Polymer electrolyte membrane fuel cells (PEMFCs) are
one of the
most promising energy conversion devices with the advantages of clean
and efficient, low-temperature operation, abundant fuel sources, and
portability. Due to limited availability and high cost, the use of
noble metal Pt catalysts in oxygen reduction reaction (ORR) must be
reduced in order to achieve large-scale commercial application of
PEMFCs. Alloying with transition metals to reduce the Pt usage and
increase the utilization of Pt is by far the main way to develop ORR
catalysts with high activity and superior stability. Particularly,
diminishing the sizes can increase the Pt atom utilization as well
as the Pt mass activity, but the smaller nanocrystals with higher
surface energy are typically thermodynamically and electrochemically
unstable. How to acquire smaller and even ultrafine Pt alloy nanocrystals
with high stability is thus especially important. Herein, we review
the research progress in this field in the very recent years. A fundamental
understanding of the size–activity and the size–durability
relationship is first introduced. Then the discussions focused on
the synthetic strategies and design concepts for reducing nanocrystal
size and simultaneously enhancing their stability. Finally, the challenges
and future perspectives on the research directions of Pt-based alloy
nanocrystal catalysts are briefly summarized.