Batteries: Primary Cells, Fuel Cells and the Li-ion Battery

Batteries take the principle of the galvanic cell and package it into a practical, self-contained, portable source of electrical energy — pre-loaded with its own reactants, unlike a fuel cell, which is supplied continuously from outside.

Batteries are broadly divided into primary cells, which cannot be recharged because their electrode reactions are not practically reversible once the original reactants are used up, and secondary cells, which can be recharged by driving the discharge reaction backwards with an externally applied current. The common dry cell (Leclanche cell), familiar from torches and remote controls, is a primary cell: its outer zinc casing serves as the anode, oxidized as $\text{Zn}(s) \to \text{Zn}^{2+}(aq) + 2e^{-}$, while a central carbon rod surrounded by a moist paste of $\text{MnO}_2$ and $\text{NH}_4\text{Cl}$ serves as the cathode, where $\text{MnO}_2$ is reduced. A single fresh dry cell delivers an EMF of approximately $1.5\ \text{V}$, though this value falls steadily as the cell discharges and its reactants are gradually consumed, since they cannot be replenished.

The hydrogen-oxygen fuel cell (covered in more detail in the concept on fuel cells) is, unlike a battery, continuously fed hydrogen and oxygen from external supplies rather than storing its reactants internally, so it does not run down in the same way as long as fuel keeps flowing. Its only chemical byproduct is water, and because it converts chemical energy directly into electrical energy without an intermediate heat-engine stage, it can reach considerably higher energy-conversion efficiencies than conventional fossil-fuel combustion. …