Proton exchange membrane water electrolysis (PEMWE) is currently limited by the sluggish kinetics and poor durability of the oxygen evolution reaction (OER). In this work, a structurally uniform IrB_160-4 catalyst was synthesized through a simple, scalable one-pot aqueous method. This template-free method enables near-quantitative yields and gram-scale preparation, with products rapidly separated via simple filtration. The catalyst consists of uniform, nanoclusters self-assembled from highly crystalline ~3 nm Ir nanoparticles. The optimized catalyst exhibits superior OER activity over commercial Ir-Black. The assembled proton exchange membrane electrolyzer, utilizing a low anodic iridium loading of 0.5 mg cm⁻², demonstrates excellent performance (2.0 A cm⁻² @ 1.79 V) and high durability (>1500 h). This synthesis strategy provides a feasible method for achieving efficient and stable PEM water electrolysis for hydrogen production.