Mesoporous Cu foams formed by a template-assisted electrodeposition process have been identified as CO2 electrocatalysts that are highly selective toward C2 product formation (C2H4 and C2H6) with C2 efficiencies (FEC2) reaching 55%. The partial current of C2 product formation was found to be higher than that of the (parasitic) hydrogen evolution reaction (HER) at any potential studied (−0.4 to −1.0 vs the reversible hydrogen electrode). Moreover, formate production could largely be suppressed at any applied potential down to efficiencies (FEformate) of ≤6%. A key point of the Cu foam catalyst activation is the in operando reduction of a Cu2O phase, thereby creating a large abundance of surface sites active for C–C coupling. The cuprous oxide phase has been formed after the Cu electrodeposition step by exposing the large-surface area catalyst to air at room temperature. The superior selectivity of the Cu foam catalyst studied herein originates from a combination of two effects, the availability of specific surface sites for C–C coupling [dominant (100) surface texture] and the temporal trapping of gaseous intermediates (in particular CO and C2H4) inside the mesoporous catalyst material during CO2 electrolysis. A systematic CO2 electrolysis study reveals a strong dependence of the C2 efficiencies on the particular surface pore size of the mesoporous Cu catalysts with a maximal FEC2 between 50 and 100 μm pore diameters.

A major concern of electrocatalysis research is to assess the structural and chemical changes that a catalyst may itself undergo in the course of the catalyzed process. These changes can influence not only the activity of the studied catalyst but also its selectivity toward the formation of a certain product. An illustrative example is the electroreduction of carbon dioxide on tin oxide nanoparticles, where under the operating conditions of the electrolysis (that is, at cathodic potentials), the catalyst undergoes structural changes which, in an extreme case, involve its reduction to metallic tin. This results in a decreased Faradaic efficiency (FE) for the production of formate (HCOO–) that is otherwise the main product of CO2 reduction on SnO x surfaces. In this study, we utilized potential- and time-dependent in operando Raman spectroscopy in order to monitor the oxidation state changes of SnO2 that accompany CO2 reduction. Investigations were carried out at different alkaline pH levels, and a strong correlation between the oxidation state of the surface and the FE of HCOO– formation was found. At moderately cathodic potentials, SnO2 exhibits a high FE for the production of formate, while at very negative potentials the oxide is reduced to metallic Sn, and the efficiency of formate production is significantly decreased. Interestingly, the highest FE of formate production is measured at potentials where SnO2 is thermodynamically unstable; however, its reduction is kinetically hindered.

Ag-foam catalysts have been developed for the electrochemical CO2 reduction reaction (ec-CO2RR) based on a concerted additive- and template-assisted metal-deposition process. In aqueous media (CO2-saturated 0.5 M KHCO3 electrolyte), these Ag foams show high activity and selectivity toward CO production at low and moderate over-potentials. Faradaic efficiencies for CO (FECO) never fell below 90% within an extremely broad potential window of ∼900 mV, starting at −0.3 V and reaching up to −1.2 V versus a reversible hydrogen electrode (RHE). An increased adsorption energy of CO on the Ag foam is discussed as the origin of the efficient suppression of the competing hydrogen-evolution reaction (HER) in this potential range. At potentials of <−1.1 V versus RHE, the FEH2 values significantly increase at the expense of FECO. Superimposed on this anti-correlated change in the CO and H2 efficiencies is the rise in the CH4 efficiency to the maximum of FECH4 = 51% at −1.5 V versus RHE. As a minor byproduct, even C–C-coupled ethylene could be detected reaching a maximum Faradaic efficiency of FEC2H4 = 8.6% at −1.5 V versus RHE. Extended ec-CO2RR reveals the extremely high long-term stability of the Ag foam catalysts, with CO efficiencies never falling below 90% for more than 70 h of electrolysis at −0.8 V versus RHE (potential regime of predominant CO production). However, a more-rapid degradation is observed for extended ec-CO2RR at −1.5 V versus RHE (potential regime of predominant CH4 production), in which the FECH4 values drop to 32% within 5 h of electrolysis. The degradation behavior of the Ag-foam catalyst is correlated to time-resolved identical-location scanning electron microscopy investigations that show severe morphological changes, particularly at higher applied over-potentials (current densities) at −1.5 V versus RHE. This study reports on the first ec-CO2RR catalyst beyond copper that demonstrates a remarkably high selectivity toward hydrocarbon formation, reaching a maximum of ∼60% at −1.5 V versus RHE. The experimental observations presented herein strongly suggest that this newly designed Ag-foam catalyst shares, in part, mechanistic features with common Cu catalysts in terms of ec-CO2RR product selectivity and catalyst degradation behavior.

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.

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.

The electrochemical reduction of CO 2 has been extensively studied over the past decades. Nevertheless, this topic has been tackled so far only by using a very fundamental approach and mostly by trying to improve kinetics and selectivities toward specific products in half-cell configurations and liquid-based electrolytes. The main drawback of this approach is that, due to the low solubility of CO 2 in water, the maximum CO 2 reduction current which could be drawn falls in the range of 0.01-0.02 A cm -2 . This is at least an order of magnitude lower current density than the requirement to make CO 2 -electrolysis a technically and economically feasible option for transformation of CO 2 into chemical feedstock or fuel thereby closing the CO 2 cycle. This work attempts to give a short overview on the status of electrochemical CO 2 reduction with respect to challenges at the electrolysis cell as well as at the catalyst level. We will critically discuss possible pathways to increase both operating current density and conversion efficiency in order to close the gap with established energy conversion technologies.

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Remarkable size-dependent activity of palladium nanoparticles (PdNPs) towards formate production is evident at very low overpotentials (-0.1 to -0.5 V vs. RHE). Size-selective PdNPs, chemically synthesized at sizes of 3.8-10.7 nm, effected an electrochemical CO reduction reaction in aqueous 0.5 m NaHCO . The faradaic efficiency of formate production (FE ) on 3.8 nm PdNPs exceeded 86 % at E=-0.1 V versus RHE, whereas on 6.5 nm PdNPs an even higher FE of 98 % was observed. However, FE decreased for larger PdNPs. The superior efficiency towards formate production at low overpotentials is rationalized in terms of a changed catalytic pathway through PdH phases. The observed maximum in the formate efficiency for a mean particle size of about 6.5 nm is discussed in terms of counterbalancing the size-dependent effects of a competing CO reduction reaction and a parasitic hydrogen evolution reaction. Production rates of formate are also remarkably high at -0.3 V versus RHE with 539.9 and 452.3 ppm h mg for the 6.5 and 3.8 nm PdNPs, respectively.