Supporting Information for Pd-catalyzed electro-hydrogenation of ...
Supporting Information for Pd-catalyzed electro-hydrogenation of carbon dioxide to formate: high mass activity at low overpotential and identification of the deactivation pathway Xiaoquan Min, and Matthew W. Kanan* *Correspondence to: [email protected] This PDF file includes: Materials and methods Figures S1 to S10 Table S1
S1
Materials and Methods Materials. 10 wt.% Pd on Vulcan XC-72 carbon black was purchased from Premetek Co. Platinum gauze (99.9%), platinum wire (99.9%), platinum foil (99.9%), sodium carbonate (99.999%), potassium carbonate (99.999%, 99%), oxalic acid (98%) and sulfuric acid (≥95%, TraceSELECT®) were purchased from Sigma-Aldrich; carbon dioxide (99.99%) was purchased from Praxair. All chemicals were used without further purification. Electrolyte solutions were prepared with deionized water (Ricca Chemical, ASTM Type I). NaHCO3 and KHCO3 solutions were prepared by vigorously bubbling CO2 gas through Na2CO3 or K2CO3 solutions for at least 12 h. N2-saturated KHCO3 was prepared by bubbling N2 through a freshly prepared KHCO3 solution for 30 min. Preparation of Pd/C electrodes. Ti foil (Sigma-Aldrich, 99.7%, thickness 0.125 or 0.25 mm) was cut into 1×2 cm2 pieces, etched in boiling 10 wt. % oxalic acid solution for 1–2 h, rinsed thoroughly with DI water, and dried in an oven overnight. Ti wires (Alfa Aesar, 99.7%, 0.25 mm dia.) were spotwelded onto one side of the Ti foils. Pd/C powder was mixed with isopropanol and 10 wt.% Nafion and vigorously sonicated for ~ 30 min. The resulting homogeneous catalyst ink was drop-dried onto the Ti substrates with Pd mass loading of 50 µg cm–2 to ensure full coverage of the substrates. The backsides of the Ti substrates were covered with epoxy and dried before electrochemical measurements. Preparation of carbon black and Ti electrodes. For control experiments as shown in Fig.S2a, carbon black electrode (C on Ti electrode) was prepared in the same way as Pd/C electrodes, except that only carbon black (Vulcan XC-72, Fuel Cell Store) mixed with 10 wt% Nafion was loaded onto the Ti substrates with carbon black mass loading of 0.5 mg cm–2. The Ti electrode was prepared in the same way except that no Pd/C or carbon black was loaded onto the Ti substrate. Electrochemical measurements and product analysis. A CH Instruments 760D (or 660D) Potentiostat was used for all CO2 reduction experiments. A piece of platinum foil or gauze was used as the counter electrode. The pH values of CO2-saturated 0.5 M NaHCO3 and N2-saturated 2.8 M KHCO3 are 7.2 and 8.5, respectively. All potentials were measured against an Ag/AgCl reference electrode (3.0 M NaCl, BASi) and converted to the RHE reference scale using E (vs RHE) = E (vs Ag/AgCl) + 0.210 V + 0.0591 V × pH. Electrolyses were performed in a gas-tight two-compartment electrochemical cell with a piece of anion exchange membrane (SELEMION®, AGC Engineering) as the separator, and the membranes were soaked in HCO3– solutions before experiments to fully exchange the anion into HCO3–. Each compartment contained 20.0 mL electrolyte and approximately 5 mL headspace. Before electrolysis, the electrolyte in the cathodic compartment was degassed by bubbling with CO2 or N2 gas for at least 30 min. The electrolyte in the cathodic compartment was stirred at a rate of 1000 rpm during electrolysis. CO2 or N2 gas was delivered into the cathodic compartment at a rate of 5.00 sccm and was vented directly into the gas-sampling loop of a gas chromatograph (SRI Instruments). A GC run was initiated every 15 or 20 min. The GC was equipped with a packed MolSieve 13X column and a packed HaySep D column. Argon (Praxair, 99.999%) was used as the carrier gas. The column effluent (separated gas mixtures) was first passed through a thermal conductivity detector (TCD) where hydrogen was quantified; it was then passed through a methanizer where CO was converted to methane and subsequently quantified by a flame ionization detector (FID). The partial current density for H2 or CO production was calculated from the GC peak area as follows: 2 Fp0 × (electrode area)–1 α RT 2 Fp0 peak area = × flow rate × × (electrode area)–1 β RT
jH2 =
jCO
peak area
× flow rate ×
where α, β are conversion factors for H2 and CO respectively based on calibration of the GC with standard samples, p0 = 1.013 bar and T = 273.15 K. HCO2– concentration was analyzed on a Varian Inova 600 MHz NMR spectrometer. A 0.5 mL aliquot of the electrolyte was mixed with 0.1 mL D2O, and 16.7 ppm (m/m) dimethyl sulfoxide (DMSO,
S2
Sigma, 99.99%) was added as an internal standard. The 1D 1H spectrum was measured with water suppression using a pre-saturation method. [HCO3–] dependence in CO2-saturated electrolytes. Electrolyses were performed as described above in CO2-saturated 0.5, 0.94, 1.8, 2.8 M KHCO3. Upon saturation with CO2, the pH values of the above solutions were 7.2, 7.8, 8.0, and 8.2 respectively. The potential applied was held at –0.15 V (vs RHE). jformate was averaged over the first hour of electrolysis. [HCO3–] dependence study under N2 condition. Electrolyses were performed in N2-saturated 0.52, 0.94, 1.5, 1.8, 2.8 M KHCO3 solutions. The pH values of these solutions were 8.5, and the potential was held constant at –0.15 V (vs RHE). jformate was averaged over the first hour of electrolysis. Activity coefficient of HCO3–. Activity coefficient values used to calculate [CO2] values in Figure 3b were extrapolated from the data in Roy et al. (ref. 27) assuming a linear relationship between concentration and activity coefficient in a narrow region. In Figure 3c, coefficient values were assumed to be 0.5 in the case of [CO32–]/[ HCO3–] = 0.01, 0.05, and 0.1, and 0.4 in the case of [CO32– ]/[ HCO3–] = 0.5. Structural characterization. A grazing incidence X-ray diffraction pattern was acquired for Pd/C using an 11.5 keV synchrotron X-ray beam with Soller slits and a photomultiplier tube detector at beamline 2-1 at the Stanford Synchrotron Radiation Lightsource. Several mg of powder were put onto a glass slide and covered with a piece of Kapton® tape. The incidence angle was optimized to be 4°. Scanning electron microscopy (SEM) images were acquired with a FEI Magellan 400 XHR Scanning Electron Microscope.
S3
Figure S1 Grazing incidence X-ray diffraction pattern of Pd/C powder. Pd diffraction peaks are labeled. The pattern was collected using an 11.5 keV synchrotron X-ray beam.
S4
Figure S2 Background activity of Ti and C black compared to Pd/C. a) Total current density vs time for a Pd/C electrode (Pd/C on Ti foil), carbon black electrode (C on Ti foil) and Ti electrode (Ti foil) at –0.25 V (vs RHE, all potentials were reported with respect to this reference, unless otherwise specified) in CO2-saturated 0.5 M NaHCO3. b) image of a Ti substrate (left), a freshly prepared Pd/C electrode (middle), and a Pd/C electrode after bulk electrolysis of several hours (right). The Ti substrate is fully covered with Pd/C even after a long electrolysis.
S5
Figure S3 Detection of CO during electrolysis with Pd/C at –0.35 V in CO2-saturated 0.5 M NaHCO3. a) geometric current density (jtot) vs time during 3 h of electrolysis. The plot was taken from Figure 2a. b) CO peaks detected from the flame ionization detector of the gas chromatograph at different timepoints during the electrolysis. The amount of CO accounted for < 0.1% Faraday efficiency.
S6
CO2-saturated 0.5 M NaHCO3 15 min
1h
2h
3h
–0.05 V
15 (98%)
13 (92%)
11 (89%)
10 (86%)
–0.15 V
60 (99%)
54 (98%)
48 (96%)
40 (94%)
–0.25 V
78 (99%)
69 (98%)
60 (94%)
50 (87%)
–0.35 V
120 (97%)
96 (91%)
67 (85%)
48 (80%)
1h
2h
3h
CO2-saturated 2.8 M KHCO3 15 min –0.05 V
48 (99%)
47 (99%)
46 (99%)
44 (99%)
–0.10 V
100 (99%)
89 (99%)
78 (99%)
70 (99%)
–0.15 V
134 (99%)
116 (99%)
98 (99%)
83 (98%)
–0.20 V
188 (99%)
155 (99%)
113 (99%)
82 (98%)
1h
2h
3h
N2-saturated 2.8 M KHCO3 15 min –0.05 V
28 (98%)
22 (98%)
19 (97%)
17 (96%)
–0.15 V
82 (97%)
67 (93%)
55 (88%)
50 (87%)
–0.25 V
84 (66%)
67 (60%)
54 (52%)
46 (48%)
Table S1 Summary of averaged mass activity (mA HCO2– synthesis per mg Pd) and average HCO2– Faraday efficiency of Pd/C electrodes in three different electrolytes. Activity values were obtained from the data in Figure 2 and represent the average mass activity for the electrolysis up to the indicated time point.
S7
Figure S4 CO poisoning and attempted recovery without exposure to air. Electrolysis was initiated with Pd/C at –0.25 V in CO2-saturated 0.5 M NaHCO3. After 4 min, a flow of 0.5 mL min–1 CO was introduced into the electrolyte to poison catalysis. After 17 min, the cell was switched to open circuit potential and the CO flow was stopped. The electrolyte was vigorously purged with CO2 for 30 min to remove the excess amount of CO without exposing the electrode to air. Electrolysis was then re-started at – 0.25 V. No recovery of activity was obtained, indicating that CO is not removed from the electrode surface without exposure to air.
S8
Figure S5 Recovering activity of a Pd/C electrode for electrolysis in N2-saturated 2.8 M KHCO3 at -0.l5 V by brief exposure to air. HCO2– Faraday efficiency was plotted by subtracting H2 Faraday efficiency, as determined by periodic GC analysis of the headspace, from the total efficiency. The electrode was exposed to air and transferred to a new electrolyte between electrolyses.
S9
Figure S6 Stability of Pd/C in the absence of CO formation. Relative current density vs time for electrolysis at – 0.25 V in N2-saturated 0.5 M Na2CO3 compared to electrolysis at –0.25 V in CO2-saturated 0.5 M NaHCO3.
S10
Figure S7 Electrolysis with polycrystalline Pd foil. a) total current density vs time for polycrystalline Pd foil at – 0.05 V and –0.25 V in CO2-saturated 0.5 M NaHCO3. The geometric surface area of the Pd foil used was 4 cm2. b) NMR analysis of the electrolyte after electrolysis. The top spectrum shows the peak size for an amount of HCO2– corresponding to 1.5 C. DMSO is used as an internal standard. The bottom two spectra show no detectable HCO2– for the two electrolyses with Pd foil.
S11
Figure S8 Effect of high HCO2– concentrations on the CO2 reduction activity of Pd/C. Relative current density vs time and Faraday efficiency for H2 determined by gas chromatrography for an electrolysis started in CO2-saturated 0.5 M NaHCO3. Concentrated NaHCO2 was added after 15 min and 45 min to raise the [HCO2–] to ~0.1 M and 0.5 M. Current density increased immediately after each addition and then resumed a gradual decline. The modest increase likely results from the increase in the ionic strength of the electrolyte (total ion concentration). The added HCO2– did not affect the FE for CO2 reduction to HCO2–, as evidenced by the low amount of H2 detected by gas chromatography. The accumulation of HCO2– in the electrolyte does not cause catalyst deactivation.
S12
Figure S9 Representative NMR spectrum of the electrolyte after CO2 reduction electrolysis with Pd/C. DMSO is used as an internal standard for quantification of HCO2–.
S13
Figure S10 Bulk electrolyses of Pd/C electrodes at –0.15 V in a) CO2-saturated 0.5 M NaHCO3 and b) CO2saturated 0.5 M KHCO3. The HCO2– Faraday efficiencies vs time were calculated by subtracting the H2 Faraday efficiency (determined by periodic GC analysis) from the total efficiency. Pd/C electrodes exhibited similar current densities and Faraday efficiencies for HCO2– production in the two conditions and similar current decay over time was also observed in both cases.
S14
S1
Materials and Methods Materials. 10 wt.% Pd on Vulcan XC-72 carbon black was purchased from Premetek Co. Platinum gauze (99.9%), platinum wire (99.9%), platinum foil (99.9%), sodium carbonate (99.999%), potassium carbonate (99.999%, 99%), oxalic acid (98%) and sulfuric acid (≥95%, TraceSELECT®) were purchased from Sigma-Aldrich; carbon dioxide (99.99%) was purchased from Praxair. All chemicals were used without further purification. Electrolyte solutions were prepared with deionized water (Ricca Chemical, ASTM Type I). NaHCO3 and KHCO3 solutions were prepared by vigorously bubbling CO2 gas through Na2CO3 or K2CO3 solutions for at least 12 h. N2-saturated KHCO3 was prepared by bubbling N2 through a freshly prepared KHCO3 solution for 30 min. Preparation of Pd/C electrodes. Ti foil (Sigma-Aldrich, 99.7%, thickness 0.125 or 0.25 mm) was cut into 1×2 cm2 pieces, etched in boiling 10 wt. % oxalic acid solution for 1–2 h, rinsed thoroughly with DI water, and dried in an oven overnight. Ti wires (Alfa Aesar, 99.7%, 0.25 mm dia.) were spotwelded onto one side of the Ti foils. Pd/C powder was mixed with isopropanol and 10 wt.% Nafion and vigorously sonicated for ~ 30 min. The resulting homogeneous catalyst ink was drop-dried onto the Ti substrates with Pd mass loading of 50 µg cm–2 to ensure full coverage of the substrates. The backsides of the Ti substrates were covered with epoxy and dried before electrochemical measurements. Preparation of carbon black and Ti electrodes. For control experiments as shown in Fig.S2a, carbon black electrode (C on Ti electrode) was prepared in the same way as Pd/C electrodes, except that only carbon black (Vulcan XC-72, Fuel Cell Store) mixed with 10 wt% Nafion was loaded onto the Ti substrates with carbon black mass loading of 0.5 mg cm–2. The Ti electrode was prepared in the same way except that no Pd/C or carbon black was loaded onto the Ti substrate. Electrochemical measurements and product analysis. A CH Instruments 760D (or 660D) Potentiostat was used for all CO2 reduction experiments. A piece of platinum foil or gauze was used as the counter electrode. The pH values of CO2-saturated 0.5 M NaHCO3 and N2-saturated 2.8 M KHCO3 are 7.2 and 8.5, respectively. All potentials were measured against an Ag/AgCl reference electrode (3.0 M NaCl, BASi) and converted to the RHE reference scale using E (vs RHE) = E (vs Ag/AgCl) + 0.210 V + 0.0591 V × pH. Electrolyses were performed in a gas-tight two-compartment electrochemical cell with a piece of anion exchange membrane (SELEMION®, AGC Engineering) as the separator, and the membranes were soaked in HCO3– solutions before experiments to fully exchange the anion into HCO3–. Each compartment contained 20.0 mL electrolyte and approximately 5 mL headspace. Before electrolysis, the electrolyte in the cathodic compartment was degassed by bubbling with CO2 or N2 gas for at least 30 min. The electrolyte in the cathodic compartment was stirred at a rate of 1000 rpm during electrolysis. CO2 or N2 gas was delivered into the cathodic compartment at a rate of 5.00 sccm and was vented directly into the gas-sampling loop of a gas chromatograph (SRI Instruments). A GC run was initiated every 15 or 20 min. The GC was equipped with a packed MolSieve 13X column and a packed HaySep D column. Argon (Praxair, 99.999%) was used as the carrier gas. The column effluent (separated gas mixtures) was first passed through a thermal conductivity detector (TCD) where hydrogen was quantified; it was then passed through a methanizer where CO was converted to methane and subsequently quantified by a flame ionization detector (FID). The partial current density for H2 or CO production was calculated from the GC peak area as follows: 2 Fp0 × (electrode area)–1 α RT 2 Fp0 peak area = × flow rate × × (electrode area)–1 β RT
jH2 =
jCO
peak area
× flow rate ×
where α, β are conversion factors for H2 and CO respectively based on calibration of the GC with standard samples, p0 = 1.013 bar and T = 273.15 K. HCO2– concentration was analyzed on a Varian Inova 600 MHz NMR spectrometer. A 0.5 mL aliquot of the electrolyte was mixed with 0.1 mL D2O, and 16.7 ppm (m/m) dimethyl sulfoxide (DMSO,
S2
Sigma, 99.99%) was added as an internal standard. The 1D 1H spectrum was measured with water suppression using a pre-saturation method. [HCO3–] dependence in CO2-saturated electrolytes. Electrolyses were performed as described above in CO2-saturated 0.5, 0.94, 1.8, 2.8 M KHCO3. Upon saturation with CO2, the pH values of the above solutions were 7.2, 7.8, 8.0, and 8.2 respectively. The potential applied was held at –0.15 V (vs RHE). jformate was averaged over the first hour of electrolysis. [HCO3–] dependence study under N2 condition. Electrolyses were performed in N2-saturated 0.52, 0.94, 1.5, 1.8, 2.8 M KHCO3 solutions. The pH values of these solutions were 8.5, and the potential was held constant at –0.15 V (vs RHE). jformate was averaged over the first hour of electrolysis. Activity coefficient of HCO3–. Activity coefficient values used to calculate [CO2] values in Figure 3b were extrapolated from the data in Roy et al. (ref. 27) assuming a linear relationship between concentration and activity coefficient in a narrow region. In Figure 3c, coefficient values were assumed to be 0.5 in the case of [CO32–]/[ HCO3–] = 0.01, 0.05, and 0.1, and 0.4 in the case of [CO32– ]/[ HCO3–] = 0.5. Structural characterization. A grazing incidence X-ray diffraction pattern was acquired for Pd/C using an 11.5 keV synchrotron X-ray beam with Soller slits and a photomultiplier tube detector at beamline 2-1 at the Stanford Synchrotron Radiation Lightsource. Several mg of powder were put onto a glass slide and covered with a piece of Kapton® tape. The incidence angle was optimized to be 4°. Scanning electron microscopy (SEM) images were acquired with a FEI Magellan 400 XHR Scanning Electron Microscope.
S3
Figure S1 Grazing incidence X-ray diffraction pattern of Pd/C powder. Pd diffraction peaks are labeled. The pattern was collected using an 11.5 keV synchrotron X-ray beam.
S4
Figure S2 Background activity of Ti and C black compared to Pd/C. a) Total current density vs time for a Pd/C electrode (Pd/C on Ti foil), carbon black electrode (C on Ti foil) and Ti electrode (Ti foil) at –0.25 V (vs RHE, all potentials were reported with respect to this reference, unless otherwise specified) in CO2-saturated 0.5 M NaHCO3. b) image of a Ti substrate (left), a freshly prepared Pd/C electrode (middle), and a Pd/C electrode after bulk electrolysis of several hours (right). The Ti substrate is fully covered with Pd/C even after a long electrolysis.
S5
Figure S3 Detection of CO during electrolysis with Pd/C at –0.35 V in CO2-saturated 0.5 M NaHCO3. a) geometric current density (jtot) vs time during 3 h of electrolysis. The plot was taken from Figure 2a. b) CO peaks detected from the flame ionization detector of the gas chromatograph at different timepoints during the electrolysis. The amount of CO accounted for < 0.1% Faraday efficiency.
S6
CO2-saturated 0.5 M NaHCO3 15 min
1h
2h
3h
–0.05 V
15 (98%)
13 (92%)
11 (89%)
10 (86%)
–0.15 V
60 (99%)
54 (98%)
48 (96%)
40 (94%)
–0.25 V
78 (99%)
69 (98%)
60 (94%)
50 (87%)
–0.35 V
120 (97%)
96 (91%)
67 (85%)
48 (80%)
1h
2h
3h
CO2-saturated 2.8 M KHCO3 15 min –0.05 V
48 (99%)
47 (99%)
46 (99%)
44 (99%)
–0.10 V
100 (99%)
89 (99%)
78 (99%)
70 (99%)
–0.15 V
134 (99%)
116 (99%)
98 (99%)
83 (98%)
–0.20 V
188 (99%)
155 (99%)
113 (99%)
82 (98%)
1h
2h
3h
N2-saturated 2.8 M KHCO3 15 min –0.05 V
28 (98%)
22 (98%)
19 (97%)
17 (96%)
–0.15 V
82 (97%)
67 (93%)
55 (88%)
50 (87%)
–0.25 V
84 (66%)
67 (60%)
54 (52%)
46 (48%)
Table S1 Summary of averaged mass activity (mA HCO2– synthesis per mg Pd) and average HCO2– Faraday efficiency of Pd/C electrodes in three different electrolytes. Activity values were obtained from the data in Figure 2 and represent the average mass activity for the electrolysis up to the indicated time point.
S7
Figure S4 CO poisoning and attempted recovery without exposure to air. Electrolysis was initiated with Pd/C at –0.25 V in CO2-saturated 0.5 M NaHCO3. After 4 min, a flow of 0.5 mL min–1 CO was introduced into the electrolyte to poison catalysis. After 17 min, the cell was switched to open circuit potential and the CO flow was stopped. The electrolyte was vigorously purged with CO2 for 30 min to remove the excess amount of CO without exposing the electrode to air. Electrolysis was then re-started at – 0.25 V. No recovery of activity was obtained, indicating that CO is not removed from the electrode surface without exposure to air.
S8
Figure S5 Recovering activity of a Pd/C electrode for electrolysis in N2-saturated 2.8 M KHCO3 at -0.l5 V by brief exposure to air. HCO2– Faraday efficiency was plotted by subtracting H2 Faraday efficiency, as determined by periodic GC analysis of the headspace, from the total efficiency. The electrode was exposed to air and transferred to a new electrolyte between electrolyses.
S9
Figure S6 Stability of Pd/C in the absence of CO formation. Relative current density vs time for electrolysis at – 0.25 V in N2-saturated 0.5 M Na2CO3 compared to electrolysis at –0.25 V in CO2-saturated 0.5 M NaHCO3.
S10
Figure S7 Electrolysis with polycrystalline Pd foil. a) total current density vs time for polycrystalline Pd foil at – 0.05 V and –0.25 V in CO2-saturated 0.5 M NaHCO3. The geometric surface area of the Pd foil used was 4 cm2. b) NMR analysis of the electrolyte after electrolysis. The top spectrum shows the peak size for an amount of HCO2– corresponding to 1.5 C. DMSO is used as an internal standard. The bottom two spectra show no detectable HCO2– for the two electrolyses with Pd foil.
S11
Figure S8 Effect of high HCO2– concentrations on the CO2 reduction activity of Pd/C. Relative current density vs time and Faraday efficiency for H2 determined by gas chromatrography for an electrolysis started in CO2-saturated 0.5 M NaHCO3. Concentrated NaHCO2 was added after 15 min and 45 min to raise the [HCO2–] to ~0.1 M and 0.5 M. Current density increased immediately after each addition and then resumed a gradual decline. The modest increase likely results from the increase in the ionic strength of the electrolyte (total ion concentration). The added HCO2– did not affect the FE for CO2 reduction to HCO2–, as evidenced by the low amount of H2 detected by gas chromatography. The accumulation of HCO2– in the electrolyte does not cause catalyst deactivation.
S12
Figure S9 Representative NMR spectrum of the electrolyte after CO2 reduction electrolysis with Pd/C. DMSO is used as an internal standard for quantification of HCO2–.
S13
Figure S10 Bulk electrolyses of Pd/C electrodes at –0.15 V in a) CO2-saturated 0.5 M NaHCO3 and b) CO2saturated 0.5 M KHCO3. The HCO2– Faraday efficiencies vs time were calculated by subtracting the H2 Faraday efficiency (determined by periodic GC analysis) from the total efficiency. Pd/C electrodes exhibited similar current densities and Faraday efficiencies for HCO2– production in the two conditions and similar current decay over time was also observed in both cases.
S14
