High purity $\left(>99.95\%\right)$ dihydrogen is obtained by

Dihydrogen of high purity $(>99.95\%)$ is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes. In electrolysis, the reaction occurs at the anode and cathode :-

At anode :- $2{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O} + \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2 + 2{\mathrm{e}}^{-}$

At cathode :- $2{\mathrm{H}}_2\mathrm{O} + 2{\mathrm{e}}^{-} \to 2{\mathrm{OH}}^{-} +{\mathrm{H}}_2$

Hence, the high purity dihydrogen is obtained by electrolysing warm aqueous barium hydroxide.