The alkaline oxygen evolution reaction is a key step in producing green hydrogen through water electrolysis, but its large-scale industrial application remains limited due to challenges with current electrocatalysts—particularly in terms of scalability, efficiency, and long-term stability. Here we show an industrially scalable synthesis of an active NiFe layered double hydroxide (NiFe-LDH) catalyst using a room-temperature, atmospheric-pressure route. The process involves homogeneous alkalinization, where chloride ions nucleophilically attack an epoxide ring, producing a low-dimensional, defect-rich NiFe-LDH with pronounced iron clustering. In-situ spectroscopy and ab-initio calculations reveal that these structural features maximize the conversion of the NiFe-LDH to the catalytic active phase and minimize the energy barrier, improving catalytic efficiency. When used as the anode in an anion exchange membrane water electrolyzer operating at 70 °C, our material delivers 1 A cm⁻² at 1.69 V in a 5 cm² full-cell setup, with notable durability compared to conventional NiFe-LDHs. This scalable approach could considerably lower the cost of green hydrogen production by enabling more efficient alkaline electrolyzers.