Relationships between the Charge-Discharge Methods and the Gas Adsorption Performance of a Supercapacitive Swing Adsorption Module for CO2 Separation
Authors: Zhu Shan , Ma Kangling , Landskron Kai 
Published: February 2020

Introduction: Supercapacitive swing adsorption (SSA) is a gas separation technology that is based on selective adsorption and desorption of a gas from a gas mixture upon capacitive charging and discharging activated carbon electrodes.1,2 SSA is a promising technology due to its facile reversibility, significant sorption capacity, complete gas selectivity, and energy efficiency. In this study, we simplified the cell design and enlarged the electrode area of SSA modules. These new cell features are expected to increase the scalability of SSA technology. We defined metrics that allow for a quantitative evaluation of the energetic and adsorption performance of SSA cycles. Using these metrics, we quantitatively determined the influence of different electrical charge-discharge methods on the energetic and CO2 adsorption performance, and identified the most favorable charge-discharge method.

Methods: The cross-sectional view of a SSA module is shown in Figure a. The SSA module was operated in gas flow-through mode and fed by a moistened 15% : 85% CO2 : N2 gas mixture serving as a flue gas simulant. In the SSA module, the gas was distributed by radial gas flow system, in which gas flowed from the inlet port in the center to the outlet port in the periphery of the SSA module. This design eliminated the use of gas flow channels, and simplified SSA module configuration. The electrode area was 49 cm2, which was 25 times of that in our previous study2. The CO2 concentration of SSA effluent gas was analyzed and recorded. The electrochemical behavior of the SSA module was controlled and measured using a potentiostat in a two-electrode configuration. Four different charge-discharge methods at voltages between 0 and -1 V were used to evaluate SSA performance, namely GCD (galvanostatic charging-discharging, Figure b), GCD+Pstat (GCD with potentiostatic holding steps between each GCD step, Figure c), GCD+OC (GCD with open-circuit holding steps between each GCD step, Figure d), and Combined (galvanostatic charging followed by holding step at open circuit, and galvanostatic discharging followed by potentiostatic holding step at 0 V, Figure e).

Results: We introduced energetic and adsorption metrics to quantitatively evaluate SSA performance for CO2 adsorption. These metrics are specific capacitance, Coulomb efficiency, energy efficiency, energy loss, sorption capacity, electron efficiency, energy consumption, adsorption rate, and time-energy efficiency. Using these metrics, we determined how different charging methods affect the energetic and adsorption performance of SSA module. The galvanostatic charge-discharge method was most time-energy efficient due to its shortest charge time and lowest energy loss. The highest time-energy efficient for CO2 adsorption (7.5 µmol.kJ-1.s-1) was obtained with the galvanostatic charge-discharge methods at 50 mA. Increasing the constant charging current improved the energetic performance but decreased CO2 adsorption capacity. The introduction of holding steps favored CO2 adsorption at the cost of higher energy loss. A holding step of 30 min duration was sufficient for the full completion of CO2 adsorption and desorption. The highest CO2 adsorption capacity (87.5 mmol.kg-1), highest electron efficiency (0.2 molecule.electron-1), lowest energy consumption (109 kJ.mol-1) and highest CO2 adsorption rate (34 µmol.kg-1.s-1) were obtained by Combined method with 30 min holding steps.

Conclusions: We demonstrated a simplified SSA module with enlarged electrode area, which is expected to benefit the scale-up of SSA technology for CO2 adsorption and separation. Our results provide systematic method to evaluate SSA performance and improve the understanding of the relationships between energetic properties and gas adsorption performance of SSA. This findings is critical to understanding the mechanism of SSA phenomena and to exploring SSA application.

References

1 B. Kokoszka, N. K. Jarrah, C. Liu, D. T. Moore and K. Landskron, Angew. Chemie - Int. Ed., 2014, 53, 3698–3701.
2 C. Liu and K. Landskron, Chem. Commun., 2017, 53, 3661–3664.

Figure 1