Which one of the following is the exhaust pipe emission from Fuel Cell Electric Vehicles, powered by hydrogen?

The correct option is Water vapour.

To understand why water vapour is the sole exhaust emission from a Hydrogen Fuel Cell Electric Vehicle (FCEV), let us examine the chemical process that occurs inside the fuel cell system.

In a hydrogen fuel cell, electricity is generated through an electrochemical reaction between hydrogen fuel (from the vehicle's onboard tank) and oxygen (drawn from the ambient air). The overall chemical reaction can be represented by the following equation:

$$2{\mathrm{H}}_2+{\mathrm{O}}_2\Rightarrow 2{\mathrm{H}}_2\mathrm{O}+\text{Electricity}+\text{Heat}$$

During this process:
1. Hydrogen gas ($H_2$) is supplied to the anode (negative electrode), where it is split into protons ($H^{+}$) and electrons ($e^{-}$).
2. The electrons flow through an external circuit to generate the electricity that powers the vehicle's electric motor.
3. The protons pass through a proton exchange membrane (PEM) to the cathode (positive electrode).
4. At the cathode, the protons and electrons recombine and react with oxygen ($O_2$) from the air.

The only byproduct of this electrochemical combination is water ($H_2$O). Because the reaction produces heat, this water is typically released from the exhaust pipe in the form of water vapour (steam) or liquid water droplets. Consequently, hydrogen fuel cell vehicles are classified as zero-emission vehicles because they produce no harmful pollutants like tailpipe carbon dioxide (CO₂), nitrogen oxides (NO_x), or particulate matter.