Abstract:

The molten salt CO₂ capture and electrochemical transformation (MSCC-ET) process is a potentially efficient method for CO₂ utilization, which can convert CO₂ into value-added carbon and oxygen with a current density of 100–1000 mA cm^-2. The electrolytic carbon (EC) prepared through the MSCC-ET process is highly electrically conductive and forms flexible microstructures. These structures show excellent adsorption ability towards environmental pollutants and high energy storage capacity when used in supercapacitors. Although the morphology, structure, and application of EC prepared under different electrolysis conditions have been previously reported, their intrinsic electrochemical properties have not yet been elucidated. Powder microelectrodes (PMEs) are useful for studying the electrochemical kinetics of various powdery materials. In this study, we systematically investigated the electrochemical properties of ECs obtained using molten Li₂CO₃-Na₂CO₃-K₂CO₃ under different temperature and electrolysis voltage conditions by cyclic voltammetry (CV) with a carbon powder microelectrode in 10 mmol L^-1 Na₂SO₄. The electrochemical behavior of the EC obtained at 450 ℃ and a cell voltage of 4.5 V (450 ℃-4.5 V-EC) differs significantly from that of other carbon materials, i.e., multi-walled carbon nanotubes, graphene, graphite, and acetylene black. In addition to a much larger charging-discharging capacity, unusual hysteresis of the charge/discharge current response of ECs in the negative potential region (-0.6 to -0.2 V vs SCE) was observed. This phenomenon was eliminated by annealing the material under Ar at 550 ℃, demonstrating that the unique electrochemical behavior of ECs is closely related to the oxygen-containing groups on its surface. Furthermore, CVs of EC-PME were compared in solutions with different pH, Na₂SO₄ concentrations, and other ions. The pH of the solution did not affect the CVs, excluding a redox mechanism involving the surface functional groups. Hysteresis was weakened by a certain degree at slower potential sweep speeds (< 10 mV s^-1) or in higher concentrations of electrolyte (100 mmol L^-1 Na₂SO₄). The onset potential for discharging was negatively shifted in electrolytes with a larger cation ((NH₄)₂SO₄) and was unaffected by larger anions (Na₂S₂O₈). This indicates that the hysteresis is more likely related to the specific adsorption of cations, caused by the unique surface properties of EC. It should be noted that the specific surface area and oxygen concentration of EC can be adjusted by the electrolysis temperature and cell voltage. Generally, the Brunauer–Emmett–Teller (BET) specific surface area and oxygen content decrease with increasing temperature and the BET-area increases with increasing cell voltage. The CVs of ECs prepared at different cell voltages were similar, but the adsorption capacity decreased for those prepared at higher temperatures (550 and 650 ℃). Interestingly, the specific capacitance of the ECs is much higher at negative potentials (-0.6 to 0 V vs. SCE) than that at positive potentials (0 to 0.6 V vs. SCE). Therefore, it is anticipated that a better capacitance performance can be achieved when the ECs are used as a negative electrode material in supercapacitors.

Key words: Electrolytic-carbon, Molten salts, Powder microelectrode, Electrochemical property, Specific adsorption