Composite aerogel and preparation method and application thereof

Abstract

A composite aerogel and a preparation method and application thereof. The aerogel takes TEMPO oxidized cellulose nanofiber as a three-dimensional skeleton and metal ion intercalated modified MXene nanosheet as a conductive phase, and has double gradient characteristics of Z-direction gradient porous structure and Z-direction gradient wettability: the pore size continuously changes from small to large along the thickness direction, and the wettability gradually transitions from weak hydrophobicity to strong hydrophilicity. The metal ion intercalation expands the interlayer spacing of MXene, constructs an ordered ion channel, and forms a coordination bond and a hydrogen bond crosslinking network with TOCNF, and the structure is stable. In the preparation, unidirectional temperature gradient section freezing combined with vacuum freeze drying is adopted to realize controllable formation of gradient pores. The aerogel utilizes gradient structure to drive directional migration of water vapor and efficient ion transmission, and outputs stable, high-voltage wet-electric conversion, and simultaneously has the functions of air permeability and moisture retention, anti-infection and promotion of wound healing, and can be applied to self-powered wet gas power generation devices and wound repair materials.

Description

Technical Field

[0001] This invention belongs to the technical field of functional nanocomposite aerogel materials, specifically relating to a composite aerogel, its preparation method, and its application. Background Technology

[0002] Electrical stimulation is a clinically recognized and highly effective physical strategy for accelerating wound healing. Normal skin can maintain an endogenous potential difference of 10-60 mV, which regulates cell polarity, migration rate, and proliferative activity to maintain skin physiological homeostasis. After skin injury, the endogenous electric field becomes disordered, which hinders the directional migration of cells and delays the wound repair process. Traditional electrical stimulation therapy requires the implantation of electrodes at the wound site, along with a large external power supply device to provide low-intensity current. This method has drawbacks such as poor portability, high cost, and difficulty in adapting to long-term home wound care.

[0003] Moisture-generated electricity can directly convert the chemical energy of environmental water vapor into electrical energy without the need for external power input. It can be continuously output at normal temperature and pressure and is suitable for the microenvironment of biological wounds, providing a new path for constructing a self-powered wound electrical stimulation system. However, existing moisture-generated electricity materials still have obvious defects: First, the porous structure lacks stability, and the ion migration channels are prone to collapse and disorder, making it impossible to form a stable ion concentration gradient, resulting in large fluctuations in output voltage and poor long-term stability; Second, the number of mobile active ions in the system is limited, the directional transport efficiency is low, and the output voltage amplitude is low, making it difficult to reach the effective electrical stimulation threshold for activating skin cell repair.

[0004] Currently, MXene / cellulose composite aerogels have been applied in the field of wet-to-electric energy conversion. However, most existing materials are homogeneous porous structures with no pore size or wettability gradient in the thickness direction, which cannot provide sufficient driving force for water vapor adsorption and directional ion migration, resulting in low conversion efficiency and weak output stability. Furthermore, related research often focuses only on power generation performance without integrating biomedical functions, making it difficult to simultaneously address gradient structure regulation, stable conductive network construction, multiphase interface bonding, and biocompatibility, thus limiting their practical application in self-powered wound repair scenarios. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide an Ion-MXene / TOCNF composite aerogel material based on a gradient porous structure (Ion-MXene represents metal ion-intercalated MXene nanosheets, and TOCNF represents CNF oxidized by TEMPO), its preparation method, and its applications. This composite aerogel, through a dual-structure design of Z-axis gradient porosity and gradient wetting, combined with metal ion intercalation modification and the synergistic effect of multiphase interfaces, can achieve efficient and stable wet-to-electric energy conversion, while also possessing excellent wound healing properties. This solves the problems of low wet-to-electric conversion efficiency, poor structural stability, and limited functionality of existing composite aerogels.

[0006] To address the aforementioned technical problems, this invention provides a composite aerogel material;

[0007] The composite aerogel uses TEMPO oxidized cellulose nanofibers as a three-dimensional porous network framework and metal ion-intercalated modified MXene nanosheets as a conductive functional phase. Metal ions are uniformly doped into the system and intercalated in the gaps between the MXene sheets. The composite aerogel has dual gradient characteristics of Z-axis gradient porous structure and Z-axis gradient wettability: the Z-axis gradient porous structure exhibits a continuous gradient change along the material thickness direction with small pore size on the upper surface and large pore size on the lower surface; the Z-axis gradient wettability exhibits a continuous gradual change along the material thickness direction.

[0008] Preferably, the TEMPO oxidized cellulose nanofibers have a diameter of 5-20 nm, a length of 500-2000 nm, and a carboxyl content of 1.2-2.5 mmol / g; their mass concentration in the precursor solution is 0.2%-0.8%, which can construct a three-dimensional framework structure with appropriate density.

[0009] Preferably, the MXene nanosheets are few-layer or monolayer structures with a lateral dimension of 0.5–3 μm, a sheet thickness of 1–5 nm, and a powder resistivity of 0.5–2.0 Ω. cm; MXene nanosheets, with a mass addition ratio of 5% to 20% relative to TOCNF, can form continuous conductive pathways in the interstices of the framework.

[0010] Preferably, the metal ions are derived from water-soluble inorganic metal salts, which may be selected from monovalent, divalent, or trivalent metal chlorides, including but not limited to one or more of LiCl, KCl, NaCl, MgCl2, and FeCl3. Monovalent metal salts are preferred to ensure uniform dispersion and biocompatibility. The molar concentration of the metal ions in the precursor solution is 0.05~0.2 mol / L.

[0011] Preferably, the composite aerogel has a uniform pore distribution in the X and Y planes, with a pore size of 200-500 nm; the upper surface has a pore size of 100-300 nm, resulting in a relatively higher water contact angle and stronger hydrophobicity; the lower surface has a pore size of 500-1000 nm, exhibiting more pronounced hydrophilicity. The material forms a dual gradient of pore size and wettability along its thickness direction, with wettability gradually increasing from top to bottom, enabling directional transport of water vapor and ions.

[0012] Preferably, the carboxyl and hydroxyl groups on the surface of TOCNF form metal coordination bonds with metal ions, and the hydroxyl and fluorine groups on the surface of MXene nanosheets form intermolecular hydrogen bonds with the polar groups of TOCNF; the metal ions also act as crosslinking bridges to achieve orderly crosslinking between MXene nanosheets and TOCNF, together forming a stable three-dimensional crosslinked network structure.

[0013] To address the aforementioned technical problem, this invention provides a method for preparing the composite aerogel material. The composite aerogel is prepared using a metal ion intercalation modification combined with a gradient freezing process, specifically including the following steps:

[0014] (1) Preparation of high-purity MXene nanosheets with few layers: Ti3AlC2 precursor powder was etched and exfoliated using a mild LiF / HCl etching system. LiF powder and Ti3AlC2 powder were added to a concentrated HCl solution and magnetically stirred until LiF was completely dissolved. The reaction was then carried out under constant temperature water bath conditions to fully remove the Al atomic layer in Ti3AlC2. After the reaction, the mixture was centrifuged and washed multiple times until the supernatant was neutral to completely remove the etching residue and acidic ions. The purified precipitate was redispersed in deionized water and ultrasonically exfoliated under ice water bath conditions to weaken the interlayer forces of MXene sheets. Finally, the upper uniform dispersion was collected and dried by gradient centrifugation to obtain high-purity MXene nanosheets with few layers.

[0015] As a preferred embodiment, in step (1), the mass ratio of LiF, Ti3AlC2 and concentrated hydrochloric acid is 1:1:(20~25); the etching reaction temperature is 35℃, the reaction time is 24h, the stirring speed is 500rpm; the ice-water bath ultrasonic peeling time is 30min; the gradient centrifugation parameters are: first centrifuge at 7500rpm for 5min, then centrifuge at 3000rpm for 30min.

[0016] (2) Preparation of MXene nanosheets modified by metal ion intercalation: MXene nanosheets and metal salt powder were dissolved and dispersed in deionized water to prepare MXene aqueous dispersion and metal ion solution. The MXene dispersion and metal ion solution were mixed in proportion and stirred continuously under constant temperature water bath conditions to allow metal ions to fully penetrate and intercalate into the gaps between MXene sheets, and to precisely control the gap between MXene sheets and the surface electronic structure. After the reaction was completed, the mixed system was centrifuged and washed multiple times to remove unreacted free metal ions, salt impurities and MXene aggregates, and the purified metal ion intercalation modified MXene (Ion-MXene) dispersion was obtained.

[0017] Preferably, the metal salt in step (2) is selected from any one of LiCl, MgCl2, and FeCl3; MXene and deionized water are mixed at a mass ratio of 1:50 to prepare a 2 mg / mL MXene aqueous dispersion, and the metal ion solution concentration is 1 mol / L; the volume ratio of MXene dispersion to metal ion solution is 4:1; the intercalation reaction temperature is 40℃, the reaction time is 48h, and the stirring speed is 500rpm; centrifugation and washing are repeated 3 to 5 times.

[0018] (3) The purified Ion-MXene aqueous dispersion and TEMPO oxidized cellulose nanofiber (TOCNF) dispersion were mixed according to a preset ratio and thoroughly mixed by magnetic stirring so that the modified MXene nanosheets were uniformly loaded on the surface and gaps of the TOCNF three-dimensional skeleton to obtain a uniform and stable Ion-MXene / TOCNF composite colloidal system. Then the colloidal system was injected into the molding mold.

[0019] Preferably, the mass ratio of Ion-MXene to TOCNF in step (3) is 1:15.

[0020] (4) Gradient freeze-drying and vacuum freeze-drying: A controllable unidirectional temperature gradient segmented freezing process was adopted to construct the Z-axis gradient porous structure of the material. A copper-plastic composite mold was used to construct a differentiated thermal conductivity temperature difference system. The characteristics of rapid cooling on the high thermal conductivity side of the copper mold and slow heat dissipation on the insulating plastic side were utilized to form a stable unidirectional temperature gradient inside the composite colloid, which induced the directional gradient growth of ice crystals and stabilized the Z-axis gradient porous precursor structure. After freezing, the sample was subjected to low-temperature vacuum freeze-drying. The gradient porous morphology was completely preserved through ice crystal sublimation. Finally, a metal ion intercalation modified gradient porous Li-MXene / TOCNF composite aerogel material with controllable pore size gradient was obtained.

[0021] As a preferred option, the specific process of gradient freezing in step (4) is as follows: First, place the copper-plastic composite mold filled with composite colloid in an environment of -20±5℃ and freeze for 4 h to initially form a gradient distribution ice crystal skeleton; then quickly transfer it to an environment of -40±5℃ and freeze for 2 h to solidify the ice crystal gradient structure and avoid the collapse and failure of the structure caused by the recrystallization of ice crystals; then transfer it to an environment of -80℃ and freeze for 6 h to completely fix the gradient porous precursor structure; finally, freeze dry it under vacuum at -80±5℃ for 72 h to completely remove the internal ice crystals by sublimation and obtain a gradient porous composite aerogel with a complete structure and uniform morphology.

[0022] The principle of this invention: Ion-MXene / TOCNF aerogels obtained by gradient freezing have pore sizes that increase with the growth of the ice layer, as the temperature difference decreases and the ice crystal size increases with height. After sublimation, they form interconnected and enlarged pores, thus constructing an aerogel with a gradient pore size structure.

[0023] The core advantage of gradient pore size aerogels lies in their unique structure with a pore size gradient distribution along the thickness direction, as well as their adaptability to applications requiring moisture-generating power generation and wound dressing exudate management.

[0024] Firstly, the gradient arrangement of large and small pores enables efficient mass transfer. The large pore side can quickly capture ambient water vapor and absorb moisture, while the small pore side can lock in water for a long time, providing a stable water medium for continuous moisture power generation.

[0025] Secondly, it can spontaneously build internal and external humidity concentration gradients without additional devices, providing the core driving force for directional ion migration and built-in potential difference, significantly improving power generation output performance;

[0026] Third, the continuous, multi-level gradient channels are free from blockage, effectively shortening the ion migration path, reducing charge transfer impedance, and improving electrochemical kinetics and power generation stability.

[0027] MXene ion intercalation modification offers several significant advantages: it effectively widens the interlayer spacing of MXene nanosheets, inhibits layer aggregation and self-stacking, increases specific surface area, and exposes more active sites; simultaneously, it constructs ordered ion transport channels between layers, shortening migration distance, reducing transport resistance, and improving electrochemical conductivity; it also increases the number of mobile protons released during material ionization, enhancing ion mobility and providing a key driving force for directional ion migration in wet gas power generation. Furthermore, ion intercalation optimizes the hydrophilicity of the material surface, increases hydrophilic sites, and strengthens hygroscopic water storage performance; through coordination with MXene surface functional groups, it enhances the cross-linking bond with the TOCNF framework, improving overall structural stability.

[0028] Advantages of this invention:

[0029] 1. An aerogel with a gradient pore structure was constructed by gradient freezing, which achieved efficient mass transfer and could spontaneously build internal and external humidity concentration gradients, providing a core driving force for directional ion migration and significantly improving the output performance of wet gas power generation; the continuous and interconnected gradient multi-level channels effectively shortened the ion migration path, reduced charge transfer impedance, and improved power generation stability.

[0030] 2. TOCNF, MXene, and metal ions form a three-dimensional cross-linked network through coordination bonds and intermolecular hydrogen bonds. The interfacial bonding is strong, effectively avoiding system delamination and framework collapse problems, resulting in excellent material mechanical properties and moisture resistance. Metal ion intercalation can increase the interlayer spacing of MXene, increase active sites, inhibit layer aggregation, and improve the system's charge density and ion transport efficiency.

[0031] 3. The aerogel prepared by this invention can be used as a self-powered moisture-driven generator, and has good application prospects in wound healing and other fields. Attached Figure Description

[0032] Figure 1 These are physical images and microstructures of composite aerogels prepared by gradient freezing and non-gradient freezing methods.

[0033] Figure 1 a and b in the figures are physical images of the non-gradient composite aerogel provided by this invention;

[0034] Figure 1 c in the figure is a SEM image of the gradient composite aerogel provided by the present invention;

[0035] Figure 1 In the figures, d and e are physical images of the non-gradient composite aerogel provided by this invention;

[0036] Figure 1 f in the figure is a SEM image of the gradient composite aerogel provided by the present invention.

[0037] Figure 2 This is a water contact angle diagram of the composite aerogel provided by the present invention.

[0038] Figure 3 This is the structural characterization of the composite aerogel provided by the present invention.

[0039] Figure 3 In this context, 'a' represents the XRD pattern of the composite aerogel provided by this invention.

[0040] Figure 3 b in the figure is the FT-IR spectrum of the composite aerogel provided by the present invention.

[0041] Figure 4 This refers to the spontaneous hygroscopicity of the composite aerogel provided by the present invention.

[0042] Figure 4 In the figure, 'a' represents the moisture absorption change curve over the entire cycle from 0 to 168 hours.

[0043] Figure 4 In the figure, b represents the short-term moisture absorption change curve from 0 to 24 hours.

[0044] Figure 5 This is the open-circuit voltage diagram of the composite aerogel provided in Embodiment 1 of the present invention under RH90% conditions.

[0045] Figure 6 This is the open-circuit voltage diagram of the composite aerogel provided in Embodiment 2 of the present invention under RH90% conditions.

[0046] Figure 7 This is the open-circuit voltage diagram of the composite aerogel provided in Embodiment 3 of the present invention under RH90% conditions.

[0047] Figure 8 This is a diagram showing the stability of the open-circuit voltage output of the composite aerogel wet-to-electric conversion provided in Embodiment 1 of the present invention.

[0048] Figure 9 This is the open-circuit voltage diagram of the composite aerogel provided in Comparative Example 1 of the present invention under RH90% conditions.

[0049] Figure 10 This is the open-circuit voltage diagram of the composite aerogel provided in Comparative Example 2 of the present invention under RH90% conditions.

[0050] Figure 11 This is the open-circuit voltage diagram of the composite aerogel provided in Comparative Example 3 of the present invention under RH90% conditions. Detailed Implementation

[0051] To further understand the present invention, the present invention will be described below with reference to embodiments. These descriptions are only for further explaining the features and advantages of the present invention and are not intended to limit the claims of the present invention.

[0052] This invention relates to an Ion-MXene / TOCNF composite aerogel material based on a gradient porous structure. The composite aerogel is composed of metal ions, MXene nanosheets, and TEMPO oxidized cellulose nanofibers (TOCNF) as its core components, exhibiting both a Z-axis gradient porous structure and Z-axis gradient wettability, enabling efficient wet-to-electrical energy conversion and demonstrating excellent wound-healing properties. The following details its raw material composition characteristics, microstructure characteristics, interfacial bonding mechanism, core properties, and mechanism of action.

[0053] The composite aerogel uses TEMPO oxidized cellulose nanofibers (TOCNF) as the overall framework of a three-dimensional porous network. Figure 1Compared to unmodified cellulose nanofibers, TOCNF exhibits superior dispersibility and interfacial bonding capabilities, enabling the formation of a stable, uniformly distributed three-dimensional framework. This avoids the framework collapse and uneven pore size issues common in traditional cellulose-based aerogels, making it a core substrate for constructing stable three-dimensional porous networks. TOCNF nanofibers range in diameter from 5 to 20 nm and length from 500 to 2000 nm. Nanofibers within this aspect ratio range can intertwine in an orderly manner, forming a continuous and mechanically stable three-dimensional network framework. This avoids both the loose framework and insufficient mechanical strength caused by excessively short fibers, as well as the aggregation and pore blockage caused by excessively long fibers, ensuring the integrity and uniformity of the aerogel's porous structure. The carboxyl content of TOCNF is 1.2~2.5 mmol / g. This range of carboxyl content gives TOCNF excellent monodispersity, effectively preventing agglomeration and sedimentation. Simultaneously, the abundant carboxyl active sites on the fiber surface can coordinate with metal ions in the system and form numerous intermolecular hydrogen bonds with the hydroxyl and fluorine groups on the MXene nanosheet surface, significantly improving the interfacial bonding strength of the three-phase components and fundamentally enhancing the structural stability of the composite aerogel. If the carboxyl content is too low, the interfacial compatibility of the raw materials decreases significantly, and the composite system is prone to delamination and failure; if the carboxyl content is too high, the fiber becomes too hydrophilic, resulting in poor moisture resistance of the aerogel. The mass concentration of TOCNF in the precursor solution corresponds to 0.2%~0.8%, which can construct a three-dimensional framework structure with appropriate density. If the framework density is too low, it will result in a sparse structure, weak mechanical properties, and easy collapse of the molded structure; if the framework density is too high, it will lead to framework densification, a significant reduction in material porosity, severely affecting water vapor adsorption and ion transport efficiency, and weakening the material's functional properties.

[0054] Composite aerogels, using MXene nanosheets as the conductive functional phase, can construct continuous conductive pathways within a three-dimensional framework, endowing the composite material with excellent conductivity and simultaneously assisting in the regulation of the aerogel's pore structure and surface wetting properties. MXene nanosheets exhibit few defects and high integrity, with lateral dimensions of 0.5–3 μm and layer thicknesses of 1–5 nm. These small-layer MXene nanosheets of this size can be uniformly intercalated within the gaps in the TOCNF framework, constructing a continuous conductive network without clogging porous channels, effectively avoiding the problems of large-size layer agglomeration and stacking, and the breakage of conductive pathways in small-size layers. The resistivity of MXene nanosheet powder is 0.5–2.0 Ω. MXene nanosheets possess excellent properties of low resistance and high conductivity, significantly reducing the overall volume resistivity of composite aerogels and providing a core foundation for rapid charge migration and wet-to-electric conversion within the system. The mass addition ratio of MXene nanosheets to TOCNF is 5%~20%. When the addition ratio is less than 5%, the content of conductive functional phase is insufficient, failing to form a continuous conductive pathway, resulting in extremely poor overall conductivity of the aerogel. When the addition ratio is greater than 20%, excessive MXene is prone to layer stacking and aggregation, directly blocking the porous channels of the aerogel, reducing the material's water vapor adsorption capacity, and simultaneously destroying the porous structure of the three-dimensional framework, leading to a significant decrease in the material's flexibility and mechanical stability.

[0055] The composite aerogel is doped with metal ions derived from the dissociation of water-soluble inorganic metal salts, which significantly enhances the spontaneous hygroscopic properties and charge density of the system, optimizes ion transport efficiency, and further strengthens the overall structural stability of the composite material through the coordination and cross-linking effects of the metal ions. The three metal ions are derived from highly water-soluble inorganic metal salts such as LiCl, MgCl2, and FeCl3, with monovalent metal salts preferred to ensure uniform ion dispersion and biocompatibility. The molar concentration of metal ions in the precursor solution is 0.05–0.2 mol / L. When the metal ion concentration is below this range, the number of hygroscopic active sites on the material surface is insufficient, the system charge density is low, and efficient water vapor ionization and charge enrichment cannot be achieved, ultimately resulting in weak wet-to-electric conversion output performance. When the metal ion concentration is above this range, excess metal salts easily crystallize and precipitate inside the aerogel pores, blocking the gradient pore structure, destroying the material's dual-gradient characteristics, and reducing biocompatibility, which is detrimental to wound healing applications. Metal ions uniformly doped in the system can be stably adsorbed on the carboxyl sites of TOCNF and the surface of MXene sheets, constructing a high-density, continuous ion transport channel and significantly improving the spontaneous moisture absorption capacity of the composite material.

[0056] The Ion-MXene / TOCNF composite aerogel possesses a unique three-dimensional anisotropic porous structure and a dual gradient characteristic in the Z-axis. The material exhibits differentiated pore size distribution patterns in the X and Y planes and the Z-axis thickness direction. This microstructure can be visually verified through SEM images. Figure 1(c and f in the text). The aerogel exhibits uniform pore distribution and size in both the X and Y directions, with no significant pore defects or density variations. The overall pore size distribution in the planar direction ranges from 200 to 500 nm. This uniform planar pore structure ensures uniform water vapor adsorption and charge distribution within the material's plane, effectively mitigating local performance differences and improving the overall stability of the material. The aerogel also exhibits a significant pore size gradient in the Z-direction thickness direction, showing a continuous gradient change from smaller pores on the upper surface to larger pores on the lower surface. The upper surface pores are concentrated in the 100–300 nm range, with a dense pore structure and a smooth, regular surface. The lower surface pores are concentrated in the 500–1000 nm range, with loose pores and excellent interconnectivity. The dense microporous structure on the upper surface can effectively slow down the loss of moisture inside the material and maintain a stable micro-humid environment inside the system; the loose and interconnected macroporous structure on the lower surface can quickly absorb environmental moisture and wound exudate, accelerate the diffusion of moisture into the material, and provide core structural support for continuous and stable moisture-electric conversion and wound ventilation and healing.

[0057] Based on the Z-axis pore size gradient structure and the gradient distribution characteristics of MXene and metal ions, the composite aerogel possesses Z-axis gradient wettability, with a regular wettability gradient difference between the upper and lower surfaces of the material. Figure 2 Specifically, all samples exhibited a uniform pattern where the water contact angle on the upper surface was higher than that on the lower surface, forming a wetting gradient that transitioned from relatively weak hydrophobic to hydrophilic along the thickness direction. The gradient amplitudes varied among the samples, with Fe-MXene / TOCNF showing the most prominent gradient effect, followed by Mg-MXene / TOCNF. Li-MXene / TOCNF was generally hydrophilic but still maintained a gradient characteristic with a higher contact angle on the upper surface. The upper surface of the aerogel is relatively more hydrophobic, which can moderately block the intrusion of excessive liquid water from the outside, alleviate excessive swelling and structural collapse of the material, and reduce the adhesion of external pollutants and bacteria, thus greatly improving the stability and anti-pollution ability of the material. The lower surface of the aerogel is more hydrophilic, which can quickly adsorb environmental moisture and wound exudate, providing a source of moisture for spontaneous moisture absorption and electricity generation, while effectively draining excess body fluid from the wound and maintaining a dry and breathable healing environment. The wettability of the material inside gradually and continuously changes to hydrophilicity from top to bottom, forming a complete wetting gradient system, which can ensure the directional and orderly transport of water vapor and ions inside the material, build a stable and continuous moisture-electric conversion channel, and avoid the problems of water vapor accumulation and ion transport interruption.

[0058] In the composite aerogel, metal ions, MXene nanosheets, and the TOCNF three-phase structure form a stable composite structure through chemical bonding and intercalation, which can be characterized and verified by XRD and FT-IR spectroscopy. XRD crystal structure testing results show that ( Figure 3(a) Pure MXene nanosheets exhibit sharp diffraction peaks characteristic of layer stacking, while pure TOCNF shows broad diffraction peaks characteristic of amorphous cellulose. However, in the Ion-MXene / TOCNF composite aerogel, the intensity of the MXene characteristic diffraction peaks significantly decreases, the peak width increases, and the diffraction peaks shift slightly. This demonstrates that TOCNF molecular chains and metal ions successfully insert into the interlayer gaps of MXene, achieving further intercalation, exfoliation, and uniform dispersion of MXene, effectively suppressing the aggregation and stacking of MXene sheets. Simultaneously, the absence of metal salt crystal characteristic peaks in the spectrum confirms that metal ions exist in the composite system in a uniformly coordinated doping form, without crystallization, ensuring the structural uniformity and long-term stability of the composite material. FT-IR results further confirm (…). Figure 3 (b) In the composite system, multiple stable interfacial interactions are constructed: On the one hand, the carboxyl and hydroxyl groups on the surface of TOCNF form stable metal coordination bonds with metal ions, which can fix free metal ions and improve the system's charge storage and transport capabilities; on the other hand, the hydroxyl and fluorine groups on the surface of MXene form a large number of intermolecular hydrogen bonds with the polar groups of TOCNF, which significantly strengthens the three-phase interfacial bonding force; at the same time, metal ions can act as cross-linking bridges to achieve ordered cross-linking and composite of MXene and TOCNF, ultimately constructing a three-dimensional cross-linked network with stable structure and high transport efficiency, which solves the technical defects of poor compatibility, easy delamination and unstable structure of traditional composite aerogel raw materials.

[0059] The Ion-MXene / TOCNF dual-gradient composite aerogel, relying on its unique gradient porous and gradient wetting structure and three-phase synergistic composite system, possesses excellent wet-to-electrochemical conversion properties and wound-healing promotion properties, both of which are supported by well-established mechanisms of action. Regarding wet-to-electrochemical conversion properties, the aerogel rapidly adsorbs environmental moisture through its hydrophilic macroporous structure on its lower surface (…). Figure 4(This describes the spontaneous hygroscopicity changes of three types of Ion-MXene / TOCNF composite aerogels under different relative humidity environments). After water vapor penetrates into the material, the uniformly distributed metal ions within the system rapidly hydrate and ionize, generating a large number of cations and anions and constructing a high-concentration ion system. Simultaneously, the unique pore size gradient and wettability gradient in the Z-axis of the material form a stable ion concentration difference driving force, promoting the directional migration of ions along the thickness direction. Combined with the continuous high-speed conductive network constructed by MXene, rapid charge transfer and collection are achieved, resulting in a stable potential difference between the upper and lower surfaces of the aerogel, ultimately completing continuous and efficient wet-to-electric energy conversion. Furthermore, the gradient structure effectively avoids disordered ion diffusion and charge recombination problems, significantly improving the wet-to-electric conversion efficiency and output stability. In terms of wound healing performance, the composite material has multi-dimensional advantages in wound repair: First, the Z-axis gradient pore structure can achieve bidirectional regulation of breathability and moisture retention. The large pores on the lower surface quickly drain wound exudate and prevent fluid accumulation and inflammation, while the hydrophobic pores on the upper surface lock in appropriate internal moisture, maintaining a moist healing environment for the wound and blocking external bacterial invasion to reduce the risk of infection. Second, the weak and stable bio-microcurrent generated by the material's wet-electro-conversion can simulate the bioelectric signals of human skin, effectively promoting fibroblast proliferation and migration, accelerating collagen deposition, and promoting rapid regeneration of wound epidermal tissue, thus shortening the healing cycle. Third, the uniformly distributed metal ions in the system can gently regulate the wound microenvironment, inhibit the reproduction of harmful bacteria, and reduce local inflammatory responses. Combined with the excellent biocompatibility of TOCNF and MXene, it can provide a clean, stable, and non-irritating repair environment for wound healing, significantly improving the wound healing effect.

[0060] This embodiment discloses a method for preparing self-powered gradient porous Ion-MXene / TOCNF aerogels based on gradient freezing technology and intercalation modification with inorganic salt metal ions of different valence states. MXene is intercalated with three metal ions of different valence states: LiCl, MgCl2, and FeCl3. A Z-axis gradient porous framework structure is constructed using controllable unidirectional temperature gradient freezing technology. Finally, a self-powered composite aerogel material with excellent wet gas power generation performance is obtained. The specific preparation steps are as follows:

[0061] (1) Preparation of few-layer high-purity MXene nanosheets

[0062] Ti3AlC2 precursor powder was etched and exfoliated using a mild LiF / HCl etching system. The entire process was carried out at a constant temperature of 35℃ and a stirring speed of 500 rpm for 24 hours to thoroughly remove the Al atomic layers from Ti3AlC2. After the etching reaction, the mixture was subjected to multiple centrifugal washings until the pH of the supernatant stabilized at 6-7, completely removing residual impurities and acidic ions. The purified MXene precipitate was redispersed in deionized water and ultrasonically exfoliated for 30 minutes under ice-water bath conditions to weaken the interlayer forces of MXene sheets and achieve multilayer exfoliation. Finally, the supernatant was collected by gradient centrifugation and dried to obtain MXene nanosheets with few layers, high purity, and excellent dispersibility.

[0063] (2) Preparation of homogeneous precursor solution

[0064] Three 1 mol / L aqueous solutions of metal salts with different valence states (LiCl, MgCl2, and FeCl3) were precisely prepared, along with a 2 mg / mL aqueous dispersion of MXene. The prepared metal salt solutions and MXene dispersions were then thoroughly stirred and ultrasonically defoamed to ensure uniform dispersion, absence of particle agglomeration, and no precipitation, thus constructing a stable and homogeneous liquid-phase reaction system. This provides a favorable reaction foundation for the subsequent uniform intercalation modification of MXene sheets with metal ions of different valence states.

[0065] (3) MXene modified by intercalation of metal ions with different valence states

[0066] A quantitatively purified MXene aqueous dispersion was mixed with LiCl, MgCl2, and FeCl3 metal salt solutions according to experimental proportions. The mixtures were placed in a constant-temperature stirred reactor, and the temperature was set at 40℃ and the stirring speed at 500 rpm for 48 hours. The reaction was carried out using different valence states of Li... + Mg 2+ Fe 3+ Through the continuous penetration and intercalation of metal ions, the spacing between MXene sheets and the surface electronic structure are precisely controlled, thereby achieving differentiated intercalation modification of MXene and ultimately obtaining three uniformly dispersed and stable metal ion-intercalated MXene composite dispersions.

[0067] (4) Intercalation-modified MXene purification treatment

[0068] The three composite mixtures that underwent the intercalation reaction were subjected to high-speed centrifugation and repeated washing with deionized water to thoroughly remove unreacted free metal ions, salt impurities, and MXene sheet aggregates, thus eliminating the influence of residual impurities on the porous structure and electrical properties of the material. Finally, the purified precipitates were resuspended in deionized water to obtain highly dispersed and high-purity Li-MXene, Mg-MXene, and Fe-MXene ion intercalation modified dispersions.

[0069] (5) Preparation of Ion-MXene / TOCNF composite colloid

[0070] The purified Ion-MXene dispersion and TOCNF (cellulose nanofiber) dispersion were precisely mixed according to a preset ratio. The mixture was thoroughly mixed by a combination of high-speed vortex oscillation and low-speed stirring, so that the modified MXene nanosheets were uniformly loaded on the surface and gaps of the TOCNF three-dimensional framework. This achieved uniform composite microstructure of the two-phase materials, and prepared an Ion-MXene / TOCNF composite colloid with good stability and no agglomeration or sedimentation. The composite colloid was then smoothly injected into a special gradient molding mold.

[0071] (6) Gradient freeze-molding preparation of gradient porous aerogels

[0072] This experiment employed a unidirectional temperature gradient segmented freezing process to construct a Z-axis gradient porous structure for the material. A stable unidirectional temperature gradient was formed by utilizing the differentiated thermal conductivity of the mold: a copper metal mold was fitted to the cold wall of a refrigerator as the cold source side, leveraging copper's high thermal conductivity to create a rapid cooling end. The other side of the mold was wrapped with insulating plastic to slow down the heat dissipation rate, creating a gentler cooling end. This resulted in a stable unidirectional temperature gradient within the colloid inside the mold, inducing the directional growth of ice crystals. The specific freezing process was as follows: First, the mold filled with the colloid was placed in a -20℃ refrigerator and frozen for 4 hours, relying on the unidirectional temperature gradient to initially form a gradient-distributed ice crystal framework. Then, it was rapidly transferred to a -40℃ environment and frozen for 2 hours to quickly solidify the initially formed ice crystal structure, preventing structural collapse and gradient failure due to recrystallization upon rewarming. Finally, it was rapidly transferred to a -80℃ low-temperature environment and frozen continuously for 6 hours to completely fix the gradient porous precursor structure of the composite colloid. After freezing, the sample was placed at -80℃ for vacuum freeze-drying for 72 hours. Internal ice crystals were completely removed by vacuum sublimation, ultimately yielding a graded porous Li-MXene / TOCNF composite aerogel with well-structured structure, controllable pore size gradient, and uniform morphology, modified by different metal ion intercalation. The micro-gradient porous morphology characteristics of the material are as follows: Figure 1 As shown in d and e.

[0073] (7) Assembly and electrical performance testing of self-powered devices

[0074] The three types of gradient porous composite aerogels were prepared and cut into standard test specimens of uniform size. Polyimide tape was used to fully encapsulate the sides of the specimens, effectively shielding them from test interference caused by lateral moisture transport and charge leakage. Conductive electrodes were tightly attached to the flat surfaces of the aerogels to complete the assembly of a moisture-driven self-powered generator. The assembled device was placed in a high-humidity environment (RH 90%) for electrical performance testing. The test results show that the modified gradient porous aerogel device can output a stable open-circuit voltage and possesses excellent and continuous self-powered output performance. Specific electrical output performance data of the device are shown in the figure.

[0075] Example 1: Preparation of LiCl intercalation modified gradient porous Li-MXene / TOCNF self-energized gas gel

[0076] This embodiment discloses a method for preparing a gradient porous Li-MXene / TOCNF self-energized gas gel modified by lithium chloride (LiCl) monovalent metal ion intercalation, using Li + Intercalation modification of MXene nanosheets was performed, and a Z-axis gradient porous framework was constructed using a controllable unidirectional temperature gradient segmented freezing technique to prepare a self-powered composite aerogel material with excellent moisture-generating performance. The specific preparation steps are as follows:

[0077] (1) Weigh 3.00 g of LiF powder into a polytetrafluoroethylene beaker, add 60.0 mL of 9 mmol / L HCl solution, and stir magnetically at 35 °C for a certain period of time to completely dissolve the LiF. Weigh 3.00 g of Ti3AlC2 powder and slowly add it to the above LiF / HCl solution to form a reaction suspension. Continue to react this reaction suspension in a water bath heating system at 35 °C for 24 h, while continuously stirring magnetically at 500 rpm. After the reaction is completed, transfer the suspension to a 250 mL polypropylene centrifuge tube, centrifuge at 7500 rpm for 5 min, and wash the precipitate with deionized water. Repeat the above steps several times until the pH of the supernatant after the last centrifugation reaches the range of 6-7. Disperse the precipitate from the last centrifugation in deionized water and transfer it to a 50 mL conical tube. The obtained precipitate was dispersed in a certain amount of deionized water and ultrasonically exfoliated for 30 min under an ice-water bath using a cell disruptor. The ultrasonically exfoliated solution was centrifuged at 3000 rpm for 30 min, the supernatant was collected, dried, and MXene nanosheets were obtained.

[0078] (2) Weigh 4.239 g of LiCl powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 1 mol / L LiCl solution; weigh 200 mg of MXene nanosheet powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 2 mg / mL MXene nanosheet dispersion. The prepared LiCl solution and MXene dispersion were thoroughly stirred and ultrasonically defoamed to ensure uniform dispersion of both systems, with no particle agglomeration or precipitation, thus constructing a stable and homogeneous liquid-phase reaction system for subsequent LiCl reaction. + Ion-modified MXene sheets provide a good reaction basis.

[0079] (3) Take the quantitatively purified MXene aqueous dispersion and mix it with the prepared LiCl metal salt solution according to the preset experimental ratio (volume ratio 4:1). Place the mixture in a constant temperature stirred reactor, set the temperature to 40℃ and the stirring speed to 500rpm, and continue the constant temperature and stirring reaction for 48h. Utilize monovalent Li + Through continuous penetration and intercalation, the spacing between MXene sheets and the surface electronic structure are precisely controlled to achieve differentiated intercalation modification of MXene, ultimately obtaining a uniformly dispersed and highly stable Li-MXene composite dispersion.

[0080] (4) The Li-MXene composite mixture after the intercalation reaction was completed was centrifuged at 7500 rpm for 5 min, and then washed repeatedly with deionized water 3-5 times to remove unreacted free Li in the system. + Salt impurities and MXene sheet aggregates were eliminated to remove residual impurities and mitigate their impact on the porous structure and electrical properties of the material. Finally, the purified precipitate was resuspended in deionized water to obtain a highly dispersed and pure Li-MXene ion-intercalated modified dispersion.

[0081] (5) The purified Li-MXene dispersion and TOCNF (cellulose nanofiber) dispersion are precisely mixed according to the preset ratio (volume ratio 1:15). The mixture is thoroughly mixed by a combination of high-speed vortex oscillation and low-speed stirring, so that the modified MXene nanosheets are uniformly loaded on the surface and gaps of the TOCNF three-dimensional framework, and the microstructure of the two phase materials is uniformly composited to prepare a Li-MXene / TOCNF composite colloid with good stability and no agglomeration and sedimentation. The composite colloid is then smoothly injected into a special mold.

[0082] (6) This experiment uses a unidirectional temperature gradient segmented freezing process to construct a Z-axis gradient porous structure, relying on the differential thermal conductivity of the mold to form a stable unidirectional temperature gradient: a copper metal mold is selected and attached to the cold wall of the refrigerator as the cold source side, and the high thermal conductivity of copper is used to form a rapid cooling end. The other side of the mold is wrapped with insulating plastic material to slow down the heat dissipation rate and form a gentle cooling end, so that the colloid inside the mold forms a stable unidirectional temperature gradient, inducing the directional growth of ice crystals. The specific freezing process is as follows: First, the mold filled with colloid is placed in a -20℃ refrigerator and frozen for 4 hours, relying on the unidirectional temperature gradient to initially form a gradient distribution ice crystal skeleton; then it is quickly transferred to a -40℃ environment and frozen for 2 hours to quickly solidify the initially formed ice crystal structure and avoid the structure from collapsing and the gradient from recrystallizing due to the temperature rise of the ice crystals; finally, it is quickly transferred to a -80℃ low temperature environment and frozen for 6 hours to completely fix the gradient porous precursor structure of the composite colloid. After freezing, the sample was placed at -80℃ for vacuum freeze-drying for 72 hours. Internal ice crystals were completely removed by vacuum sublimation, ultimately yielding a Li-MXene / TOCNF gradient porous composite aerogel with a complete structure, controllable pore size gradient, and uniform morphology (see actual image). Figure 1 (d, e in the text).

[0083] (7) The prepared Li-MXene / TOCNF gradient porous composite aerogel was cut into standard test specimens of uniform size. Polyimide tape was used to fully encapsulate the sides of the specimens, effectively shielding them from test interference caused by lateral water vapor transport and charge leakage. Conductive electrodes were tightly attached to the flat surfaces of the aerogel to complete the assembly of the moisture-driven self-powered generator. The assembled device was placed in a high-humidity environment (RH 90%) for electrical performance testing. The modified gradient porous aerogel device could output a stable open-circuit voltage (the specific voltage output trend is as follows). Figure 5 As shown, it has a maximum voltage of approximately 550mV and excellent and continuous self-powered output performance.

[0084] Example 2: Preparation of MgCl2 intercalated modified gradient porous Mg-MXene / TOCNF self-energized gas gel

[0085] This embodiment discloses a method for preparing a gradient porous Mg-MXene / TOCNF self-energized gas gel modified by divalent metal ion intercalation of magnesium chloride (MgCl2), using Mg 2+ Intercalation modification of MXene nanosheets was performed, and a Z-axis gradient porous framework was constructed using a controllable unidirectional temperature gradient segmented freezing technique to prepare a self-powered composite aerogel material with excellent moisture-generating performance. The specific preparation steps are as follows:

[0086] (1) Weigh 3.00 g of LiF powder into a polytetrafluoroethylene beaker, add 60.0 mL of 9 mmol / L HCl solution, and stir magnetically at 35 °C for a certain period of time to completely dissolve the LiF. Weigh 3.00 g of Ti3AlC2 powder and slowly add it to the above LiF / HCl solution to form a reaction suspension. Continue to react this reaction suspension in a water bath heating system at 35 °C for 24 h, while continuously stirring magnetically at 500 rpm. After the reaction is completed, transfer the suspension to a 250 mL polypropylene centrifuge tube, centrifuge at 7500 rpm for 5 min, and wash the precipitate with deionized water. Repeat the above steps several times until the pH of the supernatant after the last centrifugation reaches the range of 6-7. Disperse the precipitate from the last centrifugation in deionized water and transfer it to a 50 mL conical tube. The obtained precipitate was dispersed in a certain amount of deionized water and ultrasonically exfoliated for 30 min under an ice-water bath using a cell disruptor. The ultrasonically exfoliated solution was centrifuged at 3000 rpm for 30 min, the supernatant was collected, dried, and MXene nanosheets were obtained.

[0087] (2) Weigh 9.521 g of MgCl2 powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 1 mol / L MgCl2 solution; weigh 200 mg of MXene nanosheet powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 2 mg / mL MXene nanosheet dispersion. The prepared MgCl2 solution and MXene dispersion were thoroughly stirred and ultrasonically defoamed to ensure uniform dispersion of both systems, with no particle agglomeration or precipitation, thus constructing a stable and homogeneous liquid-phase reaction system for subsequent Mg... 2+ Uniform intercalation modification of MXene sheets provides a good reaction basis.

[0088] (3) Take the quantitatively purified MXene aqueous dispersion and mix it with the prepared MgCl2 metal salt solution according to the preset experimental ratio (volume ratio 4:1). Place the mixture in a constant temperature stirred reactor, set the temperature to 40℃ and the stirring speed to 500rpm, and continue the constant temperature and stirring reaction for 48h. Utilize divalent Mg 2+ Through continuous penetration and intercalation, the spacing between MXene layers and the surface electronic structure are precisely controlled to achieve differentiated intercalation modification of MXene, ultimately obtaining a uniformly dispersed and highly stable Mg-MXene composite dispersion.

[0089] (4) The Mg-MXene composite mixture that has completed the intercalation reaction is centrifuged at 7500 rpm for 5 min, and then washed repeatedly with deionized water 3 to 5 times to remove unreacted free Mg from the system. 2+Salt impurities and MXene sheet aggregates were eliminated to remove residual impurities and mitigate their impact on the porous structure and electrical properties of the material. Finally, the purified precipitate was resuspended in deionized water to obtain a highly dispersed and pure Mg-MXene ion-intercalated modified dispersion.

[0090] (5) The purified Mg-MXene dispersion and TOCNF (cellulose nanofiber) dispersion are precisely mixed according to the preset ratio (volume ratio 1:15). The mixture is thoroughly mixed by a combination of high-speed vortex oscillation and low-speed stirring, so that the modified MXene nanosheets are uniformly loaded on the surface and gaps of the TOCNF three-dimensional framework, and the microstructure of the two phase materials is uniformly composited to prepare a Mg-MXene / TOCNF composite colloid with good stability and no agglomeration and sedimentation. The composite colloid is then smoothly injected into a special mold.

[0091] (6) This experiment uses a unidirectional temperature gradient segmented freezing process to construct a Z-axis gradient porous structure, relying on the differential thermal conductivity of the mold to form a stable unidirectional temperature gradient: a copper metal mold is selected and attached to the cold wall of the refrigerator as the cold source side, and the high thermal conductivity of copper is used to form a rapid cooling end. The other side of the mold is wrapped with insulating plastic material to slow down the heat dissipation rate and form a gentle cooling end, so that the colloid inside the mold forms a stable unidirectional temperature gradient, inducing the directional growth of ice crystals. The specific freezing process is as follows: First, the mold filled with colloid is placed in a -20℃ refrigerator and frozen for 4 hours, relying on the unidirectional temperature gradient to initially form a gradient distribution ice crystal skeleton; then it is quickly transferred to a -40℃ environment and frozen for 2 hours to quickly solidify the initially formed ice crystal structure and avoid the structure from collapsing and the gradient from recrystallizing due to the temperature rise of the ice crystals; finally, it is quickly transferred to a -80℃ low temperature environment and frozen for 6 hours to completely fix the gradient porous precursor structure of the composite colloid. After freezing, the sample was placed at -80℃ for 72 hours of vacuum freeze-drying. The internal ice crystals were completely removed by vacuum sublimation, and finally, a Mg-MXene / TOCNF gradient porous composite aerogel with complete structure, controllable pore size gradient and uniform morphology was obtained.

[0092] (7) The prepared Mg-MXene / TOCNF gradient porous composite aerogel was cut into standard test samples of uniform size. Polyimide tape was used to fully encapsulate the sides of the samples, effectively shielding them from test interference caused by lateral water vapor transport and charge leakage. Conductive electrodes were tightly attached to the flat surfaces of the aerogel to complete the assembly of the moisture-driven self-powered generator. The assembled device was placed in a high-humidity environment (RH 90%) for electrical performance testing. The modified gradient porous aerogel device could output a stable open-circuit voltage (the specific voltage output trend is as follows). Figure 6 As shown, the maximum voltage is approximately 516mV, which provides excellent and continuous self-powered output performance.

[0093] Example 3: Preparation of FeCl3 intercalated modified gradient porous Fe-MXene / TOCNF self-energized gas gel

[0094] This embodiment discloses a method for preparing a gradient porous Fe-MXene / TOCNF self-energized gas gel modified by ferric chloride (FeCl3) trivalent metal ion intercalation, using Fe... 3+ Intercalation modification of MXene nanosheets was performed, and a Z-axis gradient porous framework was constructed using a controllable unidirectional temperature gradient segmented freezing technique to prepare a self-powered composite aerogel material with excellent moisture-generating performance. The specific preparation steps are as follows:

[0095] (1) Weigh 3.00 g of LiF powder into a polytetrafluoroethylene beaker, add 60.0 mL of 9 mmol / L HCl solution, and stir magnetically at 35 °C for a certain period of time to completely dissolve the LiF. Weigh 3.00 g of Ti3AlC2 powder and slowly add it to the above LiF / HCl solution to form a reaction suspension. Continue to react this reaction suspension in a water bath heating system at 35 °C for 24 h, while continuously stirring magnetically at 500 rpm. After the reaction is completed, transfer the suspension to a 250 mL polypropylene centrifuge tube, centrifuge at 7500 rpm for 5 min, and wash the precipitate with deionized water. Repeat the above steps several times until the pH of the supernatant after the last centrifugation reaches the range of 6-7. Disperse the precipitate from the last centrifugation in deionized water and transfer it to a 50 mL conical tube. The obtained precipitate was dispersed in a certain amount of deionized water and ultrasonically exfoliated for 30 min under an ice-water bath using a cell disruptor. The ultrasonically exfoliated solution was centrifuged at 3000 rpm for 30 min, the supernatant was collected, dried, and MXene nanosheets were obtained.

[0096] (2) Weigh 16.220 g of FeCl3 powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 1 mol / L FeCl3 solution; weigh 200 mg of MXene nanosheet powder into a beaker, add 100 mL of deionized water and dissolve thoroughly to obtain a 2 mg / mL MXene nanosheet dispersion. The prepared FeCl3 solution and MXene dispersion were thoroughly stirred and ultrasonically defoamed to ensure uniform dispersion of both systems, with no particle agglomeration or precipitation, thus constructing a stable and homogeneous liquid-phase reaction system for subsequent Fe... 3+ Uniform intercalation modification of MXene sheets provides a good reaction basis.

[0097] (3) Take the quantitatively purified MXene aqueous dispersion and mix it with the prepared FeCl3 metal salt solution according to the preset experimental ratio (volume ratio 4:1). Place the mixture in a constant temperature stirred reactor, set the temperature to 40℃ and the stirring speed to 500rpm, and continue the constant temperature and stirring reaction for 48h. Utilize trivalent Fe3+ Through continuous penetration and intercalation, the spacing between MXene sheets and the surface electronic structure are precisely controlled to achieve differentiated intercalation modification of MXene, ultimately obtaining a Fe-MXene composite dispersion with uniform dispersion and good stability.

[0098] (4) The Fe-MXene composite mixture after the intercalation reaction was completed was centrifuged at 7500 rpm for 5 min, and then washed repeatedly with deionized water 3 to 5 times to remove unreacted free Fe from the system. 3+ Salt impurities and MXene sheet aggregates were eliminated to remove residual impurities and mitigate their impact on the porous structure and electrical properties of the material. Finally, the purified precipitate was resuspended in deionized water to obtain a highly dispersed and pure Fe-MXene ion-intercalated modified dispersion.

[0099] (5) The purified Fe-MXene dispersion and TOCNF (cellulose nanofiber) dispersion are precisely mixed according to the preset ratio (volume ratio 1:15). The mixture is thoroughly mixed by a combination of high-speed vortex oscillation and low-speed stirring, so that the modified MXene nanosheets are uniformly loaded on the surface and gaps of the TOCNF three-dimensional framework, and the microstructure of the two phase materials is uniformly composited to prepare a Fe-MXene / TOCNF composite colloid with good stability and no agglomeration and sedimentation. The composite colloid is then smoothly injected into a special mold.

[0100] (6) This experiment uses a unidirectional temperature gradient segmented freezing process to construct a Z-axis gradient porous structure, relying on the differential thermal conductivity of the mold to form a stable unidirectional temperature gradient: a copper metal mold is selected and attached to the cold wall of the refrigerator as the cold source side, and the high thermal conductivity of copper is used to form a rapid cooling end. The other side of the mold is wrapped with insulating plastic material to slow down the heat dissipation rate and form a gentle cooling end, so that the colloid inside the mold forms a stable unidirectional temperature gradient, inducing the directional growth of ice crystals. The specific freezing process is as follows: First, the mold filled with colloid is placed in a -20℃ refrigerator and frozen for 4 hours, relying on the unidirectional temperature gradient to initially form a gradient distribution ice crystal skeleton; then it is quickly transferred to a -40℃ environment and frozen for 2 hours to quickly solidify the initially formed ice crystal structure and avoid the structure from collapsing and the gradient from recrystallizing due to the temperature rise of the ice crystals; finally, it is quickly transferred to a -80℃ low temperature environment and frozen for 6 hours to completely fix the gradient porous precursor structure of the composite colloid. After freezing, the sample was placed at -80℃ for vacuum freeze-drying for 72 hours. The internal ice crystals were completely removed by vacuum sublimation, and finally, Fe-MXene / TOCNF gradient porous composite aerogel with complete structure, controllable pore size gradient and uniform morphology was obtained.

[0101] (7) The prepared Fe-MXene / TOCNF gradient porous composite aerogel was cut into standard test specimens of uniform size. Polyimide tape was used to fully encapsulate the sides of the specimens, effectively shielding them from test interference caused by lateral water vapor transport and charge leakage. Conductive electrodes were tightly attached to the flat surfaces of the aerogel to complete the assembly of the moisture-driven self-powered generator. The assembled device was placed in a high-humidity environment (RH 90%) for electrical performance testing. The modified gradient porous aerogel device could output a stable open-circuit voltage (the specific voltage output trend is as follows). Figure 7 As shown, the maximum voltage is approximately 335mV, which provides excellent and continuous self-powered output performance.

[0102] Comparing the output voltages of Li-MXene / TOCNF, Mg-MXene / TOCNF, and Fe-MXene / TOCNF aerogel materials in Examples 1, 2, and 3, Li-MXene / TOCNF showed the best voltage output performance. Furthermore, stability testing revealed that after 7 days of output voltage testing, the voltage remained stable above 520mV, demonstrating good electrical signal stability. Figure 8 ).

[0103] Comparative Example 1: Preparation of Non-gradient Porous Li-MXene / TOCNF Aerogel

[0104] This comparative example uses a conventional direct cryogenic freezing process to prepare Li-MXene / TOCNF composite aerogels. Except for the cryogenic molding step, the raw material ratios, reaction parameters, purification processes, device assembly, and testing conditions are completely consistent with those in Example 1. Only the unidirectional temperature gradient segmented freezing process is omitted to verify the influence of the gradient freezing process on the material structure and self-powered performance. The specific preparation steps are as follows:

[0105] (1) to (5) are the same as in Example 1;

[0106] (6) The mixture in the mold was frozen overnight at -80°C by conventional freezing method (non-gradient freezing method), and then vacuum frozen at -80°C for 72 h. After that, the ice crystals were removed by sublimation to obtain Li-MXene / TOCNF aerogel.

[0107] (7) The obtained Li-MXene / TOCNF aerogel ( Figure 1 a and b) are cut into small pieces of the same size, the aerogel is wrapped around the edges with polyimide tape, and electrodes are attached to both ends of the aerogel to obtain a moisture-generating device. Figure 9 The diagram shows the open-circuit voltage of the aerogel material provided in Comparative Example 1 at 90% RH.

[0108] Comparative Example 2: Preparation of Non-gradient Porous Mg-MXene / TOCNF Aerogel

[0109] (1) to (5) are the same as in Example 2;

[0110] (6) The mixture in the mold was placed at -80°C overnight by conventional direct low temperature freezing process (non-gradient freezing method), and then vacuum frozen at -80°C for 72 hours. After that, the ice crystals were removed by sublimation to obtain Mg-MXene / TOCNF aerogel.

[0111] (7) Cut the obtained Mg-MXene / TOCNF aerogel into small pieces of the same size, wrap the aerogel around its sides with polyimide tape, and attach electrodes to both ends of the aerogel to obtain a moisture-generating device. Figure 10 This is a diagram showing the open-circuit voltage of the aerogel material provided in Comparative Example 2 at RH 90%.

[0112] Comparative Example 3: Preparation of Non-gradient Porous Fe-MXene / TOCNF Aerogel

[0113] (1) to (5) are the same as in Example 3;

[0114] (6) The mixture in the mold was frozen overnight at -80°C by conventional direct low temperature freezing process (non-gradient freezing method), and then vacuum frozen at -80°C for 72 hours. After that, the ice crystals were removed by sublimation to obtain Fe-MXene / TOCNF aerogel.

[0115] (7) Cut the obtained Fe-MXene / TOCNF aerogel into small pieces of the same size, wrap the aerogel around its sides with polyimide tape, and attach electrodes to both ends of the aerogel to obtain a moisture-generating device. Figure 11 The diagram shows the open-circuit voltage of the aerogel material provided in Comparative Example 3 at RH 90%.

Claims

1. A composite aerogel material, characterized in that, The composite aerogel uses TEMPO oxidized cellulose nanofibers as a three-dimensional porous network framework and MXene nanosheets as a conductive functional phase, with metal ions intercalated in the interlayer spaces. The composite aerogel has dual gradient characteristics of Z-axis gradient porous structure and Z-axis gradient wettability. The Z-axis gradient porous structure exhibits a continuous gradient characteristic of small pore size on the upper surface and large pore size on the lower surface along the material thickness direction. The Z-axis gradient wettability exhibits a continuous gradient characteristic of weak hydrophobicity on the upper surface and strong hydrophilicity on the lower surface along the material thickness direction.

2. The composite aerogel material according to claim 1, characterized in that, The TEMPO oxidized cellulose nanofibers have a diameter of 5-20 nm and a length of 500-2000 nm; the carboxyl content of the TEMPO oxidized cellulose nanofibers is 1.2-2.5 mmol / g; and the mass concentration of the TEMPO oxidized cellulose nanofibers in the precursor solution is 0.2%-0.8%.

3. The composite aerogel material according to claim 1, characterized in that, The MXene nanosheets have a lateral dimension of 0.5–3 μm and a sheet thickness of 1–5 nm; the powder resistivity of the MXene nanosheets is 0.5–2.0 Ω. cm; the mass addition ratio of the MXene nanosheets to the TEMPO oxidized cellulose nanofibers is 5%~20%.

4. The composite aerogel material according to claim 1, characterized in that, The metal ions are derived from water-soluble inorganic metal salts, which are selected from one or more of LiCl, KCl, NaCl, MgCl2, and FeCl3; the molar concentration of the metal ions in the precursor solution is 0.05~0.2 mol / L.

5. The composite aerogel material according to claim 4, characterized in that, The metal ions in the water-soluble inorganic metal salt are monovalent metal ions.

6. The composite aerogel material according to claim 1, characterized in that, The composite aerogel has a uniform pore distribution in the X and Y planes, with a pore size of 200~500 nm; the pore size on the upper surface of the composite aerogel is 100~300 nm, and the pore size on the lower surface is 500~1000 nm.

7. The composite aerogel material according to claim 1, characterized in that, The water contact angle of the upper surface of the composite aerogel is 90°~120°, and the water contact angle of the lower surface is 20°~40°.

8. The composite aerogel material according to claim 1, characterized in that, The carboxyl and hydroxyl groups on the surface of the TEMPO oxidized cellulose nanofibers form metal coordination bonds with metal ions, and the hydroxyl and fluorine groups on the surface of the MXene nanosheets form intermolecular hydrogen bonds with the polar groups of the TEMPO oxidized cellulose nanofibers. The metal ions act as crosslinking bridges to achieve orderly crosslinking between the MXene nanosheets and the TEMPO oxidized cellulose nanofibers, together forming a three-dimensional crosslinked network structure.

9. A method for preparing a composite aerogel material as described in any one of claims 1 to 8, characterized in that, Includes the following steps: (1) MXene nanosheets were intercalated with metal salts to obtain metal ion intercalated MXene nanosheets; (2) The metal ion intercalated modified MXene and TEMPO oxidized cellulose nanofiber dispersion were mixed evenly to obtain a composite colloid; (3) Apply a unidirectional temperature gradient along the thickness direction to the composite colloid and perform directional freezing. The temperature gradients are -20±5℃, -40±5℃ and -80±5℃ respectively, to obtain a frozen precursor with a Z-axis gradient structure. (4) The frozen precursor is subjected to vacuum freeze-drying to obtain Z-axis gradient porous composite aerogel material.

10. The use of the composite aerogel material according to any one of claims 1 to 8 in wet-to-electric energy conversion devices and / or wound repair materials.

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