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

Electrocatalytic and photoelectrochemical reduction of carbon dioxide (CO 2 ) to hydrocarbons remains a significant challenge, particularly for molecular catalysts that typically yield two-electron products like CO and HCOOH. Here, high selectivity toward CH 4 is achieved using a [Cu II (phen) 2 (NO 3 )] + (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 CH 4 production from CO 2 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 (NO 3 )] + , with no observed formation of Cu nanoparticles. Mechanistic studies reveal the [Cu I (phen)(H 2 O)] + species as the active intermediate for CO 2 activation. Furthermore, an assembly of the [Cu II (phen) 2 (NO 3 )] + and CNTs on a Si|TiO 2 substrate is used to construct a molecular photocathode (Si|TiO 2 |CNT-[Cu II (phen) 2 (NO 3 )] + | [Cu II (phen) 2 (NO 3 )] + ) for photoelectrochemical CO 2 reduction. The hybrid photocathode produces a photocurrent density of −5.7 mA cm −2 , achieving Faradaic efficiencies of 10 % and 5 % for CH 4 and C 2 H 4 production at −0.7 V vs. RHE, respectively. This represents the first example of photoelectrochemical CO 2 -to-hydrocarbons conversion employing a photocathode based on a low-cost molecular catalyst.