Improving the efficiency of electrochemical CO₂ reduction using immobilized manganese complexes

Abstract

Immobilization of [Mn(bpy)(CO)₃Br], (<bold>1</bold>) and [Mn(bpy(^tBu)₂)(CO)₃Br] (<bold>2</bold>, where (bpy(^tBu)₂) = 4,4′-di-<italic>tert</italic>-butyl-2,2′-bipyridine) in Nafion/multi-walled carbon nanotubes (MWCNT) on glassy carbon yielded highly active electrodes for the reduction of CO₂ to CO in aqueous solutions at pH 7. Films incorporating <bold>2</bold> have significantly improved selectivity towards CO₂, with CO : H₂ ∼ 1 at −1.4 V <italic>vs.</italic> SCE, exceeding that for the previously reported <bold>1</bold>/MWCNT/Nafion electrode. Furthermore, we report the synthesis and subsequent electrochemical characterization of two new substituted Mn(<sc>i</sc>) bipyridine complexes, [Mn(bpy(COOH)₂)(CO)₃Br] (<bold>3</bold>) and [Mn(bpy(OH)₂)(CO)₃Br] (<bold>4</bold>) (where (bpy(COOH)₂) = 4,4′-di-carboxy-2,2′-bipyridine and (bpy(OH)₂) = 4,4′-di-hydroxy-2,2′-bipyridine). Both <bold>3</bold> and <bold>4</bold> were found to have some activity towards CO₂ in acetonitrile solutions; however once immobilized in Nafion membranes CO₂ reduction was found to not occur at significant levels.