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 155.2h −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.

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Highly porous 3D Cu skeletons (sponges) modified by electropolishing, thermal annealing, and foam electrodeposition have been studied as catalysts for the electrochemical conversion of CO2 with a particular emphasis on C2 products formation. These catalyst materials appear to be promising for future applications where gaseous CO2 reactants can be transported through the 3D catalyst thereby tuning the mean residence time of reaction intermediates inside the catalyst, which crucially influences the final product distribution. In particular, the annealed skeleton (300 °C, 12 h) and the one modified by Cu foam electrodeposition show profound activities toward C2 product formation (C2H4, C2H6) with faradaic efficiencies reaching FEC2 = 32.3% (annealed skeleton sample, −1.1 V vs RHE) and FEC2 = 29.1% (electrodeposited sample, −1.1 V vs RHE), whereas the electropolished Cu skeleton remains largely inactive for both the C1 and the C2 pathway of hydrocarbon formation. This effect is discussed on the basis of residual impurities that are left behind from the investment casting approach on which the fabrication of these Cu skeleton support materials is based. In addition, a higher FEC2H4 /FEC2H6 ratio is observed for the annealed Cu skeleton as compared to the electrodeposited Cu foam. Such a switching in the C2 product distribution (FEC2H4 /FEC2H6 ratio) is discussed on the basis of particular morphological effects (residence time of intermediates inside the catalyst) related to the three-dimensional nature of the used catalysts.

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