Fuel cells generate electricity through a redox reaction producing water

Steps:

• Identify the reactants: hydrogen (H₂) at the anode and oxygen (O₂) at the cathode.
• Recall the half-reactions: anode oxidation (2H₂ → 4H⁺ + 4e⁻) and cathode reduction (O₂ + 4H⁺ + 4e⁻ → 2H₂O).
• Combine reactions: overall equation 2H₂ + O₂ → 2H₂O, showing water as the chemical product.
• Confirm: fuel cells output electricity, heat, and water vapor or liquid.

Why D is correct:

• The electrochemical reaction in a hydrogen-oxygen fuel cell follows 2H₂ + O₂ → 2H₂O, directly producing water as the byproduct.

Why the others are wrong:

• A: Electricity is the useful energy output, not a chemical product.
• B: Hydrogen is a reactant consumed at the anode.
• C: Oxygen is a reactant supplied at the cathode.

Final answer: D