Latest Progress in Pd-Based Metallenes: From Formation Mechanism, Structural Design, Optimization Strategies to Electrocatalytic Applications.

Recently, two-dimensional (2D) metallene nanomaterials have attracted widespread attention in the field of electrocatalysis, owing to their unique physical and chemical properties. Especially, palladium (Pd)-based metallenes, with their excellent conductivity, high specific surface area, unsaturated atom coordination environment, and tunable electronic structure, have emerged as promising electrocatalysts for green energy technologies such as fuel cells and hydrogen production via water electrolysis. In the review, it systematically elaborates on the recent research progress of Pd-based metallene electrocatalysts. At first, the formation mechanism of their 2D ultrathin sheet structure has been thoroughly analyzed from the perspectives of thermodynamics and kinetics. Second, the key synthesis methods, for example, wet-chemical, templating, and electrochemical synthesis, as well as the modification strategies developed for further optimizing performance, are summarized. Subsequently, based on their element composition and structural characteristics, they are classified into several categories: single-atom metallenes, intermetallic metallenes, high-entropy metallenes, and multimetallic metallenes; meanwhile, their structural advantages are discussed in detail. Finally, the review focuses on the application performance and catalytic mechanisms of Pd-based metallenes in fuel cell reactions such as the oxygen reduction reaction, formic acid oxidation reaction, ethanol oxidation reaction, hydrogen evolution reaction, and oxygen evolution reaction. This review offers a new perspective on the challenges and future development directions of 2D Pd-based metallenes for clean energy electrocatalysis.