Fuel cells are being developed as power sources for many applications. Fuel cells generate power, without combustion, by extracting the chemical energy of hydrogen from hydrogen containing fuels. Advantages of fuel cells include high efficiency and very low release of polluting gases (e.g., NOx) into the atmosphere. Of the various types of fuel cells, the proton exchange membrane (PEM) fuel cell is receiving considerable attention for transportation applications due to its low weight, low temperature operation, and its considerable potential for mobile and residential applications. The heart of the PEM fuel cell is a membrane electrode assembly (MEA), which is a sheet of a proton-conducting polymeric material (e.g., Nafion) with thin coatings of platinum containing electrocatalysts (anodes and cathodes) on opposite faces. Several MEAs are stacked with interposed electrically conductive elements (current collectors) that contain appropriate channels for distributing the gaseous reactants over the surfaces of the anode and cathodes. PEM fuel cells operate most efficiently when hydrogen is the anode reactant (fuel) and oxygen as the cathode reactant (oxidant). However, for more practical applications, air is used as the oxidant and a hydrogen rich gas (derived from hydrocarbons) is used as the fuel. For transportation applications, the use of liquid hydrocarbon fuels for fuel cells, such as gasoline, is most attractive due to transportability, high energy density, and existing infrastructure.
One of the problems of using hydrocarbons to produce the hydrogen required for operating the PEM fuel cell is that carbon monoxide is a poison to the platinum electrocatalysts in the anode of the MEA. Performance of the PEM fuel cell can be degraded when CO is present at levels as low as 20 parts per million, and considerable performance degradation is observed when the CO content is higher than 100 parts per million. Thus, the hydrocarbon fuels must be converted into a hydrogen rich gas containing little or no carbon monoxide (since trace amounts of CO will degrade PEM fuel cell performance). Fuel processors, utilizing multiple catalytic reactor stages, are being developed to meet this requirement. For the automotive application, especially, it will be imperative to have a sensor that monitors the amount of carbon monoxide at various stages of the fuel processing system, and to monitor CO content of the hydrogen-rich gas exiting the fuel processor.
The importance of carbon monoxide sensors for automotive PEM fuel cell systems is illustrated by a schematic of an automotive fuel processor, shown in FIG. 1. Fuel processing 10 typically involves three or four catalytic stages. The first reforming step 14 involves the reaction of gasified hydrocarbons 12 with air 11 (partial oxidation or POX), or with air 11 and steam 13 (autothermal reforming or ATR), to convert gasoline, methanol or other hydrocarbons into a gas mixture rich in hydrogen and carbon monoxide. This reformed gas mixture then is subjected to the water-gas-shift reaction (CO+H2O→CO2+H2) to reduce carbon monoxide levels and increase hydrogen content. The water-gas-shift (WGS) reaction is usually performed in two separate reactions, the first 15 at relatively high temperature (to convert most of the carbon monoxide), and the second 16 at a lower temperature (where equilibrium CO contents are lower). After exiting the WGS reactors 15 and 16, the hydrogen-rich reformate gas enters the preferential oxidation (PROX) reactor 17 where the gas is mixed with air 11 to oxidize remaining carbon monoxide to carbon dioxide. A key technical challenge facing developers of fuel processors for automotive applications is the requirement to maintain low carbon monoxide contents during operational transients, such as those that would occur during acceleration and deceleration. Transients can cause spikes in the carbon monoxide content of the reformed gas. The primary use for the CO sensors under development in this program is to measure the CO content of the reformate gas 18 exiting the PROX reactor 17. There are two potential benefits of this type of CO sensor:                (1) The sensor can provide feedback to the PROX reactor. This will allow the optimum amount of air to be fed into the PROX reactor (and minimize any wasted hydrogen); and        (2) The sensor will protect the PEM fuel cell stack. When a high CO content is detected the reformate gas would be diverted from the stack (with power being provided by a battery) until the CO level returns to tolerable levels.        
Existing carbon monoxide sensors cannot meet the requirements of the fuel cell application. Commercial CO sensors, typically based on semiconducting oxides (e.g., tin oxide), operate on the basis of a resistance change due to oxidation of CO to CO2 (carbon dioxide). This type of sensor cannot work for the fuel cell application because of the absence of oxygen in the reformate gas. Further, even if oxygen were available, it would be difficult for the tin oxide sensor to detect low levels of carbon monoxide in the presence of a high concentration of hydrogen (because oxidation of hydrogen also will occur). With current technology, optical sensors are the only current option for rapid and accurate detection of CO in a hydrogen-rich atmosphere. However, optical sensors are bulky and extremely expensive, and it is doubtful that the size and cost of these systems can be reduced sufficiently for the fuel cell application.
It is therefore a goal of the present invention to provide a sensor that can detect carbon monoxide in a hydrogen-rich oxygen-deficient environment. That is to say, it is an object of the present invention to provide a sensor that can detect carbon monoxide in a reducing environment. It is a further goal of the present invention to provide a sensor that can detect carbon monoxide in a hydrogen-rich gas stream so as not to poison the catalyst of a PEM fuel cell.