Correct Option (D)
Fuel Cell Electric Vehicles (FCEVs) generate electricity through an electrochemical reaction within a fuel cell. In this process, hydrogen (H₂) reacts with oxygen (O₂) from the air. The primary and sole by-product of this reaction is water (H₂O), which is typically expelled as water vapour from the exhaust pipe. This characteristic makes FCEVs a zero-emission technology at the tailpipe, contributing to cleaner air.
Incorrect Options
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Hydrogen peroxide (H₂O₂): This compound is not a direct product of the standard electrochemical reaction occurring in a hydrogen fuel cell. Hydrogen peroxide is a reactive oxygen species and is not formed as a tailpipe emission.
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Hydronium (H₃O⁺): Hydronium ions are formed internally within the proton exchange membrane of a fuel cell as part of the proton transport mechanism. However, they are not emitted externally as an exhaust product.
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Oxygen: Oxygen (O₂) is a reactant in the fuel cell, consumed during the electrochemical process to combine with hydrogen. Therefore, it is not emitted from the exhaust pipe. Any unreacted oxygen is typically vented safely but is not considered a tailpipe emission.