Electrocatalytic reduction of CO₂ with H₂O into C₂₊ compounds is a very attractive but challenging research target. Despite significant progress in this field, high Faradaic efficiency of C₂₊ products can only be achieved at low current density, limiting the yield of C₂₊ products. In a recent article published in Nature Catalysis, we present a new strategy to boost CO₂ eletroreduction to ethylene and ethanol through hydrogen-assisted C−C coupling. An ultrahigh current density of 1.6 A cm^-2 at C₂₊ (mainly ethylene and ethanol) Faradaic efficiency of 80% could be achieved for electrocatalytic CO₂ reduction in a flow cell over a fluorine-modified copper catalyst, which outperforms other reported electrocatalysts. The C₂₋₄ selectivity reaches 85.8% at a single-pass yield of 16.5%, significantly better than those reported for thermocatalytic hydrogenation of CO₂ under harsh conditions. 

Catalytic transformation of C1 molecules into C₂₊ compounds via controllable C−C coupling is the core of C1 chemistry. The synthesis of ethylene and ethanol from CO₂ is particularly attractive due to their versatility in the chemical and energy industries. Traditionally, the hydrogenation of CO₂ with H₂ by modified Fischer-Tropsch (FT)-synthesis catalysts could produce C₂₊ products at high temperatures and pressures. However, the selectivity of a specific C₂₊ product on the modified FT catalysts is low because of the limitation by Anderson-Schulz-Flory distribution. Moreover, the thermocatalytic hydrogenation of CO₂ suffers from the formation of considerable CO and CH₄. The activation of stable CO₂ molecules under mild conditions and the precise control of C−C coupling of C1 molecules are two of the biggest challenges in chemistry.

Electrocatalytic CO₂ reduction reaction (CO₂RR) with H₂O using renewable electricity offers a promising route for CO₂ utilization under ambient conditions. Better control of C₂₊ products via electrocatalytic CO₂RR may be possible because the C−C coupling on electrocatalysts proceeds via mechanism different from that in thermocatalytic reactions. Accompanying with CO₂RR, the hydrogen evolution reaction (HER) also occurs as a competitive reaction. The current consensus is that the inhibition of HER is essential to obtain high CO₂RR selectivity and C₂₊ selectivity. However, the catalysts developed based on this consensus are typically less active in spite of high selectivity, and the C₂₊ yield by electrocatalytic CO₂RR is generally lower than those reported for thermocatalytic processes. Therefore, it would be a significant step forward to develop a new methodology to accelerate the activity while keeping the high C₂₊ selectivity.

Here, we have succeeded in developing a powerful fluorine-modified copper catalyst for electrocatalytic CO₂RR to C₂₊ products with high selectivity at high activity. An ultrahigh current density of 1.6 A cm^-2 was achieved at a C₂₊ (mainly ethylene and ethanol) Faradaic efficiency of 80% in a flow cell, resulting in a C₂₊ formation rate of ~4.0 mmol h^-1 cm^-2, significantly higher than those reported so far. The C₂₊ selectivity on a molar carbon basis reaches 85.8% (ethylene 65.2%, ethanol 15.0%) with a single-pass C₂₊ yield of 16.5%. The comparison with thermocatalytic hydrogenation of CO₂ to C_2-4 products demonstrates that the present electrocatalytic system is significantly more selective for the formation of C₂₋₄ products, in particular ethylene and ethanol, on a comparable or higher level of C_2-4 yield.

From DFT calculations and in situ electrochemical attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIRS) measurements, we propose a new hydrogen-assisted C−C coupling mechanism for electrocatalytic CO₂RR to C₂₊ products. In brief, CO₂ is first reduced to *CO via *COOH intermediate, and then the hydrogenation of *CO forms *CHO intermediate that subsequently undergoes C−C coupling to *OCHCHO*, leading to the formation of ethylene. The *CO and *CHO species are key reaction intermediates. The activation of H₂O to unique H species is an important step. The presence of fluorine on copper surface promotes water dissociation, CO adsorption and the formation of *CHO intermediates.

The present work not only presents a highly active and selective electrocatalytic CO₂ reduction catalyst with potential for practical applications, but also offers a new methodology for precisely controlling and promoting the C−C coupling of C1 molecules.

Article link: https://www.nature.com/article...