(490a) Continuous CO Synthesis from the Air at Ambient Temperature and Pressure by an Integrated Direct-Air-Capture and Direct-Carbonate-Reduction System

The escalation in atmospheric CO₂ concentration has been identified as the primary driver of the climate change and concomitant environmental disruption. To solve these problems, CO₂ diminishment with capture and utilization of CO₂, preferably driven by renewable energies, are strongly required. For this purpose, we have developed a new system that integrates two units: direct-air-capture (DAC) and direct-carbonate-reduction (DCR). This system successfully demonstrated continuous and stable synthesis of carbon monoxide (CO) from atmospheric CO₂ [1].

DAC is one of the highly promising approaches to reduce atmospheric CO₂. However, conventional DAC techniques using CO₂ absorbers require high temperatures (>800˚C) to release the absorbed CO₂ gas [2] that will be converted to valuable chemical species in the subsequent process. Therefore, it would be difficult to drive these DAC devices, requiring frequent start-stop operations, by only time-fluctuating renewable energies. DCR could eliminate the high-temperature process for DAC by directly reducing carbonates in the liquid CO₂ absorber supplied from the DAC devices using electrochemical means. Furthermore, DCR operates at ambient temperature and pressure, in contrast to thermocatalytic CO₂ reduction processes that require high temperatures and pressures. However, most of previous DCR studies have used electrolytes with carbonate concentrations significantly higher than those at atmospheric equilibrium [3]. To address this, a DAC-DCR system has been reported to adopt a batch process to increase the carbonate concentration in the electrolyte to be equilibrated with the air over an extended duration [4]. However, the batch process requires switching operations for each batch, making the system more complex. In the present study, we have devised an innovative method, which enables the circulation of a CO₂-absorbing electrolyte between the DAC and DCR units, capturing CO₂ from the air and converting it into CO via electrochemical carbonate/bicarbonate reduction, thus realizing continuous operation at ambient temperature and pressure.

The DAC unit consists of a gas-liquid contactor, pumps, and CO₂ gas meters. The CO₂-absorbing electrolyte of a mixed aqueous solution of potassium carbonate (K₂CO₃) and potassium bicarbonate (KHCO₃) is sprayed into the gas-liquid contactor where air flow is supplied simultaneously. The CO₂ capture rate is determined from the difference between the CO₂ concentrations at the intake and exhaust of the air flow. A part of the electrolyte ejected from the gas-liquid contactor is transferred to the DCR unit, while the remainder is recirculated within the DAC unit. The DCR unit, composed of a carbonate-reducing electrolyzer, a power supply, pumps, and a CO/CO₂ gas meter, reduces the carbonate ion to CO gas. The electrolyzer assembly includes a nanoporous gold cathode formed by etching a gold-silver alloy, a Nafion™ 117 film solid electrolyte, and a platinum mesh anode. The CO concentration measured at the exhaust of the electrolyzer using a gas analyzer is converted to the CO synthesis rate.

To realize our concept of the continuous operation using the DAC-DCR system at a constant CO synthesis rate, it is crucial to balance the CO₂ capture rate and the CO synthesis rate. Therefore, both rates were monitored in real time with a time resolution of 1 s, and the CO synthesis rate was fine-controlled throughout the operation. As an initial state, the composition of the CO₂-absorbing electrolyte was adjusted to be close to that at atmospheric equilibration, leading to a CO₂ capture rate of 4×10^-8 mol/s. The CO synthesis rate was controlled by changing the current supplied to the DCR electrolyzer to match the CO₂ capture rate. This control successfully balanced the two rates over 1 h, as is clear from Fig. 1, showing continuous CO synthesis from CO₂ in the air. However, both rates slightly increased during the operation. One possibility for this is that the current control was insufficient to handle the slow response of the DAC-DCR integration. If the excess current is supplied to the DCR electrolyzer, the carbonate concentration in the CO₂-capturing electrolyte decreases with an increase in pH, which in turn further increases the CO₂ capture rate. An improvement of the current control makes it possible to balance the CO₂ capture rate and the CO synthesis rate with the desired CO production rate constant maintained over a long duration.

In conclusion, we demonstrated continuous and stable operation of the newly developed DAC-DCR system at ambient temperature and pressure, in contrast to conventional DAC and CO₂ reduction devices that require high temperatures and high pressure. This great feature enables intermittent operation, making the system suitable to be driven by time-fluctuating renewable energies including solar power.

References

[1] Y. Sakamoto, Y. Nishimura, Y. Mizutani, S. Mizuno, R. Hishinuma, K. Okamura, Y. Takeda, M. Iwasaki, “Continuous CO synthesis from ambient air by integrating direct air capture and direct carbonate reduction using an alkaline CO₂-absorbing electrolyte operating at room temperature”, Carbon Capture Sci. Technol., 12 (2024) 100225.

[2] D. W. Keith, G. Holmes, D. St. Angelo, K. Heidel, “A process for capturing CO₂ from the atmosphere”, Joule, 2 (2018) 1573-1594.

[3] Y. C. Xiao, C. M. Gabardo, S. Liu, G. Lee, Y. Zhao, C. P. O’Brien, R. K. Miao, Y. Xu, J. P. Edwards, M. Fan, J. E. Huang, J. Li, P. Papangelakis, T. Alkayyali, A. S. Rasouli, J. Zhang, E. H. Sargent, D. Sinton, “Direct carbonate electrolysis into pure syngas”, EES Catal., 1 (2023) 54-61.

[4] O. Gutiérrez-Sánchez, B. de Mot, N. Daems, M. Bulut, J. Vaes, D. Pant, T. Breugelmans, “Electrochemical conversion of CO₂ from direct air capture solutions”, Energy Fuels, 36 (2022), 13115-13123.