling the role of substrate materials in governing the carbon/carbide growth of molten carbonate electrolysis of CO2

The interface interaction between deposited carbon and metallic electrode substrates in tuning the growth of CO2-derived products (e.g., amorphous carbon, graphite, carbide) is mostly unexplored for the high-temperature molten-salt electrolysis of CO2. Herein, the carbon deposition on different transition-metal cathodes was performed to reveal the role of substrate materials in the growth of cathodic products. At the initial stage of electrolysis, transition metals (e.g., Cr, Fe, Ni, and Co) that exhibit appropriate carbon-binding ability (in range of -30 to 60 kJ mol-1) allow carbon diffusing into and then dissociating from metal to form graphite, as the carbon-binding ability can be determined by the Gibbs free energy of formation of metallic carbides. The catalytic cathodes showing super strong (e.g., Ti, V, Mo, and W) or weak (e.g., Cu) carbon-binding ability produce stable carbides or amorphous carbon, respectively. However, the subsequent deposited carbon is immune to the catalysis of the substrate, forming amorphous carbon nanoparticles and nanofibers on the surface of carbides and graphite, respectively. This paper not only highlights the role of the catalytic cathodes for carbon deposition, but also offers a material selection principle for the controllable growth of CO2-derived products in molten salts.

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.

Electrochemical conversion of CO2 into value-added carbon with desirable structures via molten carbonates electrolysis

Direct conversion of CO₂ to high value-added carbon products based on molten salt electrochemistry has been proven to be a feasible approach to solve the climate problem and achieve carbon neutrality. In this work, carbon nanotubes (CNTs), carbon spheres (CSs) and honeycomb carbon are synthesized by electrolysis of a single or multiple alkali metal carbonate electrolyte. The elemental composition, morphology and structure, crystallinity and graphitization degree of carbon products are characterized by electron dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman microspectroscopy (RAM). The results demonstrate that a high yield of CNTs is obtained in Li₂CO₃ electrolyte by regulating the electrolysis temperature and current density. Compared to pure Li₂CO₃, Li-Na carbonate electrolyte with 1 wt% stannic oxide/cerium oxide (SnO₂/GeO₂) favors CS formation rather than CNT formation. Additionally, honeycomb carbon products are generated in Li-Na-K electrolyte, when the electrolysis temperature is lower than 600 °C. Overall, this work provides a novel carbon neutral strategy where high value-added carbon products are synthesized using CO₂ as a carbon source.

Degradation of 2,4-DCP using persulfate and iron/E-carbon micro-electrolysis coupling system

The greenhouse gas carbon dioxide (CO₂) was converted to a novel CO₂ conversion material (electrolytic carbon, EC) by molten salt electrochemical conversion, which served as the carbon source to prepare an iron-carbon composite (Fe-EC). The composite was used to activate persulfate (PS) and degrade 2,4-dichlorophenol (2,4-DCP) in an aqueous solution. The effects of several essential operating parameters such as PS dosage and pH on 2,4-DCP degradation were investigated. The removal efficiency of 2,4-DCP (20 mg L^-1) was 97.8% in the presence of Fe-EC (50 mg L^-1) and PS (1 mmol L^-1). Moreover, the average % reaction stoichiometric efficiency (RSE) (calculated for all selected times 5-60 min) was maintained at 23.07%. Electron paramagnetic resonance (EPR), classical radical scavenging experiments, and density functional theory (DFT) calculations were integrated for a mechanistic study, which disclosed that the active species in the system were identified as SO₄^⦁-, •OH, and O₂^⦁-. Moreover, the iron-carbon micro-electrolysis/PS (ICE-PS) system had a high tolerance to a wide range of pH, which would provide theoretical guidance for the treatment of organic pollutants in practical industrial wastewater.

Effectively removing tetracycline from water by nanoarchitectured carbons derived from CO2: Structure and surface chemistry influence

Understanding of the correlation between physico-chemical property of adsorbent and the adsorption performance of contaminant is very significant for developing high-efficient materials to remove antibiotic contamination from water. In this work, a novel kind of carbon adsorbent (EC) derived from CO₂ and activated ECs with modified structure via a facile chemical method using H₂ and KOH were prepared. The synthetic carbon materials (EC, EC-H₂, and EC-KOH) were then applied to remove tetracycline (TC). The kinetics of adsorption for these three carbon materials all well fitted the pseudo-second-order kinetic model. The experimental data of adsorption isotherm had good compatibility with Langmuir and Freundlich models (R² > 0.90), but the Temkin model was the most applicable for all adsorbents (R² > 0.98). A super-high adsorption capacity of EC-KOH obtained from Langmuir fitting was 933.56 mg g^-1, which was much higher than that of EC-H₂ (538.91 mg g^-1) and EC (423.30 mg g^-1), possibly due to its larger specific surface area (S_BET), pore volume, and specific surface chemical structure. Moreover, it was found that surface functional groups and large aperture of adsorbents had a positive effect on adsorption rate. More adsorption sites and surface functional groups of adsorbents were beneficial to enhance the adsorption affinity. These results are of great benefit to the directional control of carbon structure to increase the adsorption performance in rate, capacity, and affinity of antibiotics.

Sustainable Carbons and Fuels: Recent Advances of CO2 Conversion in Molten Salts

The massive release of the greenhouse gas CO₂ has resulted in numerous environmental issues. In searching for advanced technologies for CO₂ capture/conversions, recent advances in electrochemical reduction of CO₂ in molten salts shed a light on potential solutions to CO₂ mitigation. Electro-reduction of CO₂ in molten salts exhibits features like high selectivity and efficiency towards sustainable carbons and fuels, low toxicity, and possibility to combine with in situ CO₂ capture. In this Minireview, we highlight the tuning of the products in this process and mainly discuss two categories of electrolyte, carbonate-based molten salts (CMS) and those based on halides (HMS). Depending on the synthetic conditions, fuels such as CO or hydrocarbons (in the presence of hydrogen source, i. e., LiOH, NaOH, or KOH in the electrolyte) as well as high-value nanostructured carbons including carbon nanotubes, carbon nanofibers, carbon nano-onions, and graphene can be obtained with high efficiency. The synthesis parameters are compared, and the applications of as-obtained carbons are briefly summarized. Additionally, some perspectives on this technology are also discussed.

Carbon dioxide reduction to multicarbon hydrocarbons and oxygenates on plant moss-derived, metal-free, in situ nitrogen-doped biochar

Electrochemical reduction of carbon dioxide (CO₂) is considered a promising renewable energy conversion technology, but it remains challenging to find active, stable, low-cost, and highly efficient electrocatalysts for the CO₂ conversion. Here, we develop an in situ nitrogen-doped, metal-free, porous biochar from plant moss to catalyze the electrochemical reduction of CO₂ into methane (CH₄), methanol (CH₃OH) and ethanol (C₂H₅OH) at high current densities and low overpotentials. Using this metal-free biochar electrocatalyst, production rates of approximately 36.1, 32.1, and 18.1 μg h^-1 cm^-2 towards CH₄, C₂H₅OH, and CH₃OH are obtained with Faradaic efficiencies of 56.0%, 26.0% and 10.5%, respectively. In addition, the total faradaic efficiency reaches 92.6% at -1.2 V (vs. Ag/AgCl) with good stability. A favorable pathway for the electrochemical reduction of CO₂ over the metal-free biochar is also provided. This study presents a new approach to produce cost-effective, in situ nitrogen-doped porous biochars with excellent efficiency and durability for the electrochemical reduction of CO₂.

Green Carbon Material for Organic Contaminants Adsorption

Eco-friendly and economical adsorbents are desirable for removing organic pollutants from the environment. Herein, a kind of green carbon material, electrolytic carbon (EC) prepared by the electrochemical conversion of greenhouse gas (CO₂) in molten carbonate, is verified as an effective adsorbent for aniline and other small aromatic organic molecules. The EC consists of nanoparticles and nanoflakes, featuring the specific surface area of ∼641 m²/g with an enriched micropore structure. It exhibits a large adsorption capacity (Q_max > 114.1 mg/g) for aniline, especially in water with a lower contamination level. The adsorption conforms to the pseudo-second-order equation kinetically and the Freundlich model thermodynamically in the temperature range of 303-323 K. Moreover, it is found that the adsorption performance of the material can be further improved through reducing surface oxygen functional groups by a simple thermotreatment. Its adsorption capacity for aniline is enhanced by 1.7 times, demonstrating that the π-π dispersive interaction plays a primary role for the efficient adsorption. This adsorption mechanism is further confirmed by the excellent adsorption performance of the carbon materials for other analogue aromatic compounds (phenol, nitrobenzene). The super performance of the CO₂-derived carbon adsorbents will be helpful for capturing CO₂ as well as for removing organic pollutants.

Interactions of molten salts with cathode products in the FFC Cambridge Process

Molten salts play multiple important roles in the electrolysis of solid metal compounds, particularly oxides and sulfides, for the extraction of metals or alloys. Some of these roles are positive in assisting the extraction of metals, such as dissolving the oxide or sulfide anions, and transporting them to the anode for discharging, and offering the high temperature to lower the kinetic barrier to break the metal-oxygen or metal-sulfur bond. However, molten salts also have unfavorable effects, including electronic conductivity and significant capability of dissolving oxygen and carbon dioxide gases. In addition, although molten salts are relatively simple in terms of composition, physical properties, and decomposition reactions at inert electrodes, in comparison with aqueous electrolytes, the high temperatures of molten salts may promote unwanted electrode-electrolyte interactions. This article reviews briefly and selectively the research and development of the Fray-Farthing-Chen (FFC) Cambridge Process in the past two decades, focusing on observations, understanding, and solutions of various interactions between molten salts and cathodes at different reduction states, including perovskitization, non-wetting of molten salts on pure metals, carbon contamination of products, formation of oxychlorides and calcium intermetallic compounds, and oxygen transfer from the air to the cathode product mediated by oxide anions in the molten salt.

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 BaCO3 addition on the CO2-derived carbon deposition in molten carbonates electrolyzer

Atmospheric carbon dioxide is facilely transformed into carbon materials in Ba-containing or Ba-free carbonates eutectic.

Effect of molten carbonate composition on the generation of carbon material

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

Novel process for producing hierarchical carbide derived carbon monolith and low carbon ferromanganese from high carbon ferromanganese

In this work, remelted high carbon ferromanganese was chosen as a consumable anode to produce porous carbon monolith and low carbon ferromanganese at the same time by molten salt electrolysis.

An investigation into the carbon nucleation and growth on a nickel substrate in LiCl-Li2CO3 melts

The electrochemical deposition of carbon materials has been performed in LiCl-Li2CO3 melts using a Pt anode and a nickel cathode. Cyclic voltammetry and constant voltage electrolysis are conducted to investigate the electrode reactions, and the results prove that solid carbon is the only product from the cathodic reduction. Short-term electrolysis at 750 °C for 3, 10 and 20 s has been applied to study the formation and growth of the varied carbon microstructures. All of the results demonstrate that the morphologies of the deposited carbon are significantly affected by the cathode substrates, which may show different catalyzing effects on carbon nucleation. Two primary morphologies, quasi-spherical and nanofiber structures are observed at the nickel plate cathodes during the electrolysis and the formation and growth of carbon nanofibers are easily enhanced by using a high cell voltage. However, only a quasi-spherical structure is found on the molybdenum cathode substrate.

Direct Conversion of Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis

Producing graphene through the electrochemical reduction of CO2 remains a great challenge, which requires precise control of the reaction kinetics, such as diffusivities of multiple ions, solubility of various gases, and the nucleation/growth of carbon on a surface. Here, graphene was successfully created from the greenhouse gas CO2 using molten salts. The results showed that CO2 could be effectively fixed by oxygen ions in CaCl2-NaCl-CaO melts to form carbonate ions, and subsequently electrochemically split into graphene on a stainless steel cathode; O2 gas was produced at the RuO2-TiO2 inert anode. The formation of graphene in this manner can be ascribed to the catalysis of active Fe, Ni, and Cu atoms at the surface of the cathode and the microexplosion effect through evolution of CO in between graphite layers. This finding may lead to a new generation of proceedures for the synthesis of high value-added products from CO2, which may also contribute to the establishment of a low-carbon and sustainable world.

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.

Carbon electrodeposition in molten salts: electrode reactions and applications

Carbon dioxide can be electrochemically reduced to carbon in molten carbonate salts, promising affordable energy, materials and environmental explorations.