An amperometric electrochemical sensor configured to be operable in an oxidizing atmosphere and under an applied bias to exhibit enhanced reduction of oxygen molecules at the sensing electrode in the presence of one or more target gas species and a resulting measurable increase in oxygen ion flux through the cell. The sensor has an electrolyte membrane, a sensing electrode on the electrolyte membrane, and a counter electrode on the electrolyte membrane, wherein the sensing electrode includes at least one molybdate or tungstate compound. An electrochemical sensor system is also provided, along with a method of detecting the concentration of one or more of NOx and NH3 in a gas sample or stream.