Solubility of CO2 in molten Li2Co3-LiCl

Solubility of CO2 in molten Li2CO3-LiCl was measured by using a
 pressure differential method and the enthalpy change of the solution was
 calculated on that basis. The relationships between the solubility and the
 enthalpy change, and the temperature and the composition of the melts were
 discussed. The results showed that when the temperature was 873-923K and the
 Li2CO3content was 10-50 mol%, the solubility of CO2 increased with decreasing
 temperature and/or increasing Li2CO3 content.The maximum solubility was 3.965
 ? 10?7gCO2/gmelt at 873Kwhen the content of Li2CO3 was 50 mol%. The solution
 of CO2was exothermic. With increasing temperature and Li2CO3content,more
 enthalpy was needed for CO2 solution.


Introduction
Carbon fuels have been exploited rapidly as human industrialisation has progressed. This has resulted in large emissions of CO2, the most concerning greenhouse gas.
Accordingly, there has been much attention on how to effectively decrease CO2 emissions and carbon fuel use.
Electrochemical transformation is an important method for CO2 utilisation. A higher temperature is more helpful for the electrochemical reduction of CO2 from the viewpoint of both thermodynamics and kinetics. Therefore, molten salt is a better medium for the electrochemical transformation of CO2 than is an aqueous solution.
Furthermore, molten salt often has a wider electrochemical potential window. As a result, the molten salt CO2 capture and electrochemical transformation (MSCC-ET) process has become a research hotspot. This process produces carbon and oxygen, with high value-added carbon materials obtained by improving the electrode, electrolyte and/or process conditions [1][2][3], and oxygen generation achieved from utilising solar energy, enabling humans to breathe and survive in outer space [4][5].
Wakamatsu et al. studied the solubility of CO2 in molten LiCl-Li2O using the mass measurement technique, reporting a 95% solution of the molar quantity of Li2O into the molten salts in the range of 0-60 mol% Li2O at 923 K [20]. Our previous study on the same melts using Raman spectroscopy indicated that the solubility of CO2 was 0.1105 gCO2/gmelt for the melts containing 8 wt% of Li2O at 923 K and the conversion rate of Li2O to Li2CO3 was 94.19% [17].
Shi et al. studied the solubility of CO2 in molten LiF-Li2CO3. The maximum solubility was 6.8 × 10 −4 molCO2/molmelt at 913 K when the mole fraction of LiF was 50% [21]. Deng found the absorption of CO2 in molten LiCl-KCl to be negligible; however, CO2 could be rapidly captured when Li2O or CaO was added into molten LiCl-KCl, with a conversion efficiency of Li2O to Li2CO3 of around 94%. CO2 solubility in molten Li-Na-K carbonates was much higher than in molten LiCl-KCl, with approximately 6 mmol CO2 absorbed by 100 g Li-Na-K carbonate. More time was required to reach the absorption equilibrium in molten carbonates with CaO or Li2O, and the conversion efficiency of Li2O was approximately 45% [22].
On the other hand, considering the laborious experimental conditions to determine the CO2 solubility, some theoretical models were developed and used to model the CO2 solubility in the electrolyte, such as Statistical Associating Fluid Theory (SAFT) [23,24], Cubic-Plus-Association (e-CPA) [25,26], and Mixed-Solvent Electrolyte (MSE) [27]. However, these models have all been applied for the aqueous systems or ionic liquids and few models were focused on the molten salts systems.
Compared with other molten salts systems used in MSCC-ET process, molten chloride-carbonates are getting more attention for their lower operating temperature and cost. However, there have been few research publications on the solubility of CO2 in molten chloride-carbonates. In the present study, the solubility of CO2 in molten Li2CO3-LiCl was studied using the pressure differential method, and the effects of temperature and Li2CO3 content were comprehensively determined. Data on the thermodynamics of CO2 solution was also calculated.

Experimental
The purity and company of the regents are listed in Table S1. All solid chemicals were dried at 423 K for 24 h to remove water and then stored in an argon-filled glove box with water and oxygen contents less than 1 ppm.
The solubility of CO2 in molten Li2CO3-LiCl was measured using a pressure differential method. The apparatus used for the experiment is shown in Figure 1. The same apparatus and method were used to measure the solubility of CO2 in molten Li2CO3-LiF [21]. The molten salts sample was held in a corundum crucible which was placed in a stainless steel container. The container was put in a furnace connecting to a temperature controller for heating to a certain temperature. The valves were used to control gas flows, the two pumps were for evacuating the air, and the digital pressure gauge was for reading the pressure. The volume ( ) of the container and connecting tube was measured using CO2 according to the following procedure. First, the volume ( 1 ) in the bellow was measured by the water displacement method. Second, the air in the whole apparatus was evacuated and the container and connecting tube filled with CO2, giving a pressure reading of 0 . Third, Valve 8 was opened to allow CO2 to flow into the bellow, giving a pressure reading of 0 , . Finally, the value was calculated using equation (1).
With thus obtained, the concentration of CO2 in the melts could be measured.
First, a sample with a certain mass (m) was placed in the container and heated to the target temperature. The volume of the melt was calculated by knowing the value of m and the density, which was measured using the Archimedes' principle [28]. Second, CO2 was allowed to flow in by opening Valve 7. Finally, the Soave-Redlich-Kwong equation [29] of state was used to calculate the number of moles of CO2 dissolved into the melt. Then, the number of moles was converted to the mass of CO2.
Results and discussions

Determination of dissolution equilibrium duration time
To determine the dissolution equilibrium duration, the pressure changes in the container for the Li2CO3-LiCl melts with different compositions were measured at 873 K, as shown in Figure 2 (The related data were listed in Table S2). When the pressure in the container remained constant, CO2 dissolution had reached equilibrium.
The reaction between CO2 and the melts is given in equation (2) [30]. The amount of CO2 dissolved in the melts increased with increasing time. However, after 60-80 minutes there was little further change, indicating that dissolution equilibrium had been reached. In the following solubility measurements, the pressure was read after 80 minutes.

Solubility of CO2 in molten Li2CO3-LiCl
The relationship between the solubility of CO2 in the melts with various compositions and temperature is shown in Figure 3 (The related data were listed in Table S3).
Solubility decreased with increasing temperature, which indicated the equilibrium between Li2CO3 and Li-C2O5-Li in equation (2) shifted left. Therefore, the temperature must not be set too high when Li2CO3-LiCl melts are used as the medium for CO2 electrolysis. Additionally, solubility increased as Li2CO3 content increased, indicating the very low solubility of CO2 in the molten chloride, whereas the molten carbonate allowed CO2 dissolution. The effect of LiCl in the melts was only to decrease the liquidus temperature and thereby decrease the working temperature.
Overall, the solubility of the Li2CO3-LiCl melts was much lower than that of the Li2O−LiCl melts (approximately 10 -5 times lower), in the studied ranges of temperature and sample compositions. The maximum solubility of CO2 was 3.965 × 10 −7 gCO 2 /gmelt at 873 K when the content of Li2CO3 was 50 mol%.
where, 2 is the equilibrium pressure of CO2 (MPa) and Ф 2 ( , ) is the fugacity coefficient of CO2, which can be calculated using the Soave method [31].
The relationship between and Li2CO3 content in the melts at different temperatures is shown in Figure 4 (The related data were listed in Table S4).  Figure 4 it can be seen that the value of was negative; therefore, CO2 dissolution was an exothermic process. The value of decreased with increasing temperature and Li2CO3 content, which indicated more enthalpy was needed for CO2 solution. From equation (1),

Conclusions
The solubility of CO2 in molten Li2CO3-LiCl with 10-50 mol% Li2CO3 content at 873-923 K was measured using a pressure differential method. This also enabled the enthalpy change of solution to be calculated. The following additional conclusions were drawn: (1) When the temperature was decreased and the concentration of Li2CO3 was increased, the solubility of CO2 increased, with a maximum solubility of 3.965 × 10 −7 gCO 2 /gmelt at 873 K when the content of Li2CO3 was 50 mol%.
(2) CO2 dissolution in the Li2CO3-LiCl melts was an exothermic process. The enthalpy change of CO2 solution decreased with increasing temperature and Li2CO3 content.

Conflicts of interest
There are no conflicts of interest to declare.