Carbon Capture: Garlic Roots Power Breakthrough in Supercapacitive Swing Adsorption

There is an urgent need to address climate change, primarily driven by CO₂ emissions from fossil fuel combustion. This has led to an intensive worldwide research efforts towards developing efficient and cost-effective carbon capture methods. Electrochemical CO₂ capture methods, particularly those harnessing supercapacitive swing adsorption (SSA), are gaining traction due to their lower energy requirements and compatibility with renewable energy sources. Unlike conventional methods, SSA leverages the use of activated carbon electrodes, which are robust, inexpensive, and environmentally friendly, while offering the benefits of longevity, high energy efficiency, and rapid charge/discharge cycles. It is believed that SSA relies mostly on an ionic liquid-solid adsorptive mechanism in which CO₂ hydrolyses in the aqueous electrolyte of the supercapacitor to form H⁺, HCO₃^–, and CO₃^2- ions which adsorb electrostatically to the negative and positive electrode, respectively. This process is selective for CO₂, and easily reversible upon discharge of the electrodes.

In a new study published in the Journal Small by PhD candidate Muhammad Bilal, Jiajie Li, Dr. Hao Guo, and led by Professor Kai Landskron from the Lehigh University, the researchers conducted a series of innovative experiments to explore the efficiency of SSA using garlic roots-derived activated carbon (GR-AC) for carbon dioxide capture. The team synthesized GR-AC using potassium carbonate and air as activators. This process was an improvement over traditional methods, offering a cheaper, less toxic, and more efficient way to produce activated carbon. The electrodes were then prepared by mixing this activated carbon with a binder and solvent, followed by hot-pressing to achieve the desired thickness and mass loading. They characterized GR-AC using nitrogen adsorption-desorption isotherms. The results showed a combination of type I and type IV isotherms, indicating the presence of both micro and mesopores. This porosity is crucial for effective CO₂ adsorption. The specific surface areas and pore volumes were also determined. The team conducted galvanostatic charge-discharge (GCD) cycles while measuring CO₂ concentrations to test the SSA performance of the GR-AC electrodes. These experiments demonstrated that charging the supercapacitor led to CO₂ adsorption, while discharging resulted in CO₂ desorption. The researchers varied the voltage windows to observe the effects on CO₂ capture and release. Moreover, the adsorption capacity and energy consumption of the GR-AC electrodes were evaluated at different voltage levels. Remarkably, at 1.0 V, the electrodes achieved a record adsorption capacity of 312 mmol kg⁻¹ with an energy consumption of only 72 kJ mol⁻¹. Increasing the voltage to 1.4 V further increased the adsorption capacity to 524 mmol kg⁻¹, albeit with higher energy consumption (130 kJ mol⁻¹). The researchers tested the cycle stability of the GR-AC electrodes for over 130 hours to ensure their durability and consistent performance over time.  They assessed the electrochemical behavior and performance limits of the GR-AC electrodes using cyclic voltammetry and electrochemical impedance spectroscopy. These tests confirmed the capacitive nature of the electrodes and identified the optimal voltage window for SSA operation.

The Lehigh University team’s innovative approach hinges on the use of activated carbons derived from garlic roots. These activated carbons are produced using potassium carbonate and air as activators in a simple, cost-effective process. This method not only leads to a significant enhancement in CO₂ sorption capacity but also maintains low energy consumption and high cycle stability.  The pivotal role of the voltage in this process cannot be overstated. The new study demonstrated that increasing the voltage window leads to enhanced CO₂ capture without significantly compromising energy efficiency. This is attributed to the superior properties of the GR-AC electrodes, which are prepared by an optimized synthesis method and subsequent heat treatment to introduce functional groups that improve capacitance. These electrodes exhibit excellent adsorptive and energetic performance, even at high mass loadings. The findings suggest that the micro-mesoporous structure of this activated carbon conducive to efficient CO₂ capture.

In terms of both energy and adsorptive metrics, the GR-AC electrodes outperform previous SSA electrode materials. They exhibit remarkable gravimetric and volumetric adsorption capacities, attributable to their high specific capacitance values. Up to 1.4 V  voltage windows, there are no significant parasitic redox processes that lead to unacceptable energy losses or electrode degradation, indicating their potential for practical applications. The capacitive electrochemical behavior was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. The study also compares the performance of GR-AC electrodes with traditional BPL carbon electrodes, highlighting the superiority of the former in terms of surface area, pore volume, and adsorption capacities. This comparison underscores the importance of electrode material and preparation method in enhancing SSA performance.

In conclusion, the research work by Professor Kai Landskron and colleagues represents an important advancement in carbon capture technologies. The new high-performance GR-AC electrodes for SSA is a promising route towards achieving efficient, scalable, and cost-effective CO₂ capture. The simplicity of the electrode preparation process, coupled with the remarkable performance metrics, positions this approach as a viable solution in the pursuit of a sustainable and carbon-neutral future.