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In a galvanic cell, a spontaneous oxidation-reduction reaction supplies electrical work: ${\Delta }_{r}G<0$ for discharge, the anode is negative, and the cathode positive. A discharging electric battery and a hydrogen–oxygen fuel cell are two practical examples. The first stores its reactants internally; the second is supplied continuously with fuel and oxidant.

An electrolytic cell consumes externally supplied electrical work to drive electrode reactions or material transfer under an applied potential. Its anode is positive and cathode negative. water electrolysis for hydrogen production and metal electroplating are two practical examples. oxidation always occurs at the anode and reduction at the cathode in both cell types; the electrode signs change. The electrolyte conducts ions, while electrons flow through the external circuit. A rechargeable battery can operate in either regime depending on whether it is discharging or being charged.

Water electrolysis uses electrical work to convert water into hydrogen and oxygen. For the net reaction, two water molecules produce two hydrogen molecules and one oxygen molecule; four electrons pass through the external circuit per reaction as written.

The reversible voltage follows from the reaction's Gibbs free energy under specified conditions. Finite operating electric current additionally requires kinetic and transport driving forces and incurs resistive losses, so operating voltage exceeds the reversible requirement.