Surface-Immobilized Rhodium Complex in Vertically Aligned Mesoporous Silica Films for Direct Electroreduction of Diluted CO ₂

Performing direct CO 2 reduction reaction (CO 2 RR) from flue gas streams containing low-concentrated CO 2 (4–25% v/v) represents an opportunity to obtain added value products while reducing anthropogenic emissions. The heterogenization of molecular complexes offers a pathway to scale up CO 2 RR systems, especially under CO 2 diluted conditions where the reactant is mass transport limited and the competitive hydrogen evolution reaction (HER) might be boosted. In this work, a formate-selective rhodium complex ([Rh(bpy′)(Cp*)Cl]Cl, where bpy′ = 4-(2-propyn-1-yloxymethyl)-4′-methyl-2,2′-bipyridine and Cp* = pentamethylcyclopentadienyl) is immobilized at high loading within tridimensional (3D) vertically aligned mesoporous silica films (VAMSF) electrochemically grown on glassy carbon electrodes. The resulting modified 3D electrodes allow direct CO 2 RR under both pure and diluted (10% v/v) CO 2 streams, remaining selective for formate production in both organic and aqueous media and minimizing the contribution of HER from the support. In acetonitrile/1% H 2 O, Faradaic efficiencies (FE) for formate production of 66% in 100% CO 2 and 47% in 10% CO 2 were achieved, while in KHCO 3 (pH= 6.8), FE of 47% and 43% were reached under pure and diluted CO 2 streams, respectively. Finally, in a more acidic aqueous solution (pH= 3.8), FE of 59% was achieved under pure CO 2 . The electrodes work without preactivation or auxiliary overlayers and show recyclability and stability over multiple electrolysis cycles, indicating no structural degradation under operation conditions. To our knowledge, this is the first formate-selective heterogenized molecular complex tested under a diluted CO 2 gas stream. These results reveal VAMSF as an attractive platform for bringing molecular CO 2 RR catalysis closer to realistic applications.