Which is correct about fuel cells? — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Introduction to Fuel Cells Fuel cells are electrochemical cells (galvanic cells) designed to convert the chemical energy of combustion of fuels (such as hydrogen, methane, or methanol) directly into electrical energy. Unlike conventional primary or secondary batteries, where the reacting chemicals are stored inside the cell, a fuel cell is an open thermodynamic system. Step 2 - Analysis of Statement (A): Continuous Operation In traditional batteries, once the active materials stored within the casing are fully consumed, the cell becomes "dead" and must be discarded or recharged externally. In a fuel cell, however, the reactants are fed continuously to the electrodes from an external source: * **Anode:** Fuel (typically $\ce{H2(g)}$) is continuously supplied and oxidized. * **Cathode:** Oxidant (typically $\ce{O2(g)}$) is continuously supplied and reduced. As long as the fuel and oxidant are continuously supplied, and the products are continuously removed, the electrochemical reactions proceed, and the cell runs indefinitely without requiring recharging. Therefore, Statement (A) is **correct**. Step 3 - Analysis of Statement (B): High Efficiency and Pollution-Free Nature * **Thermodynamic Efficiency:** Conventional thermal power plants convert chemical energy into heat, then into mechanical work (via steam turbines), and finally into electrical energy. Because of these intermediate conversions, they are strictly limited by the Carnot cycle efficiency, typically achieving only $35\text{--}40\%$ efficiency. Fuel cells convert chemical energy directly into electrical energy without any thermal intermediates, bypassing the Carnot limitation. Their practical operating efficiency is highly superior, usually ranging between $60\text{--}70\%$. * **Pollution-Free Operation:** In a standard hydrogen-oxygen ($\ce{H2-O2}$) fuel cell, the electrode half-reactions and the net cell reaction are: $$\text{Anode Reaction:} \quad \ce{2H2(g) + 4OH^-(aq) -> 4H2O(l) + 4e^-}$$ $$\text{Cathode Reaction:} \quad \ce{O2(g) + 2H2O(l) + 4e^- -> 4OH^-(aq)}$$ $$\text{Overall Reaction:} \quad \ce{2H2(g) + O2(g) -> 2H2O(l)}$$ The only product of this reaction is pure water ($\ce{H2O}$). Unlike burning fossil fuels, it does not release harmful greenhouse gases or pollutants such as sulfur oxides ($\ce{SO_x}$), nitrogen oxides ($\ce{NO_x}$), or particulate matter. Therefore, Statement (B) is **correct**. Step 4 - Analysis of Statement (C): Power and Water Supply in Space Programs The hydrogen-oxygen ($\ce{H2-O2}$) fuel cell was famously utilized in the Apollo space missions. In these spacecraft, it served a critical dual purpose: 1. **Power Generation:** It served as the primary source of clean, reliable electrical power for onboard instruments and life support. 2. **Drinking Water:** The water vapor formed as a byproduct of the electrochemical reaction was condensed and collected, providing high-purity drinking water for the astronauts. Therefore, Statement (C) is **correct**. Step 5 - Conclusion and Final Choice Since all statements—(A), (B), and (C)—are scientifically accurate descriptions of the features, advantages, and applications of fuel cells, the correct option is (D). $$\text{Correct Option: } \boxed{\text{D}}$$