4.2. Batteries and Fuel Cells

Batteries are compact galvanic cells (or assemblies of cells) that convert stored chemical energy into electrical energy on demand. Common types include:

• Primary (nonrechargeable) batteries: e.g., alkaline cells (zinc–manganese dioxide), alkaline‐carbon cells, lithium primary cells. They undergo irreversible chemical changes and cannot be recharged.
• Secondary (rechargeable) batteries: e.g., lead‐acid batteries (Pb–PbO2), nickel–cadmium (Ni–Cd), nickel–metal hydride (Ni–MH), lithium‐ion (Li‐ion) batteries. Applying an external current reverses the redox reactions, regenerating the reactants.

Batteries are devices that store chemical energy and convert it into electrical energy when needed. There are two main types of batteries:

• Primary batteries: These are non-rechargeable and can only be used once because the chemical reactions inside them are irreversible. An example is the alkaline battery, which is commonly used in remote controls and flashlights.
• Secondary batteries: These can be recharged by applying an external electric current, which reverses the chemical reactions inside them. An example is the lead-acid battery found in cars, which can be repeatedly charged and discharged as needed.

Think of primary batteries like disposable cameras. Once you finish the film, you can't reuse it; you have to get a new one. In contrast, secondary batteries are like rechargeable toothbrushes. After the battery runs low, you can plug it in and recharge it to use again.

Example: Lead‐acid battery half‐reactions (in 1.2 M sulfuric acid):

• Discharge (galvanic) mode:
  • Anode (oxidation): Pb(s) + SO4^2−(aq) → PbSO4(s) + 2 e−
  • Cathode (reduction): PbO2(s) + 4 H+(aq) + SO4^2−(aq) + 2 e− → PbSO4(s) + 2 H2O(l) Overall: Pb(s) + PbO2(s) + 2 H2SO4(aq) → 2 PbSO4(s) + 2 H2O(l)

In a lead-acid battery, two main reactions occur during discharge:

• At the anode, lead (Pb) reacts with sulfate ions (SO4^2−) to form lead sulfate (PbSO4), releasing electrons in the process. This is called oxidation because it involves the loss of electrons.
• At the cathode, lead dioxide (PbO2) reacts with sulfate ions and hydrogen ions (H+) to form lead sulfate and water. This is called reduction because it involves the gain of electrons. The overall reaction combines these two half-reactions and demonstrates how the chemical energy is converted to electrical energy.

Imagine this battery as a kitchen recipe. The lead and sulfate at the anode are like your ingredients, mixing together to make a dish (lead sulfate). At the cathode, lead dioxide acts like a cooking pot that combines different ingredients (SO4 and H+) to serve a final dish (again, lead sulfate plus water). The process releases energy (electricity) just like cooking can release delicious aromas!

Fuel cells are similar to batteries but consume a continuous supply of reactants (fuel and oxidant). The most common is the hydrogen–oxygen fuel cell:

• Anode (oxidation): H2(g) → 2 H+(aq) + 2 e−
• Cathode (reduction): ½ O2(g) + 2 H+(aq) + 2 e− → H2O(l) Overall: H2(g) + ½ O2(g) → H2O(l)

Because hydrogen is oxidized to protons and oxygen is reduced to water, the electron flow through the external circuit provides electricity. Fuel cells can achieve high efficiency and produce only water as a by‐product if pure hydrogen fuel is used.

Fuel cells convert chemical energy directly into electrical energy through redox reactions involving hydrogen and oxygen. Here’s how it works:

• At the anode, hydrogen gas (H2) is oxidized, releasing electrons and forming protons (H+ ions).
• At the cathode, oxygen gas is reduced by combining with protons and electrons to produce water (H2O). This process generates a steady supply of electricity as long as hydrogen and oxygen are continuously supplied, making fuel cells efficient and environmentally friendly, as their only emission is water.

Think of a fuel cell like a car engine that uses hydrogen instead of gasoline. Just like an engine needs fuel to keep running, a fuel cell needs a constant supply of hydrogen and oxygen to generate electricity. Just like cars release exhaust, fuel cells only release clean water vapor!

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