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Investigating the Electrochemical Reduction of Nitrogen to Ammonia

Sheets, Benjamin Lee
http://orcid.org/0000-0002-4431-6221
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1642101372368919

Abstract Details

2022, Doctor of Philosophy (PhD), Ohio University, Chemical Engineering (Engineering and Technology).
Electrochemical nitrogen reduction has been proposed as a potential means of generating ammonia sustainably in comparison with the current industrial process. The possibility of using green hydrogen sources to produce nitrogen would allow for the production of this vital molecule while producing much less greenhouse gas. Significant challenges must be addressed for an electrochemical ammonia synthesis process to become feasible, most notably the dominance of the competing hydrogen evolution reaction which prevents the efficient reduction of nitrogen to ammonia. The effect of catalyst loading, temperature, and applied potential was tested on the nitrogen reduction reaction (NRR) using Pt and Ir catalysts in an electrochemical cell using an alkaline polymer gel electrolyte. It was hypothesized that the polymer gel electrolyte would limit the transport of water in the electrolyte and thus inhibit the competing HER. Pt and Ir nanoparticle electrocatalysts were synthesized and deposited on carbon gas diffusion electrodes. The ammonia was quantified by conversion to indophenol and testing via UV-Vis absorption spectroscopy. The generation of ammonia with both catalysts was demonstrated, and the reaction parameters tested were determined not to have a statistically significant impact on the results. While considering alternative catalysts for the NRR, it became clear that some contamination was contributing to false positive signals, thus a number of different possible sources of false positive signal were tested. Residual ammonia in the humidifier in the NRR reaction setup was quantified, and steps were taken in the experimental process to mitigate this issue. Degradation of the ionomer binding agent used in the electrodes was also tested, and deemed unlikely to be a contributor to the observed contamination. Finally, a series of blank tests were run which revealed intermittent ammonia contamination, which was attributed to ambient atmospheric ammonia contamination. Ultimately, further modifications to the process to try and mitigate the ammonia contamination problem was deemed unfeasible. Further improvement to the NRR synthesis process by the inclusion of a quasi-reference electrode was tested to better control the potential and observe an increase in the current from the NRR distinct from the current due to the HER. The potential shifts of the QRE were deemed to be quite drastic, and its inclusion was not successful in allowing the differentiation of the NRR current from the HER. Because of the issues encountered with quantifying the ammonia synthesis, a qualitative approach to investigating the reaction was pursued. An in situ sum frequency generation spectroscopy (SFG) electrochemical approach was tested to monitor the presence of vibrational modes of reaction intermediates of the NRR at the electrode-electrolyte interface. The NH vibrational mode was observed for adsorbed ammonia gas molecules on a gold surface with a peak positioned at ~ 3279 cm-1, demonstrating the ability of the method. Additionally, when applying a potential to a gold surface in an aqueous electrolyte saturated with N2, an NH vibrational signal was detected at ~ 3220 cm-1, suggesting that an NRR intermediate was detected with this method on the working electrode surface. These observations indicate that further study with in situ SFG can be used to elucidate the reaction mechanism of the NRR on a gold surface.
Gerardine Botte (Advisor)
Valerie Young (Advisor)
Savas Kaya (Committee Member)
Howard Dewald (Committee Member)
John Staser (Committee Member)
Katherine Cimatu (Committee Member)
137 p.
Chemical Engineering
Ammonia Synthesis; Nitrogen Reduction; Chemical Engineering; Electrochemistry; SFG Spectroscopy; Sum Frequency Generation Spectroscopy; Electrochemical Engineering

Recommended Citations

Citations

  • Sheets, B. L. (2022). Investigating the Electrochemical Reduction of Nitrogen to Ammonia [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1642101372368919

    APA Style (7th edition)

  • Sheets, Benjamin. Investigating the Electrochemical Reduction of Nitrogen to Ammonia. 2022. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1642101372368919.

    MLA Style (8th edition)

  • Sheets, Benjamin. "Investigating the Electrochemical Reduction of Nitrogen to Ammonia." Doctoral dissertation, Ohio University, 2022. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1642101372368919

    Chicago Manual of Style (17th edition)

ohiou1642101372368919
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This open access ETD is published by Ohio University and OhioLINK.