The new strategy of “pseudo-nanophase separation” constructs a high-speed ion transport channel

On October 28, 2022, Professor Peng Sangshan/He Qing of Hunan University, together with the team of Professor Yu Guihua of UT Austin, published a new work entitled “Supramolecular interactions enable pseudo-nanophase separation for constructing an ion-transport highway” in the journal Chem.

This work reports a strategy of “pseudo-nanophase separation” to construct a high-speed ion transport channel: supramolecular action is used to replace the traditional covalent bonds to “graft” the hydrophilic side chains on the polymer backbone, induce the formation of “pseudo-nanophase separation structures” in the membrane, achieve high-speed and highly selective ion transport, and avoid the problem of deterioration of membrane chemical stability caused by covalent bond modification. This strategy simplifies the filmmaking process, which is expected to reduce costs and have the potential for large-scale production.

The corresponding authors of the paper are Peng Sangshan, He Qing, Yu Guihua; The first author is Xiong Jiao.

In order to make full use of renewable energy and achieve carbon neutrality, the demand for high-efficiency ionic conductive films in energy devices has grown rapidly in recent years. Given the intermittent nature of renewable energy, energy conversion and storage technologies are needed to integrate low-carbon energy sources into the grid. To date, a variety of energy conversion and storage technologies have been implemented worldwide, such as hydrogen production from electrolyzed water, fuel cells, electrochemical hydrogen pump reactors, and redox flow batteries (RFBs). In all these electrochemical techniques, ionic conduction membranes play a crucial role, such as transporting carriers and blocking positive and negative active species. Among them, RFBs have strict requirements for ion conduction membranes, namely high ion conductivity, high selectivity, good chemical and mechanical stability and low cost. Hydrophilic/hydrophobic nanophase separation structures are widely used to construct fast ion transport channels in polymer membranes. However, traditional nanophase separation structures are usually induced by side chains of covalently bonded grafts, which complicates the membrane making process and often leads to a decrease in the chemical stability of polymer membranes through covalent bond modification.

In this work, the researchers proposed a “pseudo-nanophase separation structure” based on supramolecular interaction to design an ion conduction membrane with high ion conductivity, high selectivity, high chemical stability and low cost. Different from traditional covalent bond modification, this strategy connects the polymer backbone and hydrophilic “side chains” through multiple supramolecular interactions, which has the following advantages: (1) it induces the formation of “pseudo-nanophase separation structures” to construct fast and highly selective ion transport channels; (2) The problem of deterioration of membrane chemical stability caused by covalent bond modification is avoided; (3) Simplify the membrane preparation process and reduce costs; (4) The channel size can be adjusted at the nanoscale by changing the “side chain” content. In this work, polybenzimidazole (PBI) was selected as the membrane material and tris(2-aminoethyl) amine (TAA) was selected as the “side chain”, and the supramolecular interaction mechanism between PBI and TAA was elucidated by combining experiments and theoretical calculations. Fast and highly selective ion transport channels were successfully constructed in the membrane, making VRFB highly efficient and good cycle stability.

Figure 1: Schematic of “pseudo-nanophase separation structure”.

Figure 2: Study of supramolecular interaction mechanism between PBI and TAA.

Figure 3: Microstructure characterization of membranes.

Experimental research and theoretical calculations fully reveal the multiple hydrogen bond interactions between PBI and TAA, and the hydrogen bond with PBI as the proton donor is stronger than the hydrogen bonding effect with PBI as the proton acceptor.

TAA “grafted” through supramolecular interaction successfully induced the formation of hydrophilic/hydrophobic nanophase separation structures in PBI membranes, and the size and connectivity of hydrophilic channels in the membranes could be adjusted at the nanoscale by adjusting the content of TAA. The research team called this phenomenon “pseudo-nanophase separation” and established three criteria: (1) obvious hydrophilic/hydrophobic nanophase separation structures must be formed in the membrane; (2) The polymer backbone and hydrophilic side chains are connected by non-covalent bonds; (3) In practical applications, the “side chain” does not necessarily remain in the membrane.

Figure 4: Electrolyte doping and ion-selective transport.

Figure 5: VRFB battery performance over a wide current density range.

“Pseudo-nanophase separation” successfully constructs a high-speed ion transfer channel in the membrane, which reduces the surface resistance of the membrane by an order of magnitude compared with the original PBI membrane. And due to the presence of ion channels, PBI-3 has a 5-fold increase in proton conductivity compared to the original PBI membrane at the same acid doping level. By measuring the proton conductivity of the membrane at different temperatures, the activation energy of the membrane was obtained in the range of 0.1-0.4 eV, indicating that the proton transport mechanism was dominated by Grotthuss. Although the ion channel size (6-8 nm) is comparable to that of Nafion, the protonated benzimidazole group blocks vanadium ions through the Donnan effect, so this series of membranes also achieves high vanadium ion blocking ability. PBI-3 was applied to VRFB, and the energy efficiency (EE) reached 80.1% at 200 mA cm-2, which was the highest value reported by dense PBI film. And it maintains excellent cycle stability in long cycles, and the capacity decay rate is as low as 0.05%/turn.

This study proposes a new concept of “pseudo-nanophase separation” based on supramolecular interaction, which avoids the problem of deterioration of membrane chemical stability caused by extensive covalent bond modification while constructing fast and highly selective ion transport channels in polymer membranes. (Source: Web of Science)

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