Optimization of Proton Source for Li-Mediated Nitrogen Reduction with Water
Authors: Suzuki Kotaro , Kondo Yasuyuki , Tort Romain , Spry Matthew , Stephens Ifan , Katayama Yu , Yamada Yuki 
Published: December 2024

Introduction
Ammonia has been used for diverse applications, and its worldwide demand is significantly high. Therefore, it is desirable to develop ammonia synthesis methods that do not emit carbon dioxide, unlike the Haber-Bosch process. In this context, lithium-mediated electrochemical nitrogen reduction reaction (Li-NRR) has received much attention as an alternative method. Previous research1) has shown that ethanol works as a good proton source for Li-NRR. However, there are some concerns about the use of ethanol, which is associated with its cost and sustainability. Alternatively, N2 reduction would be paired with water oxidation in full-cell operation. Hence, Li-NRR with water as a proton source is ultimately desired. Recently, our group revealed that the addition of a certain amount of water improves Faradaic efficiency (FE) to ammonia in Li-NRR, mainly due to the modification of the solid electrolyte interphase (SEI)2). On the other hand, previous reports3) show that other proton carriers could replace ethanol, and also proton carriers influence SEI composition and proton activity. Therefore, combining proton sources other than ethanol with water may tune the SEI composition and proton activity, making it suitable for operating in the presence of water. In this study, we evaluated the performance of Li-NRR in the presence of various amounts of water in electrolytes containing a variety of proton carriers. From this study, we aim to elucidate the factors that enable electrochemical ammonia synthesis in the presence of water, which is required to achieve Li-NRR using water as a proton source.
Electrolytes were prepared by mixing (CF₃SO₂)₂NLi (LiTFSI) and tetrahydrofuran to make the 1 mol dm-3 electrolyte solution. Subsequently, various proton sources, such as ethanol, propanol, and phenol, as well as water were added to the electrolytes. The electrochemical measurements were conducted using electrolysis cells composed of a working electrode: Mo plate, counter electrode: Pt wire, and reference electrode: Li1-x
FePO4. During chronoamperometry (CA) at -4.5 V vs. Li1-x
FePO4, the applied charge was set at 10 C. After the electrolysis, the FE of ammonia formation was calculated using the quantification method reported elsewhere4). The FE obtained with water varied with the type of proton carrier used, indicating the importance of optimizing the proton source to enable the operation under the co-existence of water. In the presentation, we will show the results of Li-NRR with other proton carriers, and the correlation between FE and properties of proton source and/or SEI will be discussed.
Acknowledgments
A part of this work was supported by the NEDO Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration (JPNP20005).
(1) A. Tsuneto et al., J. Electroanal. Chem., 367, 183-188, (1994)
(2) M. Spry et al., ACS Energy Lett, 8, 1230-1235, (2023)
(3) H. Du et al., Energy Environ. Sci., 16, 1082, (2023)
(4) R. Tort et al., ACS Energy Lett., 8, 1003-1009, (2023)