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

Abstract

We constructed an integrated system comprising direct air capture (DAC) and direct carbonate reduction (DCR) to facilitate the industrial implementation of negative CO2 emissions. The DAC-DCR system demonstrated continuous CO synthesis from ambient air at room temperature, in contrast to conventional methods, including high-temperature processes and previously reported a batch-type method that connect DAC and DCR. The CO2-absorbing electrolyte, a K2CO3-KHCO3 aqueous solution, is circulated between the DAC and DCR subsystems. The CO2 captured from the air with the CO2-absorbing electrolyte is converted to carbonate in the DAC; subsequently, the carbonate is electrochemically reduced to CO in the DCR. This system monitored the CO2 capture rate, carbonate reduction rate, and pH of the CO2-absorbing electrolyte in real time. Controlling the carbonate reduction rate by adjusting the DCR reactor current enabled the balancing of carbon capture and carbonate reduction rates. Consequently, stable and continuous operation of a DAC-DCR system was achieved for over one hour for the first time. The DAC-DCR is more suitable for combination with intermittent renewable electricity, including photovoltaic and wind electricity, than the previous methods because all the processes in the system work at room temperature. Thus, the present proof-of-concept study is an important step toward the widespread use of DAC.