NOVELTY - An electrochemical sensor comprises a fuel cell (10) having an anode (110) and an anode catalyst facilitating a chemical reaction in a component of a fluid stream; and an electrical circuit (500) having a voltage source (510), a variable resistor (520) and a monitor (530). The voltage source and variable resistor are operable to provide an overvoltage to the anode of the fuel cell; and the monitor is operable to detect and monitor a time-dependent response of the anode to the overvoltage. The time-dependent response is correlatable to a contaminant bound to the anode catalyst.    USE - For detecting the presence and concentration of a contaminant e.g. carbon monoxide (CO) within a fluid stream such as hydrogen gas supplied to the fuel cell (claimed).    ADVANTAGE - The sensor requires no additional CO sensing hardware; provides high sensor sensitivity in the low to sub ppm range, and rapid response to sudden changes in CO content; can be used in any system requiring accurate CO level quantification; and provides robust and reusable design. The sensor detects the true level of CO damage to a catalyst surface of a PEFC, when integrated with a power generating fuel cell, so that optimized remediation via air-bleed or other method can be employed. The sensor can also be used as an external device with limited hardware requirements. The principles of symbolic dynamics, finite state machines or simplified peak response-to-asymptotic value measurements can be used to achieve a high degree of precision for measuring CO concentrations. Using the techniques, CO poisoning of a fuel cell can be monitored and diagnosed before reaching a critical condition, thereby allowing early implementation of mitigation or graceful degradation strategies. The techniques include the analysis of time series data from a polymer electrolyte fuel cell (PEFC) using a technique for sensing time-varying CO levels in the fuel flow and allow for early prediction of incipient faults due to CO poisoning. The effective sensitivity and full-scale measuring of the sensor can be tailored to a particular application. The sensor need not be the actual power generating fuel cell itself, but can include an external component located inline with the incoming fuel stream. The configuration of sensor is less expensive, can be used to protect an entire stack of fuel cells, can be easily replaceable and calibrated, and affords for the use of the sensor in any application requiring accurate knowledge of CO levels. The sensor has a rapid response capable of detecting sudden transients common from fuel reforming stations. The sensor is extendable to the measurement of other low concentration species, e.g. sulfur, providing a suitable catalyst with adequate absorption/desorption characteristics is used.    DETAILED DESCRIPTION - An electrochemical sensor for a chemical species, which a contaminant in a fluid stream, comprises a fuel cell (10) having an anode (110) and an anode catalyst facilitating a chemical reaction in a component of a fluid stream, where the anode catalyst is capable of being poisoned by a contaminant in the fluid stream; and an electrical circuit (500) having a voltage source (510), a variable resistor (520) and a monitor (530). The voltage source and variable resistor are operable to provide an overvoltage to the anode of the fuel cell; and the monitor is operable to detect and monitor a time-dependent response of the anode to the overvoltage. The time-dependent response is correlatable to a contaminant bound to the anode catalyst. The overvoltage is sufficient to oxidize molecules of the contaminant bound to the anode catalyst. The circuit is operable to apply overvoltage to the anode for a time period of between 0.5 and 50 seconds. An INDEPENDENT CLAIM is included for detecting the presence and/or a concentration of a contaminant within a fluid stream, involving exposing a catalyst to a fluid stream, the fluid stream having a contaminant in it, the contaminant poisoning the catalyst; applying a stimulus to the catalyst; and monitoring the time-dependent catalyst performance to the stimulus, the time-dependent catalyst performance being correlatable to presence and/or concentration of the contaminant bound to the catalyst.    DESCRIPTION OF DRAWING(S) - The figure shows a polymer electrolyte fuel cell (PEFC).    fuel cell (10)    anode (110)    cathode (210)    electrical circuit (500)    voltage source (510)    variable resistor (520)    monitor. (530)