Imidazolium-based Ionic Liquid – Ag Interfaces on the CO2 Electroreduction studied by Sum Frequency Spectroscopy Natalia Garcia Rey and Dana D. Dlott University of Illinois at Urbana-Champaign International Symposium on Molecular Spectroscopy 72nd Meeting, Champaign-Urbana

Room Temperature Ionic liquid (RTIL) materials for the electrochemical challenges of the future Designed RTILs for specific applications Batteries Aprotic Protic Fuel Cells Supercapacitors https://www.jst.go.jp/act-c/en/research_area/ongoing/umeda_en.html Nature Materials 8, 621 - 629 (2009)  http://www.explainthatstuff.com/how-supercapacitors-work.html

Sum Frequency Spectroscopy to Study Electrochemical Interfaces SFG=IR+VIS SFG Intensity in liquid-metal interfaces has two contributions: Metal-liquid interface: nonresonant-SFG (NR-SFG) Molecule observed, resonant with IR : resonant-SFG (RES-SFG) IR VIS SFG Gouy-Chapman double-layer model: Diffuse layer Inner and outer layer double layer bulk electrolyte electrode CaF2 window E

Potential dependence SFG Intensity IR VIS SFG surface sensitive bulk sensitive double layer bulk electrolyte electrode CaF2 window  Ez integrated over the Debye Length  potential on the interface E <<<1 linear  offset  *Garcia Rey & Dlott, J. Electroanal. Chem, 2016, DOI: 10.1016/j.jelechem.2016.12.023

CO2electroreduction RTIL-Ag Simplified model SF intensity-potential dependence Li-ion batteries CO2electroreduction RTIL-Ag time (s) *Garcia Rey & Dlott, J. Electroanal. Chem, 2016, DOI: 10.1016/j.jelechem.2016.12.023

Water enhancement of the CO2 conversion on Ag in room temperature ionic liquids (RTIL) RTIL: EMIM-BF4 0.3% 45% 77%  in EMIM-BF4 - Ionic liquid mediated the CO2 electroreduction into CO at low-overpotential1,2. - Water increases Faradaic efficiency3. pH=6.5 pH=3.2 pH=7.6 [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

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 CO2 CO2 CO2 *Garcia Rey & Dlott, PCCP, 2017, 19, 10491

Water-RTIL Ag Interface: Potential-dependent NR-SFG 0.3 mol % H2O 45 mol % H2O 77 mol % H2O (a) (b) (c) -1.3 V -1.2 V -0.9 V *Garcia Rey & Dlott, PCCP, 2017, 19, 10491

at different water concentrations RTIL CO adsorbed on Ag at different water concentrations RTIL 0.3 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

Take Home Message: Water-RTIL Ag Interface -1.33 V -0.92 V A structural transition of ionic liquid promotes the CO2 electroreduction, when the onset of CO2 occurs: The CO Stark Shift doubles in magnitude. There is a minimum of the potential- dependence NR-SFG Intensity. At higher water concentrations: the Stark shift is weaker: the water molecules screen the e- screening from Ag. CO frequency increases: stronger CO bond, loosely bond to Ag. The Potential-dependence NR-SFG shifts to lower potentials. 0.3% 77%

Role of Imidazolium based promoter for the CO2 electroreduction on Ag http://www.nature.com/nmat/journal/v8/n8/full/nmat2448.html G.P.S. Lau, M. Schreier, D. Vasilyev, R. Scopelliti, M. Grätzel, P.J. Dyson, New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode, Journal of the American Chemical Society 138(25) (2016) 7820-7823. 1a. Tetrafluoborate 1b. N,N-bis(trifluoromethane)sulphonamide 1c. Trifluoromethane-sulfonate 1-methyl-2,3-dimethylimidazolium TFSI

CO2 electroreduction mediated by imidazolium-based ionic liquids on Pb EMIM-Tf2N L. Sun, G.K. Ramesha, P.V. Kamat, J.F. Brennecke, Switching the Reaction Course of Electrochemical CO2 Reduction with Ionic Liquids, Langmuir 30(21) (2014) 6302-6308.

CO2 electroreduction mediated by imidazolium-based ionic liquids Addition of water Anion change Water enhances the CO2 electrochemical reduction into CO mediated by imidazolium-based ionic liquids Rosen, B. A. et al. J Electrochem. Soc. 2013, 160, H138. Reference: Accelerated CO2 transport on surface of AgO nanoparticles in ionic liquid BMIMBF4 Exchange C2 imidazolium ring. Anion change Higher efficiency CO2 reduction CO2 electroreduction on Pb The protons at the C4 and C5 positions are essential for efficien catalysis Lau et. Al. New Insights Into the Role of Imidazolium-Based Promoters for the Electroreduction of CO2 on a Silver Electrode, Journal of the American Chemical Society 138 (2016) 7820 L. Sun, G.K. Ramesha, P.V. Kamat, J.F. Brennecke, Switching the Reaction Course of Electrochemical CO2 Reduction with Ionic Liquids, Langmuir 30(21) (2014) 6302-6308. Cation exchange: Piridinium. Cation tuning: Butyl chain.

Imidazolium based RTIL-Ag Interface Potential-dependent NR-SFG Anion change Cation tuning Cation exchange

Imidazolium based RTIL-Ag Interface CO atop Stark Shift - CO atop not seeing on the surface Cation exchange Cation tuning

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 potential-dependent NR-SFG Intensity monitors the changes in the double-layer RTIL-Ag interface. Tuning the electrolyte shows different dependency on the potential. 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. The atop CO vibrational mode allows to monitor the changes on the interfaces by the Stark shift. The CO FID mode relaxes faster on the RTIL-Ag than other electrochemical metal interfaces.

Thank you all for your attention! Acknowledgments Prof. Dana D. Dlott Bruno G. Nicolau Dlott’s group Bruno Dana Thank you all for your attention! Any questions?

Cation exchange Cation tuning Potential vs. Ag/AgCl(V) -0.5 -1.8 -0.5 -1.8 Potential vs. Ag/AgCl(V) IR frequency (cm-1) IR frequency (cm-1)