(24e) H2 Detection Via Polarography

Polarography is the measurement of the current that flows in solution as a function of an applied voltage. The actual form of the observed polarographic current depends upon the manner in which the voltage is applied and on the characteristics of the working electrode. The new gas polarographic H2 sensor shows a current level increment at temperature with concentration of the gaseous H2 similar to those relating to metal ions in liquid electrolytes in well-known polarography. This phenomenon is caused by the fact that the diffusion of the gaseous H2 through a gas diffusion hole built in the sensor is a rate-determining step in the gaseous-hydrogen sensing mechanism. The diffusion hole artificially limits the diffusion of the gaseous H2 toward the electrode located at the sensor cavity.

This gas polarographic H2 sensor is actually an electrochemical-pumping cell since the gaseous H2 is in fact pumped via the electrochemical driving force generated between the electrodes. Gaseous H2 enters the diffusion hole and reaches the first electrode (anode) located in the sensor cavity to be transformed into an H+ ions or protons; H+ ions pass through the solid electrolyte operated at low temperature and reach the second electrode (cathode) to be reformed to gaseous H2.

Having a gas binary system and assuming steady-state conditions, no reaction, stagnant background gas, and mass transfer only in one direction and rate-limited at the diffusion hole, the limiting output current is predicted to be a linear function of the logarithm of the background gas concentration. This linear relation between the output limiting current and the logarithm of the background molar fraction has a slope that includes the area and the length of the gas diffusion hole, the gas pressure, the gas temperature, the diffusion coefficient of the gaseous specie in the binary gas mixture, the Faraday constant, and the number of electrons per mole of the gaseous specie transferred between electrodes (this number is 4 for O2 and 2 for H2).

Gas polarographic O2 sensors are commercially available; a commercial gas polarographic O2 sensor was used to prove the feasibility of building a new gas polarographic H2 sensor operated at low temperature. As expected, the output limiting current proved to be dependent on H2 composition; the output limiting current is increased as the gaseous H2 concentration increases. As in the case of the gas polarographic O2 sensing, a linear relation between the output limiting current and the logarithm of the mole fraction of the background gas was also observed for the gas polarographic H2 sensing as shown in figure 3; further test runs and precise measurements of the diameter and the length of the diffusion hole are required to corroborate this linear relation