CO_2熔盐电化学转化碳材料的电化学特性

基于对熔融碳酸盐体系中电化学还原CO_2所得碳材料(electrolytic-carbon,EC)的形貌、结构、组成的认识,以粉末微电极循环伏安法测试为基础,在稀溶液中对EC的本征电化学行为进行了考察,以揭示这类碳材料的界面电化学特性。实验发现,在典型条件(450°C、4.5 V槽压)下制备的电解碳(450°C-4.5 V-EC)的伏安行为有别于多壁碳纳米管、石墨烯、石墨、乙炔黑等常见碳材料,在负电位区表现出显著的"双电层充放电响应迟滞"现象。通过考察溶液pH值、电位扫描速率、阴阳离子种类对这一现象的影响,发现pH和电解液组成都不影响这一现象的出现;电解液浓度提高和低扫描速率时滞后现象减弱,表明迟滞充放电是这类碳材料的本征特性,与其表面含氧官能团及其对阳离子的特性吸附密切相关。实验进一步研究了不同电解条件下制备的EC所展现的电化学特性吸附及电容性质,发现随着熔盐温度的升高,EC对电解液中阳离子的特性吸附能力降低,而相同温度不同槽压下制备的EC特性吸附能力相近,表现出相似的电容特性,这与EC的含氧量和比表面积有关。电解碳所展现的独特电容特性对其潜在的应用或可提供有价值的线索和指导。

Electrochemical Features of Carbon Prepared by Molten Salt Electro-Reduction of CO2

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.