Capturing carbon dioxide from air with charged-sorbents

The study addresses the need for low-cost, low-temperature-regenerable sorbents for direct air capture (DAC) of CO2. Conventional hydroxide-based DAC processes (aqueous KOH or solid Ca(OH)2) require high regeneration temperatures (~900 °C), driving high energy use and costs. The authors hypothesize that electrochemical insertion of reactive hydroxide ions (OH−) into porous, conductive activated carbons can create 'charged-sorbents' that chemisorb CO2 at low partial pressures and can be regenerated at low temperatures. Furthermore, because the sorbents are electrically conductive, they may be regenerated rapidly via direct Joule heating using renewable electricity, enabling a fully electrified DAC process.

Hydroxide-based scrubbers are industrially mature for DAC but require high-temperature regeneration (~900 °C) due to carbonate lattice energies and often rely on natural gas. Dispersing hydroxides within porous matrices can lower regeneration temperatures. Hydroxide-functionalized metal-organic frameworks have demonstrated promising DAC performance at ~100 °C regeneration, but suffer from limited stability and high sorbent costs. Amine-based sorbents often have oxidative stability challenges. Electrically driven temperature swing approaches using conductive supports have been explored for rapid regeneration, motivating a conductive sorbent that can be directly Joule-heated without auxiliary supports. The work builds on electrochemical ion insertion in porous carbons (supercapacitor charging mechanisms) to create tailored sorbents.

Sorbent synthesis: Activated carbon cloth electrodes were immersed in 6 M KOH(aq) and positively polarized at +0.565 V vs SHE for 4 h to accumulate hydroxide ions within nanometre-sized pores via electric double-layer formation and faradaic processes. After charging, electrodes were removed, washed to minimize residual KOH/salts, and dried to yield the positively charged-sorbent PCS-OH. A negatively charged control (NCS-K) was prepared by applying −0.235 V vs SHE for 4 h to accumulate K+. Characterization: Powder X-ray diffraction confirmed absence of crystalline KOH, indicating pore-confined OH−. 1H solid-state NMR showed a strong ~1.5 ppm resonance for PCS-OH consistent with pore OH species; controls showed weaker signals. Acid–base titration required 1.2 mmol g−1 HCl to neutralize PCS-OH vs 0.2 mmol g−1 for NCS-K, consistent with higher OH content. Combustion analysis indicated increased hydrogen content in PCS-OH. N2 sorption gave a BET surface area of 920 m2 g−1 for PCS-OH vs 1,175 m2 g−1 for blank cloth (~20% reduction) attributed to pore occupancy by OH− and electrochemical oxidation of surface groups. Gas sorption and calorimetry: CO2 adsorption isotherms at 25 °C demonstrated enhanced low-pressure uptake for PCS-OH relative to controls (blank, soaked cloth, NCS-K, and a conventionally impregnated 'dripped cloth'). Microcalorimetry measured adsorption heats: for PCS-OH, −137 kJ mol−1 at zero coverage decreasing to −33 kJ mol−1 at 0.8 mmol g−1, indicative of CO2 chemisorption via (bi)carbonate formation; blank cloth showed −28 to −20 kJ mol−1, consistent with physisorption. Stability testing: Thermogravimetric analysis (TGA) under flowing dry air at 150 °C for 12 h showed minimal change in CO2 capacity and kinetics. Repeated TGA cycling (150 cycles) under 30% CO2 in N2 indicated stable performance. A ~10% capacity loss was observed after 14 months of storage under pure CO2 test conditions. Solid-state 13C NMR DAC mechanistic studies: Quantitative 13C MAS NMR with 0.9 bar CO2 dosing resolved chemisorbed (bicarbonate-like) and physisorbed species, enabling quantification of chemical vs physical adsorption contributions and supporting bicarbonate formation at OH− sites. Direct air capture tests: DAC was evaluated by TGA under simulated dry air (400 ppm CO2, 30 °C; desorption in N2 at 130 °C). Ambient air box tests (37% RH) with an in situ CO2 sensor compared PCS-OH and controls. Joule heating regeneration: Electrodes were directly contacted (no external support) and a 7–8 V DC bias applied across a 2 × 1 cm PCS-OH piece, heating to ~90 °C within ~1 min. 13C NMR on 13CO2-predosed samples confirmed complete CO2 release after Joule heating. Multi-cycle DAC with ambient air at varying RH employed Joule heating regeneration under N2 between cycles. Humidity impact: DAC capacities were measured across RH of 11–38%. Additional 13C NMR at 0.9 bar CO2 with RH 53% and 85% probed reduced chemisorption under humid conditions. H2O uptake isotherms and heat capacity (0.62 J g−1 K−1) were measured. Minimum electrical energy for sorbent heating during desorption at 90 °C was estimated (6.5–11.4 GJ per ton CO2 for 11–38% RH).

• Electrochemical charging inserts reactive hydroxide ions into activated carbon pores, creating PCS-OH with accessible chemisorption sites for CO2.
• CO2 uptake: At 0.4 mbar CO2 and 25 °C, PCS-OH achieved 0.26 ± 0.06 mmol g−1 (n=10), substantially exceeding control samples.
• DAC capacity under simulated dry air (400 ppm CO2, 30 °C): ~0.2 mmol g−1, stable over repeated cycles.
• Ambient air box test (37% RH): 120 mg of PCS-OH reduced CO2 from ~500 ppm to ~25 ppm within ~25 min; controls showed much smaller decreases.
• Adsorption thermodynamics: Microcalorimetry showed heats decreasing from −137 kJ mol−1 at zero coverage to −33 kJ mol−1 at 0.8 mmol g−1 for PCS-OH, consistent with bicarbonate formation at a distribution of OH− sites; blank cloth exhibited −28 to −20 kJ mol−1 typical of physisorption.
• Stability: After heating to 150 °C in dry air for 12 h, CO2 capacity and kinetics were maintained. 150 adsorption–desorption cycles under 30% CO2 in N2 showed stable cycling. A ~10% capacity loss over 14 months was observed under pure CO2 test storage.
• Joule heating regeneration: Direct application of 7–8 V to a 2 × 1 cm PCS-OH sample heated it to ~90 °C in ~1 min and fully released adsorbed CO2 (confirmed by 13C NMR). Multi-cycle DAC with Joule heating regeneration demonstrated reversible capture.
• Humidity effects: DAC capacity decreased from ~0.14 to ~0.08 mmol g−1 as RH increased from 11% to 38%. 13C NMR at 0.9 bar CO2 showed reduced chemisorption at RH 53% and 85% (0.19 and 0.21 mmol g−1 vs 0.95 mmol g−1 at 0% RH). Joule heating restored capacity after humid exposure.
• Energy estimate for regeneration: Minimum electrical energy for sorbent heating at 90 °C was estimated as 6.5 GJ ton−1 CO2 at 11% RH and 11.4 GJ ton−1 CO2 at 38% RH (≈1,800 and 3,200 kWh ton−1 CO2), comparable with ranges for existing DAC processes, with the advantage of full electrification.

The results demonstrate that electrochemical insertion of OH− into porous, conductive carbons produces a hydroxide-functionalized 'charged-sorbent' that chemisorbs CO2 via (bi)carbonate formation, markedly enhancing low-pressure uptake relevant to DAC. The sorbent exhibits good oxidative and thermal stability and can be regenerated at low temperatures. Critically, its intrinsic electrical conductivity enables rapid, direct Joule heating regeneration without auxiliary conductive supports, opening a pathway to a fully electrified DAC process powered by renewable electricity. Although humidity reduces capacity by filling hydrophilic pores and limiting access to OH− sites, capacities are recoverable by Joule heating, indicating reversible water competition rather than sorbent degradation. The approach is general and tunable through choice of electrode carbon and electrolyte, suggesting broad applicability in separations and catalysis. Increasing hydroxide loading correlates with higher low-pressure CO2 uptake, pointing to clear levers for performance improvement.

Charged-sorbents created by electrochemical insertion of hydroxide ions into activated carbons provide low-cost, tunable materials that capture CO2 from ambient air via chemisorption to form (bi)carbonates and regenerate at 90–100 °C. Their conductivity enables direct, rapid Joule heating regeneration without additional conductive supports, facilitating a fully electrified DAC process. The materials show promising oxidative stability and repeated cycling performance. Future work should focus on increasing sorbent CO2 capacity (e.g., optimizing pore structure and OH− content), mitigating humidity-induced capacity losses (e.g., pore environment engineering or hydrophobic/hydrophilic balance), improving energy efficiency of regeneration, and scaling synthesis to practical modules. The tunability of electrodes and electrolytes suggests a family of designer sorbents for diverse separation and catalytic applications.

• Humidity sensitivity: Water co-adsorption in hydrophilic pores reduces CO2 capacity (e.g., ~0.14 to ~0.08 mmol g−1 from 11% to 38% RH), likely via pore blocking; although reversible, it impacts practical DAC performance in humid air.
• Capacity retention over time: Approximately 10% capacity loss after 14 months under pure CO2 test conditions indicates some long-term decline.
• Moderate saturation capacity: While low-pressure uptake is enhanced, overall capacities are lower than some benchmark sorbents, limiting energy efficiency.
• Surface area reduction: ~20% BET surface area decrease relative to blank cloth suggests potential trade-offs from electrochemical oxidation and pore occupancy by OH−.
• Energy consumption: Estimated electrical energy for regeneration (6.5–11.4 GJ ton−1 CO2) remains significant; further optimization is required to improve efficiency.