Electrochemical Synthesis of Ammonia for Large Scale Energy Storage
Authors: Garzon Fernando H , Kreller Cortney R , Matanovic Ivana , Mukundan Rangachary 
Published: February 2020

Large-scale energy storage technology is critical for achieving a high rate of penetration of renewable energy, given the intermittency of wind and solar.  With the world energy consumption of ~100,000 TWh/yr, energy density is a critical parameter in the economic viability of any energy storage system.  The energy densities of oxidizable liquid fuels are 10 to 100 times higher than batteries. Liquid ammonia has many attributes that make it a viable energy storage compound. The feedstocks, N2 and H2 (derived from water) are plentiful, ammonia is easily liquefied and routinely stored in large volumes in cheap containers, and it has exceptional energy density for grid-scale electrical energy storage.  Ammonia can be oxidized efficiently in fuel cells or advanced Carnot cycle engines yielding water and nitrogen as end products. Because of the high energy density and low reactivity of ammonia, the capital cost for grid storage could be lower than any other storage application. However, the economical conversion of renewable electricity to carbon-free fuel still presents significant technical challenges. Almost all of the world’s ammonia production is via the Haber Bosch process. The current large-scale Haber-Bosch technology needs to run at constant inputs of energy and reactants. The economics dictate very large process reactors and separation systems, consequently the technology does not downscale well to small sized plants, nor does it work with interruptible energy.

The electrochemical synthesis of NH3 from N2, H2O or H2 provides the most direct and theoretically most efficient conversion process. Many scientific challenges exist towards achieving efficient electrochemical ammonia production [1]. The dinitrogen bond is one of the most stable (946kJ/mole) diatomic bonds in nature. Efficient bond breaking via associative or dissociative mechanisms is a major challenge. The electrocatalysts also have to minimize the parasitic energy loss due to dihydrogen evolution. The keys to successful implementation of an ammonia cycle for energy storage are to develop and optimize new materials for electrolytes and electrocatalysts for ammonia electrosynthesis, and to demonstrate improved efficiency of scalable ammonia electrosynthesis concepts.

We are pursuing electrochemical processes for efficient storage of electrons in N-H bonds, and an understanding of the fundamental science of the electrochemical reduction of nitrogen, a multi-electron processes applicable to energy storage, catalysis, and biological systems. We have investigated a variety of non-aqueous electrolytes including: proton conducting ionic liquids, crystalline solid electrolytes, polymer electrolytes and composite electrolytes [2,3]. Electrocatalysts studied include noble metals, and molybdenum nitrides and carbides [4]. Theoretical studies of nitrogen dissociation on these materials were also performed [5]. Electrochemical cells were produced using the aforementioned materials and ammonia production rates were investigated [6]. Modest rates of ammonia production were observed however at low conversion efficiencies.

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