Batteries and Fuel Cells: The Electrochemistry Behind Stored Energy
A battery carries its reactants inside it and eventually runs down. A fuel cell is fed from outside and does not. That single difference explains most of their properties.
The classification, and what decides it
- Primary cells contain a fixed amount of reactant and cannot be recharged. The cell reaction is not reversible in practice, either because a product escapes or because the electrode is physically consumed.
- Secondary cells can be recharged by driving current backwards, which regenerates the original reactants. This requires the discharge products to stay in place on the electrodes.
- Fuel cells are supplied with reactants continuously from an external store, so they produce current for as long as the fuel flows and are never described as charged or discharged.
The dry cell (Leclanché cell)
The container is zinc and acts as the anode. A graphite rod surrounded by manganese dioxide and carbon acts as the cathode, and the electrolyte is a moist paste of ammonium chloride and zinc chloride.
The cell gives about 1.5 V. It cannot be recharged: the zinc case is consumed as it discharges, and the ammonia produced is a gas that does not remain available for the reverse reaction. Ammonia is complexed by the zinc ions as the diamminezinc species, which prevents pressure build-up.
The lead storage battery
The standard example of a secondary cell, used in vehicles because it delivers a very large current briefly.
Two features make recharging possible. Lead sulfate is insoluble, so it stays deposited on the electrode where it formed and is available to be converted back. And sulfuric acid is consumed during discharge, so the density of the electrolyte falls measurably — which is how the state of charge is tested with a hydrometer. On charging, the reactions reverse exactly and the acid is regenerated.
The nickel–cadmium cell
Also a secondary cell, with a cadmium anode and nickel(IV) oxide cathode in alkaline electrolyte.
It has a longer working life than the lead battery and is much lighter, but cadmium is toxic, which is why it has largely been displaced by nickel–metal hydride and lithium-ion chemistry in consumer devices.
The hydrogen–oxygen fuel cell
Hydrogen and oxygen are fed continuously to porous carbon electrodes containing a catalyst such as finely divided platinum or palladium, with concentrated potassium hydroxide as the electrolyte.
The only product is water, which is why these cells were used in the Apollo spacecraft, where the water was drunk by the crew. The electrodes are porous to maximise the area of contact between gas, electrolyte and catalyst, since the reaction can only occur where all three meet.
Why the efficiency is so high
A fuel cell converts chemical energy directly into electrical energy. A thermal power station burns fuel to make heat, uses the heat to raise steam and the steam to turn a turbine, and every heat engine in that chain is limited by the Carnot efficiency. Fuel cells are not heat engines, so that limit does not apply, and practical efficiencies of around 70 per cent are achieved against roughly 40 per cent for a thermal station.
| Cell | Type | Voltage | Distinguishing feature |
|---|---|---|---|
| Dry cell | Primary | About 1.5 V | Zinc container is the anode and is consumed |
| Mercury cell | Primary | About 1.35 V | Constant voltage over its life, used in hearing aids |
| Lead storage | Secondary | About 2 V per cell | Acid density indicates state of charge |
| Nickel–cadmium | Secondary | About 1.2 V | Long life, but cadmium is toxic |
| H2–O2 fuel cell | Fuel cell | About 0.9–1.2 V | Water is the only product; no Carnot limit |
The mercury cell is worth noting for one reason that is regularly asked: its electrode reactions involve no ions whose concentration changes, so the cell potential stays essentially constant throughout its life instead of drifting downwards.
Frequently asked questions
Why can a lead storage battery be recharged but a dry cell cannot?
In the lead cell both discharge products are insoluble lead sulfate deposited on the electrodes, so they remain in place and can be converted back by reversing the current. In the dry cell the zinc container is physically consumed and ammonia is produced as a gas, so the original reactants cannot be restored.
Is the anode positive or negative in these cells?
Negative, because all of them are galvanic cells. Oxidation releases electrons at the anode, which makes it the negative terminal. The anode is positive only in an electrolytic cell, where an external supply drives the reaction.
Why does a fuel cell not run down?
Its reactants are stored outside the cell and fed in continuously. Nothing inside is consumed, so the cell keeps producing current for as long as hydrogen and oxygen are supplied. It is neither charged nor discharged in the way a battery is.
Why is fuel cell efficiency higher than that of a thermal power station?
A fuel cell converts chemical energy to electrical energy directly, so it is not a heat engine and the Carnot limit does not apply. A power station converts chemical energy to heat and then to work, and every stage in that chain loses energy.
Why are the electrodes in a fuel cell made porous?
The reaction can only occur where the gas, the electrolyte and the catalyst are all in contact. A porous electrode provides a very large area of such three-phase contact, which raises the current the cell can deliver.
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