(Invited) Insights on Decentralized Electrochemical Reduction of Nitrate

Abstract

Ammonia is a chemical commodity with demonstrated global importance given its clear food-energy-water nexus. Ammonia large-scale production relies on the Haber-Bosch process which employs hydrogen and nitrogen as reagents under high pressure (10 MPa) and high temperature (500 °C). This process is responsible for more than ~180 million tons of ammonia production per year to satisfy industrial and agricultural needs. Sustaining the ammonia production through the Haber-Bosch process has a huge carbon footprint as sustainability toll. The Haber-Bosch process generates over 450 million tons of CO2 annually, which represents almost 1.5 % of global emissions per year. The fossil fuel reliance of this high energy demand process of ammonia synthesis defines the world economy in many ways. For instance, recent cost increase on ammonia production has had a direct impact on food price inflation worldwide. Ammonia shortage has different causes such as geopolitical conflicts around the world, and increased natural gas price, among others. Thus, a fossil fuel-free green chemical synthesis alternative is required to alleviate the ammonia demand while diminishing the carbon footprint.Agriculture is strongly dependent on ammonia-based fertilizers generating a nitrogen-imbalance in the environment due to N-species (e.g., NO3 -, NO2 -, NH4 +) runoff to ground and surface water. Among these N-species, high concentrations of nitrate and nitrite are highlighted as frequent water quality violations in the US. Concentrations way above the regulated maximum contaminant levels (MCL) of 10 mg L-1 for NO3 --N and 1 mg L-1 for NO2 --N are commonly found in many water resources. Implementation of electrochemical reduction of nitrate (ERN) to produce ammonia can address environmental pollution while producing ammonia under mild conditions with less carbon footprint. The ERN process can become a decentralized ammonia production system alleviating needs at small and even medium scale production. Selectively reducing nitrate towards ammonia has gained attention due to several advantages respect to direct N2 gas reduction such as higher solubility and lower energy barrier. This electrochemically driven process can be coupled with renewable energy systems and therefore holds the promise to enable fossil-free ammonia production off-grid.The mechanisms for electrochemical conversion of nitrate to nitrogen gas are highly complex, involving numerous reactions, products and stable intermediates (e.g., ammonia, nitrite, hydrazine, hydroxylamine, nitric oxide, nitrous oxide) spanning the many nitrogen oxidation states (from -III up to +V). Controlling the reaction selectivity of a complex mechanisms that involve different charge transfer and chemical reactions (e.g., hydrogenation) is challenging. This invited presentation aims to showcase some of the advances in terms of electrode engineering design to steer selectivity towards ammonia production as well as some of the challenges that will be faced when translating lessons learnt towards application under real environmental samples.