Stacking Fault-Enriched MoNi 4 /MoO 2 Enables High-Performance Hydrogen Evolution.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum-based catalysts show good performance towards hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, we show a non-platinum, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault defects for MoNi 4 /MoO 2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121 mV for HER at current densities of 500 and 1000 mA cm -2 in 1 M KOH, respectively. The defect-rich d-MoNi exhibits 4 times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (>100 h), making it one of the best-performing non-platinum catalysts for HER. The experimental and theoretical results reveal that the abundant stacking faults in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications. This article is protected by copyright. All rights reserved.