Water may be electrolytically decomposed in a cell similar to the one illustrated below. To improve electrical conductivity without introducing a different redox species, the hydrogen ion concentration of the water is typically increased by addition of a strong acid. The redox processes associated with this cell are
anode: 2H2O(l) ⟶ O2(g) + 4H+(aq) + 4e− Eanode°= +1.229 V
cathode: 2H+(aq) + 2e− ⟶ H2(g) Ecathode°= 0 V
cell: 2H2O(l) ⟶ 2H2(g) + O2(g) Ecell°= −1.229 V
Again, the cell potential as written is negative, indicating a nonspontaneous cell reaction that must be driven by imposing a cell voltage greater than +1.229 V. Keep in mind that standard electrode potentials are used to inform thermodynamic predictions here, though the cell is not operating under standard state conditions. Therefore, at best, calculated cell potentials should be considered ballpark estimates.
