In situ electrochemical conversion of CO ₂ in molten salts to advanced energy materials with reduced carbon emissions

Abstract

Fixation of CO2 on the occasion of its generation to produce advanced energy materials has been an ideal solution to relieve global warming. We herein report a delicately designed molten salt electrolyzer using molten NaCl-CaCl2-CaO as electrolyte, soluble GeO2 as Ge feedstock, conducting substrates as cathode, and carbon as anode. A cathode-anode synergy is verified for coelectrolysis of soluble GeO2 and in situ–generated CO2 at the carbon anode to cathodic Ge nanoparticles encapsulated in carbon nanotubes (Ge@CNTs), contributing to enhanced oxygen evolution at carbon anode and hence reduced CO2 emissions. When evaluated as anode materials for lithium-ion batteries, the Ge@CNTs hybrid shows high reversible capacity, long cycle life, and excellent high-rate capability. The process contributes to metallurgy with reduced carbon emissions, in operando CO2 fixation to advanced energy materials, and upgraded conversion of carbon bulks to CNTs.

Electrochemical Reduction of Carbon Dioxide and Iron Oxide in Molten Salts to Fe/Fe3C Modified Carbon for Electrocatalytic Oxygen Evolution

“…The co‐reduction of CO 2 and Fe 2 O 3 shows a much higher cathodic current efficiency (Supporting Information, Figure S7a). In the mixed molten salt, Li 2 CO 3 is essential for CO 2 conversion due to the thermodynamically more favorable decomposition of Li 2 CO 3 to Li 2 O rather than Li metal [11, 13, 14] . As shown in the Supporting Information, Figure S7b, LiFeO 2 is detected as the intermediate.…”

Electrochemical Fixation of Carbon Dioxide in Molten Salts on Liquid Zinc Cathode to Zinc@Graphitic Carbon Spheres for Enhanced Energy Storage

“…In one case, the liquid Zn was soaked in the 450 °C Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 molten salt for 3 h without electrolysis to understand the chemical reactions between Zn with the molten salts. The current efficiency (η, %) and energy consumption (E C , kWh kg C −1 ) were calculated by the following equations [8]…”

“…[57] Therefore, restraining the capacitive process by increasing the graphitization degree of CO 2 -derived carbon is one way to further enhance the aluminum storage of Zn@C. It is acknowledged that the graphitization of the CO 2 -derived carbon herein needs to be further increased. The strategy on tailoring the microstructure of the derived carbon during electrochemical reduction of CO 2 in molten salts have been well addressed, [6] with higher graphitization of carbon occurring in the presence of liquid metal (the herein protocol), sulfur oxides, [18,59] elevated temperature, [58,59] higher overpotential, and metallic catalysts [8,58] Another way to further improve the capacity is to optimize the content of Zn in Zn@C. As shown in Figure 5c and Figure S9b in the Supporting Information, the "Zn@C (6.4 wt% Zn)" possesses much higher capacity than "Zn@C (25 wt% Zn)," revealing that optimizing the Zn content is an effective way to further enhance the energy storage ability of CO 2 -derived carbon. Excess content of Zn is not beneficial for enlarging the overall capacity of Zn@C because of the low capacity of pure Zn powder (Figure 5c).…”

“…[1][2][3][4][5] CO 2 is also an exploitable carbon and oxygen resource. [6][7][8][9][10][11][12] Fixation of CO 2 into valuable chemicals could simultaneously realize carbon reduction and reutilization of CO 2. [6,13] For example, high-efficiency fixation of CO 2 enabling a high capacity of ≈123 mAh g −1 at 5 A g −1 , a high Coulombic efficiency of more than 98%, and a long-term stability up to 10 000 cycles as well as a high rate performance.…”

Electrochemical Reduction of Carbon Dioxide and Iron Oxide in Molten Salts to Fe/Fe3C Modified Carbon for Electrocatalytic Oxygen Evolution

“…The acid-soluble metallic Fe still remains even after leaching in 0.5 M H 2 SO 4 solution for 8 h (see the Experiment Section in the Supporting Information), implying that the metallic Fe is wrapped by the more acid-resistant Fe 3 C. The metallic Fe prepared by the electrolysis of Fe 2 O 3 alone (Supporting Information, Figure S5) and the Fe/Fe 3 C prepared by the combustion of Fe/Fe 3 C-MC in air (Supporting Information, Figure S6) both show morphology of microsize nodules, highlighting the important role of the CO 2derived carbon matrix on restraining Fe species from sintering and agglomeration. [11,13,14] As shown in the Supporting Information, Fig Figure S7c), resulting in a higher current efficiency of the coreduction process. Figure S7c also shows that Fe is firstly generated in the co-reduction, with the following deposition of carbon over Fe.…”