In the last century, the indiscriminate use of fossil energy to power the industrial revolution and technological progress of humankind, has led to the depletion of limited natural resources and, most importantly, the emission and accumulation of alarming levels of pollutants and greenhouse gases (GHG) in the atmosphere. One of the major consequences of these emissions is the climate crisis that we are currently facing. As our society is in constant need for energy to live and progress, we are urged to find more sustainable and renewable energy sources, and to decrease the environmental impact of industrial processes. Electrochemistry can be used to temporary store intermittent renewable electricity, to then be reconverted back to electrons, or it can be used to produce chemicals. As such, the electrification of the chemical industry offers the opportunity to reduce its GHG footprint. The variable supply of renewable electricity can be used by the chemical industry to generate artificial fuel and feedstock. In this way, the synergy between the chemical industry and the energy sector can boost market access, scale and competitiveness.
In particular, this thesis focuses on one of the largest processes in chemical industry, i.e. the ammonia production. An introduction on the topic is given in Chapter 1. Ammonia is produced at large scale (178 million tons per year) and it is a commodity essential for the fertiliser and food sector. The current production of ammonia, via the Haber-Bosch process, relies on fossil fuels and hydrogen derived from steam-methane reforming. Consequently the sector is responsible for releasing 1.4 % of the global CO2 emissions. The implementation of a fully renewable powered Haber-Bosch process is limited by its large reactor scale and its continuous and steady operation, which clashes with the intrinsic intermittency of sources such as solar and wind. This is one of the reasons why a direct electrochemical route for ammonia synthesis has recently attracted significant attention in the scientific and industrial communities. The concept entails the direct synthesis of ammonia from water, dinitrogen and renewable electricity. Moreover, the possibility of producing ammonia in a sustainable manner may enable a new scenario where ammonia can also be used as carbon free energy carrier, thus playing a key role in a decarbonised energy landscape powered by renewables. However, the lack of a selective catalyst and the arduous competition with side reactions, as the hydrogen evolution reaction, make this process extremely challenging.
The aim of this thesis is to expand the current understanding of the nitrogen reduction reaction at near ambient conditions, addressing both fundamental and practical challenges. The first part of this thesis (Chapter 2-4) provides insights on the implementation of reliable electrochemical nitrogen reduction experiments and sensitive operando ammonia detection. Chapter 2 provides a fast and reliable ammonia detection method to speed-up catalyst screening and development of novel sustainable ammonia evolution devices, as it requires significantly less sample handling and preparation compared to other reported methods. The proposed method is based on a gas chromatography technique, and it allows for in situ monitoring of ammonia evolution, down to 150 ppb, from -but not limited to- electrochemical devices. Chapter 3 presents an isotope sensitive gas chromatography-mass spectrometry method for the quantification of NH3 at low concentration level, typically encountered in electrochemical ammonia synthesis applications. This method allows the discrimination of 15/14NH3, necessary for the required 15N2 isotope labelling control experiments. Additionally, this method can directly and simultaneously measure other species in the analyte, thus it allows researchers to directly assess reaction selectivity by measuring reaction by-products, as well as the presence of gaseous/volatile contaminants in the experimental setup. Chapter 4 investigates the impact of contaminations on electrochemical nitrogen reduction experiments, with the aid of multiple analytical techniques and instrumentation, such as ion chromatography, gas chromatography, mass spectrometry, NOx chemiluminescence analyser and UV-Vis spectrophotometry. This chapter not only provides a comprehensive identification and quantification of the contaminations, but it also critically analyses the effectiveness of different cleaning strategies, establishing a series of guidelines to perform reliable experiments.
The second part of this thesis (Chapter 5-7) investigates the room temperature spontaneous dinitrogen activation on selected metallic surfaces and its hydrogenation to ammonia via electrochemical atomic hydrogen permeation, using a solid metallic hydrogen permeable membrane electrode. Chapter 5 demonstrates a novel strategy for ambient condition ammonia synthesis from water and dinitrogen, designed to limit the competition between nitrogen activation and other competing adsorbates at the catalytic surface. As such, a hydrogen permeable nickel membrane electrode is used to spatially separate the electrolyte and the hydrogen activation side from the nitrogen activation and hydrogenation sites. With this approach, ammonia is produced catalytically directly in the gas phase and in the absence of electrolyte. Gaseous nitrogen activation at the nickel electrode is confirmed with 15N isotope labelling control experiments and it is attributed to a Mars-van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. Chapter 6 reports on the interactions of adsorbing N and permeating H at the catalytic interface of nickel, iron and ruthenium based hydrogen permeable electrodes during electrolytic ammonia synthesis. In situ near ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure modifications in the surface electronic structure of the catalyst and the nature of the adsorbed molecules. This chapter shows that permeating atomic hydrogen reduces surface Ni oxide and hydroxide species, under conditions at which gaseous H2 does not. Moreover, the results demonstrate that the availability of surface Ni0 sites is a primary requirement for the chemisorption of gaseous N2. In situ XPS measurements reveal that nitrogen gas chemisorbs on the generated metallic sites, followed by hydrogenation via permeating H, as adsorbed N and NH3 are found on the Ni surface. Our findings indicate that the first hydrogenation step to NH and the last NH3 desorption step might be limiting at the utilised operating conditions. Finally, the study was then extended to Fe and Ru surfaces. However, the formation of surface iron oxide and nitride species on iron blocks the H permeation and prevents the reaction to advance; while on ruthenium the stronger Ru-N bond might favour the recombination of permeating hydrogen to H2 over the hydrogenation of adsorbed nitrogen. Chapter 7 provides a systematic investigation of the effect of operating temperature (in the range 25 to 120 °C) and H permeation flux on the N2 reduction reaction on Ni, leading to a considerably improved NH3 synthesis process. At 120 °C a stable operation was achieved for over 12 h with a 10 times higher cumulative NH3 production and almost 40-fold increase in faradaic efficiency compared to the room temperature operation reported in chapter 5. The results obtained in this chapter indicate that increasing operating temperatures enhances nitrogen adsorption and NH3 desorption, maintaining a steady N surface coverage throughout the NH3 synthesis cycle. Moreover, to operate the nitrogen reduction reaction in a stable and efficient manner, the control over the population of N, NHx and H species at the catalyst surface is critical, as well as the capability of oxides to be reduced by permeating H. As such, the adoption of H permeable electrodes allows to independently control the N activation and H permeation, by a large extent.