Electrochemical Reduction of CO2 into Multicarbon Alcohols on Activated Cu Mesh Catalysts: An Identical Location (IL) Study

Abstract

Potential-dependent CO 2 reduction reactions (CO 2 RR) were carried out on technical Cu mesh supports that were stepwise modified by (i) electrodeposition of dendritic Cu catalysts under mass transfer control of Cu(II) ions followed by (ii) an extra 3 h thermal annealing at 300 °C in air. The initial electrodeposition of dendritic Cu activates the technical supports for highly efficient formate production at low overpotentials (FE Formate = 49.2% at −0.7 V vs RHE) and in particular for C–C coupling reactions at higher overpotentials (FE C 2 H 4 = 34.3% at −1.1 V vs RHE). The subsequent thermal annealing treatment directs the CO 2 RR product selectivity toward multicarbon alcohol formation (ethanol/EtOH and n-propanol/n-PrOH) resulting into a total Faradaic yield of FE alcohol = 24.8% at −1.0 V vs RHE (FE EtOH = 13%). Moreover, the EtOH and n-PrOH production rate of 155.2 μML electrolyte –1 cm ECSA –2 h –1 and 101.4 μML electrolyte –1 cm ECSA –2 h –1 (normalized with respect to the electrolyte volume and the electrochemically active surface area ECSA), respectively, are the highest ones observed so far for Cu catalysts modified by a Cu 2 O/CuO surface precursor phases. The maximum of the n-PrOH efficiency is observed at slightly less negative potentials of −0.9 V with FE n-PrOH = 13.1%. Identical location (IL) SEM analysis was applied prior to and after the annealing preparation steps and in addition prior to and after CO 2 RR to monitor severe morphological changes which go along with the formation of Cu 2 O/CuO surface phases upon thermal annealing and their subsequent electroreduction under operando conditions of the CO 2 RR. Fringe pattern in the HR-TEM analysis confirms the existence of Cu/Cu oxide planes on the corresponding annealed catalysts. IL-SEM and HR-TEM analyses further identify nanodendritic Cu as being the active component for the desired production of multicarbon alcohols. In addition, such nanodendritic Cu shows a remarkably high resistance against degradation with alcohol efficiencies that can be maintained on a high level (FE alcohol = ∼24% at −1.0 V) over 6 h, whereas the electrodeposited catalyst suffers from a rapid and drastic drop-down in the ethylene efficiency from 33% to 15%. The extraordinary stability of the annealed Cu catalyst can be assigned to a changed CO 2 RR mechanism and related to the complete suppression of the coupled C1/C2 hydrocarbon pathway, thereby avoiding the accumulation of poisoning surface carbon species or other C1 intermediates. The introduced multistep approach of catalyst activation was successfully applied also to other support materials, e.g. Au and Ag meshes, resulting in similarly high yields of C2 and C3 alcohols as observed for the Cu mesh support. These results further support the robustness of the proposed catalyst preparation procedure.