Commercial, Lithium & Fuel Cells (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about commercial cells, lithium cells and fuel cells.

Non-rechargeable, rechargeable and fuel cells are the three ways electrochemical cells are used to generate electricity commercially, and that's why these three topics are covered together.

Electrochemical cells can be used as a commercial source of electrical energy, and the type used depends on the voltage, current, size and cost required. Cells fall into three categories: non-rechargeable cells, whose reaction can't be reversed; rechargeable cells, whose reaction is reversed by charging; and fuel cells, which are never recharged because fuel is supplied to them continuously.

This video starts with the non-rechargeable and rechargeable types covered in commercial cells, then looks at lithium cells as a modern rechargeable example, before finishing with fuel cells.

Non-rechargeable cells use a reaction that can't be reversed. The Daniell cell was one of the earliest examples: a zinc rod in zinc sulfate solution and a copper cylinder in copper sulfate solution, separated by a porous pot, generating an EMF of 1.1 Volts. Though its hazardous liquids make it impractical as a portable device.

Zinc-carbon cells are the most common non-rechargeable type, with a zinc casing as the negative electrode and an ammonium chloride paste as electrolyte and positive electrode, generating 1.50 Volts. As they discharge, the zinc casing wears away and the corrosive paste can leak. This is the category where the chemistry only runs one way, which is exactly what the rechargeable types don't share.

Rechargeable cells use reactions that can be reversed by applying an external voltage greater than the cell's own, pushing electrons back the other way. Lead-acid batteries join six cells in series, using lead and lead(IV) oxide electrodes in sulfuric acid, and generate about 12 Volts combined. In a car, the generator reverses the reaction to recharge them, though they're heavy and contain toxic materials.

Ni Cad cells use cadmium and nickel(II) hydroxide-oxide electrodes and can be recharged hundreds of times, but they lose capacity through the memory effect if not fully discharged before recharging. Both trade the one-way limitation of non-rechargeable cells for their own drawbacks.

Lithium cells are the rechargeable type that powers laptops and mobile devices, using lithium's low density and high electrode potential. The cell uses a positive lithium cobalt oxide electrode, a negative carbon electrode, and a solid polymer electrolyte that can't leak, generating an EMF of between 3.5 and 4.0 Volts. Unlike Ni Cad cells, lithium-ion cells don't suffer from the memory effect, so they can be topped up without losing charge. Their main limitation is supply: global demand for lithium already exceeds what's available, and cells lost to landfill make that worse.

Fuel cells are the third category, and they're never recharged because fuel is supplied to them continuously rather than stored inside them. In the hydrogen-oxygen fuel cell, hydrogen is oxidised at one electrode and oxygen is reduced at the other, combining to produce water as the only product and an overall EMF of 1.23 Volts. Because the reaction happens at room temperature without combustion, more of the chemical energy converts to electrical energy than in a combustion engine, and no nitrogen oxides form. The risk is hydrogen itself: it's highly flammable, needs thick-walled storage, and is currently produced mostly as a by-product of the crude oil industry.

When asked why a fuel cell doesn't need recharging, say that fuel is supplied continuously — don't say the cell recharges itself. And when describing the porous separator, say that it lets ions flow to complete the circuit, not electrons — electrons flow only through the external circuit.

Whether a cell can be recharged, and how, is what separates these three types. Non-rechargeable cells like the Daniell cell and zinc-carbon can't reverse their reaction. Rechargeable cells like the lead-acid, NiCad and lithium cells can, each with its own trade-off in weight, memory effect or lithium supply. Fuel cells sidestep recharging altogether by taking a continuous supply of fuel, producing water as their only by-product.