Polybenzoxazine composite bipolar plates with hierarchical sandwich architecture of freestanding laser-induced graphitized sheet core and MWCNT-dispersed expanded graphite skin layers for PEMFCs

Polymer composite bipolar plates offer weight and cost advantages over graphite and metallic alternatives for proton exchange membrane fuel cells (PEMFCs), yet simultaneously meeting all U.S. Department of Energy (DOE) targets at moderate filler loadings remains challenging. This work presents a sandwich-architecture composite bipolar plate comprising thermally expanded graphite (EGT), multiwalled carbon nanotubes (MWCNTs), and a laser-induced graphitized sheet (LIGS) core in a polybenzoxazine (PBZ) matrix — the first reported use of LIGS in PEMFC bipolar plates — fabricated via hot compression molding at 10 MPa (60 min in-mold pressure time), substantially milder than conventional systems (15–40 MPa, 30–250 min). Beyond performance, LIGS can be generated in-situ from polymer precursors already present in the fabrication workflow via a laser-based automated process, opening a pathway toward on-demand, supply-chain-independent conductive interlayer production suited to mass manufacturing, with further research into throughput scaling and high-volume process integration representing the natural next step. A 2 × 4 factorial study (± LIGS × 0–4 wt% MWCNT, constant 30 wt% PBZ / 70 wt% filler) identified 2 wt% MWCNT as optimal through convergent property maxima across all DOE-relevant domains, with LIGS contributing an approximately invariant in-plane conductivity enhancement (+ 11–13%) and MWCNT-dependent compensatory through-plane bridging, confirmed by FE-SEM showing defect-free interfaces and a mechanically interlocked hybrid core. The optimal formulation (A6: 68 wt% EGT / 2 wt% MWCNT / LIGS / 30 wt% PBZ) exceeded all measured DOE targets — flexural strength ~ 99 MPa, in-plane conductivity 322.8 S/cm, ASR 7.91 mΩ·cm², H₂ permeability 9.0 × 10⁻¹¹ cm³·cm/(cm²·s·Pa), corrosion current ~ 0.58 µA/cm², thermal conductivity 18.1 W·m⁻¹·K⁻¹, and water contact angle 115.6° — at 5–10 wt% lower filler loading than reported PBZ-based competitors, with single-cell PEMFC testing confirming performance comparable to bulk graphite across kinetic and ohmic operating regions.