We have developed a new scheme of gas-liquid electrochemical reaction to convert CO₂ to formic acid, and achieved a Faradaic efficiency (FE) as high as 70% in a gas flow of 10% CO₂ concentration. This value is extremely higher than that around 30% of conventional electrochemical reaction in a CO₂-solved electrolyte.

 Electrochemical reduction of CO₂ to hydrocarbon fuels is one of the promising technologies to produce useful chemicals in addition to fix discharged CO₂. Among various products [1], formic acid is transportable liquid, and thus it is suitable also to store renewable energies including solar and wind powers. A polymeric ruthenium (Ru) complex, Ru(C3-pyrrol-dcbpy)(CO)₂Cl₂, is an excellent catalyst in high selectivity of formic acid production with a low overvoltage when it is used in an electrolyte of a saturated CO₂ solution [2]. However, the selectivity, i.e., the FE would lower under a low CO₂ concentration, because of a low CO₂ diffusivity in the electrolyte and resultant insufficient supply of the reactive substrate.

 To improve the supply capacity of CO₂, we adopted a carbon paper loaded with the polymeric Ru complex [3] as a cathode gas diffusion electrode (GDE), and supplied CO₂ gas directly to the GDE. We used an H-type two-compartment cell with a three-electrode configuration to evaluate the activity of the GDE, which was sandwiched by the two half-cells with an aperture of 1 cm square. One of the half-cell, in which a Pt anode electrode and an Ag/AgCl reference electrode were placed, was filled with a 0.1 M phosphate buffer electrolyte (pH6.9). CO₂ gas of various concentrations (5-100 vol.%) diluted with N₂ was supplied in the other half-cell. A chronoamperometry was carried out at a constant potential of -0.37 V vs. RHE for 60 min. at each CO₂ concentration. The electrolyte was sampled twice every 30 min to quantify the formic acid production by ion-chromatography.

 The electrochemical cell using the cathode GDE realized CO₂ reduction to formic acid over the whole CO₂ concentration range. The FEs were higher than 0.8 at 100% CO₂ for both the GDE scheme and a conventional electrochemical cell using a Ru-carbon-paper cathode electrode immersed in the electrolyte. However, the FE of the conventional cell rapidly decreased with decreasing CO₂ concentration, and eventually reached only 0.3 at 10% CO₂. In contrast, high FE was secured using the GDE. The FE was as high as 0.7 at 10% CO₂, because CO₂ is sufficiently supplied from the diluted ambient gas through the thin GDE. Immediate collection of the produced formic acid to the electrolyte also ensures the continuous reaction with a high FE.

 The new scheme developed here that works in a low CO₂-concentration gas with a high FE opens the door toward the practical application of electrochemical fixation and recycling of CO₂ in industrial discharge.