Electrochemical Nitrogen Reduction: The Energetic Distance to Lithium

Abstract

Energy-efficient electrochemical reduction of nitrogen to ammonia could help in mitigating climate change. Today, only Li- and recently Ca-mediated systems can perform the reaction. These materials have a large intrinsic energy loss due to the need to electroplate the metal. In this work, we present a series of calculated energetics, formation energies, and binding energies as fundamental features to calculate the energetic distance between Li and Ca and potential new electrochemical nitrogen reduction systems. The featured energetic distance increases with the standard potential. However, dimensionality reduction using principal component analysis provides an encouraging picture; Li and Ca are not exceptional in this feature space, and other materials should be able to carry out the reaction. However, it becomes more challenging the more positive the plating potential is.

Figure 1

(a) Calculated binding energies of nitrogen (ΔE_*N) versus standard reduction potential. Horizontal lines indicate Li and Ca (working electrodes). A histogram and a probability density distribution are plotted together with the Z-score values. Neither Li nor Ca is exceptional in that regard, as a Z-score >2 gives a data point outside of 95% of the data assuming a normal distribution. (b) Calculated distance to Li, with m being the metals and f the features, such as the formation energies and binding energies, plotted as a function of the standard reduction potential. Ca is the material closest to Li, and with increasing standard potential the further away the materials energetics are. “Cleaved phase” means that the material forms a nitride phase with isolated nitrogen atoms.

Figure 2

Principal component analysis of the nitrogen reduction feature space: formation energies (ΔH), binding energies (ΔE), and the standard reduction potential (V_SHE) for (a) DFT energies and (b) including data with linear regression predicted values noted by star points. “Cleave phase” means that the material forms a nitride phase with isolated nitrogen atoms. Data are shown in Tables S1 and S2.