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15 February 2026

Self-Powered Flexible Humidity Sensor Based on HACC/LiCl Composite Electrolyte

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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China
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Materials2026, 19(4), 760;https://doi.org/10.3390/ma19040760
This article belongs to the Special Issue Fabrication, Characteristics and Applications of Multifunctional Polymer Nanocomposites (Second Edition)
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Abstract

To address the challenges of traditional flexible humidity sensors, such as reliance on external power supply, complex fabrication processes, and poor adaptability to energy-limited scenarios, this study successfully developed a low-cost, easily scalable, self-powered flexible humidity sensor based on hydroxypropyl trimethyl ammonium chitosan/lithium chloride (HACC/LiCl) composite electrolyte using a screen-printing process. The device employs A4 paper as the flexible substrate, and interdigitated manganese dioxide (MnO2) positive electrodes, zinc (Zn) negative electrodes, and HACC/LiCl composite electrolyte layers are sequentially fabricated via screen-printing, ultimately constructing a simple primary battery structure. Through a series of performance screening and optimization, 0.1 mol/L LiCl-modified HACC (HL-1) is identified as the optimal electrolyte system. The test results show that the HL-1 sensor exhibits a wide humidity detection range of 11~97% relative humidity (RH), with the output voltage displaying a good quadratic function relationship with humidity (R2 = 0.996), and a peak output voltage of up to 1.2 V. The device possesses excellent cyclic stability and long-term stability, with no significant fluctuation in output voltage under different bending deformation states. This sensor demonstrates broad application prospects in fields such as respiratory monitoring and non-contact sensing, providing a feasible technical path for the development of low-cost passive humidity monitoring equipment.

1. Introduction

Humidity is a critical parameter in various fields such as environmental monitoring [1,2], smart healthcare [3,4], and intelligent agriculture [5,6], where accurate and passive monitoring is essential. Passive humidity monitoring, which does not require active power input, is a key technological enabler for the intelligent development of these fields. Traditional humidity-sensing technologies primarily rely on resistive [7,8,9], capacitive [10,11], or quartz crystal microbalance sensing principles [12,13]. While these devices offer a certain degree of detection accuracy, they universally depend on external power sources for signal conversion. This limitation not only increases system energy consumption and size, but also hinders their adaptability to energy-constrained environments such as remote distributed monitoring systems and passive wearable devices, severely restricting their broader application in emerging intelligent monitoring fields [14]. In contrast, self-powered humidity sensors can directly convert environmental humidity energy into electrical signals, thereby integrating the function of sensing and power generation. This dual functionality provides a promising solution to the aforementioned technical challenges and has become a focal point of research in the field of sensing [15].
Currently, the self-powered humidity-sensing mechanisms reported in the literature mainly include triboelectric nanogenerators (TENG) [16,17], piezoelectric nanogenerators (PENG) [18,19], ion gradient diffusion [20,21,22], and electrochemical redox reactions [23,24]. Among them, TENG- and PENG-based sensors require external mechanical stimuli (e.g., vibration and deformation) to facilitate energy conversion, which limits their applications in static or low mechanical energy environments. Although sensors based on ion gradient diffusion do not require mechanical inputs, they typically require the construction of complex functional group gradients or heterogeneous structures, resulting in cumbersome fabrication processes and challenges in large-scale production.
Electrochemical reaction-based humidity sensors, which simultaneously generate electrical output and respond to humidity through redox reactions between electrodes and water molecules, offer significant advantages such as simple structure, no requirement for mechanical stimuli, and fast response time; therefore, they have emerged as a key development direction in the field of self-powered humidity sensing. Jiang’s team has developed a series of planar-structured self-powered sensors [25,26,27], which exhibit both a wide humidity response range and excellent output performance. Based on these findings, a wireless self-powered humidity monitoring system was constructed [28]. Furthermore, the team conducted a systematic investigation into the anodic reaction mechanism of such sensors, observing the coexistence of hydrogen evolution and oxygen reduction reactions, and confirming the positive regulatory role of oxygen in the performance of self-powered sensors [29]. Zhao et al. fabricated all-printed flexible self-powered humidity sensors, which combine the benefits of simple fabrication, a wide humidity response range (11–95% relative humidity (RH)) and a high voltage output (1.03 V). In addition, the team systematically studied the cathode and anode reaction processes, and clarified the self-powered working mechanism of the sensor [30].
The electrolyte layer plays a pivotal role in determining the core performance of self-powered sensors [31]. Hydroxypropyl trimethyl ammonium chitosan chloride (HACC) contains a high density of hydrophilic hydroxyl groups (−OH) and quaternary ammonium cation groups (−N+(CH3)3) along its molecular chain. These functional groups enable HACC to efficiently adsorb water molecules from the environment via hydrogen bonding and electrostatic interactions, while also dissociating to provide Cl− ions. In addition, HACC is biodegradable and biocompatible, making it an ideal candidate material for the electrolyte layer in self-powered sensors.
In this study, the primary objectives are low cost and scalable fabrication, and A4 paper is selected as the flexible substrate. Utilizing the screen-printing technique, interdigitated positive and negative electrodes are first fabricated by sequentially printing manganese dioxide (MnO2) as the positive electrode ink and zinc (Zn) as the negative electrode ink. Subsequently, the HACC/LiCl composite humidity-sensitive electrolyte layer is printed into the electrode gaps, forming a simple primary battery structure of positive electrode–composite electrolyte–negative electrode. This approach significantly simplifies the fabrication process and reduces production costs. Moreover, by optimizing the electrolyte composition, the study regulates the ion carrier concentration and migration efficiency, thereby enhancing the humidity response performance of the device. The sensor developed in this work not only achieves a wide humidity detection range of 11–97% RH, and a high response voltage of 1.2 V, but also exhibits excellent cyclic stability, long-term performance, and mechanical flexibility. Additionally, the output voltage can be flexibly adjusted through device array design, and its application potential in respiratory monitoring and non-contact sensing is experimentally validated. These findings lay a solid foundation for the practical deployment of the sensor in passive humidity monitoring systems and as a power source for low-power electronic devices.

2. Materials and Methods

2.1. Materials

Hydroxypropyl trimethyl ammonium chitosan chloride (HACC), sodium alginate (SA, low viscosity), polyvinyl alcohol (PVA), hydroxyethyl cellulose (HEC), manganese dioxide (MnO2), lithium chloride (LiCl), sodium bromide (NaBr), sodium chloride (NaCl), potassium chloride (KCl), and iron (Fe) powder were purchased from Tianjin Heowns Biochemical Technology Co., Ltd. (Tianjin, China). Magnesium chloride hexahydrate (MgCl2·6H2O), potassium iodide (KI), potassium carbonate (K2CO3), and potassium sulfate (K2SO4) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Carbon black (CB) was supplied by Unigram Carbon Graphene Material Co., Ltd. (Baoding, China), zinc (Zn) powder was purchased from Jiangsu Shenlong Zinc Industry Co., Ltd. (Xinghua, China), multi-functional A4 paper was purchased from Deli Group Co., Ltd. (Ningbo, China). All materials were used directly without further purification.

2.2. Preparation of Electrolyte Layer and Electrode Inks

To determine the optimal electrolyte system, a series of electrolyte layer inks were prepared. Specially, 5% HACC and SA solutions were, respectively, prepared by stirring in a constant-temperature water bath at 60 °C, and a 10% PVA solution was also prepared simultaneously, with the water bath temperature set to 90 °C. To investigate the effect of different salt-based electrolytes on the sensing performance, four salt solutions (LiCl, NaCl, KCl, and NaBr), each with a concentration of 0.1 mol/L, were separately added to the HACC solution. The resulting composite systems were designed as HL-1, HN-1, HK-1, and HB-1, respectively. To further determine the optimal doping concentration of LiCl, LiCl solutions with concentrations of 0.1, 0.2, and 0.3 mol/L were incorporated into the HACC matrix. The corresponding composite electrolyte inks were labeled HL-1, HL-2, and HL-3 for subsequent performance characterization.
The preparation of electrode inks was as follows. First, a 2% HEC binder solution was prepared by dispersing HEC powder in deionized water, followed by continuously stirring in a constant-temperature water bath at 60 °C until a homogeneous and transparent solution was obtained. Using the above HEC binder solution as a base, the positive and negative electrode inks were then formulated, respectively: each component was accurately weighed according to the mass ratio of HEC binder:CB:Zn = 1:2:20, and fully stirred until the system was uniformly dispersed without obvious particle agglomeration to obtain the negative electrode ink; the Fe-based negative electrode ink could be prepared by replacing Zn powder with an equal amount of Fe powder; for the positive electrode ink, each raw material was mixed according to the mass ratio of HEC binder:CB:MnO2 = 2:5:30, and after high-speed stirring treatment, a well-dispersed positive electrode ink was obtained.

2.3. Fabrication of Humidity Sensors

A4 paper was selected as the flexible substrate. The positive and negative electrode inks were sequentially printed onto the pre-defined interdigitated electrode pattern on the substrate. After each printing step, a drying process was carried out to remove the solvent from the system. Once the interdigitated electrode structure was formed, the electrolyte ink was precisely printed into the gaps between the electrodes, completing the fabrication of the A4 paper-based humidity sensor. The fabrication process is shown in Figure 1.
Figure 1. Schematic diagram of printing process for humidity sensors.

2.4. Characterization and Performance of Humidity Sensors

The surface morphology of the sensor was analyzed using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Tokyo, Japan) at an accelerating voltage of 15 kV. The phase composition of the electrolyte layer was characterized by X-ray diffraction (XRD, Smartlab, Rigaku, Tokyo, Japan). Fourier-transform infrared spectroscopy (FTIR, iS50, FEI, Hillsboro, OR, USA) was used to analyze the chemical composition of the electrolyte layer. Water contact angle measurements were conducted using a droplet shape analyzer (JC 2000D1, Zhongchen Digital Technology, Shanghai, China) to assess the hydrophilicity of the electrolyte surface.
To evaluate the humidity-sensing performance of the self-powered sensor, a Keithley (Cleveland, OH, USA) 2400 digital source meter was employed. A two-electrode test configuration was adopted throughout the testing, with no external bias voltage applied. The signal acquisition relied entirely on the self-generated potential of the sensor. During the test, the sensor was placed in sealed containers maintained at different constant RH levels, and its output voltage, defined as the sensing response signal, was continuously recorded. Two key kinetic parameters were defined for performance evaluation: the response time was defined as the time required for the voltage to rise to 90% of its peak value, and the recovery time was defined as the time required for the voltage to drop to 10% of its peak value. To establish stable and well-defined humidity environments, saturated salt solutions with known equilibrium RH values were placed in the sealed containers. The corresponding equilibrium RH values are as follows: LiCl (11% RH), MgCl2 (33% RH), K2CO3 (43% RH), NaBr (57% RH), KI (68% RH), NaCl (75% RH), KCl (85% RH), K2SO4 (97% RH) [32].

3. Results and Discussion

3.1. Materials and Sensor Characterization

Based on the humidity-sensing and power-generation performance test, the HL-1 humidity sensor exhibited the best overall performance, and was selected for detailed characterization.
Figure 2a shows the local SEM morphology of the HL-1 sensor, demonstrating that the Zn/MnO2 electrodes and the electrolyte layer have been uniformly printed on the A4 paper substrate. Due to the thin and transparent nature of the electrolyte layer, the fiber structure of the A4 paper substrate is clearly visible. From the magnified SEM image of the electrolyte layer (Figure 2b), it can be observed that the surface of the electrolyte layer is generally flat and smooth, with only a few microcracks. These microcracks do not compromise the structural integrity or ion transport pathways of the electrolyte layer, but instead facilitate the rapid penetration and adsorption of water molecules in the environment, thereby shortening the sensor’s response and recovery times. The SEM image of the MnO2 electrode (Figure 2c) reveals that MnO2 and CB particles are uniformly dispersed in the electrode matrix, with no obvious agglomeration, and the particles interlock to form a continuous conductive network, which is beneficial to the charge transport. Figure S1 shows the SEM image of the Zn electrode, from which it can be clearly observed that the Zn particles have a spherical morphology, uniform size distribution, and are well dispersed. The magnified image (Figure 2d) further shows that Zn and CB particles are uniformly coated by the HEC binder, forming a continuous bonding network.
Figure 2. (a) SEM image of the sensor surface. (b) SEM image of the HACC/LiCl electrolyte layer. (c) SEM image of the MnO2 electrode. (d) SEM image of the Zn electrode. (e) FTIR spectrum of the HACC/LiCl electrolyte layer. (f) Water contact angle curve of the HACC/LiCl electrolyte layer.
Figure 2e presents the FTIR spectrum of the HL-1 electrolyte layer. The 3300 cm−1 peak corresponds to the stretching vibrations of O−H and N−H, the 2936 cm−1 peak is attributed to the stretching vibration of C−H, the 1653 cm−1 peak corresponds to the bending vibration of N−H, the 1477 cm−1 peak is attributed with the C−H bending vibration of methyl groups, and the 1062 cm−1 peak is due to C−O−C stretching vibration in glycosidic bonds. These results confirm that the introduction of LiCl does not alter the fundamental chemical structure of HACC. Figure S2 shows the XRD pattern of the HL-1 electrolyte layer, where a broad diffraction peak appears at 2θ ≈ 20°, indicating that the HACC/LiCl composite electrolyte layer is amorphous in structure. Water contact angle is one of the key indicators of the surface hydrophilicity [33]. As shown in Figure 2f, when a water droplet first contacts the HL-1 electrolyte layer, the water contact angle is 69.6°, and it decreases gradually over time, reaching to 43.9° at 60 s, which indicates good hydrophilic properties.

3.2. Performance Characterization of Humidity Sensors

To screen suitable electrolyte layer substrates, single- and double-layer sensors were fabricated using different substrate materials: HACC, SA, and PVA. A comparative analysis of their humidity response behaviors (Figure 3a for HACC, Figure S3a for SA, and Figure S3b for PVA) reveals that the HACC-based sensor with a single-layer electrolyte layer exhibited significantly better response characteristics than the SA- and PVA-based sensors with similar structures. This is attributed to the high density of hydrophilic groups in the HACC molecular chain, which allows for efficient water molecule adsorption and generation of more ion carriers, providing a favorable foundation for humidity-sensitive response. Further, it was found that double-layer sensors based on HACC and SA showed much higher response voltages than their single-layer counterparts across the entire humidity range, while PVA-based double-layer sensors showed no significant improvement. In-depth mechanism analysis revealed that the response performance of humidity-sensitive materials is closely related to the concentration of internal mobile ion carriers. Both HACC and SA matrices contain relatively high concentrations of Cl− and Na+ ions, and the double-layer structure further enhances the ion carrier concentration. The increase in ion carrier concentration improves the ion migration capability in a humid environment, thereby optimizing the response voltage signal. In contrast, the PVA matrix has a low ion content, and the double-layer structure does not overcome this limitation, so it cannot effectively enhance the response voltage. These findings confirm that increasing the ion carrier concentration in the electrolyte layer by adding inorganic salts is an effective strategy for optimizing humidity-sensing performance [34].
Figure 3. (a) Dynamic response curves of HACC-based single/double-layer humidity sensors under different RHs. (b) Response curves of HACC-based humidity sensors modified with different salt solutions. (c–e) Dynamic response and recovery curves of HL series humidity sensors (HL-1, HL-2, HL-3). (f) Polynomial fitting curve between output voltage and RH of HL-1 sensor.
To screen the optimal modifier, HACC was used as the base material, and a series of modified sensors were prepared using four typical inorganic salts (LiCl, NaCl, KCl, and NaBr). The humidity response performance of these sensors was evaluated. The results (Figure 3b) showed that the humidity sensitivity of HACC-based sensors was significantly improved after modification, with the response voltage increasing from 1.07 V (unmodified) to approximately 1.2 V. Among the four modifiers, LiCl-modified sensors exhibited the best dynamic performance, with the shortest response and recovery times, and were therefore selected as the optimal modification strategy.
To determine the optimal LiCl concentration, HL-1, HL-2, and HL-3 sensors were fabricated with LiCl concentrations of 0.1, 0.2, and 0.3 mol/L, respectively, and their dynamic response and recovery characteristics were tested under various RH conditions (Figure 3c–e). The results showed that the HL-1 sensor demonstrated excellent humidity discrimination capability across the entire humidity range: at 11% RH, the output voltage remained stable at 0 V; at 33% RH, it increased to 0.16 V; at 43% RH, it reached 0.4 V. The voltage steadily increased with rising RH, reaching a peak of 1.2 V at 97% RH. This indicates a wide detection range of 11%~97% RH, which is suitable for most real-world monitoring scenarios. However, HL-2 and HL-3 sensors, with higher LiCl concentrations, exhibited excessive moisture absorption, leading to voltage saturation at RH values above 68%. This not only narrowed the effective humidity range, but also caused swelling and deformation of the electrolyte layer, disrupting the stability of the charge migration channel. Fitting analysis (Figure 3f) revealed that the output voltage of the HL-1 sensor follows a quadratic function relationship with RH (R2 = 0.996), enabling accurate estimation of unmeasured humidity values and satisfying accuracy requirements. In summary, the HL-1 sensor was selected for further performance testing and application verification due to its optimal balance between sensitivity, stability, and fabrication feasibility.
A comprehensive performance evaluation was conducted on the HL-1 sensor. The cyclic stability test demonstrated excellent reproducibility. Figure 4a,b present the typical experimental results of the output voltage and response/recovery time under cyclic environmental humidity changes, respectively. A complete test consisted of 10 alternating humidity cycles, ranging from 11% to 97% relative humidity. Statistical analysis of these 10 alternating cycles revealed a relative standard deviation (RSD) of only 0.62% for the output voltage (Figure 4a), and the response time and recovery time (Figure 4b) were determined to be 51.3 s (RSD = 4.60%) and 48.2 s (RSD = 5.60%), respectively. These low-dispersion results confirm the stable dynamic response of the device. The hysteresis test (Figure 4c) revealed that the humidity hysteresis of the HL-1 sensor is approximately 5% RH, demonstrating excellent sensing performance. The long-term stability test (Figure 4d) showed that when the sensor was continuously exposed to 97% RH for 6 h, the output voltage slightly decreased and stabilized at 1.14 V, indicating reliable long-term operation. Owing to the flexibility and mechanical robustness of the A4 paper substrate, the HL-1 sensor exhibits excellent bending stability. As shown in Figure 4e, the output voltage under various bending states only slightly attenuates compared to the flat state, indicating good mechanical flexibility.
Figure 4. (a) Response and recovery curves for 10 cycles of the HL-1 humidity sensor. (b) Response and recovery time for 10 cycles of the HL-1 humidity sensor. (c) Hysteresis curve of the HL-1 humidity sensor. (d) Continuous output voltage of the HL-1 humidity sensor over 6 h at 97% RH. (e) Voltage output of the HL-1 sensor at different RHs under different bending states. (f) Dynamic response and recovery curves of Fe-based humidity sensors. (g) Response and recovery curves with forward and reversed connections of the sensor. (h) Output voltage and current curves of the HL-1 sensor under different loading resistances at 97% RH. (i) Output voltage of the series-connected sensors at 97% RH and the linear fitting line.
To investigate the impact of electrode materials on sensor performance, Zn powder was replaced with Fe powder to fabricate an Fe-based humidity sensor (response curve shown in Figure 4f). Due to the lower standard electrode potential of Zn (−0.763 V vs. SHE) compared to Fe (−0.440 V vs. SHE), the Fe-based sensor produced a maximum output voltage of 0.8 V, which is significantly lower than the Zn-based sensor. However, the overall humidity response trend remained consistent with that of the Zn-based sensor, confirming that electrode material selection strongly influences output performance.
To clarify the energy conversion mechanism, the response and recovery curves of the sensor were tested under both forward and reverse connections to the Keithley 2400 (Figure 4g). The results showed that the voltage peaks were identical, but the polarity was reversed, which is consistent with the behavior of a primary battery, confirming that the device relies on electrochemical redox reactions for energy output. To further explore the energy output characteristics, the HL-1 sensor was placed in a 97% RH environment, and its output power was measured under varied load resistances (Figure 4h). When the load resistance was 113 kΩ, the maximum output power reached 2.88 μW.
To enhance the output voltage for practical applications, sensor arrays with different numbers of devices were constructed. Under 97% RH, the five-device series array achieved an output voltage of 6 V, with excellent linear correlation (R2 = 0.99) (Figure 4i). Figure S3a shows the charge–discharge curve of the capacitor powered by the sensor array, while Figure S3b presents the real-time voltage curve of the capacitor when used to drive an LED, directly proving the strong energy supply potential of the sensor array and laying the groundwork for applications in passive monitoring and low-power electronics.
Table 1 compares the key performance of the HL-1 humidity sensor with other recently reported self-powered humidity sensors of the same type. The HL-1 sensor demonstrates a high output voltage of 1.2 V over a wide sensing range of 11–97% RH, although its response/recovery times (51.3/48.2 s) still require further improvement.
Table 1. Comparisons of the HL-1 sensor and recently reported self-powered humidity sensors.

3.3. Working Mechanism of Humidity Sensors

The core operating mechanism of the self-powered humidity sensor developed in this study is a humidity-regulated primary battery reaction, with the HACC/LiCl composite electrolyte layer serving as the key component for humidity sensing. Specifically, the HACC molecular chain is rich in hydrophilic groups such as −N+(CH3)3 and −OH, which efficiently adsorb ambient water molecules and facilitate ion transport pathways. Meanwhile, LiCl acts as an ion source, significantly increasing the carrier concentration in the electrolyte layer.
The output performance of the sensor is closely related to three key processes: water molecule adsorption/desorption behavior of the electrolyte layer, ion migration efficiency, and the extent of electrode redox reactions. These processes are interconnected via charge transfer mechanisms, and they synergistically influence the overall sensor performance. In low-humidity environments, the electrolyte layer adsorbs only a small amount of water, and thus cannot form a continuous water film. Due to the limited water content, the dissociation of water into H+ and OH− ions is minimal, and LiCl cannot fully dissociate, generating only trace amounts of Li+ and Cl− ions. The limited number of charge carriers prevents efficient proton hopping transport through the discontinuous water film, which inhibits the progress of redox reactions at both electrodes, resulting in weak electron transfer in the external circuit and insignificant voltage output. As environmental humidity increases, a large number of water molecules are adsorbed by the electrolyte layer, and a continuous water film is gradually formed. The dissociation of water molecules in the film proceeds as follows:
H2O → H+ + OH−
This process creates a favorable interface environment for ion transport and electrode reactions, and also promotes the complete dissociation of LiCl, which follows the reaction:
LiCl → Li+ + Cl−
The synergistic effect of the continuous water film and high carrier concentration significantly reduces the internal impedance and ion migration resistance, switching the ion migration mode from discrete hopping to continuous diffusion, thereby enhancing the redox reactions at both electrodes and significantly increasing the output voltage of the sensor.
According to the principles of primary battery reactions, clear redox reactions occur at both electrodes, as illustrated in Figure 5.
Figure 5. Schematic diagram of the operating mechanism of the HACC/LiCl-based humidity sensor in humid environment.
Negative electrode (Zn) oxidation reaction:
Zn − 2e− → Zn2+
Positive electrode (MnO2) reduction reaction [28,30]:
2MnO2 + 2H+ + 2e− → Mn2O3 + H2O
Overall reaction:
Zn + 2MnO2 + 2H+ → Zn2+ + Mn2O3 + H2O
The electron transfer loop is completed via the external circuit, from the negative electrode to the positive electrode. At the same time, cations (Li+, H+, Zn2+) in the electrolyte layer migrate toward the positive electrode, while anions (Cl−, OH−) migrate toward the negative electrode, in order to balance the electrode surface charges and establish a stable primary battery operation cycle. This mechanism clearly demonstrates the inherent relationship between humidity and sensor output performance and provides a solid theoretical foundation for performance optimization and application expansion.

3.4. Multifunctional Applications of Humidity Sensor

The HL-1 sensor developed in this study shows promising application potential in human physiological signal monitoring, particularly in respiratory rate detection and non-contact switch control.
Respiratory rate is a critical physiological parameter with important clinical relevance [38,39]. The humidity of exhaled air is much higher than that of the ambient environment. By leveraging this humidity gradient, the HL-1 sensor can be placed 2 cm in front of the mouth and nose of a normal adult to accurately monitor respiratory rate by detecting real-time humidity fluctuations. The test results in Figure 6a–c show that the sensor can effectively distinguish different breathing patterns, such as bradypnea (8 times/min), eupnea (14 times/min), and tachypnea (20 times/min), demonstrating reliable physiological signal monitoring capabilities.
Figure 6. (a–c) Real-time breathing monitoring curves for simulated bradypnea, eupnea, and tachypnea states. (d) Output voltage response curves at different distances between the finger and the sensor.
The human skin contains numerous sweat glands, forming a natural humidity field. Based on this characteristic, a non-contact switch system can be developed [35,40]. Figure 6d presents the voltage response of the sensor when a finger is placed at different distances above it. The voltage increases sharply as the finger approaches the sensor and drops rapidly as the finger moves away. This indicates that the sensor’s response intensity is inversely proportional to the distance between the finger and the sensor: the closer the distance, the more pronounced the voltage response, endowing the sensor with distinct graded switching characteristics.

4. Conclusions

This study successfully developed a self-powered flexible humidity sensor based on a HACC/LiCl composite electrolyte and screen-printing technology, utilizing A4 paper as the substrate. The sensor significantly reduces preparation costs and offers excellent scalability, facilitating large-scale production. The HL-1 humidity sensor demonstrates excellent sensing performance, with a wide humidity detection range (11–97% RH) and a peak output voltage (1.2 V), along with outstanding long-term stability, cyclic stability, and mechanical flexibility. Mechanistic studies reveal that the sensor regulates the adsorption of water molecules, LiCl dissociation, and ion carrier migration in the electrolyte layer to control the Zn-MnO2 primary battery reaction, generating a voltage signal related to humidity changes. The sensor can accurately distinguish different breathing states and enable non-contact graded switch control, showing strong practical application potential. In summary, this study provides a feasible strategy for developing low-cost, portable health monitoring and humidity-sensing devices, offering valuable insights for future sensor design and application.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma19040760/s1, Figure S1. Low-magnification SEM image of the Zn electrode. Figure S2. XRD patterns of the HACC/LiCl electrolyte layer. Figure S3. Dynamic response curves: (a) SA-based single/double-layer humidity sensors. (b) PVA-based single/double-layer humidity sensors. Figure S4. (a) Charging and discharging curves of the sensor array for capacitor charging at 97% RH. (b) Real-time voltage curve of the sensor array for capacitor charging and LED driving at 97% RH.

Author Contributions

Conceptualization, H.Z. and C.Y.; Methodology, B.Z., H.Z. and C.Y.; Validation, B.Z. and F.Y.; Formal analysis, B.Z.; Investigation, B.Z. and S.G.; Resources, C.Y.; Data curation, B.Z.; Writing—original draft preparation, B.Z.; Writing—review and editing, H.Z. and C.Y.; Visualization, B.Z.; Supervision, H.Z. and C.Y.; Project administration, C.Y.; Funding acquisition, C.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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