Highly Efficient and Durable Anode Catalyst Layer Constructed with Deformable Hollow IrO x Nanospheres in Low‐Iridium PEM Water Electrolyzer

Abstract Reducing iridium packing density (g Ir  cm −3 electrode) represents a critical pathway to lower geometric Ir loading in proton exchange membrane water electrolyzers (PEMWEs), yet conventional approaches often cause performance issues of anode catalyst layer due to decreased structural stability and limited electron/mass transport efficiency. Here, we present deformable hollow IrO x nanospheres ( dh ‐IrO x ) as a structural‐engineered catalyst architecture that achieves an ultralow Ir packing density (20% of conventional IrO 2 nanoparticle‐based electrodes) while maintaining high catalytic activity and durability at reduced Ir loadings. Scalable synthesis of dh ‐IrO x via a hard‐template method—featuring precise SiO 2 nanosphere templating and conformal Ir(OH) 3 coating—enables batch production of tens of grams. Through cavity dimension and shell thickness optimization, dh ‐IrO x demonstrates excellent mechanical resilience to necessary electrode fabrication stresses, including high‐shear agitation, ultrasonic processing and hot‐pressing. In the anode catalyst layer, the quasi‐ordered close packing of dh ‐IrO x nanospheres simultaneously maximizes electrochemically active surface area, suppresses particle migration and agglomeration, and establishes percolated electron highways and rapid mass transport channels. The architected anode delivers high PEMWE performance (e.g., 1 A cm −2 @1.60 V and 2 A cm −2 @1.75 V@80 °C), while demonstrating excellent operational durability with <1.5% voltage loss over 3000 h.