Electrospining: One-step electrospinning method for preparing LiCl/cellulose nanofiber membranes for flexible humidity sensors with high sensitivity and wide range

Background

The detection and management of humidity plays a vital role in human life and the operation of electronic products. Humidity sensors are used in various fields, including atmospheric and soil treatment, industrial manufacturing, agricultural production, and physiological health management. In addition, the human body continuously loses water through explicit and implicit sweating. As a biomarker, the ubiquitous humidity in the human body provides support for the development of non-contact sensor networks for emerging wearable digital medical technologies. Therefore, the creation of high-performance humidity sensors designed for non-contact and respiratory monitoring has attracted great attention. Cellulose is considered an ideal candidate for green humidity sensors because of the abundant hydrophilic groups on its molecular chain, which gives it good water absorption and swelling properties. Due to the flexibility and versatility of cellulose in material design, cellulose-based sensors can be used to achieve sensing through a variety of pathways, including resistance, capacitance, colorimetry, gravity, microwave, etc. Among them, resistance sensors are considered to have the most development potential due to their simple structure, convenient reading, and high accuracy. However, since cellulose is not conductive, many conductive materials, including CNT, PPY, PANI, Ag, RGO, NaCl, KOH, etc., are used to composite with cellulose to improve the conductivity of cellulose, and its conductive mechanism is divided into electronic conductivity and ionic conductivity.

The main point of this paper

Importance of humidity sensors:

Humidity sensors play an important role in many fields such as atmospheric and soil treatment, industrial manufacturing, agricultural production and physiological health management. They are essential for applications such as monitoring human humidity, non-contact sensing, etc.

Application of nanomaterials in humidity sensing:

Nanomaterials are excellent in conductivity due to their large surface area and porosity, making them very suitable for humidity sensing

Cellulose nanofiber humidity sensors:

Cellulose nanofibers (CNF) are considered to be ideal humidity sensor materials due to their large specific surface area and high hydrophilicity

LiCl/cellulose nanofiber membrane humidity sensors:

LiCl/cellulose nanofiber (LCNF) membranes prepared by electrospinning technology have high sensitivity (up to 4191%) and wide detection range (5-98% RH), as well as fast response and recovery rates (99/110 seconds) and minimal hysteresis (2.9%)

Hydrophilicity and conductivity of LiCl:

As a stable ionic compound, LiCl has strong hydrophilicity and can improve the detection limit of moisture sensing of cellulose fibers, especially under low humidity conditions

Advantages of electrospinning technology:

Electrospinning technology has simple equipment, a wide range of raw materials, and easy control of fiber structure. The prepared nanofibers have the characteristics of small diameter, large specific surface area, adjustable pores, and good pore connectivity.

Ultrathin LiCl/cellulose nanofiber membranes: high-performance humidity sensors fabricated by one-step electrospinning

Advantages of cellulose as a humidity sensor material:

Due to its green and renewable nature, rich surface groups and strong hydrophilicity, cellulose is considered to be an ideal material for flexible electronic humidity sensors

Limitations of traditional cellulose-based humidity sensors:

Traditional cellulose-based humidity sensors cannot have a wide detection range, high sensitivity and fast response time at the same time, which limits their development and application

Preparation of LiCl/cellulose nanofiber membrane:

Ultra-thin LiCl/cellulose nanofiber membranes were prepared as humidity-sensitive materials by one-step electrospinning using N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) solvent system to dissolve cellulose

LiCl enhances humidity sensing performance:

As a highly humidity-sensitive inorganic salt, LiCl has strong hydrophilicity, which improves the humidity-sensitive detection limit of cellulose fibers, giving the sensor excellent sensitivity (up to 4191%) and a wide detection range (5-98%) RH)

Fast response and low hysteresis:

The nanometer size of cellulose fibers, extremely low thickness and large pores of cellulose membrane accelerate the mass exchange between air and membrane, giving the sensor fast response/recovery time (99/110s) and low hysteresis (2.9%)

Long-term stability and durability:

The performance of humidity sensor has not decreased even after long-term use (more than 30 days), high and low temperature (73.6/0.1℃) and thousands of bending cycles

Summarize

LiCl was immobilized in cellulose nanofibers by a one-step electrospinning method to prepare LiCl/cellulose nanofiber membranes, which were used to assemble high-performance humidity sensors. The LCNF humidity sensor has the characteristics of high sensitivity (up to 4190% at 98% RH), wide detection range (5-98% RH), fast response/recovery time (99/110s), and low hysteresis (2.9%), which is mainly due to the cellulose nanofiber and microporous network structure formed by it and the synergistic effect of LiCl and cellulose. Compared with the cellulose-based humidity sensors reported in the literature, the LCNF humidity sensor has significant advantages in detection range and sensitivity. In addition, the performance of the LCNF humidity sensor did not decrease after long-term use, high and low temperature, and thousands of bending cycles. It has applications in non-contact humidity detection, breathing detection, sleep apnea detection, etc. These results provide a basis for the application and development of renewable and sustainable humidity sensors in the future.