Dihydrogen is prepared by the electrolysis of water using platinum electrodes in the presence of a small amount of acid or alkali. $\mathrm{2H_2O}_{(l)}$ $\xrightarrow[\text{acid/base}]{\text{Electrolysis}}$ $\mathrm{2H_{2(g)}+O_{2(g)}}$ During electrolysis, the reactions that take place are : $\mathrm{H_2O}$ ⇌ $\mathrm{H}^+ + \mathrm{OH}^-$ At cathode : $\mathrm{H}^+ + \mathrm{e}^-$ → H H + H → $\mathrm{H_{2(g)}}$ ↑ At anode : $\mathrm{4OH}^-$ → $\mathrm{4OH} + 4\mathrm{e}^-$ 4OH → $\mathrm{2H_2O}_{(l)} + \mathrm{O_{2(g)}}$ The hydrogen prepared by this method is of very high purity.