Understanding Interfaces Using Neutron Reflectometry in Calcium-Mediated Nitrogen Reduction for Electrochemical Ammonia Synthesis
Authors: Niemann Valerie Anne , Fu Xianbiao , Doucet Mathieu , Benedek Peter , Wang Hanyu , Nielander Adam C. , Chorkendorff Ib , Tarpeh William Abraham , Jaramillo Thomas F. 
Published: August 2024

Ammonia (NH3) synthesis from N2 via the Haber-Bosch (HB) process addressed the major challenge of finite fertilizer sources in the early 20th century,and today, it continues to sustain the human population of 8 billion.1 Furthermore, NH3 has been identified as a promising, carbon-free energy vector.2 However, HB operates at elevated temperatures and pressures in centralized facilities, accounting for 1.3% of carbon emissions.1 To decentralize and decarbonize NH3 synthesis, there is a need to develop a synthetic method that operates at ambient conditions.

Methods for N2 fixation to NH3 at ambient conditions include via the nitrogenase enzyme, homogenous catalysts, and an electrochemical lithium-mediated process.1,3 Of these methods, the lithium-mediated process (Li-NRR) has demonstrated impressive gains in selectivity and stability, and it has been engineered into a continuous process.3 In Li-NRR, metallic Li is plated onto an electrode from a Li-containing salt dissolved in N2-saturated, organic solution. Electroplated lithium reacts with the saturated N2 to form surface lithium nitride (Li3N), and a proton donor, such as ethanol, provides protons to form NH3. Of note, a solid-electrolyte interphase (SEI) passivating layer forms at the Li-electrolyte interface from electrolyte decomposition products. The SEI layer moderates proton, Li+, and N2 transport, and therefore, its structure and composition have become a major focus of study.4 While much remains to be understood in the Li-NRR system, a broader question has been posed as to whether a non-Li material can mediate ammonia synthesis with a similar mechanism.5

Recently, our groups have demonstrated that Ca, the fifth most abundant element in the earth’s crust, can mediate nitrogen reduction to ammonia in a process henceforth referred to as calcium-mediated nitrogen reduction to ammonia (Ca-NRR).6 Inspired by recent advancements in Ca-metal batteries for Ca plating at room temperature, we have demonstrated that calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) in tetrahydrofuran with ethanol can be used as the electrolyte to plate Ca and produce NH3. We hypothesized that Ca-NRR likely operates similarly to Li-NRR, where the SEI passivating layer plays a crucial role in the selective transport of species such as N2, protons, and Ca2+,6 however, much remains to be determined about the structure and composition of these interfaces.

A useful method for understanding interfaces is neutron reflectometry. This technique involves the measurement of neutron reflectivity as a function of scattering vector that is fit to a model to determine the compositional and structural properties of a series of interfaces, i.e., scattering length density, thickness, and roughness of each layer.4 Neutron sensitivity to light-elements, e.g., H, and scattering contrast between isotopes, including H and D, enable tracking proton dynamics and incorporation into the layers. Of particular importance is the formation of a hydride which may lead to major side reactions, including the formation of H2. Herein, we measured the Ca-NRR system in-situ at varied applied current densities, and observed changes in layer properties. This technique, paired with ex-situ characterization, elucidates properties of the Ca-NRR plated and SEI passivation layers that can inform the rational, molecular design of nitrogen reduction systems.

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