Recent advances in platinum-based intermetallic nanocrystals: Controlled synthesis and electrocatalytic applications

Fuel cells, whose energy source can be hydrogen, formic acid, methanol, or ethanol, have received considerable attention in recent years because of their environmentally friendly characteristics. A high Pt loading is often required to achieve a practical power density in fuel cells, thus leading to high costs and limited applications. Meanwhile, the high Pt loading promotes aggregation during cycling under harsh electrocatalytic conditions, which reduces the surface area of the catalyst and leads to a decrease in catalytic activity. The formation of alloy or intermetallic nanocrystals via the addition of non-precious metals along with precious metals is one strategy to effectively reduce the cost. Due to the electronic and geometric effects introduced by the non-precious metals, the catalytic performance of these bimetallic nanocrystals can be retained or even improved. Compared to the metallic alloy nanocrystals, the intermetallic ones are more stable in critical catalytic conditions. Due to their highly ordered structures, Pt-based intermetallic nanocrystals are widely used as electrode materials for various electrocatalytic reactions in fuel cells, and they show high stability against oxidation and etching. PtCo intermetallic nanocrystals have attained performances that exceed the 2020 target of the U.S. Department of Energy (DOE) for Pt activity and stability for the cathode reaction of fuel cells (oxygen reduction reaction). Decreasing the size of intermetallic compounds to the nanometer scale can significantly increase their active site densities due to the large specific surface area. However, the preparation of intermetallic nanocrystals is more complicated than that of alloys. Therefore, to further improve the electrocatalytic properties of intermetallic nanocrystals, an in-depth study of the factors affecting the electrocatalytic properties of nanocrystals is necessary. This review summarizes recent advances in Pt-based intermetallic nanocrystals. First, we highlight the controlled synthesis strategies, including direct liquid-phase synthesis, the thermal annealing approach, and chemical vapor deposition. Of these strategies, direct liquid-phase synthesis is the most common approach to prepare the intermetallic nanocrystals. Second, the diverse potential applications of different electrocatalytic reactions are summarized. The reactions include the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and hydrogen oxidation reaction (HOR), as well as the oxidation reactions of formic acid (FAOR), methanol (MOR), and ethanol (EtOR). Of these reactions, ORR is the most important, and it has been widely studied. Some advanced characterization techniques and machine learning research based on density functional theory (DFT) are also mentioned. Finally, the challenges and the future perspectives of intermetallic nanocrystals are outlined.