Cells That Do Work: Batteries, Fuel Cells and Corrosion

A battery is a galvanic cell that has been carefully engineered so its spontaneous redox reaction delivers a steady, useful current. Batteries come in two families. Primary batteries, like the alkaline cells in a flashlight, run their reaction once and are then thrown away. Secondary batteries, like the lead-acid battery in a car or the lithium-ion battery in a phone, are rechargeable: driving current backward through the cell reverses the electrode reactions and restores the starting chemicals. In a lead-acid battery, discharge turns lead and lead dioxide into lead sulfate at the two plates; charging pushes that reaction in reverse. Every battery has limits. Its voltage is set by the chemistry of its electrodes, its stored energy is limited by how much reactant it holds, and side reactions slowly wear it out, which is why rechargeables lose capacity over hundreds of cycles. A fuel cell works differently. Instead of storing its reactants inside a sealed case, it is fed fuel and oxidizer continuously from outside, usually hydrogen and oxygen. As long as fuel keeps flowing in, the cell keeps producing electricity, with water as the product. A fuel cell never needs recharging, but it needs a fuel supply, and its real voltage always falls short of the ideal cell potential because of slow reaction kinetics at the electrodes and resistance losses inside the cell.

Any galvanic cell could in principle be a battery, but a practical battery has to do a job: hold enough reactant to last, keep its voltage steady, and survive being carried around. An ideal battery would never run down and would deliver the same voltage forever, but real batteries fall short of that ideal in predictable ways. Their voltage drops as reactants are used up, and drawing a large current makes the voltage sag because the electrode reactions cannot keep up instantly. This is the difference between a cell potential on paper, which predicts what a cell could deliver, and a working cell, which has to keep supplying current. Primary cells like zinc-carbon and alkaline batteries are used once. Secondary cells can be recharged because their electrode reactions are chemically reversible, so an external power supply can run them backward. Fuel cells sidestep storage entirely: reactants flow in from tanks and products flow out, so the cell runs as long as it is fed. The hydrogen-oxygen fuel cell powers spacecraft and some vehicles, producing only water, but its operating voltage is limited by electrode kinetics and internal resistance, so engineers stack many cells together to reach useful voltages.