A LiCl / cellulose nanofiber membrane and its preparation method, and an ion-conducting humidity sensor.

Abstract

This invention provides a LiCl / cellulose nanofiber composite membrane and its preparation method, as well as an ion-conducting humidity sensor, belonging to the field of biomass nanomaterials technology. Using cellulose pulp as raw material, the invention dissolves cellulose in an N,N-dimethylacetamide (DMAc) / lithium chloride (LiCl) solvent system. During electrospinning, DMAc volatilizes into the air, while LiCl remains within the cellulose nanofibers. This method allows for the preparation of LiCl / cellulose nanofiber films with varying LiCl contents. The humidity sensor prepared in this way exhibits high sensitivity, a wide detection range, and a fast response / recovery time, making it suitable for applications such as human respiration monitoring and non-contact humidity testing. This invention provides a new approach for constructing novel cellulose-based humidity sensors.

Description

Technical Field

[0001] This invention belongs to the field of biomass nanomaterials technology, specifically relating to a LiCl / cellulose nanofiber membrane and an ion-conducting humidity sensor. Background Technology

[0002] Humidity detection and management play a crucial role in human life and the operation of electronic products. Applications of humidity sensors range from atmospheric and soil treatment to industrial manufacturing, agricultural production, and physiological health management. Furthermore, moisture constantly evaporates from the human body due to primary and secondary perspiration. As a biomarker, ubiquitous human body humidity provides a non-contact human area sensor network for emerging wearable digital healthcare. Research on a high-performance non-contact, wearable humidity sensor has attracted widespread attention. Cellulose molecules contain abundant hydrophilic groups, exhibiting good water absorption and swelling properties, thus being considered an ideal candidate material for green humidity sensors. Due to the flexibility and versatility of cellulose in material design, cellulose-based sensors can achieve sensing through various methods such as resistance, capacitance, colorimetry, gravity, and microwave. Among these, resistance sensors are considered the most promising due to their simple structure, convenient readings, and high accuracy. However, since cellulose is non-conductive, many conductive materials, including CNTs, PPY, PANI, Ag, RGO, NaCl, and KOH, have been used to endow cellulose compounds with improved conductivity. Their conductivity mechanisms are divided into electronic conductivity and ionic conductivity.

[0003] Traditional cellulose-based humidity sensors are typically designed as two-dimensional materials, such as paper-based humidity sensors and cellulose nanofiber membrane-based humidity sensors. For humidity sensors, a wide response range, fast response time, and high sensitivity are key characteristics. Research shows that the size and structure of cellulose, as well as the content of its hydrophilic groups, significantly affect the sensing performance of cellulose humidity sensors. To improve the sensing performance of cellulose sensors, two strategies can be adopted: one is to nano-size cellulose to expose more hydroxyl groups, and then design the nanofiber structure. Recent studies have shown that cellulose nanofibers have a large specific surface area and high hydrophilicity, making them more suitable as raw materials for humidity sensors. Patent CN202410153128.5 discloses a cellulose nanofiber membrane and its preparation method, a cellulose nanofiber / carbon nanotube composite membrane and its preparation method, and a humidity sensor. The method involves preparing a cellulose nanofiber membrane using electrospinning cellulose spinning solution, washing and drying it, and then filtering carbon nanotubes onto the surface of the cellulose nanofibers as a conductive material to prepare a cellulose nanofiber / carbon nanotube humidity sensor with a fast response time. The second strategy is to improve the humidity sensitivity of the nanofibers by adding hydrophilic substances. Wang et al. published an article titled "Flexible and Transparent Cellulose-Based Ionic Film as a Humidity Sensor" in ACS Applied Materials & Interfaces (ACS Appl. Mater. Interfaces 2020, 12, 6, 7631–7638), which fabricated a cellulose / KOH composite ion-exchange membrane (CKF) humidity sensor. Due to the good solubility and ionization of KOH in water, the resistance of CKF is reduced, thus enabling a rapid humidity response.

[0004] Electrospinning is one of the fastest-growing methods for preparing nanofibers in recent years. It offers advantages such as simple equipment, a wide range of raw materials, and easy control over fiber structure. The prepared nanofibers have small diameters and large specific surface areas, and the resulting fiber membranes possess adjustable pore sizes and good pore connectivity. LiCl is a stable ionic compound. With increasing relative humidity, the amount of water adsorbed by LiCl and the number of conductive ions in LiCl also increase, leading to a decrease in its resistance. When the vapor pressure of LiCl exceeds the partial pressure of water vapor in the air, LiCl releases moisture, causing its resistance to increase. Therefore, LiCl is considered the optimal electrolytic material for ion-conductive humidity sensors.

[0005] Electrospun nanofibers possess advantages such as high specific surface area, large aspect ratio, light weight, and ease of modification. Furthermore, the diameter of electrospun nanofibers is less than 1 μm, which greatly exposes the hydroxyl groups contained in cellulose nanofibers. However, cellulose-based materials lack electrical conductivity due to their structural characteristics. Therefore, combining cellulose and its derivatives with other components, including carbon materials and polymers, has become a research hotspot for designing high-performance humidity sensors that meet various application requirements.

[0006] This invention utilizes electrospinning to prepare an ultrathin LiCl / cellulose nanofiber membrane with a macroporous structure based on cellulose material. Cellulose is dissolved in a DMAc / LiCl solvent system. During electrospinning, DMAc evaporates into the air, while LiCl remains within the cellulose nanofibers. The LiCl retained in cellulose exhibits strong hydrophilicity, enhancing the humidity detection range of the cellulose nanofibers (especially under low humidity conditions), resulting in a cellulose humidity sensor with excellent sensitivity (up to 4191%) and a wide detection range (5%–98% RH). The nanoscale size of the cellulose fibers, the extremely low thickness of the cellulose membrane, and the large pore size accelerate moisture exchange between the air and the membrane, thus giving the cellulose humidity sensor a fast response / recovery time (99 / 110 s) and low hysteresis. The sensor's performance does not degrade after long-term use, under high / low temperature conditions, and after thousands of bending cycles. The humidity sensor also has wide applications in non-contact humidity detection, respiration detection, and sleep apnea detection. Furthermore, based on the experimental results, the mechanism by which structural optimization enhances the humidity-sensitive performance of the sensor was analyzed. Finally, the application potential of the prepared sensor in the field of practical humidity measurement was explored, providing new ideas and references for the construction of novel cellulose-based humidity sensors. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a LiCl / cellulose nanofiber membrane and its preparation method, as well as an ion-conductive humidity sensor. The LiCl / cellulose nanofiber membrane provided by this invention has excellent hygroscopic capacity and good biodegradability. The humidity sensor prepared from it has high sensitivity, a wide detection range, and its performance does not degrade after long-term use, under high / low temperature conditions, and after thousands of bending cycles.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] (1) The hardwood pulp board is decomposed and dried to obtain cellulose pulp;

[0010] (2) Cellulose pulp was mixed with different proportions of DMAc and LiCl to obtain cellulose spinning solutions of different concentrations;

[0011] (3) The cellulose solutions of different concentrations are electrospun and then dried to obtain LiCl / cellulose nanofiber membranes with different LiCl contents.

[0012] Preferably, the hardwood pulp in step (1) comprises chemical pulp and / or dissolving pulp. The hardwood pulp has an α-cellulose content of 90-99%, a defatting time of 5-60 min, a drying temperature of 50-100℃, and a drying time of 8-15 h.

[0013] More preferably, the hardwood pulp is a dissolving pulp. The hardwood pulp has an α-cellulose content of 95-98%, a dissolving time of 15-30 min, a drying temperature of 80-100℃, and a drying time of 10-15 h.

[0014] Preferably, in step (2), the mass of DMAc accounts for 80-95% of the total mass of DMAc and LiCl, and the cellulose spinning solution of different concentrations is 1-10 wt%.

[0015] More preferably, in step (2), the mass of DMAc accounts for 90-95% of the total mass of DMAc and LiCl, and the cellulose spinning solution of different concentrations is 1-5 wt%.

[0016] Preferably, in the preparation process of LiCl / cellulose nanofiber membranes with different LiCl contents in step (3), the electrospinning conditions include: an applied voltage of 20-30KV, a cellulose spinning solution injection speed of 0.2-5mL / h, a receiving distance of 8-25cm, a hollow receiver as the collecting device, a spinning time of 2-10h, a drying temperature of 100-200℃ after forming, and a drying time of 0.5-3h. The LiCl / cellulose nanofibers obtained by the preparation method have a diameter of 20-800nm ​​and a thickness of 18-300μm. The LiCl / cellulose nanofibers interweave to form a porous structure with an average pore size of 2-10μm.

[0017] More preferably, in the preparation process of LiCl / cellulose nanofiber membranes with different LiCl contents in step (3), the electrospinning conditions include: the applied voltage is 23-28KV, the cellulose spinning solution injection speed is 2-3mL / h, the receiving distance is 10-20cm, the drying temperature after forming is 150-200℃, and the drying time is 1-3h.

[0018] This invention provides a LiCl / cellulose nanofiber composite membrane, comprising a cellulose nanofiber membrane and LiCl loaded on the surface of the cellulose nanofiber membrane; the LiCl / cellulose nanofiber membrane is the LiCl / cellulose nanofiber membrane described in the above scheme or a LiCl / cellulose nanofiber membrane prepared by any of the preparation methods described in the above scheme. The humidity sensor prepared thereby is expected to perform two functions: (1) cellulose is dissolved using a DMAc / LiCl solvent system, and during electrospinning, DMAc evaporates into the air, while LiCl is retained in the cellulose nanofibers. The LiCl retained on the cellulose has strong hydrophilicity, which improves the humidity detection range of the cellulose nanofibers (especially under low humidity conditions), giving the cellulose humidity sensor excellent sensitivity (up to 4191%) and a wide detection range (5% to 98% RH). (2) The nanoscale size of cellulose fibers, the extremely low thickness and large pore size of cellulose membrane accelerate the exchange of moisture between air and membrane, thus enabling the cellulose humidity sensor to have a fast response / recovery time (99 / 110s) and low hysteresis. The performance of the sensor does not degrade after long-term use, high / low temperature conditions and thousands of bending cycles, providing a new idea for the construction of novel cellulose-based humidity sensors.

[0019] This invention provides a method for preparing a LiCl / cellulose nanofiber membrane using cellulose pulp as raw material. Cellulose is a humidity-sensitive material because its molecular chain contains numerous hydroxyl groups that can form hydrogen bonds with water molecules. The principle behind the humidity-responsive LiCl / cellulose nanofiber composite membrane prepared by this invention is as follows: Under low humidity conditions, cellulose-based humidity sensors are insensitive to humidity; when humidity is <75%, there is no significant current change. This indicates that under low humidity conditions, although water molecules can partially ionize in an electrostatic field, no current is formed. However, LiCl in the cellulose nanofibers can enhance the absorption of water molecules by cellulose, giving the LiCl / cellulose-based humidity sensor a low humidity detection limit as low as 5%. This is because LiCl has strong water absorption and can ionize into LiCl under low humidity conditions. + and Cl - Ions. Under high humidity conditions, water molecules aggregate into a liquid water layer through physical adsorption, and further adsorbed water molecules form a layer during the physical adsorption process according to the Grotthuss chain reaction mechanism. At this point, for cellulose-based humidity sensors, some water molecules ionize into H+ and OH-. - An electric current is generated in the water layer on the surface of cellulose fibers; for LiCl / cellulose-based humidity sensors, an electric current is generated in the H+ formed by water ionization. + / OH - Li and LiCl ionization to form Li + / Cl -Under the synergistic effect of these factors, the sensor current increases significantly due to the intensified directional migration of ions.

[0020] The advantages of this invention are as follows: 1. The LiCl retained in cellulose has strong hydrophilicity, which improves the humidity detection range of cellulose fibers (especially under low humidity conditions), thus endowing the cellulose humidity sensor with excellent sensitivity. 2. The nanoscale size of cellulose fibers, the extremely low thickness of the cellulose membrane, and the large pore size accelerate the mass exchange of moisture between the air and the membrane, thereby giving the cellulose humidity sensor a fast response / recovery time. 3. The LiCl / cellulose nanofiber composite membrane prepared by this invention has high sensitivity (ΔI / I0 = 4190%), a wide response range of 5%RH to 98%RH, a response / recovery time of 99 / 110s, and a low hysteresis of 2.9%. Furthermore, even after long-term use (>30 days), high / low temperature (73.6 / 0.1℃), and thousands of bending cycles, the performance of the humidity sensor will not decrease. 4. Compared with traditional materials such as multi-walled carbon nanotubes, graphene, and polypyrrole as humidity-responsive materials, this invention directly utilizes LiCl in the DMAc / LiCl system of dissolved cellulose as a conductive material, and prepares a LiCl / cellulose nanofiber composite membrane in one step through electrospinning. By using LiCl and cellulose nanofibers together as humidity-responsive materials, the response range of cellulose nanofibers is expanded, its humidity sensitivity is improved, its cost is lower, the manufacturing process is simpler, and this humidity-responsive material is more environmentally friendly.

[0021] The humidity sensor has also proven to be suitable for non-contact humidity detection, respiration detection, and sleep apnea detection. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 SEM images of LiCl / cellulose nanofiber membranes with different LiCl contents (where a is the SEM image of Example 1; b is the SEM image of Comparative Example 1; c is the SEM image of Comparative Example 2; d is the SEM image of Comparative Example 3).

[0024] Figure 2The current-voltage curves of LiCl / cellulose nanofiber membrane-based humidity sensors with different LiCl contents are shown below (where a is the current-voltage curve of the LCNF08 humidity sensor in Example 2; b is the current-voltage curve of the LCNF07 humidity sensor in Comparative Example 4; c is the current-voltage curve of the LCNF09 humidity sensor in Comparative Example 5; and d is the current-voltage curve of the LCNF10 humidity sensor in Comparative Example 6).

[0025] Figure 3 The humidity sensitivity performance of the LCNF08 humidity sensor is shown in Figure 3 (where a is the humidity sensitivity curve of the LCNF08 in Example 3, and b is the response / recovery time curve of the LCNF08 humidity sensor).

[0026] Figure 4 The fabrication process and sensing mechanism of a LiCl / cellulose nanofiber composite membrane humidity sensor. Detailed Implementation

[0027] The following embodiments are only for further elaboration of the present invention. It should be noted that all techniques and scientific terms used in this invention, unless otherwise stated, have the same meaning as those in the technical field to which this invention pertains. Experimental methods in the following embodiments that do not specify specific conditions all employ conventional techniques in this technical field or follow the conditions recommended by the manufacturer; reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0028] To enable those skilled in the art to better understand the technical solution of the present invention, the following detailed description, in conjunction with the accompanying drawings and embodiments, provides a LiCl / cellulose nanofiber membrane and its preparation method, as well as an ion-conductive humidity sensor, but these descriptions should not be construed as limiting the scope of protection of the present invention.

[0029] Example 1

[0030] A method for preparing a porous and transparent LiCl / cellulose nanofiber membrane includes the following steps:

[0031] (1) Preparation of raw materials: Cellulose bleached pulp is used as raw material, and it is decomposed and dried by a decomposition machine to obtain cellulose pulp;

[0032] (2) Cellulose dissolution: The mass ratio of DMAc / LiCl was controlled at 92:8. DMAc solvent was first added to the cellulose pulp for activation for 2 hours, and then LiCl reagent was added to dissolve the cellulose. Finally, the mixture was stirred at room temperature for 8 hours to obtain a LiCl / cellulose spinning solution with a cellulose pulp concentration of 3 wt%. An 8% LiCl / cellulose solution was then prepared.

[0033] (3) Electrospinning: The LiCl / cellulose solution with a LiCl concentration of 8% was electrospinned using a hollow metal receiver as a collection device. The electrospinning parameters were as follows: applied voltage 28KV, flow rate (i.e., the injection speed of the cellulose spinning solution) 2.5mL / h, receiving distance 15cm, and spinning time 6h. After spinning, the film was peeled off from the collection net to obtain a LiCl / cellulose nanofiber membrane with a LiCl content of 8%. Based on the LiCl content in the DMAc / LiCl system, it was recorded as an LCNF08 film.

[0034] Comparative Example 1

[0035] The preparation steps are the same as in Example 1, except that the mass ratio of DMAc / LiCl is controlled to be 93:7, and the resulting cellulose membrane is designated as LCNF07 membrane.

[0036] Comparative Example 2

[0037] The preparation steps are the same as in Example 1, except that the mass ratio of DMAc / LiCl is controlled to be 91:9, and the resulting cellulose membrane is designated as LCNF09 membrane.

[0038] Comparative Example 3

[0039] The preparation steps are the same as in Example 1, except that the mass ratio of DMAc / LiCl is controlled to be 90:10, and the resulting cellulose membrane is designated as LCNF10 membrane.

[0040] Example 2

[0041] The LCNF08 thin film from Example 1 was used to fabricate a humidity sensor, and the steps are as follows:

[0042] Using the LCNF08 film from Example 1 as the humidity-sensitive layer, the film was cut to a size of 1cm × 2cm. The LCNF08 film and the copper foil electrode were connected by conductive silver paste. The copper foil electrode was connected to the test leads of the digital source meter to prepare the LCNF08 humidity sensor.

[0043] Comparative Example 4

[0044] Using the LCNF07 film of Comparative Example 1 as the humidity-sensitive layer, the film was cut to a size of 1cm×2cm. The LCNF07 film and copper foil electrodes were connected by conductive silver paste. The copper foil electrodes were connected to the test leads of the digital source meter to prepare the LCNF07 humidity sensor.

[0045] Comparative Example 5

[0046] Using the LCNF09 film of Comparative Example 2 as the humidity-sensitive layer, the film was cut to a size of 1cm×2cm. The LCNF09 film and copper foil electrodes were connected by conductive silver paste. The copper foil electrodes were connected to the test leads of the digital source meter to prepare the LCNF09 humidity sensor.

[0047] Comparative Example 6

[0048] Using the LCNF10 film of Comparative Example 3 as the humidity-sensitive layer, the film was cut to a size of 1cm×2cm. The LCNF10 film and copper foil electrodes were connected by conductive silver paste. The copper foil electrodes were connected to the test leads of the digital source meter to prepare the LCNF10 humidity sensor.

[0049] Structural and performance characterization

[0050] 1) The LiCl / fiber membranes prepared in Example 1 and Comparative Examples 1-3 were characterized by SEM and their pore size was analyzed.

[0051] Figure 1 SEM images of LiCl / CNF cellulose membranes with different LiCl contents. Figure 1 In the image, 'a' is the SEM image of Example 1; Figure 1 In the middle, b is the SEM image of comparative example 1; Figure 1 c is the SEM image of Comparative Example 2.

[0052] ; Figure 1 In the diagram, d is the SEM image of Comparative Example 3. For example... Figure 1 As shown, the fibers in Example 1 are uniform and smooth, with an average fiber diameter of 574.72 nm. In contrast, the cellulose membrane prepared in Comparative Example 1 has severely interwoven fibers, is not smooth, exhibits obvious beading, and has a sparse fiber distribution. This is because the low LiCl content in the DMAc / LiCl system results in insufficient cellulose dissolution, leading to poor spinning performance.

[0053] like Figure 1 As shown, the fibers in Example 1 are uniform and smooth, with an average fiber diameter of 574.72 nm. In contrast, the cellulose membrane obtained in Comparative Example 2 exhibits severe fiber adhesion and swelling. This is related to the LiCl on the surface of the cellulose, which can absorb moisture from the air, leading to fiber swelling.

[0054] like Figure 1 As shown, the fibers in Example 1 are uniform and smooth, with an average fiber diameter of 574.72 nm. In contrast, the cellulose membrane prepared in Comparative Example 3 exhibits more severe fiber adhesion and swelling. This is related to the excessively high LiCl content in the DMAc / LiCl system. Excessive LiCl on the cellulose surface can absorb moisture from the air, leading to severe fiber swelling.

[0055] 2) The humidity sensors of Example 2 and Comparative Examples 4-6 were placed in different saturated salt solutions, and the current changes were measured using a digital source meter.

[0056] Figure 2 Current-voltage curves for different LiCl / cellulose nanofiber-based humidity sensors. Figure 2 In Figure 'a', the current-voltage curve of the LCNF08 humidity sensor in Example 2 is shown. Figure 2 In Figure b, the current-voltage curve of the LCNF07 humidity sensor in Comparative Example 4 is shown. Figure 2 In the middle c, the current-voltage curve of the LCNF09 humidity sensor in Comparative Example 5 is shown. Figure 2 In the figure, d is the current-voltage curve of the LCNF10 humidity sensor in Comparative Example 6.

[0057] from Figure 2 It can be seen that when the concentration of LiCl is 8%, the current is relatively large (e.g. Figure 2 As shown in Figure a); when the concentration of LiCl is too low, the cellulose is not fully dissolved, resulting in poor film formation of the cellulose membrane, making it difficult to form a uniform and complete conductive network in the LiCl / cellulose composite membrane, and thus failing to accurately reflect changes in humidity; when the content of LiCl is too high, its water absorption capacity is too strong, causing the cellulose nanofibers to swell severely, thus affecting its sensitivity to humidity.

[0058] 3) The humidity sensors of Example 2 and Comparative Examples 4 to 6 were tested for their humidity sensitivity performance.

[0059] The LCNF08 humidity sensor with 8% LiCl content used in Example 1 has the best conductivity.

[0060] Figure 3 This refers to the humidity-sensitive performance of the LCNF08 in Example 2. Figure 3 In the diagram, a is the humidity sensitivity curve; b is the response / recovery time curve. Figure 3 As shown, the LCNF08 humidity sensor has a humidity detection range of 5 to 98% RH, with the highest sensitivity of 4190% at 98% RH, and a response / recovery time of 99 / 110s.

[0061] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a LiCl / cellulose nanofiber membrane, characterized by, The LiCl / cellulose nanofiber membrane is used as a humidity sensitive layer for an ion conductive humidity sensor; the preparation method comprises the following steps: broadleaf wood pulp plates are treated by defibration and drying to obtain cellulose pulp; the cellulose pulp is mixed with different proportions of DMAc and LiCl to obtain cellulose spinning solutions with different concentrations; in the preparation process of the cellulose spinning solutions with different concentrations, the mass of the DMAc accounts for 80-95% of the total mass of the DMAc and the LiCl; the concentration of the cellulose in the cellulose spinning solutions with different concentrations is 1-10wt%; the cellulose spinning solutions with different concentrations are formed by electrospinning and then dried to obtain LiCl / cellulose nanofiber membranes with different LiCl contents; In the preparation process of the LiCl / cellulose nanofiber membranes with different LiCl contents, the conditions of the electrospinning include: the applied voltage is 20-30KV, the injection speed of the cellulose spinning solution is 0.2-5mL / h, the receiving distance is 8-25cm, the collection device is a hollow type receiver, the spinning time is 2-10h, the drying temperature after forming is 100-200℃, and the drying time is 0.5-3h; the LiCl / cellulose nanofiber obtained by the preparation method has a diameter of 20-800nm, the LiCl / cellulose nanofiber is interwoven to form a porous structure with an average pore size of 2-10μm, and the thickness of the LiCl / cellulose nanofiber membrane is 18-300μm.

2. The production method according to claim 1, characterized by, In the preparation process of the cellulose pulp, the broadleaf wood pulp comprises chemical pulp and / or dissolving pulp; the content of alpha-cellulose of the broadleaf wood pulp is 90-99%, the defibration time is 5-60min, the drying temperature is 50-100℃, and the drying time is 8-15h.

3. An ionically conductive humidity sensor, characterized in that, The LiCl / cellulose nanofiber membrane prepared by the preparation method of any one of claims 1-2 is used as a humidity sensitive layer, a copper sheet is used as a contact electrode, conductive silver paste is used to connect the humidity sensitive layer and the contact electrode, the contact electrode is connected with a digital source table, and the ion conductive humidity sensor is obtained.

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