Phase diversity driven mechanistic landscapes for nitrate to ammonia electrosynthesis

Electrocatalytic nitrate reduction to ammonia (NO₃RR) is a promising alternative to the Haber-Bosch process, offering reduced energy consumption and lower environmental impact. However, challenges like low selectivity, poor catalyst stability, and side reactions (e.g. nitrogen and nitrous oxide formation) hinder practical application. Traditional strategies, such as compositional tuning and surface modification, have made progress but are often insufficient, as they overlook the structural and electronic factors affecting catalyst performance. Phase engineering, however, allows precise control over crystalline phases, defects, interfaces, and metastable phases to optimize performance at the atomic level. This review emphasizes the structural aspects of phase engineering, analyzing how strategies like phase transition regulation, defect engineering, heterostructure design, and manipulation of metastable phases impact electrocatalytic NO₃RR. Electrocatalysts are classified into six types: metals and alloys, metal oxides, metal hydroxides, metal sulfides and phosphides, metal carbides and nitrides, and other emerging materials. Unlike traditional reviews that focus on macroscopic performance or surface modifications, this work offers a deeper understanding of the phase-engineering-performance relationship. The insights provided push electrocatalytic nitrate reduction forward and offer a framework for advancing phase engineering in electrocatalysis, guiding future development of efficient, scalable, and sustainable ammonia production.