CO₂ electrolysis to value-added products is a promising technology to close the carbon cycle and sequester anthropogenic CO₂ into chemical feedstocks; increasing the current density for multi-carbon products is one of the requirements for practical implementation. [1] The use of gas diffusion electrodes (GDEs) that allow the CO₂ reduction reactions (CO₂RR) to occur at the solid-catalyst/liquid-electrolyte/gaseous-CO₂ interface, effectively accelerates the CO₂RR by solving the problem of the mass transport limitation due to the inherently low diffusion and solubility of CO₂ in water. However, CO₂RR at the three-phase interface is complex, and guidelines for catalyst design toward its high activity have not been fully established. Since macrostructure significantly influences activity, it is challenging to distinguish between surface reactivity at the microscale (molecular level) and material transport properties at the macroscale (three-phase interface level). This presentation summarises our recent studies on high-rate CO₂RR from the point of view of both novel electrocatalyst designs and appropriate electrode assembly.

We successfully applied various metal-doped covalent triazine frameworks (M-CTFs) as platforms for CO₂RR electrocatalysts on GDEs [2-5]. In addition, we conducted systematic first-principles calculations and found that this reaction selectivity correlates with the adsorption energy of the intermediates. As M-CTFs possess the same framework, it is possible to vary only the metal center (catalytic active center) with the identical macrostructure of the catalyst layer. Thus, as we can directly compare the catalytic activity of the metal centers, M-CTFs serve as a model catalyst for establishing design guidelines for gaseous CO₂ electrocatalysts consisting of single metal centers.

We also achieved the ultra-high-rate CO₂ electrolysis to multicarbon products (C₂₊) by designing the triple phase interface composed of ordinary materials. We successfully increased the partial current density for C₂₊ over cupric oxide (CuO) nanoparticles on gas diffusion electrodes in neutral electrolytes to a record value of 1.7 A/cm² [6,7]. Specifically, we highlighted that the thickness of the catalyst layers is a crucial parameter that impacts the maximum current density for C₂₊. Although the GDE and electrocatalyst used in this case are not unique, the optimized assembly elicits their potential.