Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management

Flexible electronics are increasingly used in health management for continuous monitoring of physiological signals, such as respiration. Most flexible devices rely on separate power supplies and wiring, complicating integration and affecting comfort and aesthetics. Self-powered electronics mitigate these issues by integrating energy supply, enabling continuous operation without external power. Among self-powered schemes, mechanisms like triboelectric, piezoelectric, thermoelectric, and hydroelectric require external mechanical or thermal stimuli. In contrast, metal-air redox reactions, typically utilized in metal-air batteries, convert chemical energy directly to electricity without external stimulation, offering high stability. Prior studies indicate that outputs of metal-air systems can be modulated by external stimuli (e.g., pressure, light, glucose, NO₂), suggesting feasibility for self-powered sensing if humidity can influence the redox process. This work targets the development of a self-powered chemoelectric humidity sensor whose signal depends on humidity-controlled ion mobility in an electrolyte, aiming for high sensitivity, rapid response, and applicability in health monitoring and human–machine interfaces.

Self-powered sensors have been reported based on triboelectric, piezoelectric, thermoelectric, and hydroelectric effects, driven by friction, compression, temperature gradients, and humidity gradients, respectively. These systems typically harvest external mechanical or thermal energy. Metal-air batteries, leveraging spontaneous redox reactions between an active metal anode and oxygen reduction at a cathode, provide a different pathway to stable self-powering without external excitation. Previous research has shown that metal-air battery outputs can be influenced by stimuli such as pressure, light, glucose, and NO₂, indicating the potential for sensing modalities beyond traditional mechanisms. However, translating this into humidity sensing requires an electrolyte whose ion mobility is strongly humidity-dependent. Materials like graphene oxide (GO) and silk fibroin (SF), rich in hydrophilic functional groups, combined with hygroscopic salts (e.g., LiBr), can provide such humidity-responsive electrolytes. The present study builds on these insights to realize a chemoelectric humidity sensor driven by a metal-air redox reaction.

Materials: Lithium bromide (LiBr), sodium bicarbonate (NaHCO₃), LiCl, MgCl₂, K₂CO₃, NaBr, NaCl, KCl, silk cocoons, graphite paper, copper foil/wire/mesh were commercially sourced and used as received. Deionized water was used throughout. Preparation of SF/LiBr solution: Bombyx mori silk cocoons were cut and degummed by boiling with 0.5 wt% NaHCO₃ for 30 min. Degummed silk (2.0 g) was dissolved in 10 mL 9.3 M LiBr at 80 °C for 1 h to obtain an SF/LiBr solution. Preparation of GO/SF/LiBr ink: Commercial GO powder was mixed with the SF/LiBr solution to achieve 2.5–10 wt% GO content using a speed mixer (3500 rpm, 10 min). Unless stated, 7.5 wt% GO was used. The GO and SF interact via hydrogen bonding and π–π interactions, yielding uniform, processable dispersions with viscosity increasing with GO content, enabling multiple printing techniques. Electrolyte printing and patterning: GO/SF/LiBr inks with tuned viscosities were patterned on substrates/electrodes by direct writing (Chinese brush), screen printing (stencil masks), or extrusion 3D printing (Anycubic i3 MEGA; syringe nozzle 150 µm; extrusion 200 µL/min; nozzle speed 10 mm/s). Designs were created in 3ds Max and converted to G-code via Ultimaker Cura. Unless noted, screen printing with 7.5 wt% GO ink was used for custom electrolytes. Sensor assembly: The chemoelectric humidity (CEH) sensor comprised a GO/SF/LiBr gel electrolyte layer sandwiched between a flexible inert cathode (graphite paper) and an active metal anode (typically aluminum foil; copper variants also tested). Copper leads were attached with silver paste for measurements. Characterization of materials and structure: GO/SF/LiBr flakes and films were characterized using TEM/AFM and Cryo-SEM to confirm sheet-like GO with SF/LiBr adsorption and layered, porous film morphology. Raman spectroscopy assessed I_D/I_G changes indicating SF/LiBr adsorption on GO. XPS characterized elemental composition and functional groups. Rheology measured ink viscosity vs shear rate. Water contact angle evaluated hydrophilicity. Thermogravimetric analysis assessed water content and thermal stability. Electrical testing and humidity control: An electrochemical workstation (CHI760E) recorded open-circuit voltages and currents. Humidity was controlled using saturated salt solutions to achieve specific RH levels (11.3% LiCl, 32.8% MgCl₂, 43.2% K₂CO₃, 57.6% NaBr, 75.3% NaCl, 84.3% KCl) at 25 °C, 1 atm unless stated. For temperature tests, 0–80 °C was used. Air pressure/O₂ concentration effects were tested in a vacuum chamber from 1 atm down to 0.01 atm with 30 min holds. Lifetime tests were conducted at 25% RH. Non-contact sensing used controlled distances above 37 °C water via a SHIMADZU AG-IS tester. Data were analyzed with Origin 2018 edu. Working mechanism analysis: The device operates as a metal-air system. In humidity, LiBr dissociates to Li⁺/Br⁻; the metal anode oxidizes (M → Mⁿ⁺ + ne⁻), electrons flow to the graphite cathode where oxygen is reduced (O₂ + 4e⁻ + 2H₂O → 4OH⁻). Ion mobility in the GO/SF/LiBr electrolyte increases with absorbed water, reducing internal resistance and increasing external current. The current change rate (CCR, dI/dt) reflects adsorption/desorption dynamics of water in the electrolyte matrix. Demonstrations: A CEH sensor was integrated into a mask for respiratory monitoring with data acquisition via Arduino and cloud transmission (ESP8232 Wi‑Fi to bemfa.com). A non-contact human–machine interface was fabricated for applications such as elevator control, with virus simulation using UV-fluorescent phosphors to compare contamination between contact and non-contact operation.

- A self-powered chemoelectric humidity (CEH) sensor based on a metal-air redox reaction was developed using a GO/SF/LiBr gel electrolyte between graphite paper (cathode) and aluminum (anode). - Performance: quick response (1.05 s), fast recovery (0.80 s), and high sensitivity with CCR of 0.0933 µA/s per 1% RH (reported also as 0.09 µA/s/1%) over a wide RH range (11.3–84.3%). - Open-circuit voltage (OCV): ~0 V in a completely dry environment; ~0.9 V at 30% RH. Across 0–80 °C, OCV increased from 1.12 to 1.29 V, with stable humidity response signals maintained. - Pressure/O₂ robustness: From 1 to 0.01 atm, OCV fluctuation was <1%, indicating negligible influence of air pressure and O₂ concentration on sensing performance. - Anode versatility: Sensors with Cu anodes (foil, wire, mesh) also showed stable humidity responses, indicating flexibility in anode materials. - Stability and lifetime: Average Al anode consumption ~0.18 mg/cm² over 12 h (approx. $0.01/m²/day at $2.82/kg). OCV showed minimal decrease in first 36 h, then decayed from 1.24 to 0.51 V over the next 36 h. Effective sensing persisted after 72 h continuous operation; sensors can be regenerated by replacing anode/electrolyte. Stable after 180 days dry storage; good mechanical stability over 300 bending cycles. - Mechanistic evidence: GO/SF/LiBr films are highly hydrophilic (water contact angle ~18°); water uptake increased from 33.76% to 66.54% and relative resistance change ΔR/R0 decreased from -88.63% to -98.50% as RH increased from 11.3% to 84.3%, confirming humidity-enhanced ion mobility. - Applications: Demonstrated an integrated system for respiratory monitoring/diagnosis/treatment (e.g., SAS) and a non-contact HMI (elevator control). Non-contact operation prevented simulated virus contamination compared to contact-based control.

The study demonstrates that humidity can effectively modulate the ion mobility within a GO/SF/LiBr electrolyte, thereby regulating the current output of a metal-air cell to realize self-powered sensing. The abundant hydrophilic functional groups in GO and SF, together with the hygroscopic nature of LiBr, enable rapid water adsorption/desorption, producing strong, RH-dependent electrical signals without external power input. The layered, brick-and-mortar alignment of GO flakes increases steric hindrance for ion transport, which, while permitting sensitive RH response, slows ion depletion and extends operational lifetime. Robust performance across a broad temperature range, negligible sensitivity to air pressure/O₂ variation, and compatibility with different metal anodes underline the reliability and adaptability of the platform. Practical demonstrations in respiratory monitoring and non-contact human–machine interfaces show the sensor’s relevance to health management, including infection prevention and disease diagnosis, addressing the need for integrated, wire-free, and comfortable wearable systems.

A flexible, self-powered chemoelectric humidity sensor driven by a metal-air redox reaction was realized using a GO/SF/LiBr gel electrolyte between an active metal anode and an inert graphite cathode. The device achieves high sensitivity (≈0.09 µA/s per 1% RH), rapid response (1.05 s) and recovery (0.80 s), and operates over a wide RH range (≈11–84%). Its humidity sensitivity arises from hydrophilic GO/SF matrices and LiBr hygroscopicity, while a layered GO/SF structure helps prolong operational life by hindering fast ion migration. The platform supports facile printing/processing and was validated in a smart respiratory system for monitoring/diagnosing/treating SAS and a non-contact HMI that mitigates pathogen transmission. This approach to leveraging metal-air redox reactions for sensing may inspire broader classes of power-free, portable, and wearable devices.

- Finite operational lifetime due to consumption of the active anode metal; continuous operation showed performance decay after ~36 h and effective sensing up to ~72 h before regeneration (anode/electrolyte replacement) is needed. - Sensitivity (CCR) decreases at elevated temperatures above ~70 °C due to accelerated water desorption reducing overall water content. - Device output depends on ambient humidity; in completely dry environments the OCV is ~0 V (beneficial for storage but implying no signal without humidity).