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Bi, Wei

University of British Columbia

Ammonia (NH₃) is critical to sustaining agriculture for the rapid growth of global population. This thesis focuses on NH₃ electrosynthesis via the electrochemical nitrogen reduction reaction (ENRR) and develops customized flow electrolyzers for aqueous and non-aqueous ENRR conditions. The ENRR performance with commercial or research electrocatalysts was firstly evaluated in aqueous media, but the prevalence of false positives from contaminated cell components masked the genuine ENRR assessment. Contributing to the observed false positives, NH₃ or ammonium (NH₄⁺) crossover through the separator was then investigated. Despite frequent appearances in the literature, Nafion as a cation-exchange membrane is incapable of completely separating NH₃/NH₄⁺ and acts as a possible source or sink of contamination. The behaviors of NH₃/NH₄⁺ that include crossover, volatilization, and adsorption in Nafion were systematically evaluated under a variety of electrolyte compositions, operation conditions, concentration gradients, and feed locations. Additionally, an anion-exchange membrane and microporous polypropylene separators were evaluated as alternatives to the Nafion separator. However, only the hydrophobic Celgard separator demonstrates satisfactory NH₃/NH₄⁺ separation at open-circuit conditions. Lastly, a three-compartment cell was used to evaluate the non-aqueous lithium-mediated ENRR performance of gas-diffusion electrodes with a carbon-based reaction layer: carbon-supported Pt, carbon black, and graphite. The ENRR results achieved in tetrahydrofuran-based electrolyte can generally achieve one-order of magnitude higher NH₄⁺ concentration than in aqueous media, thus avoiding false positives. The current density (−5 mA cm⁻²) and the EtOH concentration (0.5 vol%) were optimized with a current-cycling operation strategy using the C45 carbon black cathode. Replacing the Nafion binder with polyvinylidene difluoride further improves the LNRR performance to 3.11 ± 0.41 μmol h⁻¹ cm⁻² in NH₃ production rate and 5.0 ± 0.65% in Faradaic efficiency that are comparable to or exceeding previous literature reported results. Since Li plating causes exfoliation of the electrode, poor LNRR durability was observed at −5 mA cm⁻². However, reducing the current density to −3 mA cm⁻² achieves a stable operation for over 8 h owing to the balanced Li plating and dissolution rates that mitigates the reaction layer delamination.

Text

2024-04-08

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87708

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/