Dissertation
Optimization of electrolyte media on pure and alloyed electrodes for the electrochemical reduction of carbon dioxide
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2020
DOI: 10.17077/etd.005297
Abstract
Recycling atmospheric carbon dioxide (CO2) is a potential source of renewable carbon-based fuels and chemicals. Developing affordable electrocatalysts to facilitate carbon dioxide reduction reaction (CO2RR) to high-value products with high selectivity, efficiency, and large current densities, is a critical step for the production of liquid carbon-based fuels. However, the discovery and development of catalysts that can incorporate all the desirable properties afore mentioned remains a challenge. In the studies presented here efforts were undertaken to optimize the electrolyte media using room temperature ionic liquids (RTILs) in combination with common molecular solvents. Optimization of the cathode material was also investigated with pure and alloyed metallic electrocatalysts.
One drawback to the utilization of RTILs for efficient CO2RR is their high viscosity, which can further increase significantly when saturated with CO2. Their high viscosity hinders mass transport and reduces conductivity, reducing reaction kinetics. To address the high viscosity of RTILs and the subsequent reduction to mass transport; experiments were carried out to systematically decrease the viscosity of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) by means of dilution with pure water. Water is an ideal solvent due its low cost and dual purpose as a potential proton source, however its disadvantage arises from competition with hydrogen evolution reaction (HER). The results from this work show that [EMIM][OTf] can be diluted up to an optimal volume concentration of 10 % (v/v) with pure water. At the optimal dilution the over-potential to reduce CO2 is reduced by 400 mV. CO2 reduction in [EMIM][OTf] is further improved by using a buffering co-solvent (10% 0.1 M NaHCO3) and CO2RR is achieved with high selectivity for carbon monoxide (CO) with 93.0 ± 4.6 % Faradaic efficiency. HER is suppressed in the range of neat [EMIM][OTf] (500 ppm water content) to [EMIM][OTf]:H2O mixture of 10 % aqueous content. The reduction process and products are largely insensitive to pH. Conductivity and viscosity of the [EMIM][OTf]:H2O mixtures suggest that the ionic liquid ion pair fully dissociates; analogous to dilute KCl solution, where each ion is completely hydrated by water molecules.
To further investigate the potential of co-solvent for RTILs for CO2RR the potential use of Acetonitrile as a co-solvent were investigated. The results of this work show that water reduced the overpotential with little to no improvement to current density, on the other hand, acetonitrile shows no improvement to the over-potential but significantly improves the current density up to 50 mA/cm2; matching some of the best performance data currently reported for this reaction. These results clearly show the difference in using a polar protic solvent, water and using a polar, amphiphilic, aprotic solvent, acetonitrile. We find that adding water to the ionic liquid significantly reduces the overpotential for the reduction, and that adding acetonitrile significantly increases the current density. These results suggest that a three-part mixture of ionic liquid, water, and acetonitrile may lead to further improvements in reaction efficiency. This idea is further explored in the third project.
Alloying has been used as a means to tune reactivity and selectivity of electrocatalysis for decades. In the third work, we show that inexpensive post-transition metals (tin (Sn) and lead (Pb)) and their alloys (PbSn) are excellent cathode materials to reduce CO2 in an ionic liquid/acetonitrile/water electrolyte media. Electrochemical impedance spectroscopy (EIS) measurements show that the PbSn alloys exhibit lower charge-transfer resistance when compared to the pure metal electrodes, as supported by electronic structure calculations. Current densities as high as 60 mA/cm2 are observed with optimal mixtures of ionic liquid, acetonitrile, and water. Reduction product analysis identifies carbon monoxide (CO) and formate (HCOO-) as primary reduced products, with higher selectivity towards formate. Faradaic efficiency for formate on pure Pb and pure Sn was determined to be 80 ± 4% and 86 ± 3% respectively. FE% improves as either Pb is incorporated into Sn or vice versa, and a maximum FE of 91 ± 3% for both 50 % and 40 % Pb composition.
To further explore the benefits of alloying pure metals for CO2RR, experiments were carried out to investigate alloys of copper (Cu) and silver (Ag) in the fourth study presented here. In this work we present investigations of electrochemical reduction of carbon dioxide on pure copper, pure silver, and copper/silver alloys in pure acetonitrile with added water. EIS measurements show that the copper/silver alloys exhibit lower charge-transfer resistance when compared to the pure metal electrodes. Reduction product analysis identifies carbon monoxide (CO) as the major product on pure Ag, Ag with only 10% Cu, and Cu with only 10% Ag with Faradaic efficiencies over 60%. Formate (HCOO-) is the major reduced product close to the 50:50 (Cu:Ag) ratio with Faradaic efficiencies over 30%. These results suggest that changes in the structural strain of the bimetallic electrodes influences reduced product selectivity.
In conclusion, this dissertation presents a variety of experimental work and technique development which directly led to the combination of various solvents to provides insight towards logical formulation of electrolyte media for CO2RR. Furthermore, the work presented here led to fundamental insights into the activity and selectivity of pure and alloyed metal electrodes for the electrochemical reduction of CO2.
Details
- Title: Subtitle
- Optimization of electrolyte media on pure and alloyed electrodes for the electrochemical reduction of carbon dioxide
- Creators
- Amanuel H. Hailu
- Contributors
- Scott K Shaw (Advisor)Scott R Daly (Committee Member)Johna Leddy (Committee Member)Sara E Mason (Committee Member)Alexei V Tivanski (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Spring 2020
- DOI
- 10.17077/etd.005297
- Publisher
- University of Iowa
- Number of pages
- xviii, 123 pages
- Copyright
- Copyright 2020 Amanuel H. Hailu
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 114-123).
- Public Abstract (ETD)
Since the industrial era we have burned fossil fuels and other carbon-based fuels to power our transportation and energy needs. While this has been an essential step to advance our society, it has released a significant amount of carbon dioxide (CO2) into our atmosphere. The long-term effects of such high concentration of a greenhouse gas in our atmosphere is still unknown. Over the years many technologies have been developed and proposed to isolate CO2 from our atmosphere and store it in abandoned mines and perhaps even deep within the oceans.
The question we have asked ourselves, along with many scientists and chemists around the world, is what if we took the captured CO2 and converted it to a useful fuel or chemical. For one it would reduce our dependence on fossil fuels and two, if we are capturing and converting as much CO2 as we are releasing into the atmosphere, we would be able to achieve a carbon neutral economy.
My research investigates various methods to efficiently convert CO2 to useful fuels and chemicals using electrochemistry. I generally do so by inserting electrodes in a solution, bubble CO2 through the solution, and apply a voltage to the electrodes and then identify the products and energy efficiency of the process. I experiment with different mixtures and also investigate different metals of the electrodes to improve the process. The outcome of these projects creates new knowledge/insight into CO2 recycling to be readily accessible to chemists and engineers via publications and presentations.
- Academic Unit
- Chemistry
- Record Identifier
- 9983949696102771
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