MXene-based hydrovoltaic electricity generators and their coupling with other energy harvesting systems
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Abstract
MXenes, a class of two-dimensional transition metal carbides and nitrides, have garnered significant attention for their unique properties, making them promising candidates for next-generation energy harvesting technologies. Among these, emerging MXene-based hydrovoltaic electricity generators (HEGs), including moisture electricity generators, evaporation electricity generators, reverse electrodialysis electricity generators, and droplet electricity generators, have demonstrated exceptional performance in converting energy in environmental water such as moisture, water, wave, and droplets into electricity. Additionally, the synergistic coupling of MXene HEGs with other energy harvesting systems, such as triboelectric nanogenerators and thermoelectric generators, offers new avenues for enhancing power generation performance and expanding application scenarios. This review systematically examines the structures and properties of MXenes, their application in various HEGs, and the recent advancements in their integration with other energy harvesting systems. Furthermore, we discuss the challenges and future opportunities for MXene-based devices in multifunctional energy harvesting platforms.
Keywords
INTRODUCTION
With the intensification of the global energy crisis and environmental pollution, the development and utilization of sustainable clean energy have become imperative[1-11]. To date, various energy harvesting devices have been developed to capture and utilize green energy from nature. These include photovoltaic (PV) cells for harnessing solar energy, thermoelectric generators (TEGs) for harvesting temperature gradient energy, triboelectric nanogenerators (TENGs) for converting mechanical energy, microbial electrochemical systems (MESs) that employ microbial metabolism to generate bioelectricity or produce value-added chemicals, and, more recently, hydrovoltaic electricity generators (HEGs) that generate electricity through direct interactions between different forms of water and nanomaterials[12-16]. However, devices such as PV cells, TEGs, TENGs and MESs often face limitations related to time or space. For instance, PVs struggle to operate during nighttime or cloudy conditions, while TEGs are typically constrained to environments with significant temperature differentials, such as industrial waste heat or automobile exhaust. In contrast, HEGs stand out due to their ability to utilize water in diverse forms through different mechanisms, enabling them to generate electricity without being restricted by time or location[17-21]. This makes HEGs a promising candidate for long-term and versatile energy harvesting applications.
HEGs, named in analogy to PV, combine "hydro" referring to water, and "voltaic" denoting the generation of electricity. As their name implies, they specifically refer to "the generation of electricity through direct interactions between nanomaterials and various forms of water". Water exists in multiple forms, including moisture, flowing water, waves, droplets, and water with varying salinity, all of which can be utilized for electricity generation. To date, four representative types of HEGs have been developed to exploit these different forms of water: moisture electricity generators (MEGs), evaporation electricity generators (EEGs), electrodialysis electricity generators (REGs), and droplet electricity generators (DEGs) [Figure 1]. The theoretical and experimental foundations of hydrovoltaic phenomena can be traced back over a century. For instance, the observation of streaming potential in 1859 laid the groundwork for modern EEGs. Similarly, the concept of reverse electrodialysis (RED), proposed in 1945, provided the theoretical basis for REGs. Although the principles behind HEGs have a long history, progress in this field remained slow until the development of nanomaterials and nanotechnology in the 21st century, which spurred rapid advancements and broader interest. The term "hydrovoltaic" was only introduced in 2018 to unify and describe devices capable of directly generating electricity from water in various forms. This term has since gained widespread recognition. Unlike traditional large-scale hydroelectric power plants, which rely on gravitational potential energy and classical electrodynamics to indirectly generate electricity through electromagnetic effects, HEGs are typically small, lightweight, and potentially wearable, making them highly promising for diverse applications. However, their current power generation efficiency remains low, significantly limiting their further development and practical implementation.
Figure 1. The overall outline of the review.
To enhance the power generation performance of HEG devices, various active materials have been explored in device design, including inorganic nanomaterials such as graphene oxides (GO), MXene, and SiO2, natural biomaterials such as cellulose and proteins, and polyelectrolytes such as Poly(styrene sulfonic acid) (PSSA) and Poly(diallyldimethylammonium chloride) (PDDA)[22]. Among these, two-dimensional (2D) MXene materials have emerged as particularly promising for HEGs due to their unique advantages, including tunable chemical compositions, modifiable surface structures, easily adjustable porous nanochannels, large specific surface area, and excellent conductivity[23-25]. The ultrathin nanosheets of MXene, with their atomic-level thickness, expose all their surfaces, significantly increasing the interaction area with water molecules. Their adjustable chemical compositions and surface structures enhance interfacial interactions with water, improving charge separation capabilities. Additionally, their controllable assembly enables the creation of more nanochannels, accelerating ion diffusion. Furthermore, their relatively high conductivity facilitates stronger interactions between electrons and ions. These characteristics make 2D MXene highly sensitive to water molecules, with enhanced water-solid phase interactions that improve energy conversion efficiency and significantly boost the power generation performance of HEGs.
Moreover, coupling HEGs with other energy harvesting technologies in the design of MXene-based systems can further enhance the utilization of environmental energy for better power generation [Figure 1]. This approach not only complements and significantly improves power generation performance but also expands the scope of potential functional applications. For instance, integrating HEGs and PVs can substantially increase the efficiency of environmental energy utilization. Combining HEGs with TENGs holds promise for applications in self-powered multimodal sensors and personalized healthcare. Meanwhile, coupling HEGs with TEGs could enable dual-energy recovery from waste heat and exhaust gases in industrial plants and automobiles. These synergistic strategies provide exciting opportunities for versatile and efficient energy harvesting systems.
From the above analysis, MXene materials exhibit tremendous potential for applications in HEG devices. However, a comprehensive review of MXene in this field is currently lacking. Therefore, this review aims to summarize the latest progress of MXene in the domain of HEGs [Figure 1]. We will begin by discussing the structural characteristics and physicochemical properties of MXene. Following this, we will explore its applications in four key areas: MEGs, EEGs, DEGs, and REGs. For each category, we will delve into the power generation mechanisms, the roles of MXene, and emphasize the structure-function relationships within HEG devices. Subsequently, we will highlight recent advancements in coupling MXene-based HEGs with other energy harvesting systems. Finally, the review will conclude with a summary of the key insights and a forward-looking discussion on promising future research directions.
OVERVIEW OF MXENES
Structures of MXenes
MXene was first reported in 2011 by Professors Naguib et al., and their collaborators at Drexel University[26]. The earliest identified MXene material is Ti3C2Tx, which remains the most widely used due to its high electrical conductivity and well-established synthesis methods[27-30]. MXenes represent a family of 2D layered materials composed of transition metal carbides, nitrides, and carbonitrides. Their general chemical formula is Mn+1XnTx, where n ranges from 1 to 4, M denotes an early transition metal such as Ti, Zr, V, Nb, or Mo, X represents either carbon (C) or nitrogen (N), and Tx refers to surface terminations. These terminations can include oxygen (O), hydroxyl (OH), amines, halogens, chalcogens, and, more recently, antimony (Sb) and phosphorus (P) [Figure 2A and B].
Figure 2. Structures and properties of MXene. (A) Elements constituting MXene. Reproduced with the permission from Ref.[28] Copyright 2022 Springer Nature. (B) Atomic structure model of MXene. Reproduced with the permission from Ref.[29] Copyright © 2023 American Chemical Society. (C) Schematic of MXene synthesis. Reproduced with the permission from Ref.[30] Copyright © 2021 The American Association for the Advancement of Science. (D) Properties of MXene.
Given the wide range of transition metals, carbon and nitrogen options, four archetypical MXene structures, and various monatomic surface terminations, over a thousand stoichiometric compositions are theoretically possible. To date, more than 50 distinct MXenes have been reported. When considering solid solutions with varying element ratios and diverse surface terminations, which significantly influence MXene properties, the total number of experimentally synthesized MXenes likely approaches a hundred. The diverse composition provides a rich library of materials for exploring solid-liquid-gas interfacial interactions and constructing various HEG structures.
Preparation of MXenes
The synthesis methods of MXenes are generally classified into two major categories: the top-down method and the bottom-up method [Figure 2C][30,31]. The top-down approach, which is the most widely used for MXene synthesis, involves selectively etching the A-layer elements from MAX phase materials to produce MXene nanosheets with rich surface chemistry. This section focuses on this widely adopted method. In contrast, the bottom-up approach has emerged more recently and is primarily based on chemical vapor deposition (CVD). However, research and practical applications of this method remain limited. Therefore, this section will provide a brief overview of the top-down synthesis of MXenes.
The top-down synthesis of MXenes typically involves three key steps: (1) Preparation of MAX Phase Precursors. The synthesis of MAX phase materials is a prerequisite for the top-down method. MAX phases are layered materials with the general structure Mn+1AXn, where n = 1-4, M is a transition metal, A is a group IIIA or IVA metal (e.g., aluminum or silicon), and X is carbon or nitrogen. MAX phases are commonly synthesized through solid-state reactions; (2) Selective Etching of A-layer Atoms. This step is the core of the top-down process, where the A-layer elements are selectively removed to produce weakly bonded MXene multilayers. Two primary etching routes are commonly employed: Fluoride ion-containing acids: These acids selectively etch the A-layers, resulting in multilayered MXene particles or in situ delaminated 2D flakes. Molten salts: MAX phases are etched in molten salts, typically yielding multilayered MXene particles[32]; (3) Delamination of Mn+1XnTx Multilayers into Single Sheets. A common approach is the intercalation of molecules or ions to expand the interlayer spacing and weaken interflake interactions, followed by agitation or sonication to exfoliate the layers into a stable colloidal solution of single- to few-layered MXenes. These obtained single-layered or few-layered structures lay the foundation for further modification and the assembly of HEGs with diverse architectures.
Properties of MXenes
MXene, with its unique advantages in electrical conductivity, hydrophilicity, specific surface area, and mechanical properties, demonstrates significant potential for applications in HEG materials and devices. Additionally, its distinctive photothermal properties open new possibilities for enhancing HEG performance and integrating with other energy harvesting systems [Figure 2D][33,34].
In terms of electrical conductivity, MXene exhibits exceptionally high conductivity as high as 20,000 S cm-1, primarily attributed to the d-electron orbitals of its transition metal elements, which confer metallic-like conductive properties[35,36]. Moreover, its conductivity can be tuned by modifying its surface terminations. These terminations influence the density of states (DOS) and shift the Fermi level, enabling MXene to transition from a conductor to a semiconductor[30,37]. This tunability enhances the versatility of MXene, making it not only suitable as an electrode material but also as an active material in HEGs[38].
Secondly, MXene exhibits strong hydrophilicity, due to its surface polar groups, such as -OH, -O, and -F. These functional groups not only enhance MXene’s hydrophilicity but also ensure its stable dispersion in aqueous solutions. This greatly improves its processability, enabling the fabrication of various materials with different dimensions, including fibers, films, and aerogels, thereby broadening its application scenarios[39]. More importantly, these surface functional groups also impart a high zeta potential (approximately -30 mV) to MXene, facilitating strong interactions with water molecules. This enhances the adsorption of water molecules on its surface and promotes efficient charge separation. Additionally, when MXene membranes are used in REGs, their charged surfaces endow them with excellent ion selectivity for high-performance osmotic energy harvesting.
Additionally, the single-layered or few-layered MXene units provide an abundance of exposed surface area, while their controllable assembly prevents layer stacking and generates hierarchical pore structures with a large specific surface area. Such structure significantly increases the number and efficiency of interaction sites between MXene and water molecules, thereby enhancing charge generation capacity. Moreover, the hierarchical channels within the structure facilitate the rapid diffusion of water molecules and ions, significantly improving charge transfer efficiency. As a result, these characteristics enable MXene to excel in HEG applications.
Furthermore, MXene possesses exceptional mechanical flexibility. Thanks to its graphene-like 2D layered structure, MXene can withstand bending, stretching, and other deformations without losing its functional properties. This flexibility makes it particularly well-suited for use in wearable devices or integration onto flexible substrates, allowing it to adapt to a wide range of application scenarios[40-42].
RESEARCH PROGRESS OF MXENE-BASED HEGS
MXene-based MEGs
Mechanism of MEGs
Moisture, the gaseous form of water, is ubiquitous and abundant on Earth, comprising approximately one-tenth of the total water content of all the planet’s lakes. MEGs can adsorb moisture from the air and convert it into electrical energy, garnering significant interest over the past decade. In 2015, Zhao et al. pioneered the development of a GO-based MEG and investigated the underlying mechanism of moisture-to-electricity conversion[43]. Since then, extensive advancements have been made in the design and development of MEG devices, incorporating diverse active materials (e.g., inorganic materials, organic materials, organic-inorganic hybrid materials), device dimensions (e.g., fibers, films, hydrogels, and aerogels), and device structures (e.g., sandwich-type, bilayer-type, and asymmetric-type configurations).
MEGs operate based on ion concentration gradient diffusion, a widely accepted mechanism. To create such a gradient, two primary strategies are commonly employed: one involves directly constructing a functional group gradient within the power-generating material, while the other relies on directional water infiltration into the material. The latter approach has garnered broader research attention due to its simplicity in both methodology and structure and the wide availability of compatible materials. In this review, we will illustrate the power generation mechanism using the most prevalent design, specifically sandwich-structured MEG devices [Figure 3A][44]. These devices typically feature a top electrode that is either a mesh or partially covers the middle active material, while the bottom electrode completely covers the active material [Figure 3A]. To achieve asymmetric moisture adsorption, the bottom and sides of the device are generally encapsulated, leaving only the top exposed to air for moisture adsorption. A sandwich-structured MEG, utilizing sulfonated polyacrylamide (PAM) hydrogel as the electricity-generating functional layer, is presented as a representative example to elucidate the detailed power generation process [Figure 3B], which can be distinctly divided into three sequential steps[45]: (1) Moisture adsorption. The excellent water absorption of hydrogel, due to the introduction of LiCl, allows a significant number of water molecules to adsorb onto its upper surface. These water molecules then permeate into the hydrogel’s interior through swelling; (2) Ion dissociation. The sulfonic acid groups on the polymer chains exhibit strong ionization in the presence of water, leading to the dissociation of abundant H+ ions. Due to the asymmetric moisture adsorption structure, H+ ions predominantly accumulate on the top surface of the hydrogel, creating a vertical ion gradient; and (3) Ion migration and charge separation. The accumulated H+ ions begin to diffuse automatically toward the bottom side of the hydrogel driven by the ion concentration difference. When an external circuit is connected, the internal ion diffusion generates an electric current, enabling the conversion of moisture into electrical energy. Notably, in this system, Li+ ions play a crucial role by breaking the hydrogen bonds within the polymer chains and creating additional ion transport pathways via the Hofmeister effect, thereby accelerating the diffusion of H+ ions to achieve better output performance.
Figure 3. Working mechanism of MEGs and typical MEGs using MXenes as active materials. (A) Typical sandwich-type device structure of MEG. Reproduced with the permission from Ref.[44] Copyright © 2023 Elsevier Ltd. (B) Schematic illustration showing the power-generation processes in a representative MEG. Reproduced with the permission from Ref.[45] Copyright © 2023 Wiley-VCH. (C) Schematic illustration of the preparation of MC and SCS fabrics. (D) Voc and Isc curves of Al-MC-SCS MEG tested over
For MEG devices, the number of dissociated ions generated by the active material and their diffusion efficiency within the porous network of the active material play a pivotal role in determining power generation performance. MXene, with its abundant oxygen-containing functional groups, can dissociate a significant number of mobile hydrogen ions upon contact with water molecules, serving as effective charge carriers for MEG devices. Moreover, the assembly of MXene can generate a highly porous structure that facilitates ion diffusion, significantly enhancing its power generation performance. These unique properties position MXene as a highly promising active material for MEG applications. Table 1 provides a detailed summary of the MEG properties of recently reported MXene-based MEGs and highlights the specific roles MXene plays in their operation.
Summary of recent reported MXene-based MEGs
| Functional layers | Electrodes | Roles of MXene | Area (cm-2) | Stabilized Voc (V) | Stabilized Jsc (µA cm-2) | Lasting time | Maximum power density (µW cm-2) | RH (%) | Temperature | Ref. |
| MXene cotton + SnO/SnO2/carbonized silk | Al/Carbon | Proton dissociation | 1.5 | 0.932 | 289 | 20,000 s | 26.66 | / | Room temperature | [46] |
| MoS2/MXene Cotton + (MoS2)-carbonized silks | Al/Carbon | Proton dissociation | 4 | 1.16 | 170 | 25,000 s | 32.26 | / | Room temperature | [49] |
| CMC/MXene/LiCl aerogel | Steel/Steel | Proton dissociation | / | 0.12 | / | / | / | 77 | 25 °C | [57] |
| Cellulose/MXene/PSSA film | Ag/Ag | Proton dissociation | 1 | 0.3 (peak) | 1.2 (peak) | 100 s | 81.2 uW/cm-3 | 80 | 20 °C | [55] |
| CMC/MXene/Al3+ composite film | Cu/Cu | Proton dissociation | 14 | 0.055 | / | 1,000 s unchanged (Voc) | / | 90 | Room temperature | [56] |
| CNF/MXene/CMC composite film | Cu/Cu | Proton dissociation | 4.5 | 0.167 | / | / | / | / | Room temperature | [58] |
| Nb2CTx/sodium alginate composite film | Cu-Ni/Cu-Ni | Proton dissociation | 0.5 | 0.5 | 1.6 | 6 h unchanged (Voc) | 0.5 | 91.5 | Room temperature | [50] |
| PAM hydrogel + MXene aerogel | Cu/Cu | Proton dissociation | 1 | 0.6 | 1,160 (peak) | 500 h unchanged (Voc) | 24.8 | 20 | 20 °C | [47] |
| PAM organic-ionic hydrogel + MXene sponge | Ag/Ag | Proton dissociation | 4 | 0.3 | 450 | 30 min unchanged (Voc) | 9.15 | 20 | 20 °C | [51] |
| PANI-PVA-MXene films | CNT/CNT | Proton dissociation | 2 | 0.4 | 1,800 (peak) | 300 s unchanged (Voc) | 3 | / | Room temperature | [48] |
| TiO2@Ti3C2 film | Ni/CNT | Precursor of TiO2 | 1 | 0.8 | 50 | 1,000 s unchanged (Voc) | / | 90 | Room temperature | [52] |
| MXene-CNCs-tamarind gum-PAM hydrogel | Cu/Cu | Conductivity | 5 | 0.164 | / | / | / | / | Room temperature | [53] |
| Agar/PDACl/MXene + Agar/PSSNa/MXene organohydrogel | Cu/Cu | Conductivity | 0.785 | 0.068 | / | 30 min unchanged (Voc) | / | 99 | 25 °C | [54] |
Advances in MXene-based MEGs
To date, MXene has been utilized in MEG devices in multiple roles - not only as an active electricity-generating material but also as a precursor for deriving other highly active MEG materials. Additionally, it has been employed as a conductive additive to reduce internal resistance. To enhance the power generation performance of MXene-based MEGs, various device architectures have been developed, including simple sandwich structures, bilayer structures with a water-absorbing layer serving as a reservoir, and asymmetric structures featuring dual-ion gradients. Based on the unique characteristics of these architectures, diverse applications have been explored, with representative examples including power supply units, self-powered sensors, and actuators.
In a simple sandwich-structured MEG device, MXene sheets can serve directly as an active material for dissociating mobile H+ ions to establish ionic gradients[46,49,50]. For example, Luo et al. reported a fabric-based MEG device comprising MXene-coated cotton (MC) fabrics and SnO/SnO2-grown carbonized silk (SCS) fabrics [Figure 3C][46]. The device configuration featured an active Al electrode as the top electrode, with SCS fabrics serving as the bottom electrode. MC fabrics acted as the active layer responsible for building proton gradient. In addition, the negatively charged pores in the SCS layer facilitate ion selectivity and migration, while the active Al electrode generates more mobile Al3+ ions through chemical reactions. These factors result in the Al-MC-SCS MEG exhibiting a high open-circuit voltage (Voc) of 0.934 V and a high short-circuit current (Isc) of 0.434 mA [Figure 3D].
Although the simple sandwiched MEGs can achieve exceptionally high Voc and Isc outputs, their performance is highly susceptible to interference from relative humidity (RH) and demonstrates minimal electrical output under low-humidity conditions. To overcome this limitation, bilayer structure is introduced, which includes a water-absorbing layer that acts as a reservoir to mitigate the impact of external humidity fluctuations and a functional layer containing MXene to enable efficient charge separation and ion diffusion. Zhao et al. developed a humidity-resistant bilayer MEG device comprising MXene aerogel and a PAM ionic hydrogel [Figure 3E][47]. In this design, the MXene aerogel serves as the electricity-generating layer, while the underlying PAM hydrogel acts as a water reservoir. As water from the PAM ionic hydrogel infiltrates MXene channels, hydrolysis of oxygen-containing groups renders them negatively charged. This selective transport allows only cations to pass through, leading to charge accumulation at the top and bottom of the MXene aerogel, forming an internal electric field [Figure 3E]. Based on this bilayer structure, the MEG maintains a stable voltage across a wide RH range [Figure 3F]. Similarly, Kim et al. introduced a bilayer moisture-resistant MEG comprising a MXene-coated sponge and PAM hydrogel[51]. Due to the excellent flexibility and stretchability of the sponge and hydrogel, along with their strong interfacial bonding, the MEG maintains a stable Voc output under various mechanical deformations.
Besides, fabricating asymmetric structures with dual-ion gradients has also proven to be an effective strategy for enhancing the performance of MXene-based MEGs. For example, Li et al. reported an asymmetric MEG consisting of negatively charged MXene (NCM) and positively charged polyaniline (PANI) membranes
Additionally, MXene can serve as a precursor for synthesizing TiO2 with MEG activity. For instance,
Figure 4. MXene as precursor and conductive additive for MEGs. (A) MD calculations displaying the process of water molecules absorption on (101) crystal planes of TiO2. (B) Schematic illustration showing triple roles of water molecules during power generation. Reproduced with the permission of Ref.[52] Copyright © 2024 Wiley-VCH. (C) Schematic illustration of the double-network structure of MCTP hydrogel. Reproduced with the permission of Ref.[53] Copyright © 2021 Elsevier B.V.
MXene can also function as a conductive additive to reduce the internal resistance of the device, thereby enhancing its responsiveness to humidity[53,54]. He et al. developed a multifunctional conductive double-network (DN) hydrogel for wearable strain sensors and MEGs[53]. This DN hydrogel consists of ionically crosslinked tamarind gum networks and covalently crosslinked PAM networks [Figure 4C]. MXene/CNC composites were incorporated as conductive additives, as CNCs effectively disperse MXene nanosheets. The resistance of the DN hydrogel was sensitive to various external stimuli. Under humidity stimulation, the MEG generates a peak Voc output of 164 mV.
Applications of MXene-based MEGs
Benefiting from the material modifications and device design, MXene-based MEGs exhibit diverse application potential [Figure 5]. MXene-based MEGs can serve as power sources for driving small electronic devices, such as calculators, alarm clocks, and light-emitting diode (LED) lamps, with their output performance enhanced by connecting multiple units in series and parallel [Figure 5A-C][47]. Additionally, due to the high moisture absorption and sensitivity of MXene-based MEG devices to external humidity, they have been explored for potential applications in self-powered sensors[47], actuators[47,56,58], and water-collection devices[57].
Figure 5. Diverse applications of MXene-based MEGs. (A-C) Flexible and wearable power source to drive small electronic devices, including (A) calculator, (B) clock and (C) LED lights. (D-F) Self-powered sensors for healthcare monitoring, including (D) wearable strain sensing, Reproduced with the permission of Ref.[47] Copyright © 2023 American Chemical Society. (E) finger humidity sensing, and (F) respiration humidity detection. Reproduced with the permission of Ref.[52] Copyright © 2024 Wiley-VCH. (G) Schematic illustration of the structure of MCPM film and corresponding humidity-driven actuating. Reproduced with the permission of Ref.[55] Copyright © 2021 American Chemical Society. (H) The bending-unbending mechanism of CMC/MXene/Al3+ composite film under asymmetric RHs. Reproduced with the permission of Ref.[56] Copyright © 2022 Elsevier B.V. (I) Schematic illustration of MXene@LiCl aerogel. (J) Schematic illustration of water absorption mechanisms. Reproduced with the permission of Ref.[57] Copyright © 2024 American Chemical Society.
When an MEG device is attached to the body, it can monitor the degree of finger flexion in real time
For moisture-driven actuators, Li et al. developed a multifunctional MXene/cellulose/Polystyrene sulfonic acid membrane (MCPM) capable of both moist-electric generation and humidity-driven actuation
MEGs are typically accompanied by moisture adsorption during the power generation process, wherein gaseous water is converted into liquid water stored within the active material. This process opens up a novel application field for MEG devices: the integration of power generation and water harvesting to achieve hydropower cogeneration[59-64]. Cai et al. demonstrated this potential with a vertically channeled MXene@LiCl aerogel designed for water harvesting [Figure 5I][57]. LiCl was uniformly dispersed within the aerogel walls without disrupting its structure, significantly enhancing water adsorption. At 90% RH, it achieved a water collection efficiency of 3.12 g g-1. This improvement stems from LiCl hydration, forming LiCl·H2O, which lowers the water adsorption barrier and facilitates migration. Combined with hydrophilicity of MXene, this dual mechanism enables exceptional water collection [Figure 5J]. Besides water harvesting, the MXene@LiCl aerogel serves as an active MEG material, generating a Voc of 0.12 V at 77% RH. This dual functionality enables hydropower cogeneration, highlighting its multifunctional potential.
In summary, MXene materials are mainly used as active layers in MEG devices. To enhance power generation performance and stability, various strategies have been employed, including introducing negatively charged ion channels, constructing asymmetric structures, designing bilayer systems for separate water collection and power generation, and electrode modulation. Nevertheless, the performance of current MXene-based MEGs remains significantly limited compared to those utilizing other materials. Most of the existing strategies focus on device design, while research involving the chemical modification of MXene itself is relatively scarce. Therefore, future studies should place greater emphasis on this aspect, aiming to enhance MEG performance through more fundamental material-level design. Additionally, long-term stability assessments of MEG devices, spanning several weeks or months, remain rare, despite their critical importance for practical applications. This issue warrants substantial attention and further investigation.
MXene-based EEG
Mechanism of EEG
Water evaporation is a ubiquitous and continuous natural phenomenon, where the phase transition from liquid to gas occurs spontaneously by absorbing ambient thermal energy. It was not until 2017 that
We will use this EEG device as an example to elaborate on its structural design and power generation mechanism. In this device, a rectangular carbon black (CB) film was deposited onto an insulating substrate, with multi-walled carbon nanotubes (MWCNTs) positioned at both ends of the CB film as electrodes. The entire device was partially immersed in bulk water [Figure 6A][65]. On the water-contacting side, liquid water infiltrated the CB film through capillary action, allowing water molecules to flow through the porous network formed between CB flakes. This natural evaporation process ensured a continuous capillary flow, enabling long-term operation with a sustained output of a Voc of up to 1 V and a Isc of 150 nA.
Figure 6. Working mechanism of EEGs and representative MXene-based EEGs. (A) Schematic illustration of typical experimental setup for EEGs and water flow between carbon black nanosheets induced by evaporation. Reproduced with the permission of Ref.[65] Copyright © 2017 Springer Nature. (B) Working mechanism of EEG device. Reproduced with the permission of Ref.[44] Copyright © 2023 Elsevier Ltd. (C) Photograph showing the operation of MXene-based EEG induced by droplets. (D) Voc, Isc, and resistance curves of MXene-based EEG with one water drop wicking. (E) Schematic illustration of working mechanism including rapid wicking and slow diffusion. (F) Two typical Isc peaks initiated by wicking and diffusion. (G) Surface charge density of water molecules and different ions on the surface of MXene sheets calculated by DFT. Reproduced with the permission of Ref.[67] Copyright © 2022 Royal Society of Chemistry. (H) Schematic illustration of device structure of MPCF and corresponding channel’s structure. (I) Schematic of the upstream proton diffusion mechanism. (J) MD simulations for protons dissociation and diffusion in the MXene channel. Reproduced with the permission of Ref.[69] Copyright © 2024 Springer Nature.
Studies have shown that the working mechanism of EEGs is closely related to streaming potential. As early as 1859, Quincke experimentally observed that when an electrolyte flows through a narrow channel under pressure, a potential difference - known as the streaming potential - is generated. The power generation mechanism of EEGs is highly analogous to this phenomenon. In EEGs, continuous water evaporation and capillary action drive water flow through the porous channels formed by the stacked nanomaterials. Due to surface functional groups or electrostatic interactions, these nanomaterial-based porous channels are typically surface-charged, leading to the formation of an electric double layer (EDL) at the water-channel interface [Figure 6B][44]. Within the diffuse layer of the EDL, a large number of counterions accumulate, while co-ions are nearly absent. As water evaporates, counterions undergo directional migration, inducing an electric current in the external circuit.
Similar to carbon-based materials, MXene nanosheets feature surfaces rich in oxygen-containing functional groups, enabling the formation of EDLs when water molecules flow across them[66]. This makes MXene an excellent candidate for EEG applications. Table 2 summarizes the performance metrics of MXene-based EEG devices reported in recent years and highlights the critical role played by MXene in their operation.
Summary of reported MXene-based EEGs
| Functional layers | Electrodes | Roles of MXene | Area (cm-2) | Stabilized Voc (V) | Stabilized Jsc (µA cm-2) | Lasting time | Maximum power density (µW cm-2) | Conditions | Temperature (°C) | Ref. |
| Ti3C2Tx/polyaniline-coated fabric | Cu/Cu | EDL effect | 3.5 | 0.69 (peak) | 2,157 (peak) | 2,500 s | 371 | 1 M NaCl | / | [67] |
| MXene-coated fabric | Zn/Zn | EDL effect | 2 | 0.687 (peak) | 1,500 (peak) | 240 s | 684 | Sweat | / | [68] |
| GO/CNT/Ti3C2TX-coated fabric | Ni/Ni | EDL effect | 30 | 0.25 | 17.3 | 600 s unchanged (Voc and Isc) | 2.08 | 3.5 wt% NaCl | Room temperature | [66] |
| PVA/MXene film | Pt/Pt | Proton dissociation | 1.5 | 0.4 | 2.67 | 300 min (Voc) | / | DI water | 25 °C | [69] |
| Ti3C2TX-coated fabric | Fe/Fe | EDL + photothermal effect | 72 | 0.33 | 0.118 | 3,000 s unchanged (Voc) | / | 3.5 wt% NaCl | Room temperature | [72] |
| Ti3C2TX/GQDs aerogel | Pt/CNT | EDL + photothermal effect | 0.25 | 0.8 | 184 | 12,000 s unchanged (Voc and Isc) | 45.6 | 3.5 wt% NaCl | Room temperature | [74] |
| NaV3O8/Ti3C2TX film | Au/Au | EDL + Photothermal effect | 4 | 0.8 | 14.5 | 250 s unchanged (Voc and Isc) | 2.6 | 0.1 M NaCl | Room temperature | [73] |
| MXene/GO@PVDF/SA film | Ag/Ag | EDL + Photothermal effect | 0.28 | 0.218 | 0.85 nA | 400 s unchanged (Voc and Isc) | / | 3.5 wt% NaCl | 25 °C | [70] |
| Biological fibrils/Ti3C2Tx aerogel | Pt/Pt mesh | EDL + photothermal effect | 0.785 | 0.35 | 7.64 | 200 min unchanged (Voc) | / | 10-3 M NaCl | 25 °C | [71] |
Advances in MXene-based EEG
Owing to the sensitive response of Mxene to water molecules, the constructed EEGs are not only capable of generating electricity when immersed in conventional liquid water but can also directly harvest energy from water droplets and sweat. This capability allows MXene-based EEGs to overcome the dependency on large volumes of liquid water, significantly expanding their potential applications, particularly in flexible and wearable sensor devices. Furthermore, MXene exhibits excellent photothermal effects, enabling enhanced water evaporation efficiency under light irradiation, which further improves power generation performance.
Several planar MXene-based EEGs utilizing water droplets as the water source have been developed, demonstrating unique power generation performance and promising application potential. These droplets first wick into one side of the active layer and gradually diffuse to the other side via capillary action and water evaporation. During this process, streaming potential is generated as water molecules flow along the surface of MXene nanosheets. Bae et al. reported a high-performance Ti3C2Tx-based EEG, where Ti3C2Tx nanosheets coated on cotton serve as the electricity-generating layer [Figure 6C][67]. As shown in Figure 6D, the EEG achieved a maximum Voc of 0.24 V and Isc of 120 μA as a 30 μL water droplet wicked and diffused through MC. The current profile exhibited two peaks: a large initial peak during rapid wicking due to hydrophilicity and capillary channels of Ti3C2Tx, and a smaller peak at the drying stage, attributed to reduced resistance from water molecule removal
Unlike the streaming potential-based power generation mechanism observed in all previous EEG devices, which relies on downstream ion diffusion, a novel upstream proton diffusion mechanism has been discovered in MXene-based EEG devices. This device utilizes an MXene/PVA composite film (MPCF) as the power-generating material, with two inert Pt electrodes positioned at opposite ends of the MPCF[69]. When 5 μL of water is applied to one electrode, it exhibits a higher potential of 400 mV for over 330 min, opposite to conventional EEG behavior [Figure 6H]. Unlike fast wicking of MC, MPCF shows slower water permeation due to dense PVA molecules impeding diffusion through nanopores. Water entering MXene channels releases protons, creating a proton gradient that drives protons back into bulk water [Figure 6I]. When connected, electrons flow from the dry to the wet Pt electrode, completing the circuit. MD simulations confirmed this proton diffusion mechanism [Figure 6J], revealing a novel upstream proton transport in MXene/PVA films and offering new insights into EEGs.
To further enhance the power generation performance of EEGs, the photothermal effect of MXene was integrated into the device, thereby enhancing water evaporation rates and improving power generation efficiency[70,71]. More interestingly, when seawater is used as the working fluid, this device not only generates electricity but also enables seawater desalination. Inspired by tree water transport and leaf evaporation,
Figure 7. Photothermal effect-enhanced EEG devices. (A) Schematic illustration of pump-inspired MXene-based EEG. Reproduced with the permission of Ref.[72] Copyright © 2021 American Chemical Society. (B) Schematic illustration of photothermal effect-enhanced MXene/SVO EEG. Reproduced with the permission of Ref.[73] Copyright © 2024 American Chemical Society. (C) Schematic diagram of the working process of IEHVG. (D) Device structure of IEHVG. (E) The comparison of positive ion concentration for IEHVG and IENG. (F) Schematic representation of the hydrovoltaic contribution in the IEHVG system. Reproduced with the permission of Ref.[74] Copyright © 2024 Wiley-VCH.
The biomimetic multilayer design of EEGs, combined with the integration of photothermal effects, holds great potential for further enhancing the power generation performance of the device. Inspired by lotus leaves, Chen et al. developed an interfacial evaporation-driven hydrovoltaic generator (IEHVG) with a multilayered biomimetic structure [Figure 7C][74]. The device consists of three layers: a bottom aerogel with vertically aligned channels for rapid water diffusion, a middle MXene/graphene quantum dots (GQD)-coated aerogel for electricity generation, and a top octadecyltrichlorosilane (OTS)-coated gradient hydrophobic layer to enhance evaporation [Figure 7D]. The MXene/GQDs layer provides high electronegativity, photothermal properties, and conductivity, ensuring strong power generation. The OTS layer creates a thermal and ionic concentration gradient, confirmed by numerical simulations and experiments [Figure 7E]. The high energy-harvesting efficiency of the IEHVG stems from rapid evaporation, the EDL effect, and gradient thermal diffusion, with the contribution of each factor summarized in Figure 7F.
MXene-based EEG devices have been extensively studied across various aspects, including material composition, power-generating water sources, multilayer structural designs, novel power generation mechanisms, and photothermal coupling, yielding significant research progress. However, it is important to note that most EEG devices still rely on large volumes of liquid water as the primary power-generating source, which poses a major limitation for practical applications.
MXene-based REGs
Working mechanism of REGs
The osmotic pressure difference between river water and seawater is a promising renewable energy source, known as osmotic energy, first proposed for power generation by Loeb in 1975. The RED method is one of the most promising approaches for harnessing osmotic energy[75]. Devices that operate on the RED principle are referred to as REGs.
An REG system consists of selectively permeable membranes placed between two salt solutions with different concentrations, connected to an external circuit via non-polarizable Ag/AgCl electrodes. Within this salinity gradient, ions from the high-concentration solution migrate naturally toward the low-concentration solution. When ions selectively diffuse through the membranes, osmotic potential energy is released, captured, and converted into electrical energy. However, if the channel lacks ion selectivity for cations and anions, the net ion flux will be zero, as the flows of cations and anions cancel each other, resulting in no current generation. This highlights two critical requirements for REG electricity generation: (1) A salinity gradient between the solutions; and (2) Differential diffusion rates for cations and anions. The channels of selectively permeable membranes are the primary determinants of REG performance.
Enhancing power output requires improving both ion selectivity and permeability, which depends on designing channels with optimal structural and chemical properties. In charged nanochannels, the EDL effect promotes preferential transport of counterions while repelling co-ions. When the channel radius approaches or is smaller than the Debye length, EDL overlap occurs, excluding co-ions and allowing only counterions to diffuse through the channel. This dramatically increases ion selectivity. However, reducing the channel radius to enhance selectivity often decreases ion permeability, creating a trade-off between the two parameters. Addressing this trade-off - achieving high selective transport while ensuring efficient ion passage - is critical for optimizing osmotic energy harvesting. To maximize power density, channel density, size, and charge characteristics must be carefully optimized. By striking a balance between ion selectivity and permeability, high-performance REG systems can be designed to harness salinity gradient energy effectively.
Since MXene was introduced into REGs in 2018, it has garnered extensive research attention. To enhance the performance of MXene-based REGs, significant efforts have been made across multiple dimensions. This section provides a comprehensive review of modification strategies, focusing on the following three aspects: (1) Surface modification and structural assembly of pure MXene materials; (2) Development of various MXene-based composite materials; and (3) Incorporation of additional effects, such as photothermal effects, to boost REG performance.
Advances in pure MXene-based REGs
MXene lamellar membranes with different compositions can be directly utilized in REGs. Hong et al. prepared Ti3C2Tx MXene lamellar membranes via vacuum filtration and systematically studied the influence of various parameters on REG performance, including salinity gradient, pH, membrane thickness, and temperature[76]. From the measured conductance in a 10 mM KCl solution at pH 6.3, they observed a surface charge density as high as ≈100 mC·m-2, surpassing those of GO laminates (50-60 mC·m-2), perforated graphene (~40 mC·m-2), and MoS2 nanopores (20-80 mC·m-2 at pH 5). This high surface charge density plays a critical role in enabling highly cation-selective ion flow through the MXene nanochannels. Additionally, as the pH increases, the dissociation of terminal groups results in more negative surface charges and higher surface charge density on individual MXene nanosheets. When evaluating the MEG performance of membranes with varying thicknesses, it was found that power density strongly decreases with increasing membrane thickness. This decline is likely due to the longer channel length in thicker membranes, which hinders ion flux beyond a certain thickness. Under an optimized salinity gradient of 1000-fold, the membrane achieved a record-high output power density of 21 W·m-2 at room temperature and an energy conversion efficiency (η) of up to 40.6%.
MXenes with other composition such as Mo2TiC2Tx have also been studied for their application in REGs. Chang et al. fabricated lamellar membranes using Mo2TiC2Tx MXene through a similar method and further enhanced their REG performance via alkali immersion treatment[77]. After soaking the membrane in an alkaline solution with pH = 9, the Voc increased significantly to 83 mV, compared to 48 mV at pH = 5. A similar trend was observed for the Isc. Additionally, the membrane's cation transference number (t+), defined as the ratio of current carried by cations to the total current and indicative of ion selectivity, improved from 0.90 under neutral conditions to 0.95 after alkaline treatment. This indicates that the treated membrane achieved a remarkably high level of cation selectivity. The study revealed that the pH-dependent REG performance is closely linked to the deprotonation and protonation processes on the MXene surface, which significantly affect surface charge density and ion transport properties. Moreover, the membrane demonstrated excellent long-term stability, maintaining its performance over 120 h of continuous testing, showcasing its potential for durable and efficient energy harvesting applications.
MXene lamellar membranes have also been utilized to harvest energy from proton gradients, laying the groundwork for using acidic industrial wastewater with varying proton concentrations in REGs. To generate electricity from acidic solutions, it is essential to ensure that REG devices can maintain long-term stability in acidic environments. MXene offers inherent advantages in this regard, as its nanosheets are produced via acid-assisted etching processes, granting them exceptional acid resistance. Qin et al. investigated the REG performance of Ti3C2Tx MXene lamellar membranes in various acidic solutions, including HCl, KCl, H2SO4, and H3PO4, and found that the output voltage and current followed the trend HCl > KCl > H2SO4 > H3PO4[78]. This trend indicates that under the same proton gradient, anions in the interlayer channels impede cation transport, with the blocking effect intensifying as the anion radius increases (Cl- < SO42- < PO43-), leading to reduced output voltage and current. MD simulations further revealed that under the same anion type and concentration, hydrogen ions with smaller hydrated ion diameters move significantly faster than potassium ions. Consequently, HCl solutions exhibit superior REG performance among the tested acids. Under optimized conditions with a 1,000-fold concentration gradient in HCl, the membrane achieved a high power density of 6.5 W m-2, outperforming the commercial benchmark (5 W m-2) by 30%, and demonstrated stability for over 200 h at pH = 0.
Tuning the ion transport pathways within MXene membranes is an effective strategy to enhance REG performance. Dong et al. demonstrated the fabrication of an ultrathin MXene membrane with horizontal transport (H-MXM) through ultrathin sectioning[79]. This unique membrane design incorporates straight and ultrashort subnanochannels, functioning as an “ion freeway” that enables ultrafast ion transport. When employed for osmotic energy conversion, H-MXM achieved an impressive power density of 93.6 W·m-2 under a 50-fold NaCl concentration gradient, outperforming most macroscopic 2D membranes. Further studies on KCl ion transport revealed that the diffusion current density in the horizontal transport mode is nearly 40 times higher than that in the vertical transport mode, highlighting the exceptional ion transport efficiency of the horizontal “ion freeway”. In another approach, Hong et al. developed porous MXene membranes by intentionally introducing nanosized holes into MXene sheets using a facile, scalable H2SO4-based etching method[80]. The resulting membranes featured shortened and continuous charge-transport pathways, enabling faster ion transport across the lamellar structure. This design simultaneously enhanced permeability and ion selectivity, overcoming the inherent trade-off between the two properties. The nanoporous Ti3C2Tx MXene membranes achieved a maximum power density of 17.5 W·m-2 under a 100-fold KCl concentration gradient at neutral pH and room temperature, representing a 38% increase compared to pristine membranes. Surface modification for better selectivity of cations is another common strategy to enhance the REG performance of MXene membranes. Given that coexisting anions have equal value as cations in osmotic energy conversion, the development of anion-selective membranes is both significant and promising. Wang et al. introduced an amination strategy using 3-aminopropyltriethoxysilane (APTES) to fabricate positively charged MXene (PCM) nanosheets, thereby imparting anion selectivity to the resulting MXene membranes[81]. The power density of these modified membranes reached a maximum of 10.98 W·m-2 under a natural salinity gradient, surpassing the commercial benchmark of 5 W·m-2. When operating under optimized environmental conditions, the power density further increased to 20.66 W·m-2 through Cl- transmembrane transport. The modified membranes also exhibited excellent stability, with an output power density attenuation of only 3% over 10 h of continuous operation.
Combining or assembling MXene materials with differently charged surfaces has also been explored to enhance REG performance. Ding et al. prepared self-supporting flexible membranes by vacuum filtering pristine MXene nanosheets with negatively charged surfaces (N-MXene) and surface-modified MXene nanosheets with positively charged surfaces (P-MXene), termed N-MXM and P-MXM, respectively[82]. These membranes were then assembled into an energy harvesting system comprising a three-chamber electrochemical cell. When the middle chamber was filled with 0.01 M NaCl and the side chambers with
Advances in MXene composite-based REGs
In addition to using pure MXene, the introduction of additional components to form composite materials has been widely studied to synergistically enhance REG performance. To date, a variety of materials, including inorganic materials, organic materials, and organic-inorganic hybrid materials such as metal-organic frameworks (MOFs), have been combined with MXene to create composites for REGs[85-100].
Inorganic materials, such as 2D membranes similar to MXene, including GO, boron nitride (BN), and Co-Al layered double hydroxides (LDH), have become popular choices for synthesizing composite materials for REGs[101-106]. Gao et al. combined 2D GO membranes with layered MXene membranes using vacuum filtration to create a 2D-2D composite membrane, which exhibits high mechanical strength, a high surface charge density, and well-defined transport channels, making it an ideal candidate for energy conversion[106]. The study revealed that incorporating GO nanosheets into MXene membranes facilitates the formation of hydrogen bonds, enhancing the binding force and significantly improving the mechanical strength of the membrane. As the GO content increased, the zeta potential of the composite membrane slightly decreased. However, the high negative zeta potential still provided excellent selective permeability for cations such as Na+. Inspired by the efficient vertical transport of water and minerals in tree trunks, Wang et al. developed a bioinspired vertically aligned MXene/GO membrane, where partial reduction of GO was used to intensify the alignment[103]. In simulated seawater and artificial river water, this vertically aligned MXene/GO membrane achieved a remarkable power density of 372 W·m-2, nearly an order of magnitude higher than that of horizontally assembled membranes under similar conditions. Numerical calculations and theoretical analyses revealed that the ultrafast ion transport in the vertically aligned MXene/GO membrane is attributed to the short transport pathways, which minimize resistance during ion migration.
Yang et al. constructed a Ti3C2Tx MXene-BN composite membrane for REGs, leveraging the small size and high surface charge density of BN nanosheets[105]. Incorporating BN into the pristine MXene membrane significantly reduced its internal resistance. A composite membrane containing 44 wt% BN nanosheets achieved an output power density of 2.3 W·m-2, nearly double that of the pristine MXene membrane. In another approach, Yang et al. sequentially vacuum-filtered negatively charged Ti3C2Tx MXene and CoAl-LDH nanosheets to fabricate a 2D Janus MXene/CoAl-LDH membrane[102]. This membrane demonstrated efficient asymmetric ion transport and osmotic energy harvesting without requiring additional modification. The MXene/LDH ratio played a critical role in determining the ion rectification factor, osmotic voltage, ion selectivity, and energy conversion efficiency. The Janus membrane containing
MOF materials, with their tunable structures and exceptional regularity, hold significant potential for REGs[107-110]. Their precisely angstrom-sized channels provide an ideal confined space, offering ultrahigh ion selectivity due to the strong overlap of the EDL and the dehydration effect of hydrated ions caused by the ultra-small pores of the MOF. However, due to the challenges in forming freestanding films and the limited surface charge density of MOFs, they are more suitable to combine with MXene to create hybrid membranes. Zhou et al. developed MXene/ZIF-8 hybrid membranes featuring specialized angstrom/nano-scale channels using a fast current-driven method for high-performance REG [Figure 8A and B][111]. The synergistic effect between the 3.4 Å pore size of ZIF-8 and the surface charge of MXene significantly enhanced the cation selectivity. Moreover, the incorporation of ZIF-8 crystals strengthened the interlayer interactions between MXene nanosheets and reduced the interlayer spacing, facilitating faster ion transport and higher ion permeability. As a result, the hybrid membrane achieved a high current density of
Figure 8. MOF/MXene composites for REGs. (A) Schematic of MXene/ZIF-8 hybrid membrane preparation and its microstructures. (B) Numerical simulation of ion concentration profile for cations (Cp) and anions (Cn) across 2D channels with varying distances (1.0, 1.4, and 1.8 nm). Reproduced with the permission of Ref.[111] Copyright © 2022 Wiley-VCH. (C) Schematic of oriented growth of HKUST-1 between MXene layers. Reproduced with the permission of Ref.[112] Copyright © 2024 Elsevier B.V. (D) Bioinspired design and corresponding schematic of the nano-hierarchical porous MIL-53-COOH/MXene membrane. (E) Steady-state concentration distribution of K+ and Cl- near MIL-53-COOH/MXene hybrid membrane. (F) Steady-state concentration distribution of K+ near MIL-53/MXene hybrid membrane and MIL-53-NH2/MXene hybrid membrane. Reproduced with the permission of Ref.[113] Copyright © 2024 Elsevier Ltd. (G) Schematic of MXCT membrane preparation process. (H) Schematic of interlayer structures of MXCT. (I) Schematic of the enhanced ion transport across the MXCT membrane via the incorporation of the 2D Cu-TCPP MOFs with rich sub-2 nm ion channels. Reproduced with the permission of Ref.[114] Copyright © 2024 Elsevier Ltd.
Yao et al. utilized an anodic electrodeposition method to construct HKUST-1@MXene composite membranes [Figure 8C][112]. The uniquely oriented porous MOFs expanded the interlayer spacing of MXene, while the inherently NCM surface facilitated a synergistic effect that significantly enhanced both ion selectivity and flux. This synergy enabled the composite membrane to achieve a remarkable diffusion potential of 340.2 mV. These advancements translated into impressive power density outputs, reaching
Lin et al. developed an electrolyte-stable 2D composite membrane by self-assembling Cu-tetrakis(4-carboxyphenyl)porphyrin (TCPP) nanosheets with MXene [Figure 8G-I][114]. The unique feature of this composite lies in the orderly framework channels of the Cu-TCPP nanosheets, which impart ultralow resistance to the membrane. This innovative composite membrane achieved a high power density of
Advances in photothermal effect-enhanced MXene-based REGs
As discussed above, significant efforts have been devoted to developing REGs based on MXenes and their composites, yet their performance remains constrained. Taking advantage of excellent photothermal properties of MXene, researchers have begun leveraging this feature to introduce additional enhancement strategies aimed at improving REG performance[115].
The photothermal effect of MXene can be utilized to regulate ion transport, enabling enhanced REG performance under concentration gradients and even reverse ion transport against concentration gradients[116]. This expands the application scenarios of MXene membranes. Liu et al. demonstrated that in a system with no applied voltage and equal concentrations of KCl solution (0.01 M), there was negligible ionic current initially[117]. Upon partial light irradiation of the MXene membrane (light intensity ~200 mW·cm-2), the temperature of the illuminated region rapidly increased by ~19.91 °C due to MXene’s excellent photothermal properties, causing the ionic current to rise sharply to approximately 37.5 nA. When the light was turned off, both the current and temperature returned to their original states, confirming that light irradiation induces ion transport. Interestingly, the recorded ionic current flowed from the non-illuminated (low-temperature) region to the illuminated (high-temperature) region, aligning with the temperature gradient in the device. Further tests with light at wavelengths between 350-700 nm showed that at
Integrating light irradiation into an REG system containing wastewater can simultaneously achieve water treatment and osmotic power harvesting, as demonstrated by Xia et al. In this system, light irradiation not only enhanced the high permeability of Na+ ions, increasing by 429% to 62.6 mol·m-2·h-1, but also demonstrated high selectivity for heavy metal ions, achieving a separation ratio of up to 2050, thus effectively treating wastewater[118]. As a result, the membrane stably generated osmotic power from simulated industrial wastewater, with the power density enhanced fourfold under light illumination at approximately one sun intensity. At a concentration gradient ratio of 500, the output power density reached an impressive 19 W·m-2.
Integrating photothermal and photoelectric effects in REG systems is expected to further enhance energy harvesting capability. Wang et al. developed a Janus PMMT/MXene membrane by sequentially vacuum-filtering montmorillonite (MMT) decorated with photoelectric molecules (PMMT) and MXene nanosheets[101]. The REG system constructed with this membrane enabled light-driven active ion transport, facilitating ionic energy harvesting even in electrolyte systems with equal concentrations. Under light illumination, the system generated an intramembrane electric field and a temperature gradient due to efficient charge separation and localized thermal excitation. These effects produced a PV-driven force and thermo-osmotic flow, promoting preferential ion transport. This unidirectional active transport mechanism allowed the system to achieve an output power density of 2.0 mW·m-2 and a high-performance energy conversion efficiency of 8.3 × 10-4%.
Overall, as shown in Table 3, research on MXene-based REGs is the most extensive among all MXene-based HEGs, with a diverse range of performance enhancement strategies. These include surface modification, structural regulation, biomimetic assembly, composite formation, and photothermal coupling. These continuous advancements are driving progress in the field, offering broad application prospects and significant practical value. However, it is also important to note that current MXene materials used in REGs are predominantly Ti3C2Tx, while the MXene family comprises over a hundred different compositions. Investigating how different compositions influence interface properties, ion transport, and REG performance remains an important avenue for future research. Additionally, given the diverse range of modification strategies, conducting quantitative studies to systematically evaluate their effectiveness in enhancing device performance would be highly meaningful. Such an approach could provide valuable insights into the relative impact of different modification methods, facilitating the development of more optimized and efficient strategies.
Summary of reported MXene-based REGs
| Functional layers | Electrodes | Thickness | Area (mm2) | Stabilized voltage (V) | Stabilized current | Power density (W m-2) | Maximum energy conversion efficiency | Long-term operation | t+ | Condition | Ref. |
| Ti3C2Tx MXene lamellar membrane | Ag/AgCl | 1.3 μm | 1.63 × 10-2 | 0.175 | 30 μA | 21 | 40.6% | 20 h | 0.95 | 1 mM/1 M KCl, room temperature | [76] |
| Mo2TiC2Tx MXene lamellar membrane | Ag/AgCl | 4.3 μm | 1.41 × 10-2 | 0.083 | 8.9 μA | 13.1 | 40.5% | 120 h | 0.95 | 0.5/0.01 M alkali KCl (pH = 9) | [77] |
| Ti3C2Tx MXene lamellar membrane | Ag/AgCl | 2.5 μm | 0.03 | 0.3 | 15 μA | 6.5 | 18.1% | 200 h | 0.801 | 1 mM /1 M HCl | [78] |
| ultrathin MXene membrane with horizontal transport | Ag/AgCl | 3 μm | 1.89 × 10-3 | - | - | 93.6 | - | 15 days | - | 0.5/0.01 M NaCl | [79] |
| oppositely charged Ti3C2Tx MXene membrane assembly | Ag/AgCl | - | - | 0.176 | 30.2 μA | 4.6 | - | 2 months | 0.96 | 0.5/0.01 M NaCl | [82] |
| Ti3C2Tx MXene-Based Ionic Diode Membrane | Ag/AgCl | 4 μm | 0.03 | 0.089 | - | 8.6 | 39.2% | 20 days | 0.943 | 0.5/0.01 M NaCl | [84] |
| Porous Ti3C2Tx MXene lamellar membrane | Ag/AgCl | 0.6 μm | 0.025 | 0.1 | 16 μA | 17.5 | 46% | 120 h | 0.98 | 0.5/0.005 M KCl | [80] |
| oppositely charged Ti3C2Tx MXene fiber assembly | Ag/AgCl | Diameter 30 and 20 μm | - | 0.172 | 356 nA | 12.3 | 45.7% | 3 months | 0.98 | 0.5/0.01 M NaCl | [83] |
| Aminated Ti3C2Tx MXene membrane | Ag/AgCl | 678 nm | 180 | - | - | 10.98 | - | 10 h | - | 0.5/0.01 M NaCl, 30 °C | [81] |
| Ti3C2Tx MXene membrane+light | Ag/AgCl | ~4 μm | - | - | 34.39 nA | 1.68mW/m2 | - | 300 s | - | 0.5/0.01 M NaCl; Light intensity: ~200 mW·cm-2 | [117] |
| Ti3C2Tx MXene membrane+light | Ag/AgCl | 2.2 μm | 0.03 | 0.1 | 6.3 μA | 5 | - | 400 s | - | 0.5/0.01 M NaCl; Light intensity: 100 mW·cm-2 | [118] |
| Ti3C2Tx MXene/GO composite membrane | Ag/AgCl | 420 nm | 710 | - | 3.72 μA | 3.7 | - | 10 h | - | 0.5/0.01 M NaCl, 25 °C | [106] |
| Ti3C2Tx MXene/GO membrane with vertical channels | Ag/AgCl | 18.5-30 μm | 0.26 | - | - | 372 | - | 12 h | - | 0.5/0.01 M NaCl, 27 °C | [103] |
| Ti3C2Tx MXene/BN composite membrane | Ag/AgCl | 10 μm | 0.03 | 0.14 | 78 A/m2 | 2.3 | ~25% | 10 h | 0.85 | 0.5/0.01 M NaCl, 48 °C | [105] |
| Janus Ti3C2Tx MXene/ CoAl-LDH membrane | Ag/AgCl | 4.5 μm | 0.03 | 0.13 | - | 8.75 | - | - | - | 1 M HCl/1 M NaOH, room temperature | [102] |
| Janus Ti3C2Tx MXene/ montmorillonite membrane | Ag/AgCl | 17.7 μm | 0.2 | 0.0166 | 22.3 μA/cm2 | 0.96 mW/m2 | 8.3 × 10-4% | - | - | 10-3 M KCl, Light intensity: 25 mW·cm-2 | [101] |
| Ti3C2Tx MXene/ZIF-8 composite membrane | Ag/AgCl | 2 μm | 3.1 | 0.11 | - | 7.18 | 32.97% | 16 days | 0.906 | 0.5/0.01 M NaCl | [111] |
| HKUST-1@ Ti3C2Tx MXene composite membrane | Ag/AgCl | - | - | 0.342 | 1,400 A/m2 | 25.3 | 45.1% | - | 0.92 | 106-fold KCl concentration | [112] |
| MIL-53-COOH/ Ti3C2Tx MXene membrane | Ag/AgCl | 4 μm | 0.03 | - | - | 35.04 | 45% | 30 days | 0.95 | 0.5/0.01 M NaCl | [113] |
| Ti3C2Tx MXene/Cu-TCPP membrane | Ag/AgCl | 8 µm | 0.03 | 0.09 | 10 μA | 8.29 | - | - | 0.775 | 0.5/0.01 M NaCl | [114] |
MXene-based DEG
Working mechanism of DEG
The DEG utilizes a droplet sliding on the surface of an active material to generate a drawing potential. In 2014, Yin et al. reported for the first time DEG devices utilizing graphene film as active material[119]. When a NaCl droplet slides on the surface of graphene, it can generate a voltage of about 0.15 mV, which is named the drawing potential [Figure 9A]. The authors further provide an explanation for the generation of the drawing potential by means of a pseudocapacitive mechanism. When a droplet slides across a graphene surface, cations at the leading edge tend to adsorb onto the graphene due to the difference in contact angle between the front and rear ends of the droplet. This adsorption induces the accumulation of electrons in the graphene, facilitating pseudocapacitive charging. Conversely, at the trailing edge, cations desorb from the graphene surface, releasing the accumulated electrons and resulting in pseudocapacitive discharging. Consequently, the electron density at the trailing edge of the droplet remains higher than that at the leading edge throughout the sliding process, creating a potential gradient along the sliding direction of the droplet [Figure 9B].
Figure 9. MXene-based DEG. (A) Schematic illustration of setup for generating electricity via sliding droplet on the graphene. (B) Schematic diagram showing the potential difference along the droplet sliding. Reproduced with the permission of Ref.[119] Copyright © 2014 Springer Nature. (C) Photograph showing the MXene film attached to the glass substrate to form the DEG device. (D) Voc and (E) Isc generated by droplets sliding (4 cm s-1) on the MXene film with a height of 15 cm. Reproduced with the permission of
MXene, another 2D material such as graphene, has also been explored for its potential in DEGs. Table 4 summarizes and compares in detail the performance of reported MXene-based DEGs.
Summary of reported MXene-based DEGs
| Functional layers | Electrodes | Roles of MXene | Area (cm-2) | Peak Voc (V) | Peak Jsc (µA cm-2) | Lasting time | Droplet composition | Volume | Speed | Ref. |
| MXene film | Fe/Fe | EDL effect | 4 | 0.0015 | 1.5 | / | 4 mg L-1 NaCl + 4 mg L-1 CaCl2 | 25 µL | 4 cm s-1 | [120] |
| MXene film | Ag/Ag | EDL effect | 2.5 | / | 3.2 | / | 1 M NaCl | / | 3 cm s-1 | [121] |
| TiO2 (Magneli phase) film | Cu/Cu | Precursor of TiO2 | 10 | 0.6 | / | 10,000 s unchanged (Voc) | 0.1 M NaCl | 100 µL | 14 mL min-1 | [122] |
Advances in MXene-based DEG
Bai et al. fabricated a 1 cm × 4 cm MXene film on a glass substrate with electrodes connected at the top and bottom ends to construct a DEG device [Figure 9C][120]. Then 25 µL droplets of an electrolyte containing
However, the Voc remained low due to the high electrical conductivity of the MXene film. To overcome this, Si et al. used Ti3C2Tx-derived materials to fabricate oxygen vacancy-rich TinO2n-1 films via calcination[122]. Peroxo-titanic acid gels were synthesized by mixing Ti3C2Tx with H2O2, then calcined in hydrogen at 800 °C to form TiO2 films. At 1,000 °C, TiO2 transformed into Ti4O7 due to oxygen removal. Oxygen vacancies on the Ti4O7 surface enabled denser EDL formation, enhancing cation adsorption and DEG performance [Figure 9F]. Raising the calcination temperature from 800 to 1,000 °C increased Voc from 17 to 83 mV and Isc from 0.015 to 0.52 µA. To further improve performance, the authors designed a 10 cm tubular DEG coated with TiO2 films. Using 0.1 M NaCl and droplets sliding at 14 mL min-1, the device achieved a stable 0.6 V Voc over 10,000 s [Figure 9G], highlighting the potential of MXene-based DEG as a reliable power source.
COUPLING OF MXENE-BASED HEGS WITH OTHER ENERGY-HARVESTING SYSTEMS
In the preceding sections, we reviewed various strategies for enhancing MXene-based HEGs, primarily focusing on improvements to HEG materials and device design. However, due to the inherent limitations in energy input and conversion efficiency, their power generation performance remains far from satisfactory.
Integrating other energy harvesting systems such as TENGs, TEGs, and PVs, with HEGs holds great promise for introducing additional energy inputs, thereby significantly boosting power generation performance[123-127]. More importantly, such coupled designs could enable multimodal self-powered sensing applications, effectively expanding their potential use cases[128-135].
Coupling of MXene-based HEG with TENGs
Since a TENG converts external mechanical stimuli into AC output based on the coupling of triboelectrification and electrostatic induction, it exhibits high voltage (> 100 V) and low current (< 10 µA), complementing the output of MEGs. However, the challenge in integrating these generators lies in their differing humidity dependencies. MEGs typically generate lower output at low humidity but significantly improve under high humidity. In contrast, excessive moisture easily neutralizes triboelectric charges in TENGs, reducing charge transfer efficiency at the friction interface, leading to optimal performance under low humidity.
To achieve complementary integration of TENGs and MEGs, Kim et al. reported a flexible hybrid energy generator (HEG) based on a MXene/organo-ionic hydrogel foam (MOHF) [Figure 10A][51]. Benefiting from this bilayer structural design, the hydrogel provides sufficient water to the MXene active layer, allowing the MEG to function independently of external humidity. Even at 20% RH, the device achieves a Voc of 0.3 V and an Isc of 100 µA. The top surface of the MOHF, unwetted by the bottom hydrogel, serves as the friction layer for the TENG. When a Perfluoroalkoxy (PFA) film undergoes contact-separation with MOHF, the electronegativity difference between PFA and MOHF induces electron transfer at the interface, generating a corresponding Voc or Isc change in the external circuit [Figure 10B]. For MOHF, the power generation processes of TENG and MEG operate independently, with MEG relying on internal cations migration and TENG on surface electron transfer [Figure 10C]. Therefore, to achieve hybrid output, both signals can be simultaneously collected using a parallel circuit [Figure 10D]. Comparison tests showed that when TENG and MEG operate simultaneously, the MOHF hybrid generator achieves a Voc of 55 V and an Isc of 100 µA, demonstrating complementary integrated output [Figure 10E and F].
Figure 10. Coupling of MXene-based HEG and TENG. (A) Schematic showing working mechanism of MXene-based MEG. (B) Working principle of contact separation type TENG consisting of PVA/MXene-melamine foam and PFA film. (C) Schematic diagram explaining the structure of complementary MEG-TENG-MOHF device. (D) Schematic diagram and photograph of the external circuit integrating the TENG and MEG outputs. (E) VOC and (F) ISC for the MEG-MOHF, TENG-MOHF, and complementary MEG-TENG-MOHF. Reproduced with the permission of Ref.[51] Copyright © 2024 Royal Society of Chemistry.
Coupling of MXene-based HEGs with thermoelectric generators
TEGs based on Seebeck effect convert thermal energy directly into electrical energy by utilizing a temperature gradient to drive charge carrier migration, generating an electric current. Meanwhile, the MEGs operate based on asymmetric water molecule adsorption, which creates an ion gradient within the active material. Ion diffusion generates an ionic current, inducing an electron current in the external circuit. Synthesizing an active material with both TEG and MEG properties is key to achieving TEG-MEG coupling. Xue et al. introduced a sandwiched Janus-structured thermo-hydroelectric generator (THEG) based on 1T-MoS2/Csilk (MCs) and 1T-MoS2/MXene-Cotton (MMC)[49]. Due to van der Waals interactions between MoS2 and MXene nanosheets, MMC exhibits n-type conductivity, while MC shows p-type conductivity due to the presence of MoS2. As a result, the MMC/MC structure enables TEG performance detection under a temperature gradient [Figure 11A]. Within a temperature difference range of 0-20 K, the Seebeck coefficient is 2.35 µV K-1 [Figure 11B]. Additionally, due to the oxygen-containing functional groups on MXene nanosheets, MMC can serve as the active layer for MEG, with MC and Al acting as MEG electrodes. Under these conditions, the THEG exhibits MEG output [Figure 11C]. Due to the Janus-type MMC/MC active layer possessing both TEG and MEG properties and sharing a common external circuit, both generate a continuous DC output. As a result, THEG enables coupled output under simultaneous temperature and moisture gradients. Under dry conditions, the device generates a Voc of 0.825 mV via the TEG effect at a ΔT of 30 K. Upon sweating, the MEG effect significantly enhances the Voc to 1.236 V [Figure 11D]. These findings underscore the potential of integrating TEG and MEG mechanisms in wearable electronics, providing a versatile and efficient solution for self-powered systems.
Figure 11. Coupling of MXene-based HEG and TEG. (A) Schematic of thermoelectric measurement for MCs-MMCs THEGs. (B) Corresponding Seebeck coefficient with the ΔT range of 0-20 K. (C) Voc and Isc output of HEG based on the structure of Al-MMC-MCs (D) Voc curves demonstrating the wearable THEG exhibits TEG and HEG power generation in dry and wet states, respectively. Reproduced with the permission of Ref.[49] Copyright © 2024 Wiley-VCH.
CONCLUSION AND PERSPECTIVES
In this review, we have provided a comprehensive summary of the development of MXene in the field of HEGs, focusing on four key aspects: MEGs, EEGs, DEGs, and REGs. We have detailed various strategies for MXene, including surface modification, structural assembly, and composite formation, for boosting HEG performance.
Overall, in terms of research volume, current HEG studies utilizing MXene have advanced more extensively and rapidly in MEGs and REGs, whereas research on DEGs remains in its early stages. Regarding device configurations, most MXene-based HEGs are fabricated in the form of 2D thin films, while devices with other dimensional structures are still rare, potentially limiting their application scope. In terms of applications, the primary focus has been on power generation and sensing, yet the number of related studies remains relatively limited. Regarding the role of MXene, it has demonstrated versatility, serving multiple functions across different HEG devices: it can serve as an active material for power generation, as a conductive additive, and as a precursor material for HEG components. Among these, its role as an active material is the core focus of current research, accounting for the majority of studies, while its other applications remain in early stages of exploration. Besides, the coupling of MXene-based HEGs with other types of energy harvesting devices has only recently attracted attention, with only a few scattered studies reported to date.
As an emerging field, MXene-based HEGs are still in the early stages of development. Consequently, the performance of MXene-based HEGs is currently limited, with relatively simple structures and functionalities that require further enhancement. Looking ahead, future research should prioritize the following areas:
(1) Exploring the HEG properties of different MXene family members. Currently, Ti3C2Tx is the primary material studied for HEGs, and its structure-property relationships have been partially understood. However, the MXene family potentially includes hundreds of members, each with unique compositions that could significantly alter their physicochemical properties. These changes may impact interactions with water molecules and the overall HEG performance. Future research should systematically explore how different MXene compositions influence HEG performance to uncover potential material candidates with enhanced capabilities.
(2) Clarifying the mechanisms and principles of MXene-based HEGs. Although the power generation mechanisms of various HEGs have been extensively studied, the understanding of MXene-based HEGs remains superficial. Theoretical models often fail to fully align with experimental observations, leading to multiple hypotheses about power generation mechanisms. To improve HEG performance and facilitate practical applications, it is crucial to develop a clear understanding of the fundamental mechanisms and principles underlying MXene-based HEG operation through combined experimental and theoretical studies.
(3) Identifying the intrinsic performance of MXene-based HEGs. In many studies, various electrodes, including non-inert electrodes such as Zn and Cu, have been used. However, such electrodes may inadvertently introduce battery-like reactions, artificially enhancing HEG performance, particularly in MEGs. This makes it challenging to accurately assess the intrinsic activity of MXene materials, potentially hindering the field’s development. Establishing standardized testing protocols, such as using symmetric inert electrodes, is essential for accurately evaluating the intrinsic performance of MXene-based HEGs and ensuring the field’s healthy and robust growth.
(4) Expanding the dimensions and forms of MXene-based HEG devices. The hydrophilicity and excellent dispersion of Mxene in water provide it with superior processability, enabling the fabrication of fibers, thin films, aerogels, hydrogels, and other structures. However, most MXene-based HEG devices are currently utilized in 2D thin-film forms. Future research should diversify the dimensions and configurations of MXene-based HEGs to better align with functional and application needs.
(5) Coupling with other energy harvesting devices. As demonstrated earlier, coupling MXene-based HEGs with other energy harvesting technologies not only improves power generation performance but also broadens application scenarios. However, research on coupled devices is still in its infancy, with only a few case studies available. This area presents significant opportunities for exploration.
DECLARATIONS
Authors’ contributions
Conceptualization, methodology, visualization, formal analysis, writing - original draft: Li, S.; Zan, G.
Conceptualization, investigation, writing - original draft: Li, S.; Liu, J.
Methodology, writing - original draft: Zhao, K.
Software, visualization: Kim, H.
Investigation, visualization: Shin, E.
Visualization: Kim, G.
Conceptualization, funding acquisition, resources, supervision, writing - reviewing and editing: Zan, G
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (No. 22205165).
Conflict of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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