Hierarchical Microstructure Engineering in Composite Bipolar Plates: From Catalytic Curing Behavior to Multifunctional Performance

Abstract The high performance and long-term durability of proton exchange membrane fuel cells (PEMFCs) demand bipolar plates with exceptional electrical conductivity, mechanical strength, and corrosion resistance. While composite bipolar plates (CBPs) are promising, their performance is dictated by the microstructure formed during curing. Here, we employ an interrupted curing strategy combined with multiscale characterization and molecular simulation to establish a hierarchical structure–property relationship. We show that 2-methylimidazole catalytically accelerates epoxy cross-linking via a proton-transfer mechanism. This accelerated network formation, in turn, drives a transition at S15, where the composite transforms from a porous, weakly bonded assembly into a dense, continuous architecture. This hierarchical causal chain, from molecular reactivity to network development to microstructure consolidation, directly enables the simultaneous achievement of electrical conductivity (>186 S·cm–1), flexural strength (>48 MPa), and gas barrier properties that meet or exceed U.S. DOE targets. Our findings establish the degree of cure as a hierarchical control variable, demonstrating that rational design of the curing pathway offers a predictable route to high-performance composite bipolar plates.