Controlling selectivity in electrocatalytic CO2 reduction with an external magnetic field.

Author

Nawaraj Karki, University of LouisvilleFollow

Date on Master's Thesis/Doctoral Dissertation

12-2025

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Wilson, Andrew J.

Committee Member

Zamborini, Francis

Committee Member

Thompson, Lee

Committee Member

Mendes, Sergio

Committee Member

Sathitsuksan, Noppadon

Author's Keywords

Electrocatalytic; magnetic field; controlling; reduction

Abstract

My dissertation describes how to alter the speciation and concentration of molecules at the electrode-electrolyte interface, and how those alterations affect reactivity and selectivity. This dissertation presents two distinct approaches to control the mass transport of neutral molecules such as carbon dioxide (CO₂) at the electrode-electrolyte interface by using an external magnetic field. The first finding demonstrates that magnetic fields indirectly affect the mass transport of target analytes such as CO₂ molecules by selectively interacting with charged species, such as hydroxide ions (OH^-). Such interaction induces magnetohydrodynamic (MHD) effects, which generate localized hydrodynamic flow and alter the pH near the electrode surface, thereby modulating the reaction selectivity. The second finding reveals that magnetic fields directly affect the mass transport of the CO2 analyte by using charged molecules, such as imidazolium-based ionic liquids (ILs), to form IL-CO₂ complexes. The magnetic field directly enhances the mass transport of CO₂ molecules, resulting in increased selectivity of the CO₂ reduction reaction over the H₂O reduction reaction. The electrocatalytic reduction of CO₂ in aqueous systems was monitored by observing the change in current density using linear sweep voltammetry (LSV) in the presence and absence of an external magnetic field. The reduction potential slightly beyond the onset potential was used to measure the steady-state current density using chronoamperometry (CA), and current density was measured across different magnetic field strengths. Faradaic efficiencies of gaseous products such as CO and H₂ were measured and observed that CO selectivity increased on increasing the magnetic fields. As the magnetic field increases, the OH^- ions transports away from the electrode surface, reducing local pH and favoring CO₂ reduction over water reduction reaction. Similarly, CO selectivity was higher when the magnetic field was kept constant while varying the reduction potential. Bulk pH studies confirmed that the magnetic field enhances the transport of OH^- species away from the electrode surface, lowering the local pH and suppressing water reduction. Finite-element simulation studies further revealed that magnetic field transports OH^- species away from the electrode surface via Lorentz force, making water reduction less favorable thereby increasing the interfacial CO^₂ concentration and promoting CO₂ reduction. A magnetic field directly interacts with charged species, the second finding of this dissertation describes how an external magnetic field affects the selectivity of CO₂ reduction when mass transport of CO₂ molecules is directly affected. This part of the work not only discusses improved selectivity but also enhanced mass transport of CO₂. A charged analyte such as an ionic liquid (IL) was used to form an IL-CO₂ complex through physisorption or chemisorption. NMR and FTIR characterizations confirmed the formation of carboxylate moieties bonded to the imidazolium ring. Faradaic efficiency measurements revealed that at higher concentrations of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄), the selectivity of CO decreased due to ILs forming a layer and blocking the active sites for CO₂ reduction. However, at lower concentrations of EMIMBF₄, the selectivity for CO₂ reduction was higher in the presence of a magnetic field. This finding demonstrates that neutral molecules can be charged so that magnetic fields can directly interact with them, increases mass transport via Lorentz force and enhances CO₂ reduction reaction over H₂O reduction reaction. The findings in this dissertation discuss and offers more degree of freedom to control mass transport at the electrode-electrolyte interface. Furthermore, they provide an opportunity to enhance the mass transport of other neutral species such as N₂ or CH₄ and to control reaction selectivity in electrocatalysis through magnetic field modulation.

Recommended Citation

Karki, Nawaraj, "Controlling selectivity in electrocatalytic CO2 reduction with an external magnetic field." (2025). Electronic Theses and Dissertations. Paper 4655.
Retrieved from https://ir.library.louisville.edu/etd/4655