(391a) Modeling the Reactions and Transport in Gas Diffusion Electrode (GDE) for the Electrochemical Reduction of Nitrogen to Ammonia

Electrochemical reduction of N₂ to NH₃ is a promising alternative to the conventional Haber-Bosch process as the later is highly energy intensive and causes excess CO₂ emissions. The electrochemical synthesis of ammonia suffers from two major setbacks: 1) mass transfer limitations due to low solubility of N₂ in aqueous medium 2) Competing H₂ evolution reaction. The former can be overcome by depositing the catalyst in a porous Gas Diffusion Electrode (GDE). [1, 2] GDE has two parts, the catalyst layer which is hydrophilic to water and the gas diffusion layer which is hydrophobic to water. N₂ enters through the gas diffusion layer and reaches the catalyst surface. It has been experimentally observed that the current density of ammonia is two orders of magnitude higher than the planar electrode when GDE is used.

The improvement in current density is due to the higher surface area, the suppression of mass transfer limitation of N₂ to reach the catalyst surface, and minimum concentration polarization near the cathode surface. In this work we developed a multi-physics continuum model for the entire reactor system, that solves the transport of N₂ and water across the GDE, transport of N₂ in the electrolyte, charge transport and the kinetics of the Nitrogen Reduction Reaction (NRR) and the competing HER (Hydrogen Evolution Reaction). The model has been compared with the experimental results. The effect of various operating conditions such as pH, flow rates of N₂ and electrolytes has been studied. The effect of GDE parameters such as porosity, thickness of the catalyst and permeability has been investigated. Different reactor configurations have been simulated at various operating conditions to improve the existing performance of the catalyst.

References

1. Weng, L.C., A.T. Bell, and A.Z. Weber, Modeling gas-diffusion electrodes for CO2 reduction. Phys Chem Chem Phys, 2018. 20(25): p. 16973-16984.
2. Higgins, D., et al., Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm. ACS Energy Letters, 2018. 4(1): p. 317-324.