Free-Standing Hierarchical Porous Membranes as Robust Cathode Supports for High-Performance Zinc-Air Batteries.

Hierarchical porous structures are essential for regulating mass transport in electrochemical systems, yet precise control over pore architecture across multiple length scales remains challenging. Herein, we report a free-standing hierarchical porous membrane (FHPM) fabricated via linear and bottlebrush block copolymer (BBCP)-directed self-assembly. By integrating linear and bottlebrush copolymers with tunable molecular weights, ordered porous carbon membranes consisting of three layers with independently controllable pore sizes ranging from 10 to 100 nm and adjustable layer thicknesses are achieved while maintaining high surface area and structural integrity. When applied as air cathodes in Zn-air batteries, the FHPM exhibits strong structure-dependent performance. The hierarchical architecture significantly influences discharge behavior and cycling stability. In particular, the outermost layer regulates oxygen transport and electrolyte management. Systematic tuning of outer-layer thickness, which contains the smallest pores, reveals a clear trade-off between power density and durability, where thinner layers enhance oxygen diffusion and discharge performance, while thicker layers improve flooding resistance and extend cycling lifetime. This work provides a versatile strategy for designing hierarchical porous electrodes with tunable transport properties.