An Artificial-Photosynthetic System Consisting of Solar-Driven Reaction and Product Isolation Processes to yield only Pure Formic Acid

We proposed a concept of an artificial-photosynthetic system to produce pure formic acid using only CO₂, water, and solar energy, generating no waste.¹ The system consists of a solar-driven reaction process and a product isolation process. We proved the concept by small-scale experiments, and constructed a practically large-sized reactor as the first step toward widespread use of artificial photosynthesis.^2,3<br/>We adopted a single-compartment configuration of the solar-driven reactor without ion-exchange membranes and a near-neutral pH electrolyte of a potassium phosphate buffer aqueous solution to simplify the reactor structure. The large-sized reactor was composed of eight-stacked cathode/anode-electrode pairs of 1 m×1 m in size and powered by crystalline-silicon solar cells on the reactor housing via a DC-DC converter. The raw material of 100% CO₂ gas was dissolved in the electrolyte and supplied to the cathode-electrode surfaces uniformly with the help of well-designed flow channels. The electrochemically synthesized formic acid was dissolved in the electrolyte and ejected from the reactor. We developed a Ru-complex-based cathode catalyst and a IrO<i>_x</i>-based anode catalyst, and achieved a Faradaic efficiency as high as 96% for the formic-acid synthesis at an extremely low operating voltage of 1.65 V (overpotential of only 0.22 V) and a large current of 65 A, resulting in a high solar-to-chemical energy conversion efficiency of 10.5%. In addition, we tackled to improve both activity and durability of the cathode and anode electrodes,^4,5 and to realize direct reduction of CO₂ in a flue gas to eliminate CO₂ capture processes.⁶ Further, we designed tandem solar modules suitable for direct coupling with the reactors using organic-inorganic hybrid perovskite solar cells.⁷<br/>The next challenge was isolation of the synthesized formic acid dissolved in the aqueous electrolyte. The formic acid cannot be isolated by distillation because of its boiling point (101 °C) almost the same as that of water. Thus, we developed an isolation process on the basis of reactive extraction, including three sequential steps.¹ The first step was reactive extraction of the formic acid from the electrolyte using an extraction solution composed of an organic base (trioctylamine, TOA, boiling point 366 °C) as an extractant and an organic solvent (dichloromethane, DCM, boiling point 40 °C) as a diluent. Although formic acid is highly miscible in water, it was extracted into DCM because the formic acid formed a complex salt with TOA that is insoluble in water. The second step was removal of DCM from the mixture of the extracted formic acid, TOA, and DCM produced in the first step, by evaporation at 40 °C. Finally, the residue was heated at 160 °C for isolation of the formic acid by distillation in the third step. The large differences in the boiling points among these three chemicals secured few contaminations in the second and third steps. In addition, the composition of the electrolyte was tuned for promotion of the formation of the formic acid-TOA complex salt. Thus, we achieved an over 90% isolation yield of pure formic acid. The new electrolyte was confirmed to lower the conversion efficiency by only 10%(relative) and to secure similarly high durability compared with the original electrolyte. The great feature of the isolation process is that all the electrolyte, TOA, and DCM are reused for the next synthesis and isolation after the formic acid is isolated. Thus, we established a highly sustainable artificial-photosynthetic system that consumes no chemicals other than the raw materials of CO₂ and water, or generates no waste.<br/>References<br/>1. M. Shiozawa, et al., in preparation.<br/>2. N. Kato, et al., Joule 5, 687 (2021).<br/>3. N. Kato, et al., ACS Sustain. Chem. Eng. 9, 16031 (2021).<br/>4. M. Shiozawa, et al., Electrocatalysis 13, 830 (2022).<br/>5. N. Kato, et al., in preparation.<br/>6. Y. Takeda, et al., J. CO₂ Util. 71, 102472 (2023).<br/>7. Y. Takeda, J. Appl. Phys. 132, 075002 (2022).