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.
Received: 15 January 2018
Published: 12 February 2018
Fund: the National Natural Science Foundation of China(21673162);the National Natural Science Foundation of China(51325102);the International Science & Technology Cooperation Program of China(2015DFA90750)
Table 1Specific surface area and oxygen content of different EC.
Fig 1 Metallographic micrograph of powder microelectrode before (a) and after (b) filled with carbon, and the measurement of the microcavity depth (c), (d).
Fig 2 The schematic diagram of the powder microelectrode and the experimental setup.
Fig 3 Cyclic voltammogram of different carbon materials. Condition: in 10 mmol·L?1 Na2SO4 aqueous solution at 50 mV·s?1.
Fig 4 Cyclic voltammogram of 450 ℃-4.5 V-EC after annealing at 550 ℃ under argon atmosphere. Condition: in 10 mmol·L-1 Na2SO4 solution at 50 mV·s-1.
Fig 5 Cyclic voltammograms of 450 ℃-4.5 V-EC in different pH of 10 mmol·L-1 Na2SO4 aqueous solution at 50 mV·s-1 (a) and in 10 mmol·L-1 Bu4NClO4 at 100 mV·s-1 (b).
Fig 6 Cyclic voltammograms of 450 ℃-4.5 V-EC at different scan rate in 10 mmol·L-1 Na2SO4 aqueous solution (a) and at the scan rate of 50 mV·s-1 in 100 mmol·L-1 Na2SO4 aqueous solution (b).
Fig 7 The schematic diagram of different ion distribution state on EC surface. No polarization (OCP), the adsorption of cations (?0.5 V vs. SCE), ion exchange (?0.1 V vs. SCE) and desorption of large amount of cations (0.1 V vs. SCE), respectively.
Fig 8 Cyclic voltammogram of 450 ℃-4.5 V-EC in 10 mmol·L-1 (NH4)2SO4 (a) and 10 mmol·L-1 Na2S2O8 aqueous solution (b). The scan rate is 50 mV·s?1.
Fig 9 Cyclic voltammograms of EC synthesized at different electrolysis temperature (a) and different cell voltages (b). The scan rate is 50 mV·s?1 in 10 mmol·L?1 Na2SO4 aqueous solution.
Bai X. F. ; Chen W. ; Wang B. Y. ; Feng G. H. ; Wei W. ; Jiao Z. ; Sun Y. H. Acta Phys. -Chim. Sin. 2017, 33, 2388.