Fixing nitrogen: understanding ammonia electrosynthesis through advanced characterisation
File(s)
Author(s)
Westhead, Olivia
Type
Thesis
Abstract
Despite being hailed as one of the most influential inventions of the twentieth century, ammonia production via the Haber-Bosch process is one of the most environmentally damaging industries. A sustainable, distributed mode of ammonia synthesis would democratise fertiliser access, as well as reducing the environmental impact of both use and production of nitrogen-based fertilisers. Electrochemical nitrogen reduction could provide the answer. Non-aqueous lithium-mediated nitrogen reduction has been rigorously verified as capable of true nitrogen reduction to ammonia. Here, a lithium based active surface is deposited in-situ on a metallic current collector in an organic electrolyte. Similar to a lithium-ion battery, a Solid Electrolyte Interphase (SEI) is formed via decomposition of the organic electrolyte. This SEI is critical for stable and selective nitrogen reduction. Although much progress has been made, relative to other electrochemical reactions, there is still comparatively little is known about the fundamental science behind lithium-mediated nitrogen reduction.
This Thesis seeks to improve fundamental understanding of both the anodic and cathodic reactions in lithium mediated nitrogen reduction. A technique for the determination of the reversible hydrogen electrode (RHE) potential in the lithium-mediated nitrogen reduction electrolyte is described, allowing for the determination of the overpotential past the thermodynamic equilibrium and measurement of energy efficiency. Further discussions of hydrogen redox reactions in the lithium-mediated nitrogen reduction electrolyte are presented in the context of a proof-of-concept study aiming to eliminate the poisoning of platinum based electrodes from organics and the salt anion. Bulk electrolyte properties are linked to SEI formation and nitrogen reduction performance, and advanced characterisation techniques used to gain further insight into the nitrogen reduction SEI. The primary takeaway from the SEI characterisation studies is that the lithium-mediated nitrogen reduction system sits within a narrow Goldilocks zone, where the conditions are just right for optimal performance.
This Thesis seeks to improve fundamental understanding of both the anodic and cathodic reactions in lithium mediated nitrogen reduction. A technique for the determination of the reversible hydrogen electrode (RHE) potential in the lithium-mediated nitrogen reduction electrolyte is described, allowing for the determination of the overpotential past the thermodynamic equilibrium and measurement of energy efficiency. Further discussions of hydrogen redox reactions in the lithium-mediated nitrogen reduction electrolyte are presented in the context of a proof-of-concept study aiming to eliminate the poisoning of platinum based electrodes from organics and the salt anion. Bulk electrolyte properties are linked to SEI formation and nitrogen reduction performance, and advanced characterisation techniques used to gain further insight into the nitrogen reduction SEI. The primary takeaway from the SEI characterisation studies is that the lithium-mediated nitrogen reduction system sits within a narrow Goldilocks zone, where the conditions are just right for optimal performance.
Version
Open Access
Date Issued
2023-12
Date Awarded
2024-06
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Stephens, Ifan
Jervis, Rhodri
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/S023259/1
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)