(404d) Modeling the Transport of Nitrogen in Gas Diffusion Electrode (GDE) for the Electrochemical Ammonia Synthesis at Ambient Conditions

Ammonia is as an effective hydrogen storage medium as it has higher hydrogen storage capacity (17.65 %) when compared to water (11.11%) and it can be transported easily for mobile applications. Ammonia can be electrochemically oxidized to hydrogen using renewable energy whenever required. So, the excess renewable energy can be used to synthesize ammonia electrochemically, apart from storing in batteries.

Ammonia is synthesized electrochemically from nitrogen and water in an alkaline medium in the presence of an electro catalyst. The challenge in electrochemical synthesis of ammonia is the low Faradaic efficiency of ammonia, which can be overcome by efficient design of electrochemical cell and an appropriate electro catalyst. The nitrogen, water and electrocatalyst must meet at the interface for the reaction to occur. Typically, planar electrode is employed by placing it in the alkaline medium saturated with nitrogen. This configuration suffers from low efficiency, as the solubility of nitrogen in water is extremely small and it is severely limited by the mass transport resistance across the boundary layer. Alternatively, gas diffusion electrode (GDE) can be used to overcome this issue. The catalyst is electrodeposited on a gas diffusion layer. Nitrogen and electrolyte are introduced on the opposite sides of the GDE. Only gas diffuses through the electrode and the electrolyte diffusion is prevented by using a hydrophobic coating on the GDE. Since hydrodynamics plays an important role in such a system, it is important to study its effect on electrochemical ammonia synthesis

In this work, a multi-physics model of nitrogen transport through a GDE is developed and solved in COMSOL. The model encompasses Nernst-Planck equation to capture the species transport in the liquid electrolyte, Darcy’s law for the flow of gas in a porous medium and Tafel equations to predict the charge transfer kinetics at high over potentials. The cell performance is dependent on several parameters such as the flow rates of the gas and the liquid, orientation of the GDE, catalyst loading, thickness of the catalyst layer deposited and porosity of the catalyst. The interplay between the reaction kinetics and the species transport is studied and the operating conditions are determined to optimize the performance of the electrochemical cell. The simulations are used to study the inherent trade-offs between mass transport and reaction kinetics and to design an effective electrocatalyst-electrochemical cell system.