Efficient electrocatalytic CO 2 reduction on a three-phase interface

Transcription

1 SUPPLEMENTARY INFORMATION Articles In the format provided by the authors and unedited. Efficient electrocatalytic CO 2 reduction on a three-phase interface Jun Li 1,2, Guangxu Chen 1, Yangying Zhu 1, Zheng Liang 1, Allen Pei 1, Chun-Lan Wu 1, Hongxia Wang 1, Hye Ryoung Lee 3, Kai Liu 1, Steven Chu 4,5 and Yi Cui 1,6 * 1 Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. 2 Department of Chemistry, Stanford University, Stanford, CA, USA. 3 Department of Electrical Engineering, Stanford University, Stanford, CA, USA. 4 Department of Physics, Stanford University, Stanford, CA, USA. 5 Department of Molecular and Cellular Physiology, Stanford University, Stanford, CA, USA. 6 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, USA. * yicui@stanford.edu Nature Catalysis

2 Supplementary Information Efficient Electrocatalytic CO2 Reduction on a Three-phase Interface Jun Li, 1,2 Guangxu Chen, 1 Yangying Zhu, 1 Zheng Liang, 1 Allen Pei, 1 Chun-Lan Wu, 1 Hongxia Wang, 1 Hye Ryoung Lee, 3 Kai Liu, 1 Steven Chu 4,5 and Yi Cui 1,6* 1 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA. 2 Department of Chemistry, Stanford University, Stanford, CA 94305, USA. 3 Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA. 4 Department of Physics, Stanford University, Stanford, California 94305, USA. 5 Department of Molecular and Cellular Physiology, Stanford University, Stanford, California 94305, USA. 6 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA. *Correspondence to: yicui@stanford.edu

3 Supplementary Figures Supplementary Figure 1. Schematic of the positions of the gaseous CO2, high alkalinity electrolyte, and low alkalinity electrolyte. (a) For Au/carbon-based GDL, both the gas phase (brown) and the electrolyte (blue) are constantly flowing. (b) Single-layered structure with the gas phase and low alkalinity electrolyte. (c) Bilayer structure with gaseous CO2 (brown), high alkalinity electrolyte (green) and low alkalinity electrolyte (blue). (d) Flat structure with the low alkalinity electrolyte. Supplementary Figure 2. SEM image and (inset) photograph of a Au-coated PE membrane. 2

4 Supplementary Figure 3. XRD patterns of a Au/PE membrane (red curve) and a pristine nanope membrane (black curve). The characteristic peaks of Au were marked with triangles. 3

5 Supplementary Figure 4. (a-j) Detailed fabrication process and the experimental setup. A bilayer electrode with a spacing of 0.5 mm was prepared as follows. (a) A piece of commercial nanope was cut into suitable sizes (e.g cm 2 ). (b) Au was deposited on the nanope membrane by direct current magnetron sputtering of a gold target. The Au/PE was cut into suitable sizes (e.g. 2 5 cm 2 ). (c) The Au/PE was rolled into a bilayer structure. (d) The end of the Au/PE was trimmed and sealed with Kapton tape. (e) 0.5 mm thick spacers were placed in the interlayers at the bottom (e.g., three spacers needed in total for a bilayer structure). (f) The 4

6 bottom was sealed with a hand-press impulse sealer (e.g. ULINE, Model: H-306) and the total thickness was measured as ~ 1.5 mm. (g) The deformed Au/PE was covered by Kapton tape at the bottom. (h) After ~50 μl of electrolyte was added to the interlayer, the CO2 inlet tube was inserted into the middle of the electrode. The top was sealed using the same method as steps (e) (g). The electrode was clamped and functioned as the working electrode. (i) The outer layer of the bilayer Au/PE membrane was punched with several pores using a 300-μm-diameter needle. (j) All the electrochemical experiments were conducted in an H-type electrochemical cell separated by a selemion anion exchange membrane. The working electrode (WE) and reference electrode (RE) were enclosed in one compartment. The counter electrode (CE, carbon rod) was placed into another enclosed compartment. The bilayer-inner layer coating and bilayer-outer layer coating samples were coated with Au on only the inner layer and only the outer layer, respectively. The other steps were fabricated using the same procedure. 5

7 Supplementary Figure 5. SEM image of (a, b) Au/C and (c, d) Au/Si. Supplementary Figure 6. Contact angle measurements to compare the hydrophobicity change of (a, b) Au/carbon-based GDL and (c, d) Au/PE membrane, respectively, before and after a 24-h continuous electrochemical testing under -1.0 V. 6

8 Supplementary Figure 7. EDX mapping of the cross-section of a Au/carbon-based GDL after 24-h electrochemical testing in a K + -containing electrolyte. The high density dotted area shows the location of the C and K elements. Supplementary Figure 8. (a) Cross-section SEM image and (b) photograph of a bending Au/PE. (c) Cross-section SEM image and (d) photograph of a bending Au/carbon-based GDL. 7

9 Supplementary Figure 9. FEs of CO (red dots) and H2 production (black dots) from (a) flat Au/C, and (b) flat Au/Si electrodes. The error bars represent the average of three independent samples. Supplementary Figure 10. The FEs dependence of the flat, single-layered, bilayer, or trilayered Au/PE. The error bars represent the average of three independent samples. 8

10 Supplementary Figure 11. CV curves of different catalysts measured by an oxygen monolayer chemisorption method to calculate their electrochemical surface areas. Cyclic voltammograms (CVs) of the samples were measured repeatedly from 0.7 to 2.4 V (vs. RHE) in 50 mm H2SO4 at a scan rate of 50 mv s -1 until the traces converged. In the forward scan, a monolayer of chemisorbed oxygen was formed, which was subsequently reduced in the reverse scan. The surface area was calculated by integrating the reduction peak (~1.48 V vs. RHE) to obtain the reduction charge. The reduction charge per microscopic unit area was experimentally measured as 448 μc cm -2. 9

11 Supplementary Figure 12. (a) LSV curves of the flat structure, the single-layered structure, and the bilayer with only outer layer coated structure (designated as bilayer-outer layer coating). FEs and structural schematics of (b) the flat structure, (c) the single-layered structure, and (d) the bilayer with only outer layer coated structure (designated as bilayer-outer layer coating). The error bars represent the average of three independent samples. 10

12 Supplementary Figure 13. Comparison of the saturated normalized current densities of CO production versus catalyst thickness for representative Au catalysts in the literature. Supplementary Figure 14. LSV curves of the flat Au/C under different CO2 flow rates. 11

13 Supplementary Figure 15. (a) Time-dependent ph change in the interlayer space between the outer and inner PE membrane layers. The error bars represent the average of three independent samples. (b) ph profile in the boundary layer at 5 ma cm -2 in 0.5 M KHCO3 (Supplementary Ref. 6). Supplementary Figure 16. Photographs of the fabrication process of (a) bilayer-inner layer coating and (b) bilayer-outer layer coating. The fabrication steps were described in the Method section. 12

14 Supplementary Figure 17. (a) LSV curves of the bilayer structure with both layers coated with Au (designated as bilayer-both layer coating), the bilayer structure with only inner layer coated with Au (designated as bilayer-inner layer coating), the single-layered structure, and the bilayer structure with only outer layer coated with Au (designated as bilayer-outer layer coating). All the three bilayer structures were tested in CO2-saturated 0.5 M KHCO3 (ph 7.4), and the single- 13

15 layered structure was tested in CO2-saturated 6 M KOH (ph ). FEs and structural schematics of (b) bilayer-both layer coating, (c) bilayer-inner layer coating, (d) single-layered structure, and (e) bilayer-outer layer coating. The error bars represent the average of three independent samples. The bilayer-inner layer coating sample exhibited slightly higher FE but lower total current density than the bilayer-both layer coating one. It also showed similar activity and selectivity as the single-layered sample tested in high alkaline electrolyte. The bilayer-outer layer coating sample showed much inferior current density and FE compared with the other three samples, which was similar to a flat structure activity. Supplementary Figure 18. LSV curves of the flat and bilayer Au/PE membranes with same catalyst loading mass. 14

16 Supplementary Figure 19. (a) Liquid and gas separated by a membrane. (b) Pinned contact line at the pore entrance. (c) Advancing contact line inside the pore when θa > 90 and θa < 90 respectively. Supplementary Figure 20. (a) Schematic and (b) measurement of the advancing contact angle of 0.5 M KHCO3 solution on a smooth PE surface using the tilting stage method. 15

17 Supplementary Figure 21. (a) Membrane with vertically aligned cylindrical walls. (b) Membrane with catalyst deposited into the pores for a short distance (< membrane thickness). (c) Membrane with irregular shaped pores where Pore i forms a re-entrant cavity, Pore ii has a vertical sidewall, Pore iii has a tapered sidewall. 16

18 Supplementary Figure 22. Photographs of (a) a bilayer Au/PE membrane and (b, c) a flat Au/PE membrane under an applied potential of -1.1 V in a CO2-saturated 0.5 M KHCO3 solution containing phenolphthalein. 17

19 Supplementary Tables Supplementary Table 1. The onset potentials and overpotentials for the bilayer pouch-type Au/PE, flat Au/C, and flat Au/Si Initial current density (ma cm -2 ) Onset potential (V) [a] Overpotential (mv) vs V Bilayer pouch-type Au/PE Flat Au/C Flat Au/Si [a] Onset potential ma cm -2 higher than the initially stabilized current density 18

20 Supplementary Table 2. The geometric surface area, electrochemical surface areas (ECSAs), and roughness factor (RF) of different electrocatalysts. Geometric area (cm 2 ) ECSA (cm 2 ) Roughness factor Bilayer pouch-type Au/PE Flat Au/PE Flat Au/C Flat Au/Si

21 Supplementary Table 3. Comparison of a bilayer Au/PE electrode with other Au catalysts for CO2 reduction. Sample Thickness Potential FE Geometric Roughness Normalized Saturated (μm) vs jco (ma/cm 2 ) factor jco normalized RHE (mv) (ma/cm 2 ) jco (ma/cm 2 ) Bilayer % V [This work] Au/PE Au/C [This work] % V Au/Si [This work] % V Au needle [ref. 35] 2~5 600 ~95% ~ ~1.1 V Au rod [ref. 35] 0.5~1 600 ~60% ~ ~0.11 Au particle [ref. 35] 0.5~1 600 ~23% ~ ~0.09 Oxide-derived 1~ ~17 72 ~0.24 [ref. 10] Au Meso-Au [ref. 17] ~78% ~5.7 [a] [a] Meso-Au [ref. 17] ~66% ~2.3 [a] [a] 20

22 [a] ir-compensated: Electrode potentials were corrected for the uncompensated ohmic loss (iru) in situ via the current interrupt method. 21

23 Supplementary Notes 1 For the liquid and gas separated by a porous membrane (Supplementary Fig. 19a), whether the liquid will wet the pores of the membrane depends on the wettability and pore geometry. Consider a membrane with pores that are perpendicular to its surface (Supplementary Fig. 19b). Assuming the liquid-vapor interface is initially pinned at the pore entrance, the mean radius of curvature R of the interface is determined by the difference in the pressure across the interface according to the Young-Laplace equation: P = Pliq-Pgas=2σ /R (4) where Pliq and Pgas are liquid and gas pressure respectively, σ is the surface tension of the liquid, and R is the mean radius of curvature of the interface. Equation (4) indicates that the shape of the liquid-gas interface in the pinned state solely depends on the relative pressure (i.e., the interface curves towards the side with a lower pressure). For the contact line to depin and enter the pores, the contact angle that the liquid makes on the sidewall of the pore needs to be the intrinsic advancing contact angle θa (Supplementary Fig. 19c). For a non-ideal surface, θa is typically greater than the static contact angle 2. If θa is greater than 90 degrees, the pressure on the liquid side must be greater than that on the gas side according to the Young-Laplace equation (equation (4)). This critical burst-through pressure ΔPc (the pressure difference between the liquid and the gas) is related to θa as, Pc = 2σ sin(θa-90)/r (5) 22

24 where r is the pore radius. We have assumed that the pore is cylindrical. Equation (5) indicates that the more hydrophobic the membrane surface is (i.e., higher θa), a higher pressure on the liquid side is required to push liquid into the pores. On the contrary, if θa is less than 90 degrees, for liquid advancing in the pores requires the liquid pressure to be less than the gas pressure, also governed by equation (5). In fact, in a system where Pliq,bulk Pgas, an advancing contact angle lower than 90 means that Pliq,bulk is always greater than Pliq at the liquid-gas interface. In other words, liquid will spontaneously enter and wet the entire pore. To investigate the wetting state of the KHCO3 solution on the nanope membrane, we measured the intrinsic advancing contact angle of the KHCO3 solution on a smooth PE surface with a tilting stage method 2 (Supplementary Fig. 20a-b). We added the KHCO3 solution to a smooth PE substrate on a tilted stage and obtained the advancing contact angle just before the contact line moved downhill. The advancing contact angle is 97, which agrees reasonably well with previous literature (92 ) 1. Based on the measured θa, the surface tension of 78 mn/m for 0.5 M KHCO3 solution 3. Based on the measured θa, and a surface tension of 78 mn/m for 0.5 M KHCO3 solution (3), we calculated the critical burst-through pressure as a function of the pore radius. The result is shown in Figures 2c and 2d. In practice, the catalyst can be deposited to a short degree into the pores rather than remaining solely on the membrane surface (Supplementary Fig. 21a-b). Though this may cause the electrolyte to wet the nanope pores to the depth of the catalyst deposited, eventually the liquid will be pinned where it meets the bare PE. In addition, the shape of the pores is more irregular than a perfect cylinder (Supplementary Fig. 21c) 4. In this case, the pores can form (i) re-entrant cavities on the surface of the membrane, which requires an even higher critical burst- 23

25 through pressure because the curvature of the liquid-vapor interface is higher for θa to remain the same 1 ; (ii) a vertical sidewall, similar to Supplementary Fig. 21a; or (ⅲ) a tapered sidewall where liquid may enter due to reduced burst-through pressure. However, liquid will be pinned when it meets the first reentrant cavity and will not wet the rest of the membrane. To evaluate the gas-liquid-solid three-phase contact line position and prove that liquid does not flood the nanope membrane, the critical burst-through pressure ΔPc (i.e., the minimum pressure difference between the liquid and the gas for liquid to enter the pores, assuming cylindrical pores) was calculated based on the Young-Laplace equation (Fig. 2d, inset), which is 2sin (θa - 90 )/r. Here, θa is the intrinsic advancing contact angle of the electrolyte on PE (measured to be 97 ), and the pore radius r is in the range of nm based on SEM images of the nanope membrane (Fig. 2a). This calculation result suggests that the liquid pressure has to be at least 37 kpa higher than the gas pressure for the liquid to enter the pores (Fig. 2d), which is significantly greater than the hydraulic pressure exerted by the electrolyte on the nanope membrane in this system. Therefore, the liquid should not enter the pores of the membrane based on this model. The critical burst-through pressure ΔPc of the GDL is also calculated based on the Young-Laplace equation 2sin (θa - 90 )/r. Here, θa is the intrinsic advancing contact angle of the electrolyte on the PTFE/C (120 according to Ref. 5), and the pore radius r is in the range of μm based on the SEM images of the PTFE/C membrane (Fig. 2c, inset). The critical burstthrough pressure of the GDL is calculated to be 0.88 kpa for the largest pore radius of 85 μm (Fig. 2c), which is far less than that of PE (37 kpa for the largest pore radius of 500 nm). 24

26 Supplementary Notes 2 Calculation for the electrode surface ph 6. A previous theoretical study shows that the local concentration near the electrode surface is significantly different from the bulk concentration and depends on the stirring rate, electrolyte buffer capacity, and the current density of operation. For example, the calculation shows that at a current density of 5 ma cm -2 in 0.5 M KHCO3 solution and a boundary layer (the layer between the electrode surface and the bulk solution where the velocity gradients or convective effects are assumed to be negligible) thickness of 0.01 cm, the system takes roughly s to reach a steady state after the current is turned on. The calculated ph near the electrode surface at steady state is 9.3 (Supplementary Fig. 15b), which provides a very constructive support to the saturated local ph value ~9.6 measured in our work (Supplementary Fig. 15a). In addition, this theoretical work also indicates that the stirring rate, in particular, results in a significant change in the local ph. If the electrolyte is stirred at a higher rate or if the flow velocity is increased in a given flow cell, the boundary layer thickness will decrease and thereby alter the electrode surface ph. Thus, limiting the electrolyte flux can effectively retain a steady-state local high ph as shown in the bilayer pouch-type device structure of our work. Ion distribution profile evolution during the reaction. We conducted inductively coupled plasma mass spectrometry (ICP-MS) measurements to study the ion distribution profile in the interlayer spacing (between the outer and inner PE membrane layers) during the reaction. It showed that the original K + concentration (~ 0.5 M) did not change (within the instrument error) over 5 h since the OH - concentration increase was much smaller comparatively (i.e., ph increases from 7.4 to 9.6). Although the bicarbonate (HCO3 - ) and carbonate (CO3 2- ) anions could not be quantitatively measured by ICP due to equilibrium shifting when diluted, previous work 25

27 calculated this buffer action within the boundary layer and indicated the decrease of bicarbonate anion concentration and the increase of carbonate anion concentration to reach a steady state 6. Phenolphthalein color transition. The local environment ph trapping effect was demonstrated by a simple test using phenolphthalein as an indicator (1 ml of 0.05 M phenolphthalein in ethanol), which has a colorless-to-pink transition point at ph ~ 8.3. Several pinholes (1 mm in diameter) were punched on the outer membrane of a bilayer Au/PE. To clearly see the color change, the Au-coated side of the PE membrane was rolled to face inside, and the white color of the pristine PE membrane was facing outwards. Initially, phenolphthalein in a CO2-saturated 0.5 M KHCO3 solution (ph ~ 7.4) appeared colorless (Supplementary Fig. 22a left). With an applied voltage of -1.1 V vs. RHE, a slight pink color was observed through the pinholes within 1 min (Supplementary Fig. 22a middle and right, and Supplementary Video 1), suggesting a fast increase of the local ph inside the sealed bilayer Au/PE membrane due to both the CO2RR and HER. The electrolyte trapped between the PE layers had limited exchange with the equilibration of the bulk electrolyte, thus effectively retaining an increased alkaline environment. In comparison, a flat Au/PE membrane hardly generated any color change in the bulk electrolyte during 20 min of continuous electrolysis at the same applied voltage due to the lack of the OH - trapping effect by a flat structure (Supplementary Fig. 22b). The flat Au/PE gradually turned pink after 30 min of continuous electrolysis (Supplementary Fig. 22c). The same phenomenon of pink color emergence was also observed in the Au-coated PE membrane with the Au side facing outwards. 26

28 Supplementary References 1. Matsunaga, M. & Whitney, P. Surface changes brought about by corona discharge treatment of polyethylene film and the effect on subsequent microbial colonization, Polym. Degrad. Stab.. 70, (2000). 2. de Gennes, P., Brochard-Wyart, F., Quere, D., Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. Springer, (2003). 3. Chemistry Dashboard Arora, P. & Zhang, Z. Battery separators. Chem. Rev. 104, 4419 (2004) 5. Zhang, J., Li, J. & Han, Y. Superhydrophobic PTFE Surfaces by Extension. Macromol. Rapid Commun. 25, 1105 (2004). 6. Gupta, N., Gattrell, M. & MacDougall, B. Calculation for the cathode surface concentrations in the electrochemical reduction of CO2 in KHCO3 solutions. J. Appl. Electrochem. 36, (2006). 27