In recent years, porous two-dimensional (2D) materials such as graphene, carbon nitride, and boron nitride have garnered a lot of interest due to their unique structure, capabilities, and intriguing implications.
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Combining the benefits of 2D materials with porous structure, the synthesized porous 2D materials exhibited large surface areas, diverse compositions, and increased electronic conductivity, endowing them with a great potential for use in electrochemical, photocatalytic, and energy applications.
Potential applications of porous two-dimensional (2D) materials, future outlooks, and the main challenges associated with them are discussed in this article.
Porous Two-Dimensional (2D) Materials
Materials containing nanopores on a 2D plane are porous 2D materials. High physical and chemical activation create in-plane pores in 2D materials. These in-plane pores provide the materials with extra absorption and reactive sites and create alternative pathways for mass and charge transfer, which is crucial for energy storage and conversion devices.
Using nanoscale carbons as the main building blocks, several porous 2D materials have been produced that are unrivaled compared to conventional carbon materials. Consequently, it is recognized that nanoscale carbon may serve as one of the fundamental building elements for porous carbon 2D materials.
Examples of porous 2D materials include graphene, boron nitride (BN) and metal oxides. BN is an amorphous, non-metallic material that can be manufactured into a 2D porous structure. Metal oxides, such as NiO, Co3O4, Mn2O3, Fe2O3, and NiFe2O4, are useful materials that can be utilized to produce porous structures.
Properties of Porous Two-Dimensional (2D) Materials
Due to their unique characteristics and high surface area, porous 2D materials have shown significant benefits over pure 2D or porous structures. The high mechanical rigidity of 2D materials could improve the stability of porous structures and prevent their contraction or disintegration.
Thermal and chemical sustainability of 2D materials can give porous materials with enhanced resistance to specific extreme environments. The porous materials' channels and pores facilitate the rapid transport of electrolytes, resulting in increased electrical conductivity. It has been claimed that interactions between molecules and pore walls can accelerate the diffusion rate of electrolytes and increase their electrical conductivity.
Applications of Porous Two-Dimensional (2D) Materials
Two-dimensional (2D) porous materials offer tremendous potential for usage in many practical applications.
Electrocatalytic Applications of Porous 2D Materials
Electrocatalysis is a procedure that speeds up electrochemical processes on the electrode surface. The electrocatalytic performance is primarily governed by the number of active sites, atomic configuration states, and conductance of the electrode material. Because of their morphological and structural benefits, porous 2D materials are anticipated to exhibit excellent electrocatalytic performance.
Porous 2D materials are useful in many domains of electrocatalysis, including applications for HER, OER, overall water splitting, electrocatalysts CO2 reduction, and organic oxidation reaction.
Materials with nanostructured pores have a significant surface area and pore size. In addition, they include more active sites than bulk materials.
Due to high active sites, acid stability, and cheap cost, 2D transition-metal-dichalcogenide (TMD) nanosheets are considered one of the most attractive materials to produce noble-metal electrocatalyst for electrocatalytic HER.
Porous 2D nanosheets are explored for the organic oxidation reaction due to their desirable characteristics, which include a high exposed surface area for copious catalytic active sites and a large interfacial electrolyte area for accelerated electron transfer and mass transfer of reactants to the electrocatalyst.
Photocatalytic Applications of Porous 2D Materials
Due to the environmental compatibility and cheap cost of photocatalysis, it has been extensively explored for use in energy applications. The energy conversion efficiency of photocatalysis is significantly affected by light absorption, dissociation, and distribution of the charge carrier, the number of active surface sites, and band structures (the edge placement of the CB and VB) of photocatalysts.
Photo-corrosion is one of the key challenges for photocatalysts. It has been reported that porous 2D-structured materials exhibit good photo-corrosion stability and superficial resistance.
Applications as Energy Materials
Owing to their unique structure, porous 2D materials offer several exceptional qualities, including a high specific surface area, low weight, strong mechanical strength, and chemical inertness, and have shown considerable promise in the energy industry.
Various functional groups in the derivatives of 2D materials might be employed as intriguing supports to combine with both organic and inorganic species, demonstrating the tremendous potential for the fabrication of 2D porous materials with varying degrees of complexity.
These benefits give porous 2D materials a wide range of applications in the environmental and energy sciences, such as high-performance components for constructing nanosystems for energy storage and conversion, particularly as the key elements for battery cells, supercapacitors, and fuel cells.
There are currently several materials that could be used for hydrogen storage, including carbon compounds, MOFs, and organic materials. Porous 2D materials are the most often utilized materials for hydrogen storage and display exceptional performance. It has been shown that porous boron nitride microspheres (BNMSs) are exceptional hydrogen storage materials. Due to their exceptional electrochemical behavior, some porous materials, including porous carbon, NiO, and FeMnO composites, could be exploited in producing supercapacitors.
In the realm of fuel cells, porous 2D materials might potentially be employed as electrodes.
Challenges Associated with Porous 2D Materials
Despite great advancements, porous 2D materials and associated research continue to face major obstacles. for example, the presence of large pores might decrease packing density and conductivity. In addition, in porous 2D materials, many exposed active sites and a lower charge transport barrier may stimulate unwanted adsorption or chemical reactions.
The structure and morphology of porous 2D materials, such as the surface to volume (SSA) and pore size, had a substantial impact on the fabrication of lithium batteries, supercapacitors, and fuel cells. To achieve porous 2D materials with enhanced SSA, conductivity, and electrochemical characteristics, the manufacturing processes, homogeneity, and durability of the porous structures should be further enhanced.
Future Outlook
In the future, by incorporating multiple elements, new porous 2D porous materials could be produced. The connection between microstructures and structural characteristics should be studied, and more emphasis should be placed on the theoretical study (computer simulations) of 2D porous materials to comprehend the material properties.
Moreover, as useful building blocks for varied nanostructures, porous 2D materials may be developed further by linking with other 2D architectures or combined into 3D structures with specified structures (3D monoliths, foams, etc.), depending on the actual application needs.
In photocatalysis, porous 2D/2D heterojunctions can produce improved charge carrier segregation and transport, as well as quicker surface reaction kinetics. To maximize catalysis performance, various heterostructure types with varying band structures and processes can be examined.
Currently, the research of porous 2D materials focuses mostly on a narrow subset of substances, such as transition metal oxides, dichalcogenides, and LDHs. A greater variety of materials with distinct features merits further study. For this reason, additional porous 2D materials with a smaller band gap should be produced, either by proposing revolutionary deposition or exfoliation procedures or by using powerful computational algorithms to search for new materials.
References for Further Studies
He, Y., Zhuang, X., Lei, C., Lei, L., Hou, Y., Mai, Y., Feng, X., 2019. Porous carbon nanosheets: Synthetic strategies and electrochemical energy-related applications. Nano Today 24, 103–119. https://doi.org/10.1016/j.nantod.2018.12.004
Liu, T., Ding, J., Su, Z., Wei, G., 2017. Porous two-dimensional materials for energy applications: Innovations and challenges. Mater Today Energy 6, 79–95. https://doi.org/10.1016/j.mtener.2017.08.006
Wang, H., Liu, X., Niu, P., Wang, S., Shi, J., Li, L., 2020. Porous Two-Dimensional Materials for Photocatalytic and Electrocatalytic Applications. Matter 2, 1377–1413. https://doi.org/10.1016/j.matt.2020.04.002
Salloum, K.S., Hayes, J.R., Friesen, C., Posner, J.D., 2008. Sequential flow membranes microfluidic fuel cell with porous electrodes: Sensors, Actuators, and Microsystems. ECS Trans, 21–38. https://doi.org/10.1149/1.3007996
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