Theoretical Approaches Towards Selective Electrochemical Nitrogen Reduction

Book excerpt: Anthropogenic climate change poses an extraordinary risk to all aspects of life on the planet in the coming decades. There is an urgent need to diminish the world's reliance on fossil fuels. However, transitioning industrial chemical processes to renewable energy inputs presents a unique challenge to net-zero emission targets. Many processes struggle with the intermittency of renewable energy sources, such as wind and solar, and even worse, several rely on fossil fuels as chemical inputs. Ammonia synthesis is one such chemical process. Ammonia plays a vital role as fertilizer in the world's global food supply chain. At 182 M tonnes of production volume per year, the Haber- Bosch process for synthesizing ammonia is the largest chemical process operating today. However, inherent drawbacks to the Haber-Bosch process make it unsuitable for a sustainable future. The process utilizes fossil-fuel derived hydrogen as an input, accounting for 1.6% of global carbon dioxide emissions. Additionally, the process's centralization and high capital costs make it challenging to deploy in the developing world, resulting in prohibitively high fertilizer costs in regions of the world with the most food scarcity. Developing alternative, sustainable forms of ammonia production is essential towards attaining global emissions goals and decentralizing agriculture. Electrochemical ammonia synthesis is an attractive alternative to the Haber-Bosch process. By removing the need for fossil-fuel derived hydrogen and replacing the prohibitively high pressures and temperatures with moderate reducing potentials, the electrochemical nitrogen reduction reaction (NRR) addresses many of the Haber-Bosch process's fundamental drawbacks. However, electrochemical ammonia synthesis is hindered by prohibitively low selectivity; hydrogen production is kinetically favored over ammonia production on every potential electrocatalyst currently known. In this thesis, we show how insights from electronic structure calculations and theoretical kinetic modeling can be used to (1) understand the activity and selectivity challenges in electrochemical NH3 synthesis and (2) propose and improve alternative ammonia synthesis systems. In particular, we examine the role that metal nitrides will play in achieving these goals by developing a theoretical understanding of their formation, activity, and stability. We also develop kinetic models for understanding non-aqueous NRR, with a focus on lithium-mediated nitrogen reduction. These theoretical frameworks are further improved through successful experimental collaborations. The resulting systems demonstrate a promising pathway towards selective and sustainable ammonia production.