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 (Pt)‐based catalysts show good performance toward hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, a non‐Pt, high‐performance electrocatalyst for HER achieved by engineering high fractions of stacking fault (SF) defects for MoNi 4 /MoO 2 nanosheets (d‐MoNi) through a combined chemical and thermal reduction strategy is shown. 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 four 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‐Pt catalysts for HER. The experimental and theoretical results reveal that the abundant SFs 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.