Selective Catalytic Electroreduction of CO₂ at Silicon Nanowires (SiNWs) Photocathodes Using Non-Noble Metal-Based Manganese Carbonyl Bipyridyl Molecular Catalysts in Solution and Grafted onto SiNWs

The electrocatalytic reduction of CO₂ to CO in hydroorganic medium has been investigated at illuminated (λ > 600 nm; 20 mW cm^–2) hydrogen-terminated silicon nanowires (SiNWs–H) photocathodes using three Mn-based carbonyl bipyridyl complexes as homogeneous molecular catalysts ([Mn­(L) (CO)₃(CH₃CN)]­(PF₆) and [Mn­(bpy) (CO)₃Br] with L = bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine). Systematic comparison of their cyclic voltammetry characteristics with those obtained at flat hydrogen-terminated silicon and traditional glassy carbon electrodes (GCE) enabled us to demonstrate the superior catalytic efficiency of SiNWs–H in terms of cathodic photocurrent densities and overpotentials. For example, the photocurrent densities measured at −1.0 V vs SCE for [Mn­(bpy) (CO)₃(CH₃CN)]­(PF₆) at SiNWs–H exceeded 1.0 mA cm^–2 in CO₂-saturated CH₃CN + 5% v/v H₂O, whereas almost zero current was measured at this potential at GCE. Such characteristics have been supported by the energetic diagrams built for the different SiNWs|Mn-based catalyst interfaces. The fill factor FF and energy conversion efficiency η calculated under catalytic conditions were higher for [Mn­(bpy or dmbpy) (CO)₃(CH₃CN)]­(PF₆) (FF = 0.35 and 0.34; η = 3.0 and 2.0%, respectively). Further preparative-scale electrolysis at SiNWs–H photocathode with Mn-based complex catalysts in electrolytic solution evidenced the quantitative conversion of CO₂ to CO with a higher stability of the [Mn­(dmbpy) (CO)₃(CH₃CN)]­(PF₆) complex. Finally, in order to develop technologically viable electrocatalytic devices, the elaboration of SiNWs–H photoelectrodes modified with a Mn-based complex has been successfully achieved from an electropolymerizable catalyst, and it was shown that the electrocatalytic activity of the complex was retained after immobilization