In this present work, a task specific ionic liquid (TSIL) was encapsulated into the framework of ZIF-8 to enhance its CO₂ capture capacity and CO₂/N₂ selectivity at post-combustion conditions. 1-Ethyl-3-methylimidazolium amino-acetate [EMIM][Gly] was selected as TSIL. TSIL@ZIF-8 composite sorbents were prepared by varying the loading of TSIL and the sorbents’ thermal stability, porous structure and crystal nature were investigated. Incorporation of TSIL into ZIF-8 has shown dramatic impact on CO₂ uptake especially at below 1.0 bar. At this low-pressure range, CO₂ uptake was higher than the pristine ZIF-8 for all TSIL loadings. TSIL@ZIF-8 composite with 30 wt.% TSIL reached a CO₂ uptake capacity of 0.55 mmol·g^-1 at 0.2 bar which was 6 times higher than the pristine ZIF-8 at the same condition. TSIL incorporated composites also exhibited much higher selectivity than the pristine ZIF-8 at all studied pressures. CO₂/N₂ ideal selectivity at 313 K for 30 wt.% [EMIM][Gly] loading was 28 and 19 at 0.1 and 0.2 bar, respectively. This composite sorbent with significantly higher CO₂ uptake and better CO₂/N₂ selectivity at low pressure region (<1.0 bar) can play an important role in post-combustion CCS processes

​This study presents an innovative approach to enhancing power-to-gas (P2G) systems by integrating high-temperature solid oxide electrolysis (SOE) with molten carbonate fuel cells (MCFCs) for efficient CO₂ capture in power plants. The proposed hybrid system aims to improve energy efficiency and reduce carbon emissions, addressing critical challenges in sustainable energy production.​

High-Temperature Electrolysis and Energy Efficiency

High-temperature electrolysis, particularly through solid oxide electrolyzer cells (SOECs), offers significant advantages over low-temperature methods. Operating at elevated temperatures (700–800 °C), SOECs facilitate more efficient water splitting, resulting in lower electricity consumption per unit of hydrogen produced. This efficiency stems from the favorable thermodynamics at higher temperatures, which reduce the electrical energy required for the electrolysis process. ​

Molten Carbonate Fuel Cells for CO₂ Capture

MCFCs operate at approximately 650 °C and are capable of internal reforming, allowing them to utilize fuels like natural gas directly. A notable feature of MCFCs is their ability to capture CO₂ from flue gases. In this system, flue gas is mixed with air and introduced to the MCFC, where CO₂ is transferred from the cathode to the anode side, effectively separating it from other gases. Experimental studies have demonstrated that CO₂ separation rates exceeding 90% are achievable by adjusting the cathode inlet flow.

Integration of SOEC and MCFC in P2G Systems

The integration of SOEC and MCFC technologies within a P2G framework offers multiple benefits:​

Enhanced Energy Efficiency: The synergy between SOECs and MCFCs leads to improved overall system efficiency. The waste heat generated by the MCFC can be utilized to maintain the high operating temperatures required by the SOEC, creating a thermally integrated system that minimizes energy losses.​

Effective CO₂ Utilization: The CO₂ captured by the MCFC can be recycled and used in the methanation process to produce synthetic natural gas (SNG). This not only reduces greenhouse gas emissions but also contributes to the production of valuable fuels, aligning with carbon capture and utilization (CCU) strategies.​

Modular and Scalable Design: The proposed system's design is straightforward and compact, allowing for modular implementation. This modularity facilitates scalability, enabling the system to be adapted for various applications, from small-scale industrial settings to larger power plants.​

Thermal Management and System Optimization

Effective thermal management is crucial for the optimal performance of the integrated system. All components operate at elevated temperatures: the Sabatier reactor at 300 °C, the MCFC at 650 °C, and the SOEC at 700–800 °C. Proper integration ensures that the heat generated by the MCFC and the exothermic methanation reaction in the Sabatier reactor is effectively utilized to sustain the SOEC's operating temperature. This internal heat exchange reduces the need for external heating sources, thereby enhancing the system's overall efficiency.​

Environmental and Economic Implications

The adoption of this hybrid system has significant environmental and economic implications:​

Reduction in CO₂ Emissions: By capturing and utilizing CO₂, the system contributes to lowering greenhouse gas emissions from power plants, aiding in the mitigation of climate change.​

Cost-Effective Hydrogen Production: The improved efficiency of high-temperature electrolysis reduces the electricity required for hydrogen production by approximately 25%, leading to cost savings and making the process more economically viable.​

Production of Synthetic Fuels: The system enables the production of SNG, which can be injected into existing natural gas infrastructure, providing a renewable energy source and enhancing energy security.​

Challenges and Future Directions

While the proposed system offers numerous advantages, several challenges need to be addressed:​

Material Durability: The high operating temperatures necessitate the use of materials that can withstand thermal stress and corrosion over extended periods. Ongoing research focuses on developing and testing materials that meet these stringent requirements.​

System Integration: Achieving seamless integration of SOECs and MCFCs requires careful design and control strategies to manage the interactions between components and ensure stable operation.​

Economic Viability: While the system has the potential for cost savings, initial capital investment and maintenance costs must be considered. Economic analyses and pilot projects are essential to demonstrate the system's commercial feasibility.​

Conclusion

The integration of solid oxide electrolysis cells and molten carbonate fuel cells presents a promising pathway for enhancing P2G systems. By improving energy efficiency, enabling effective CO₂ capture, and facilitating the production of synthetic fuels, this hybrid system addresses key challenges in sustainable energy production. Further research and development efforts are necessary to overcome existing challenges and realize the full potential of this innovative approach.

Vanadium is a metal of strategic significance, essential to contemporary industries because of its distinctive physical and chemical characteristics. Three dynamic optimization problems are solved to control ionic valence states, regulate irregular product morphologies, and eliminate carbon dioxide (CO₂) emissions in this study. The simple and innovative method for the preparation of metallic vanadium via nitrogen-doped vanadium consumable anode (VC_xN_yO_z) electrolysis was proposed and confirmed. This research compared the polarization behavior and the reduction mechanism of vanadium ions to metallic vanadium of consumable anodes (VC_xN_yO_z and VC_xO_y). Nitrogen doping stabilizes the V²⁺ ion through sp²-hybridized C-N coordination, contributing to the formation of a stable [VN₆]^3- octahedral configuration facilitated by V–N bond coordination. This study introduces the VC_xN_yO_z anode as a viable option for regulating the valence state of vanadium ions in molten salt applications, achieving a regular dendritic morphology and a 30% reduction in CO emissions compared to the VC_xO_y anode.

Molten salts containing ZrCl₄ generally have a high vapor pressure. However, there are concentration ranges with a relatively low vapor pressure. And such melts are quite suitable for industrial use. In this work we have considered molten of MCl – ZrCl₄ mixtures (where M is an alkali metal) with a relatively low saturated vapor pressure (P ⩽ 1 atm) of highly volatile ZrCl₄. These mixtures can be divided into high-temperature regions with a ZrCl₄ concentration of 0–30 mol. % and low-temperature regions with a ZrCl₄ concentration of 50–75 mol. %. 

The aim of this work is to review the available experimental data on the electrical conductivity of ZrCl₄-containing salt melts with vapor pressures below atmospheric ones [1–6].

It was found that the electrical conductivity of all molten ZrCl₄ - containing mixtures increases as the temperature increases, zirconium tetrachloride concentration decreases, and the molten salt-solvent is replaced in a series from CsCl to LiCl.

As the concentration of zirconium tetrachloride in melts increases, the concentration of its relatively low-mobility complex anion groups, (in solutions with ZrCl₄ concentrations up to 33 mol.%) or and (in solutions with higher ZrCl₄ concentrations) also increases. This leads to a decrease in the concentration of the main current carriers: alkaline cations and mobile Cl^- ions, which are gradually replaced by bulky complex Zr(IV) groups that make a small contribution to the transfer of electricity. As a result, the electrical conductivity of molten mixtures decreases as the ZrCl₄ concentration increases.

The electrical conductivity of all studied molten mixtures decreases not only with an increase in the concentration of ZrCl₄, but also with a decrease in temperature as a result of a decrease in the mobility of ions (both simple and complex) and an increase in the viscosity of the melt. As a result, the electrical conductivity of high-temperature MCl - ZrCl₄ melts (M is an alkali metal, with 0 - 30 mol. % ZrCl₄) is in the range of 0.6 - 3.1 S/cm, is significantly higher than that of low-melting molten mixtures of the same chlorides (0.1 - 0.5 S/cm) with a high ZrCl₄ content (55 - 75 mol. %).

It was found that the use of low-melting salt solvents such as the LiCl - KCl eutectic allows for a significant (by hundreds of degrees) expansion of the existence range of ZrCl₄ - containing melts towards lower temperatures and saturated vapor pressures at sufficiently high electrical conductivity values ​​(0.9 - 2.8 S/cm), which provides additional advantages for various technological processes.

The electrical conductivity of the majority of molten salts increases with temperature. However, the rate of conductivity growth decreases with temperature. It is hypothesized that the electrical conductivity polytherms of all molten salts pass through their maximums in the temperature range from the melting point to the critical point [1–3]. For the majority of molten salts, the maximum of electric conductivity is difficult to achieve experimentally, because, as a rule, it is reached at the temperature when the value of the salt vapor pressure is equal to tens of atmospheres. 

It is known that there is a very small number of halides that have negative temperature coefficients of electrical conductivity, starting from the melting point of salt [1–4]. One such salt is InCl₃. According to Klemm et al. [4], the electrical conductivity of molten InCl₃ decreases with increasing temperature in the studied range of 867–967 K (i.e., immediately after melting). Our experimental data on the electrical conductivity of molten InCl₃, obtained using a specially designed hermetically sealed quartz cell of capillary type [5], in the temperature range of 862–1009 K confirm the Klemm’s information [4] on the negative temperature coefficient conductivity of the melt, but deviate by 2–7% at higher values:

k(S·cm^–1) = –5.408 + 5.2114·10^–2∙T – 5.6407·10^–5∙T² + 2.0028·10^–8·T³.

In addition to InCl₃, we have measured the electrical conductivity of molten ZrCl₄ and HfCl₄ for the first time. These salts exist in a liquid state only in narrow temperature ranges (68 or 17 K, respectively, from their melting points to the critical points):

ZrCl₄:   k·10⁴/S·cm^–1 = –2.0970·10³ + 8.7463∙T – 1.2119·10^–2∙T² + 5.5819·10^–6·T³

(in the temperature range studied 710–744.5 K the electrical conductivity decreases 1.8 times),

HfCl₄:   k·10⁶/S·cm^–1 = 735.06 – 1.9473∙T + 1.2966·10^–3∙T²

(in the temperature range studied 704.5–713.5 K the electrical conductivity decreases by about 1.17 times).

The following InCl₃, ZrCl₄ and HfCl₄ chlorides have a high vapor pressure already at the melting point (13–46 atm). These three salts form melts, which electrical conductivity falls on the descending branch of the general curve of electrical conductivity immediately after the melting [1, 2]. Possible causes of the anomalous dependence of electrical conductivity of molten salts on temperature are discussed.

Tantalum (Ta) has garnered significant attention due to its exceptional physicochemical properties and mechanical performance, enabling its widespread use in electronics, energy, metallurgy, and chemical industries. It is particularly strategic for high-temperature alloys, electronic components, and chemical processing equipment. Currently, the main methods for tantalum production include thermal reduction and molten salt electrolysis. Among them, molten salt electrolysis has emerged as a promising approach for the extraction and purification of refractory metals, owing to its advantages of low energy consumption, high product purity, and minimal environmental impact [1].

Several molten salt electrolysis methods have been developed for tantalum, including the FFC process (molten salt electro-deoxidation method), SOM process (solid oxygen-permeable membrane method), and the USTB process. The USTB method, pioneered by the University of Science and Technology Beijing, integrates carbothermal reduction with molten salt electrolysis. This two-step process employs metal carbon–oxygen solid solutions as soluble anodes to enable the efficient reduction and extraction of metals in a NaCl–KCl electrolyte. In 2006, Prof. Hongmin Zhu’s team first reported the successful electrolytic production of titanium metal using TiC_xO_y solid solutions as anode materials, laying a foundation for the advancement of soluble anode electrolysis technology [2].

In this approach, conductive compounds containing the target metal serve as soluble anodes, releasing metal ions into the molten salt under controlled potentials or current densities, followed by cathodic reduction and deposition [3]. Compared to traditional oxide anodes, soluble anodes offer advantages including higher energy efficiency, lower emissions, and sustained anodic dissolution. The core requirement lies in developing anode materials with excellent electrical conductivity and controllable electrochemical dissolution behavior. Carbon–oxygen solid solutions release C and O in the form of CO or CO₂ gas during electrolysis, thus avoiding electrode passivation caused by carbon buildup—a common issue in conventional anodes—making them a topic of current research interest. Given the similar thermodynamic and electrochemical properties of tantalum and titanium, it is reasonable to anticipate that TaC_xO_y solid solutions also hold promise as anode materials for molten salt electrolysis. However, research in this area is still in its infancy, with limited studies on material design, electrochemical behavior, and process optimization.

Additionally, the electrolyte composition plays a critical role in determining current efficiency, electrode stability, and product quality. Existing molten salt systems are primarily categorized into chlorides, fluorides, and mixed fluoride–chloride salts. The nature of the ionic species significantly affects the coordination environment and electrochemical behavior of metal ions [4]. Studies have shown that introducing an appropriate amount of fluoride ions (F^-) into NaCl–KCl molten salts can form more stable coordination structures with metal ions, enhancing their solubility and electrochemical stability. This facilitates continuous cathodic reduction, thereby improving current efficiency and process controllability [5].

Building upon previous research and the urgent demand for green, efficient tantalum extraction, this study proposes two innovative strategies: (1) Development of a molten salt electrolysis system based on TaC_xO_y solid solution anodes: This aims to assess the feasibility and advantages of using TaC_xO_y as a soluble anode for tantalum extraction. (2) Fluoride ion regulation in the electrolyte: To address the low Ta ion concentration and poor reduction efficiency, we propose modulating the F^- content in the molten salt to precisely control the solubility and coordination environment of Ta ions, enhancing the anodic dissolution–cathodic deposition equilibrium and improving extraction efficiency.

Finally, TaC_xO_y solid solutions with excellent electrical conductivity and structural stability were successfully synthesized and validated as effective soluble anode materials for the efficient electrolytic extraction of metallic tantalum in a NaCl–KCl molten salt system. Experimental results revealed that in the absence of fluoride ions, tantalum ions exhibited high volatility, significant current fluctuations, and low electrolysis efficiency. Conversely, the introduction of KF significantly reduced tantalum volatilization, simplified the reduction pathway from a two-step to a one-step process, and substantially enhanced both deposition efficiency and current stability. The obtained electrolytic products consisted primarily of spherical tantalum clusters with fine particle size, uniform morphology, and high purity, further confirming the effectiveness of the proposed approach. Overall, the “soluble anode + fluoride ion regulation” strategy developed in this study presents a promising and sustainable route for the green and efficient extraction of tantalum.

In this work, we have studied the possibility of the electrochemical synthesis of titanium borides from ionic-organic melt based on imidazole: C₃H₄N₂ (t_m = 91°C) [1]. The solubility of (NH₄)₂TiF₆ and NH₄BF₄ in imidazole melts at 120°С reaches 5 and 10 wt.%, which makes it possible to carry out voltammetric studies and electrolysis experiments. In the binary system: C₃H₄N₂ (t_m = 91°C)–(NH₄)₂TiF₆ a new process, which is more electropositive than the decomposition of imidazole as been observed. The scan rate independence of potential characterizes the observed processes as reversible charge transfer. This process corresponds to the one-electron irreversible charge exchange: Ti(IV)/Ti(III). In the binary system C₃H₄N₂ –NH₄BF₄ only the decomposition of imidazole has been observed. In the ternary system C₃H₄N₂–(NH₄)₂TiF₆–NH₄BF₄ a new process, which is more electropositive than the decomposition of imidazole and more electronegative than the one-electron charge exchange: Ti(IV)/Ti(III) has been observed. 10 micron coatings have been obtained on nickel and stainless steel by the electrolysis from a C₃H₄N₂–(NH₄)₂TiF₆–NH₄BF₄ melt at 120°C at current densities of 20-40 mA/cm². The X-ray phase analysis of samples after the electrolysis ofan ionic-organic melt did not allow us to determine the composition of the coating because it was very fine-crystalline. In order to coarsen the crystal structure of the coating, the samples were annealed in a furnace at 600°C in Ar stream. The research has established that the XRD patterns of the products obtained by electrolysis C₃H₄N₂–(NH₄)₂TiF₆–NH₄BF₄ to nickel cathode at 120°C and after annealing at 600°С in an Ar stream exhibits peaks corresponds to the Ni (COD - 96-901-3025) and TiB (COD - 96-151-1333) [2]. TiB crystallizes in the orthorhombic Pnma space group. The sample was obtained without extraneous phases inclusions. On the basis of XRD analyses it may be assumed that the stoichiometry of the compound deposited from ionic-organic melts is TiB.

The (LiCl-KCl)_eut.- PbCl₂ melts can be used as a reaction medium for “soft chlorination” during pyrochemical reprocessing of spent nuclear fuel, as well as for electrolytic extraction and refining of metallic lead and its alloys.

Previously, we measured the specific electrical conductivity of molten (LiCl-KCl)_eut.- PbCl₂ mixtures in the entire concentration range and in the temperature range from the liquidus to 994 K. In this work we calculated the density and molar volumes of the quasi-ternary (LiCl−KCl)_eut. − PbCl₂ system in the entire range of PbCl₂ concentrations. This system may be used in a vast number of technological operations. The calculation was performed by an original method using experimental data [1]. Based on the obtained results, the molar electrical conductivity and its activation energy were also calculated.

It was found that the specific electrical conductivity of molten (LiCl-KCl)_eut. – PbCl₂ mixtures decreases with the addition of PbCl₂, while the molar electrical conductivity, on the contrary, increases. The molar electrical conductivity (Λ) of molten PbCl₂ is higher than the electrical conductivity of the LiCl-KCl eutectic by approximately 30%. For all molten mixtures, the ln Λ vs. 1/T dependence is not linear. For example, at 773 K, the molar conductivity has positive deviations from additive values (~ +1.6%). However, with increasing temperature, the deviations decrease, become negative and further increase in the negative direction with the increasing temperature (-5.3 % at 973K). This indicates different predominant mechanisms of electrical transport at different temperatures.

In general, the isotherms of molar conductivity in the (LiCl-KCl)_eut. - PbCl₂ system are close to linear. This indicates weak interaction in the system. The data on electrical conductivity and molar volumes of the (3LiCl-2KCl) - PbCl₂ melts are compared with those earlier obtained in our studies on the (3LiCl-2KCl) - CdCl₂ [2] and (3LiCl-2KCl) - SrCl₂ molten mixtures [3].

When adding PbCl₂, the activation energy of molar electrical conductivity decreases until the PbCl₂ concentration of 30-40% is reached and then increases. The available data on the structure of the melts [4] indicate that, along with simple ions, complex chloride anion groups of various compositions are present. These groups were also considered for the analysis of the results. A relatively complex behavior of deviations of molar electrical conductivity depending on the temperature, activation energy and the PbCl₂ concentration is associated with the presence of two complex-forming ions with almost equal ionic potentials (Li⁺ and Pb²⁺) in the system.

According to the obtained electrical conductivity data, the liquidus line of this system has been built. It is in good agreement with the literature data.