(Invited) Zirconia-Based Electrochemical Oxygen Sensor for Monitoring Humidity at the Cathode of PEM Stacks

Abstract

One of the greatest challenges associated with PEMFCs is the water balance in thefuel cell stack. Depending upon the load and the operating conditions, the fuel cell membranes have a propensity to both flood and dry-out. If the PEM is not adequately humidified, the conductivity of the protons decreases, which causes reduced performance. Excess water also can create an issue by inhibiting the reactants from diffusing to the catalyst sites by flooding of the electrodes and gas channels if the water removal is insufficient. Current humidity sensing technologies are not capable of meeting the demands required for this harsh environment. Zirconia-based electrochemical sensors for measuring the oxygen concentration in the automotive exhaust have been used on vehicles for nearly 40 years. This oxygen sensor is an electrochemical cell based on oxygen-ion conducting zirconia operated at elevated temperatures (> 400°C). The sensor is a simple Nernst cell that consists of a single electrochemical cell with one electrode exposed to a reference atmosphere (typically air) and the other electrode exposed to the measurement (exhaust) gas. Although successfully used for stoichiometric A/F control for many years, Nernst-type oxygen sensors are not useful for applications away from stoichiometry because of their weak logarithmic dependence of the emf on oxygen partial pressure. Single and double-zirconia-cell sensors have been developed which have much higher sensitivity and thus a wider range of operation. These are amperometric sensors are based on oxygen pumping, which involves the transfer of oxygen from one side of a zirconia cell to the other by passing an electric current through the device. Since these sensors operate over a wide range, they are referred to as “wide-range oxygen” or “universal exhaust gas oxygen” (UEGO) sensors and can be used over a broader range of A/F measurement and control. All gasoline vehicles, as well as diesel powertrains which operate lean, employ UEGO sensors to meet the emission requirements. The double-cell amperometric sensor also can be used to measu­re the concentration of other oxygen-containing molecules, e.g. H2O, CO2, SO2, and NOx. In order to mea­sure the H2O concentration for example, a voltage is applied across the pump-cell that is greater than the dissociation potential for H2O. The resulting oxygen generated from the decomposition of water is then measured via an increase in pumping current by the UEGO sensor and is proportional to the H2O concentration in the gas. If other oxygen-containing molecules are present in the measurement gas, the voltage across the pump cell can be varied over the dissociation potentials of these gases. For H2O/air mixtures, the concentration of oxygen can be measured at a lower potential. A second voltage larger than the dissociation potential of the measurement gas, H2O, is applied to measure the concentration of oxygen plus water. The water concentration can be determined by subtracting the pumping current due to oxygen from the pumping current due to the combination of the two gases. Other gases can also be measured in a similar fashion. These measurements can be achieved without any modification to the existing sensor hardware. A simple modification to the electronic control circuitry is all that is required to perform this type of measurement. Figure 1 shows the sensitivity of a commercially available UEGO sensor for the dissociation of H2O as a function of dissociation potential. A typical operating value for the sensing voltage Vs for oxygen measurement is 450 mV. The plot below was generated from data measured in the lab at various water concentrations in a carrier gas of dry nitrogen. From the plot it is evident that water dissociation is occurring at potentials less than 800 mV. Additionally, the output begin to saturate at a potential approaching 1100 mV, suggesting that full dissociation of the water has been achieved. Figure 2 shows the sensitivity (pumping current) of the same sensor as a function of water concentration for Vs = 1080 mV. Once again the carrier gas is dry nitrogen. The output is linear in water concentration. The water balance at the cathode is particularly difficult to manage because water accumulates due to the reaction and the clogging of the small channels. Controlling this water balance requires accurate water concentration measurements at temperatures approaching 100°C and pressures up to 3 bar absolute. The application of this humidity sensing technology to measure the water concentration at the cathode of PEMFCs will be discussed. Figure 1