Adaptive Crystallization of NiFe‐MIL‐55 Nanorods: Atomically Precise Shape Control and Dual‐Function Electrocatalytic Performance

ABSTRACT NiFe‐based MIL‐55 metal‐organic frameworks (MOFs) face challenges in achieving atomically precise composition and reliable methods for uniform growth. This study addresses these challenges by applying a slow evaporation crystallization technique to crude colloidal MIL‐55 products from hydrothermal synthesis, resulting in highly uniform, hexagonal pyramid‐shaped NiFe‐MIL‐55 nanorods (NRs). In contrast, isolating MIL‐55 through antisolvent precipitation leads to irregular shapes, highlighting the critical role of the crystallization method. This shape control enables single‐particle analysis, revealing tunable atomic compositions ranging from Ni 1 Fe 9 to Ni 9 Fe 1 by adjusting the Ni 2+ /Fe 3+ precursor ratios. A volcano‐type morphology plot illustrates the role of the Ni‐BDC framework in shape control, where Ni 4 Fe 6 to Ni 8 Fe 2 compositions produce uniform NRs, whereas others yield mixed or irregular morphologies. The electrocatalytic oxygen evolution reaction (OER) and supercapacitor performances of these NRs were evaluated, with Ni 7 Fe 3 ‐MIL‐55 showing the best OER performance, including a low overpotential of 230 mV at 10 mA cm −2 and a Tafel slope of 64 mV dec −1 . It also demonstrates excellent stability, retaining 82% of its OER activity over 50 h, as well as superior supercapacitor performance (571 F g −1 at 1 A g −1 ), making it a promising dual‐function material. Density functional theory (DFT) calculations reveal that Ni 7 Fe 3 has the lowest limiting potential (0.52 V) compared to its single‐metal counterparts, explaining its enhanced OER activity. This work provides a strategy for atomically precise MOF nanostructure design, offering valuable insights into electrocatalytic behavior and guiding the development of materials for renewable‐energy technologies.