How the electric double layer impacts nitrate reduction to ammonia

Abstract

Electrochemical nitrate reduction (ENR) is an appealing method for remediating nitrate contamination in wastewater and producing ammonia using renewable electricity. However, a mechanistic understanding of coupled mass transfer and electrocatalysis at the electrode–electrolyte interface, which dictates ENR efficiency, is limited. In this study, we develop an experimentally-validated multiphysics model of the Stern, diffuse, and diffusion layers near the surface of a polycrystalline titanium catalyst to investigate the effect of the electric double layer on ENR. The developed model couples the generalized-modified-Nernst–Planck equation with Frumkin–Butler–Volmer kinetics and numerical optimization to quantify the effect of applied potential and bulk electrolyte concentration on the ammonia formation rate. Our results reveal how dynamic driving forces at the polarized interface give rise to experimentally observed trends in ENR. Guided by this insight, we show that a more negative potential-of-zero-charge increases the limiting current density for ammonia synthesis by enabling faster migration of nitrate towards the cathode surface. The results motivate the development of multi-scale models that link transport phenomena with molecular-scale modelling to design and tailor interfaces for efficient ENR.