Electrochemical and photoelectrochemical CO2 reduction to hydrocarbons by a copper-based molecular catalyst

Abstract

Electrocatalytic and photoelectrochemical reduction of carbon dioxide (CO₂) to hydrocarbons remains a significant challenge, particularly for molecular catalysts that typically yield two-electron products like CO and HCOOH. Here, high selectivity toward CH₄ is achieved using a [Cu^II(phen)₂(NO₃)]⁺ (phen = 1,10-phenanthroline) molecular catalyst immobilized on multi-walled carbon nanotubes (CNTs) in aqueous solution. At −1.42 V versus the reversible hydrogen electrode (RHE), the electrode drives partial current densities of −10.8 mA cm⁻² for CH₄ production from CO₂ electroreduction, corresponding to a Faradaic efficiency of 50 %. This value is among the highest reported for molecular catalysts. Conventional characterization and in situ measurements are conducted to verify the molecular identity of [Cu^II(phen)₂(NO₃)]⁺, with no observed formation of Cu nanoparticles. Mechanistic studies reveal the [Cu^I(phen)(H₂O)]⁺ species as the active intermediate for CO₂ activation. Furthermore, an assembly of the [Cu^II(phen)₂(NO₃)]⁺ and CNTs on a Si|TiO₂ substrate is used to construct a molecular photocathode (Si|TiO₂|CNT-[Cu^II(phen)₂(NO₃)]⁺| [Cu^II(phen)₂(NO₃)]⁺) for photoelectrochemical CO₂ reduction. The hybrid photocathode produces a photocurrent density of −5.7 mA cm⁻², achieving Faradaic efficiencies of 10 % and 5 % for CH₄ and C₂H₄ production at −0.7 V vs. RHE, respectively. This represents the first example of photoelectrochemical CO₂-to-hydrocarbons conversion employing a photocathode based on a low-cost molecular catalyst.