Combining catalysts design and reactor engineering to enhance the electrochemical synthesis of ammonia

Objective

In 2020, the total global production of ammonia is up to 144 million metric tons via the Haber-Bosch process that is energy-intensive in that it requires a substantial driving force (e.g. 500 oC and 200 atm) to break the highly inert N-N triple bond, which consumes 1-2% of the world’s annual energy output and generates over 1% of global carbon dioxide emissions. The electrochemical nitrogen (N2) reduction reaction (NRR) has attracted much attention to circumvent the carbon-intensive Haber-Bosch process because of the decreasing renewable electricity prices. In contrast to the Haber-Bosch process that produces ammonia in large and centralized factories, the electrocatalytic route can achieve on-site ammonia synthesis in small-scale devices with the expectation to reduce the price of fertilizer and realize a neutral carbon footprint. However, electrochemical ammonia synthesis suffers from extremely low partial current density and Faradaic efficiency towards ammonia in aqueous conditions due to the competition of the hydrogen evolution reaction (HER). There are two challenges to NRR in aqueous conditions: (1) the HER competitive reaction is much faster than NRR in kinetics and the activation of N2 is therefore difficult, (2) low solubility of N2 in aqueous electrolytes. To overcome the challenges mentioned above, in this project, I aim to combine electrocatalysts design (i.e. suppress the HER and activate N2) and electrochemical reactor engineering (i.e. overcome the mass transport limits of N2) to improve the ammonia yield rates and current efficiency. Theory-guided preparation of singe-atom catalysts and diluted surface alloy will be performed in flow-cell/MEA electrolyzers (i.e. designed three different setups) to obtain a clear structure-activity relationship with the assistance of in-situ spectroscopy and theoretical calculations. The obtained structure-activity relationship could guide the rational development of high-performance catalysts for efficient NRR.