Efficient Selective Electroreduction of CO ₂ to CO by a Quaterpyridine Molecular Copper Complex Immobilized onto Carbon

We report herein the synthesis, spectroscopic and electrochemical characterization, as well as electrocatalytic properties toward heterogeneous electrochemical CO₂ reduction of two mononuclear copper(II) complexes, namely, [Cu(<b>L</b>)]²⁺ (<b>1</b>) and [Cu(<b>QTPy</b>)]²⁺ (<b>2</b>), where <b>L</b> = 8-([2,2'-bipyridin]-6-yl)-2-(pyridin-2-yl)quinoline and <b>QTPy</b> = 2,2':6',2″:6″,2‴-quaterpyridine. The two complexes <b>1</b> and <b>2</b> have been first successfully immobilized onto multiwalled carbon nanotubes (MWCNTs) grafted on glassy carbon electrodes, as shown by X-ray photoelectron spectroscopy and cyclic voltammetry. Electroreduction of carbon dioxide by the grafted complexes has been investigated in an aqueous KHCO₃ medium at neutral pH. Comparative studies highlight the better catalytic performance of the complex bearing <b>L</b> rather than <b>QTPy</b> under the same experimental conditions. Controlled potential electrolysis (CPE) at low overpotential (-0.55 V vs reversible hydrogen electrode (RHE)) displays high selectivity for carbon monoxide production with complex <b>1</b> (Faradaic efficiency (FE_CO = 75%)). Furthermore, in situ and ex-situ analyses of the electrolysis evidence no modification of the immobilized mononuclear complex upon CPE, which is rare for grafted copper systems as well as no leach of copper in the electrolyte solution. Noteworthy, the implementation of the complexes into zero-gap electrolyzers allows the strong enhancement of FE_CO for complex <b>1</b>, reaching 100% at -0.60 V vs RHE, whereas complex <b>2</b> remains poorly selective, hence clearly highlighting the strong impact of the ligand topology on the catalytic properties of the complexes.