Fuel Cell Sensor Technology and Electrolytic Cell Oxygen Analyzer
Fuel Cell Sensor Technology
At present, oxygen analyzers widely adopted in the market are based on the following working principles:
● Paramagnetic (utilizing the extremely high paramagnetic property of oxygen compared with general gases in a magnetic field).
● Zirconia (utilizing the doped zirconia, a solid electrolyte, to deliver excellent conductivity for oxygen ions at high temperatures).
● Laser: (essentially utilizing the absorption of specific-wavelength infrared spectra by oxygen molecules).
● Fuel cell type. Oxygen analyzers adopting the first three principles feature long service life, excellent long-term stability and fast response when measuring constant or high-concentration oxygen. However, for oxygen concentration below 1000ppm, their performance is inferior to that of fuel cell oxygen analyzers.
Consequently, whether industrial plants or laboratories, fuel cell oxygen sensors are predominantly applied as the core detection components for trace oxygen analyzers.
A fuel cell oxygen sensor is essentially a type of galvanic cell oxygen sensor which consists of three basic parts: electrodes, electrolyte solution and a housing. When oxygen-containing sample gas enters the housing, an oxidation reaction occurs at the anode, forming oxides of the anode material. This reaction is similar to the combustion of oxygen; hence, such galvanic cells are commonly referred to as "fuel cells", and this type of galvanic cell oxygen sensors is also named fuel cell oxygen sensors. According to the properties of the electrolyte solution inside the sensor, fuel cell oxygen sensors are classified into alkaline fuel cell oxygen sensors and acidic fuel cell oxygen sensors. The electrolyte solutions can be liquid or paste-like (referred to as solid electrolyte in some literatures). The following introduces the alkaline and acidic fuel cell oxygen sensors respectively.
Alkaline Fuel Cell Oxygen Sensor
An alkaline fuel cell consists of a silver cathode, a lead anode and KOH alkaline electrolyte. As the electrolyte is alkaline, when the measured gas contains acidic components such as CO₂, H₂S, Cl₂ and SO₂, neutralization reactions will occur and the silver electrode will be corroded. This leads to performance degradation of the fuel cell oxygen sensor, manifested as slower response time, reduced sensitivity and decreased measurement accuracy. Therefore, alkaline fuel cell oxygen sensors are not suitable for measured gas with acidic components.
>> Structure and Operating Principles
Figure 6-1 shows the schematic structure of the Model CI113 alkaline fuel cell oxygen sensor manufactured by CHANGAI Electronic Science & Technology Co., Ltd. It is composed of a silver cathode, a lead anode and liquid KOH electrolyte solution. The contact plate in the figure is made of metal and serve to connect the electrodes (silver cathode and lead anode). The thin electrolyte layer is formed by the overflow of electrolyte through a number of round holes on the upper surface of the cathode. The gas permeable membrane, made of polytetrafluoroethylene (PTFE) , allows the measured gas to pass through and covers the top of the thin electrolyte layer.
During operation, the measured gas passes through the PTFE membrane and enters the thin electrolyte layer, where the oxygen in the sample gas undergoes an electrochemical reaction inside the cell.
| Silver cathode | O₂+2H₂O+4e-→4OH- |
| Lead anode | 2Pb+ 4OH-→2 PbO+2H₂O+4e- |
| Overall cell reaction | O₂+ 2Pb→2 PbO |
The current generated by OH- ion flow is proportional to the oxygen content in the sample gas. It can be seen from the above chemical reactions that if no oxygen is present, no reaction occurs and no current is generated. Therefore, the sensor theoretically has an absolute zero point. Nevertheless, similar to concentration-cell zirconia sensors, whose theoretical electromotive force in air should be zero but usually delivers a non-zero output due to materials, the signal of fuel cell oxygen sensors generally cannot reach a zero even after being supplied with high-purity nitrogen treated by deoxygenation technology, and may even produce negative signals. Since the lead at the anode is continuously converted into lead oxide, the service life of the sensor terminates once the lead electrode is completely consumed.
>> Performance Analysis
In an alkaline electrolyte solution, the reduction of oxygen to OH- at the silver cathode can be expressed by the following formula.
In formula:
I - Current flowing through the electrodes of a galvanic cell
K - Constant
[O₂] The oxygen concentration in the measured sample gas
[OH-] The activity (effective concentration) of OH⁻ ions in the electrolyte
e - Base of the natural logarithm
φ- Polarization reaction potential of the silver electrode
F - Faraday constant
R - Gas constant
S - Thermodynamic temperature
This formula covers all reactions of alkaline fuel cell oxygen sensors, but can be used for the qualitative interpretation of the characteristics of fuel cell oxygen sensors.
As can be seen from the formula and Figure 6-2
① The higher the oxygen concentration, the more obvious the nonlinear relationship.
② Temperature Characteristics: The discharge current of the fuel cell oxygen sensor exhibits an exponential relationship with the thermodynamic temperature T. As the temperature rises, the discharge current increases significantly.
Therefore, to ensure measurement accuracy, two methods can be employed: constant temperature maintenance or temperature compensation. At present, most oxygen analyzers on the market equipped with fuel cell oxygen sensors employ thermistors with a negative temperature coefficient for temperature compensation, while those using a constant-temperature method are less common.
③ Effect of KOH solution on fuel cell oxygen sensors
It can be concluded from the formula that OH- exhibits a negative exponential relationship with the current signal output by the sensor. Studies have shown that when the concentration of KOH solution is around 6 mol/L (mass fraction: 26.8%), the electrical conductivity reaches a maximum, meaning that the activity of OH⁻ is also at its maximum at this point. Further research indicates that when the KOH concentration is maintained within the range of 5.5~6.9 mol/L, the variation of conductivity caused by fluctuations in solution concentration and temperature is minimized. This corresponds to the smallest variation in OH⁻ activity, thereby minimizing the impact on the sensor's sensitivity. Therefore, the preparation of KOH solution for the sensor shall comply with the above principles.
④ Effect of sample gas flow rate
Variations in sample gas flow rate generally have no significant effect on the discharge current of fuel cell oxygen sensors. This is because the current signal output of the sensor is correlated with the oxygen partial pressure in the measured gas. When the sample gas flow rate changes but the oxygen content in the sample gas stays constant, the partial pressure of oxygen also remains unchanged.
>> Major Technical Specifications
Taking the CI-PC90 trace oxygen analyzer from CHANGAI Electronic Science & Technology Co., Ltd. as an example, the main technical specifications are as follows:
| Sensor | CI213 | |
| Accuracy | 0.01～9.99ppm O₂ | ±5% FS |
| 10.0～99.9ppm O₂ | ±3% FS | |
| 100～1000ppm O₂ | ±2% FS | |
| 0～21.00% O₂ | ±2% FS | |
| Repeatability | 0.01～9.99ppm O₂ | ±2.5% FS |
| 10.0～99.9ppm O₂ | ±1.5% FS | |
| 100～1000ppm O₂ | ±1% FS | |
| Stability | 0.01～9.99ppm O₂ | ±2.5% FS/7d |
| 10.0～99.9ppm O₂ | ±1.5% FS/7d | |
| 100～1000ppm O₂ | ±1% FS/7d | |
| Response Time | T90＜60S(25℃) | |
| Recovery Time | It takes 60 minutes to reduce the concentration from the ambient level (20.94%) to 10 ppm | |
| Calibration Cycle | One year(recommended) | |
| Ambient Temperature | 0～45℃ | |
| Ambient Humidity | ＜80%RH | |
| Sample Gas Pressure | Normal pressure ±10% (air outlet must be vented) | |
| Sample Gas Flow | 1.5～2L/min | |
| Sensor Service Life | More than 2 years(normally use) | |
>> Precautions for Use
① Studies have shown that the service life of fuel cell oxygen sensors is related to the following factors:
● Volatilization and leakage of electrolyte;
● Passivation effect caused by lead oxide deposition from surface reaction of the lead anode metal;
● Gas permeability and water repellency of the permeable membrane. The passivation of lead oxide is related to the measured oxygen content. The higher the oxygen concentration, the greater the anode consumption and the shorter the service life of the sensor. Therefore, it is recommended to equip a spare sensor.
② Oxygen analyzers equipped with fuel cell oxygen sensors as the detection unit require low routine maintenance. Calibration shall be performed once every six months with high-purity nitrogen (≥99.999%) and oxygen-in-nitrogen standard gas at 90% of the measurement range.
③ When the production equipment is shut down for maintenance and the analyzer is out of service, it is recommended to purge the fuel cell oxygen sensor of the analyzer with high-purity nitrogen (≥99.999%) for approximately 8 - 10 minutes, and then set the analyzer to the purge mode ( (at which point the sensor is sealed). After production equipment maintenance is completed and the analyzer is being restarted, purging the gas circuit with the measured sample gas for 3–5 minutes before switching the analyzer to the measurement mode. This operation provides two advantages: firstly, it extends the service life of the sensor; secondly, it results in faster response and stabilization times when resuming measurements. This measure is especially applicable to scenarios requiring rapid measurement, such as the production of high-purity nitrogen and high-purity argon, and CO₂ recovery in breweries.
④When storing a fuel cell oxygen sensor, place it in a nitrogen-filled protective bag and short-circuit the terminals with a shorting ring. Do not damage the protective bag during storage. The bag should only be opened when replacing the sensor. After removing the shorting ring, install the sensor into the analyzer immediately.
⑤The pressure range of fuel cell oxygen sensors is generally 35~210 kPa. If the gas supply pressure is excessively high, a pressure reducing valve must be used first to adjust the pressure within the above-mentioned safe range.
Acidic Fuel Cell Oxygen Sensor
The acidic fuel cell oxygen sensor consists of a gold cathode, a lead anode and liquid acetic acid electrolyte. It is suitable for environments where the measured atmosphere contains acidic substances (such as CO₂ and H₂S), such as the measurement of trace oxygen in CO₂ recovery at breweries and the measurement of trace oxygen under nitrogen-protected in brazing furnaces. A typical acidic fuel cell oxygen sensor is the XLT-12-333 from AII. Its schematic structure is similar to the alkaline fuel cell oxygen sensor shown in Figure 6-1, with differences only in electrode materials and electrolyte. The figure below illustrates the schematic structure of the acidic fuel cell oxygen sensor produced by CITY. Despite structural differences, both sensors share the same operating mechanism.
When oxygen in the measured gas passes through the PTFE permeable membrane (also referred to as an oxygen diffusion membrane in some literature) and enters the fuel cell, the following redox reactions occur at the electrodes.
The main difference between alkaline and acidic fuel cell oxygen sensors lies in their electrolytes.This design is intended to accommodate various application scenarios.With the advancement of technology, some companies have developed fuel cell oxygen sensors using neutral electrolytes, such as the CI213 model from Changai, which is suitable for applications where the measured atmosphere contains acidic or alkaline gases.
| Cathodic reduction reaction | O₂+2H₂O+4e-→4OH- |
| Anodic oxidation reaction | 2Pb+ 4OH-→2 PbO+2H₂O+4e- |
| Overall cell reaction | O₂+ 2Pb→2 PbO |
Electrolytic Cell Oxygen Analyzer
Essentially, an electrolytic cell converts electrical energy into chemical energy. The electrolytic cell oxygen sensor belongs to the electrolytic cell category. Therefore, in principle, its electrochemical reaction requires an external power supply for normal operation. Compared with fuel cell oxygen sensors, its anode is non-consumable and generally does not need replacement. Electrolytic cell oxygen sensors are mainly used for trace oxygen measurement, with a detection limit down to the ppb level (currently, the vast majority of fuel cell-type oxygen sensors used for trace oxygen measurement can only achieve the ppm level). A typical electrolytic oxygen analyzer is the Delta F trace oxygen analyzer manufactured by GE (see Figure 6-4 for the schematic structural diagram of the sensor). Its sensor is based on the coulometric electrolysis principle. A DC voltage of approximately 1.3 V is applied to the electrolytic cell to supply energy for redox reactions. When trace oxygen in the sample gas passes through the permeable membrane into the cathode, oxygen molecules are reduced to OH⁻ at the cathode. With the aid of KOH electrolyte, OH⁻ migrates to the anode where an oxidation reaction takes place to generate oxygen, which is then discharged.
| Cathodic reduction reaction | O₂+2H₂O+4e-→4OH |
| Anodic oxidation reaction | 4OH-→O₂+2H₂O+4e |
As can be seen from the above electrode reaction equations, there is no consumption of the electrolytic cell or electrodes. Therefore, users do not need to replace the electrodes or electrolytic cell during operation; they only need to periodically replenish the distilled water and electrolyte (the electrolyte decreases due to natural evaporation). This is different from the aforementioned fuel cell oxygen sensors, which generally need to be replaced every 1 to 2 years.
When introducing alkaline fuel cell-type oxygen sensors, it is emphasized that they must not be used in applications where the measured gas contains acidic components. The Delta F electrolytic oxygen sensor uses an alkaline KOH solution as its electrolyte. To overcome interference caused by acidic gases and prevent electrode corrosion, a pair of Stab-EL auxiliary electrodes is designed inside the sensor. The function of these auxiliary electrodes is to remove these harmful gases after the sample gas containing acidic gases enters the electrolytic cell, thereby preventing damage to the sensor and ensuring the accuracy of the analyzer's readings.
Figure 6-4 Schematic diagram of the Delta F trace oxygen sensor
