High-Performance Proton Exchange Membrane for Vanadium Redox Flow Battery Reinforced by Amphoteric Graphitic Carbon Nitride Nanosheets as Proton Conductor

To balance the contradiction between proton conductivity and vanadium ion permeability of sulfonated aromatic polymer proton exchange membranes (PEMs), sulfonated poly(ether ether ketone) (SPEEK)-based hybrid membranes (S/SGN) were prepared, incorporating amphoteric graphitic carbon nitride nanosheets (SGN) with triangular nanopores. The SGN nanosheets were prepared by grafting 1,4-butane sultone onto graphitic carbon nitride (g-C3N4) sheets. Structural characterization revealed that the triangular nanopore architecture, along with amino, imino, and sulfonic acid functional groups, as well as the acid–base interactions formed at the interface with the hybrid membrane, provide the S/SGN hybrid membrane with excellent physicochemical properties and battery performance. Notably, the S/SGN-1 membrane containing 1 wt% SGN loading demonstrated optimal electrochemical performance, achieving a proton conductivity of 27.9 mS cm–1 and an ion selectivity of 21.3 × 103 S min cm–3. In vanadium redox flow battery (VRFB) evaluations, the S/SGN-1 demonstrated significantly higher energy efficiency (86.2–71.5%) at a current density of 60–200 mA cm–2 compared to Nafion 212 (82.2–64.0%) and longer self-discharge time (91.2 h vs 23.2 h of Nafion 212). This indicates that a balanced proton conductivity and vanadium-ion resistance enhances membrane performance. Additionally, the S/SGN-1 hybrid membrane has a charge capacity retention of 71.0% after 50 cycles at a current density of 150 mA cm–2, and the energy efficiency stabilizes at 76.0 ± 1.0% after 500 charge–discharge cycles. These results suggest that the combination of physical barriers and acid–base interactions of SGN nanofillers synergistically improve the battery efficiency of the S/SGN hybrid membrane. The above results validate that developing two-dimensional nanomaterials with self-nanopores and surface functional groups to construct high-performance proton exchange membranes is a simple and feasible method.