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Abstract Single atom catalysts (SACs) achieve 100% utilization of metal atoms and have versatile support effects, whereas single atom alloys (SAAs) with metallic bonds own the free‐atom‐like electronic structure. Herein, surface single atom alloys (SSAAs) are developed that integrate the advantages of SACs and SAAs via incorporating an ultrathin metallic layer during the synthetic process of SACs. It is shown that the Pt single atom preferentially coordinates with metallic Mo nanolayer, thereby forming a Pt 1 ‐Mo L surface atom alloy on Mo 2 C (marked as Pt 1 ‐Mo L ‐Mo 2 C SSAAs). Comprehensive spectroscopic and theoretical calculations reveal that the Mo nanolayer in SSAAs not only functions as an electron buffer between Pt 1 and Mo 2 C, leading to a free‐atom‐like d state at Pt 1 sites and thereby balancing the adsorption and desorption of H, but also enhances the aggregation, adsorption, and activation of H 2 O. Consequently, the Pt 1 ‐Mo L ‐Mo 2 C SSAAs exhibit superior alkaline hydrogen evolution reaction (HER) performance compared to Pt 1 /Mo 2 C SACs, achieving a low overpotential of 12 mV at 10 mA cm −2 and a low Tafel slope of 17 mV dec −1 . This work provides novel insights into the design of advanced single‐site catalysts.
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