Enhancement of electrocatalytic abilities toward CO₂ reduction by tethering redox-active metal complexes to the...

Tethering metal complexes, like [Ru(bpy)₂Cl₂] (bpy = 2,2'-bipyridine), which are redox-active at low reduction potentials and have the ability to transfer electrons to another complex, to a [Ni(cyclen)]²⁺ electrocatalyst enhanced the reduction of CO₂ to CO at low overpotentials. The [Ni(cyclen)]²⁺ electrocatalyst was modified by tethering redox-active metal complexes via 4-methylpyridyl linkers. The redox-active metal complexes were reduced after CO₂ bound to the active site. In controlled potential electrolysis (CPE) experiments in 95 : 5 (v/v) CH₃CN/H₂O, [{([Ru]pic)₄cyclen}NiCl]⁵⁺ ([Ru]⁺ = {Ru(bpy)₂Cl}⁺; pic = 4-methylpyridyl) could be used to reduce CO₂ into CO at a turnover frequency (TOF) of 708 s^-1 with a faradaic efficiency (FE) of 80% at an onset potential of -1.60 V vs. NHE. At the same time, this electrocatalyst was active at an onset potential of -1.25 V vs. NHE, which is the reduction potential of one of the bpy ligands of the [Ru]⁺ moieties, with FE = 84% and TOF = 178 s^-1. When the electrocatalysis was performed using [bn₄cyclenNiCl]Cl (bn = benzyl) without tethered redox-active metal complexes, the TOF value was determined to be 8 s^-1 with FE = 77% at an onset potential of -1.45 V vs. NHE. The results show that tethering redox-active metal complexes significantly improves the electrocatalytic activities by lowering the potential needed to reduce CO₂.