Researchers from Peking University, Beijing Normal University and KU Leuven recently reported a novel method to substantially reinforce large-area graphene membranes. Their work provides a facile method to fabricate large-area graphene membranes and paves the road to practical application in the membrane separation field.
Nanoporous graphene membranes are attractive for molecular separations, but it remains challenging to maintain sufficient mechanical strength during scalable fabrication and module development. In this work, the team drew inspiration from the composite structure of cell membranes and cell walls, and designed a large-area atomically thin nanoporous graphene membrane supported by a fiber-reinforced structure with strong interlamellar adhesion. It was found that factors like fracture stress, fracture strength, and tensile stiffness of the composite membranes can be enhanced compared with other graphene-based membranes of large scale.
The researchers drew inspiration from plant cell biology. Plant cells are wrapped in a sturdy composite structure with the cell membrane surrounded by the fibrous cell wall. This provides mechanical strength to withstand osmotic pressure gradients for water transport. Adapting such bioinspired principles, the team sandwiched graphene between a nanoscale polymer adhesion layer and a porous nonwoven support matrix.
This composite reinforcement enhanced the graphene membrane's fracture stress and strength by factors of 17 and 67 respectively compared to previous graphene membranes. The membrane's stiffness rose 94-fold. Tests demonstrated stability across repeated bending and handling.
Unlike past attempts, no tears formed even at extreme curvature. The membrane withstood over 10,000 bending cycles with high graphene coverage retained, far exceeding typical polymer films. The surprisingly robust performance results from synergies between the polymer intermediary layer and fibrous network support matrix surrounding the mechanically fragile graphene.
To enable selective molecular transport, the team introduced nanopores into the graphene via argon plasma etching. Tests revealed the nanoporous graphene membrane completely blocked liquid water permeation up to 5 bar pressure. This extraordinary impermeability results from water's surface tension within graphene's angstrom-scale pores. Yet the membrane demonstrated a remarkably high gas permeation over 6 orders of magnitude greater than commercial polymer films.
Specifically, the graphene membrane exhibited an ultra-high gas permeance of 8.6-23 L m-2 d-1 Pa-1 along with an exceptionally low water vapor transportation rate of 23-129 g m-2 d-1. This adjustable "breathing" performance mirrors stomata in plant leaves. Varying the plasma process tunes the nanopore density to tailor permeability as needed for different separations.
The team's novel reinforced graphene membrane architecture overcame Achilles' heels that have long hindered real-world application of these promising materials. The improved scalable fabrication method and module integration capability mark a major milestone for deploying graphene membranes.
Looking ahead, the approach could be adapted for different 2D materials like molybdenum disulfide to expand options for membrane materials with desirable separation capabilities. The researchers underscored that their robust graphene membranes still require further development and testing before commercial viability. Nonetheless, their work provides a significant foundation and opens exciting prospects for next-generation membranes to make water purification far more energy efficient.