Question

A fuel cell operates at constant current, with H$_2$ fuel (1 bar) and O$_2$ oxidant (1 bar). The electrolyte used is 0.001 M HCl and the product(s) of the reaction are confined inside the fuel cell. Which of the following is true about the electrolyte?

• A. Boiling point of the electrolyte decreases with increase in the duration of fuel cell operation
• B. Boiling point of the electrolyte increases with increase in the duration of fuel cell operation
• C. Open circuit voltage of the fuel cell remains constant with increase in duration of operation
• D. Open circuit voltage of the fuel cell increases with increase in duration of operation
• E. Both (A) and (C)

Hint:

Water production in a confined fuel cell always leads to the dilution of the electrolyte, which in turn reduces the boiling point and increases the freezing point of the solution.

Answer 1

The Correct Option is A

Concept: In a hydrogen-oxygen fuel cell, the overall reaction produces water ($2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$). If the products are confined, the concentration of the electrolyte changes over time.

Step 1: {Analyze the chemical change in the fuel cell.} As the fuel cell operates, water is continuously produced as the primary byproduct. Since the products are confined inside the cell, this water mixes with the 0.001 M HCl electrolyte.

Step 2: {Determine the effect on electrolyte concentration.} The addition of water to the HCl solution results in dilution. As the duration of operation increases, the concentration of HCl decreases.

Step 3: {Relate concentration to boiling point.} Boiling point elevation is a colligative property proportional to molality. As the electrolyte becomes more dilute, the boiling point elevation decreases, causing the boiling point of the electrolyte to decrease toward that of pure water.