Nanoporous Graphene Based Nanofluidics, From Mechanisms to Applications

Nanofluidics has not only drawn significant research interest due to its abnormal behavior compared to bulk fluids, but also has numerous applications such as separation, sensing, and energy conversion. Graphene provides an ideal two-dimensional platform to investigate the transport of nanofluids owing to its ordered structure, chemical stability, and easy modification.<br/> Nanopores in graphene membranes provide 0D confined spaces to study transport mechanisms. Controlling the pore size in graphene membranes is a prerequisite, as even a small variation in pore size can result in a significant difference in transport properties. By decoupling defect site nucleation and defect growth with two successive plasma treatments, we achieved a narrow pore size distribution from gas-selective sub-nanometer pores to a few nanometers in size for small molecule separation. Moreover, we used a new strategy and achieved high solvent flux via organic solvent forward osmosis. After the precise introduction of the confined space, we then focus on designing functional groups, which provide a new degree of freedom. Through controlled nitrogen plasma, a highly proton-selective membrane was fabricated. By controlling the grain boundary density, proton permeance can be tuned within a range of 2 orders of magnitude with high selectivity and high proton conductivity. Through the synergistic regulation of the pore size and chemical properties of <i>in-situ</i> covalent modification, asymmetric ion transport behaviors and efficient sieving of monovalent metal ions (K⁺/Li⁺ selectivity ~ 48.6) can be achieved. Meanwhile, it also allows preferential transport for cations. The resulting membranes exhibit a K⁺/Cl^- selectivity of 76 and an H⁺/Cl^- selectivity of 59.3. The synergistic effects of steric hindrance and electrostatic interactions impose a higher energy barrier for Cl^- or Li⁺ to cross nanopores, leading to ultra-selective H⁺ or K⁺ transport.<br/> Besides the proof-of-concept experiments, we tried to bridge the gap between theory and reality to utilize graphene membranes in specific applications. For large-scale production, decimeter-scale (∼15 × 10 cm²) large-area nanoporous single-layer graphene membranes and stability-enhanced double-layer graphene membranes were fabricated. An application paradigm of graphene in a membrane-based precision instrument with higher precision and better stability was achieved. One step further, regarding realizing the module of graphene membrane, the poor resistance to deformation under tension and bending of composite membrane is the limit. We designed a large-area nanoporous graphene separation membrane supported by a fiber-reinforced structure, which exhibits excellent tensile and bending capabilities. The fracture stress, fracture strength, and tensile stiffness have been increased by 1-2 orders of magnitude compared to existing research. It can maintain stability under different curvature conditions and remain intact after 10,000 repeated bends, laying a foundation for the long-term development of graphene films in practical separation applications.