The proton exchange membrane (PEM) – the key component of PEM fuel cell – separates the anodic and cathodic compartments and at the same time acts as a proton conductor. Therefore, considerable efforts have been devoted to the development of high-performance PEM, which should exhibits high permselectivity (the ratio of permeabilities of protons) and proton conductivity (the tendency of proton passing through the membrane). The micrometer-thick polymer-based membrane used nowadays, such as Nafion, shows decent proton permselectivity, with however limited proton conductivity. Theoretical and experimental works have demonstrated that atomically thin membranes with nanometer pores exhibit ultra-permeability as well as high selectivity, making them promising candidates for fuel cells as well as water desalination and reverse electrodialysis. In practice, nanopores can be introduced into 2D materials by physical methods such as electron beam, plasma etching and ion bombardment, with the drawback that even if the chemical functionality of the pores is poorly controlled, the scalability of the process remains elusive. Two dimensional metal-organic frameworks and covalent-organic frameworks were also explored recently, with the major drawbacks that the chemical and mechanical stability over a large area in solutions limits the application of such ion selective membrane.
Supported by the ERC proof of concept, we demonstrate for the first time the synthesis of atomically thin carbon membranes with native, tight size distribution, nanometer pores from core-rim structure monomers via a two steps approach. The core-rim monomer is crucial for the membrane preparation: the core is thermally stable which forms the frame of the membrane; the rim is thermally unstable which decomposes and crosslinks the cores. The mechanically robust, centimeter-sized membrane has a pore size of 3.6 ± 1.8 nm and a thickness of 2.0 ± 0.5 nm. Remarkably, ion current rectification was observed which is unprecedented in atomically thin, large area porous membranes. Additionally, the fact that the membrane is ultra-permeable to cations makes it a good candidate for reverse electrodialysis research. By using a diversity of core-rim monomers, our strategy paves a road towards the controllable design and synthesis of atomically thin porous membranes with tight pore size distribution, varied frame structure, and importantly the possibility for the large-scale fabrication of nanopore arrays yielding remarkable ionic selectivity and conductivity which are fundamental elements for PEM fuel cell, desalination and water purification.
