A Bidirectional Nanomodification Approach for Synthesizing Hierarchically Architected Mixed Oxide Electrodes for Oxygen Evolution

Abstract Several transition‐metal oxides and hydroxides based on earth‐abundant elements, such as Fe, Ni, and Co, have emerged as a new generation of oxygen evolution reaction (OER) catalysts due to their low cost, favorable activity, and multifunctional behavior. However, the relatively complicated surface structuring methods, high Tafel slope, and low stability hinder their practical applications to replace the conventional Ir‐ and Ru‐based catalysts. Herein, a strategy to construct hierarchically architected mixed oxides on conductive substrates (e.g., ITO and Ni foam) via a nanosheet (NS) deposition and subsequent bidirectional nanomodification approach, with metal salts in an aprotic polar solvent (e.g., acetone) as the primary modifying reactants is reported. This strategy is used to prepare NiO‐based NSs with nanopores, nanobranches, or a combination of both, containing up to four transition metal elements. Record‐low Tafel slope (22.3 mV·dec⁻¹, ≈lowest possible by computational predictions) and week‐long continuous operation durability are achieved by FeMnNi‐O NSs supported on Ni foams. Taken together, properly designed hierarchical mixed oxide electrodes may provide a cost‐effective route to generating high, reliable, and stable OER catalytic activities, paving the way for both new electrocatalyst design and practical water‐splitting devices.