High-Flux CO Reduction Enabled by Three-Dimensional Nanostructured. Copper Electrodes

Transcription

1 Supporting Information High-Flux CO Reduction Enabled by Three-Dimensional Nanostructured Copper Electrodes Yuxuan Wang, David Raciti, Chao Wang * Department of Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD * chaowang@jhu.edu 1

2 Experimental Methods Synthesis. In a typical experiment, copper foam (99.99% mesh from MTI) was rinsed with ethanol twice and sonicated in ethanol for 1 min. After drying in vacuo, the copper foam was placed on a quartz plate and heated in a Muffle furnace to the desired temperature at a ramping rate of ~35 C/min. After the allotted time, the reaction was stopped and the sample was allowed to cool down to room temperature in air. The derived CuO nanostructures were reduced to Cu by applying 0.4 V vs. RHE in 0.1 M KOH until a steadystate current had been achieved. Characterization. Morphology of the 3D Cu nanostructures were examined using a scanning electron microscope (SEM, JEOL JSM-6700F, 15kV). X-ray diffraction (XRD) patterns were collected on a PANalytical X Pert3 Powder X-ray diffractometer equipped with a Cu Kα source (λ= ). Electrocatalysis. Electrocatalytic properties of the 3D Cu nanostructures were measured using a custom-made gas-tight electrolysis cell equipped with an Autolab 302 potentiostat (Metrohm). A Hg/HgSO 4 electrode (Hach) and a Pt mesh (VWR) were used as the reference and counter electrode, respectively. A solution of 0.1 M KOH (ph 13) was used as the electrolyte. CO was bubbled through a glass frit to the cathode compartment at a constant rate of 20 sccm and purged for 10 minutes prior to each measurement. The cathode and anode compartments were separated with an AHO anion exchange membrane (Selemion Inc.) to avoid (or minimize) possible diffusion of chemical species (e.g., Pt dissolved from the counter electrode, products produced from the cathode) across the cell. Current densities and faradaic efficiencies presented in the main text are the average of the measurements over >30 min and potentials are versus the reversible hydrogen electrode (RHE) with ir drop correction. Gas- and liquidphase products were analyzed by using GC-MS and NMR, respectively. Estimation of Surface Roughness. The surface roughness factors of the 3D Cu nanostructures were estimated by measuring the double-layer capacitance in a potential region free of Faradaic processes. 1 Cyclic voltammograms (CVs) were recorded between 0.3 and 0.05 V (vs. RHE) at various scan rates in the electrolysis cell with 0.5 M KHCO 3 as the electrolyte (purged with Ar). The capacitance was determined 2

3 by evaluating the slope of the double-layer width versus the scan rate. Surface roughness factors were estimated by comparing the capacitance measured for the nanostructured Cu electrodes to the values obtained for the pristine Cu foam or a polycrystalline Cu disk electrode with a flat surface (Cu-poly). The specific activities (current normalized by the ECSA, ma/cm 2 Cu) were calculated by dividing the surface roughness factors versus Cu-poly from the geometric current density (ma/cm 2 geo). Gas and Liquid Product Analysis. Gas products of the electrochemical reaction were analysed using a GCMS-QP2010SE (Shimadzu) equipped with a Plot-Q column (Restek). Helium was used as the carrier gas. Calibration curves were generated for the GC-MS using a custom gas mixture (Airgas) diluted with various amounts of Ar. NMR experiments were performed using a Bruker 300 MHz. Our sampling and quantification methods followed the previous report from our group. 2 Calculation of CO Conversion Rates. CO conversion rates were calculated using nn CCOO cccccccc = jj FF nn ii ii=1 FFFF% ii zz ii, where CO conv stands for the rate of CO conversion (mols s -1 cm -2 ), j is the current density (A/cm 2, either geometric or specific), F is faraday s constant, n i is the moles of CO consumed for the production of one mole of species i, FE% i is the Faradaic efficiency of product i and z i the number of electrons transferred for each molecule i. Estimation of CO Solubility. CO solubility was estimated using the Henry s Law: [CCOO] = KK 0 ff 0 where [CO] stands for the solubility of CO in water, K 0 is the Henry s constant and f 0 is the fugacity of CO. The value of K 0 is ca (mol kg -1 bar -1 ) at room temperature for CO in water (NIST Chemistry Web Book). The fugacity is assumed to be approximately the partial pressure of CO in the cell (~1 atm). 3

4 Supplemental Results from the Electrocatalytic Studies Table S1. Total geometric current density towards CO reduction products (J CO ) and Faradaic efficiencies toward each product for the pristine Cu foam. The carbonaceous products were below the detection limits due to the low current densities. Faradaic Efficiency (%) E J CO V vs RHE ma/cm 2 H 2 Acetate Ethanol C 2 H 4 C 2 H Table S2. Total geometric current density toward carbonaceous products (J CO ) and Faradaic efficiencies toward each product for Cu-NS-500C. Faradaic Efficiency (%) E J CO V vs RHE ma/cm 2 H 2 Acetate Ethanol C 2 H 4 C 2 H

5 Table S3. Summary on surface roughness factors estimated from the double-layer capacitance. It is noticed that the ECR-Cu-mesh has a roughness factor of 356 versus Cu-poly, 3 which is line with Cu-NS-500C. 5

6 Figure S1. SEM images of ECR-Cu-mesh. 6

7 i (ma/cm 2 ) Time (s) Figure S2. Electrode current recorded during reduction of a 2.5 cm x 2.5 cm piece of Cu-NS-500C at 0.4 V vs. RHE in 0.1 M KOH purged with Ar gas. 7

8 Cu-NS-500C Cu Cu 2 O Intensity(a.u.) θ(degree) Figure S3. XRD patterns for Cu-NS-500C, prepared by the electrochemical reduction of CuO-NS-500C. The small Cu 2 O peak could be attributed to the re-oxidation of surface Cu when the reduced nanostructures were exposed to air. 2 8

9 Figure S4. (a) CVs taken over a range of scan rates in a potential region where only double-layer charging and discharging are involved. The scan rates are 150, 175, 200, 225, 250, 300 and 350 mv/s, respectively. (b) Plot of double-layer width against the scan rate, with the electrochemical capacitance estimated from the slope. 9

10 10 ECR-Cu-mesh i (ma/cm 2 geo ) E (V vs. RHE) Figure S5. CV recorded for ECR-Cu-mesh in Ar-purge 1 M KOH. 10

11 Figure S6. (a) Faradaic efficiencies and (b) geometric current densities measured for Cu-NS-500C. 11

12 Figure S7. Comparison of the electrocatalytic performance among the 3D Cu nanostructures (this study), ECR-Cu-mesh 2 and OD-Cu electrode 4 : (a) Overall Faradaic efficiency of CO reduction (FE CO ), (b) Faradaic efficiency toward ethanol (FE eth ), geometric current densities of (c) overall CO reduction (J CO ), (d) ethanol and (e) acetate production, and (f) calculated CO conversion rates. 12

13 Figure S8. (a) Faradaic efficiencies and (b) CO conversion rate for the electroreduction of CO on Cu-NS- 700C at more negative potentials. 13

14 Figure S9. Gaussian deconvolution of background-subtracted OH ad peaks for (a) Cu-NS-500C, (b) Cu- NS-700C and (c) Cu-NS-1000C. (d) Calculated areal ratios of peaks for each catalyst. 14

15 References 1. Li, C. W.; Kanan, M. W., CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu 2 O Films. J. Am. Chem. Soc. 2012, 134, Raciti, D.; Cao, L.; Livi, K. J. T.; Rottmann, P. F.; Tang, X.; Li, C.; Hicks, Z.; Bowen, K. H.; Hemker, K. J.; Mueller, T.; Wang, C., Low-Overpotential Electroreduction of Carbon Monoxide Using Copper Nanowires. ACS Catal. 2017, 7, Raciti, D.; Mao, M.; Park, J. H.; Wang, C., Mass Transfer Effects in CO 2 Reduction on Cu Nanowire Electrocatalysts. Catal. Sci. & Tech. 2018, 8, Li, C. W.; Ciston, J.; Kanan, M. W., Electroreduction of carbon monoxide to liquid fuel on oxidederived nanocrystalline copper. Nature 2014, 508,