Proton Donors Induce a Differential Transport Effect for Selectivity toward Ammonia in Lithium-Mediated Nitrogen Reduction

Abstract

Alternative approaches for producing ammonia are necessary to reduce the environmental impact of its production. The lithium-mediated electrochemical nitrogen reduction reaction (LM-NRR) is one attractive alternative method for producing ammonia at small scales in a distributed process. This process requires a proton donor in the electrolyte to produce ammonia from nitrogen, but the role of the proton donor in selective ammonia production is not well understood. In this work, we experimentally tested several classes of proton donors for the ability to promote LM-NRR. We found that a wide array of alcohols can promote nitrogen reduction and that n-butanol leads to the highest ammonia Faradaic efficiencies. Among the tested proton donors, even slight changes in the proton donor structure can significantly affect the yield of ammonia. In addition, most active proton donors exhibit a thresholding behavior as a function of their concentration, where the selectivity toward ammonia increases dramatically above a certain concentration of the proton donor. We found evidence to imply that these effects could be due to the proton-donor-induced changes in the properties of the solid electrolyte interphase (SEI), which lead to changes in the diffusion of relevant species through the SEI to the reactive electrode. By selectively allowing for diffusion of nitrogen over the proton donor to the electrode, the SEI can promote selective nitrogen reduction to ammonia. A coupled kinetic transport model of the process was proposed to explain the observed trends and to predict ammonia production as a function of operating conditions.

Lithium-Mediated Electrochemical Nitrogen Reduction: Tracking Electrode–Electrolyte Interfaces via Time-Resolved Neutron Reflectometry

“…Although the model SLDs for the single layer were consistent between electrolyte conditions, the modeled layer thicknesses began to exhibit differences after t = 240 s, with the layer in the 0 M-EtOH electrolyte becoming thicker than that in the 0.17 M-EtOH electrolyte. This indicates that, although the presence of EtOH does not appear to detectably affect the composition of the layer that forms during the initial stages of chronopotentiometry, EtOH may, as expected, cause decomposition of some portion of the layer via the ethanolysis of Li 3 N and prevent it from growing as quickly as the layer without EtOH. , This is consistent with recent work that suggests that the presence of a proton donor increases the permeability of the formed SEI at the electrode surface, improving diffusion of nitrogen and the proton donor to the electrode surface, and prevents side reactions of Li with the electrolyte to increase ammonia production …”

“…13,40 This is consistent with recent work that suggests that the presence of a proton donor increases the permeability of the formed SEI at the electrode surface, improving diffusion of nitrogen and the proton donor to the electrode surface, and prevents side reactions of Li with the electrolyte to increase ammonia production. 55 Thus, using neutron reflectometry, we have demonstrated that, following chronopotentiometry for 30 min, a diffuse SEI layer remains; the neutron reflectometry data, in tandem with previous reports, suggest that this layer may consist predominantly of carbonaceous electrolyte degradation products at the electrode surface. Time-resolved neutron reflectometry measurements support a model of the electrode−electrolyte interface in which, during the initial states of NRR, a single, Li-containing layer forms atop the Mo electrode, with no separate carbonaceous SEI layer observed during the first 6 min of chronopotentiometry.…”

The Role of Ion Solvation in Lithium Mediated Nitrogen Reduction

“…The reduction in RCT with LiClO4 concentration, as shown in Figure 5 and Table S3, may suggest an increase in proton mobility through the SEI with increased salt concentration. However, the increasing thickness, and perhaps density, may also inhibit the transfer of reactants to the catalytically active surface 44 . It may be that the combination of these factors also results in the suppression of Faradaic efficiency at higher LiClO4 concentrations.…”

Enhancing Electrocatalytic Nitrogen Reduction on Few-Layer Antimonene in an Aqueous Potassium Sulfate Electrolyte

“…Transition metal-based catalysts have drawn much attention as they can weaken NN bonds and strengthen metal–nitrogen bonding through σ donation and π backdonation between transition metals and N 2 . , However, the electrocatalytic activity and selectivity are still far from satisfactory because most transition metal-based catalysts favor competitive adsorption of H + over N 2 . , Recent breakthroughs have been made on suppressing the parasitic hydrogen evolution reaction (HER), such as interface engineering, nonaqueous electrolyte regulation, and cation incorporation . Nevertheless, protons participate in steps of proton-coupled electron transfer (PCET) in NRR, and the inhibition of proton transfer will inevitably limit the activation of N 2 molecules . Hence, it is desirable to develop novel electrocatalysts with balanced thermodynamics and kinetics to promote nitrogen fixation.…”

“…12 Nevertheless, protons participate in steps of proton-coupled electron transfer (PCET) in NRR, 13 and the inhibition of proton transfer will inevitably limit the activation of N 2 molecules. 14 Hence, it is desirable to develop novel electrocatalysts with balanced thermodynamics and kinetics to promote nitrogen fixation.…”