Electrolyte Descriptors for Current Efficiency and Stability for Lithium-Mediated Electrochemical Ammonia Synthesis
Authors: Iriawan Haldrian , Herzog Antonia , Shao-Horn Yang 
Published: December 2024

Ammonia (NH3) is a critical chemical that is used primarily as fertilizers (80%) as well as the source of N atom in all synthetic chemicals. With a demand >170 million metric tons annually, ammonia is among the “Big Four” industries that release half of the industrial emissions in the world [1]. Electrochemical strategies have emerged as an attractive technology to replace the conventional synthesis via Haber-Bosch process. As one of the only reliable room temperature systems to date [2-3], the electrochemical lithium-mediated N2 reduction to ammonia (LiNRR) has emerged as a promising candidate for renewables-based and decentralized ammonia production. In LiNRR, the activation of molecular nitrogen (N2) is preceded by the electrochemical deposition of metallic lithium in an organic solvent, which then reacts with N2 and undergoes protonation to release NH3, while significant fractions of the active material undergo various parasitic reactions [4]. Progress in recent years aimed at improving the faradaic efficiency, energy efficiency and long-term stability have been achieved via changing the salt [5-6] and proton donor compositions [7], which have been correlated with the structure and composition alterations of the Solid Electrolyte Interphase (SEI). However, fundamental understanding of the interplay between N2 reduction, parasitic reactions and speciation at the interface that drive efficiency and system instability is lacking.
In this study, we aim to develop electrolyte design principles that govern the faradaic efficiency and electrode stability of lithium-mediated electrochemical N2 reduction. Taking inspiration from Li-metal battery [8], a variety of lithium salts and electrolyte compositions in tetrahydrofuran and ethanol were employed to evaluate their efficacy for ammonia synthesis. Physical descriptors to probe the solvation structure around lithium ions (Li+) were established and quantified via electrochemical (Li/Li+ redox potential) characterization, Raman spectroscopy and Infrared spectroscopy of the bulk electrolytes and Li interface characterization. Our study revealed a clear correlation between the NH3 faradaic efficiency, the extent of Li+-anion and Li+-solvent pairing and the driving force for electrolyte decomposition. By drawing a comprehensive connection between ammonia efficiency against bulk and interfacial descriptors, we could reveal the fundamental origin of different NH3 synthesis performances in different electrolytes. An order-of-magnitude improvement in faradaic efficiency can be obtained at the optimal electrolyte composition compared to state-of-the-art perchlorate or tetrafluoroborate electrolytes. Moreover, the transability of ammonia performance enhancement on a continuous flow cell setup is evaluated. Such fundamental insights that connect the bulk properties with interfacial chemistry during LiNRR will aid in engineering stable interfaces, resulting in a long-lasting, high-performing system for practical applications.

References

[1] Georgina Gustin. MIT Climate Grant Challenge - Going after the big four. MIT Technology Review (2022).
[2] Andersen, S. Z., Čolić, V., Yang, S., Schwalbe, J. A., Nielander, A. C., Mcenaney, J. M., Enemark-Rasmussen., ... Chorkendorff, I. Nature. 570, 504–508 (2019).
[3] Iriawan, H., Andersen, S. Z., Zhang, X., Comer, B. M., Barrio, J., Chen, P., Medford, A. J., Stephens, I. E. L., Chorkendorff, I., & Shao-Horn, Y. 1(1), 56. (2021).
[4] Cai, X., Fu, C., Iriawan, H., Yang, F., Wu, A., Luo, L., Shen, S., Wei, G., Shao-Horn, Y., & Zhang, J. IScience, 24(10), 103105. (2021).
[5] Li, S., Zhou, Y., Li, K., Saccoccio, M., Sažinas, R., Andersen, S. Z., ..., Nørskov, J. K. & Chorkendorff, I. Joule, 6(9) (2022).
[6] Du, H. L., Chatti, M., Hodgetts, R. Y., Cherepanov, P. v., Nguyen, C. K., Matuszek, K., MacFarlane, D. R., & Simonov, A. N. Nature, 609, 7928 (2022).
[7] Fu, X., Xu, A., Pedersen, J. B., Li, S., Sažinas, R., Zhou, Y., ..., Nørskov, J. K., & Chorkendorff, I. Nature Communications 15:1, 15, 1 (2024).
[8] Hobold, G. M., Lopez, J., Guo, R., Minafra, N., Banerjee, A., Shirley Meng, Y., Shao-Horn, Y., & Gallant, B. M. (2021). Nature Energy, 6, 10 (2021)