Use a reaction to generate electricity

A simple cell contains two different electrodes in an electrolyte connected through an external circuit. A more reactive metal tends to lose electrons more readily. In a zinc/copper cell using dilute acid, zinc is oxidised: Zn(s) -> Zn²⁺(aq) + 2e^-. Electrons travel through the wire towards copper, where hydrogen ions can gain them: 2H⁺(aq) + 2e^- -> H₂(g). The electrolyte carries ionic current and completes the circuit.

Which metal is which electrode? The more reactive metal is the negative electrode. It loses electrons more readily, so it is oxidised and pushes electrons into the wire. The less reactive metal is the positive electrode, where reduction happens. In the zinc/copper cell, zinc is negative and copper is positive. Electrons in the wire always flow from the negative electrode to the positive electrode.

The voltage depends on how far apart the two metals are in the reactivity series. The bigger the difference in reactivity, the bigger the voltage. So magnesium with copper gives a bigger voltage than zinc with copper, because magnesium is more reactive than zinc. Iron with copper gives a smaller voltage than zinc with copper, because iron is less reactive than zinc. Two pieces of the same metal give no voltage at all, since neither one pushes electrons harder than the other.

Hydrogen can be obtained from water by electrolysis or from hydrocarbons. In a hydrogen fuel cell it reacts with oxygen to produce water and electricity directly: 2H₂(g) + O₂(g) -> 2H₂O(l). Water is the point-of-use product, but overall environmental impact depends on how the hydrogen and electricity were produced. Detailed fuel-cell construction is not required.