1. CO Stark Shift to Probe the Ionic Liquid-Ag Interface
on the CO2 electroreduction
Natalia Garcia Rey and Dana D. Dlott
University of Illinois at Urbana-Champaign
International Symposium on Molecular Spectroscopy
72nd Meeting, Champaign-Urbana

2. Take-Home Message
The CO Stark shift monitors the Ionic Liquid-Ag structural transition, that promotes the CO2 electroreduction on Ag.
At higher water concentrations the Stark shift is weaker, indicating the CO bonds less strongly to Ag and enhancing the CO2 electroreduction.
Vibrational Sum Frequency Spectroscopy to study the CO2 electroreduction into CO promoted by ionic-liquid (electrolyte) on Ag (electrode).
Tuning the ionic liquid by:
Mixing with water
Exchange the anion
Exchange the cation
How by tuning the ionic liquid affects:
the Stark shift of the CO adsorbed on Ag.
the relaxation of the CO mode (Free Induction decay).
the CO adsorption site on Ag.

3. Artificial Photosynthesis
CO2 Filter
Air H2
Filtered Air
Fuel
Water electrolyser
Synthesis
Energy
Energy
CO2
Electrolyzer CO
Efficient CO2 electroreduction:
Recycling CO2
Production of hydrocarbon fuels

4. Water enhancement of the CO2 conversion on Ag in room temperature ionic liquids (RTIL)
0.3%
45%
77% 
RTIL: EMIM-BF4
- Ionic liquid mediated the CO2 electroreduction into CO at low-overpotential1,2.
- Water increases Faradaic efficiency3.
in EMIM-BF4
[1] Rosen, B. A. et al., Science 2011, 334, 643, [2] A. Salehi-Khojin et al. J. Phys. Chem. C, 2013, 114, 1627, [3] Right Fig. adapted from Rosen, B. A. et al. J Electrochem. Soc. 2013, 160, H138.
*Studied concentrations

5. Sum Frequency Spectroscopy to Study Electrochemical Interfaces
SFG=IR+VIS
SFG Intensity in metal-liquid interfaces has two contributions:
Metal-liquid interface: nonresonant-SFG (NR-SFG)
Molecule observed, resonant with IR : resonant-SFG (RES-SFG)

6. Water enhancement of the CO2 conversion on Ag in room temperature ionic liquids (RTIL)
0.3 mol% H2O
45 mol% H2O
77 mol% H2O
-1.33 V
-1.21 V
-0.92 V Ar Ar Ar
(a)
(b)
(c)
CO2
CO2
CO2
Explain how to read the CV
Compare the 3 CVs
Remark the onset potential at different water concentrations

7. Water-RTIL Ag Interface:
Potential-dependent NR-SFG
0.3 mol % H2O
45 mol % H2O
77 mol % H2O
-1.3 V
-1.2 V
-0.9 V
these figures highlights the coincidence between the parabolic minimum and the CO2 reduction threshold, and the increase in parabolic
curvature in the CO2 reduction region at all water concentrations.
*Garcia Rey & Dlott, PCCP, 2017, 19, 10491

8. Take-Home Message
A minimum in the potential-dependent NR-SFG Intensity was found at the same potential where the CO2 electroreduction occurs, indistinctly of the Ar or CO2 is on the electrochemical cell; indicating the RTIL structural transition on Ag mediated the reduction.
At higher water concentrations, the RTIL structural transition occurs at less negative potentials, which similarly lowers the CO2 reduction threshold.

9. at different water concentrations RTIL
CO atop adsorbed on Ag
at different water concentrations RTIL O C Ag
0.3 mol% H2O
45 mol% H2O
77 mol% H2O
1st cycle
2nd cycle
3rd cycle
Potential vs. Ag/AgCl(V)
C:\Users\Natalia\Documents\Data\EC-SFG\AgEMWater\160331_AgEM77CO2.pxp
IR frequency (cm-1)
*Garcia Rey & Dlott, PCCP, 2017, 19, 10491

10. CO atop vs. bridged bonded on Ag
at 77 mol% water in RTIL
IR frequency (cm-1)
1st cycle
2nd cycle
3rd cycle
Potential vs. Ag/AgCl(V)
*Garcia Rey & Dlott, PCCP, 2017, 19, 10491

11. Stark Shift of the CO atop on Ag
at different water concentrations RTIL
*Stark Shift probes the local field on the electrode
0.3 mol% water
77 mol% water
24 cm-1/V
55 cm-1/V
15 cm-1/V
Stark shift is a measurement of the local field at the electrode surface. The SF abruptly increases by double at the onset of the CO2 electroreduction.
27 cm-1/V
Garcia Rey & Dlott, JPCC, 2015, 119, 20892; Garcia Rey & Dlott, PCCP, 2017, 19, 10491
Water molecules  e- screening on Ag
 e- screening lowers the workfunction,  the probability of the e- transfer at lower overpotentials.
 H+ (pH) produce  e- screening?
The double layer with more H+, opens new adsorption sites and lower energy reaction paths, decreasing the CO2 overpotential.

12. CO Stark Shift on RTIL-Ag electrified interface
-1.33 V
-0.92 V
The CO Stark shift also monitors the RTIL structural transition occurs on Ag, and it doubles in magnitude.
At higher water concentrations:
the Stark shift is weaker: the water molecules decrease the e- screening from Ag.
CO frequency increases: stronger CO bond, loosely bond to Ag.
0.3%
77%

13. Free Induction decay of CO atop on Ag at different cell potentials
FID of CO adsorbed on:
Cu, Pt (UHV) ~ 1-2 ps
0.1ML H2SO4 on Pt ~675 fs
RTIL/Ag ~250 fs

14. Take home message
The RTIL structural transition mediated the CO2 electroreduction. A minimum in the NR-SFG was found at the same potential where the CO2 electroreduction occurs, and where the CO Stark shift also changes and doubles.
At higher water concentrations, the RTIL structural transition occurs at less negative potentials, which similarly lowers the CO2 reduction threshold.
Water adsorbed on the interfaces reduces the electron screening from the Ag, reducing the electric field felt by the CO molecule.
The atop CO vibrational mode relaxes faster on the RTIL-Ag than other metal surfaces, such as Pt which is chemically bonded in electrochemical cells.

15. Thank you all for your attention!
Acknowledgments
Prof. Dana D. Dlott
Shuichi Toyouchi
Alexander K. Moore
Dlott’s group
Thank you all for your attention!
Shuichi
Dana
Any questions?
*Part II tomorrow: P2570 Noyes 161 at 9:55 am

16. Free Induction decay of CO atop on 0.1ML H2SO4 Pt