Unconventional phase engineering of fuel-cell electrocatalysts.

[Display omitted] • Influences of unconventional phase on catalytic performance are manifested. • Methods for unconventional phase engineering and phase transformation are also summarized. • Representative fuel cell catalysts with unconventional phases are also provided. • Perspective sights and challenges for the unconventional phase engineering are added. The commercialization of fuel cells is greatly hampered by the poor stability, sluggish kinetics, and limited catalytic activity of electrocatalysts of both the cathodic oxygen reduction reaction (ORR) and anodic hydrogen oxidation reaction (HOR) or small molecules oxidation reaction (SMOR). Thus far, the exploration of highly active and durable electrocatalysts has emerged as a promising strategy for improving the performance of fuel cells. Accordingly, increasing attention has been devoted to optimizing the geometric and electronic properties of catalysts and modifying the adsorption/desorption of intermediates via phase engineering, especially for unconventional phase engineering. Herein, in this review, unconventional phase engineering is introduced in terms of the phase definition, classification, characterization, and understanding. Subsequently, we highlight the recent advances in the unconventional phase engineering by focusing on the rational design, preparation and transformation of different phases in nanomaterials. Moreover, the latest progress of electrocatalysts with unconventional phase for driving electrocatalytic ORR, HOR or SMOR are scientifically and systematically summarized, along with a deep understanding on the structure–activity relationship. Furthermore, we also provide personal perspectives on challenges and opportunities in this emerging field, covering the exploitation of phase-dependent properties of electrocatalysts. [ABSTRACT FROM AUTHOR]