Is Ethanol Essential for the Lithium‐Mediated Nitrogen Reduction Reaction?

Abstract

The lithium‐mediated nitrogen reduction reaction (Li‐NRR) is a promising method for decentralized ammonia synthesis using renewable energy. An organic electrolyte is utilized to combat the competing hydrogen evolution reaction, and lithium is plated to activate the inert N2 molecule. Ethanol is commonly used as a proton shuttle to provide hydrogen to the activated nitrogen. In this study, we investigate the role of ethanol as a proton shuttle in an electrolyte containing tetrahydrofuran and 0.2 M lithium perchlorate. Particularly designed electrochemical experiments show that ethanol is necessary for a good solid‐electrolyte interphase but not for the synthesis of ammonia. In addition, electrochemical quartz crystal microbalance (EQCM) demonstrates that the SEI formation at the onset of lithium plating is of specific importance. Chemical batch synthesis of ammonia combined with real‐time mass spectrometry confirms that protons can be shuttled from the anode to the cathode by other species even without ethanol. Moreover, it raises questions regarding the electrochemical nature of Li‐NRR. Finally, we discuss electrolyte stability and electrochemical electrode potentials, highlighting the role of ethanol on electrolyte degradation.

Operando investigations of the solid electrolyte interphase in the lithium mediated nitrogen reduction reaction

“…Intermediate species of the multi-step oxidation of EtOH such as acetic acid were detected previously with GC–MS, 28 providing a possible pathway to Li 2 CO 3 formation. Decomposition products of THF previously detected by NMR, 16,19,27 forming only in the presence of EtOH such as 2-ethoxy-THF and γ-butyrolactone provide other possible precursors for carbonate formation. Li 2 CO 3 could be observed during the OCV before any current was applied, suggesting that it can also be formed without an electrochemical step, possibly facilitated by the X-ray beam.…”

“…The observed FEs for ammonia without EtOH, 0.1 ± 0.1% and 0.4 ± 0.4% for LiClO 4 and LiBF 4 respectively, are in agreement with previously reported values for unpressurized batch reactors. 6,7,10,26,27 The stability of the plated Li indicates that also the competing reduction of protons by Li is limited.…”

In situ neutron reflectometry reveals the interfacial microenvironment driving electrochemical ammonia synthesis

“…In the current case, the outer layer is still distinguishable from the background electrolyte; the filling of its pores could enable the transfer of protons from either proton donor or electrolyte decomposition products to the Li-containing layer. 56 When comparing morphology, the experiments with EtOH have similar thicknesses in inner and outer layers. Meanwhile, the experiments without EtOH exhibit a thicker inner layer that dominates over the outer layer, such that the associated SLD profile in Figure 3c shows one, rough layer on top of the Cu working electrode.…”

“…47,51 These results are consistent with the understanding that EtOH modifies SEI composition and morphology but also reacts with deposited Li. 26,31,51,56 Meanwhile, some variations in the SLD in the outer layer could result from the disordered nature of disrupted SEI formation due to the presence of EtOH. 26 As far as the composition of the outer layer, it could comprise THF degradation products resulting from ring-opening reactions as previously observed.…”

“…We also expect that proton-donor derived species would be incorporated along with solvent degradation products into the outer layer after continued cycling, though for the experiments here, the outer layer composition is largely dominated by the solvent. 26,31,56 The outer layer is highly porous and can be filled with electrolyte. It is within this SEI microenvironment that the desired path of NH3 production takes place.…”