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.

Aqueous Proton Transportation in Graphene-Based Nanochannels.

Graphene oxide (GO) has been unveiled to exhibit high proton conductivity in a humidified or aqueous environment, making it a promising candidate to construct proton conduction nanochannels. In this work, we systematically investigate how the confinement effect and surface chemistry influence the proton transportation behavior in graphene-based nanochannels via extensive ReaxFF MD simulations. Graphene (GE), graphane (GA), and hydroxygraphane (HG) sheets were employed to mimic the graphitic and functionalized region of GO and construct nanochannels with different interlayer distances. We find that confined water molecules are stratified and their orientation is influenced by the surface chemistry, thus impacting the distribution of protons. Surface chemistry makes the compression of the hydrogen-bond network induced by the confinement effect more variable. The hydrogen-bond network between GE slabs is crushed by extreme confinement and ultrafast proton transportation behavior mainly achieved via vehicle mechanism. Meanwhile, the hydrogen-bond network and solvation structure can be kept more complete with the existence of functional groups. The hydrogen bonds formed with surface functional groups impede the transportation of water molecules but allow more Grotthuss hopping of protons to different extents. Our work clarified the proton transportation mechanism in graphene-based nanochannels with different interlayer distances and surface chemistry and can guide the future design of proton conduction devices such as proton exchange membranes.

Anomalous proton conduction behavior across a nanoporous two-dimensional conjugated aromatic polymer membrane.

We investigate aqueous proton penetration behavior across a newly synthesized nanoporous two-dimensional conjugated aromatic polymer (2D-CAP) membrane using extensive ReaxFF reactive molecular dynamics simulations. We found that the proton penetration energy barrier across 2D-CAP is twice as high as that of graphtetrayne, even though 2D-CAP exhibits a larger pore size. Detailed analysis indicates that the anomalous high proton conduction energy barrier of 2D-CAP originates from its unique atomic nanopore structure. The hydrogen atoms at the periphery of the 2D-CAP nanopores can form a stable local hydrogen bond network with water molecules inside or surrounding the nanopores. The mobility of water molecules involved in this local hydrogen bond network will be significantly lowered, and the proton transportation process across the nanopores will thus be impeded. Our results show that the proton penetration behavior across nanoporous 2D materials is influenced not only by the pore size, but also by the decorated atoms or functional groups at the pore edges. Hydrogen atoms at the periphery of nanopores with certain geometry can form a stable local hydrogen bond network with neighboring water molecules, further hampering the proton conductivity.

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.