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

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.

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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

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)

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

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

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.

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

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

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.

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%

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

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.

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

Free Induction decay of CO atop on 0.1ML H2SO4 Pt