SUPPORTING INFORMATION. for the paper

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1 SUPPORTING INFORMATION for the paper Electrochemical CO 2 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 EXPERIMENTAL Materials. The Cu skeleton (sponge) material has been purchased from Goodfellow Cambridge Limited in form of pieces having dimensions of 150mm x 150 mm 6.35mm (Figure S1). The mass density of the Cu skeleton amounts to = 0.8 g cm -3 which corresponds to a porosity of 91%. Key structural motif of the Cu skeleton is an open-cell architecture of interconnected pores having diameters in the range of 1-2mm (Figure S1). According to the supplier the purity of the Cu material amounts to 99.9% with trace contaminations of O (400 ppm), Ag (500 ppm), Bi ( < 10 ppm), Pb (< 50 ppm), and other metals (< 300 ppm). However, not indicated in the technical data sheet are particular zircon contaminations on and inside the struts of the Cu skeleton most likely originating from the investment casting route that is typically used for the fabrication of these sponge type of metallic materials. Clearly visible in the SEM images (Figure S2) are particles on the Cu struts which are identified as zircon by means of EDX mapping (Figure S2) and EDX point analysis (Figure S3) and by XPS (Figure S4). Zircon is a zirconium silicate (ZrSiO 4 ) which is insoluble and even highly resistant against chemical digestion. This is most likely why this particular contamination is overlooked in standard analysis procedures, e.g. by means of ICP- MS of the oxidatively dissolved material (e.g. in aqua regia). For the electrolysis experiments the Cu skeleton was cut into smaller pieces having dimensions of 30mm x 8mm 2mm (Figure 1). However, not the entire Cu skeleton samples were immersed into the electrolyte for the electrolysis reactions but only sections having dimensions of 5mm x 8mm 2mm. To ensure that always the same volume of the Cu skeletons were exposed to the electrolyte, parts of the Cu skeleton samples were covered with Teflon tape (see Figure 1, electrodeposited sample). Electropolishing of the Cu skeletons was carried in orthophosphoric acid (50w%, ACS grade, Sigma-Aldrich) in a twoelectrode arrangement with a graphite foil serving as counter electrode (anode). A potential of -2 V was applied. The Cu skeleton was gently moved in the phosphoric acid to remove oxygen bubbles formed on the skeleton sample during the 2 min. electropolishing procedure. Electrolytes. For the electrodeposition of the functional Cu foam (Figure S5-S7), a Cu plating bath was used containing 1.5 M sulfuric acid (ACS grade, H 2 SO 4, Sigma-Aldrich) and 0.2 M Cu(II) sulfate (ACS grade CuSO 4 5H 2 O, Sigma-Aldrich). The electrochemically active surface areas (denoted as ECSA) of the Cu skeleton samples was determined by cyclic voltammetry (CV) using di-methyl viologens (DMV 2+ ) as reversible redoxprobes. Scan-speed depending CVs were carried out in aqueous 1M Na 2 SO 4 (ACS grade, Sigma-Aldrich) solution containing 10 mm DMVCl 2 (Sigma-Aldrich). Similar results are obtained when other redox probes were used, e.g. Ru(NH 3 )] 6 Cl 3 (see Figures S12 and 13). Characterization methods. X-ray diffraction. The crystallinity of the Cu skeleton samples was studied prior and after the CO 2 electrolysis by means of powder XRD techniques (Bruker D8) with CuKα radiation (λ = nm, 40 ma) generated at 40 kev. Scans were recorded at 1 min 1 for 2θ values between 20 to 90. Obtained XRD pattern were analyzed and compared with JCPD (Joint Committee on Powder Diffraction) standards for Cu, Cu 2 O and CuO. X-ray photoelectron spectroscopy (XPS). XPS studies were carried out using Al-K radiation sources operated at 150 W with an Omicron Multiprobe (Omicron Nano Technology) spectrometer coupled to an EA 125 (Omicron) hemispherical analyzer. Samples for the XPS inspection were dried under Ar-stream after the electrochemical treatment and used for the XPS without any further modification (e.g. by sputtering). Scanning Electron Microscopy (SEM) and Energy-Dispersive X- ray spectroscopy (EDX). Catalyst samples were subjected to SEM and 2D-EDX analysis using a Hitachi S-3000 N Scanning Electron Microscope and a Noran SIX NSS200 energy-dispersive X-ray spectrometer. For the high-resolution SEM imaging a Zeiss DSM 982 instrument was used. Electrochemical experiments. The electrodeposition of the functional Cu foam was carried out in a glass-beaker with a Pt-wire serving as counter and a leakless double-junction Ag/AgCl 3M as reference electrode. For the galvanostatic deposition process a current density of J = -3.0 A/cm 2 was applied. As plating bath we used an additive-free electrolyte containing 1.5 M sulfuric acid and 0.2 M Cu(II) sulfate.

2 A custom-built, air-tight glass cell (H-type) was used for all voltammetric measurements and CO 2 electrolysis experiments. The three-electrode arrangement consisted of a leakless Ag/AgCl 3M reference electrode (EDAQ), a bright Pt-foil (15mm x 5mm) serving as counter electrode and the Cu skeleton catalysts serving as working electrodes. Possible chloride ion cross-contaminations in the working electrolytes originating from the Ag/AgCl 3M reference electrode were monitored and excluded by ion exchange (IC) chromatography (IC detection limit: 100 ppb Cl). Prior to electrolysis, the cathodic and the anodic compartments were both filled with 30 ml of 0.5M NaHCO 3 (ACS grade, Sigma-Aldrich) electrolyte solution and saturated with CO 2 gas (99.999%, Carbagas, Switzerland). Catholyte and anolyte were separated by a polymer membrane (Nafion 117, Sigma-Aldrich). All electrochemical measurements (galvanostatic Cu deposition, CV and potentiostatic CO 2 electrolysis) were carried out using a potentiostat/galvanostat (Metrohm Autolab 128N, The Netherlands). The current interrupt approach (Autolab Nova) has been used to determine and compensate the ir drop. Cell resistances in the range of were determined by the current interrupt method. Similar values are obtained on the basis of electrochemical impedance spectroscopy (Figure S17). For the sake of comparability, all potentials measured versus Ag/AgCl 3M were converted to the RHE scale by using the equation: E RHE (V) = E Ag/AgCl(3M) (V) V + (0.059 V ph) The ph of the CO 2 saturated 0.5 M NaHCO 3 solution at the beginning of the electrolysis was determined to be 7.2. The electrochemically active surface areas (denoted as ECSA) of the Cu skeleton catalysts was determined by cyclic voltammetry (CV) using di-methyl viologens (DMV 2+ ) as reversible redoxprobes (Figure S12). Scan-speed depending CVs were carried out in aqueous 1M Na 2 SO 4 (ACS grade, Sigma-Aldrich) solution containing 10 mm DMVCl 2 (Sigma-Aldrich). The electrochemically surface area was determined on the basis of the Randles- Sevcik equation i p = 2.69 x 10 5 n 3/2 A c D 1/2 1/2 with i p representing the peak current, and the number of transferred electrons (n = 1), c the concentration of the redox-active DMV 2+ species, D the DMV 2+ diffusion coefficient and the potential sweep rate. The DMV 2+ diffusion coefficient has been measured by 1 H-DOSY-NMR. The electrochemically active surface area A could be determined by linear regression of the respective i p vs 1/2 plots with A serving as free fit parameter. Important to note is that the surface area determination is done in a separate measurement campaign (ideally after the CO 2 electrolysis) in order to avoid any cross-contamination originating from the chloride counter-ions of the viologens. We are aware of the fact that the equations used were originally derived only for planar surfaces and not for three-dimensional ones. We consider the thus determined ECSAs as estimation. Our results were further crosschecked with an alternative redox-probe (Ru(NH 3 )] 6 Cl 3 ). Both approaches result in very similar ECSA values (see Figures S12 and S13). Gas chromatography (GC). The headspace of the catholyte compartment was continuously purged with CO 2 gas thereby transporting volatile reaction products from the headspace into the sampling loops of the gas chromatograph (GC, SRI Instruments, Multi-Gas Analyzer #3). The partial current density for a given gaseous product was determined using the equation: I 0 (i) = x i n i F v m where xi represents the volume fraction of the products measured via online GC using an independent calibration standard gas (Carbagas, Switzerland), ni is the number of electrons involved into the reduction reaction to form a particular product, v m represents the molar CO 2 gas flow rate and F the Faraday constant. The partial current density for a given reaction product was normalized with respect to the total current density thus providing the FE for a given reaction product. Gas aliquots were analyzed in intervals of 20 minutes during steady-state CO 2 electrolysis in terms of an online measurement (see details of the GC analysis section in the SI). Ion exchange chromatography (IC). Non-volatile liquid products of the CO 2 electrolysis which accumulate in the catholyte during CO 2 RR were analyzed by means of ion exchange chromatography (Metrohm Advanced Modular Ion Chromatograph: L pump, Metrosep A Supp column, conductivity detector). SUPPLEMENTARY RESULTS SEM and EDX characterization of the Cu skeleton. An important experimental observation rationalizing the poor electrocatalytic activity of the electropolished Cu skeleton (Figure 6, Figure S1) is related to residual contaminations originating from the investment casting fabrication procedure of the Cu skeleton (sponge) fabrication. 1 The SEM, EDX and XPS inspection reveals the presence of zircon (ZrSiO 4 ) particles inside and on the struts of the Cu skeleton (Figure S2-S4). It should be noted that these contaminations are not listed by the supplier (Goodfellow) in the technical data sheet. A plausible explanation for that lack in accuracy in the determination of contaminations is related to the low solubility of zircon and its resistance against various forms of chemical digestion. This is why zircon cannot be detected by standard ICP-MS analysis of the dissolved Cu material. An EDX analysis integrated over a large area revealed a Zr content of 3.69w% and 2.55 at%. For Si integrated contents of 1.44w% and 2.55at% were detected, even after an electropolishing treatment of the Cu skeleton. It s most likely this contamination which is the origin for the poor electrocatalytic activity of the electropolished Cu skeleton. This also explains why the Cu skeleton can be activated for the CO 2 RR by electrodeposition and by thermal annealing. In both cases a new catalytically active skin is formed on the surface of the electropolished Cu skeleton. Electrodeposition of functional Cu foams on the skeleton support. Functional Cu foams can be deposited on the 3D Cu skeleton support in a similar fashion as known from planar electrode surfaces (Figure S5). 2,3 As visible in Figure S5 the pore size of the Cu foam is bigger at the outer part of the Cu skeleton as compared to the skeleton interior. This effect might be related to a Cu deposition that is more severely limited by mass transport of cupric ions inside the skeleton as compared to the peripheral sections of the Cu skeleton. In consequence, this leads to a lower partial current density of Cu deposition inside the skeletal structure. An undesired sideeffect is a broader pore size distribution in case of the skeleton (Figure 3) as compared to the planar Cu supports. 3 A precise determination of the pore size distribution becomes, however, be hampered because of the three-dimensional nature of the skeleton

3 support and the covering functional Cu foam. A tilted geometry of the SEM detector with respect to the pores leads to severe uncertainties in the pore size determination which cannot be overcome experimentally. The mean surface pore diameters in Figure S5 have been determined solely on planar sections of the catalysts oriented in direction of the SEM detector. An important observation from the SEM analysis is that the porous structure of the Cu foam electrodeposited onto the skeleton support remains largely unaffected by the CO 2 RR (Figure S6), at least on a µm-scale. The dendritic pores do not collapse even under massive HER/CO 2 RR at -1V vs RHE. Post-electrolysis XRD inspection of the electrodeposited sample indicates that the crystalline Cu 2 O surface phase has disappeared almost completely (Figure S9). It has already been stated that the electrodeposited Cu foam is highly sensitive towards oxidations in particular shortly after emersion from the Cu plating bath. 3 REFERENCES (1) Kranzlin, N.; Niederberger, M. Mater. Horiz. 2015, 2, (2) Shin, H. C.; Liu, M. Chem. Mater. 2004, 16, (3) Dutta, A.; Rahaman, M.; Luedi, N. C.; Mohos, M.; Broekmann, P. ACS Catal. 2016, 6, Corresponding Author * peter.broekmann@dcb.unibe.ch Notes The authors declare no competing financial interests. A.D. and M.R. contributed equally to this work. Annealing of the Cu skeleton. Various annealing temperatures have been applied to the Cu skeleton as indicated in Figure S8. Only the sample annealed at 300 C has been subjected to CO 2 RR. Further and more systematic results on the temperature depending performance of the annealed skeleton catalysts will be published separately in form of a full paper. An evolution of the surface morphology is clearly visible by going from 150 C to 750 C. A fine granular surface morphology is visible on the 150 C sample with small clusters having dimension in the nm range (Figure S8). The corresponding XRD pattern (Figure S10) does only show an improved crystallinity of the Cu skeleton support (recrystallization effect) but no contribution from Cu 2 O or CuO phases. It can, however, be assumed that the Cu skeleton annealed at 150 C is covered by an ultrathin Cu 2 O film whose translational order is still poor thus explaining the absence of any oxide contribution to the XRD pattern (Figure S10). For the 300 C anneal temperature (Figure S8) a superposition of a granular structure (with grain dimensions in the order of µm) and ultrathin fibers is visible. Only the combination of XPS (Figure 5) and XRD analysis reveals that the near surface regime of the sample annealed at 300 C is composed by a mixed Cu 2 O/CuO phase. The fibers, which are commonly assigned to one modification of cupric CuO, increase in density and size by going from 300 C to 600 C anneal temperature (Figure S8). 600 C seems to be the optimum annealing temperature for the formation of this fiber type of CuO. Further increasing the anneal temperature to 750 C lets these fibers almost completely disappear (Figure S8). Instead, a rough granular surface morphology becomes visible (Figure S8). The corresponding XRD demonstrates that all skeleton samples annealed between 450 C and 750 C are covered by mixed and crystalline Cu 2 O/CuO films. The fraction of the crystalline CuO is thereby steadily increasing by going from 450 C to 750 C. The XRD result clearly indicates that the formation of crystalline CuO requires much higher anneal temperature than the formation of crystalline Cu 2 O. Post-electrolysis XRD inspection of the skeleton sample annealed at 300 C (Figure S11) indicates an almost complete disappearance of the oxide phases after the CO 2 RR (see also Figure S9). The morphologies of the annealed samples presented in Figure S8 do therefore not represent the catalyst being active during CO 2 RR.

4 Figure S1. Optical micrograph (left) and SEM image of the as received Cu skeleton. Figure S2. Combined SEM inspection and EDX mapping of the electropolished Cu skeleton sample proving the presence of zircon particles on and inside the struts of the Cu skeleton.

5 Cu skleteton after electropolish Line Wt.% Error At% Error O K / / Si K / / Cu K / / Zr K / / Total Figure S3. EDX point analysis proving the presence of zircon particles on the Cu skeleton as residuals of the investment casting preparation of the Cu skeleton. The table shows the elemental composition at point 3 which can be assigned to zirconium

6 Figure S4. XPS survey spectrum of the electropolished skeleton sample prior to CO 2 RR. A small Zr 3d peak is highlighted.

7 Figure S5. Series SEM images demonstrating the increase of mean surface pore size of the functional Cu foam for increasing deposition times. The graph shows a linear relation between the mean surface pore size and the deposition time. The mean surface pore size was determined from selected (in plane) areas of the SEM images. Areas that were tilted were not considered for the determination of the mean pore size.

8 Figure S6. SEM inspection of the foam catalysts (20 s deposition time) after CO 2 RR. The pore and dendritic side wall structure of the electrodeposited Cu foam remain unaffected by the CO 2 electrolysis reaction at any potential for 1 hour studied herein.

9 Figure S7. Representative high-resolution SEM images demonstrating the presence of crystalline nanoparticles as main constituents of the dendrites in the Cu foam sample (20s).

10 Figure S8. SEM inspection of the Cu skeleton catalysts after anneal for 12h at various temperatures (prior to CO 2 RR). Figure S9. SEM images of the annealed (300 C) Cu skeleton catalyst after CO 2 RR at E = -1.0 V vs RHE for 1 hour. The fiber structure disappeared after the electrolysis reaction.

11 Figure S10. XRD data of the electropolished and the annealed Cu skeleton samples as a function of the anneal temperature.

12 Figure S11. XRD data of the electropolished, the annealed Cu skeleton and the electrodeposited samples after CO 2 RR for 1h at -1 V vs RHE.

13 Figure S12. Cyclic voltammetric (CV) experiments serving as basis for the determination of the electrochemically active surface area (ECSA) of the catalysts. (a) (c) Scan rate dependent CVs of (a) the electropolished, (b) the electrodeposited and (c) the annealed Cu skeleton samples in aqueous 10 mm DMVCl 2 (dimethyl-viologen-dichloride) + 1M Na 2 SO 4 solution. (d) Comparison of three CVs for a given scan rate of 50 mv s -1 ; (e) Plot showing the ECSAs for the three catalysts used in this study for the CO 2 electrolysis.

14 Figure S13. Cyclic voltammetric (CV) experiments serving as basis for the determination of the electrochemically active surface area (ECSA) of the catalysts. (a) (c) Scan rate dependent CVs of (a) the electropolished, (b) the electrodeposited and (c) the annealed Cu skeleton samples in aqueous 10 mm hexa-amin ruthenium(iii) chloride ([Ru(NH 3 )] 6 Cl 3 in 0.1M KCl. (d) Comparison of three CVs for a given scan rate of 25 mv s -1 ; (e) Plot showing the ECSAs for the three representative catalysts used in this study for the CO 2 electrolysis.

15 Figure S14. Potential transient measurements (chronoamperometry) of the CO 2 RR on (a) the electropolished; (b) the electrodeposited and (c) the annealed Cu skeleton.

16 Figure S15. Representative gas chromatograms of the CO 2 RR product analysis.

17 Figure S16. Deconvoluted XPS spectrum of the annealed sample (300 C) focusing on the Cu 2p 3/2 region. The peak can be fitted by assuming two components of CuO (black curve) and Cu/Cu 2 O (red curve).

18 -Z" / ohm Measured data Fitting data Z' / ohm Figure S17. Representative determination of the cell resistance by means of the current interrupt approach (upper section) and electrochemical impedance spectroscopy (EIS, lower section) The cell resistance lies typically between 11 and 12 in case of the current interrupt method. Slightly higher resistances (~13 ) are obtained from the EIS measurements. The relatively high cell resistance is a typical feature of the skeleton supports. The ir compensation is based on the values from the current interrupt method

19 0 j / ma. cm -2 ECSA Cu 3D Skeleton in 0.5 M NaHCO 3 (CO 2 sat. 20s Cu foam on Cu 3D in 0.5 M NaHCO 3 (CO 2 sat.) Cu (Annealed) 3D in 0.5 M NaHCO 3 (CO 2 sat.) E / V vs. RHE Figure S18. Representative LSVs normalized to the ECSA. The on-set of the reduction process is more positive in case of the annealed sample. This is due to an on-set of the Cu oxide reduction which takes place at potentials before HER and CO 2 RR become dominant.

20 Figure S19. Representative CO 2 RR product analysis of 1h electrolysis from CO 2 saturated 0.5M NaHCO 3 solution at ph = 7.2 using a Cu foil as catalyst. Note that polycrystalline Cu is active for the C1 pathway. Further note that the Cu foil catalyst yields higher FEs of CO 2 RR products at higher overpotentials as compared to the highsurface area catalysts (annealed and electrodeposited) studied in the paper. CO 2 mass transfer limitations are much more severe in case of the skeleton/sponge catalysts modified by annealing and electrodeposition.