Electrochemical Ammonia Oxidation in Water Treatment: A Comprehensive Review on Mechanisms, Catalysts, and Implementation Challenges

The discharge of ammonia-rich wastewater poses significant threats to water quality and ecosystem health, driving the need for efficient and sustainable treatment technologies. The electrochemical ammonia oxidation reaction (eAOR) has emerged as a promising alternative to conventional biological and physicochemical methods, offering advantages such as in situ oxidant generation, tunable product selectivity, and applicability under challenging water matrices. This comprehensive review systematically examines the mechanisms, catalyst design, and environmental factors influencing eAOR performance. Two primary pathways are detailed: direct eAOR, involving stepwise dehydrogenation of NH3 on the electrode surface, and indirect eAOR, mediated by electrogenerated reactive chlorine species (RCS). The mechanisms—including the Oswin-Salomon and Gerischer-Mauerer pathways for direct oxidation, as well as breakpoint chlorination and radical-mediated routes for indirect oxidation—are critically discussed alongside experimental and theoretical evidence. Recent advances in electrocatalyst development are highlighted, covering noble metals, non-noble transition metal oxides, alloys, and hybrid materials, with an emphasis on enhancing activity, selectivity toward N2, and durability. Key operational parameters such as pH, chloride concentration, and coexisting ions are analyzed for their impact on reaction kinetics and byproduct formation. Finally, the review identifies current challenges—including catalyst poisoning, toxic byproduct generation, and scalability—and outlines future research directions aimed at advancing eAOR toward energy-efficient, resource-recovering water treatment systems.