I. Core Working Principle and Technical Nature
(I) Fundamental Mechanism
Solid-electrolyte gas sensors realize gas detection based on electrochemical catalytic reactions. Their core structure consists of a three-electrode system (working electrode, counter electrode, reference electrode) and a solid-electrolyte layer. The detection process includes three key steps:
- Gas diffusion: Target gas diffuses through the porous membrane to the surface of the working electrode.
- Electrochemical reaction: Oxidation/reduction reactions of gas take place under electrode catalysis (e.g., CO is oxidized into CO₂), generating a current signal proportional to gas concentration.
- Signal conversion: In accordance with Fick’s First Law (i = nFDC), the current signal is converted into a standard output via an external circuit to achieve quantitative concentration measurement.
(II) Typical Reaction Example
Take carbon monoxide detection as an example. The electrode reactions are shown below:
- Working electrode: CO + H2O → CO2 + 2H^{+} + 2e^{-}
- Counter electrode: 2H^{+} + 2e^{-} + 1/2O2 → H2O
Protons migrate through the solid electrolyte to complete charge balance, and the resulting current signal directly reflects CO concentration.
II. Technical Classification and Key Characteristics
(I) Classification by Electrolyte Material
| Electrolyte Type | Representative Materials | Core Advantages | Application Scenarios |
| Solid polymer | Polymeric conductive materials | Leak-proof, compact size, good low-temperature adaptability | Portable devices, indoor air monitoring |
| Oxygen-ion conductor | Yttria-Stabilized Zirconia (YSZ) | High-temperature resistance, excellent chemical stability | Motor vehicle exhaust, industrial flue-gas monitoring |
| Sodium-ion conductor | NASICON (Sodium Super-Ionic Conductor) | Response to multiple gases, outstanding ionic conductivity | Toxic gas detection (SO2/NOx) |
(II) Classification by Detection Principle
- Potentiometric type (mixed–potential type) Gas concentration is detected according to the potential difference between the sensing electrode and the reference electrode. YSZ-based sensors can detect multiple components such as NOx and NH3. Construction of Triple-Phase Boundaries (TPB) is required to improve sensitivity.
- Amperometric type Gas concentration is quantified by measuring electrolytic current under constant potential. Suitable for accurate detection of gases including O2 and H2.
- Impedimetric type Detection is realized based on electrolyte impedance changes induced by gas adsorption, mostly applied for high-concentration gas monitoring.
(III) Core Performance Advantages
Compared with traditional liquid-electrolyte sensors, they have the following prominent strengths:
- Environmental adaptability: Stable operation within -40 ℃ ~ 70 ℃ and 15 % ~ 95 % RH (non-condensing). Free from freezing at low temperatures and drying-out at high temperatures.
- Long–term reliability: No liquid-leakage risk; solid electrolyte is non-consumable. Service life can reach 10 years (e.g., Model FS00303 CO2 sensor).
- Detection performance: Accuracy of CO2 sensor reaches ±(50 + 3 % of reading); batch consistency ≥90 %; strong anti-moisture-interference capability.
- Engineering compatibility: Compatible with printing processes and automated manufacturing; compact dimensions (e.g., 17.8 × 20.6 × 15.4 mm), easy for integration.
III. Main Application Scenarios and Practical Cases
(I) Environmental Monitoring
- Industrial flue–gas monitoring: YSZ-based sensors are deployed in chemical industrial parks for real-time measurement of SO2 and NOx. Combined with IoT technology, emission source tracing can be realized.
- Motor–vehicle exhaust detection: Potentiometric sensors are integrated into exhaust analyzers for fast response to variations of hydrocarbons (HC) and carbon monoxide.
- Indoor air quality: Model FS00303 CO2 modules are used in office buildings and schools. They link with fresh-air systems to regulate gas concentration (measuring range: 400 ~ 10000 ppm).
(II) Special-Scenario Applications
- Agriculture and aquaculture: Monitor CO2 concentration in greenhouses to optimize photosynthesis; detect H2S in aquaculture ponds to prevent water quality deterioration.
- Medical diagnosis: Detect NH3 and VOCs in exhaled breath to assist diagnosis of diabetes and asthma, relying on the ppb-level high sensitivity of sensors.
- Harsh–environment applications: NASICON-based sensors are applied in tunnels and mines. They withstand high-humidity and high-dust conditions and detect toxic-gas leakage.
IV. Technical Challenges and Future Outlook
(I) Existing Bottlenecks
- Insufficient selectivity: Prone to cross-interference in complex gas atmospheres (e.g., mutual interference between NO and NO2).
- Limitation of low–temperature activity: Response speed of YSZ-based sensors drops below -20 ℃.
- Cost constraints: Precious-metal electrodes and precision manufacturing lead to high costs for high-end models.
(II) Innovation Directions
- Material improvement: Develop mixed-metal-oxide sensing electrodes and nano-coated electrolytes to enhance selectivity and anti-interference performance.
- Structural optimization: Adopt laser processing and electrospinning to construct high-efficiency Triple-Phase Boundaries (TPB) and boost sensitivity.
- Technology integration: Combine photosensitization technology with sensor arrays, and apply algorithms to realize simultaneous multi-component detection.
- Intelligent upgrading: Integrate with 5G and edge-computing techniques to develop low-power, self-calibrating intelligent sensor nodes.
V. Conclusion
Benefiting from core merits including zero liquid-leakage risk, wide-temperature-range adaptability and long service life, solid-electrolyte gas sensors have achieved large-scale deployment in environmental monitoring, industrial safety, public health and other fields. Driven by material innovations (e.g., NASICON composite electrolytes) and upgraded manufacturing workflows (e.g., fully-automated mass production), further breakthroughs will be made in detection accuracy, environmental tolerance and cost control. They are expected to become a mainstream technical solution in gas-sensing domains and deliver core support for precise monitoring and intelligent management.
