TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Component: The electrochemical cell is an electrochemical cell gas sensor (26) assembly comprising a 3-electrode arrangement with a working electrode, a reference electrode and a counter electrode. It can be electrochemical fuel cell assembly. The gas sensor assembly can be 2- or 4-electrode arrangement. It includes a chemically selective filter (24) arranged to remove interferent gas components that would interfere with or preclude measurement of a target gas species in a gas environment being monitored by the electrochemical cell gas sensor assembly. The electrochemical fuel cell assembly includes an ionic liquid electrolyte compartment containing the ionic liquid electrolyte; an anode and cathode in contact with the ionic liquid electrolyte in the compartment, both anode and cathode have backing layers providing passageways for gas access to the cathode; an anode current collector in contact with the anode backing layer; a cathode current collector in contact with the cathode backing layer; and an electrical circuit-forming mechanism coupled with the anode current collector and the anode current collector, to form an electrical circuit enabling output of electrical energy from the electrochemical cell. The anode current collector has fuel flow passages for flow of fuel during operation of the electrochemical fuel cell assembly. The cathode current collector has oxidant flow passages for flow of oxidant during operation of the electrochemical fuel cell assembly. The electrochemical fuel cell assembly has a fuel cell stack assembly. The electrochemical cell has a measuring electrode made of silver (Ag), a reference electrode made of Ag, and a counter electrode made of Ag or platinum. Preferred Property: The ionic liquid electrolyte comprises salt(s) having a melting point less than 100degreesC and a negligible vapor pressure. It is weight-stable after 5 days at 80degreesC, an electrical conductivity of at least 1 x 10 power 3 OMEGA-1.cm-1.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The salt is liquid at room temperature and has a formula of C+A-. It includes methyl-octyl-imidazolium-chloride, or and butyl-methyl-imidazolium-bis-trifluoromethane-sulfonimide. It can be imidazolium cations or pyridinium cations; or alkyl and/or aryl substituents (preferably halo, lower alkyl (1-4C), hydroxyl, or amine). The salt in the electrolyte can be halides (chlorides, iodides, bromides, fluorides), nitrates, nitrites, tetrafluoroborates, hexafluorophosphates, polyfluoroalkanesulfonates, bis(trifluoromethylsulfonyl)imides, methylsulfates, acetates, anions of fluoroalkanoic acid, or preferably aluminum chloride.C+ = quaternary ammonium salt or phosphonium salt;A- = anion.