Electrocatalytic Activation of CO2 in Room-Temperature Ionic Liquids
Final Report Abstract
The electrochemical conversion of CO2 into higher-value chemical products, i.e. through power-to-X processes, can make an important contribution to addressing global environmental and energy storage challenges. However, high overpotentials and low product selectivity in conventional aqueous electrolytes hinder this process, necessitating the investigation of alternative electrolytes. Room-temperature ionic liquids (IL) have proven to be promising as they lower CO formation potentials and reduce the energy demand for CO2 electrolysis. The results of this project show that the molecular structure of the interface between the electrode and the IL is crucial for CO2 electrolysis. Using in situ sum-frequency generation spectroscopy (SFG), the structures of 1-butyl-3-methylimidazolium [BMIM] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BMMIM][NTf2] as well as water molecules on single-crystalline Pt(111) electrodes were analyzed as a function of electrode potential. The cations of these ILs reorient differently, creating partially open structures that facilitate water adsorption at the interface and enable additional reaction pathways for CO2 electrolysis. Experiments on the electrochemical reduction of CO2 revealed that the CO formation potential for [BMMIM][NTf2] and 1-butyl-1-methylpyrrolidinium [BMPyrr][NTf2] (-0.7 V vs. SHE) is only weakly dependent on the H2O concentration (< 1.5 M), yet CO2 electrolysis can still be slightly enhanced. In contrast, [BMIM][NTf2] shows a strong dependence on the H2O concentration, reducing the CO formation potential to -0.4 V and significantly lowering the existing overpotentials. Operando IR absorption spectroscopy provided insights into the mechanism: an acidic C2 position at the imidazolium ring favors the formation of an imidazolium-2-carboxylic acid intermediate, which reacts further with water to form CO as a reaction product. For [BMMIM][NTf2]- and [BMPyrr][NTf2]-based electrolytes with a blocked C2 position, it is suggested that the reduction of CO formation potentials is due to electrostatic stabilization of the CO2- radical anion at the interface. Once the reaction mechanisms were clarified, the investigation focused on additional catalysts, particularly Au and Cu electrodes, due to their application potential. Both well-defined Cu- Au(111) surfaces and AuCu intermetallic compounds in contact with [BMIM][NTf2] electrolytes were examined. Excellent product selectivity for H2 and CO was demonstrated. In addition, by systematically varying the copper content on the catalyst surface, an adjustable H2/CO ratio between 1.8 (Cu-Au(111)) and 3.2 (AuCu3) was achieved. These ratios are particularly relevant for industrially important Fischer-Tropsch synthesis and methanation.
Publications
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Cations of Ionic Liquid Electrolytes Can Act as a Promoter for CO2 Electrocatalysis through Reactive Intermediates and Electrostatic Stabilization. The Journal of Physical Chemistry C, 125(30), 16498-16507.
Ratschmeier, Björn & Braunschweig, Björn
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Role of imidazolium cations on the interfacial structure of room‐temperature ionic liquids in contact with Pt(111) electrodes. Electrochemical Science Advances, 3(4).
Ratschmeier, Björn & Braunschweig, Björn
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Influence of interfacial water and cations on the oxidation of CO at the platinum/ionic liquid interface. Physical Chemistry Chemical Physics, 25(2), 1014-1022.
Ratschmeier, Björn; Roß, Gina; Kemna, Andre & Braunschweig, Björn
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Cu/Au(111) Surfaces and AuCu Intermetallics for Electrocatalytic Reduction of CO2 in Ionic Liquid Electrolytes. ACS Catalysis, 14(3), 1773-1784.
Ratschmeier, Björn; Paulsen, Christian; Stallberg, Klaus; Roß, Gina; Daum, Winfried; Pöttgen, Rainer & Braunschweig, Björn
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Electrocatalysis and self assembly at solid/liquid interfaces studied by nonlinear optical spectroscopy. Encyclopedia of Solid-Liquid Interfaces, 254-266. Elsevier.
Ratschmeier, Björn; Kemna, Andre & Braunschweig, Björn
