Electrochemical Reduction of CO2 into Multicarbon Alcohols on

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Electrochemical reduction of CO into multicarbon alcohols on activated Cu mesh catalysts: An identical location (IL) study Motiar Rahaman, Abhijit Dutta, Alberto Zanetti, and Peter Broekmann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02234 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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Electrochemical Reduction of CO2 into Multicarbon Alcohols on Activated Cu Mesh Catalysts: An Identical Location (IL) Study Motiar Rahaman‡, Abhijit Dutta‡, Alberto Zanetti, and Peter Broekmann* Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern 3012, Switzerland Supporting Information Placeholder ABSTRACT: Potential-dependent CO2 reduction reactions (CO2RR) 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 3h 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 (FEFormate = 49.2% at -0.7 V vs RHE) and in particular for C-C coupling reactions at higher overpotentials (FEC2H4 = 34.3% at -1.1 V vs RHE). The subsequent thermal annealing treatment directs the CO2RR product selectivity towards multicarbon alcohol formation (ethanol/EtOH and n-propanol/n-PrOH) resulting into a total Faradaic yield of FEalcohol = 24.8% at -1.0 V vs RHE -1 -2 -1 -1 (FEEtOH = 13%). Moreover, the EtOH and n-PrOH production rate of 155.2 µMLelectrolyte cmECSA h and 101.4 µML-1electrolyte cm-2 ECSA h (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 Cu2O/CuO surface precursor phases. The maximum of the n-PrOH efficiency is observed at slightly less negative potentials of -0.9 V with FEn-PrOH = 13.1%. Identical location (IL) SEM analysis was applied prior and after the annealing preparation steps and in addition prior and after CO2RR to monitor severe morphological changes which go along with the formation of Cu2O/CuO surface phases upon thermal annealing and their subsequent electroreduction under operando conditions of the CO2RR. Fringe pattern in the HR-TEM analysis confirms the existence of Cu/Cu oxide planes on the corresponding annealed catalysts. ILSEM and HR-TEM analyses further identify nano-dendritic Cu as being the active component for the desired production of multicarbon alcohols. In addition, such nano-dendritic Cu shows a remarkably high resistance against degradation with alcohol efficiencies that can be maintained on a high level (FEalcohol = ~24% at -1.0 V) over 6 hours 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 CO2RR mechanism and related to that 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. KEYWORDS: CO2 electroreduction, copper, mesh support, identical location study, ethanol, n-propanol

Introduction The electrochemical conversion of CO2 into products of higher value can be considered as a seminal approach1-6 that has great technological potential for a future closing of the anthropogenic carbon cycle. Such CO2 electroconversion process (in the following referred to as CO2RR) offers not only the unique chance to reduce the amount of environmentally harmful CO2 in our atmosphere, it provides in addition means of storing intermittently produced excesses of electricity originating from renewable energy sources like wind, solar and hydro. Major challenges that currently prevent such electrochemical CO2 conversion technology from being implemented into industrial applications are related to the enormous overpotentials needed for CO2 activation, thus typically resulting into a poor energy efficiency of the entire full cell-level process.5 Most desired CO2RR products are chemical feedstocks serving as starting material for large-scale synthesis processes in the chemical industry (e.g. CO/syngas7-11 or unsaturated hydrocarbons like C2H412-21) or liquid fuels of high energy density22 such as methanol23, 24 (MeOH), ethanol13, 14, 21, 25 (EtOH) or npropanol26, 27 (n-PrOH). Recently, Zhao et al. synthesized oxide derived Cu catalysts on a carbon support from Cu-MOFs that demonstrated high activity towards alcohol production (MeOH and EtOH) at particularly low-overpotentials.28 A major challenge to be addressed in the CO2 technology development is related to the product selectivity of the CO2RR which strongly depends on the choice of the catalyst material

that is needed for the CO2 activation.29 Whereas the electrosynthesis of CO on Ag-based catalysts can nowadays already be operated with Faradaic efficiencies reaching 92% at high reduction rates and partial current densities,30 the production of unsaturated hydrocarbons (C2H4, C3H6) and alcohols (EtOH, n-PrOH) from CO2 or CO reactants remains much more challenging. So far only Cu-based catalysts are reported to yield C-C coupled hydrocarbons and alcohols in significant amounts.25, 29, 31, 32 Early work by Hori et al.13, 25 using electropolished Cu foils as catalysts already demonstrated ethylene and ethanol formation (C2 pathway) with maximum Faradaic yields of 48.1% and 18.2%, respectively, whereas only minor amounts of the C3 coupled n-propanol were obtained reaching a maximum of in the Faradaic yield of 4.2%.13, 25 In these pioneering studies, CO2 was converted under strictly galvanostatic conditions (-5 mA cm-3, E = -1.1 V vs NHE).13, 25 The required high overpotentials needed for the CO2 activation not only reduce the energy efficiency of the overall process, they are also the major origin of poor product selectivity of the CO2RR on Cu catalysts. Under these extreme conditions the kinetic activation barriers are sufficiently low for a variety of different competing hydrogenation and C-C coupling reactions which particularly involve chemisorbed *CO (the asterisk * indicates an adsorption state) as major intermediate species.25 Strategies towards an improvement of the CO2RR product selectivity go therefore often along with a decrease of the required overpotentials. It is well known that activity and selectivity characteristics of the Cu catalysts strongly depend on their chemical and

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physical pretreatment such as sputtering33, electropolishing33, electrodeposition34 and anodization27, 34. Most successful activation strategies of catalyst activation rely on the formation of a thin oxide skin on the respective catalyst surface which gets reduced to metallic Cu under conditions of CO2RR.33, 35 Such operando activation of the Cu catalysts via the electroreduction of cupric and cuprous precursor species creates those specific low-coordinated sites on the catalyst surface that are particularly mandatory for the stabilization of (i) chemisorbed ·-

*COଶ radical anions as key intermediates in the initial phase of the catalytic CO2 conversion and (ii) of the chemisorbed *CO which is the key intermediate for the subsequent hydrocarbon and alcohol formation process.36 Stabilizing chemisorbed *CO on the Cu catalyst is beneficial in a twofold sense, namely (i) it increases the *CO surface concentration thereby promoting those CO2RR pathways involving C-C coupling reactions and (ii) it simultaneously blocks reaction sites for the parasitic hydrogen evolution reaction (HER).13, 25 A variety of treatments have been proposed to produce the cuprous/cupric precursor thin films including electrochemical oxidation27, 34, thermal annealing26, 35, 37, 38 or oxygen plasma treatment20 all leading to so-called oxide-derived (OD) Cu catalysts. Besides molecular oxygen or more reactive oxygen plasma species20 also other corrosive agents like bromine or chlorine are known to oxidize Cu giving rise to the formation of intermediate cuprous halides or poorly soluble azurite (Cu3(CO3)2(OH)2) and malachite (Cu2CO3(OH)2) phases.39 Similar precursors can be obtained by an electrode anodization in the presence of halides in solution.18 The intermediate presence of Cu halide phases has been discussed in literature as beneficial for the production of ethylene.31, 40, 41 However, it needs to be pointed out that similar to the cuprous/cupric oxides also the respective halide and carbonate phases are thermodynamically unstable under CO2RR conditions.39 Kinetic effects, e.g. due to the poor electric conductivity of massively precipitated cuprous oxides/halides, might, however, lead to an intermediate (kinetic) stabilization of those cupric/cuprous phases even under CO2RR conditions. Similar kinetic effects are known from other oxidic CO2RR catalyst materials, e.g. SnO2-NPs whose structural and compositional stability was probed by potential-dependent operando Raman spectroscopy.42 Operando X-ray absorption spectroscopy in combination with cross-sectional TEM analysis have further shown that oxygen species can remain embedded to some extend at subsurface sites of the Cu matrix even under such harsh cathodic conditions typically applied for the CO2RR.20 What all these different catalyst activation treatments have in common are severe morphological changes as consequence of mass transfer and volume changes which accompany the electro-reduction of the cuprous and cupric oxide/halide precursor phases.27 Typically, metallic Cu nanoparticles (Cu-NPs) or Cu nanocrystallites (Cu-NCs) form during this final reductive activation step which can be considered as the actual catalyst species promoting C-C coupling or alcohol formation.37, 39 A prime example thereof was given by Li et al.37 reporting on the production of multicarbon oxygenates (EtOH, n-PrOH and acetate) from CO reactants on oxide-derived nanocrytsalline Cu reaching a Faradaic efficiency of FEoxygenates = 57% (maximum FEn-PrOH = 10.0%, maximum FEEtOH = 42.9%) at considerably low cathodic potentials ranging from -0.25 to -0.5 V vs RHE. Both the activity and the durability of the OD Cu-NP catalyst were shown to decisively depend on the thickness of the initially formed Cu2O surface phase which actually corre-

Figure 1. Optical images and SEM micrograph of the Cu mesh used as support/catalyst for the CO2RR. Left: Cu mesh before electrodeposition; Right: Cu mesh after electrodeposition. Only the upper parts of the meshes (dimension: 8.00 mm x 6.25 mm) were immersed into the electrolyte for the electrodeposition and the CO2RR.

lates with the amount of precursor material available for such operando catalyst formation.38 A further promising approach was recently introduced by Ren et al.27 who electrosynthesized nano Cu crystallites (Cu-NCs) by an anodization of Cu-NPs under alkaline conditions followed by the reductive decomposition of the formed cuprous/cupric oxide surfaces phases thereby leaving highly active Cu-NCs behind. These showed an even higher efficiency towards n-PrOH production from CO2 reactants reaching values of FEn-PrOH = 10.58% at 0.85V vs RHE. The nature of those reaction sites promoting in particular CC coupling reactions in combination with the alcohol formation are, however, still under debate. Fundamental studies on single crystalline Cu catalysts demonstrated that the C-C coupling (C2 pathway) is more efficient on (100) textured catalyst whereas the C1 pathway is preferred on (111)-type of facets.43-46 The formation of multicarbon alcohols seems to be favored in particular when additional low-coordinated kink sites are present, e.g. at stepped Cu surfaces.43 Besides the presence of certain facets also grain boundaries between adjacent crystal facets as one form of interfacial defects seem to play an eminent role in particular for the production of longerchain CO2RR products as stated by Li et al..37 Cu-NPs and CuNCs that form during the precursor reduction are typically not isolated from each other, instead, they tend to agglomerate thereby forming highly reactive grain–boundaries.27, 37 In this current study we report a multi-step activation process of technical 2D Cu mesh catalysts involving an initial Cu electrodeposition which activates C-C coupling reactions followed by an annealing step at moderate temperatures of 300°C that further initiates the desired multicarbon alcohol production. The use of well-defined 2D mesh supports allows for identical location (IL) inspection of the Cu catalysts on the individual stages of their preparation process. Furthermore the particular impact of the CO2RR on the catalyst morphology can be visualized by this approach. IL-SEM and IL-TEM investigations have already been successively applied in the context of catalyst degradation studies for fuel cell applications47, 48 whereas systematic IL studies on the activation of CO2RR catalysts have, to the best of our knowledge, not yet been reported. By means of such an IL approach we will correlate the high Faradaic efficiencies towards C2 and C3 alcohols (total FEalcohol = 24.8% at -1.0V vs RHE, FEn-PrOH = 13.1%


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ACS Catalysis

at -0.9V) to the appearance of nano-dendritic Cu as direct result of the reductive degradation of the oxidic precursor phases under CO2RR conditions. Experimental Figure 1 shows SEM and optical micrographs of the Cu mesh support prior (left) and after (right) modification by Cu electrodeposition. The nominal aperture and the wire diameter of the mesh amount to 380 µm and 250 µm, respectively. For both the electrodeposition and the CO2RR experiments Cu meshes were cut into pieces having dimensions of 8mm x 25mm. Note that not the entire mesh sample was immersed into the electrolyte but only their front sections having dimensions of 8mm x 6.25mm. To ensure that always the same geometric surface area was exposed to the working electrolyte, middle parts of the Cu mesh samples were blanket with insolating Teflon tape. In a first preparation step, Cu was electrodeposited potentiostatically onto electropolished mesh substrates at Edepo. = -1.1 V vs RHE. A CO2 saturated and Cu2+ ion containing 0.5 M KHCO3 buffer solution served as electrolyte for the Cu electrodeposition. The low Cu2+ concentrations ranging from 100

to 500 µmol L-1 in combination with the extremely negative deposition potential of Edepo. = -1.1 V vs RHE guaranteed a Cu electrodeposition that was limited by Cu2+ diffusional mass transfer. Under these experimental conditions fine dendritic Cu deposits form on the Cu mesh having a characteristic black appearance in the visual inspection (“Copper black”, Figure 1). For a fixed deposition time of tdeposition = 2400s the Cu2+ ion concentration was systematically varied between 100 and 500 µM thereby producing different loadings of the catalytically active Cu (for more details see Supporting Information file). In a second preparation step, the formed dendritic Cu catalysts were subjected to thermal annealing in air for 3h at 300°C in a tube furnace (GERO Hochtemperaturofen GmbH, Germany). Technical details on the physical characterization of the Cu catalysts and the electrochemical electrolysis experiments are provided in the Supporting Information file. Results and Discussion Step 1: Catalyst activation by electrodeposition. Figure 2 summarizes all main morphological features of the Cu meshes modified either by electrodeposition (denoted in

Figure 2. Matrix showing the experimental results on the Cu catalysts produced by electrodeposition of Cu (samples M1-M5) onto the Cu mesh support (M0). The respective amount of Cu deposited under potentiostatic conditions at -1.1 V vs RHE (tdepo = 2400 s) is indicated in row R1. The electrochemically active surface area (ECSA) is presented in row R2. R3-R5 represent the structural/morphological characterization of the Cu catalysts by SEM at different magnifications. R6 shows the corresponding XRD data (the numbers indicate the intensity ratio of the (111), (200) and (220) diffraction peaks).


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the following as samples M1 to M5) or by electropolishing (sample M0), the latter serving as internal reference. The amount of deposited Cu, determined gravimetrically, increased linearly from 160 to 790 µg for a constant deposition time of 2400s as function of the concentration of cupric ions in solution (row R1 in Figure 2). This trend is expected due to the linear relation between the deposition current and the concentration under diffusion limited conditions. Important to note is that the electrochemical active surface area (denoted in the following as ECSA, for details see Supporting Information file, Figure S1a) increases from 1.12 cm2 (M0, reference) to 3.07 cm2 (M5) along with the amount of electrodeposited Cu. This corresponds to an increase of the ECSA by 274% with respect to the electropolished sample (row R2 in Figure 2). ECSAs have been determined by probing the reversible redoxtransition of dimethyl-viologens.49 Note that this “viologen” approach of surface area determination was further confirmed by corresponding capacity measurements (Figure S1b) yielding roughness factors that agree well with the ones obtained on the basis of the viologen approach (Figure S1a). Large-scale SEM images clearly demonstrate mesh apertures and wire diameters which remain largely unaffected by the Cu electrodeposition (see e.g. R3 in Figure 2). This experimental approach provides means of depositing an ultra-thin and catalytically active catalyst layer on such a technical support without altering its overall geometric characteristics. It can therefore be considered as promising alternative to Cu foam electrodeposition processes where also dendritic deposits are formed which, however, are part of the pore sidewall structure of the resulting foam (see Figure S2). Both the mesh aperture and the wire diameter are significantly altered in this latter case by the electrodeposited dendritic foams. Such metal foam deposition is typically carried out at high current densities (e.g. j = -3 A cm-2) from highly concentrated metal ion electrolytes.50 Dutta et al. have already reported a high catalytic activity of these dendritic foams towards C2 product formation which, however, suffers from a relatively high content of non-desired saturated C2H6 products. This effect has been rationalized in terms of a prolonged residence time of unsaturated C2H4 intermediates inside the pores thus increasing the probability for intermediate re-adsorption and further reductive hydrogenation reactions (intermediate trapping effect).51 The SEM inspection of the electropolished Cu mesh (M0, Figure 2) reveals a completely featureless and smooth surface. By contrast to that, dendritic Cu features are clearly visible on samples M2 to M5 whose size and area density on the mesh support increase along with the amount of deposited Cu. Interestingly there is only a fine granular Cu film present on sample M1 but no dendritic features. Diameters of the Cu clusters range from 40-60 nm. A similar film is also visible on sample M2 which, however, is located underneath the dendritic features. Compared to M1 the clusters on sample M2 are slightly bigger in size with diameters ranging from 60-80 nm. These experimental observations are indicative for a two-step growth process of Cu on the mesh with an initial (granular) film formation followed by the nucleation of individual dendritic features on top which then spread over the entire surface thereby hindering or even preventing the further growth of the granular Cu film underneath (Row R4 and R5 in Figure 2). Post-deposition XRD inspection (R6 in Figure 2) indicates the presence of polycrystalline Cu with the expected prominent (111), (200) and (220) diffraction peaks for all samples. Note that the observed XRD pattern result from a superposi-

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Figure 3. Steady-state linear sweep voltammograms (LSVs) of the Cu mesh support (orange, sample M0) and the electrodeposited catalyst (red, sample M4) in Ar saturated (dashed lines, pH = 8.15) and CO2 saturated (solid lines, pH = 7.2) 0.5 M KHCO3 electrolyte solutions. dE/dt = 25 mV s-1. The potential scale has been normalized according to the corresponding pH value (for details see Supporting Information file).

tion of the polycrystalline Cu mesh support and the covering electrodeposited Cu. Only minor satellite peaks are visible in the XRD spectra originating from a surface confined Cu2O phase which naturally forms when the Cu catalysts get exposed to ambient air. Oxide formation during the electrodeposition at pH of about 7.2 plays only a marginal role due to the rather negative deposition potential of -1.1 V. A certain minor difference between the 6 Cu samples presented in Figure 2 is related to the relative intensities of the (111), (200) and (220) diffraction peaks (R6). Whereas the (111) diffraction peak is clearly dominant in the XRD pattern of sample M0 and the (200) diffraction peak becomes slightly more prominent after the electrodeposition. The electrochemical characteristics and the electrolysis performances are compared in more detail for samples M0 (electropolished, reference) and M4 (dendritic). For this purpose, steady-state linear sweep voltammograms (LSVs) in Ar (blankets) and CO2 saturated potassium bicarbonate electrolytes are presented in Figure 3 (please find the complete set of LSVs in Figure S3). The exponential increase of the cathodic currents in case of the Ar saturated electrolytes originates predominantly from the reductive hydrogen evolution reaction (HER). Higher reaction rates observed for M4 (red dotted line) can be rationalized on the basis of surface area effects with an ECSA that is higher by a factor of 2.5 in case of M4 with respect to the reference sample M0. The presence of a saturation concentration of CO2 (≈ 35 mM) leads in both cases to an downward shift of the voltammetric curves indicative for an slightly increased activity towards HER and in addition for CO2RR that sets in at slightly less negative potentials as compared to the parasitic HER. This effect is more pronounced in case of sample M4 where an additional slight hump (denoted as P, Figure 3) is visible in the LSV that can be rationalized in terms of a primary CO2RR that reaches mass transport limitations under potentiodynamic conditions before the parasitic hydrogen evolution reaction (HER) becomes dominant.13 Systematic CO2RRs have been carried out for 1h on catalyst samples M0 and M4 under potentiostatic conditions in the potential range from -0.6 V to -1.2 V vs RHE (Figure 4). The measured currents (colored squares) and the corresponding current densities normalized to the respective ECSAs (black circles) are presented in Figure 4a and 4b, respectively. Alt-


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ACS Catalysis To the best of our knowledge, the FEFormate value of 49.2% (-0.7 V) described herein is the highest one reported so far for bare Cu catalysts exposed to CO2 saturated aqueous bicarbonate solutions. The highest catalytic activity towards formate production in our study is further reflected by the maxi-1 mum production rate (312 µM L-1electrolyte cm-2 ECSA h ) at -0.7 V, normalized with respect to the ECSA (see Figure S5 and S6 in the supporting information file). High efficiencies of formate production have also been reported by Kanan et al.38 (FEFormate ≈ 33%) for oxide-derived Cu catalysts and by Sen et al.52 for electrodeposited Cu foam catalysts (FEFormate ≈ 37%). Changing the electrolyte composition, e.g. by using water/ionic liquid mixtures as electrolyte, can, however, further increase the FEFormate values up to ~83% as demonstrated by Huan et al..53, 54

Figure 4. a) - b) Potential dependent steady state currents and current densities derived from the current/time traces of the 1h CO2 electrolysis (see Figure S4 in the supporting information file), the colored squares indicate the electrolysis currents as measured, the black circles represent current densities normalized to the ECSA (see Figure 2); c) Potential dependent Faradaic efficiencies (FEs) of the HER/CO2RR products as derived from sample M0 (electropolished Cu mesh); d) Potential dependent faradaic efficiencies (FEs) of the HER/CO2RR products as derived from sample M4.

hough the overall reaction rates normalized to the ECSA are similar in both cases there are pronounced differences visible in the potential dependent product distributions as demonstrated by the Faradaic efficiency (FE) plots in Figure 4c and 4d. A first remarkable difference between the electropolished sample M0 and the one modified by the electrodeposited dendritic catalyst (M4) concerns the efficiencies of hydrogen production which are significantly higher at low and medium overpotentials (up to -0.9 V vs RHE) for the electropolished reference sample M0. FEH2 values gradually drop down from 78% at 0.6 V to 36% at -1.1 V. The highest FEH2 value on sample M4 amounts, by contrast to that, only to 57% at -0.6 V followed by a quasi-plateau in the potential regime from -0.7 to -1.1 V with FEH2 values scattering between 42% to 39%. At higher negative overpotentials (below -1.1 V) the Faradaic efficiencies for hydrogen production are almost the same for both catalyst samples. With regard to the formate production the potential dependent trend is very similar for both catalysts with a maximum in the FEFormate at low overpotentials of -0.7 V (on M4) and -0.8 V vs RHE (on M0), respectively. The selectivity towards formate production is, however, significantly higher for the dendritic Cu catalyst M4 with a maximum of 49.2% at -0.7 V followed by a continuous drop of the FEFormate to 5% at -1.2 V. On the electropolished reference sample M0, by contrast, the FEFormate never exceeds 24% (-0.8 V). Also it becomes evident from Figure 4c and 4d that the potential dependent FEFormate values are anti-correlated to the efficiencies of hydrocarbon formation which continuously increase at higher negative overpotentials (below -0.8 V) on the expense of FEFormate.

The most important difference between reference sample M0 and sample M4 lies in their preference towards C1 and C2 product formation. On the electropolished sample M0 it is the C1 pathway with a preferential CH4 production which is clearly dominating the CO2RR product distribution. FECH4 reaches its maximum of 44% at highest overpotentials of -1.2 V even in a half-cell configuration. Similar high efficiencies for methane production were reported by Kuhl et al.31 (FECH4 ≈ 41% at -1.15V) and by Mistry et al.20 (FECH4 ≈ 47% at -1.0V) also for electropolished Cu samples. A further increase in the FECH4 was achieved by applying an H2 plasma pre-treatment to the planar Cu catalyst.20 By contrast to that, the C2 pathway is less dominant on the electropolished sample M0 with an FEC2H4 maximum of 8% at

Figure 5. a) Cathodic partial current densities of formate production derived from CO2RR on samples M0-M5 at -0.7 V vs RHE; b) FE plot for H2, CO, and formate derived from CO2RR on samples M0-M5 at -0.7 V vs RHE; c) Cathodic partial current densities for CH4 and C2H4 derived from CO2RR on samples M0-M5 at -1.1 V vs RHE; d) FE vs E plot for H2, CO, CH4, C2H4 and formate derived from CO2RR on samples M0-M5 at 1.1 V vs RHE.


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-1.1 V. The situation is inversed in case of the dendritic catalyst M4. Also the FECH4 continuously rises with increasing overpotentials but reaches a value of only 18.9% at -1.2 V. Obviously the C1 pathway gets partially suppressed in the presence of the Cu dendrites. Instead, the C2 pathway is clearly favored as evidenced by the maximum in the C2H4 efficiency of 34.3% at -1.1 V. A noteworthy detail of the product analysis concerns the entire absence of any C2H6 as the final hydrogenation product of unsaturated C2H4. The C2H6 has been identified as the major C2 product of the CO2RR when dendritic Cu foams (see Figure S2) were used as catalysts.51 This unique feature of the highly porous Cu foam catalysts was rationalized in terms of particular trapping effects of the gaseous CO2RR products and intermediates inside the pores thereby increasing their residence time inside the threedimensional catalyst and thus the probability of subsequent hydrogenations reactions.51 Obviously, the residence time of the C2H4 at the non-porous dendritic catalyst on the Cu mesh support is significantly reduced thus explaining the high yield of unsaturated C2 products. Our findings presented herein therefore further support the scenario of certain “trapping” effects as hypothesized by Dutta et al. for the porous foam catalysts.51 These catalyst performance changes by the electrodeposition have been examined in more detail for two particular electrolysis potentials indicated by the grey bars in Figure 4d. This choice of electrolysis potentials is because of the predominant formate production at lower overpotentials of -0.7 V and the favored hydrocarbon formation (CH4 or C2H4) at -1.1 V. In Figure 5, changes of the catalytic activity going along with the increasing amount of electrodeposited Cu are represented in terms of partial current densities normalized to the ECSA (major products, 5a and 5c) and in terms of Faradaic efficiency (5b and 5d). By normalization to the ECSA all contributions originating from specific surface area changes are eliminated. At -0.7 V, the catalytic activity towards formate production is the lowest for the electropolished reference sample M0 (jFormate = -0.29 mA cm-2) but increases gradually up to a maximum value of jFormate = -0.49 mA cm-2 (M3) before it slightly drops down to jFormate = -0.485 mA cm-2 (M4) and further decreases to jFormate = -0.43 mA cm-2 (M5). The corresponding FE plot (Figure 5b) indicates (i) a strong anti-correlation between FEFormate and FEH2 and (ii) a weaker anti-correlation between FEFormate and FECO. Formation of formate at this potential is thus highly selective. At low overpotentials (–0.7 V vs RHE) the dendritic Cu catalysts are obviously less active for HER than the electropolished reference sample M0. At higher overpotentials (-1.1 V vs RHE) it is the preference towards the C1 and the C2 pathways which drastically changes by going from M0 to M4 (Figure 5c). The electropolished sample M0 shows the highest catalytic activity towards CH4 production with a partial current density of jCH4 = -4.67 mA cm-2 which continuously drops down to jCH4 = -1.08 mA cm-2 by going from M0 to M5. Just the opposite trend is observed for the C2 pathway. It is the electropolished samples M0 which shows the lowest activity towards C2 product formation with jC2H4 = -1.02 mA cm-2. This values increases to a maximum of jC2H4 = -3.87 mA cm-2 for sample M4 before it drops down to jC2H4 = -2.70 mA cm-2 for sample M5. The switching from CH4 (sample M0, C1 pathway) to C2H4 selectivity (in particular sample M4, C2 pathway) is also reflected by the respective FE plot presented in Figure 5d. Representative chromatograms are shown in Figure S7.

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Figure 6. XRD pattern of the sample M4annealed (a) before and (b) after CO2RR at -1.0 V vs RHE. JCPDS files for Cu, Cu2O and CuO are 46-1043, 00-001-1242 and 45-0937, respectively. The relative intensities of the Cu fcc (111), (200) and (220) diffractions are indicated.

As expected, the FECO values remain at -1.1 V for all samples on a relatively low level ranging from 1 to 5% only. This is a clear indication for an efficient conversion of the chemisorbed CO* intermediate into C1 or C2 hydrocarbons. Step 2: Catalyst activation by thermal annealing. A thermal annealing of the catalyst has already been proven as beneficial to improve the activity and selectivity towards C2 product formation.55 Herein we demonstrate that a thermal annealing of the dendritic Cu catalyst for 3h under relatively mild conditions (300°C) is already sufficient to significantly alter the product distribution in particular with respect to the formation of primary alcohols. To demonstrate this improvement we used sample M4 which already demonstrated the highest selectivity towards C2 hydrocarbon (C2H4) formation during the prescreening (Figure 5d). Annealing sample M4 leaves the relative intensities of the Cu related (111), (200) and (200) diffraction peaks largely unchanged (Figure 6). Weak and broadened diffraction features of a cuprous Cu2O phase appear in the diffractogram after the annealing treatment with the Cu2O(111) diffraction reflection at 2θ = 36.3° being the most intensive one. Cu2O(200) and Cu2O (220) peaks can be found in the diffractogram at 2θ = 42.15° and 61.16°, respectively. The extra weak diffraction peak at 2θ = 31.6° is indicative for a minor contribution of crystalline cupric CuO to the mixed oxide phase. Identical location (IL) SEM inspection prior and after anneal demonstrates that the overall morphology of the catalyst and in particular the µm-sized dendrites remain fully intact upon the thermal treatment (Figure 7). However, on the nm length scale the crystalline fine structure has disappeared and the pristine Cu crystallites seem to be coalesced after the annealing. Corresponding HR-TEM inspection revealed the formation of a Cu2O/Cu composite layer on the Cu surface of the dendritic catalyst structure. Clearly visible in Figure 7 are Cu2O(111) and Cu2O(200) related fringe pattern in full agreement with results of our XRD analysis (Figure 6a). Further IL-SEM analysis carried out after the CO2RR at -1.0 V demonstrates the formation of oxide derived Cu nanoparticles/nanocrystals on the Cu dendrites having diameters in the range from 10 – 50 nm (Figure 7). Their presence leads to a nano-dendritic appearance in the respective HR-TEM image (Figure 7). These findings are in agreement with the further increase of the ECSA from 2.76 cm2 (for sample M4, as prepared, Figure 2) to 3.04 cm2 (Figure S8) as consequence of the


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Figure 7. Identical location (IL) SEM and high resolution (HR) TEM analysis of the Cu catalyst (sample M4 ) after the electrodeposition (topmost row), after the 3h annealing treatment at 300°C (middle row) and after the subsequent CO2RR at -1.0 V (bottom row).

reduction of the oxide precursor. Note that the electrochemical surface area determination was carried out by probing the reversible redox-transition of di-methyl viologens. The CuxO phases were electrochemically reduced prior to the ECSA determination (Figure S8). From our HR-TEM analysis we have no indication for the presence of Cu2O or CuO nano-crystals left after the CO2RR, thus proving the structural and compositional instability of oxide-precursors under strong reductive conditions of CO2RR (-1.0 V). These findings are further confirmed by our ex situ post-electrolysis XPS results (see Figure S13). A survey FE plot of the potential dependent product distribution of the CO2RR derived from the annealed sample M4annealed is shown in Figure 8a. Most prominent differences to the non-annealed sample M4 (Figure 4d) are related to (i) the complete suppression of CH4 production and (ii) to the appearance of primary alcohols (EtOH and n-PrOH) as new CO2RR products. The formate efficiencies FEFormate qualitatively show a similar trend as for the non-annealed sample (Figure 4d) but reach a maximum of only 33% at -0.7 V whereas a maximum value of 49.2% was observed for the non-annealed sample M4. Similar qualitative trends are observed for the CO efficiencies which are in both cases highest at lowest overpotentials (-0.6 V vs RHE) and drop down with increasing overpotentials. However, in particular at -0.6 V and -0.7 V much higher CO efficiencies are observed for the annealed sample M4annealed with 24.1% and 16.9% as compared to 13% and 8.2% for the non-annealed one (Figure 4d). Also the C2H4 efficiencies follow qualitatively the same trend as observed for the nonannealed sample M4 but reach at -1.0 V a significantly lower maximum of only 17.8% as compared to 34.3% on the nonannealed sample at -1.1 V (Figure 4d). Not only is the appearance of primary alcohols as CO2RR products after the thermal treatment intriguing but also the

formation of C3 products such as n-propanol and propylene (C3H6). The propylene efficiency FEC3H6, however, never exceeds 2% (Figure 8a). Propylene can therefore be considered as a minor by-product of the CO2RR. Figure 8b demonstrates the potential dependent formation of CO2RR products grouped according to the number of C atoms in the respective product. The C1 efficiencies (CO and formate) are continuously decreasing with increasing overpotentials whereas the C2 and C3 efficiencies pass both a maximum at -1.0 V and -0.9 V, respectively. These results clearly prove that the nano-dendritic Cu is capable to stabilize not only chemisorbed *C1 intermediates (e.g. *CO) as crucial prerequisite for *CO dimerization but in addition also adsorbed *C2 intermediates (*C2HxOy). C3 product formation requires the selective C-C coupling of *C1 and *C2 intermediates which most likely results first in the formation of propionaldehyde as already stated previously by Hori at al.56 and recently by Ren et al.27. In a second reaction step the metastable aldehyde gets rapidly reduced to the corresponding C3 alcohol. The total C3 efficiency reaches a value of 14.5 % at remarkably low potentials of -0.9 V. Total CO2RR efficiencies vary only marginally within the potential range from -0.6 V to -1.1 V from 49% to 56% (quasi-plateau regime) before they drop down to 31% at -1.2 V mainly due to the increasingly dominating HER (Figure 8a). Alcohol production sets in at 0.8 V vs RHE (Figure 8c). Maximum efficiency for EtOH production is reached at -1.0 V (FEEtOH = 13%) whereas the maximum of n-PrOH efficiency is observed at slightly less negative potentials of -0.9 V with FEn-PrOH = 13.1% (chromatograms for alcohol formation are provided in Figure S9). The total Faradaic efficiency for alcohol production at -1.0 V amounts to intriguing 24.8% which corresponds to a partial current density for alcohol formation of -2.81 mA cm-2 normalized to the ECSA (Figure 8d). The partial current density for n-PrOH production reaches a remarkable value of -1.33 mA cm-2ECSA, normalized to the electrochemical surface area.


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Figure 9. Time dependence of the FEs (main products of the C1/C2/C3 reaction pathways and parasitic HER) during CO2RR. a) Sample M0 (electropolished mesh) at -1.1 vs RHE; b) Sample M4 (electrodeposited) at -1.1 V vs RHE; c) Sample M4annealed (3h anneal at 300°C in air) at -1.0 V vs RHE.

Figure 8. a) Survey FE vs E plot of the CO2RR product distribution derived for the annealed (300°C in air, 3h) sample M4annealed; b) FE vs E plot showing the potential dependent changes of the C1 (CO, formate), C2 (C2H4, EtOH) and the C3 (n-PrOH, C3H6) products of CO2RR; c) FE vs E plot highlighting the potential dependent alcohol efficiencies; d) Corresponding plot of cathodic partial current densities for alcohol production normalized to ECSA.

The high catalytic activity of our annealed Cu dendrite catalyst becomes further evidenced by the rate of alcohol formation of -2 -1 155.2 µMLelectrolyte cmECSA h-1 for EtOH (-1.0V vs RHE) and 101.4 µMLelectrolyte cmECSA h-1 for n-PrOH (-1.0 V vs RHE) as shown in Figure S16. Note that the production rates were normalized to the respective ECSA of the catalyst. Particularly promising is the high yield of n-propanol whose energy density is significantly higher than the one of ethanol. The heat of combustion at 25°C amounts to ∆Hc = 1336.8 kJ mol-1 for EtOH compared to ∆Hc = 2021.3 kJ mol-1 for n-PrOH. Faradaic efficiencies of alcohol production (Figure 8c) exceed the maximum efficiencies reported by Ren et al.27 with FEEtOH = 7.7% and maximum FEn-PrOH = 10.58% obtained at slightly more positive potentials of -0.85 V vs RHE than in our present study. The high activity of the Cu catalyst can clearly be attributed to the formation of nanocomposite-dendritic Cu in the course of the oxide precursor reduction. Identical location (IL) SEM inspection (Figure 7) shows presence of nano-cavities and smaller Cu-NPs after completing the CO2RR which have diameters in the range of 10 nm to 50 nm. This observation was further supported by our HR-TEM analysis of the annealed catalyst sample after CO2RR (Figure 7). The thermal oxidation/electrochemical reduction treatment obviously leads to a further fragmentation and cracking of the pristine nm-sized crystallites. This fragmentation leaves nano-dendritic catalyst -1

-2

morphologies behind. These results are also in full agreement with the observation of an increased ECSA after the annealing treatment (Figure S8). Further control IL-SEM experiments carried out after CO2RR over the electrodeposited sample M4 reveal that the pristine dendritic morphology of the electrodeposited Cu catalyst is, by contrast, fully conserved upon 6h CO2RR (Figure S10). No formation of smaller Cu-NPs was observed in this case in the respective IL-SEM inspection. From this result one can safely conclude that the appearance of the Cu-NPs in case of the annealed sample has to be associated solely to the electro-reduction of the oxide precursors in the initial stage of CO2RR and is not a direct consequence of the CO2RR itself. Long term stability and mechanistic insights A further interesting difference between the three studied Cu catalysts concerns their stability against degradation as demonstrated in Figure 9. For this degradation study the electrolysis time was extended to 6h (the corresponding long term current transient plots are given in Figure S11). Actually, it is the electropolished reference sample which is most prone to degradation (Figure 9a). Within 4h electrolysis time the FECH4 drops down from 43% to 3% only. After 6h electrolysis methane is hardly detectable any more in the GC analysis. Qualitatively similar degradation trends are observed for the electrodeposited sample (Figure 9b). In this case, it is in particular the C2H4 efficiency which significantly drops down from initial 33% (1h) to 15% (6h) whereas the methane production remains on a fairly stable level of 14.8% after 1h and 14% after 6h electrolysis time. This demonstrates that it is predominantly the C2 hydrocarbon pathway which is severely affected by the long time electrolysis on the electrodeposited sample whereas the respective C1 hydrocarbon pathway remains active, at least on the time scale studied herein. Qualitatively we consider the reaction pathways for the hydrocarbon formation on the electropolished and the electrodeposited samples as mechanistically closely related to each other. Our experimental results clearly support the assumption of a coupled C1/C2 hydrocarbon pathway similar to the one discussed by Nie et al..57 Their DFT results discuss C1 and C2 pathways that share key intermediates such as *CO, *COH and *CH2 (see Figure 10a adopted from ref. 57) where the adsorbed *CH2 species are essential for the dimerization step.


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Figure 10. Reaction schemes proposed for the CO2RR on the electropolished and electrodeposited catalyst samples (a: coupled C1/C2 pathway) and on the annealed sample (b: coupled C2 hydrocarbon/alcohol pathway) starting from the *CO intermediate. The schemes were adopted from ref. 36 and 57.

The scenario of a coupled C1/C2 pathway became recently further extended by Xiao et al. particularly focusing on the pH dependence of the C1 and C2 hydrocarbon formation.58 It was confirmed by their DFT study that at neutral pH C1 and C2 pathways share adsorbed *COH species as common intermediates. In addition a new dimerization pathway was postulated occurring via the coupling of *CO and *COH leading to *COCOH species.58 In an attempt to rationalize the observed quantitative differences in the C1 and C2 product selectivity of the electropolished and the electrodeposited samples with C1/C2 ratios of 4.43 and 0.37 (at -1.1.V vs RHE, derived from Figure 4) we have to consider various aspects. A first structural effect might be related to the density of catalytically active sites. Cu catalysts active for C-C coupling reactions are much more demanding in terms of providing a sufficient number of lowcoordinated binding sites in closest proximity to each other thereby forming a critical ensemble of surface sites capable for single or even multiple C-C coupling reactions. It can be assumed that the density of those binding sites is higher on the electrodeposited sample than on the smoother electropolished one (Figure 2). This structural effect might contribute to the higher probability of C-C coupling on the electrodeposited sample in comparison to the electropolished one, at least in the initial stage of the CO2RR. Furthermore one has to consider local pH changes which might guide the product selectivity either into the C1 or the C2 direction. Both from experimental work and from calculations it is known that at lower pH the C1 pathway (methane production) is favored whereas at more elevated pH the C2 pathway (ethylene production) becomes more dominant.58, 59 Starting from an almost neutral pH of 7.2 in the CO2 saturated 0.5 M KHCO3 solution one could assume that local pH changes due to the parasitic HER (pre-dominant

water splitting involving OH- production) are more severe in case of the significantly roughened electrodeposited sample (Figure 2) than on the smoother electropolished one. This would further direct the CO2RR selectivity on the electrodeposited sample towards ethylene production whereas the C1 pathway gets more suppressed (kinetic effect).58 It needs to be pointed out, however, that local pH changes cannot rationalize the degradation behavior of the electropolished and the electrodeposited samples (Figure 9). The observed drop down of the FEC2H4 with electrolysis time (Figure 9b) has to be assigned to a catalyst poisoning/blocking effect pre-dominantly caused by the C1 hydrocarbon pathway. According to Hori et al., Akhade et al. and Dewulf et al., this C1 pathway might involve either an irreversible chemisorption of *C surface species on the catalyst surface25, 60, 61 (see Figure 10a) or the blocking of those sterically demanding ensembles of reactive sites that are required for the *CH2 dimerization57 or *CO/*COH coupling reaction58. Not only the changed product distribution but also the extraordinarily high stability of the M4annealed catalyst (Figure 9c) points to a fundamental change in the CO2RR mechanism.36 It can be assumed that the C-C coupling takes already place in a rather early stage of the catalytic reaction most likely via chemisorbed *CO intermediates (Figure 10b, adopted from ref. 36).44, 57, 58 A key structural element of the OD catalyst is the appearance of nano-dendritic Cu in form of Cu NPs and nm-sized cavities and cracks on the larger dendrites (see HRTEM analysis in Figure 7). One could assume that a local pH change in the interior of these nano-cavities is even more pronounced than in the case of the electrodeposited sample. For strongly alkaline surface conditions Xiao et al.58 predict indeed a full (kinetic) suppression of the C1 pathway and the opening of the of an extra reaction branch via the *COCOH


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nm-sized Cu particles/ Cu crystallites and nano-cavities at the surface of the dendritic Cu after the oxide precursor reduction. These Cu-NPs/-NCs can be identified as those catalyst species that are particularly active for the alcohol formation and the CC coupling reactions that even lead to C3. Our degradation study supports the hypothesis that the hydrocarbon formation on the electropolished and the electrodeposited Cu catalysts relies on a coupled C1/C2 reaction pathway whereas on the annealed Cu catalyst a coupled C2 hydrocarbon/alcohol pathway is operative.

Figure 11. Comparison of the product distribution of catalyst sample M4 (a) and M4annealed (b) that were deposited on Cu, Ag and Au mesh supports. The CO2RR product analysis was carried out after 1h electrolysis at -1.1V (M4, optimal conditions for ethylene production) and at -1.0V (M4annealed, optimal conditions for alcohol production.)

intermediate leading to C2 or even C3 products.58 The long term stability of the OD dendritic Cu catalyst (Figure 9c) in combination of the absence of methane as CO2RR product confirms such a coupled C2 hydrocarbon/alcohol reaction pathway which would even allow for further C-C coupling.58 Obviously it is the creation of highly active catalyst ensembles by the combined electrodeposition/annealing approach which enables (i) more complex C-C coupling reactions, (ii) the stabilization of CxHyOz oxygenate intermediates and further prevents severe catalyst degradation at the same time.

(The Supporting Information is available free of charge on the ACS Publications website).

AUTHOR INFORMATION Corresponding Author PD Dr. Peter Broekmann Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern 3012, Switzerland E-Mail: [email protected]

Author Contributions ‡ M.R. and A.D. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Transfer of the catalyst preparation to other support materials In order to prove the robustness of our proposed multistep Cu catalyst preparation procedure we have successfully transferred the most active catalysts (M4 and M4annealed) to Au and Ag meshes (for details see Supporting Information file, Figure S17-20). Their intrinsic selectivity towards CO as CO2RR product gets fully suppressed by the deposition of the dendritic M4 Cu catalyst. Figure 11a and b demonstrates almost the same product distribution for M4/M4annealed on the Au and Ag meshes as observed before on the technical Cu mesh substrate. For further details on these catalysts see the Supporting Information file. Conclusions A multi-step preparation procedure comprising (i) initial electrodeposition, (ii) thermal annealing (3h at 300°C) and (iii) reductive degradation of such formed cuprous/cupric oxide precursors has been developed. The proposed procedure is ideally suited for the activation of technical CO2RR catalysts/supports such as Cu meshes but can also be applied to other support materials (e.g. Au or Ag). The electrodeposited Cu dendrite show high selectivity towards formate and C2H4 production at lower and higher overpotentials, respectively. The oxide derived Cu catalysts reveals a remarkably high selectivity towards ethanol (FEEtOH = 13.0%, -1.0 V vs RHE) and n-propanol (FEn-PrOH = 13.1%, -0.9 V vs RHE) production, both highly added value CO2RR products potentially interesting as high energy density fuels. IL-SEM and HR-TEM have been applied to follow the morphological changes which go along with the subsequent electrodeposition and thermal oxidation of the catalyst. Key result of the identical location analysis is the appearance of smaller

The financial support by the CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage) is gratefully acknowledged. M. R. gratefully acknowledges the financial support by Swiss Government Excellence Scholarships for Foreign Scholars (ESKAS). P.B. acknowledges financial support from the 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.

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