What Is a Fuel Cell
We already know from galvanic cells that a spontaneous redox reaction can be harnessed to push electrons through an external wire and do electrical work. An ordinary battery does exactly this, but it carries a finite store of reactants sealed inside its casing; once those are consumed the cell is exhausted. A fuel cell removes that limitation.
NCERT defines it precisely: galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane and methanol directly into electrical energy are called fuel cells. The defining feature is continuous supply — the reactants are fed continuously to the electrodes and the products are removed continuously from the electrolyte compartment, so the cell runs as long as fuel keeps arriving.
The word "combustion" is doing careful work here. In a furnace, hydrogen burning in oxygen releases its energy as heat. In a fuel cell the same overall change, $\ce{2H2 + O2 -> 2H2O}$, is split into two spatially separated half-reactions so that the electrons travel through an external circuit. The chemical energy of the combustion is captured as electrical work rather than scattered as heat.
"Fuel cell" is not the same as "fossil-fuel power plant"
A thermal power plant burns a fossil fuel to raise steam and is a major source of pollution. A fuel cell is an electrochemical device — there is no flame and no turbine. Both consume fuel continuously, but only the fuel cell converts chemical energy directly into electricity.
Fuel cell = continuous-feed galvanic cell. No combustion flame, no turbine, no large-scale pollution.
Why Not Just Burn the Fuel
NCERT motivates the fuel cell by contrasting it with the dominant way we make electricity today. In a thermal plant the chemical energy (heat of combustion) of a fossil fuel — coal, gas or oil — is first used to convert water into high-pressure steam; this steam runs a turbine to produce electricity. The chain is therefore chemical → heat → mechanical → electrical, and energy is lost at every conversion.
A galvanic cell, by contrast, converts chemical energy directly into electricity and is highly efficient. The insight behind the fuel cell is to keep that direct conversion but feed the reactants continuously, so the device can run indefinitely like a power plant while keeping the efficiency of a cell.
The thermal route loses energy at every chemical→heat→mechanical→electrical hand-off; the fuel cell skips straight from chemical to electrical energy.
The H2–O2 Cell: Construction
One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water. In NCERT's description, hydrogen and oxygen are bubbled through porous carbon electrodes into a concentrated aqueous sodium hydroxide solution (the NIOS account uses aqueous KOH; both are strong alkalis, so the device is an alkaline fuel cell). The electrolyte is therefore basic, which is why $\ce{OH^-}$ ions appear in every half-reaction.
The electrode reactions are intrinsically slow, so catalysts — finely divided platinum or palladium — are incorporated into the porous carbon to increase the rate of the electrode reactions. The porosity matters: it maximises the three-phase contact between the gas, the liquid electrolyte and the solid catalyst where the reaction actually occurs.
Hydrogen feeds the anode and oxygen feeds the cathode through porous catalysed carbon. Electrons run through the external load; $\ce{OH^-}$ ions carry the current inside the cell.
Electrode Reactions
This is the part NEET most often tests, so commit the alkaline half-reactions to memory. Hydrogen is the fuel and is oxidised at the anode; oxygen is the oxidant and is reduced at the cathode. In the basic electrolyte, $\ce{OH^-}$ is the species exchanged.
Anode (oxidation):
$\ce{2H2(g) + 4OH^-(aq) -> 4H2O(l) + 4e^-}$
Cathode (reduction):
$\ce{O2(g) + 2H2O(l) + 4e^- -> 4OH^-(aq)}$
Overall:
$\ce{2H2(g) + O2(g) -> 2H2O(l)}$
Notice that when the two half-reactions are added, the $4\,\ce{OH^-}$ and $4\,\ce{H2O}$ terms on each side cancel down, leaving the clean overall change $\ce{2H2 + O2 -> 2H2O}$ — the cell runs continuously as long as the reactants are supplied. The only chemical product is water; there is no $\ce{CO2}$ or other emission, which is why the device is described as pollution-free.
Four electrons leave the anode through the wire; four hydroxide ions return through the electrolyte. The two loops together give a balanced four-electron transfer per cycle.
Don't write the acidic half-reactions for the alkaline cell
Because the electrolyte is NaOH/KOH, the correct species is $\ce{OH^-}$, not $\ce{H+}$. A frequent error is to write the anode as $\ce{H2 -> 2H+ + 2e^-}$ — that is the acidic-medium version. In the NCERT alkaline cell the anode is $\ce{2H2 + 4OH^- -> 4H2O + 4e^-}$ and the cathode produces $\ce{OH^-}$.
Alkaline electrolyte ⇒ $\ce{OH^-}$ in both half-reactions. Overall change is still just $\ce{2H2 + O2 -> 2H2O}$.
Confused about why a battery dies but a fuel cell does not? Compare the storage devices in Batteries — Primary & Secondary.
Efficiency vs Thermal Plants
The headline number to remember: fuel cells produce electricity with an efficiency of about 70%, compared to thermal plants whose efficiency is about 40%. The reason is structural, not incidental — the fuel cell avoids the heat-engine step entirely.
A thermal plant is a heat engine, and any heat engine is bound by thermodynamic limits on how much heat can be turned into work. The fuel cell sidesteps this because it never converts the chemical energy into heat first; it draws the energy out directly as electrical work, much like the maximum work relationship $W_{max} = -nFE^\circ$ that governs any galvanic cell.
| Property | H2–O2 Fuel Cell | Thermal Power Plant |
|---|---|---|
| Energy conversion | Chemical → electrical (direct) | Chemical → heat → mechanical → electrical |
| Approximate efficiency | ~70% | ~40% |
| Pollution | Pollution-free (product is water) | Major source of pollution |
| Mechanism | Electrochemical, no flame | Combustion + turbine |
| By-product | $\ce{H2O}$, usable as drinking water | $\ce{CO2}$, ash, flue gases |
Advantages & Applications
The fuel cell's appeal is the combination of high efficiency, clean operation, and continuous running. NIOS puts it neatly: galvanic cells have high efficiency but can be used only once and then discarded; thermal plants run continuously but with low efficiency; the fuel cell combines the advantages of the two — it is both efficient and able to run continuously.
| Advantage | Why it matters |
|---|---|
| High efficiency | ~70% vs ~40% for thermal plants — direct chemical-to-electrical conversion |
| Pollution-free | Only product of the H2–O2 cell is water; no $\ce{CO2}$ or particulates |
| Continuous operation | Runs as long as fuel and oxidant are supplied; no recharge cycle |
| Useful by-product | Water produced was condensed and added to astronauts' drinking supply |
The flagship application is the Apollo space programme, where the H2–O2 cell provided onboard electrical power; the NIOS text records the cell potential as about 0.9 V. NCERT also notes that, as new electrode materials, better catalysts and improved electrolytes are developed, fuel cells have been tried in automobiles on an experimental basis, and a variety of fuel cells have been fabricated in view of their future importance.
In the H2–O2 fuel cell, how many electrons are transferred per molecule of $\ce{O2}$ consumed, and what mass of water forms per mole of $\ce{O2}$?
From the cathode half-reaction $\ce{O2 + 2H2O + 4e^- -> 4OH^-}$, each $\ce{O2}$ accepts 4 electrons. The overall reaction $\ce{2H2 + O2 -> 2H2O}$ shows that 1 mol $\ce{O2}$ yields 2 mol $\ce{H2O}$, i.e. $2 \times 18 = \mathbf{36\ g}$ of water. This is exactly the water the Apollo crews recovered for drinking.
Fuel Cell vs Battery
Both fuel cells and batteries are galvanic cells, so the distinction is conceptual rather than a difference in fundamental chemistry. The dividing line is where the reactants live and what happens when they run low.
| Feature | Fuel Cell | Battery |
|---|---|---|
| Reactant storage | Fed continuously from outside | Sealed inside the cell |
| Lifetime | Runs as long as fuel is supplied | Primary: dies when reactants used up; Secondary: needs recharge |
| Product handling | Removed continuously | Accumulates inside the cell |
| Recharging | Not required — just refuel | Secondary cells must be recharged electrically |
| Typical example | $\ce{H2}$–$\ce{O2}$ cell (Apollo) | Dry cell, lead storage, Ni–Cd |
The cleanest way to phrase it for an exam: in a battery the chemical energy is stored and slowly depleted, whereas in a fuel cell the chemical energy is delivered on demand from an external fuel stream. This is why a fuel cell never "runs down" in the way a torch battery does.
Other Fuels & the Hydrogen Economy
Hydrogen is the cleanest fuel, but it is not the only option. NCERT's own intext question asks for fuels other than hydrogen, and the chapter itself names methane and methanol as combustion fuels that can be fed into fuel cells. These hydrocarbon-based cells trade some of hydrogen's cleanliness for easier fuel handling.
NCERT closes the topic with the vision of the Hydrogen Economy. Because burning hydrogen yields only water, it is an ideal non-polluting fuel — provided the hydrogen itself is produced cleanly, for instance by splitting water using solar energy. Both halves of this cycle, the production of hydrogen by electrolysis of water and its combustion in a fuel cell, are built on electrochemical principles, tying the fuel cell back to the whole chapter.
Fuel Cells in one screen
- A fuel cell is a galvanic cell that converts the energy of combustion of a fuel directly into electricity, with reactants fed in continuously.
- The classic device is the alkaline H2–O2 cell: porous carbon electrodes with Pt/Pd catalyst dipping in concentrated NaOH/KOH.
- Anode: $\ce{2H2 + 4OH^- -> 4H2O + 4e^-}$; Cathode: $\ce{O2 + 2H2O + 4e^- -> 4OH^-}$; Overall: $\ce{2H2 + O2 -> 2H2O}$.
- Efficiency ≈ 70% vs ≈ 40% for thermal plants; pollution-free, only product is water.
- Powered the Apollo missions (~0.9 V); water was used as astronauts' drinking water.
- Unlike a battery, reactants are not stored inside and the cell never needs recharging — it just needs refuelling.