Phase-Regulatable Synthesis of Single-Atom Alloy Nanocages for Efficient Alkaline Hydrogen Evolution

Crystal phase engineering of metal nanocatalysts presents a promising strategy to modulate the catalyst–adsorbate interaction for enhanced catalysis. However, conventional synthetic methods have faced substantial challenges in achieving regulatable crystal phases and lack precise control over catalyst composition at the atomic level, which is detrimental, especially for reactions involving multiple intermediates. Here, we report a facile strategy for simultaneously regulating the crystal phase and composition (Pt single-atom alloying) of ultrathin Ru nanocages (<2 nm in thickness), enabling efficient hydrogen evolution reaction (HER) in alkaline electrolytes. In situ characterizations and theoretical calculations reveal that both the metastable face-centered cubic (fcc) Ru phase and isolated Pt atoms contribute to stabilizing metallic Ru, facilitating Pt–Ru synergy for optimized adsorption of H* and *OH intermediates and accelerated HER kinetics. Consequently, the Pt–Ru_fcc single-atom alloy nanocages exhibit impressive alkaline HER performance, with an overpotential as low as 8.5 mV at 10 mA cm^–2, an 18.0-fold enhancement in mass activity relative to commercial Pt/C and commendable stability over 400 h of operation at ampere-level current densities. This work provides insights into the atomic-level design and preparation of metal nanocrystals with unconventional phases for advanced catalysts.