Large-Scale Artificial-Photosynthetic Production of Formate and Isolation of Pure Formic Acid
Authors: Kato Naohiko , Mizuno Shintaro , Shiozawa Masahito , Kikuzawa Yoshihiro , Nojiri Natsumi , Morikawa Takeshi , Takeda Yasuhiko 
Published: August 2024

We proposed a concept of an artificial-photosynthetic system to produce pure formic acid using CO2, water, and solar energy, generating no waste, as illustrated in Fig. 1.1 The system consists of an artificial-photosynthetic reaction process and a product isolation process. We proved the concept by small-scale experiments, and constructed a practically large-sized solar-driven CO2 electrolyzer as the first step toward widespread use of artificial photosynthesis.2,3

We adopted a single-compartment configuration of the solar-driven CO2 electrolyzer 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 electrolyzer was composed of eight-stacked cathode-anode-electrode pairs of 1 m×1 m in size and powered by crystalline silicon solar cells installed on the electrolyzer 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 electrolyzer. Insertion of nanoporous separator between the facing anode/cathode electrodes prevented O2 bubbles produced on the anodes from reaching the cathodes, suppressing the crossover reaction of O2 reduction. We developed a novel Ru complex polymer (RuCP)-based cathode catalyst loaded on carbon supports and an IrO
x
-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%. This significant performance was realized by the low-resistive Ti substrates of the cathode and anode electrodes, sufficient CO2 supply to whole of the cathode surfaces, etc., which have greater impacts on a larger electrolyzer, in addition to the high catalytic activity of the RuCP and IrO
x
. Further, we tackled to improve both activity and durability of the anode and cathode electrodes,4,5 and to realize direct reduction of CO2 in a flue gas to eliminate energy-consuming CO2 capture processes.6 Moreover, we designed tandem solar modules suitable for direct coupling with the electrolyzers using organic-inorganic hybrid perovskite solar cells.7

The next challenge was isolation of the synthesized formic acid dissolved in the aqueous electrolyte. The formic acid cannot be isolated by distillation because its boiling point (101 °C) is virtually 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 (tri-n-octylamine, 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 at 0.03 MPa. Finally, the residue was heated at 160 °C at 0.02 MPa 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 energy 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.
References

M. Shiozawa,et al., Energy & Fuels, submitted.

N. Kato, et al., Joule, (2021) 5, 687.

N. Kato, et al., ACS Sustain. Chem. Eng.,(2021) 9, 16031.

M. Shiozawa, et al., Electrocatalysis, (2022)13, 830.

N. Kato, et al., submitted to 245th ECS Meeting Abstract, Symposium I04: Electrosynthesis of Fuels 8 (2024).

Y. Takeda, et al., J. CO2 Util., (2023)71, 102472.

Y. Takeda, et al., J. Appl. Phys., (2022)132, 075002.

Figure 1