# On-Water Surface Synthesis of Two-Dimensional Polymer Membranes for Sustainable Energy Devices

## TL;DR

This paper reviews the development of 2D polymer membranes for energy devices, focusing on their synthesis and performance in sustainable technologies.

## Contribution

The paper introduces a surfactant-monolayer-assisted interfacial synthesis method for creating high-quality 2D polymer membranes.

## Key findings

- 2D polymer membranes overcome the permeability-selectivity trade-off in ion transport.
- The SMAIS method enables tunable thickness and large crystalline domains in 2DPMs.
- Applications include osmotic power generators and metal ion batteries with improved efficiency.

## Abstract

Ion-selective membranes are
key components for sustainable energy
devices, including osmotic power generators, electrolyzers, fuel cells,
and batteries. These membranes facilitate the flow of desired ions
(permeability) while efficiently blocking unwanted ions (selectivity),
which forms the basis for energy conversion and storage technologies.
To improve the performance of energy devices, the pursuit of high-quality
membranes has garnered substantial interest, which has led to the
exploration of numerous candidates, such as polymeric membranes (e.g.,
polyamide and polyelectrolyte), laminar membranes (e.g., transition
metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes
(e.g., single-layer MoS2 and porous graphene). Despite
impressive progress, the trade-off effect between ion permeability
and selectivity remains a major scientific and technological challenge
for these membranes, impeding the efficiency and stability of the
resulting energy devices.

Two-dimensional polymers (2DPs), which
represent monolayer to few-layer
covalent organic frameworks (COFs) with periodicity in two directions,
have emerged as a new candidate for ion-selective membranes. The crystalline
2DP membranes (2DPMs) are typically fabricated either by bulk crystal
exfoliation followed by filtration or by direct interfacial synthesis.
Recently, the development of surfactant-monolayer-assisted interfacial
synthesis (SMAIS) method by our group has been pivotal, enabling the
synthesis of various highly crystalline and large-area 2DPMs with
tunable thicknesses (1 to 100 nm) and large crystalline domain sizes
(up to 120 μm2). Compared to other membranes, 2DPMs
exhibit well-defined one-dimensional (1D) channels, customizable surface
charge, ultrahigh porosity, and ultrathin thickness, enabling them
to overcome the permeability-selectivity trade-off challenge. Leveraging
these attributes, 2DPMs have established their critical roles in diverse
energy devices, including osmotic power generators and metal ion batteries,
opening the door for next-generation technology aimed at sustainability
with a low carbon footprint.

In this Account, we review our
achievements in synthesizing 2DPMs
through the SMAIS method and highlight their selective-ion-transport
properties and applications in sustainable energy devices. We initially
provide an overview of the SMAIS method for producing highly crystalline
2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity
on the water surface. Subsequently, we discuss the critical structural
parameters of 2DPMs, including pore sizes, charged sites, crystallinity,
and thickness, to elucidate their roles in selective ion transport.
Furthermore, we present the burgeoning landscape of energy device
applications for 2DPMs, including their use in osmotic power generators
and as electrode coating in metal ion batteries. Finally, we conclude
persistent challenges and future prospects encountered in synthetic
chemistry, material science, and energy device applications within
this rapidly evolving field.