Book Description

Due to the development of the Haber-Bosch process, humanity accounts for over half of the nitrogen fixation that occurs on Earth, the other half being microbial activity and lightning. This leads to a large footprint in terms of runoff, energy usage, and plant growth. Due to its use of methane as a source of hydrogen, the Haber-Bosch process emits 1% of all anthropogenic carbon dioxide. The high temperature and pressure required by the Haber-Bosch process also necessitates large, centralized plants. In this thesis, I explore a possible alternative, electrochemical ammonia synthesis. This process uses proton-electron pairs to reduce nitrogen at a cathode in an electrochemical cell. The only inputs are air, water, and electricity. If successful, an electrochemical ammonia cell could operate at ambient temperature and pressure due to the high driving force provided by the electric potential. The challenge of electrochemical ammonia synthesis comes from the inert nature of dinitrogen and the ease of the competing hydrogen evolution reaction. In the Haber-Bosch process, high temperatures are used to help break the N-N triple bond. In electrochemical ammonia synthesis, voltage and reactive catalysts are used. Unfortunately, at these conditions protons can be readily reduced to hydrogen rather than added to the nitrogen to form ammonia. Not only is ammonia synthesis difficult from a fundamental standpoint, it is also difficult from the standpoint of experimental validation. Because the amounts of ammonia detected are generally small, contamination is easily mistaken for positive results. For example, a human breath contains enough ammonia to produce what appears to be a promising result. Many common materials, epoxy, nitric acid, rubber gloves, etc., contain nitrogen compounds that can be reduced more easily than dinitrogen. Working with our collaborators in Ib Chorkendorff's group at DTU and other SUNCAT researchers at Stanford University we developed rigorous protocols to overcome these challenges. The only way to definitively prove ammonia synthesis is with quantitative isotopically labeled experiments with purified 15N2 gas. However, the contamination found in many commercially available cylinders of 15N2 means that such an experiment could easily be fooled. Our work on proton limitations combined with rigorous verification led to the first unambiguous demonstration of electrochemical ammonia synthesis using a lithium mediated strategy, described in chapter 3. The lithium-mediated protocol was first explored in 1993 by Tsuneto et. al., but they did not possess the techniques necessary to truly prove ammonia synthesis. Our definitive result is important for two reasons. First, it validated our theoretical framework and should inspire future research into non-aqueous systems for nitrogen reduction and other challenging reactions. Second, it allowed for a positive demonstration of ammonia synthesis. we investigated the nature of the ammonia synthesis as well as the reaction at the counter electrode. The lithium mediated system uses a THF electrolyte with dissolved lithium perchlorate as a lithium source and electrolyte and ethanol as a proton donor. The cell is operated at a voltage where lithium is plated onto the cathode. The nature of the catalyst is not obvious. Using electrochemical characterization and density functional theory, we show that lithium metal, lithium nitride, and lithium hydride are all plausible candidate catalysts. Experiments show that appreciable amount of the current ends up as lithium on the surface. Surprisingly, relatively little of this reacts to form fixed nitrogen on the cathode. Density Functional Theory experiments support this observation by showing that lithium nitride should be unstable relative to lithium metal and ammonia at the operating conditions. These calculations also suggest that lithium hydride and nitride can both act as catalysts for ammonia synthesis, opening up exciting options for materials that are stable at less reducing conditions.