Oxygen reduction on iron macrocycles kinetics study and engineering of the electrochemical interface
File(s)
Author(s)
Favero, Silvia
Type
Thesis
Abstract
Hydrogen fuel cells have garnered significant attention as a promising alternative to conventional energy sources. However, their commercialization has been hindered by the requirement of platinum to catalyse the oxygen reduction reaction. Catalysts based on FeN4 active sites have emerged as a promising alternative, but the activity and stability of these catalysts is still too low for cost-effective applications.
In this thesis, the kinetics of oxygen reduction is studied on well-defined iron macrocycles, used as model FeN4 catalysts. Two groups of macrocycles were identified, differentiated by the presence of a high potential Faradaic peak in the cyclic voltammogram, which was found to control both activity and peroxide selectivity. Operando UV-Vis spectroscopy, operando X-ray adsorption, DFT simulations and electrochemical techniques were combined to show that this peak is also an experimental probe of *OH binding energy.
Iron phthalocyanine is then modified with the addition of a thin layer of ionic liquids, to understand how ionic liquids can affect the activity of FeN4 catalysts. Oxygen concentration and thermodynamic activity is experimentally determined and the results are used to develop a first-principle model for oxygen transport. The effect of the ionic liquid on the kinetics of oxygen reduction is also studied. In conclusion, it was found that optimal ionic liquid layers should maximize oxygen solubility, while minimizing their thickness.
A polymerized form of one of the ionic liquids, poly-co-(styrene)-(vynilimidazole TFSI), was also tested as ionomer. It was found that this poly(ionic liquid) can combine the binding properties of polymers with the oxygen permeability of ionic liquids. Dynamic light scattering and rheology measurements showed that the PIL can improve iron phthalocyanine dispersion and prevent its aggregation in the catalyst layer. Finally, studies in gas diffusion electrodes showed that the hydrophocity provided by the styrene units can improve activity at low overpotentials.
In this thesis, the kinetics of oxygen reduction is studied on well-defined iron macrocycles, used as model FeN4 catalysts. Two groups of macrocycles were identified, differentiated by the presence of a high potential Faradaic peak in the cyclic voltammogram, which was found to control both activity and peroxide selectivity. Operando UV-Vis spectroscopy, operando X-ray adsorption, DFT simulations and electrochemical techniques were combined to show that this peak is also an experimental probe of *OH binding energy.
Iron phthalocyanine is then modified with the addition of a thin layer of ionic liquids, to understand how ionic liquids can affect the activity of FeN4 catalysts. Oxygen concentration and thermodynamic activity is experimentally determined and the results are used to develop a first-principle model for oxygen transport. The effect of the ionic liquid on the kinetics of oxygen reduction is also studied. In conclusion, it was found that optimal ionic liquid layers should maximize oxygen solubility, while minimizing their thickness.
A polymerized form of one of the ionic liquids, poly-co-(styrene)-(vynilimidazole TFSI), was also tested as ionomer. It was found that this poly(ionic liquid) can combine the binding properties of polymers with the oxygen permeability of ionic liquids. Dynamic light scattering and rheology measurements showed that the PIL can improve iron phthalocyanine dispersion and prevent its aggregation in the catalyst layer. Finally, studies in gas diffusion electrodes showed that the hydrophocity provided by the styrene units can improve activity at low overpotentials.
Version
Open Access
Date Issued
2023-10-05
Date Awarded
01/02/2024
License URL
Advisor
Titirici, Maria Magdalena
Stephens, Ifan E. L.
Sponsor
Engineering and Physical Sciences Research Council (Great Britain)
Grant Number
DPT scholarship (EP/R513052/1)
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)