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Electrochemical technologies are projected to play a key role in decoupling human needs from the climate-disrupting consumption of fossil fuels. Their ability to directly convert electrical energy into chemical bonds and vice versa enables the integration of renewable energy generated by wind and solar into chemical production. In the introduction of this thesis, the scale of the challenge at hand, decarbonizing human consumption, is put into the context of planet earth’s natural biogeochemical cycles, and the relevant electrochemical technologies are introduced to the reader. From this, it is evident that the exponentially growing anthropogenic contribution to earth’s biogeochemical cycles is reaching its planetary boundaries. However, this introductory analysis also shows that the select electrochemical technologies needed to emulate the natural biogeochemical cycles in the form of techno-chemical cycles are technically and economically feasible. The scientific work of this thesis then focuses on the development of new electrochemistry mass spectrometry methods to analyze, understand and eventually improve the electrochemical reactions at the heart of these technologies.First, electrochemistry mass spectrometry as a technique is introduced. In electrochemistry mass spectrometry experiments, the measured mass spectrometer signal typically does not directly correspond to the interfacial reaction rate at the electrode interface due to mass transport effects. Consequently, a mathematical method is developed to obtain this interfacial reaction rate from a measured mass spectrometer signal. The method is thereby grounded in a full mass transport model of the analyte between the electrode surface and mass spectrometer. After validation of the mass transport model by means of impulse response measurements, the method is applied to hydrogen and oxygen evolution on platinum electrodes in an acidic electrolyte. Dynamic reaction phenomena that occur within a few seconds can thereby be resolved.The main chapter of the thesis then switches the focus from aqueous electrochemistry to nonaqueous electrochemistry. In particular, the adaption of the instrument to be capable of measuring small amounts of electrosynthesized ammonia from nitrogen reduction in non-aqueous electrolytes is presented. A lowering of the ionization energy of the ion source in the mass spectrometer to 22 eV is shown to enable ammonia detection. Quantification and successful validation of lithium-mediated nitrogen reduction to ammonia are then demonstrated at a faradaic efficiency of up to 46 %. Moreover, the mass spectrometer is able to detect and quantify side products, such as hydrogen and methane. With this, a range of different electrolytes is screened and their stability in regard to side product generation as well as their ability to enable lithium-mediated nitrogen reduction is studied. Only the ether-based electrolytes diglyme, and tetrahydrofuran show ammonia production. Tetrahydrofuran shows less side product generation compared to any other of the screened electrolytes. Thereafter, the dynamic capabilities of the electrochemistry mass spectrometry instrument are leveraged to uncover how ammonia is being generated even after lithium plating has ceased. The evidence obtained from dynamic measurements points towards the presence of small clusters of metallic lithium detached from the electrode so-called dead lithium, that is responsible for ongoing ammonia production. The following sensitivity study on the effect of current density provides further evidence for a strong relationship between lithium morphology and nitrogen reduction as well as side product generation. In the final section of the main chapter, electrolyte oxidation and hydrogen oxidation are studied due to their relevance as anode reactions in lithium-mediated ammonia synthesis. Using cyclic voltammetry, it is shown that anode reactions lead to the generation of readily reducible protons that consequently acidify the electrolyte over the course of an experiment. In the last part of this thesis, the application of the electrochemistry mass spectrometry instrument to study gas generation during the operation of lithium batteries is briefly outlined. As a proof of concept, gas generation in an anode-free lithium metal battery electrode is analyzed. The high time resolution of the instrument thereby allows for resolving gassing behavior previously not detectable. Finally, the findings of the previous chapters are summarized and an outlook for future research on the respective topics is given. |
