# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 2 Catalyst Combustion Hydrogen Sensors
### Principle of Concentration Measurement  
The structure schematic of the catalyst combustion-type hydrogen sensor is shown in Figure 1. It consists of a sensitive element and a compensation element, in which the pill-beaded sphere is its sensitive body, and the sensitive body of the sensitive element contains a catalyst. The sensitive body of the compensation element contains no catalyst and is used to compensate for the effect of ambient temperature changes on the hydrogen sensor output. The working principle of the catalyst combustion-type hydrogen sensor is that hydrogen reacts with oxygen on the surface of the catalytic sensor to release heat. Using a sensitive element, a compensating element, and a fixed resistor to form a bridge, the heat generated via the catalytic combustion of the combustible gas is conducted to the wrapped platinum (Pt) coil, causing the resistance of the coil to rise and, thus, causing a change in voltage in the bridge path of the sensing signal. This principle can be used to detect any combustible gas, including hydrogen. When the hydrogen sensor works normally, the Pt wire of the sensor itself generates heat under the condition of applied voltage, meaning that the temperature of the sensitive body ranges from 200 to 300 \({}^{\circ}\)C. In this temperature range, a certain volume fraction of hydrogen reacts with oxygen in the air under the action of a catalyst, and the reaction heat is generated, meaning that the resistance value increases accordingly. In general, the volume fraction of hydrogen in the atmosphere that is measured is not too high, i.e., below 4%, and the hydrogen can be burned completely, with its heat generation being related to the volume fraction. The larger the volume fraction of hydrogen in the environment that is measured, the greater the heat of the reaction, the greater the temperature increase in the platinum wire, and the greater the increase in its resistance value. Therefore, the volume fraction of hydrogen in the air can be detected via measuring the change in resistance of the Pt wire \(\Delta R\). The bridge circuit of the catalyst combustion-type hydrogen sensor is shown in Figure 2, where F1 is the sensitive element and F2 is the compensation element. When F1 is in contact with hydrogen, heat is released due to violent oxidation, which causes the temperature of F1 to rise and the resistance value to increase accordingly. The bridge circuit is no longer balanced, and a potential difference \(E\) is generated between A and B. Next, there is  
\[E=k(R_{F2}/R_{F2})\Delta R_{F} \tag{1}\]  
where \(\Delta R_{F}\) is the rate of change in the resistance of the sensitive element, and \(k\) is a constant related to the applied voltage. The potential difference \(E\) between points A and B is  
Figure 1: Schematic diagram of structure of catalytic combustion hydrogen sensor. Figure 2: Bridge circuit diagram of thermoelectric catalytic hydrogen sensor. approximately proportional to \(\Delta R_{F}\) in the range where the resistance ratio \(R_{F2}/R_{F1}\) of the compensating element \(F2\) and the sensitive element \(F1\) is close to 1. Moreover, \(\Delta R_{F}\) is due to the heat of combustion released via hydrogen combustion, which is proportional to the heat of combustion and can be expressed using Equation (2)  
\[\Delta R_{F}=\alpha\Delta T=\alpha(\Delta H/C)=\alpha\cdot a\cdot m(Q/C) \tag{2}\]  
where \(\alpha\) is the resistance temperature coefficient of the sensitive element, \(\Delta T\) is the value of temperature increase generated via hydrogen combustion, \(\Delta H\) is the heat generated via the catalytic combustion of hydrogen, \(C\) is the heat capacity of the sensitive element, \(Q\) is the heat of combustion of hydrogen, \(a\) is the volume fraction of hydrogen, and \(a\) is the constant determined via the catalyst on the sensitive element. The values of \(\alpha\), \(C\) and, \(a\) are related to the material, shape, and surface treatment of the sensitive element.