Graphitic Carbon Nitride‐Engineered SPEEK Composite Membranes With Regulated Ion Transport and Suppressed Borohydride Crossover for Direct Borohydride Fuel Cells

Direct borohydride fuel cells (DBFCs) offer high energy density and low‐temperature operation, yet their practical application is constrained by borohydride crossover and the trade‐off between ionic conductivity and membrane stability. Herein, sulfonated poly(ether ether ketone) (SPEEK)/graphitic carbon nitride (GCN) composite membranes were fabricated via solution casting to regulate ion transport while restricting borohydride permeation. Structural and morphological analyses confirmed successful GCN incorporation and favorable interfacial interactions within the SPEEK matrix. Among the investigated compositions, SPEEK/GCN‐1.0 achieved the most balanced membrane properties, exhibiting an ionic conductivity of 1.28 mS cm ⁻¹ , borohydride permeability of 1.53 × 10 ⁻⁶ cm ² s ⁻¹ , and a tensile strength of 26.85 MPa. In single‐cell DBFC testing, SPEEK/GCN‐1.0 delivered a maximum power density of 15.93 mW cm ⁻² , representing a 7.28% improvement over pristine SPEEK (14.77 mW cm ⁻² ), while increasing the open‐circuit voltage from 0.911 to 1.037 V. These findings demonstrate that optimized GCN incorporation establishes a favorable structure–transport balance by combining ion‐conducting functionality, borohydride‐barrier characteristics, and mechanical reinforcement, positioning SPEEK/GCN composites as promising cation exchange membranes for efficient DBFC operation.