Principles, Structure, and Technical Evolution of Three-Electrode Electrochemical Gas Sensors

Colorless and odorless gas molecules in the air often affect our safety and health without notice. From carbon monoxide and hydrogen sulfide emitted by factories, to nitrogen oxides and ozone in urban air, and formaldehyde and carbon dioxide indoors, precise monitoring has become essential in industrial production, environmental protection, and daily life. Among various gas detection technologies, three-electrode electrochemical gas sensors are widely adopted due to high sensitivity, excellent selectivity, low power consumption, and mature industrial application. Nexisense, with over 40 years of expertise in gas sensing technology, continuously optimizes the design and performance of three-electrode systems. This article systematically analyzes their basic principles, structural features, technical advantages, key performance metrics, application scenarios, and future trends to provide a comprehensive understanding of this technology.

Basic Principles: Extending Fuel Cell Technology to Gas Sensing

The principle of a three-electrode electrochemical gas sensor directly derives from the electrochemical reaction mechanism of fuel cells. Essentially, the sensor is a miniature electrochemical cell composed of an electrolyte, a gas-permeable membrane, and three electrodes. The target gas diffuses through the selective membrane, reacts with the catalyst on the working electrode, undergoes oxidation or reduction, and generates electron transfer, producing a measurable current signal.

For example, in carbon monoxide (CO) detection, the typical reactions are:

This reaction occurs at room temperature without external heating, resulting in extremely low power consumption, typically in the milliwatt range. The generated current is linearly proportional to gas concentration, with sensitivity reaching tens to hundreds of nA/ppm. Although the signal is weak, it is highly precise, forming the basis of the sensor’s high sensitivity.

From Two-Electrode to Three-Electrode: A Structural Leap

Early electrochemical sensors used a two-electrode structure where the working and counter electrodes were shared. The counter electrode handled both current conduction and potential reference, leading to polarization and significant potential drift, limiting measurement accuracy and stability.

The three-electrode system resolves these issues, consisting of:

• Working Electrode (WE): The core site of gas reactions, coated with noble metal catalysts (Pt, Au, Pd), generating detection current.

• Counter Electrode (CE): Provides the electron transfer path for current, usually with a large area to reduce polarization.

• Reference Electrode (RE): Establishes a stable potential reference, does not participate in main reactions, used to monitor and control WE potential.

The three-electrode structure works with a potentiostat circuit: the circuit monitors the WE-RE potential difference and dynamically adjusts the WE-CE voltage, keeping WE potential constant. This closed-loop control eliminates interference from CE polarization, electrolyte concentration changes, and temperature fluctuations, greatly improving accuracy, repeatability, and long-term stability. Nexisense optimizes electrode materials, electrolyte formulas, and potentiostat design to ensure RE drift is minimal, typically <±2 mV/year.

Key Technical Elements and Core Performance Metrics

Performance depends on:

• Catalyst and Electrode Material: Particle size, dispersion, and crystal orientation of noble metal nanoparticles directly affect sensitivity and selectivity.

• Electrolyte System: Acidic, alkaline, or neutral electrolytes determine gas type compatibility and anti-interference ability.

• Gas-Permeable Membrane & Diffusion Control: Controls gas diffusion rate, ensuring linear range and response time.

• Sealing and Packaging: Prevents electrolyte evaporation and external contamination.

Typical performance metrics:

Applications and Practical Value

Applications include:

• Industrial Safety: Real-time monitoring of CO, H₂S, SO₂, NH₃ in oil, coal, and metallurgical industries.

• Environmental Monitoring: Track NO₂, O₃, SO₂ for AQI calculation.

• Indoor Air Quality: Detect CO₂, formaldehyde, VOCs, enabling smart ventilation control.

• Medical: Breath analysis for auxiliary diagnosis.

• Automotive: In-car air quality sensors triggering purification systems.

  
  

Technical Evolution and Future Directions

From two-electrode to three-electrode was a major revolution. Current trends include:

• Miniaturization & MEMS integration: Millimeter-scale sensors for wearable and IoT nodes.

• Smart upgrades: Integrated temperature/humidity compensation, ADCs, microprocessors for digital output, auto-zero, and self-diagnostics.

• Multi-gas arrays: Electrode arrays with different catalysts for simultaneous detection.

• Long-life technology: Ionic liquid electrolytes, solid electrolytes, improved sealing for >5-year lifespan.

• Low-power optimization: Reduced standby current for battery-powered deployment.

FAQ

1. Difference between three- and two-electrode sensors? Three-electrode adds RE and potentiostat control, eliminating polarization interference, improving precision and stability.

2. Role of reference electrode? Provides stable potential reference to maintain WE potential.

3. How does the potentiostat work? Monitors WE-RE potential difference and adjusts WE-CE voltage to keep WE potential constant.

4. Unit of sensitivity? Typically nA/ppm.

5. Why is T90 important? Time to reach 90% of final reading, affects alarm timeliness.

6. Typical sensor lifetime? 2–3 years under normal conditions, affected by environment.

7. Common interfering gases? e.g., CO on H₂. Optimized catalysts and membranes minimize interference.

8. Support for digital output? Yes, some models have ADC and RS485/Modbus interfaces.

9. Detected gases? CO, H₂S, SO₂, NO₂, NH₃, O₃, Cl₂, H₂, etc.

10. Future focus? Miniaturization, smart features, multi-gas detection, ultra-long life.

Conclusion

Three-electrode electrochemical gas sensors, based on fuel cell electrochemistry, with reference electrode innovation and potentiostat control, are a pillar technology in modern gas detection. They provide room-temperature operation, low power, high accuracy, and easy integration, serving industrial safety, environmental monitoring, and indoor health. Nexisense leverages 40 years of expertise to advance miniaturization, intelligence, and long-life design, offering reliable, adaptable solutions. Choosing a stable, reliable three-electrode sensor is both a technical decision and a commitment to safety and foresight.