Two-Dimensional Mesoporous Materials for Energy Storage and Conversion: Current Status, Chemical Synthesis and Challenging Perspectives

Abstract

Two-dimensional (2D) mesoporous materials (2DMMs), defined as 2D nanosheets with randomly dispersed or orderly aligned mesopores of 2–50 nm, can synergistically combine the fascinating merits of 2D materials and mesoporous materials, while overcoming their intrinsic shortcomings, e.g., easy self-stacking of 2D materials and long ion transport paths in bulk mesoporous materials. These unique features enable fast ion diffusion, large specific surface area, and enriched adsorption/reaction sites, thus offering a promising solution for designing high-performance electrode/catalyst materials for next-generation energy storage and conversion devices (ESCDs). Herein, we review recent advances of state-of-the-art 2DMMs for high-efficiency ESCDs, focusing on two different configurations of in-plane mesoporous nanosheets and sandwich-like mesoporous heterostructures. Firstly, a brief introduction is given to highlighting the structural advantages (e.g., tailored chemical composition, sheet configuration, and mesopore geometry) and key roles (e.g., active materials and functional additives) of 2DMMs for high-performance ESCDs. Secondly, the chemical synthesis strategies of 2DMMs are summarized, including template-free, 2D-template, mesopore-template, and 2D mesopore dual-template methods. Thirdly, the wide applications of 2DMMs in advanced supercapacitors, rechargeable batteries, and electrocatalysis are discussed, enlightening their intrinsic structure–property relationships. Finally, the future challenges and perspectives of 2DMMs in energy-related fields are presented.

Graphical Abstract

In this review, the recent advances of 2DMMs (including in-plane mesoporous nanosheets and sandwich-like mesoporous heterostructures) for energy storage and conversion are systematically summarized.

1 Introduction

The growing environmental problems and limited fossil fuel supply have intensively stimulated the continuous exploitation of renewable and clean energy (e.g., wind, solar, tidal, geothermal and biomass energy) and the rapid development of energy storage and conversion technologies (e.g., supercapacitors, rechargeable batteries, and fuel cells) [1,2,3,4]. These sustainable energy storage and conversion devices (ESCDs) not only reduce the detrimental economic effects of intermittent energy sources, but also satisfy the commercial requirements of electronic products, electric vehicles, power grids, etc., thus attracting immense attention recently [5,6,7,8,9]. To develop high-performance ESCDs, designing novel functional materials with advanced nanostructures and physicochemical properties as active electrodes, special additives and catalysts is crucial yet challenging.

As one of the most promising electroactive components for ESCDs, two-dimensional (2D) materials such as graphene [10,11,12] and its analogues (e.g., transition metal oxides/hydroxides/dichalcogenides [13,14,15], MXenes [16,17,18], phosphorene [19,20,21], hexagonal boron nitrides [22, 23] and graphitic carbon nitrides [24, 25]) have been widely reported, due to their intriguing physicochemical features such as atomic thickness, tunable catalytic sites, large specific surface areas (SSAs), and excellent mechanical flexibility [26,27,28,29]. However, these 2D materials self-stack rather easily because of van der Waals forces, resulting in reductions in SSA and active sites. This blocks ion/mass transport to a certain extent and results in degraded electrochemical performance. On the other hand, mesoporous materials with tailored pore sizes between 2 and 50 nm possess high SSAs, large pore volumes and diverse geometries, and have been considered as potential candidates for active materials of ESCDs [30,31,32]. Their appropriate pore size accommodates solvated electrolyte ions, provides adsorption/reaction spaces, reduces ion/mass diffusion resistance, and eventually enhances the overall electrochemical properties of ESCDs. Nonetheless, the poor structural stability and long ion transport distances of bulk mesoporous materials hinder the achievement of high-performance ESCDs. By combining 2D materials and mesoporous structures to construct 2D mesoporous materials (2DMMs), the merits of both kinds of materials can be synergistically coupled while their individual drawbacks (e.g., easy self-stacking of the 2D materials and long ion transport paths in the bulk mesoporous materials) can be maximally eliminated [33]. The unique features such as large lateral size, abundant mesopores, ample active sites, interconnected ion–electron transport paths, and multifunctional components make 2DMMs suitable for not only advanced electrodes but also special additives and catalytic materials for boosting the overall performance of ESCDs. Although the progress of porous graphene and MXenes for ESCDs has been summarized recently [34,35,36], there are still few reports with comprehensive overviews on the gigantic achievements of 2DMMs with well-defined mesoporous structures including but not limited to graphene and MXenes.

In this review, a recent development of the state-of-the-art 2DMMs for high-efficiency ESCDs is summarized (Fig. 1). In particular, these 2DMMs are classified into in-plane mesoporous nanosheets (with vertical mesopores in the basal planes of the 2D materials) and sandwich-like mesoporous heterostructures (with in-situ coating mesoporous composition on the surface of the 2D materials). Firstly, the structural importance and uniqueness of 2DMMs are emphasized for high-performance ESCDs. Secondly, the chemical synthesis and fabrication strategies, including template-free, 2D-template, mesopore-template and 2D mesopore dual-template methods, for 2DMMs with random and ordered mesopores are thoroughly described. Thirdly, the recent progress of 2DMMs in typical ESCD applications, e.g., supercapacitors, rechargeable batteries, and electrocatalysis, is discussed. Lastly, the outlook on challenges and opportunities of 2DMMs in emerging and promising energy-related fields is proposed.

Fig. 1

Schematic illustration of the main synthesis strategies and applications of 2DMMs for energy storage and conversion

2 Structures and Advantages of 2DMMs

In general, 2DMMs are defined as 2D nanosheets with mesopore diameters of 2–50 nm, which present common strengths of developed porous structures, larger SSAs, enriched active sites, more admissible free spaces, and enhanced mass/ion diffusion, compared to homologous 2D non-mesoporous materials and bulk mesoporous materials [33, 37]. More specifically, the well-developed mesoporous networks can provide continuous migration paths for relevant species and electrolyte ions, ensuring short ion transmission distances and rapid mass transport. The large SSA and rich active sites enable sufficient penetration of electrolyte ions, thus increasing the effective contact areas between electrodes and electrolytes. Furthermore, the large pore volume and enough free spaces not only anchor extra active components for boosting the electrochemical performance of corresponding materials, but also effectively buffer the volume variation of electrodes during charge/discharge process for improving the cycling stability of ESCDs. 2DMMs are classified into in-plane mesoporous nanosheets and 2D sandwich-like mesoporous heterostructures according to the design principles of chemical composition and microstructure configuration, both of which display distinctly different characteristics, as discussed below.

In-plane mesoporous nanosheets possess vertical mesopores in basal planes, including substances such as mesoporous graphene [35, 38, 39], carbon [37, 40], metal oxides [41,42,43], and MXenes [36]. This unique structure enables fast migration of ions or molecules along both vertical and horizontal directions, thus greatly improving mass transfer rates in active materials. Furthermore, in-plane mesoporous nanosheets exhibit more defects and edges on surfaces, which significantly increases available SSAs and exposed active sites for ESCDs. As a prime example, graphene nanomeshes (GNMs) with pore sizes of 5–20 nm, as electrode materials for supercapacitors, exhibited higher capacitance, rate capability, frequency response and cycling stability than graphene without pores, because of the unique structural advantages of in-plane mesopores in the GNMs [39]. As another example, single-layered ordered mesoporous CeO₂ (m-CeO₂), possessing in-plane mesopores of ~ 20 nm and a uniform thickness of ~ 15 nm, delivered highly accessible surfaces, more active sites, enhanced mass diffusion, and thus superior CO oxidation performance compared to bulk mesoporous CeO₂, which is attributed to the effective coupling of the 2D morphology and the vertical holey structure [43]. Hence, in-plane mesoporous nanosheets used as active materials could realize high-rate energy storage and fast reaction kinetics in catalysis.

Sandwich-like mesoporous heterostructures are denoted as layer-by-layer stacked 2D hybrids with abundant mesopores, usually produced by in-situ self-assembly of mesoporous functional components on both sides of 2D substrates [36, 44, 45]. Different from in-plane mesoporous nanosheets with single composition, sandwich-like mesoporous heterostructures can introduce single-, dual-, or multifunctional components to extend the physicochemical properties of the primary materials. Hence, they can synergistically couple 2D substrates and mesoporous functional components to satisfy multifunctional requirements of materials and even devices. For instance, Yang et al. [46] first reported 2D sandwich-like graphene-based mesoporous silicon/carbon/Co₃O₄ nanosheets (defined as GM-silica, GM-C and GM-Co₃O₄) in 2010, which could effectively alleviate self-aggregation and random stacking of graphene. The valid combination of highly conductive graphene and mesoporous carbon coatings endowed GM-C based lithium-ion batteries (LIBs) with enhanced electrochemical performance (in terms of capacity, rate capability and cycle life), in contrast to traditional non-graphitic carbon or porous graphitic carbon. Subsequently, 2D mesoporous graphene-based carbon nitride (G-CN) nanosheets with mesopore diameters below 5 nm were demonstrated to serve as high-efficiency electrocatalysts for oxygen reduction reactions (ORRs) [47]. The enhanced electrocatalytic performance was ascribed to the ample electrocatalytic active sites (C–N bonds) in the mesoporous carbon nitride and the outstanding electrical conductivity of the graphene frameworks. Therefore, sandwich-like mesoporous heterostructures could combine conductive 2D substrates (e.g., graphene and MXenes) and highly active mesoporous layers to endow electrodes/catalytic materials with simultaneously enhanced conductivity and activity, eventually producing high-performance ESCDs.

Based on the above understanding of structures and corresponding advantages, plenty of 2DMMs with diverse chemical composition, structural configurations and mesopore geometries have been developed in this field (Table 1). The chemical constituent, lateral size, sheet thickness, mesopore diameter, mesopore distribution, SSA and adjustability are considered as critical metrics of 2DMMs. In a sense, 2DMMs are offering a new material paradigm for versatile energy storage and conversion. To sufficiently explore underlying synthesis–structure–property relationships, a systematic summary and deep analysis about controllable synthesis strategies and promising energy-related applications of 2DMMs are urgently needed.

Table 1 Summary of the state-of-the-art 2DMMs for diverse ESCD applications

3 Chemical Synthesis of 2DMMs

The reliable production of 2DMMs with desirable architectures involves two steps: assembling 2D morphology and constructing mesoporous structures. It is well known that 2D materials can normally be generated through a “top-down” or “bottom-up” strategy [26, 27, 48], and that porous materials can be obtained by a “soft-template”, “hard-template”, or “template-free” method [30, 32, 35]. To effectively distinguish and visually depict the synthesis routes, current strategies for 2DMM production are divided into four categories, including template-free, 2D-template, mesopore-template and 2D mesopore dual-template methods (Table 2).

Table 2 Comparison of the main synthesis methods for 2DMMs

3.1 Template-Free Method

The template-free method is a facile strategy to prepare 2DMMs without using 2D and mesoporous structure directing agents, which includes two different routes of bottom-up growth from molecular precursors and top-down etching of existing 2D materials (Table 2). Via the former route, 2D frameworks are spontaneously formed through self-assembly, ion/molecule intercalation, pyrolysis, or gas-induced exfoliation, accompanied by mesoporous voids derived from structural vacancies, nanoparticle aggregation, precursor decomposition, and releasing of gas. As an example, Fuertes et al. [49] reported a template-free one-pot strategy to synthesize microporous/mesoporous carbon nanosheets with uneven thicknesses from 40 to 200 nm and hierarchical pores of spherical mesopores (10–30 nm) and micropores (< 1 nm), in which the 2D morphology and porous structure were generated by straightforward carbonization of an organic salt, i.e., sodium gluconate (Fig. 2a, b). As active electrodes of supercapacitors, the porous carbon nanosheets can effectively guarantee short ion-transport paths stemming from the 2D porous structure, as well as enhanced electric double-layer (EDL) capacitance benefiting from the mesopores acting as ion buffering reservoir and micropores as ion accommodators. For instance, ultrathin mesoporous Li₄Ti₅O₁₂ nanosheets (UM-LTONS) were synthesized through a template-free solvothermal process and subsequent thermal annealing [50]. Notably, the UM-LTONS exhibited an ultrathin thickness of ~ 1 nm, a large SSA up to 200 m² g^–1, and uniform mesopores of ~ 10 nm (Fig. 2c, d), demonstrating tremendous potential as high-performance anodes for LIBs. Recently, Fu and coworkers presented a facile molecular self-assembly approach to synthesize 2D porous C₃N₄ for photoredox catalysis (Fig. 2e) [51]. Specifically, melamine molecules and hydrolyzed products first self-assembled layered microrod precursors with a large interlayer spacing of 0.315 nm and rich functional groups of –NH₂ and –OH. Such characteristics enabled an easy intercalation of polar ethanol and glycerol molecules into interlayers. After calcination at 500 °C, the microrod precursors were delaminated into in-plane mesoporous C₃N₄ nanosheets, caused by gas release and volume shrinkage. As shown in Fig. 2f, g, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirmed the ultrathin 2D structure of ~ 1 nm and plentiful in-plane mesopores of 2–15 nm for the as-prepared few-layered C₃N₄, which offered increased active sites and enhanced charge mobility for boosting hydrogen evolution activity compared with bulk C₃N₄ (Fig. 2h). As these examples illustrate, the template-free synthesis of 2DMMs through the bottom-up growth of molecular precursors is simple but uncontrollable, and that only in-plane porous nanosheets with heterogeneous sizes, thicknesses and pores have been generated.

Fig. 2

Reproduced with permission from Ref. [49]. Copyright © 2015, American Chemical Society. c SEM image, and d TEM image of UM-LTONS under a calcination temperature of 600 °C. Reproduced with permission from Ref. [50]. Copyright © 2019, The Royal Society of Chemistry. e Scheme for the fabrication, f SEM image, g TEM image, and h photocatalysis mechanism of porous few-layered C₃N₄. Reproduced with permission from Ref. [51]. Copyright © 2019, American Chemical Society

Template-free synthesis and characterization of 2DMMs through the bottom-up growth of molecular precursors. a SEM image, and b TEM image of microporous/mesoporous carbon nanosheets.

As a top-down method, physical etching (i.e., photo-/electron-/plasma-etching) or chemical etching (e.g., KOH, HNO₃, H₂O₂ and metal oxide etching) has been used to create in-plane mesopores in pristine 2D materials [34,35,36,37]. Between them, chemical etching is regarded as a more universal and reliable way to achieve 2D mesoporous graphene [35, 52], carbon [37, 53], and MXenes [36]. For instance, graphene nanomeshes (GNMs) with adjustable nanoperforations of 5–20 nm and pore densities of 500–5 000 µm⁻², were produced from graphene oxide (GO) by metal oxide catalytic carbon gasification (Fig. 3a–d) [39]. Specifically, metal oxide (SnO₂, Fe₃O₄ or RuO₂) nanoparticles were employed as catalysts to selectively etch adjacent graphene at 450 °C in air, and then HI was chosen to remove the metal oxide and reduce GO. Due to the well-developed perforations and edge-enriched structures, the as-prepared GNMs showed excellent electrochemical performance for supercapacitors. Similarly, porous Ti₃C₂T_x MXene (p-MXene) flakes with through holes about tens of nanometers, were prepared at room temperature (Fig. 3e–g) [54]. This chemical etching process was conducted in aqueous solutions of transition metal salts followed by a subsequent acid treatment. Compared with pristine Ti₃C₂T_x, the p-MXene flakes revealed significantly enhanced Li-ion storage capability due to the existence of ample in-plane pores. Therefore, the template-free synthesis of 2DMMs by the top-down etching of existing 2D materials offers in-plane mesoporous nanosheets with relatively uniform 2D morphology and ample mesopores. However, the chemical composition and mesopore geometry of 2DMMs are still unmanageable.

Fig. 3

Reproduced with permission from Ref. [39]. Copyright © 2016, The Royal Society of Chemistry. e Scheme for the formation, f TEM image, and g HRTEM image of p-MXene flakes. Reproduced with permission from Ref. [54]. Copyright © 2016, Wiley–VCH

Template-free synthesis and characterization of 2DMMs by the top-down etching of existing 2D materials. a Illustration of synthesis routes, and b–d ex-situ TEM images at different synthetic stages of GNMs.

Based on the above, it can be concluded that the template-free method is simple and scalable for large-scale preparations of in-plane mesoporous nanosheets with a wide range of available precursors and 2D materials. However, it is difficult to precisely control lateral dimensions, sheet thicknesses, and mesoporous architectures of as-prepared 2DMMs. Ultimately, non-uniform 2D morphology and randomly distributed mesopores are often produced in 2DMMs by using the template-free method (Table 2).

3.2 2D-Template Method

The 2D-template method to fabricate in-plane mesoporous nanosheets or sandwich-like mesoporous heterostructures commonly relies on the growth of mesoporous inorganic/organic layers on 2D sacrificial or directed templates (Table 2). Based on the strong forces of hydrogen bonding, electrostatic attraction, or coordination interaction between 2D templates and inorganic/organic precursors, sandwich-like precursor-template hybrids are firstly assembled. Then, the target sandwich-like mesoporous nanosheets can be formed after a specific post-processing, and the in-plane mesoporous nanosheets can be successfully obtained by removing the 2D templates. It is worth noting that the mesoporous structures come from growth vacancies, nanoparticle accumulation, and chemical reactions. Hitherto, 2D GO [41, 55], lipid bilayers [56,57,58,59,60], g-C₃N₄ [61], metal oxides [38, 62] and layered double hydroxides [40], have proven as sacrificial 2D templates for preparing in-plane mesoporous nanosheets. For instance, Peng et al. [41] proposed a universal strategy to controllably synthesize holey transition metal oxide (TMO) and mixed transition metal oxide (MTMO) nanosheets for efficient alkali-ion storage (Fig. 4a–c). Due to abundant functional groups (e.g., –OH, –COOH, –O), GO served as a 2D template and covalently anchored precursors on it. After a thermal treatment in air, TMO/MTMO nanoparticles interconnected to form holey TMO/MTMO nanosheets with variable pore sizes (5–20 nm), with the GO decomposed at the same time. Typically, our group demonstrated a bottom-up supramolecular self-assembly method to fabricate in-plane mesoporous manganese dioxide (m-MnO₂) nanosheets for planar asymmetric micro-supercapacitors (MSCs, Fig. 4d) [60]. With 2D lipid bilayer (octadecylamine, OTA) templates, MnO₂ crystalline grains firstly nucleated and then continuously grew to form an interconnected porous film, leading to the formation of m-MnO₂ nanosheets with a uniform thickness of ~ 10 nm, abundant mesopores of 5–15 nm and a high SSA of 128 m² g⁻¹ (Fig. 4e, f). Prominently, the m-MnO₂ revealed higher specific capacitance and a longer cycle life compared to non-mesoporous MnO₂ nanosheets, confirming improved ion diffusion and structure stability of the m-MnO₂ stemming from the unique in-plane mesopore features. These metal oxide-based in-plane mesoporous nanosheets demonstrated well-maintained 2D morphology from the 2D sacrificial templates. The random mesopores produced from the growth of the crystalline grains and the subsequent volume shrinkage are beneficial for realization of high-performance electrodes/catalysts and corresponding devices.

Fig. 4

Reproduced with permission from Ref. [41]. Copyright © 2017, Springer Nature. d Scheme for the fabrication, e TEM image, and f HRTEM image of m-MnO₂ nanosheets. Reproduced with permission from Ref. [60]. Copyright © 2019, Elsevier

Synthesis of in-plane mesoporous nanosheets with the 2D-template method. a Schematic showing the synthesis process for holey MTMO nanosheets, b SEM image, and c scanning transmission electron microscope (STEM) image of a holey ZnMn₂O₄ nanosheet.

Using this approach, various 2D mesoporous polymer [63, 64], carbon [63,64,65,66,67], metal oxide/hydroxide/sulfide [68, 69] based sandwich-like heterostructures have also been synthesized, in which the mesopores are disordered interparticle/interpolymer voids. In 2014, Feng and coworkers reported simple graphene-directed synthesis for Schiff-base-type porous polymer (TPP) and derived nitrogen-doped porous carbon (TPC) sandwich-like nanosheets (Fig. 5a) [63]. It was demonstrated that the TPC nanosheets possess uniform 2D morphology with a large aspect ratio and abundant pores (Fig. 5b, c). They also exhibited superior electrochemical performance for supercapacitors compared with graphene-free nitrogen-doped porous carbon and other reported heteroatom-doped porous carbon, which could be attributed to the average mesopore diameter of ~ 10 nm, an ultrahigh nitrogen content of up to 12.3%, and a high SSA > 300 m² g⁻¹. Besides, 2D sandwich-like MoS₂-coupled conjugated microporous polymers (M-CMPs) and derived nitrogen-doped porous carbon (M-CMPs-T) were successfully synthesized by using functionalized MoS₂ nanosheets as 2D templates and substrates (Fig. 5d) [64]. Owing to the synergistic effect of the 2D MoS₂ and the conductive carbon layers with a N content of 3.6 wt% (wt% means weight percentage), high SSAs of 1 058–1 625 m² g⁻¹, and hierarchical porous structures of 1.1 and 3.5 nm (Fig. 5e), the M-CMPs-T nanosheets presented impressive ORR and supercapacitor performance. Therefore, the 2D-template synthesis of sandwich-like heterostructures can efficiently combine 2D substrates/templates and porous functional layers to pursue improved electrochemical performance for ESCDs.

Fig. 5

Reproduced with permission from Ref. [63]. Copyright © 2014, Wiley–VCH. d Schematic diagram of the fabrication of M-CMPs-T nanosheets, and e pore size distributions of M-CMPs-T nanosheets with different precursors and a pyrolysis temperature of 800 °C. Reproduced with permission from Ref. [64]. Copyright © 2016, Wiley–VCH

Synthesis of sandwich-like mesoporous heterostructures with the 2D-template method. a Scheme of the synthesis process for TPP and TPC nanosheets with varying monomers, and b, c TEM images of TPC-1 nanosheets.

In short, the 2D-template method is universal and relatively simple, as evidenced by the great variety of in-plane and sandwich-like mesoporous nanosheets derived (Table 2). Superior to the free-template approach, the composition, microstructure and 2D morphology of target 2DMMs can be well designed and controlled by the 2D-template method. Nevertheless, the realization of narrow distributions and well-defined sizes of mesoporous architectures in 2DMMs is still challenging through this method.

3.3 Mesopore-Template Method

To precisely tailor the ordered mesopores, a mesopore-template method has been developed, which mainly involves the usage of flexible molecules or rigid nanoparticles as soft or hard templates to construct mesopores inside the counterpart of 2D materials. In this regard, the mesopore geometry can be readily regulated by selecting different templates, while the 2D morphology is derived from the original 2D precursors or self-confinement growth of molecules (Table 2). For instance, Bai et al. [70] demonstrated porous graphene nanomeshes (GNMs) with adjustable periodicity and neck widths (5–20 nm) by using a block copolymer lithography technology, in which the graphene was used as 2D precursors, SiO_x films served as protecting layers and subsequent hard masks, and poly(styreneblock-methyl methacrylate) (P(S-b-MMA)) block copolymers were employed as sacrificial templates (Fig. 6a–d). Although it is intrinsically scalable for the fabrication of continuous semiconducting GNM thin films, this method was limited by high cost, low yield, and hazardous conditions due to the utilization of CHF₃ plasma based reactive-ion etching (RIE) and O₂ plasma etching. To explore facile chemical approaches for controllable preparations of 2DMMs, Zhao’s group developed a strategy of hydrothermal-induced solvent-confined monomicelle assembly to fabricate in-plane mesoporous TiO₂ nanosheets (Fig. 6e) [42]. To be specific, spherical triblock copolymer (Pluronic F127) monomicelles surrounded by TiO₂ oligomers were first assembled and dispersed in an ethanol/glycerol mixed solution. Owing to the parallel network domain confinement of the viscous glycerol solvent, a single-layered mesoporous TiO₂ was successfully obtained after a subsequent hydrothermal process. As exhibited in Fig. 6f, g, the as-obtained TiO₂ nanosheets revealed a uniform 2D size of ~ 500 nm, a single-layer thickness of 5.5 nm, ordered mesopores of 4.0 nm, and a large SSA of 210 m² g⁻¹, which presented remarkable performance for sodium-ion storage. Moreover, using diblock copolymer (polystyrene-b-poly(ethylene oxide), PS-b-PEO) or Pluronic P123 micelles as mesoporous templates, 2D in-plane mesoporous metallic Ir [71], IrO_x [72] and layered double hydroxides (LDHs) [73], have also been fabricated, confirming the possibility to construct 2DMMs by this mesopore-template method. However, the mesopore-template synthesis of 2DMMs without 2D templates remains scarce, because of the difficulty in constructing mesopores in combination with 2D morphology simultaneously. Despite the superior controllability of mesostructures, only a few in-plane mesoporous nanosheets with less uniform 2D morphology have been achieved so far by using this approach (Table 2).

Fig. 6

Reproduced with permission from Ref. [70]. Copyright © 2010, Springer Nature. e Schematic diagram of the formation process, f SEM image, and g TEM image of single-layered mesoporous TiO₂ nanosheets. Reproduced with permission from Ref. [42]. Copyright © 2018, American Chemical Society

Synthesis of 2DMMs with the mesopore-template method. a Scheme of the fabrication, b structural diagram, and c, d TEM images of GNM with 39 nm mesopores (neck width: 7.1 nm).

3.4 2D Mesopore Dual-Template Method

The 2D mesopore dual-template method has proven as a most efficient and popular strategy for the generation of highly ordered 2DMMs via the synergic assembly of 2D and mesopore templates (Table 2). Particularly, 2D interfaces, mesopore templates and organic/inorganic precursors are firstly coassembled to form hybrid composites through sufficient regulation of the interactions between them. The succeeding co-solidification and selective removal of the templates result in ordered 2DMMs. Using this method, 2D in-plane mesoporous polymers [56, 57], carbon [40, 43], graphene [38], metal oxides [43, 59], metal [74] and metal–organic networks [58] with well-organized porosity have been obtained by utilizing 2D sacrificial templates of lipid bilayers, graphene, LDHs, anodic aluminum oxides, silicon, and removable inorganic salts. As a prime example, Liu et al. [56] demonstrated a supramolecular cooperative self-assembly approach for the bottom-up fabrication of 2D mesoporous polypyrrole (mPPy, Fig. 7a). Notably, the OTA and PS-b-PEO firstly self-assembled into bilayers and spherical micelles, respectively, guiding orientated polymerization of the pyrrole monomers around the 2D and mesoporous templates. After removing these soluble templates, in-plane mPPy nanosheets were successfully received. Due to the unique structure with adjustable mesopore size (6.8–13.6 nm), variable thickness (25–30 nm), and impressive SSA (up to 96 m² g⁻¹), the 2D mPPy nanosheets displayed superior electrochemical performance for Na-ion batteries (SIBs) to other reported PPy-based materials (Fig. 7b, c). Recently, an effective and scalable preparation of single-layered ordered mesoporous materials (SOMMs) has been proposed through coassembly of amphiphilic block copolymers (as soft mesoporous templates) and various organic/inorganic precursors on removable inorganic salt substrates (as 2D interfaces), combined with vacuum filtration (Fig. 7d) [43]. By changing the mesoporous templates and molecular precursors, a variety of ordered mesoporous poly(phenolic formaldehyde resin), carbon, silica and crystalline metal oxides (e.g., ZrO₂, Ce_0.5Zr_0.5O₂, Al₂O₃ and CeO₂) have been readily gained. This strategy offers a promising paradigm for the design and fabrication of novel 2DMMs with in-plane ordered mesopores.

Fig. 7

Reproduced with permission from Ref. [56]. Copyright 2016, Wiley–VCH. d Schematic illustration of the fabrication procedures of SOMMs. Reproduced with permission from Ref. [43]. Copyright © 2020, Wiley–VCH

Synthesis of in-plane mesoporous nanosheets with the 2D mesopore dual-template method. a Scheme of the synthesis of mPPy nanosheets, b SEM image, and c TEM image of mPPy-1 with PS₃₃-b-PEO₁₁₄ as the mesoporous templates.

Besides, since the first report of graphene-based mesoporous silica nanosheets in 2010 [46], sandwich-like graphene-based mesoporous carbon [46, 75], Co₃O₄ [46], TiO₂ [76, 77], SnO₂ [78], Nb₂O₅ [79], polymer [80,81,82,83,84], carbon nitride [47] and heteroatom-doped carbon [85,86,87] nanosheets have also been successfully synthesized by this 2D mesopore dual-template strategy. Representatively, an interface-induced coassembly route was reported for the preparation of ordered mesoporous carbon on graphene aerogel (OMC/GA) with oriented mesoporous carbon of ~ 9.6 nm and interconnected macroporous networks (Fig. 8a) [75]. By tuning the mass ratio of resol-F127 micelles and GA, the OMC/GA with vertical or horizontal orientation mesopores could be readily modified, resulting in different electrochemical behaviors of supercapacitors. Apart from graphene, MXenes [45, 88], MoS₂ [89, 90], TiO₂ [91], exfoliated niobate [45] and zeolitic imidazolate frameworks [45] based sandwich-like mesoporous nanosheets have been put forward to explore the full potential of their distinct structures, chemical composition, and physicochemical properties. As shown in Fig. 8b, a composite-micelle-directed interfacial assembly strategy has been proposed for the construction of mesoporous carbon layers on ultrathin 2D materials (i.e., MXene, GO, niobate, and zeolitic imidazolate frameworks) [45]. Through the interaction of block copolymer/melamine–formaldehyde resin (F127/MF) composite-micelles and MXene nanosheets, as well as the succeeding annealing, 2D sandwich-like mesoporous carbon@MXene@carbon (MXene@C) heterostructures were successfully obtained. SEM and TEM images manifested the well-maintained 2D morphology of 20 nm in thickness and ordered mesopore structures of 4–9 nm for MXene@C before and after a thermal treatment (Fig. 8c–f). As a proof-of-concept, the MXene@C mesoporous heterostructures as sulfur hosts offered greatly improved electrochemical performance for Li–S batteries, due to the synergistic effect of 2D MXenes, conductive carbon layers and mesoporous structures [92,93,94]. Thus, the synthesis of sandwich-like mesoporous nanosheets using the 2D mesopore dual-template method demonstrated an effective coupling of uniform 2D substrates, multifunctional composition and ordered mesoporous structures, which is conducive to significantly enhancing the electrochemical performance of corresponding ESCDs.

Fig. 8

Reproduced with permission from Ref. [75]. Copyright © 2015, Wiley–VCH. b Schematic diagram of the formation process for 2D mesoporous heterostructured materials, c, d SEM images of MXene@F127/MF nanosheets, and e, f TEM images of MXene@C nanosheets. Reproduced with permission from Ref. [45]. Copyright © 2020, Wiley–VCH

Synthesis of sandwich-like mesoporous heterostructures with the 2D mesopore dual-template method. a Scheme of the fabrication of OMC/GA.

In short, the 2D mesopore dual-template method can realize precise regulation of 2D morphology and mesoporous structure simultaneously, which provides an ideal platform for synthesizing various designed 2DMMs with tailored chemical composition, sheet thickness, and mesopore geometry (Table 2). Despite its advantages of plentiful feasible precursors, different available templates, and great controllability, the 2D mesopore dual-template method is still limited by tedious steps and harsh reaction conditions. Therefore, a large-scale preparation of high-quality 2DMMs under mild conditions is a goal worth pursuing. This will promote their key applications in energy storage and conversion.

4 2DMMs for Energy Storage and Conversion

Inspired by the structural advantages and the realization of chemical synthesis, 2DMMs reveal huge potential applications in the construction of high-performance ESCDs. To comprehensively explore the inherent structure–property relationships, a summary of relevant progress achieved in 2DMMs’ utilization for key energy-related applications, including various supercapacitors, LIBs, SIBs, emerging lithium/sodium metal batteries (LMBs/SMBs) and diverse electrocatalysts, is essential.

4.1 Supercapacitors

Supercapacitors represent a type of attractive electrochemical energy storage systems with rapid charge/discharge rates, high power density, ultralong lifespan, low cost, and good safety, which can be widely used in portable electronics, electric vehicles, and heavy machinery [95,96,97]. However, their relatively low energy density has triggered research into the exploration of advanced electrode materials with enhanced electrochemical performance. Based on the surface charge storage mechanism of supercapacitors, high-performance electrode materials require large ion-accessible SSAs and high electron–ion conductivity simultaneously. To this end, 2DMMs can realize an in-situ coupling of high conductivity components, highly capacitive materials, and porous structures to offer large SSAs, rich active sites, satisfactory electronic conductivity, rapid ion transport, and outstanding structural stability, ultimately realizing increased energy density of supercapacitors without sacrificing their power density and cycle life.

4.1.1 Electrochemical Double-Layer Capacitors

As a typical kind of supercapacitors, electrochemical double-layer capacitors (EDLCs) store charge via fast electrostatic adsorption of electrolyte ions at the electrode surface driven by the formation of EDL [5, 98, 99]. Since nanocarbon is the most attractive capacitive electrode material in recent years, plenty of 2D mesoporous carbon, graphene and carbon-graphene composites have been exploited for EDLCs [39, 49, 75, 100, 101]. As exhibited in Fig. 9a–c, a holey graphene framework (HGF) with abundant in-plane nanopores, a high SSA of 1 560 m² g⁻¹, high packing density of 0.71 g cm⁻³ and ultrahigh conductivity of 1 000 S m⁻¹, was demonstrated to realize high-performance EDLC electrodes with both high gravimetric and volumetric capacitance (298 F g⁻¹ and 212 F cm⁻³) as well as superior cycling stability (91% retention after 10 000 cycles) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄)/acetonitrile electrolyte [100]. Moreover, a fully packaged HGF-based EDLC delivered high energy density of 35 Wh kg⁻¹ and 49 Wh L⁻¹ comparable to lead acid batteries. In addition, porous graphene has been used as versatile building blocks to fabricate holey graphene hydrogels (HGHs) and holey graphene papers (HGPs), which offer better electrochemical performance in various electrolytes compared to non-holey graphene and other nanocarbon materials (Fig. 9d, e) [101].

Fig. 9

Reproduced with permission from Ref. [100]. Copyright © 2014, Springer Nature. d Specific capacitance comparisons, and e schematic diagram showing the ion diffusion pathway for HGP and non-holey GP based EDLCs. Reproduced with permission from Ref. [101]. Copyright © 2015, American Chemical Society. f CV curves of OMC/GA-2 obtained at varying scan rates, g specific capacitance versus current density for different samples in a 6 M (1 M = 1 mol L⁻¹) KOH electrolyte with a three-electrode system, h CV curves obtained at 100 mV s⁻¹, and i capacitance comparisons for EDLCs based on OMC/GAs, OMC and GA. Reproduced with permission from Ref. [75]. Copyright © 2015, Wiley–VCH

2DMMs for EDLCs. a Schematic illustration of an HGF electrode, b CV curves at 1 000 mV s⁻¹, and c GCD profiles at 100 A g⁻¹ for HGF and graphene framework (GF) based EDLCs.

Furthermore, sandwich-like mesoporous carbon-graphene nanosheets are expected to boost higher performance for EDLCs, because of the synergistic combination of mesoporous carbon layers and highly conductive graphene. For instance, the OMC/GA composites (Fig. 8a) with interconnected macroporous graphene skeletons and oriented mesoporous carbon layers revealed superior electrochemical performance to these counterparts of ordered mesoporous carbon (OMC) and graphene aerogel (GA) in both three-electrode and two-electrode systems (Fig. 9f–i) [75]. Moreover, the different orientations of the mesopores in the OMC/GA remarkably influenced their electrochemical behaviors, and the OMC/GA with vertically oriented mesopores (OMC/GA-2) displayed higher specific capacitance and better rate performance than those with horizontal aligned mesopores (OMC/GA-3). Overall, the uniform 2D structure, high electrical conductivity, and controllable meso-scaled porosity play key roles in the improvement of specific capacitance, rate capability and cycling stability of 2DMMs, contributing to higher energy and power density for corresponding EDLCs.

4.1.2 Pseudocapacitors

Unlike EDLCs, pseudocapacitors (PCs) store charge by rapid redox (Faradic) reactions at the surface/near-surface of electrodes, affording higher capacitance and energy density [95, 102]. Heteroatom-doped carbon, transition metal oxides and conducting polymers have been regarded as classic pseudocapacitive materials [5, 103]. Unfortunately, their large volume expansion and poor conductivity usually lead to inferior cycling stability and low power density for PCs. To address these issues, diverse in-plane mesoporous nanosheets (e.g., 2D mesoporous heteroatom-doped carbon [67, 104], MnO₂ [60]) and sandwich-like mesoporous composites (e.g., 2D mesoporous polypyrrole (PPy)-graphene [80, 81], polyaniline (PANI)-graphene [83, 84], and PANI-MoS₂ [90]) have been developed for high-performance pseudocapacitive electrodes. For example, mesoporous graphene-like N-doped carbon nanosheets (MCS-1@800) with well-defined mesopores (~ 3.5 nm), high SSA (2 607 m² g⁻¹) and large pore volume (3.12 cm³ g⁻¹) as well as high nitrogen content (~ 6%) were reported for PCs [104]. Notably, the MCS-1@800 exhibited excellent rate capability of 275.5 F g⁻¹ at 1 mV s⁻¹ and 131.4 F g⁻¹ at 2 000 mV s⁻¹ in a KOH electrolyte, surpassing that of commercial activated carbon. As another example, m-MnO₂ nanosheets synthesized by the 2D-template approach, presented higher capacitance and enhanced cycling stability than 2D MnO₂ without mesopores, confirming the superiority of in-plane mesopores for enhancing electrochemical performance of pseudocapacitive electrodes [60]. However, in-plane mesoporous nanosheets normally contain single chemical constituents, which cannot simultaneously satisfy the requirements of high activity and high conductivity for PCs.

Compared to individual 2D pseudocapacitive materials with in-plane mesopores, sandwich-like mesoporous heterostructures combining high-pseudocapacitive components and high-conductive frameworks are expected to realize advanced PCs with both high power and energy densities. As shown in Fig. 10a–c, a 2D mesoporous PANI-graphene (G-mPANI) hybrid was produced via in-situ fabricating mesoporous PANI layers on a 2D graphene for PC application [83]. The cyclic voltammetry (CV) curves of G-mPANI revealed two pairs of redox peaks, stemming from the Faradic pseudocapacitive behavior of PANI. Due to an effective integration of the mesoporous PANI layers and the 2D graphene, the G-mPANI electrode displayed high specific capacitance of 749 F g⁻¹ at 0.5 A g⁻¹, outstanding rate capability with 73% retention at 5.0 A g⁻¹, and satisfactory cycling stability of 88% retention for 1 000 cycles. Meanwhile, the G-mPANI-based PCs exhibited maximum energy density of 11.3 Wh kg⁻¹ and power density of 10.7 kW kg⁻¹, much higher than that of pure PANI-based PCs. Subsequently, 2D mesoporous PPy-GO [80], PANI-exfoliated graphene (EG) [84], and PANI-MoS₂ [90] hybrids have also been developed to further elucidate the advantages of 2D mesoporous heterostructures and explore the influence of mesopore structure on their electrochemical performance. For example, 2D mesoporous PANI-MoS₂ (mPANI/MoS₂) composites with tunable mesopore shapes and sizes were synthesized for PC application (Fig. 10d, e) [90]. Significantly, mPANI/MoS₂-1 with smaller spherical mesopores demonstrated higher specific capacitance, rate capability, and cycling stability than those of mPANI/MoS₂-2 with bigger spherical mesopores, mPANI/MoS₂-3 with cylindrical mesopores, and pure MoS₂ and PANI, which was attributed to the unique mesopore structure, smaller mesopore size, larger SSA and superior contact interface between PANI and MoS₂ in the mPANI/MoS₂-1 (Fig. 10f). In other words, the spherical mesopores and small mesopore sizes can bring about 2DMMs with higher electrochemical performance than cylindrical mesopores and larger mesopore diameters, respectively.

Fig. 10

Reproduced with permission from Ref. [83]. Copyright © 2013, Elsevier. d SEM image of mPANI/MoS₂-1, e SEM image of mPANI/MoS₂-3, and f specific capacitance comparisons for various mPANI/MoS₂, pure PANI and MoS₂ samples. Reproduced with permission from Ref. [90]. Copyright © 2017, American Chemical Society. g Diagram of an mPPy-Fe₂O₃@rGO nanosheet, h GCD profiles for mPPy-Fe₂O₃@rGO-1 with smaller mesopores, and i specific capacitance comparisons for mPPy-Fe₂O₃@rGO with varying mesopore sizes, PPy-Fe₂O₃@rGO, and PPy@rGO. Reproduced with permission from Ref. [105]. Copyright © 2018, Wiley–VCH

2DMMs for PCs. a Diagram of a G-mPANI hybrid, b CV curves of G-mPANI, and c a capacitance comparison of G-mPANI and pristine PANI.

Furthermore, sandwich-like mesoporous heterostructures can be employed as a host platform to embed or anchor highly redox species in their networks or mesopores, which allow the full utilization of their structural advantages and enhance the electrochemical performance of electrode materials and whole devices. For example, Zhu and coworkers fabricated an iron-oxide-embedded mesoporous polypyrrole on a reduced GO (denoted as mPPy-Fe₂O₃@rGO) using a FeCl₃ initiator (Fig. 10g) [105]. Serving as an electrode material of PCs, the mPPy-Fe₂O₃@rGO with smaller mesopores presented higher specific capacitance compared to those of mPPy-Fe₂O₃@rGO with larger mesopores, PPy-Fe₂O₃@rGO without mesopores, and PPy@rGO without Fe₂O₃ particles in a 6 M (1 M = 1 mol L⁻¹) KOH electrolyte (Fig. 10h, i). However, the narrow voltage window (0.36 V) of the mPPy-Fe₂O₃@rGO electrode severely limited the energy density of corresponding PCs. In this case, 2D mesoporous PPy-based graphene nanosheets anchoring redox polyoxometalate (mPPy@rGO-POM) were reported to enhance the volumetric capacitance and energy density of PCs [106]. Owing to the ultrathin morphology, wormlike mesoporous structure and uniformly anchored polyoxometalate species, the mPPy@rGO-POM based micro-supercapacitors (MSCs) displayed remarkably improved volumetric capacitance (137 F cm⁻³) and energy density (4.8 mWh cm⁻³) compared with MSCs based on mPPy@rGO without POM and non-porous PPy@rGO. These results demonstrated that sandwich-like mesoporous heterostructures are promising materials for the synthesis of new composite materials and fabrication of high-performance PCs.

4.1.3 New-Concept Supercapacitors

Along with the rapid expansion of wearable, portable/implantable electronics and their integrated systems, multiple functionalities such as flexibility, miniaturization, shape-diversity, and high-voltage output have been put forward for next-generation supercapacitors [6, 107,108,109,110,111,112]. Owing to the intriguing electrochemical and mechanical properties, 2DMMs have stimulated tremendous research interest in electrode materials for new-concept supercapacitors including flexible supercapacitors, MSCs, high-voltage supercapacitors and supercapacitor-based integrated systems [112,113,114].

The 2D flexible structure of mesoporous nanosheets makes them ideally suitable for electrode materials of flexible supercapacitors, which are characteristic of various mechanical deformations, e.g., bending, stretching, compressing, and twisting [107, 115]. For instance, 2D hierarchical porous carbon nanosheets (2D-HPCs) with ultra-thin thickness (1.5 nm), large lateral dimension (1–3 µm), high SSA (2 406 m² g⁻¹), and hierarchical micro-, meso- and macropores, have been reported as highly capacitive and flexible electrode materials for supercapacitors [116]. As illustrated in Fig. 11a, a flexible supercapacitor was fabricated with 2D-HPCs electrode materials, a cellulose separator, an EMIMBF₄ electrolyte and a poly(dimethylsiloxane) encapsulation, showing enhanced gravimetric and volumetric energy densities of 139 Wh kg⁻¹ and 8.4 mWh cm ⁻³, respectively, and a long cycle life of 96% retention for 10 000 cycles. More importantly, the flexible supercapacitor presented excellent capacitive stability at varying bending radii, and ~ 78% retention after 10 000 bending cycles (Fig. 11b, c). Furthermore, four serially connected flexible supercapacitors in a twisted configuration could light up a yellow light-emitting diode (LED) for ⩾ 13 min, confirming superior electrochemical performance and mechanical flexibility of 2D-HPCs (Fig. 11d).

Fig. 11

Reproduced with permission from Ref. [116]. Copyright © 2018, Wiley–VCH. e Schematic diagram of lithographical microfabrication of interdigital microelectrodes, f cross-sectional SEM image of a microelectrode film, g areal and volumetric capacitance as a function of scan rate, and h Ragone plot of volumetric power density and energy density for PANI-G based MSCs. Reproduced with permission from Ref. [119]. Copyright © 2017, Wiley–VCH. i Photograph of a microelectrode under bending state, and j GCD profiles at 0.3 mA cm⁻² for four serially connected linear tandem MSCs. Reproduced with permission from Ref. [125]. Copyright © 2017, Wiley–VCH. k Scheme of DM-PG nanosheets for a planar MSC-sensor integrated system, l Ragone plot of DM-PG based MSC, and m NH₃ response curves of a MSC-driven sensor at 10–40 ppm (1 ppm = 1 mL m⁻³). Reproduced with permission from Ref. [130]. Copyright © 2020, Wiley–VCH

2DMMs for new-concept supercapacitors. a Schematic diagram, b CV curves at varying bending radii, c capacitance retention for 10 000 bending cycles for a 2D-HPC-based flexible supercapacitor, and d photographs of a yellow LED powered by four serial-connected 2D-HPC-based flexible supercapacitors under a twining state.

Planar MSCs are one of the most competitive and hopeful on-chip power sources, which not only possess fascinating performance metrics of traditional supercapacitors, but also offer the merits of miniaturization, tailored size, light weight, and outstanding flexibility [6, 113, 117, 118]. By virtue of 2D flat architectures and abundant mesoporous structures, 2DMMs afford a novel and hopeful material platform for high-energy and high-power planar MSCs. For instance, Wu et al. [119] put forward a lithographical microfabrication technology to construct in-plane interdigital MSCs with alternating stacked mesoporous PANI-functionalized graphene (PANI-G) or PPy-functionalized graphene (PPy-G) nanosheets and EG compact hybrid films (Fig. 11e, f). With a synergistic effect of pseudocapacitive PANI-G and highly conductive 2D EG, the as-fabricated MSCs simultaneously delivered landmark areal and volumetric capacitance of 368 mF cm⁻² and 736 F cm⁻³ in a polyvinyl alcohol (PVA)/H₂SO₄ gel electrolyte (Fig. 11g). Further, the MSCs exhibited maximum energy density of 11.7 mWh cm⁻³ in a H₂SO₄/PVA gel electrolyte, 21.1 mWh cm⁻³ in a 1 M H₂SO₄ aqueous electrolyte, and 46.0 mWh cm⁻³ in an EMIMBF₄ ionic liquid electrolyte, much higher than those of other reported carbon-based MSCs (Fig. 11h). In addition, a planar asymmetric MSC was reported with an m-MnO₂ positive electrode and a 2D porous VN negative electrode in a SiO₂-lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) gel electrolyte [60]. Owing to the highly pseudocapacitive m-MnO₂ and VN, highly stable “water-in-salt” gel electrolyte, and strong interfacial effect between microelectrodes, electrolytes, and substrates, the resulting MSCs presented improved energy density (21.6 mWh cm⁻³) compared to symmetric m-MnO₂ and VN based MSCs, outstanding mechanical flexibility, as well as convenient serial/parallel integration. Therefore, the coupling of 2DMM electrode materials in MSCs can fully exert the structural advantages of 2DMMs and remarkably increase the overall performance of MSCs.

High-voltage supercapacitors usually involve either individual supercapacitors with electrolytes exhibiting a high voltage window and/or integrated supercapacitors connected in a serial fashion, which can satisfy the demand of certain electronic devices [120,121,122]. For the former, the highly concentrated water-in-salt electrolyte brings about a broadened voltage window close to ~ 2.5 V, the organic electrolyte can realize a steady operating voltage up to 3.0 V, and the ionic liquid electrolyte provides a wide and safe voltage window of 4.0 V [123, 124]. As previously reported [60, 100, 116, 119], these high-voltage electrolytes enable 2DMMs to play their structural advantages to the fullest and thus witness superior energy/power density and cycling stability for supercapacitors. Moreover, the integrated supercapacitors without traditional metal interconnects represent one of the most advanced all-in-one power sources. In this regard, our group demonstrated a series of new-concept linear tandem MSCs with symmetric or asymmetric configurations, and high-voltage output up to 8.0 V (Fig. 11i) [125]. Among these devices, 2D mesoporous polyaniline-graphene (PANI-G) based MSCs showed areal capacitance of 7.6 mF cm⁻², higher than that of EG based MSCs. Moreover, four linear tandem MSCs presented an increased voltage of 3.2 V from 0.8 V, manifesting excellent performance uniformity and efficient high-voltage output (Fig. 11j). In summary, 2DMMs as active electrode materials are also suitable for achieving remarkably high voltage by using individual supercapacitors with high-voltage electrolytes, or serially integrating supercapacitors.

The integrated systems of supercapacitors, energy harvesters, and/or energy consuming devices, are an increasingly important technology for advanced portable and wearable electronics [118, 126,127,128,129]. The exploitation of multifunctional materials will simplify the complicated preparation process and improve overall compatibility of integrated systems. It is worth noting that sandwich-like mesoporous nanosheets are classic heterostructure materials, which can easily realize multifunctional applications for different energy-related situations [44]. For instance, a 2D hierarchical ordered dual-mesoporous PPy/graphene (DM-PG) was demonstrated as a bifunctional material for a novel planar integrated microsystem of MSC and NH₃ sensor on one single substrate (Fig. 11k) [130]. Notably, the DM-PG nanosheets synergistically coupled highly active PPy, conductive graphene, hierarchical dual-mesopores of 7 and 18 nm, and a large SSA of 112 m² g⁻¹, simultaneously unveiling superior response for the NH₃ sensor and higher specific capacitance for supercapacitors compared with single-mesoporous and non-mesoporous PPy/graphene. Then, the DM-PG based MSCs exhibited excellent capacitive performance, e.g., a volumetric capacitance of 110 F cm⁻³ and energy density of 2.5 mWh cm⁻³ (Fig. 11l). After charging for only 100 s, the MSC-sensor integrated microsystem based on bifunctional DM-PG displayed high NH₃ sensitivity (as low as 10 ppm (1 ppm = 1 mL m⁻³), Fig. 11m) and impressive flexibility (82% response retention at 180°) at room temperature. This work opens a novel direction for creating multifunctional 2DMMs for supercapacitor-based integrated systems.

4.2 Rechargeable Batteries

Due to the higher energy density and lower self-discharge than supercapacitors, rechargeable batteries including lithium and non-lithium batteries, have been extensively developed in the past decades [8, 131, 132]. However, their relatively poor power density and cycle life have inspired researchers to explore advanced functional materials for rechargeable batteries. In this regard, introducing 2DMMs into rechargeable batteries can confer benefits of the structural features of 2DMMs, and significantly improve the electron/ion transport, mass diffusion and structural stability of rechargeable batteries. In recent years, 2DMMs have been regarded as a key class of materials, e.g., active electrodes, cathode hosts, anode protectors, and solid polymer electrolyte additives for LIBs, SIBs, as well as emerging LMBs and SMBs. While the application of 2DMMs in other new-type rechargeable batteries (e.g., potassium-ion batteries, zinc-ion batteries, magnesium-ion batteries and calcium-ion batteries) is scarce.

4.2.1 Lithium-Ion Batteries

As one of the hottest research fields, LIBs are deemed as the predominant power source for portable electronics and electrical vehicles in recent decades [133,134,135]. 2DMMs are promising active electrode materials for LIBs, overcoming the shortcomings of slow electron/ion transport and structural instability of bulk counterparts. Up to now, 2D in-plane mesoporous graphene [38], transition metal oxides [41], iron-based metal–organic networks [58], MXenes [54], and Li₄Ti₅O₁₂ nanosheets [50], have been reported as advanced anode materials for LIBs, yielding enhanced rate capability and cycle life. As revealed in Fig. 4a–c, various holey TMO and MTMO nanosheets were developed for enhanced alkali-ion storage [41]. When used as anodes of LIBs, ZnMn₂O₄ (ZMO) exhibited higher cycling stability and better rate performance in comparison with controlled ZMO + super P carbon (SP) and ZMO without GO (Fig. 12a). Further, in-situ TEM, X-ray diffraction (XRD) pattern and X-ray absorption structure (XAS) measurements were conducted to study the charge storage mechanism of the holey ZMO nanosheets. In Fig. 12b, c, the in-situ TEM images revealed the minimal structural changes of ZMO nanosheets in the lithiation/delithiation processes, proving the unique advantages of 2D in-plane porous nanostructures. As another typical example, a series of 2D metal-phosphate/phosphonate nanomeshes with ordered mesopores were reported for LIB application [58]. Owing to the monolayer thickness of ~ 9 nm and hexagonally closed-packed mesopores of ~ 16 nm, the as-obtained 2D mesoporous ferric phytate nanomeshes (mFeP-NM) could directly serve as high-performance anodes for LIBs, presenting higher capacity, rate capability and cycling stability than those of metal phosphate materials reported previously (Fig. 12d–f). These results confirmed that the 2DMMs with in-plane mesopores can increase the ion transport and structural stability of active electrodes in LIBs.

Fig. 12

Reproduced with permission from Ref. [41]. Copyright © 2017, Springer Nature. d Sketch model of mFeP-NM, e GCD profiles of mFeP-NM at varying current densities, and f cycling stability at 200 mA g⁻¹ for mFeP-NM and a blank sample. Reproduced with permission from Ref. [58]. Copyright © 2020, Wiley–VCH. g Rate capability of 2D mesoporous-carbon/MoS₂ heterostructures and pure MoS₂ nanosheets. Reproduced with permission from Ref. [89]. Copyright © 2016, Wiley–VCH. h Cycle life at 200 mA g⁻¹, and i rate capacity at different current densities for C/Si–rGO–Si/C and Si@C–rGO–Si@C electrodes. Reproduced with permission from Ref. [138]. Copyright © 2017, American Chemical Society

2DMMs for LIBs. a Rate performance comparison of 2D holey ZMO nanosheets, controlled ZMO + SP and controlled ZMO, and b, c in-situ TEM images of a 2D holey ZMO nanosheet at varying lithiation stages.

In addition, engineering sandwich-like mesoporous heterostructures is another powerful strategy to boost the electrochemical performance of anodes and whole cells of LIBs. For instance, 2D mesoporous-carbon/MoS₂ heterostructures were successfully fabricated for LIBs [89]. Dramatically, the hybrid nanosheets were composed of two-layered mesoporous carbon (~ 1.4 nm) sandwiched with one-layered MoS₂ (~ 1.6 nm), offering an abundance of accessible well-defined interfaces for efficient ion and electron transmission. Hence, the mesoporous-carbon/MoS₂ based electrode presented ultrahigh reversible capacity of over 1 400 mAh g⁻¹ at 100 mA g⁻¹ and 400 mAh g⁻¹ at 10 A g⁻¹, as well as satisfactory cycling stability, greatly exceeding that of pure MoS₂ (Fig. 12g). Moreover, silicon-based material is another attractive candidate for LIB anodes with high capacity [, 136, 137]. To address the dramatic volume expansion and poor electrical conductivity of nanoscale silicon, Yao et al. [138] developed 2D porous C/Si–rGO–C/Si heterostructures for LIB application, in which a mesoporous Si layer first grew on rGO, and a carbon coating was built subsequently. The unique porous structure of the C/Si–rGO–C/Si provided fast Li-ion diffusion, and the conductive carbon layers promoted electron transport and formation of stable solid electrolyte interphase (SEI) films. As revealed in Fig. 12h, i, the C/Si–rGO–Si/C anode presented highly reversible capacity of 1 187 mAh g⁻¹ after 200 cycles at 0.2 A g⁻¹, outstanding cycling stability of 70.3% retention for 1 000 cycles, and superior rate performance of 447 mA h g⁻¹ at 10 A g⁻¹, surpassing those of cogrowing porous hybrids (Si@C–rGO–Si@C). Therefore, sandwich-like mesoporous heterostructures are more attractive and present good potential as anode materials for practical LIBs, owing to the simultaneous improvement of electron–ion transport and structural stability.

4.2.2 Sodium-Ion Batteries

Recently, SIBs are considered as one of the most promising candidates for large-scale energy storage devices because of the low price, natural abundance, and environmental friendliness of Na [139,140,141,142]. Nevertheless, the large ionic radius of Na (1.02 Å) brings out sluggish ion transport and large volume change in continuous charging and discharging processes, leading to low reversible capacity and poor cycling stability of SIBs. In this respect, 2DMMs as suitable SIB electrode materials can accommodate large Na ions, promote diffusion of electrolyte ions, and effectively improve the reaction kinetics upon sodiation/desodiation. As shown in Fig. 13a, b, partially single-crystalline mesoporous Nb₂O₅-graphene (G-Nb₂O₅) nanosheets were reported for SIB application [79]. Because of the orthorhombic crystal structure, partially single-crystalline nature, thin thickness, and mesoporous architecture, the as-prepared G-Nb₂O₅ nanosheets presented superior reversible capacity (230 mAh g⁻¹ at 0.25 C), outstanding rate performance (100 mAh g⁻¹ at 20 C), and superior cycling stability (no distinct decay for 1 000 cycles), exceeding those of Nb₂O₅ nanosheets, commercial Nb₂O₅ powders, and mixed G/Nb₂O₅ nanosheets (Fig. 13c). Apart from orthorhombic Nb₂O₅, TiO₂-based hybrids are another representative insertion-type anode materials for high-performance SIBs [42, 91, 143]. For example, hierarchical porous nitrogen-doped graphene-wrapped TiO₂ nanosheets (TiO₂@NFG HPHNSs) were synthesized with C₃N₄ nanosheets as both 2D templates and nitrogen/carbon sources (Fig. 13d) [61]. Compared with other reported carbon-based composites, the TiO₂@NFG HPHNSs possessed a hierarchical porous structure (< 20 nm), low content of graphitic carbon (< 5 wt%), and uniformly restricted TiO₂ nanoparticles, offering enhanced ion transport and charge transfer, as well as improved structural stability for SIB anodes. Thus, the TiO₂@NFG HPHNSs delivered superior reversible capacity, ultrahigh rate performance, and excellent cyclability, remarkably better than those of other TiO₂-based anodes for SIBs (Fig. 13e, f). Furthermore, a monolayered mesoporous-TiO₂-mesoporous-carbon heterostructure (denoted as meso-hetero) was demonstrated for superior pseudocapacitive sodium storage (Fig. 13g–i) [91]. The unique 2D mesoporous heterostructure of meso-hetero could provide highly accessible electrolyte–electrode interfaces, and effectively buffer the volume changes of active electrodes in charging/discharging process. As a result, the meso-hetero based anodes showed excellent cycling stability (e.g., 77.8% retention after 20 000 cycles at 10 A g⁻¹), and high pseudocapacitive contribution (96.4% at 1 mV s⁻¹) in SIBs. These studies reveal that the adoption of 2DMMs as SIB anodes is an effective strategy for boosting Na-ion storage, owing to improved ion transport and structural stability.

Fig. 13

Reproduced with permission from Ref. [79]. Copyright © 2016, Wiley–VCH. d Diagram of TiO₂@NFG HPHNSs, e long-term cycling stability of TiO₂@NFG HPHNSs and m-TiO₂ (synthesized in air) electrodes at 2 C, and f rate capability comparisons of TiO₂@NFG HPHNSs and other reported TiO₂-based electrodes. Reproduced with permission from Ref. [61]. Copyright © 2018, Wiley–VCH. g Separation of the capacitive and diffusion currents, h contribution ratio of the capacitive and diffusion-controlled charge under different scan rates, and i mechanism model of charge storage and charge transfer in the meso-hetero. Reproduced with permission from Ref. [91]. Copyright © 2019, American Chemical Society

2DMMs for SIBs. a Diagram of G-Nb₂O₅ nanosheets, b HRTEM image of G-Nb₂O₅ nanosheets, and c cycling stability of G-Nb₂O₅ nanosheets and their counterparts at 0.25 C (1 C = 335 mA g⁻¹).

4.2.3 Lithium/Sodium-Metal Batteries

To realize the widespread applications of long-life electrical vehicles and other 3C electronics, the currently used LIBs/SIBs with limited energy density are unsatisfactory. To this end, LMBs and SMBs have gained extensive attention due to their ultrahigh theoretical specific capacities (up to 3 860 mAh g⁻¹ for Li and 1 166 mAh g⁻¹ for Na) and low redox potentials (−3.040 V and −2.714 V vs. the standard hydrogen electrode) [144,145,146,147]. Unfortunately, the poor Coulombic efficiency, unstable cyclability, and associated safety risks hinder the practical application of LMBs and SMBs, such as Li/Na–S, Li/Na–air and Li/Na–O₂ batteries. To overcome these issues, introducing 2D functional mesoporous materials into sulfur cathodes, Li/Na metal anodes or solid electrolytes has been proven as one of the most effective strategies.

Sulfur is a fascinating cathode material with the characteristics of high theoretical capacity (1 672 mAh g⁻¹), low cost, and ecofriendliness, which is commonly coupled with lithium/sodium metal anodes to construct Li/Na–S batteries [148,149,150]. However, the existing problems of low conductivity and large volume changes of sulfur along with the shuttle effect of polysulfides, need to be addressed. From this view, some sandwich-like mesoporous nanosheets with well-designed functional components and porous frameworks have been examined as advanced sulfur hosts for Li/Na–S batteries. For example, the MXene@C mesoporous heterostructures (Fig. 8b–f) served as conductive sulfur hosts to realize remarkably enhanced rate capability and superior cycling stability for Li–S batteries, originating from the synergistic interactions of MXenes and mesoporous carbon for high-efficiency utilization and encapsulation of sulfur [45]. In addition, manganous oxide nanocluster-implanted nitrogen-doped mesoporous carbon nanosheets (denoted by rGO@mC-MnO-800) with ordered mesopores of ~ 18 nm, a high SSA of 300.9 m² g⁻¹, and a large pore volume of 0.57 cm³ g⁻¹, have been developed as promising sulfur hosts for high-performance Li–S batteries (Fig. 14a) [151]. As proven by experimental results and theoretical calculations, the rGO@mC-MnO-800 provided high conductivity, strong chemisorption for lithium polysulfides, and enough buffer spacing for sulfur, thus greatly improving the specific capacity, rate capability and cyclability of Li–S batteries (Fig. 14b). These results prove that 2DMMs (especially sandwich-like mesoporous heterostructures) are advanced sulfur host materials for realization of high-performance cathodes for Li/Na–S batteries.

Fig. 14

Reproduced with permission from Ref. [151]. Copyright © 2020, American Chemical Society. c Illustration showing the Li-ion redistribution in holey metal oxide nanosheets for a Li metal anode, and d long-term cycling stability of symmetrical cells with holey MgO-protected Li and bare Li electrodes. Reproduced with permission from Ref. [155]. Copyright © 2020, Wiley–VCH. e Structure and composition of MXene-mSiO₂ containing a solid polymer electrolyte, f digital picture of MXene-mSiO₂ containing a solid polymer electrolyte, and g cycling stability for full cells with MXene-mSiO₂ containing electrolyte, mSiO₂ containing electrolyte and an ePPO solid polymer electrolyte. Reproduced with permission from Ref. [88]. Copyright © 2020, Wiley–VCH

2DMMs for LMBs. a Schematic diagram of rGO@mC-MnO-800 as a sulfur host for Li–S battery, and b cycling performance at 0.2 A g⁻¹ for Li–S batteries with S/rGO@mC-MnO-800, S/rGO@mC-MnO-900 (annealing at 900 °C) and S/rGO@mC-800 (without MnO clusters) cathodes.

Metallic lithium/sodium is considered as the most ideal anode material for high-energy rechargeable batteries [146, 152,153,154]. However, continuous formation and growth of Li/Na dendrites and huge volume changes in repeated Li/Na plating/stripping processes, result in short cycle lifetime, poor Coulombic efficiency and potential safety risks for corresponding batteries. Therefore, designing uniform Li/Na-ion flux in electrolytes and SEI layers for dendrite-free Li/Na metal anodes should be considered. With abundant chemical composition and uniform mesoporous channels, 2DMMs are expected to homogenize ion flux, regulate metal deposition, and restrain dendrite formation from chemical and physical levels. For instance, in-plane holey metal oxide nanosheets were reported for stabilizing Li metal anodes [155]. Different from other reported 2D materials-based protective layers, holey metal oxide nanosheets with parallelly aligned pores (e.g., ~ 6.4 nm for holey MgO nanosheets), not only provided rapid Li-ion transport, but also redistributed Li-ion flux in electrolytes and SEI films (Fig. 14c). As a result, the holey MgO protective layers enabled Li anodes with high Coulombic efficiency (~ 99%) and ultralong cycling stability (⩾ 2 500 h at 10 mA cm⁻²), resulting in a full cell with greatly increased cycle life of 90.9% retention for 500 cycles (Fig. 14d). Further, 2D heterostructures composed of defective GO and mesoporous PPy (mPPy-GO) were developed as dual-functional Li-ion redistributors for ultrastable dendrite-free Li metal anodes [82]. In the mPPy-GO nanosheets, the defective GO served as Li-ion nanosieves, and the mesoporous PPy was employed as continuous Li-ion transport nanochannels, as confirmed by experimental results and theoretical calculations. Observably, the mPPy-GO-Li anodes exhibited high Coulombic efficiency of 98% for 1 000 cycles, low overpotential of 70 mV at 10.0 mA cm⁻², and outstanding cyclability even under harsh conditions of 0 and 50 °C. The mPPy-GO-Li/LiCoO₂ full cell displayed significantly improved capacity and cycling stability compared with pure Li/LiCoO₂ and Cu-Li/LiCoO₂ cells. In addition, 2DMMs have received much attention for protecting Na metal anodes recently. For example, our group has developed a 2D mesoporous polydopamine-graphene (mPG) heterostructure with well-defined pore diameter and sheet thickness as a multifunctional separator coating for stable and dendrite-free Na metal anodes (Fig. 15a, b) [156]. Due to abundant oxygen-/nitrogen- containing groups, nanoporous 2D graphene, and well-designed mesoporous channels, the mPG endowed Na metal anodes with high Coulombic efficiency of > 99.5%, ultralong cycling stability of ~ 2 000 h, and excellent rate capability up to 25 mA cm⁻² and 25 mAh cm⁻² (Fig. 15c, d). The Na/Na₃V₂(PO₄)₃ full cells showed improved cyclability of 90% retention for 500 cycles, and enhanced rate performance of 75 mAh g⁻¹ at 30 C. The theoretical calculations and experimental results disclosed that the mesoporous structure and mesopore size could greatly affect the Na-ion diffusion, Na plating and SEI formation. Specifically, the mPG with 12 nm mesopores based polypropylene (mPG-12@PP) separator delivered higher electrochemical performance for both Na metal anodes and Na metal batteries than mPG with 7/22 nm mesopores and non-mesoporous polydopamine-graphene (nPG) based separators. The abovementioned works have elucidated the critical roles of 2D and mesoporous structure of 2DMMs in precisely regulating the ion flux and metal deposition for ultrastable Li/Na metal anodes.

Fig. 15

Reproduced with permission from Ref. [156]. Copyright © 2021, Springer Nature

2DMMs for SMBs. a Schematic diagram of the synthesis for an mPG heterostructure, b diagram of Na deposition behaviors with an mPG-based separator and a bare PP separator, c cycling stability of Na||Na symmetric cells with mPG-12@PP separator and counterparts, and d rate capability of Na||Na symmetric cells with an mPG-12@PP separator.

Apart from protecting the sulfur cathodes and Li/Na metal anodes, 2DMMs also hold great promise for creating solid polymer electrolytes with high safety, high ionic conductivity as well as excellent mechanical properties. To overcome the poor room-temperature ionic conductivity (10⁻¹⁰–10⁻⁵ S cm⁻¹) and low mechanical moduli (< 0.4 MPa) of traditional solid polymer electrolytes [157,158,159,160], Yang and coworkers developed sandwich-like MXene-based mesoporous silica (MXene-mSiO₂) nanosheets to produce a high-performance solid polymer electrolyte (ionic conductivity: 4.6 × 10⁻⁴ S cm⁻¹; and Young’s modulus: 10.5 MPa) for LMBs [88]. Due to the insulating nature, unique mesoporous structure, high SSA, and rich functional groups, the MXene-mSiO₂ could facilely disperse into poly(propylene oxide) elastomer (ePPO) to form MXene-mSiO₂/ePPO films, and further swell LiTFSI and propylene carbonate (PC) to generate MXene-mSiO₂ containing solid polymer electrolytes (Fig. 14e, f). Finally, the full cells with LiFePO₄ cathodes, Li metal anodes and MXene-mSiO₂ containing solid polymer electrolytes revealed better electrochemical stability than those with mSiO₂ containing electrolytes and pure ePPO electrolytes (Fig. 14g). Nonetheless, the application of 2DMMs in solid electrolytes is still at a stage of infancy, and an extensive study is highly necessary.

4.3 Electrocatalysis

Electrocatalysis is playing an increasingly important role in clean electrochemical energy conversion, which includes the hydrogen evolution reaction (HER), ORR, oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO₂RR), etc. [9, 161, 162]. To address their slow reaction kinetics and decrease the usage of noble metal catalysts, designing effective non-noble metal-based catalysts is essential. With intriguing structural and electronic features, 2DMMs can offer large accessible SSAs, exposed interfacial reaction sites, fast mass transport, well-designed composition, and adjustable electronic structures, making them promising candidates for efficient electrocatalysts of HER, ORR, OER and CO₂RR.

4.3.1 Hydrogen Evolution Reaction

In general, HER provides an effective process to generate H₂ by electrolyzing water with Pt-based electrocatalysts [163, 164]. The high price and resource shortage of Pt metal materials stimulate the exploitation of efficient electrocatalysts with low overpotential, high abundance and low price. Inspired by the large SSA and rich active sites, 2DMMs reveal great potential for efficient, low-cost HER electrocatalysts. For example, ultrathin 2D iron compounds with tunable microstructures from mesoporous nanosheets to meso-macroporous nanonets were synthesized for HER (Fig. 16a, b) [165]. Contrary to the relatively inert 2D α-Fe₂O₃, FeP nanosheets displayed superior electrocatalytic activity with low overpotential of 117 mV, a small Tafel slope of 56 mV dec⁻¹, and impressive stability of ~ 15 h in acidic media (Fig. 16c, d). Moreover, the electrocatalytic performance of FeP nanosheets exceeded their counterparts with nanonet morphology, arising from the unique 2D mesoporous structure and the enhanced active SSA. Besides, Mo₂C-based materials have been put forward as promising HER electrocatalysts, owing to their natural abundance, low price, and superior electrocatalytic activity [166, 167]. Typically, Mai’s group successfully synthesized 2D mesoporous nitrogen-doped carbon/Mo₂C/rGO hybrids (mNC-Mo₂C@rGO) with spherical mesopores (13 nm) and tiny Mo₂C particles (4 nm) distributed in the porous carbon networks, witnessing effective HER catalytic activity in alkaline media (Fig. 16e) [168]. Notably, the tunable content of Mo₂C could greatly affect the electrocatalytic performance of mNC-Mo₂C@rGO. With an optimized Mo₂C content of 28.0 wt% as well as a well-balanced SSA and exposed active sites, the mNC-Mo₂C@rGO-2 revealed low overpotential of 95 mV at 10 mA cm⁻², a small Tafel slope of 49.8 mV dec⁻¹, and good stability of 60 h at 20 mA cm⁻², surpassing most reported Mo₂C-based electrocatalysts, comparable to commercial Pt/C (Fig. 16f, g). From the above, it can be clearly seen that the development of new 2D electrocatalysts and the optimization of their functional components and porous structures are influential in boosting their HER performance.

Fig. 16

Reproduced with permission from Ref. [165]. Copyright © 2020, Royal Society of Chemistry. e Schematic illustration of the synthesis route for mNC-Mo₂C@rGO nanosheets, f polarization curves, and g Tafel slopes of Pt/C and mNC-Mo₂C@rGO with different Mo₂C contents. Reproduced with permission from Ref. [168]. Copyright © 2018, American Chemical Society

2DMMs for HER. a TEM image of FeP nanosheets, b selective area electron diffraction pattern and elemental mapping of FeP nanosheet in STEM mode, c polarization curves, and d corresponding Tafel slopes of Pt/C, FeP nanosheets, FeP nanonets, α-Fe₂O₃ nanosheets and α-Fe₂O₃ nanonets.

4.3.2 Oxygen Reduction Reaction

ORR plays a vital role in fuel cells and metal–air/O₂ batteries, which requires high-efficiency and stable catalysts in practical applications [169,170,171]. Although Pt-based materials are recognized as the best ORR electrocatalysts, the scarcity, high cost and poor durability hinder their development and industrialization. Hence, great efforts have been devoted to exploiting novel non-precious metal catalysts for ORR, wherein 2D heteroatom-doped carbonaceous materials with rich mesopores stand out from the crowd [35, 37]. As displayed in Figs. 5d, e and 17a, b, the M-CMP2-800 pyrolyzed at 800 °C revealed superior ORR catalytic activity with high peak current density (− 0.21 V vs. Ag/AgCl), large diffusion-limited current (5.4 mA cm⁻²), and ultralow half-wave potential (− 0.14 V) in alkaline conditions, exceeding those of M-CMP2-700, M-CMP2-900, and MoS₂-free counterparts [64]. The excellent catalytic performance could be derived from the abundant porous structures, a large SSA of 828 m² g⁻¹, and a high N content of 3.6 wt% in the M-CMP2-800 nanosheets. Further, 2D graphene-based mesoporous phosphorus- and nitrogen-codoped carbon (rGO@PN/C) nanosheets were developed as advanced ORR electrocatalysts (Fig. 17c, d) [172]. Owing to the high SSA, bimodal pores of micropores and mesopores, synergistic effect of phosphorus/nitrogen dual dopants, the rGO@PN/C exhibited better electrocatalytic performance than the nitrogen-doped carbon (rGO@N/C) in alkaline conditions. Meanwhile, the ORR catalytic performance of rGO@PN/C-x (x = 1–5) was significantly influenced by the activation temperature and the phytic acid concentration in the preparation process. After a systematic optimization, the rGO@PN/C-2 showed the highest onset potential and diffusion-limited current density, along with excellent durability and selectivity. These results demonstrated heteroatom-doped carbon based 2DMMs as low-cost catalysts could realize high-efficiency electrocatalysis for ORR.

Fig. 17

Reproduced with permission from Ref. [64]. Copyright © 2016, Wiley–VCH. c STEM image of rGO@PN/C-2 nanosheets, and d linear sweep voltammetry (LSV) curves of rGO@PN/C-x, rGO@N/C, and Pt/C catalysts at 1 600 rpm (1 rpm = 1 r min⁻¹). Reproduced with permission from Ref. [172]. Copyright © 2020, American Chemical Society. e Diagram of a NDCN nanosheet, f TEM image of an NDCN-22 nanosheet, g enlarged TEM image of the square region in (f), and h LSV curves of NDCN-22, NDCN-7, NDCN-2 and NDCN nanosheets at 1 600 rpm. Reproduced with permission from Ref. [85]. Copyright © 2014, Wiley–VCH. i TEM image of mNC-Fe₃O₄@rGO-1; j LSV curves, k Tafel plots, and l stability comparison of mNC-Fe₃O₄@rGO and commercial Pt/C catalysts [the inset of (l): the chronoamperometric responses with the introduction of 2% methanol at 300 s]. Reproduced with permission from Ref. [105]. Copyright © 2018, Wiley–VCH

2DMMs for ORR. a Schematic diagram of ORR process, and b CV curves in 0.1 M KOH for the M-CMPs-T hybrids.

In addition, the relationship between catalytic activity and mesopore size of 2DMMs has been revealed for ORR. For instance, Wei and coworkers reported nitrogen-doped carbon nanosheets (NDCNs) with well-defined mesopores as efficient electrocatalysts for ORR under acidic and alkaline conditions (Fig. 17e–h) [85]. Notably, NDCN-22 with a large mesopore size of ~ 22 nm exhibited higher catalytic performance in an alkaline medium, including more positive half-wave potential and higher kinetic current density, compared to those of NDCNs without mesopore templates, NDCN-2 with 2 nm mesopores, NDCN-7 with 7 nm mesopores and Pt/C. Due to the unique large mesoporous shells and highly exposed active sites, the NDCN-22 also manifested outstanding catalytic performance and long-term stability in an acidic medium. Differently, Zhu et al. [105] demonstrated that sandwich-like mesoporous nitrogen-doped carbon/Fe₃O₄/rGO (named mNC-Fe₃O₄@rGO-1) nanosheets with smaller mesopores of 12 nm presented more positive half-wave potential and higher limited current density than mNC-Fe₃O₄@rGO-2 with mesopores of ~ 16 nm and mNC-Fe₃O₄@rGO-3 with mesopores of ~ 22 nm for ORR in alkaline media (Fig. 17i–l). In the meanwhile, the mNC-Fe₃O₄@rGO-1 displayed a low Tafel slope of 67 mV dec⁻¹ and impressive durability. The better catalytic performance of the mNC-Fe₃O₄@rGO-1 was attributed to the smaller mesopores, higher SSA and more active sites compared with the other two counterparts. Through careful analysis and detailed comparison, it was found that the sheet thickness and mesopore size of these 2DMMs changed synchronously, and the opposite experimental results could be attributed to the different effects of these two factors. Therefore, the investigation of the structure–property relationships of 2DMMs should control the sheet thickness, lateral size, mesopore diameter, or mesopore geometry as single variable.

4.3.3 Oxygen Evolution Reaction

The OER is a vital oxidative half reaction for electricity-driven water splitting and fuel cells, and the noble metal-based materials, e.g., Ir, IrO₂ and RuO₂, are deemed to be the most efficient OER catalysts currently [173, 174]. To improve their catalytic activity and utilization efficiency, creating ultrathin 2D nanostructures with ample mesopores is regarded as a promising strategy. For example, Jiang et al. [71] reported 2D mesoporous metallic Ir nanosheets for high-performance OER catalysts in acidic solutions (Fig. 18a). With ordered in-plane mesopores, highly exposed surface, and rich catalytically active sites, the mesoporous Ir nanosheets displayed small overpotential of 240 mV at 10 mA cm⁻² and a Tafel slope of 49 mV dec⁻¹ in a 0.5 M H₂SO₄ electrolyte, much higher than those of other Ir-containing catalysts (Fig. 18b, c). Further, developing highly active and durable OER electrocatalysts in all-pH electrolytes is also necessary. In this regard, our group reported a confined oxygenation technique to synthesize 2D defective RuO₂/graphene heterostructures (D-RuO₂/G) for pH-universal OER electrocatalysts [175]. As shown in Fig. 18d, the 2D D-RuO₂/G revealed ultralow overpotentials of 169 mV and 175 mV at 10 mA cm⁻² in H₂SO₄ and KOH electrolytes, respectively, surpassing a majority of the reported OER electrocatalysts (Fig. 18e, f). Combining the experimental results and theoretical calculations, it was concluded that the unprecedented electrocatalytic activity of the D-RuO₂/G was derived from its ultrathin thickness (9 nm), high SSA (125 m² g⁻¹), abundant mesopores, low Ru–O coordination number (5), and enriched –OH groups.

Fig. 18

Reproduced with permission from Ref. [71]. Copyright © 2020, Elsevier. d Sketch of 2D D-RuO₂/G heterostructures, and overpotential comparisons at 10 mA cm⁻² for 2D D-RuO₂/G and other reported excellent OER catalysts in e acidic and f alkaline electrolytes. Reproduced with permission from Ref. [175]. Copyright © 2020, Elsevier. g TEM image of CoCo-LDH nanomesh (the inset: the pore size distribution), h polarization curves, and i overpotentials at 10 mA cm⁻² for CoCo-LDH nanomesh and counterparts. Reproduced with permission from Ref. [73]. Copyright © 2019, Wiley–VCH

2DMMs for OER. a Sketch model of a mesoporous Ir nanosheet, b polarization curves, and c overpotential (η) comparison at 10 mA cm⁻² of mesoporous Ir nanosheets and counterparts.

Furthermore, earth-abundant transition metal oxides/hydroxides have been widely examined as alternatives for prevailing precious metal OER catalysts in alkaline media [173, 176]. As a typical case, 2D Co²⁺–Co³⁺ layered double hydroxide (CoCo-LDH) nanomeshes were synthesized for OER application (Fig. 18g) [73]. Prominently, the extraordinary 2D and mesoporous structure could greatly enhance the active site density and facilitate the mass diffusion, delivering a lower onset overpotential of 220 mV compared with non-mesoporous CoCo-LDH nanosheets (270 mV, Fig. 18h). Furthermore, its overpotential of 319 mV at 10 mA cm⁻² was also smaller than those of CoCo-LDH nanosheets, mesoporous single crystalline Co(OH)₂, single crystalline Co(OH)₂, IrO₂ and glassy carbon (GC) (Fig. 18i). These works open many opportunities for designing new 2DMMs for effective OER electrocatalysts.

4.3.4 Carbon Dioxide Reduction Reaction

The CO₂RR represents one of the most attractive and environmentally-friendly means to convert CO₂ into value-added chemical fuels (e.g., formate, CO, CH₄, C₂H₅OH and CH₃OH) [177,178,179,180]. Compared to gaseous products, formate is considered as high-value chemical raw materials and renewable energy carriers. Thus, the development of effective and highly selective CO₂RR electrocatalysts for formate production is urgent yet challenging. In recent years, 2DMMs with unique structures have also received comprehensive attention in this field. Owing to their intrinsic composition and structural merits of low-cost, earth-abundance, eco-friendliness, high SSA, rich active sites, and good catalytic activity [59, 181,182,183], 2D mesoporous SnO₂ nanosheets loaded on flexible carbon cloths (SnO₂/CC) were developed as an electrode for efficient and selective CO₂RR (Fig. 19a) [181]. As displayed in Fig. 19b, c, the SnO₂/CC electrode delivered unprecedented current density of ~ 45 mA cm⁻² at 0.88 V, high Faraday efficiency (FE) of (87 ± 2)%, outstanding flexibility, and superior durability. The good electrocatalytic performance was originated from the unique porous structure of SnO₂/CC, which offered large SSA and rapid mass transfer. In addition, 2D ordered mesoporous SnO₂ nanosheets (mSnO₂ NTs-350) were synthesized by the 2D mesopore dual-template method and a subsequent calcination (Fig. 19d) [59]. With a uniform thickness of 22.5 nm, hierarchical mesopores of 5 and 16 nm, a large SSA of 97 m² g⁻¹ and high crystallization, the mSnO₂ NTs-350 exhibited improved current density (8.3 mA cm⁻² at −1.3 V) and Faradaic efficiency of HCOOH (90.0% at −1.3 V) and C1 (97.4% at −1.2 V) products, compared to the as-made mesoporous SnO₂ nanospheres (as-mSnO₂ NPs) and nanosheets (as-mSnO₂ NTs) as well as most previously reported SnO₂ materials (Fig. 19e, f).

Fig. 19

Reproduced with permission from Ref. [181]. Copyright © 2017, Wiley–VCH. d TEM image of as-mSnO₂ NTs, e LSV curves, and f Faradaic efficiencies of HCOOH for mSnO₂ NTs-350, as-mSnO₂ NPs and as-mSnO₂ NTs catalysts. Reproduced with permission from Ref. [59]. Copyright © 2020, Wiley–VCH. g Polarization curves, h Faradaic efficiencies of HCOO⁻, H₂, and CO for mpBi nanosheets and commercial Bi nanopowders in N₂- or CO₂-saturated 0.5 M NaHCO₃, and i voltage change during a 3 h solar-driven CO₂RR/OER electrolysis (the inset: schematic diagram of the solar-driven full-cell electrolysis). Reproduced with permission from Ref. [184]. Copyright © 2018, Wiley–VCH

2DMMs for CO₂RR. a SEM image of mesoporous SnO₂ nanosheets, b Faradaic efficiency of the SnO₂/CC electrode for formate, CO, and H₂ at varying applied potentials, and c durability of the SnO₂/CC electrode under folding or twisting 10 times at 1.6 V.

Beyond Sn-based materials, traditionally under-explored Bi also reveals great potential as efficient CO₂RR catalysts for formate production. As exhibited in Fig. 19g, h, Yang et al. [184] prepared 2D mesoporous Bi (mpBi) nanosheets for superior CO₂RR activity with high cathodic current density (~ 18 mA cm⁻² at − 1.1 V), excellent Faradaic efficiency (~ 100%), and impressive stability (almost no decay for 12 h). Ultimately, the mpBi CO₂RR electrocatalyst was coupled with an Ir-based OER electrocatalyst to fabricate a full catalytic cell, which realized impressive solar-driven splitting of CO₂/H₂O to formate/O₂ (Fig. 19i). These works unveil the tremendous potential of 2DMMs for CO₂RR electrocatalysis and CO₂RR based full-cell electrolysis.

5 Summary and Outlook

In this review, the recent advances of 2DMMs (including in-plane mesoporous nanosheets and sandwich-like mesoporous heterostructures) for energy storage and conversion are systematically summarized. By synergistically combining the advantages of 2D materials with mesoporous structures, 2DMMs present significantly enhanced electrochemical behavior, e.g., acceleration of charge transfer, ion/mass transport and reaction kinetics. Significantly, the key chemical synthesis strategies of free-template, 2D-template, mesopore-template and 2D mesopore dual-template methods have been proposed to construct 2DMMs with diverse composition, configurations and mesopore geometries. Further, their wide range of key applications in supercapacitors (e.g., EDLCs, PCs, MSCs, high-voltage supercapacitors and flexible supercapacitors), rechargeable batteries (LIBs, SIBs, LMBs and SMBs), and electrocatalysis (HER, ORR, OER and CO₂RR) have been generalized. A particular emphasis has been given to the chemical synthesis mechanisms and synthesis–structure–property relationships for 2DMMs. However, several existing challenges still remain unsolved in 2DMMs for energy storage and conversion (Fig. 20).

Fig. 20

Existing challenges related to 2DMMs for energy storage and conversion

5.1 Chemical Design

Despite the exciting progress that has been made in the rational design of 2DMMs, their chemical composition and synthetic strategies are far behind the requirements. From the view of chemical composition, most recent studies of 2DMMs are focused on common polymers (e.g., PPy, PANI and polydopamine), carbon (including graphene), oxides (e.g., SiO₂, TiO₂) and their hybrids. The development of novel 2DMMs such as carbides, nitrides, sulfides, phosphides, selenides, and metal/covalent organic frameworks, with various single components or multiple components is crucial, which can produce new physicochemical properties for ESCDs. Further, the application-oriented design of chemical composition is extraordinarily essential for 2DMMs. Considering the ESCD applications, 2DMMs should possess both superior activity and high conductivity to satisfy electron–ion transport and stability for electrodes and devices. In this case, 2D sandwich-like mesoporous heterostructures coupled with highly active/functional components and conductive network are more promising for the near future. From the perspective of synthetic strategy, simpler and more universal methods are required for the controllable synthesis of 2DMMs with well-defined composition, configurations, sheet thickness and pore structures. To meet the industry or commercialization demands, the large-scale and low-cost preparation of high-quality 2DMMs is also of primary importance. Meanwhile, the overall factors of precursors and products, such as surface property, dispersion, solubility, formation temperature and acid/base resistance, should be considered for choosing suitable synthetic strategies to produce high-performance 2DMMs for different ESCDs.

5.2 Structural Engineering

Evident from a plethora of research work, the structural features including configuration, sheet thickness, lateral size, mesopore geometry and SSA, can dramatically affect the electrochemical performance of 2DMMs for ESCDs. First, the two different configurations of in-plane mesoporous nanosheets and sandwich-like mesoporous heterostructures provide opportunities to tune their properties to cater to various application requirements. For the materials with good conductivity (such as carbon, graphene, and metal), in-plane mesoporous nanosheets should be fabricated to achieve the significant improvement of SSA, active sites and ion transport without sacrificing their conductivity. For the materials with high activity and poor conductivity (e.g., metal oxides, sulfides, polymers), it is necessary to introduce conductive substrates or mesoporous conductive layers to construct sandwich-like mesoporous heterostructures, realizing enhanced conductivity and fully exerted activity. For materials with poor stability in water/air (e.g., MXenes, phosphorene), constructing a mesoporous protective layer on their surface is very important, which can not only inhibit the stacking of 2D substrates and increase their SSAs, but also improve the physicochemical stability without affecting their activity and mass transfer. Subsequently, the nanosheet thickness, lateral size, mesopore structure and SSA of 2DMMs should be precisely controlled to optimize their electrochemical performance. In general, the nanosheet thickness and flake size can be readily tuned by the amount of precursors and size of 2D templates. The mesopore geometry changes over the aggregation form of soft template molecules (e.g., spherical, rod-like, and worm-like micelles) or the morphology of hard template particles (sphere, cube, prism, etc.). The mesopore size can be well adjusted by altering the hydrophilic/hydrophobic chain length of the soft template molecules or the particle size of the hard templates. Finally, the structural engineering of 2DMMs can realize the fine adjustment of configuration, sheet thickness, lateral size, mesopore geometry and SSA independently or synergistically, which is beneficial to developing novel 2DMMs and efficiently regulating their energy-related performance.

5.3 Structure–Property Relationships

Based on current advancements, the sheet thickness, mesopore diameter, mesopore geometry and SSA of 2DMMs have demonstrated distinct influences on their electrochemical performance for ESCDs. (i) The thinner in-plane mesoporous nanosheets can increase the active sites and improve the activity of corresponding materials and electrodes, while the medium thickness of sandwich-like mesoporous heterostructures should be chosen to balance the content of functional components and 2D substrates and maximize their properties. (ii) With varying mesopore sizes of 2DMMs, the electrochemical performance for supercapacitors and batteries, as well as the catalytic activity for different electrocatalysis show irregular changes. But the smaller mesopore size should deliver higher electroactivity originating from the larger SSA and highly exposed active sites, when the other factors (e.g., the sheet thickness and active material content) are fixed. (iii) For different mesopore geometries, 2DMMs with vertical spherical pores usually exhibit larger SSAs and thus higher specific capacitance and energy density, while 2DMMs with parallel rodlike pores display better rate capability and power density for supercapacitors. Thus, one can design various mesoporous geometries to satisfy different application scenarios of ESCDs. (iv) The ordered or disordered mesostructures also have a great effect on actual electrochemical behavior of 2DMMs. In general, the disordered 2DMMs possess simpler preparation process, interconnected mesopores, larger SSAs and thus enhanced mass/ion transport, whereas the ordered mesopores can endow 2DMMs with controllable morphology, special functions (e.g., uniform redistribution of molecules/ions [82, 156]) and tunable electrochemical properties. Thus, the ordered 2DMMs hold more potential in the fields of materials science and energy-related applications. (v) The large ion-accessible SSA is able to boost the specific capacitance and catalytic activity of 2DMMs for supercapacitors and electrocatalysts, while the lower SSA is good for battery materials to prevent side reactions, electrolyte consumption and poor stability. (vi) It is worth noting that the high mesoporosity will not affect the packing density for electrodes and volumetric performance for devices (e.g., volumetric capacitance, energy density and power density). Therefore, 2DMMs are of great value for compact, stable, and high-performance ESCDs, especially MSCs with both high energy/power densities, non-noble metal-based catalysts and electrocatalysis with rapid reaction kinetics. Additionally, the synergistic combinations of macropores as solution reservoirs, mesopores as ion transfer channels, and micropores as ion confined spaces, are being developed and will be a future direction for practical ESCD applications [185]. In short, the fundamental understanding of structure–property relationships between material parameters and ESCD performance can efficiently guide the rational design, synthesis and applications of 2DMMs.

5.4 Mechanism Studies

On account of the synthesis–structure–property principles, the growth mechanism of 2DMMs and the electron–ion transport form and redox reaction theory for energy-related applications, are critically important but challenging. Therefore, advanced simulation techniques, such as density functional theory and finite element method calculations, should be used for the investigation of chemical bonding, precursor–template interaction, molecular assembly, ion adsorption and electron transport for 2DMMs in the synthesis and ESCD applications [82, 155]. Moreover, in-situ characterizations [e.g., SEM, TEM, atomic force microscopy (AFM), infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, XRD, XAS and electrochemical impedance spectroscopy] [186,187,188], can be applied to visualize the synthesis process and energy storage/conversion mechanism of 2DMMs through real-time tracking of microstructure evolution, chemical environment, and reaction dynamics. For example, the Li-ion redistribution function of mesoporous channels can be revealed by using in-situ TEM, AFM or Raman spectroscopy [41, 187]. Undoubtedly, the combination of theoretical calculations and in-situ characterizations can verify the structure–property relationships, and eventually establish surface and nano-electrothermy models for 2DMMs in energy storage and conversion.

In brief, the structural advantages, chemical synthesis strategies and energy-related applications of 2DMMs have been discussed in detail, disclosing the brilliant prospects and existing challenges of 2DMMs for ESCDs. However, 2DMMs are still at an early stage of development and the commercialization of 2DMM-based ESCDs has a long way to go. After comprehensively understanding and effectively addressing the unsolved issues in chemical design, structural engineering, structure–property relationships, and mechanism studies, we believe that 2DMMs can open a novel space in materials science and meet the practical demands of high-efficiency ESCDs in the near future.