Modulating Intrinsic Sulfate Ions in FeOOH Nanorods for Enhanced Energy Storage and Catalytic Oxygen Evolution

Designing efficient low-cost earth-abundant metal electrodes for enhanced energy storage and sluggish oxygen evolution reactions (OERs) poses significant challenges in electrochemistry. Herein an innovative approach to boost the activity of FeOOH nanorods for energy storage and catalytic OER by initiating intrinsic sulfate ion (SO₄ ^2-) modulation is proposed. Through a one-step hydrothermal synthesis using a polymeric ferric sulfate precursor, it is successfully cultivated sulfated iron oxyhydroxide (S-FeOOH) nanorods. Remarkably, the presence of sulfate ions effectively prevented the transformation of FeOOH into less active Fe₂O₃, even under elevated temperature. Annealing induced the leaching of sulfate ions, leading to structural rearrangements with shorter Fe-O bond lengths and the formation of sulfate-textured FeOOH (ST-FeOOH) with additional active sites, consequently increasing the material's surface area. Importantly, compared with reported non-noble metal catalysts, the ST-FeOOH nanorods exhibited significantly enhanced energy storage capabilities (3684 mF cm^-2) and catalytic performance in the OER. With a low overpotential of 173 mV to achieve a current density of 10 mA cm^-2, fast OER kinetics (39 mV dec^-1), and exceptional stability exceeding 80 h, these nanorods demonstrate their potential as efficient OER catalysts. This work demonstrates sulfate ion modulation's role in tailoring FeOOH nanorods for advanced cost-effective electrodes and OER electrocatalysts.