Dissertations, Theses, and Capstone Projects

Date of Degree

6-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Elizabeth Biddinger

Committee Members

Teresa Bandosz

Michael Mirkin

Ilona Kretzschmar

Subject Categories

Catalysis and Reaction Engineering

Keywords

Electrocatalysis, CO2 electroreduction, Ionic liquids, Aqueous electrolyte, Electrode geometry, Simulation

Abstract

Excessive utilization of the fossil fuels due to the rapid growth of the global population has resulted in a dramatic increase in the carbon dioxide (CO2) level in the atmosphere which is the main reason for global warming and climate change. Therefore, green technologies are in high demand to develop carbon-neutral energy cycles. In this regard, CO2 electroreduction (CO2ER) has been proposed as a promising approach for CO2 utilization. CO2ER can mitigate the CO2 level in the atmosphere as well as produce value-added chemicals and fuels at ambient conditions. Despite the benefits of CO2 electroreduction, the low energy efficiency and poor product selectivity in CO2ER have retarded large-scale application of this process. Numerous strategies have been proposed to control the selectivity and enhance the catalytic activity in CO2ER. However, the electrode geometry and electrolyte composition in the aqueous electrolytes have been less studied in CO2ER compared to other factors such as catalyst materials and catalyst morphology.

In the first part of this work, the effect of electrode geometry on CO2ER was examined for both polycrystalline Cu and Ag metals. For this purpose, CO2ER was performed on three different electrode shapes, flag (2-D), foil coil (3-D), and wire coil (3-D), in 0.1 M potassium bicarbonate (KHCO3). In addition to the experimental study, COMSOL Multiphysics was also used to predict the current, potential, and electric field distribution. Results showed that both foil coil and wire coil have a higher CO2ER catalytic activity in relation to the flag electrodesregardless of the electrode material (Cu or Ag). By changing the electrode geometry from flag to foil coil and wire coil, a 69% and 76% increase, respectively, in faradaic efficiency (FE) for C2 products were observed. However, the FE for methane increased only on Cu foil coil (104% increase compared to Cu flag), and the Cu wire coil showed a lower FEmethanecompared to other electrode shapes. The shape of the electrode also affected the CO selectivity and activity on Ag electrodes. Ag foil coil and Ag wire coil had a 20% and 5% increase in FE for CO compared to Ag flag at -1.12 V. The observed superior performance on foil coil and wire coil electrodes can be explained by the high electric fields around them due to the larger amount ofsharp and high curvature points on the surface compared to the flag electrode. Enhanced electric field at the interface causes more cations to adsorb to the surface and stabilize the intermediates such as CO2 •− radicals which are needed for CO2ER.

In the second part of this study, the effect of anion and cation in ionic additives on the product selectivity and activity of the Cu catalyst in CO2ER was investigated. For the anion study, 10 mM of an ionic liquid (IL) with the same cation 1-butyl-3-methylimidazolium [BMIM]+ and various anions: bis(trifluoromethylsulfonyl)imide [NTF2]–, triflate [OTF]–, acetate [Ac]–, chloride [Cl]–, and dicyanamide[DCA]– was used. The results showed that although imidazolium-based ILs have a potential to enhance CO2ER due to the interaction of CO2 with imidazolium ring, the anion of IL also plays an important role in CO2ER. It was found that there is a relationship between the hydrophobicity of the anion and CO2ER activity. Higher CO2ER activity was found for more hydrophobic ILs such as [BMIM][NTF2]. In all ILs except for [BMIM][DCA], the formate FE% increased by adding the ILs to the electrolyte. The maximum increase in formate (38.7% FE) was observed for [BMIM][NTF2] at -0.92 V which has the highest hydrophobicity compared to other ILs. However, [BMIM][DCA] which has a high hydrophilicity and a low CO2 affinity shut off the CO2ER and enhanced HER at all potentials. This observation is attributed to the surface poisoning due to the strong adsorption of [BMIM][DCA] which was confirmed by X-ray photoelectron spectroscopy (XPS).

Changing the cation from [BMIM]+ to sodium (Na+) and potassium (K+) with [NTF2]– and [DCA]– anions showed that the cation of the additive also plays a role in CO2ER especially for [NTF2]–-based additives. Results showed that all [NTF2]–-based additives increased the FE for formate compared to the additive-free electrolyte (9% FE). Among [NTF2]–-baased additives, [BMIM][NTF2] had a higher FE for formate (38.7%) compared to K[NTF2] (23.2%) and Na[NTF2] (18.5%) at -0.92 V probably due to the presence of imidazolium cation which can further stabilize the intermediates on the surface and enhance CO2ER. However, the FE for C2products (ethylene and ethanol) at high negative potentials were lower for [BMIM][NTF2] and K[NTF2] compared to the additive-free and Na[NTF2] electrolytes. This observation can be due to the presence of [BMIM]+ and hydrated K+ cations on the surface and inhibiting the *CO dimerization which is needed for the formation of C2 products. Electrolytes containing [DCA]–-based additives had a very high HER activity and low CO2ER activity regardless of the cation nature. This is due to the strong adsorption of [DCA]– anions on the surface which poisons the surface for CO2ER.

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