A hybrid electro-thermochemical device for methane production from the air

The study addresses the challenge of converting atmospheric CO₂ into renewable methane (CH₄) by integrating direct air capture (DAC) with hydrogen (H₂) production and thermochemical methanation. Rising atmospheric CO₂ levels (∼423 ppm in 2024) drive the need for negative emissions and CO₂ utilization. DAC is flexible but energy intensive due to low CO₂ concentration; electrochemical DAC methods offer operation at ambient conditions and compatibility with renewable electricity. Converting captured CO₂ to CH₄ is attractive because CH₄ is a versatile chemical feedstock, an effective H₂ carrier leveraging existing natural gas infrastructure, and can be used directly as fuel. The research proposes and investigates a hybrid electro-thermochemical device that simultaneously captures CO₂ and produces H₂ in a single bipolar membrane electrodialysis (BPMED) module, then feeds them directly to a thermochemical methanation reactor. The central hypothesis is that co-generating CO₂ and H₂ in the same compartment enables H₂ to act as a sweep gas that lowers CO₂ partial pressure, enhancing CO₂ release without vacuum stripping, thereby reducing energy use and costs while achieving the 1:4 CO₂:H₂ feed ratio required for Sabatier methanation.

The paper summarizes DAC energy requirements, noting that capturing CO₂ from air (∼400 ppm) carries a thermodynamic penalty roughly 3.7× that of flue gas capture (12% CO₂), with practical DAC processes estimated at 4.5–12.2 GJ tCO₂⁻¹ versus 0.8–5.6 GJ tCO₂⁻¹ for concentrated sources. Electrochemical CO₂ capture is highlighted for potential energy and cost advantages at ambient conditions. Thermochemical methanation (Sabatier reaction) is thermodynamically favorable with >90% carbon conversion under ∼1 bar and 300 °C, but requires green H₂. Electrochemical CO₂-to-CH₄ typically requires deep reductions (8e⁻) with kinetic/selectivity challenges, though a recent BPM-based bicarbonate-fed system reported >70% Faradaic efficiency for CH₄ at 100–750 mA cm⁻² under alternating current operation, still requiring downstream treatment to recover H₂ and unreacted CO₂. Bipolar membranes can alleviate catalyst pH incompatibility and enable performance gains; BPM, AEM, and PEM electrolyzers have comparable voltage requirements. This background motivates integrating BPMED-based DAC with H₂ generation and thermochemical methanation to reduce energy and cost relative to decoupled DAC and electrolysis.

System concept and configuration: Two routes are compared: (i) a decoupled route with separate BPMED DAC (pH swing) and water electrolysis for H₂, requiring vacuum-assisted CO₂ stripping; and (ii) an integrated route using a single BPM-based electrochemical reactor (BPMED) to simultaneously acidify and basify streams, cogenerate CO₂ and H₂ in the cathode compartment, and directly feed a thermochemical methanation reactor. In the integrated route, fast reversible redox couples (K₃/K₄[Fe(CN)₆]) replace electrode water-splitting at lower current densities to reduce cell voltage and allow controlled onset of HER/OER at higher currents. The H₂ produced serves as a sweep gas to lower CO₂ partial pressure and enhance CO₂ release without vacuum. Electrochemistry and reactions: A Fumasep bipolar membrane (FBM-PK) splits water at the junction to generate H⁺ and OH⁻ (water dissociation). Acidified catholyte releases CO₂ from carbonate/bicarbonate, while the basified anolyte absorbs CO₂ from air. Electrode reactions at low current density are reversible one-electron [Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻ redox. At higher current density, HER (2H⁺ + 2e⁻ → H₂) and OER (4OH⁻ → O₂ + 2H₂O + 4e⁻) initiate as the redox couple depletes. The target gas ratio is CO₂:H₂ ≈ 1:4 for methanation (CO₂ + 4H₂ → CH₄ + 2H₂O). Device architecture and operation: The hybrid device comprises (i) a CO₂ absorption module (air bubbled through KOH base tank), (ii) a BPMED module with acid and base recirculation loops, and (iii) a fixed-bed methanation reactor with Ru/Al₂O₃ catalyst at 320 °C. Initial tests used simulated equilibrium electrolyte (100 mM KOH in equilibrium with 400 ppm CO₂, yielding 54.32 mM KHCO₃ and 22.80 mM K₂CO₃) spiked with 100–300 mM K₃/K₄[Fe(CN)₆]. Electrolyte flow rate: 2 ml min⁻¹; in-cell recirculation: 30 ml min⁻¹. Methanation temperature maintained at 320 °C by ceramic heater. Real DAC absorption: Indoor air (∼1750 ± 90 ml min⁻¹) dehydrated upstream of a mass flow meter is bubbled through 1000 ml of 100 mM KOH using a bubble disk in a base tank (ID 14.5 cm; disk effective diameter ~9 cm). After ~83 h, 860 ml carbonated electrolyte (alkalinity increased to 116.3 mM due to 14% water loss) is obtained. K₃/K₄[Fe(CN)₆] is then added to 300 mM for BPMED operation. BPMED module construction: Home-built single-stack cell with anode (base) and cathode (acid) compartments separated by FBM-PK. Each compartment volume ~5.06 ml, cross-sectional area 20.25 cm² (4.5 cm × 4.5 cm), channel thickness 0.25 cm. Electrodes: anode Ti mesh with Ru–Ir coating (8 µm); cathode Ti mesh with Pt coating (1 µm); electrode dimensions 4.5 cm × 4.5 cm × 0.1 cm. Gas/liquid outlets ~0.25 cm diameter. Operation typically at 120–140 mA cm⁻² for integrated CO₂/H₂ generation. Thermochemical methanation module: Quartz tube reactor (8 mm OD, 6 mm ID) with fiberglass wool packing and 0.2 g Ru/Al₂O₃ catalyst. Reactor placed inside 10 mm ID alumina ceramic heating tube; Type K thermocouple measures temperature; manual control maintains ~320 °C. Gas from BPMED acid tank (CO₂ + H₂) is fed directly to the reactor without intermediate separation. Analytics and calculations: Outlet gas compositions measured by GC (after argon dilution and flow-controlled sampling). Carbon removal efficiency ε_removal = r_CO2(g)/DIC × 100%. Current efficiency ε_current = (I_CO2 + I_H2)/I_applied × 100%. Energy consumption computed from measured cell voltage and gas production rates. Techno-economic analysis compares decoupled vs integrated routes, accounting for membrane counts, ohmic losses, vacuum stripping, and capital/operating costs (details in Supplementary Notes/Data).

- Integrated vs decoupled route: The integrated BPMED route eliminates vacuum-assisted CO₂ stripping and reduces membrane count, yielding a predicted 37.8% reduction in DAC energy consumption and a 36.6% reduction in DAC cost (from 372.7 to 236.3 $ tCO₂⁻¹). The levelized CH₄ production cost decreases by 12.6% (from 3065.0 to 2679.3 $ tCH₄⁻¹). - BPMED performance (simulated electrolyte, 54.32 mM KHCO₃, 22.80 mM K₂CO₃, K₃/K₄[Fe(CN)₆] 100–300 mM): • Carbon removal efficiency increases with current density and redox couple concentration; at 300 mM and 120 mA cm⁻², ε_removal reaches 96.0 ± 9.5%. At 40 mA cm⁻² and 300 mM, ε_removal ~80.9 ± 17.6%. • Acidified stream pH drops below 4 at >40 mA cm⁻²; basified stream pH increases correspondingly, enabling continued air capture in the base tank. • CO₂ output rate saturates due to DIC depletion (e.g., 3.4 ± 0.4 ml min⁻¹ at 80 mA cm⁻²; 3.5 ± 0.2 ml min⁻¹ at 140 mA cm⁻² with 300 mM redox). H₂ output rate rises monotonically with current (0.2 ± 0.1 to 17.2 ± 0.4 ml min⁻¹ from 20 to 140 mA cm⁻² at 300 mM). • H₂/CO₂ ratio is tunable via current density and redox concentration; values near 4:1 are achievable (e.g., 120 mA cm⁻² at 300 mM yields target ratio). • Current efficiency approaches ~100% at high current densities; losses at low currents attributed to co-ion leakage and acid-base speciation reactions. • Energy consumption for CO₂ release ranges from 119.4 ± 1.4 to 640.1 ± 40.0 kJ mol⁻¹ (300 mM, 10–140 mA cm⁻²). At 100 mM, 372.3 ± 18.4 to 551.0 ± 7.6 kJ mol⁻¹ (10–40 mA cm⁻²). - Cascaded methanation (simulated electrolyte, 300 mM redox, 140 mA cm⁻²): BPMED outlet stabilized at CO₂ 3.7 ml min⁻¹ and H₂ 17.0 ml min⁻¹ (H₂/CO₂ = 4.6). Methanation at 320 °C produced CH₄ at 3.4 ml min⁻¹ with CO₂ conversion of 91.1%; exhaust contained 48.7% unreacted H₂ (H₂ conversion 79.5%). Electric energy for BPMED to produce CH₄: 6412.3 kJ mol⁻¹ CH₄ (includes both CO₂ release and H₂ production). Long-term operation (30 h) showed stable outputs and BPMED cell voltage <6.0 V; separate stability run reported CH₄ ~3.6 ml min⁻¹ and CO₂ conversion 96.1%. - Real DAC operation: Air absorption into 100 mM KOH yielded carbonated electrolyte (alkalinity 116.3 mM). BPMED at 140 mA cm⁻², flow 2 ml min⁻¹, recirculation 30 ml min⁻¹ produced CO₂ ~4.1 ml min⁻¹ and H₂ ~18.6 ml min⁻¹ (H₂/CO₂ = 4.5). Methanation delivered CH₄ ~4.0 ml min⁻¹ with average CO₂ conversion 97.3%. BPMED cell voltage averaged 5.05 V over ~170 min. Energy consumption: CO₂ release 704.0 kJ mol⁻¹; H₂ production 967.4 kJ mol⁻¹; CH₄ production energy 5206.4 kJ mol⁻¹. - Energy repartition (Sankey): Overpotentials constitute the dominant share of electrical input (66.3% overall: 12.2% for CO₂ release, 54.1% for H₂ production), with additional ohmic losses and reversible enthalpy contributions. Geometric constraints (narrow channels, small outlet), bubble accumulation, and non-optimized electrocatalysts contribute to losses. - Optimization potential: Assuming state-of-the-art electrocatalysts (e.g., NiFe/NF for OER, CoP@NC for HER) and reduced BPM water dissociation overpotential (∼100 mV), projected reductions are 51.9% for CO₂ release energy (704.0 → 338.4 kJ mol⁻¹) and 52.8% for CH₄ production energy (5206.4 → 2459.4 kJ mol⁻¹).

The integrated BPMED–methanation approach directly addresses the challenge of coupling DAC with CH₄ synthesis by co-generating CO₂ and H₂ in a single compartment. The in situ H₂ sweep lowers CO₂ partial pressure, enhancing CO₂ desorption without the need for vacuum pumps or membrane contactors, thereby reducing both energy consumption and capital/operating costs relative to decoupled DAC plus electrolysis. The BPMED leverages pH-swing for efficient CO₂ absorption and release while providing an amphoteric environment that facilitates water splitting for H₂ production. Crucially, the device allows tuning of the CO₂:H₂ ratio to the stoichiometric 1:4 required for the Sabatier reaction, minimizing downstream separations and enabling high CO₂ conversion (>90%, up to 97.3% under real DAC conditions). The findings demonstrate feasibility with stable operation and show that overpotentials, largely from gas bubble effects and suboptimal catalysts, dominate energy losses. System-level implications include simplified plant design, lower ohmic resistance due to fewer membranes/compartments, and reduced DAC energy and cost. The analysis indicates substantial headroom for improvement by optimizing electrode materials, hydrodynamics to mitigate bubble coverage, BPM water dissociation kinetics, and reactor flow geometries, which together could halve the energy intensity toward more competitive renewable CH₄ production.

This work introduces a hybrid electro-thermochemical device that integrates BPMED-based DAC/H₂ co-generation with thermochemical methanation to produce CH₄ directly from air. Compared to decoupled DAC and electrolysis, the integrated route reduces DAC energy by 37.8% and DAC cost by 36.6%, lowering CH₄ production cost by 12.6%. Experiments verified tunable CO₂:H₂ ratios near 1:4, high CO₂ conversion (up to 97.3%) at 320 °C over Ru/Al₂O₃, and stable 30 h operation. Real-DAC tests reported energy consumptions of 704.0 kJ mol⁻¹ for CO₂ release, 967.4 kJ mol⁻¹ for H₂ production, and 5206.4 kJ mol⁻¹ per mol CH₄ produced, with optimization potential to reduce these by roughly half through improved electrocatalysts, BPM performance, and reactor hydrodynamics. Future research should focus on: (i) engineering electrodes and catalysts to reduce HER/OER overpotentials and bubble coverage, (ii) optimizing BPM junction layers to accelerate water dissociation at low loss, (iii) redesigning flow fields and channel/outlet geometries to enhance gas disengagement and lower ohmic/transport losses, (iv) integrating efficient H₂/CO₂/CH₄ separations and recycle to increase overall efficiency, and (v) scaling techno-economic assessments under renewable electricity coupling and real-world operating conditions.

- Overpotentials dominate energy consumption (∼66% of electrical input), driven by bubble formation, limited gas disengagement (narrow 0.25 cm channels and outlets), and non-optimized commercial electrodes; these increase cell voltage and energy intensity. - Current efficiency is below 100% at low current densities due to co-ion leakage across BPM layers and acid-base speciation reactions; performance approaches 100% only at high currents. - CO₂ output rate saturates at higher current densities due to DIC depletion; tuning feed DIC and residence time is necessary for scale-up. - Gas composition at methanation outlet retains significant unreacted H₂ in some tests (∼49%), requiring downstream separation/recycle to improve energy utilization. - Electrocatalyst choices (Pt cathode, Ru–Ir anode) are not optimized for the mixed electrolyte (e.g., HER onset shifted negatively by ~0.5 V in presence of 300 mM K₃/K₄[Fe(CN)₆] + 50 mM H₂SO₄), inflating overpotentials. - Potential catalyst poisoning by airborne impurities (CO, NO₂, SO₂) is acknowledged but not studied; concentrations are low in lab tests, yet real-world impacts need evaluation. - Lab-scale device with small compartments and limited measurements (some metrics measured once) may not capture long-term degradation phenomena or scale-dependent transport effects.