Electrochemical CO2 Conversion Using Skeleton (Sponge) Type of

Jul 14, 2017 - Motiar Rahaman , Abhijit Dutta , Alberto Zanetti , and Peter Broekmann. ACS Catalysis 2017 7 (11), 7946-7956. Abstract | Full Text HTML...
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Electrochemical CO conversion using skeleton (sponge) type of Cu catalysts Abhijit Dutta, Motiar Rahaman, Miklos Mohos, Alberto Zanetti, and Peter Broekmann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01548 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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ACS Catalysis

Electrochemical CO2 conversion using skeleton (sponge) type of Cu catalysts Abhijit Dutta≠, Motiar Rahaman≠, Miklos Mohos, Alberto Zanetti and Peter Broekmann* Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern 3012 Switzerland

Supporting Information Placeholder ABSTRACT: 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, 12h) and the one modified by Cu foam electrodeposition show profound activities towards C2 product formation (C2H4, C2H6) with faradaic efficiencies reaching FEC2 = 32.3% (annealed skeleton sample, -1.1V vs RHE) and FEC2 = 29.1% (electrodeposited sample, -1.1V 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 on. 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 threedimensional nature of the catalysts used. Keywords: CO2 conversion; Cu/CuxO catalysts; electrocatalysis, hydrocarbon formation; metal sponge

INTRODUCTION The electrochemical reduction of CO2 into products of higher value (in the following referred to as CO2RR) offers the unique chance to significantly contribute to the closing of the anthropogenic CO2 cycle and is therefore currently in the focus of research activities worldwide.1,2 Energy needed for such an electroconversion processes might originate from excesses of renewables like hydro, wind and solar energy (power to value concept).1,3-5 A key challenge of the process development remains the product selectivity of the CO2RR which can be directed by the choice of the catalyst.6,7 Among the vast number of materials screened so far, it is Cu which deserves particular attention since it is the only catalyst which is capable to convert CO2 into hydrocarbons in considerable amounts.6,7 Crucial for the performance of the Cu catalysts is their pre-treatment, e.g. by thermal annealing8-12, exposure to oxygen plasma13,14, electrodeposition12, sputtering15, electropolishing, anodization16 etc.. This pretreatment provides not only means of forming catalytically active sites and surface textures but creates in addition particular catalyst morphologies on

Figure 1. Optical images of the Cu skeleton type of catalysts used in this study: a) as received Cu skeleton; b) electropolished Cu skeleton; c) annealed Cu skeleton (at 300 °C for 12h); d) functional Cu foam electrodeposited on the Cu skeleton. Red dotted lines and arrows indicate those sections of the catalysts (5mm x 8mm x 2mm) which were immersed into the electrolyte for CO2RR. various length scales that are needed to guide the CO2RR into the desired direction. Target current densities of industrial CO2 electrolyzers are in the order of hundreds of mA/cm2.3,17,18 These high reaction rates can, however, only be achieved by gas/liquid flow approaches where the CO2 reactant is transported by convection over or even through the active catalyst thereby minimizing undesired limitations of CO2 mass transfer.1,3,17 A vast number of promising Cu catalysts were, however, tested in classical half-cell configurations from liquid, CO2-saturated electrolytes where the low solubility of CO2 in aqueous media (∼35mM) prevents reaching such high CO2 conversion rates.1,3 Concepts are therefore needed to translate those catalysts that were identified as promising in classical half-cell configurations to cathode architectures that are permeable for gas/liquid flows. The most common approach is based on the use of gas diffusion electrodes (GDEs) where the catalysts are dispersed on a gas diffusion layer (GDL), e.g. by air brush/spray painting methods,19 in form of inks containing the electrocatalytically active metal nanoparticles (NPs).17,19 A promising approach particularly interesting for future large scale CO2 electrolysis applications was recently introduced by Kas et al.20 who developed 3D hollow fiber electrodes by using a multistep sintering/CuOx reduction fabrication process. During CO2RR, reactants are transported through such porous Cu electrodes at elevated gas pressures thus leading to a highly efficient and selective CO2 conversion with faradaic efficiencies for CO reaching 85%.20 Inspired by this kind of electrode design20-22 we studied the intrinsic electrocatalytic activity of three-dimensional Cu skeletons (sponges). These catalysts combine a pronounced open-cell poros-

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Figure 2. Top-down SEM inspection of the electropolished Cu skeleton (a-e), the annealed Cu skeleton (f-j) and the Cu skeleton modified by the electrodeposited functional Cu foam (k-o). The SEM analysis was carried out prior to the CO2RR. ity (permeable for fluids) with a rigid 3D network of interconnected metallic struts that is isotropic in all three spatial directions (Figure S1).23 Also this type of porous electrode can be considered as well suited for a controlled flow of CO2 reactants though the catalysts. This design offers in addition the possibility of further catalyst activation and optimization, e.g. with regard to C2 product formation. Two approaches are compared in detail in this study, (i) a thermal annealing of the Cu skeleton at moderate temperatures (300°C) and (ii) the electrodeposition of functional Cu foams. The latter approach leads to 3D catalyst architectures revealing a hierarchical porosity on three different length scales as discussed below.

Results and Discussion Figure 1 depicts the Cu catalysts used in this study, the electropolished Cu skeleton, the annealed one and the Cu skeleton activated by electrodeposition of a functional Cu foams (Figure 1b-d). For comparison purposes also the as received Cu skeleton is shown (Figure 1a). The mass density and the porosity of the pristine Cu skeleton material amounts to ρ = 0.8 g cm-3 and 91%, respectively, thus resulting in an open-cell architecture of interconnected pores with pore diameters of up to 2 mm. Cu struts of the three-dimensional percolating network exhibit diameters ranging from 200-500 µm (Figure S1-2). Whereas the electropolished sample shows the typical bright color of copper (in contrast to the as received sample), the annealed Cu skeleton and the one modified by electrodeposition exhibit both a characteristic black appearance (Figure 1). In case of the annealed sample, this is due to the presence of a composite surface phase of largely amorphous CuO and crystalline Cu2O (see XPS and XRD section below). As demonstrated by the SEM analysis in Figure 2, all three samples reveal pronounced differences in their surface morphology on various length scales. Up to a magnification of x100 the surfaces of the electropolished and annealed Cu skeletons appear largely featureless whereas a characteristic pore structure is visible in case of the electrodeposited Cu foam. Surface pore diameters range from 50 - 80µm (Figure 2, Figure S5-6). The open-cell nature of the 3D skeleton support remains, however, unaffected by the presence of the electrodeposited Cu foam, at least if deposition times of ≤ 20s are used. The Cu foam was deposited from a acidified Cu sulfate plating bath (1.5 M sulfuric

acid and 0.2 M Cu(II) sulfate) at a current density of -3 A/cm2 (for details see the supporting information file). The porous nature of the Cu deposit is the result of an electrodeposition process under strong hydrogen evolution reaction (HER) conditions.12,24-26 Our analysis proves that this kind of functional metal foam can be deposited not only on planar supports (e.g. Cu foil or wafer) but in principle on any kind of three-dimensional objects that are sufficiently conductive (Figure S5). As known from systematic studies on Cu foils24,26 and Cu wafer coupons12 we observe also for the 3D Cu skeleton a linear increase of the mean surface pore size of the electrodeposited functional foams with increasing deposition time (Figure S5). A noteworthy difference to the planar supports is, however, a less uniform current distribution during the HER assisted Cu deposition across the 3D skeleton support thus resulting in a broader pore size distribution with smaller pores inside the skeleton and bigger ones at the outer part of the skeletal structure. This effect is exemplarily demonstrated by the identical location SEM images in Figure 3 taken prior and after the electrodeposition process of the functional Cu foam (see also yellow arrows in Figure 2k). As discussed below, this morphological characteristic has a significant impact on the resulting CO2RR product distribution. The surface of the electropolished Cu skeleton at higher magnification (>10k) appears rather blurred. The skeleton is covered by grooves revealing a certain preferential orientation (Figure 2a-e). A granular surface covered with ultrathin fibers is the main morphological motif of the annealed Cu skeleton (Figure 2f-j).

Figure 3. Identical location SEM before (a) and after (b) the electrodeposition of the functional Cu foam on the Cu skeleton support.

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ACS Catalysis x 200 nm (Figure 4, Figure S7). Not only the morphology but also the chemical surface composition of the catalysts strongly depends on the pretreatment as exemplarily shown for the spin-orbit split Cu 2p photoemission (XPS) spectra in Figure 5. Binding energies of BE(Cu 2p3/2) = 932.4 eV (FWHM = 1.4 eV) and BE(Cu 2p1/2) = 952.1 eV were determined for the electropolished sample (Figure 5a). These BEs are similar to those of the Cu foam electrodeposited on the skeleton support (Figure 5b) with BE(Cu 2p3/2) = 932.2 eV (FWHM = 1.2 eV) and BE(Cu 2p1/2) = 952.0 eV. These BEs are indicative for either pure Cu or the presence of diamagnetic cuprous oxide (Cu2O).27,28 Note that both phases are indistinguishable in the XPS analysis. In case to the annealed sample, the Cu 2p emissions are shifted towards higher BEs with BE(Cu 2p3/2) = 933.6 eV and BE(Cu 2p1/2) = 953.6 eV (Figure 5c). In addition, one observes a prominent peak broadening for the Cu 2p emissions resulting in a FWHM of 3.2 eV (Figure 5c). This clearly points to a composite nature of the annealed skeleton sample, where Cu, Cu2O and cupric oxide (CuO) coexist on the skeleton surface. A dominant contribution of the CuO to such composite phase becomes evident from the rather strong shake-up satellites of the Cu 2p3/2 emission in the range of BE = 939.3 – 945.6 eV and of the Cu 2p1/2 line in the range of BE = 959.8 – 964.6 eV (Figure 5c) which is in full agreement with the literature.27,28 The Cu 2p3/2 peak could be fitted by assuming two components exactly matching the binding energies of CuO and Cu/Cu2O (see Figure S16). Note that all features assigned to bulk CuO have disappeared in the postelectrolysis XPS inspection of the annealed Cu catalyst thus clearly pointing to an instability of bulk Cu oxide phases under such harsh reductive conditions of CO2RR (red dotted line in Figure 5c). The actually active Cu catalyst is formed under operando CO2RR conditions. More insights into the structure and composition of the skeleton type of catalysts could be gained from complementary XRD analyses also carried out prior to the CO2RR (Figure 6). The electropolished sample solely shows diffraction pattern of pure polycrystalline Cu ((111), (200) and (220) diffraction peaks). There is only a weak satellite of the (220) peak at lower 2θ values

Figure 4. High resolution SEM images showing Cu nanoparticles (NPs) with highly ordered facets on their surface as key structural motif of the Cu foam catalyst on the nm-scale. Diameters of these grains range from 500 nm to 2 µm whereas the fibers have diameters in the order of a few tens of nanometers. Their length can, however, extend to 1-2 µm. According to the literature,26,27 these fibers can be attributed to one of the structural modifications of cupric CuO formed during the thermal annealing of Cu (for more details see Figure S8). The dendritic nature of the pore side walls of the electrodeposited foam becomes evident from the SEM inspection of the electrodeposited Cu foam (magnification >1k, Figure 2k-o) in full agreement with previous studies.12,24,26 This particular Cu skeleton catalyst reveals porosity on three different length scales: (i) on the nm-scale inside the pore side-walls of the functional foam; (ii) on the µm-scale due to the pores of the functional Cu foam and (iii) on the mm-scale due to the open-cell structure of the skeleton support (Figure 2). The size of the electrodeposited Cu pores and the thickness of the Cu foam can have an influence on the diameters of the open-cells of the skeleton support when applying deposition times of ≥ 20s (Figure S5). An intriguingly new aspect of the Cu foam materials is related to the morphology on the nmscale. The high resolution SEM characterization in Figure 4 demonstrates that the dendrites of these Cu foams are built up by shaped and textured Cu-NPs as key structural motif of the dendritic pore side walls. The area of such facets can even reach 200 nm

Figure 5. XPS spectra of (a) the electropolished, (b) the electrodeposited and (c) annealed Cu skeleton samples focusing of the Cu 2p photoemission region measured prior to the CO2RR (solid lines) and after 1h CO2RR at -1.1 V (red dotted line in (c)), respectively.

Figure 6. XRD analysis of the electropolished (a), the electrodeposited (b) and the annealed (c) Cu skeleton samples prior to the CO2RR; (d –f), JCPDS references for CuO, Cu2O and Cu. .

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which might be indicative for the presence of trace amounts of crystalline Cu2O at the skeleton surface which naturally forms when exposed to air. Interesting to note is that the particular intensity distribution of the Cu related diffraction peaks which significantly differs from the one of typical polycrystalline Cu reference samples (I111 : I200 : I220 = 1 : 0.46 : 0.20). For the electropolished skeleton we observe a dominant (111) diffraction peak whereas the (200) peak appears extraordinarily weak (I111 : I200 : I220 = 1 : 0.08 : 0.22). This unusual crystallographic feature and deviation from polycrystalline Cu is a specific characteristic of the sponge material originating from the investment casting technique used for the production of this type of metal skeletons (sponges).23,29 A combined SEM, EXD and XPS inspection (Figure S2-4) proves in addition the presence of zircon impurities (insoluble zirconium silicate) on the Cu skeleton surface and within the Cu struts as a residual of the fabrication process by the investment casting technique. The diffraction pattern of the Cu skeleton modified by the electrodeposition is mainly governed by the functional Cu foam covering the Cu skeleton support. For the Cu related diffraction pattern one observes a prominent (200) diffraction peak (I111 : I200 : I220 = 1 : 1 : 0.34). Although contributions from Cu2O and Cu cannot be separated in the XPS (Figure 5b) the presence of crystalline Cu2O can clearly be deduced from the XRD analysis of the electrodeposited sample (Figure 6 and Figure S10). Broadened Cu2O related (111), (220) and (200) diffraction peaks are clearly visible in the respective diffractogram. The diffraction pattern of the annealed Cu skeleton contains contributions from pure Cu and Cu2O whereas no indication for the presence of a crystalline CuO phase is visible although CuO was unambiguously observed in the corresponding SEM (CuO fiber structure30,31, Figure 2 and Figure S8) and XPS analysis (Figure 5c). From this comparison we can safely conclude that the annealing temperature of 300°C is indeed sufficient to form CuO but insufficient to produce a high degree of translational order in the cupric oxide phase. The amount of crystalline CuO fibers does not seem to be sufficient to significantly contribute to the XRD pattern of the sample annealed at 300°C. However, applying higher annealing temperatures of 450°C and 600°C leads to an increased density of CuO fibers on the skeleton surface in accordance with the appearance of CuO related diffraction features in the respective XRD experiment (for details see Figure S8 and S10). Another important observation is related to the relative intensities of the Cu diffraction peaks (I111 : I200 : I220 = 1 : 0.89 : 0.24). A rather prominent (200) diffraction peak is observed similar to the electrodeposited sample (Figure 6). The presence of (100) textured Cu is known to facilitate C-C coupling reactions (C2 pathway) on Cu catalysts.14,15,32,33 Note that our highresolution SEM analysis of the Cu foam catalyst confirms the presence of shaped Cu-NPs with distinct (100) facets on their surface (Figure 4). Besides the surface composition and surface morphology, it is the electrochemically active surface area (ECSA) which is also different for the three Cu skeleton samples. Surface areas of 0.5 cm2, 2.5 cm2 and 6.4 cm2 were determined for the electropolished, the annealed and the electrodeposited sample, respectively. The ECSAs have been estimated on the basis of voltammetric experiments using di-methyl-viologens (DMVs) as redox-active probes (for details see discussion in the Supporting Information file and Figure S12 and Figure S13).12,34 For the initial testing of the catalyst activities, LSV measurements were recorded in both Ar- (dotted lines) and CO2-saturated (solid lines) 0.5 M NaHCO3 electrolytes (Figure 7). The reductive currents in the LSVs measured in the Ar-saturated electrolytes (blankets) are due to the pure hydrogen evolution reaction (HER) at potentials below -0.5V. The HER related reductive currents on

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Figure 7. Linear sweep voltammograms (LSVs) of the electropolished (black), the electrodeposited (green) and annealed (blue) Cu skeleton samples exposed to either Ar saturated 0.5M NaHCO3 solution at pH = 8.0 (dotted curves) or CO2 saturated 0.5M NaHCO3 solution at pH = 7.2 (solid lines). The sweep rate was 25 mV s-1. The LSVs are iR compensated (see Figure S17). LSVs normalized to the ECSA are provided in the Supporting Information file (Figure S18). the three Cu skeletons increase in the sequence IHER(electropolished) < IHER(annealed) < IHER(electrodeposited), which reflects predominantly the increase of the ECSA in the same sequence (Figure S12). The same trend of increasing cathodic currents can be observed for the CO2-saturated electrolytes (solid lines, Figure 7) but now on a higher level as compared to the Ar-saturated ones. The slight change in the pH by going from an Ar- to a CO2-saturated electrolyte cannot be main reason for the increased reduction currents (Figure 7). This significant increase in the reduction currents is due to the superposition of the HER with the CO2RR and proves that it is rather the physically dissolved CO2 than the bicarbonate anion of the supporting electrolyte which is the reactive species of the CO2RR on Cu. It is worth mentioning that the most positive (apparent) onset of the reductive processes is observed for the annealed Cu skeleton (∼ 0.25 V, blue curves). This is indicative for an onset of the Cu2O/CuO reduction process which already sets in at potentials more positive than the potential regime where HER and CO2RR become dominant. This points to a CO2RR occurring on the annealed skeleton sample under transient conditions involving severe changes in both the surface composition and morphology under operando conditions. The active catalyst is produced during CO2RR. Post-electrolysis SEM and XRD inspection proves morphological and compositional changes in particular for the annealed sample (disappearance of the CuO fibers, Figure S9) whereas the porosity of electrodeposited Cu foam remains unaffected by the CO2RR, at least on a µm-scale (Figure S6). In order to probe the catalytic activity/selectivity of the three skeleton catalysts, potentiostatic CO2 electrolysis reactions were carried for 1h from a CO2 saturated 0.5M NaHCO3 solution. The CO2RR product analysis was based on online gas chromatography for the volatile products (H2, CO, C2H4, and C2H6) whereas ion exchange chromatography was employed to quantify non-volatile products (formate). For further technical details of the product analysis see the Supporting Information file. The FE vs E plots (Figure 8) demonstrate significant differences between the skeleton samples in their potential dependent product distribution. It is the electropolished sample which shows the highest selectivity towards parasitic HER with almost constantly high FEH2 values ranging from 69% to 62% in the potential regime between -0.6 and -1.0V. At the most negative potential of -1.3V the FE of hydrogen production is tremendously increased for the electropolished sample even up to 88%. FEH2 values of the electrodeposited and the annealed samples are well below the ones of the

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ACS Catalysis

Figure 8. Potential and catalyst dependent product analysis of the CO2 electrolysis (FE vs E plots). The faradaic efficiencies (FEs) are presented for the electropolished (black), the electrodeposited (green) and the annealed (blue) Cu skeleton samples. electropolished sample with a minimum of 51% and 43%, respectively, at –0.8V. Main CO2RR product on the electropolished Cu skeleton is formate. It should be noted that formate production requires only ∙ ∙ -

a weak interaction/adsorption of the *CO radical anion as the initial CO2RR intermediate on the catalyst surface (the asterisk * indicates an adsorption state).35,36 FEformate values show only minor variations in the range between 22% and 28% with a maximum at -0.8V. Even higher FEsformate are observed for the annealed Cu skeleton with a maximum of 34% also at -0.8V whereas the electrodeposited foam shows a less pronounced trend towards formate production with a maximum of FEformate = 24% at 0.7V which drops. The FEsformate of all three samples converge at 1.3 to values close to zero. The most important difference between the skeleton catalysts becomes obvious from the efficiencies of CO (Figure 8). These remain on an exceptionally low level for the electropolished skeleton sample in the entire potential range never exceeding 3% (-0.7V). CO production can generally be considered as fingerprint for a catalytic activity towards hydrocarbon formation since temporarily chemisorbed *CO is considered as one of the key chemisorbed intermediate species for both the C1 and C2 pathway.17,35,37 The high formate efficiency in combination with the absence of any activity towards CO production point to the lack of less coordinated and catalytically active surface site in the case of the electropolished Cu skeleton which are capable to ∙ ∙ -

stabilize the initially formed *CO intermediate. This has been identified as a mechanistic prerequisite for the subsequent coupled H+/e- transfer followed by an OH- abstraction which then leaves adsorbed *CO behind as the second key intermediate required for hydrocarbon formation.36 In agreement with this reasoning we do

not observe any significant activity of the electropolished Cu skeleton towards CH4, C2H4, C2H6 formation or any other hydrocarbon production. The presence of contaminations on the electropolished skeleton sample might be one of the reasons why a CCu bond formation is suppressed (Figure S2-4). In addition we observed an unusual XRD pattern of the electropolished sample (Figure 6) also demonstrating tremendous differences with respect to polycrystalline Cu. For comparison purposes a CO2RR product analysis for a polycrystalline Cu foil is provided in the Supporting Information file (Figure S19). Both, the annealed and the electrodeposited catalyst show, by contrast, higher CO efficiencies at low overpotentials (15.1% and 19% at –0.6V) which continuously drop down to values below 1% at -1.3V. This decrease of the FECO is anti-correlated to the raise in the FEC2 values (sum of FEC2H4 and FEC2H6), at least in the potential range between -0.6V and -1.0V. Maximum FEC2 values of 32.2% (annealed skeleton) and 29.1% (skeleton modified by electrodeposition) are reached at -1.1V before these efficiencies drop down to values below 5% at -1.3V. This drastic drop down in the C2 efficiencies at potentials below -1.1V can be rationalized by the synergy of two effects, namely the increased activity of the high surface area Cu catalyst towards H2 production which is not mass transfer limited and the mass transfer limitation of the CO2 conversion at these high overpotentials. It can, however, be assumed that in particular undesired CO2 mass transfer limitations can be overcome by using a gas/liquid flow cell design. It is important to note that only C2 hydrocarbons are detected in the product analysis but no CH4. The activation of the Cu skeleton by thermal annealing (300°C) and Cu foam electrodeposition obviously lead both to a preference of the C2 reaction pathway and a complete suppression of the C1 pathway particularly at higher overpotentials. It can be speculated that this preference for C2 products is related to the presence of (100) textured Cu (Figure 4 and 6). An important difference between the annealed and the electrodeposited samples concerns, however, the relative fraction of the fully reduced C2H6 to the total FEC2 which is clearly higher in case of the electrodeposited sample (Figure 8). Fully reduced C2 hydrocarbons are reported so far only for porous Cu catalysts.12 This effect has been rationalized by particular morphological effects of the foam type of Cu catalysts where the gaseous CO2RR products and intermediates (CO and C2H4) are effectively trapped inside the pores thereby increasing their mean residence time during CO2RR inside the catalyst.12 A re-adsorption and subsequent reductive hydrogenation of ethylene becomes favored in particular for the foam type of catalyst having pore sizes in the range of 50 - 80µm. Dutta et al. already demonstrated a strong dependence of the final product distribution (C2 efficiency) on the mean surface pore size of Cu foams with an optimum at about 75µm.12 For the annealed Cu skeleton sample with open pores in the order of millimeters we therefore observe a less pronounced trend towards the fully reduced C2 product probably due to a reduced residence time of the C2H4 inside the skeleton catalyst. The idea of an enhanced trapping efficiency in case of the electrodeposited sample gets further supported by the current transients of the CO2 electrolysis (Figure S14). Whereas the transients appear smooth in case of the electropolished and annealed samples indicating an almost unhindered release of the gaseous CO2RR/HER products there is a noise visible in the transients in case of the electrodeposited sample that increases with the applied electrolysis potential. This effect has been rationalized by the trapping and release of gas bubbles from the foam type of catalyst where the noise in the current response represents the temporal changes of the surface area which goes along with the gas bubble formation in the interior of the catalyst`s pores.

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CONCLUSION The activity and product selectivity of Cu skeleton type of catalysts were studied with regard to CO2RR applications. The skeleton itself reveals only a poor catalytic activity towards CO2RR which can mainly be assigned to contaminations/residuals from the production process which can hardly be removed even by electropolishing. In addition, such Cu sponges show unusual crystallographic characteristics deviating from polycrystalline Cu. However, Cu skeletons can be considered as a promising support material for an active catalyst layer due to its open-cell porosity which in principle allows a flow-through of either gaseous or liquid media through the three dimensional catalyst, thus being important for future application on the cell level of real CO2 electrolyzers. It has been demonstrated that an activation of the largely inactive Cu skeleton can be achieved either by thermal annealing or by Cu form electrodeposition. In both cases a new catalytically active skin is formed on the surface of the Cu skeleton support. The concept of HER assisted Cu foam electrodeposition was successfully transferred from planar supports to 3D skeletal supports. Both treatments lead to active Cu catalysts which favor the C2 pathway (C2H4 and C2H6) whereas the C1 pathway (CH4) towards hydrocarbons remains fully suppressed. Both modified catalysts can be considered as oxide-derived and both show a preferential (100) texturing in their XRD analysis thus favoring the C2 pathway. Differences in their C2 reaction pathway concern the particular ratio of C2H6/C2H4 product formation which is higher in case of the electrodeposited Cu foam. This observation could be rationalized by the presence of µmsized pores within the Cu foam thus leading to a more efficient trapping of reaction intermediates (e.g. C2H4) as compared to the annealed skeleton sample and by this to a longer mean residence time of the intermediates inside the catalyst modified by the foam electrodeposition. This effect clearly favors the fully reduced C2 product. Future research will focus on the further improvement of the C2 efficiencies in particular of the Cu skeleton modified by the Cu foam deposition. The C2 efficiencies of 29.1% presented herein are below the maximum of FEC2 = 55% reported for Cu foams on planar wafer supports.12 This has been explained by the less uniform current distribution during the metal foam deposition on the 3D skeleton as compared to the planar wafer surface.

Swiss National Foundation (No. 200020_172507). This study was performed with the support of the interfaculty Microscopy Imaging Centre (MIC) of the University of Bern.

REFERENCES (1)

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ASSOCIATED CONTENT Supporting Information Further SEM/XPS/EDX characterization of the Cu skeleton catalysts, details concerning their synthesis, and results of further experiments on the reduction of CO2 are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author *Email: [email protected]

Author Contributions

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≠ A.D. and M.R. contributed equally to this work.

Notes The authors declare no competing financial interest. (15)

ACKNOWLEDGMENT The support by the CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage) is gratefully acknowledged. P.B. acknowledges financial support from the

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