Very pure hydrogen (99.9%) can be made by which of the following processes?

Electrolysis of water yields incredibly clean hydrogen.

Through the use of electric current, water is converted into O₂ and H₂ gas.

Reaction to the anode,

$6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶{\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}_3{\mathrm{O}}^{+}+4{\mathrm{e}}^{-}$

Reaction on the cathode,

 $4{\mathrm{e}}^{-}+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{H}}_2\left(\mathrm{g}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

The overall reaction is,

$2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\mathrm{\%}$