Part 4: Commercial cells, recharging and fuel cells

Combine supplied electrode reactions and evaluate batteries and hydrogen fuel cells using their chemistry.

Distinguish how the reactants are supplied

A primary cell is designed for one discharge cycle: its changes are not usefully reversed by recharging. A secondary cell is rechargeable; an external electrical supply drives the discharge reactions backwards to regenerate active materials. A fuel cell receives fuel and oxidant from outside and can operate while supplies continue and products are removed.

Rechargeable does not mean infinitely reusable. Side reactions, structural changes and losses of active material gradually reduce capacity. A fuel cell does not need electrical recharging of a fixed internal fuel store, but it still needs a continuing fuel supply and maintenance.

The specified simplified lithium model

AQA specifies a simplified lithium/cobalt-oxide model. At the negative electrode during discharge, lithium is oxidised; at the positive electrode, cobalt oxide accepts an electron while incorporating Li⁺. Summing the two changes gives Li + CoO₂ → LiCoO₂. Cobalt changes from +4 in CoO₂ to +3 in LiCoO₂.

The notation Li⁺[CoO₂]⁻ shows the combined neutral product. It must not be mistaken for a net negatively charged bulk material. Use the supplied potentials to calculate EMF. If illustrative reduction potentials are +0.60 V for the positive couple and −3.00 V for Li⁺/Li, discharge EMF is +3.60 V.

This is a syllabus model, not a full description of every modern lithium-ion battery. Many commercial rechargeable designs move lithium ions between host structures rather than depositing bulk lithium metal in normal operation. Lithium-based cells use suitable non-aqueous electrolytes because lithium metal would react with water.

Reverse the discharge equations

For a supplied rechargeable-cell model, first derive discharge as usual, balancing electrons and cancelling shared species. For charging, reverse the overall equation and both electrode reactions. The applied electrical supply must drive the non-spontaneous reverse process; practical charging also overcomes losses.

For example, a lead–acid model has discharge Pb + PbO₂ + 2SO₄²⁻ + 4H⁺ → 2PbSO₄ + 2H₂O. Charging reverses this equation, regenerating lead, lead dioxide and acid. Do not assume products that escape or permanently damage an electrode can readily be regenerated. Details of unfamiliar commercial cells can be deduced from the provided half-equations.

Learn the alkaline hydrogen–oxygen half-equations

At the negative electrode, hydrogen is oxidised using hydroxide ions and releases electrons to the external circuit. At the positive electrode, oxygen is reduced with water and produces hydroxide ions. Multiply the hydrogen half-equation by two so four electrons cancel. The electrolyte carries ions internally while electrons do useful work externally.

Calculate EMF from reduction potentials

Suppose the given reduction potentials are −0.83 V for 2H₂O + 2e⁻ → H₂ + 2OH⁻ and +0.40 V for O₂ + 2H₂O + 4e⁻ → 4OH⁻. The hydrogen equation is reversed for discharge, so E°cell = 0.40 − (−0.83) = +1.23 V. Multiplying the hydrogen equation by two does not double its potential.

An acidic hydrogen fuel cell has different half-equations: hydrogen forms H⁺ at the anode and oxygen consumes H⁺ at the cathode. Its overall reaction is still water formation. Use the electrolyte stated in the question; do not mix an acidic half-equation with an alkaline one unless you also account for the chemistry connecting the media.

Compare full systems, not slogans

Hydrogen–oxygen fuel cells produce water at the point of use and can provide efficient electrical conversion with low local pollutant emissions. Overall environmental impact depends on hydrogen manufacture, electricity sources, compression or liquefaction, transport and equipment production. Hydrogen is an energy carrier, not an automatically renewable primary energy source.

Hydrogen’s low volumetric density makes storage difficult: compressed gas needs strong vessels, liquefaction requires very low temperatures and energy, and storage in other materials adds mass or processing requirements. Flammability and leaks require appropriate engineering. Catalyst cost, fuel purity and infrastructure also affect deployment.

Battery comparisons should consider capacity, mass, voltage, cycle life, charging time, material extraction and recycling. “Higher voltage” alone does not mean “stores more energy”: charge capacity matters too. A liquid fuel such as methanol can be easier to handle volumetrically than hydrogen gas, but methanol cells can emit CO₂; compare the actual alternatives given.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources