Fuel Cells — Revision Notes

⚡ 30-Second Revision

Definition — Electrochemical device converting chemical energy of fuel/oxidant into electrical energy continuously.
Key Difference from Batteries — Continuous external supply of reactants vs. internal finite reactants.
H2-O2 Fuel Cell Reactions (Acidic Medium)

- Anode (Oxidation): $H_2(g) \rightarrow 2H^+(aq) + 2e^-$ - Cathode (Reduction): $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$ - Overall: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

Byproduct — Water ($H_2O$) for H2-O2 fuel cells.
Electrolyte Role — Conducts ions, blocks electrons, separates reactants.
Catalyst Role — Speeds up electrode reactions (e.g., Pt).
Efficiency — $eta = \frac{|Delta G|}{|Delta H|} \times 100$
Gibbs Free Energy — $Delta G = -nFE_{cell}$
Advantages — High efficiency, clean (water byproduct), quiet, continuous power.

2-Minute Revision

Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy through a continuous supply of fuel and an oxidant. Unlike batteries, they are power generators, not energy storage devices, and do not require recharging.

The most common example is the hydrogen-oxygen fuel cell. At the anode, hydrogen is oxidized ($H_2 \rightarrow 2H^+ + 2e^-$), releasing electrons that flow through an external circuit. At the cathode, oxygen is reduced, combining with these electrons and protons from the electrolyte to form water ($O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$).

The overall reaction is $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$, making water the only byproduct and highlighting their environmental friendliness. The electrolyte is crucial for ion transport and separating reactants, while catalysts (like platinum) accelerate the electrode reactions.

Fuel cells boast high theoretical efficiencies, calculated as $eta = |Delta G|/|Delta H|$, and their operation is linked to Gibbs free energy via $Delta G = -nFE_{cell}$.

5-Minute Revision

Fuel cells are sophisticated electrochemical devices designed for continuous power generation. They operate on the principle of converting the chemical energy of a fuel and an oxidant directly into electrical energy, distinguishing them from traditional batteries which store a finite amount of reactants internally. This continuous operation, fueled by external supplies, means they never 'run out' or need recharging, only refueling.

The most prominent example is the hydrogen-oxygen fuel cell. Here, hydrogen gas ($H_2$) serves as the fuel and oxygen gas ($O_2$), typically from the air, acts as the oxidant. The cell comprises two porous electrodes (anode and cathode), usually coated with a platinum catalyst, separated by an electrolyte.

At the Anode (Oxidation): Hydrogen gas is fed to the anode, where it is oxidized, releasing electrons and forming protons:

$$H_2(g) xrightarrow{\text{catalyst}} 2H^+(aq) + 2e^-$$

These electrons then travel through an external circuit, generating an electric current that powers devices.

At the Cathode (Reduction): Oxygen gas is supplied to the cathode. Here, it reacts with the protons that have migrated through the electrolyte and the electrons arriving from the external circuit to form water:

$$O_2(g) + 4H^+(aq) + 4e^- xrightarrow{\text{catalyst}} 2H_2O(l)$$

To balance the electrons for the overall reaction, the anode reaction is multiplied by two:

$$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$$

The only byproduct is water, making hydrogen fuel cells remarkably clean. The electrolyte plays a vital role by allowing ions (e.g., $H^+$) to pass through, completing the internal circuit, while simultaneously blocking electrons and preventing direct mixing of the fuel and oxidant. Catalysts are essential for accelerating the slow electrode reactions.

Fuel cells offer high efficiency because they bypass the heat engine cycle, converting chemical energy directly. Their theoretical efficiency is given by $eta = \frac{|Delta G|}{|Delta H|}$, where $Delta G$ is the Gibbs free energy change and $Delta H$ is the enthalpy change of the reaction.

The electrical work done is related to $Delta G$ by $Delta G = -nFE_{cell}$, where $n$ is the number of electrons transferred and $F$ is Faraday's constant. Key advantages include minimal environmental impact, quiet operation, and scalability.

Challenges include fuel storage and infrastructure costs.

Worked Example: If $Delta G^circ = -474.4,\text{kJ/mol}$ and $Delta H^circ = -571.6,\text{kJ/mol}$ for the H2-O2 fuel cell, its theoretical efficiency is $eta = \frac{|-474.4|}{|-571.6|} \times 100$.

Prelims Revision Notes

Fuel cells are electrochemical cells that convert chemical energy into electrical energy continuously, unlike batteries which store energy. They require a constant external supply of fuel and oxidant.

Key Characteristics:

Continuous Operation — Produce electricity as long as fuel and oxidant are supplied.
No Recharging — Unlike secondary batteries, they don't need recharging.
High Efficiency — Direct conversion of chemical to electrical energy, bypassing Carnot cycle limits.
Environmentally Friendly — Especially H2-O2 fuel cells, producing only water as a byproduct.

Hydrogen-Oxygen Fuel Cell (PEMFC - Proton Exchange Membrane Fuel Cell):

Fuel — Hydrogen gas ($H_2$)
Oxidant — Oxygen gas ($O_2$) from air
Electrolyte — Proton Exchange Membrane (e.g., Nafion) - conducts $H^+$ ions.
Catalyst — Platinum (Pt) on porous electrodes.

Electrode Reactions:

Anode (Oxidation) — $H_2(g) \rightarrow 2H^+(aq) + 2e^-$
Cathode (Reduction) — $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$
Overall Reaction — $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

Role of Components:

Anode — Site of fuel oxidation, electron release.
Cathode — Site of oxidant reduction, electron consumption.
Electrolyte — Allows ion transport ($H^+$), blocks electron flow, separates reactants.
Catalyst (Pt) — Lowers activation energy for electrode reactions.

Thermodynamics:

Gibbs Free Energy — $Delta G = -nFE_{cell}$ (Maximum electrical work)
Theoretical Efficiency — $eta = \frac{|Delta G|}{|Delta H|} \times 100$

* For H2-O2 fuel cell, $Delta G^circ = -474.4,\text{kJ/mol}$, $Delta H^circ = -571.6,\text{kJ/mol}$ at $298,\text{K}$. * $eta approx 83$

Other Fuel Cell Types (Briefly):

Alkaline Fuel Cells (AFCs) — KOH electrolyte, $OH^-$ ions.
Solid Oxide Fuel Cells (SOFCs) — Solid ceramic electrolyte, $O^{2-}$ ions, high temp.

NEET Focus: Understand the reactions, byproducts, distinction from batteries, and the role of components. Be prepared for simple calculations involving $Delta G$ and efficiency.

Vyyuha Quick Recall

Fuel Cells: For Us, Electricity Lasts Continuously, Emitting Little Liquid Simply. (Focuses on continuous operation, electricity, and water byproduct).