When an electric current is applied, water reacts at the cathode by forming hydrogen and negatively charged hydroxide ions, which diffuse through the barrier to the anode to produce oxygen.

Water molecules can break down into hydrogen ions and hydroxide ions. \(H_{2}O(l)\rightleftharpoons H^{+}(aq)+OH^{-}(aq)\) This is a reversible reaction. A small proportion of water molecules ...