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23.9: Electrolysis of Water

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    With fossil fuels becoming more expensive and less available, scientists are looking for other energy sources. Hydrogen has long been considered an ideal source, since it does not pollute the air when it burns. The problem has been finding ways to generate hydrogen economically. One new approach that is being studied is photoelectrolysis—the generation of electricity using photovoltaic cells to split water molecules. This technique is still in the research stage, but appears to be a very promising source of power in the future.

    Electrolysis of Water

    The electrolysis of water produces hydrogen and oxygen gases. The electrolytic cell consists of a pair of platinum electrodes immersed in water to which a small amount of an electrolyte such as \(\ce{H_2SO_4}\) has been added. The electrolyte is necessary because pure water will not carry enough charge due to the lack of ions. At the anode, water is oxidized to oxygen gas and hydrogen ions. At the cathode, water is reduced to hydrogen gas and hydroxide ions.

    \[\begin{array}{lll} \text{oxidation (anode):} & 2 \ce{H_2O} \left( l \right) \rightarrow \ce{O_2} \left( g \right) + 4 \ce{H^+} \left( aq \right) + 4 \ce{e^-} & E^0 = -1.23 \: \text{V} \\ \text{reduction (cathode):} & 2 \ce{H_2O} \left( l \right) + 2 \ce{e^-} \rightarrow \ce{H_2} \left( g \right) + 2 \ce{OH^-} \left( aq \right) & E^0 = -0.83 \: \text{V} \\ \hline \text{overall reaction:} & 2 \ce{H_2O} \left( l \right) \rightarrow \ce{O_2} \left( g \right) + 2 \ce{H_2} \left( g \right) & E^0_\text{cell} = -2.06 \: \text{V} \end{array}\nonumber \]

    In order to obtain the overall reaction, the reduction half-reaction was multiplied by two to equalize the electrons. The hydrogen ion and hydroxide ions produced in each reaction combine to form water. The \(\ce{H_2SO_4}\) is not consumed in the reaction.

    Figure \(\PageIndex{1}\): Apparatus for the production of hydrogen and oxygen gases by the electrolysis of water.


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