The escalating atmospheric CO₂ concentration, driven by fossil fuel combustion, necessitates active CO₂ removal strategies. Direct air capture (DAC) offers flexibility in deployment but faces high energy penalties due to the low CO₂ concentration in air. Electrochemical methods present a promising approach for energy-efficient DAC, potentially integrating with renewable electricity. Converting captured CO₂ to valuable products, such as methane (CH₄), is particularly attractive. CH₄ is a crucial chemical feedstock, a hydrogen carrier, and a direct fuel source. Thermochemical methods, based on the Sabatier reaction (CO₂ + 4H₂ → CH₄ + 2H₂O), are thermodynamically favorable and achieve high conversion efficiencies. Electrochemical methods, while showing high Faradaic efficiencies in some cases, require deep reduction processes and may yield byproducts. This study proposes a hybrid electro-thermochemical device integrating electrochemical DAC and H₂ production with thermochemical methanation to enhance efficiency and cost-effectiveness.

Existing DAC technologies, including electrochemical approaches, are reviewed to highlight their energy efficiency and cost. The challenges of capturing CO₂ directly from air (due to its low concentration) compared to concentrated sources are discussed. Existing methods for CO2 conversion to CH4, including thermochemical and electrochemical methods, are compared in terms of their energy efficiency and selectivity. The literature reveals the potential benefits of combining electrochemical DAC with thermochemical methanation for improved performance.

The research employed a hybrid electro-thermochemical device consisting of three modules: a CO₂ absorption module, a BPMED module for simultaneous DAC and H₂ production, and a thermochemical methanation module. The BPMED module uses a bipolar membrane to generate H⁺ and OH⁻ ions, facilitating CO₂ release and H₂ production. The co-generated H₂ serves as a sweep gas, eliminating the need for vacuum pumping for CO₂ extraction. The electrochemical performance of the BPMED module was studied by varying the concentration of potassium ferro/ferricyanide (K₃/K₄[Fe(CN)₆]) and the current density. The influence of these parameters on CO₂ removal efficiency, H₂/CO₂ ratio, current efficiency, and energy consumption was evaluated. The integration of the BPMED module with a thermochemical methanation reactor was demonstrated using simulated and real air capture electrolytes. The CO₂ conversion efficiency, CH₄ production rate, and overall energy consumption were assessed under both conditions. Long-term stability tests were conducted to evaluate the device's performance over extended periods. The characterization of the gas composition (CO2, H2, CH4) was done using gas chromatography.

The integrated route demonstrated significant advantages over the decoupled route, resulting in a 37.8% reduction in DAC energy consumption and a 36.6% reduction in cost. The cost of CH₄ production was reduced by 12.6%. Experiments using a simulated electrolyte showed that at a current density of 140 mA cm⁻² and 300 mM K₃/K₄[Fe(CN)₆], the BPMED module achieved stable CO₂ and H₂ output rates (3.7 ml min⁻¹ and 17.0 ml min⁻¹, respectively) with a H₂/CO₂ ratio of 4.6:1. Methanation achieved a 91.1% CO₂ conversion efficiency, with an overall energy consumption of 6412.3 kJ mol⁻¹ CH₄. Under real DAC conditions, the BPMED module achieved stable CO₂ and H₂ production rates (4.1 ml min⁻¹ and 18.6 ml min⁻¹, respectively), with a H₂/CO₂ ratio of 4.5:1. Methanation under real DAC conditions resulted in a 97.3% CO₂ conversion efficiency and an energy consumption of 5206.4 kJ mol⁻¹ CH₄. The long-term stability tests (30 h) showed consistent CH₄ production and stable operation of the BPMED module. Analysis revealed that overpotentials constituted a significant portion (66.3%) of the total energy consumption, suggesting potential for improvement via optimized electrocatalysts and electrode design. A theoretical optimization, assuming improved electrocatalyst performance, predicted a 51.9% reduction in CO₂ release energy consumption and a 52.8% reduction in CH₄ production energy consumption.

The findings demonstrate the feasibility and potential of the proposed hybrid electro-thermochemical device for direct CH₄ production from air. The significant energy and cost reductions compared to decoupled methods highlight the advantages of the integrated approach. The high CO₂ conversion efficiency and stable CH₄ production observed in both simulated and real DAC conditions confirm the practical potential of the technology. The identification of overpotentials as a major source of energy loss points to key areas for future research and development. These results contribute to advancements in carbon capture and utilization technologies and could play a role in decarbonizing energy systems.

This study successfully demonstrated a proof-of-concept hybrid electro-thermochemical device for direct CH₄ production from air. The device achieved high CO₂ conversion efficiency and stable CH₄ production, with significant energy and cost reductions compared to decoupled methods. Future research should focus on optimizing electrocatalysts and electrode design to further reduce overpotentials and enhance the overall efficiency and cost-effectiveness of the technology. This work provides a promising pathway toward sustainable fuel production from atmospheric CO₂.

The study utilized a relatively small-scale device, and scaling up the technology for industrial applications would require further research. The current device uses commercial electrodes, which may not be optimal for the integrated process. Developing custom-designed high-performance electrodes could significantly improve the efficiency of the system. Further investigation into the long-term stability of the system under continuous operation with real atmospheric air would enhance the reliability and robustness of the technology.