Impurity Ion Complexation Enhances Carbon. Dioxide Reduction Catalysis

Transcription

1 Supporting Information Impurity Ion Complexation Enhances Carbon Dioxide Reduction Catalysis Anna Wuttig, Yogesh Surendranath* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts S1

2 Index Experimental methods Calculation of estimated mass-transport limited metal deposition rate Fig. S1. Survey X-ray photoelectron spectra of copper, silver, and gold electrodes following prolonged electrolysis in untreated C i electrolyte Fig. S2. Linear sweep voltammograms of copper, silver, and gold electrodes following short and prolonged electrolysis in untreated C i electrolyte Fig. S3. Linear sweep voltammograms of copper, silver, and gold electrodes following electrolysis in C i electrolyte containing 50µM ZnSO 4, CuSO 4, or Pb(NO 3 ) 2 Page S3-S5 S5 S6 S6 S7 Fig. S4. Faradaic Efficiency for H 2 and CO production on Au foil in a pre-electrolyzed solution S7 Fig. S5. Comparison of the partial current densities for gold foil in different electrolyte media S7 Fig. S6. Comparison of the partial current densities for silver foil in different electrolyte media S8 Fig. S7. Comparison of the partial current densities for copper foil in different electrolyte media S9 S2

3 Experimental Methods Materials. Na 2 CO 3 ( % TraceSELECT, Lot # BCBL6733V, Sigma-Aldrich), ethylenediaminetetraacetic acid, EDTA, (99.995%, Lot # MKBK5436V, Sigma-Aldrich), phosphoric acid (ACS Reagent, Ward s Science), sulfuric acid (99.999%, Sigma-Aldrich), ZnSO 4 7H 2 O (99.999%, Sigma-Aldrich), CuSO 4 5H 2 O (99.999%, Strem Chemicals) and Pb(NO 3 ) 2 (99.999%, Sigma-Aldrich) were used without modification unless otherwise noted. 0.1 M NaHCO 3, ph 6.8, was prepared by sparging 0.05 M Na 2 CO 3 with CO 2 (Research Grade, Airgas) for 2 hours. 3.4 µm EDTA/0.1 M NaHCO 3 solution was prepared by adding EDTA to CO 2 -saturated 0.1 M NaHCO 3 (C i ) electrolyte. All electrolyte solutions were prepared with reagent grade water (Millipore Type 1, 18MΩ-cm resistivity). Resin-treated 0.1 M NaHCO 3 solutions were prepared by treating the electrolyte with regenerated Chelex 100 Resin (Bio-Rad, Catalog # ), according to the manufacturer s protocol, 1 with slight modifications. Chelex was regenerated by stirring the as-received material for 12 hours in 1 M HCl (ACS Reagent Grade, EMD Chemicals) followed by rinsing with 5 L of reagent grade water. Subsequently, Chelex was placed in 1 M NaOH (99.99%, semiconductor grade, Sigma-Aldrich) for 24 hours at 60 C with constant stirring. Chelex was rinsed with 8 L of reagent grade water until the ph of the filtrate was Regenerated Chelex was stirred with 0.1 M NaHCO 3 electrolyte for at least 24 hours. Electrochemical Methods. All electrochemical experiments were conducted using a Gamry REF 600 potentiostat, a double junction Ag/AgCl electrode (PINE Research Instruments), and a high surface area Pt-mesh counter electrode (Alfa Aesar, %). Ag/AgCl reference electrodes were stored in 10% KNO 3 in between measurements and were periodically checked relative to pristine reference electrodes to ensure against potential drift. All experiments were performed at ambient temperature, (21 ± 1) C. Electrode potentials were converted to the reversible hydrogen electrode (RHE) scale using E RHE = E Ag/AgCl V (pH) and corrected for the uncompensated Ohmic loss (ir u ) in situ via positive feedback or following the run using E corrected = E applied ir u. R u was measured using the R u test function in the Gamry Framework software. All current density values are reported relative to the geometric surface area of the working electrode. All electrolyte solutions were used as both the catholyte and the anolyte and with stirring of both chambers at a constant rate 300 rpm during experiments. In all cases, experiments were conducted in an airtight H-cell with 50 ml catholyte and 20 ml anolyte separated by an anion exchange membrane (AGC Selemion membrane). The H-cell was cleaned overnight in nitric acid and rinsed with MilliQ water prior to each experiment. During all experiments, the catholyte was sparged continuously with CO 2 at 20 sccm, and purged with the CO 2 at 50 sccm for 15 min prior to all measurements. The anolyte was sparged continuously with N 2. Preparation of Rotating Electrodes. A rotating disk (copper, r = 0.25cm, PINE Research Instrumentation) or rotating cone (silver, gold, r = 0.25cm, 45 cone angle, custom milled, PINE Research Instrumentation) was employed as the working electrode at a rotation rate of 2500 rpm. Electrode rotation was controlled with a Metrohm Autolab B.V. rotator that formed an air-tight seal with the working compartment of the H-cell. The electrodes were polished sequentially using 1 µm and 0.3 µm alumina and sonicated using a bath sonicator. The electrodes were cycled reductively from the open circuit potential for each metal (Cu: 90 to 123 mv vs Ag/AgCl, Ag: 53 to 75 S3

4 mv vs Ag/AgCl, Au: 80 to 96 mv vs Ag/AgCl) to 0.6 V vs Ag/AgCl five times without pause prior to each experiment. Stripping Voltammetry. Stripping voltammetry data in both untreated and EDTA-containing 0.1 M NaHCO 3 were collected without ir compensation on rotating electrodes at 2500 rpm using a Metrohm Autolab B.V. First, the potential for CO 2 reduction ( 1.60 V vs Ag/AgCl for Cu, 1.50 V vs Ag/AgCl for Ag and 1.30 V vs Ag/AgCl for Au) was held for a variable amount of time ( min). Second, cyclic voltammograms were recorded at 50 mv/s scan rate and were initiated at 1.00 V vs Ag/AgCl immediately following the conclusion of electrolysis (~1 s time delay). Since deposited metal impurities may desorb oxidatively as a result of double layer discharge, a minimal time delay between the end of CO 2 reduction catalysis and the initiation of the CV scan was found to be necessary to observe the stripping peaks (Figures 1, S2, and S3). In order to determine the identity of the stripping features, the above experimental procedure was applied to an untreated 0.1 M NaHCO 3 electrolyte solution containing 50 µm of candidate impurity ions, ZnSO 4 7H 2 O, CuSO 4 5H 2 O, or Pb(NO 3 ) 2. XPS Measurements. X-ray photoelectron spectra were collected on Cu, Ag, and Au rotating electrodes that were electrolyzed for 45min in C i or EDTA-containing C i electrolyte at 1.60 V vs Ag/AgCl for Cu, 1.50 V vs Ag/AgCl for Ag and 1.30 V vs Ag/AgCl for Au. In all cases, the working electrode was removed from the electrochemical cell while under polarization, rinsed thoroughly with MilliQ water, and dried under ambient conditions before being loaded into the ultra high vacuum chamber. XPS samples were prepared by adhering the electrodes to the sample stage with conducting carbon tape. The X-ray photoelectron spectra were collected using a Physical Electronics Model Versaprobe II with a hemispherical energy analyzer and a monochromated X-ray source (Aluminum Kα, ev). Data were collected using a 200 µm, 50 W focused X-ray beam at a base pressure of torr. Wide scan survey data were collected with a pass energy of ev and a step size of 0.8 V. Narrow scans over peaks of interest were collected with a pass energy of ev and a step size of 0.7 ev. The C 1s peak arising from adventitious hydrocarbons was assigned the energy value ev and used as an internal binding energy reference. Preparation of Foil Electrodes. 2 cm 2 foils (copper % Alfa Aesar, silver % Alfa Aesar, and gold % Alfa Aesar) were attached to corresponding Ag, Au or Cu wire (99.999% Alfa Aesar) by either welding (Au) or insertion of the wire into a drilled hole in the foil (Ag, Cu). Cu foil was electropolished prior to use in 85% phosphoric acid (ACS reagent grade) at 4V vs the Ti wire counter (99.99%, Alfa Aesar) electrode for 5min in quiescent solution. Ag foil was etched in sulphuric acid (99.999%) for 5min prior to use. Au foil was etched by dipping in aqua regia for 30 s prior to use. The foils were rinsed with MilliQ water before introduction to the electrochemical cell. The electrodes were cycled reductively from the open circuit potential for each metal (Cu: 130 to 139 mv vs Ag/AgCl, Ag: 130 to 189 mv vs Ag/AgCl, Au: 89 to 500 mv vs Ag/AgCl) to 0.6 V vs Ag/AgCl five times without pause prior to each experiment. Pre-electrolysis Procedure. Pre-electrolysis on untreated 0.1 M NaHCO 3 was conducted by applying 3.0 ma cm 2 between two gold mesh (99.999%, Alfa Aesar) electrodes for 19 hours. The cathode was removed under potential bias, and replaced with a fresh gold foil electrode for subsequent CO 2 reduction studies (Figure S4). Product Distribution Analysis. Product distribution was measured using an in-line gas chromatograph (SRI Instruments, Multi-Gas Analyzer #3) equipped with a thermal conductivity detector, methanizer, and flame S4

5 ionization detector in series following Molsieve 13x and Hayesep D columns. Prior to each experiment, the uncompensated cell resistance was measured and typically ranged from 32 to 87 Ω. Electrodes were polarized at potentials sufficient for robust CO 2 reduction catalysis ( 1.60 V vs Ag/AgCl for Cu, 1.50 V vs Ag/AgCl for Ag and 1.30 V vs Ag/AgCl for Au) for two hours, and GC traces were collected every 12 min for Au and Ag and every 20 min for Cu. The partial current density (j p ) for each CO 2 reduction product, p, was calculated using the following relationship: j p = [p]*flow rate*nfp/rt*1/area. [p] is the ppm value of the product measured via GC using an independent calibration standard gas mixture, n is the number of electrons transferred per equivalent of p, P is the pressure in the electrochemical cell headspace (1.1 atm), T is the temperature, and F is Faraday s constant. The partial current density for a given product is divided by the total current density, averaged over a 30 s span immediately prior to each GC run, to determine its partial Faradaic efficiency. Data plotted in Figures 2, 3, 4, S5, S6, and S7 are the average and standard deviation of three independent measurements for each electrode and each solution preparation method. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Analysis of Impurity Content in Electrolyte. To determine the concentration of copper, zinc, iron, and lead impurities in 0.1 M NaHCO 3, three separate 0.1 M NaHCO 3 samples were analyzed by the Evans Analytical Group using a Perkin Elmer Elan DRC II ICP-MS equipped with a Cetac ASX-520 auto sampler. Calculation of Estimated Mass Transport Limited Metal Ion Deposition Rate. The total current density for metal ion (M n+ ) deposition, j dep, can be described by the sum of reciprocals of the mass transfer-limited deposition current (j MT ) and the deposition current under activation control (j AC ), j!!!"# = j!!!" + j!!!". 2 At potentials well beyond the thermodynamic potential for M n+ deposition, as is the case for operative CO 2 reduction conditions on group 11 metals, j!!!" 0. In that case, j MT alone characterizes the deposition current, and j!" = 0.20nFD!/! ν!!/! N!/! c. 2 As an example, we calculate the j MT for Zn 2+ deposition in dilute NaHCO 3 solution on a planar rotating disk electrode. Under these conditions, n=2, D = cm 2 s 1, 3 ν = cm 2 s 1, 4 N = 2500 rpm and c = 1 µm. With these values, j MT for Zn 2+ deposition equals 1 µa. Assuming a roughness factor of a freshly polished surface of 2 2 and the charge required for monolayer (ML) electrodeposition of Zn of ~200 µc cm 2, 5 we estimate that mass transport-limited Zn deposition of a full monolayer would occur in ~400 s. We observed deposition as slightly longer time scales (tens of minutes), suggesting that metal deposition may be under mixed diffusion and activation control under CDR conditions. S5

6 Figure S1. Survey X-ray photoelectron spectra of copper (red), silver (blue), and gold (green) electrodes following 45 min electrolysis ( 1.00 V for Cu, 0.90 V for Ag and 0.70 V for Au) in untreated C i electrolyte. Black dotted lines denote peak positions of Pb, Cu, and Zn impurities detected and highlighted in Figure 1A-C. All other peaks in the spectra are unchanged relative to the initial electrode prior to CDR catalysis. Figure S2. Cyclic voltammograms of copper, silver, and gold working electrodes prior to (black) and immediately following short (red; Cu: 45 min, Ag & Au: 12 min) and prolonged (green; Cu: 120 min, Ag & Au: 45 min) electrolysis ( 1.00 V for Cu, 0.90 V for Ag and 0.70 V for Au) in untreated C i electrolyte. S6

7 Figure S3. Cyclic voltammograms (CV) of copper (A), silver (B) and gold (C) electrodes following 12 min electrolysis ( 1.00 V for Cu, 0.90 V for Ag and 0.70 V for Au) in untreated C i electrolyte with 50 µm ZnSO 4 (red), CuSO 4 (green) and/or Pb(NO 3 ) 2 (blue). CVs of each electrode following prolonged (black; Cu: 120 min, Ag & Au: 45 min) electrolysis ( 1.00 V for Cu, 0.90 V for Ag and 0.70 V for Au) in untreated C i electrolyte. Figure S4. Faradaic Efficiency for H 2 (red circles) and CO (black squares) production on Au foil in preelectrolyzed C i electrolyte. Figure S5. Partial current densities for CDR and HER product formation on silver and gold foil in electrolytes of varying purity. Activity of Au for CO (A) and H 2 (C) formation at 0.70 V in native C i electrolyte (black squares), C i electrolyte containing 3.4 µm EDTA (red circles), and Chelex-treated C i electrolyte (blue triangles). S7

8 Figure S6. Partial current densities for CDR and HER product formation on silver and gold foil in electrolytes of varying purity. Activity of Ag for CO (A) and H 2 (C) formation at 0.90 V in native C i electrolyte (black squares), C i electrolyte containing 3.4 µm EDTA (red circles), and Chelex-treated C i electrolyte (blue triangles). S8

9 Figure S7. Partial current densities for CDR and HER product formation on copper foil in electrolytes of varying purity. Activity of Cu for CH 4 (A), CO (B), C 2 H 4 (C) and H 2 (D) formation at 1.00 V in native C i electrolyte (black squares), C i electrolyte containing 3.4µM EDTA (red circles), and Chelex-treated C i electrolyte (blue triangles). S9

10 References: (1) Chelex 100 Chelex 20 Chelating Ion Exch. Resin Instr. Man. 2014, rad.com/. (2) Gileadi, E. Physical Electrochemistry, Fundamentals, Techniques and Applications; Wiley-VCH: Weinheim, (3) Vanysek, P. In CRC Handbook of Chemistry and Physics; Haynes, W. M., Ed.; CRC Press/Taylor and Francis: Boca Raton, 2014; pp (5 77) (5 79). (4) Bedekar, S. G. J. Appl. Chem. 2007, 5, (5) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons, Inc.: New York, S10