Layered Double Hydroxide Nanosheets: Synthesis Strategies and Applications in the Field of Energy Conversion.

In recent years, layered double hydroxides (LDH) nanosheets have garnered substantial attention as intriguing inorganic anionic layered clay materials. These nanosheets have captured the attention of researchers due to their unique physicochemical properties. This review aims to showcase the latest advancements in laboratory research concerning LDH nanosheets, with a specific emphasis on their methods of preparation. This review provides detailed insights into the factors influencing the anionic conductivity of LDH, along with delineating the applications of LDH nanosheets in the realm of energy conversion. Notably, the review highlights the crucial role of LDH nanosheets in the oxygen evolution reaction (OER), a vital process in water splitting and diverse electrochemical applications. The review emphasizes the significant potential of LDH nanosheets in enhancing supercapacitor technology, owing to their high surface area and exceptional charge storage capacity. Additionally, it elucidates the prospective application of LDH nanosheets as anion exchange membranes in anion exchange membrane fuel cells, potentially revolutionizing fuel cell performance through improved efficiency and stability facilitated by enhanced ion transport properties.

Preparation of Montmorillonite Nanosheets with a High Aspect Ratio through Heating/Rehydrating and Gas-Pushing Exfoliation.

Montmorillonite (MMT) is an abundant silicate mineral with ultrahigh stability. The exfoliation of stacked MMT into high-aspect-ratio nanosheets is of crucial importance for various applications such as toxic gas suppression, barrier property enhancement, flame retardancy, and ion conduction. In this work, we develop a new heating/rehydrating and gas-pushing method that can successfully exfoliate MMT into nanosheets with aspect ratios (600-5000) far higher than the currently reported values (1-120). The MMT first goes through a "starvation pretreatment" under different heating temperatures to improve its hydrophilicity and is then rehydrated in a hydrogen peroxide solution. The hydrogen peroxide in the MMT interlayer space can decompose into water and oxygen bubbles, thus finally leading to the exfoliation via gas-pushing while preserving the large lateral size (mainly in the range of 1-6 µm) of the nanosheets. By changing the pretreatment temperature and pH value of the hydrogen peroxide solution, the exfoliation performance can be tuned. This simple and low-cost exfoliation method is promising to achieve the mass production of MMT nanosheets with a high aspect ratio and may promote its application in various fields such as energy conversion, drug delivery, and photocatalysis.

Charge-induced proton penetration across two-dimensional clay materials.

Two-dimensional clay materials possess superior thermal and chemical stability, and the intrinsic tubular channels in their atomic structure provide possible routes for proton penetration. Therefore, they are expected to overcome the lack of materials that can conduct protons between 100-500 °C. In this work, we investigated the detailed proton penetration mechanism across 2D clay nanosheets with different isomorphic substitutions and counterions using extensive ab initio molecular dynamics and metadynamics simulations. We found that the presence of negative surface charges can dramatically reduce the proton penetration energy barrier to about one-third that of the neutral case, making it a feasible choice for the design of next-generation high-temperature proton exchange membranes. By tuning the isomorphic substitutions, the proton conductivity of single-layer clay materials can be altered.

Influence of stacking on the aqueous proton penetration behaviour across two-dimensional graphtetrayne.

Two-dimensional (2D) graphtetrayne (G4) with intrinsic pattern triangular nanopores has been predicted to be an excellent candidate for next-generation proton exchange membranes due to its superior proton conductivity and selectivity. However, it is technically challenging to prepare a large area single-layer intact 2D material. A multi-layer stacked 2D material is a much more suitable choice, and the stacking can effectively shield the undesired defects and tears. In this work, we investigate the aqueous proton penetration behavior across multilayer-stacked two-dimensional G4 using extensive ReaxFF molecular dynamics simulations. We found that the G4 layers prefer a slightly misplaced stacking pattern which would cause only a slight reduction in the pore size. Detailed analyses indicate that the "water wires" across G4 remain continuous and can provide a low-barrier path for proton penetration until the number of stacking layers increases to three. In triple-layer G4, the "water wires" no longer exist and the aqueous phase will be separated by a wide vacuum area, thus significantly impeding the proton penetration behavior. Based on these results, we suggest that when serving as a proton exchange membrane, the number of stacking G4 layers should be fewer than three to achieve satisfactory conductivity. Our work provides guidance for the fabrication of next-generation proton exchange membranes based on nanoporous 2D materials.