The palladium-hydrogen electrode (abbreviation: Pd/H2) is one of the common reference electrodes used in electrochemical study.[1] Most of its characteristics are similar to the standard hydrogen electrode (with platinum). But palladium has one significant feature—the capability to absorb (dissolve into itself) molecular hydrogen.[2][3]

## Electrode operation

Two phases can coexist in palladium when hydrogen is absorbed:

• alpha-phase at hydrogen concentration less than 0.025 atoms per atom of palladium
• beta-phase at hydrogen concentration corresponding to the non-stoichiometric formula PdH0.6

The electrochemical behaviour of a palladium electrode in equilibrium with H3O+ ions in solution parallels the behaviour of palladium with molecular hydrogen

$\tfrac{1}{2} \mathrm{H}_2 = \mathrm{H}_{ads} = \mathrm{H}_{abs}$

Thus the equilibrium is controlled in one case by the partial pressure or fugacity of molecular hydrogen and in other case—by activity of H+-ions in solution.

$E=E^0 + {RT \over F}\ln {a_{\mathrm{H}^+} \over (p_{\mathrm{H}2})^{1/2}}$

When palladium is electrochemically charged by hydrogen, the existence of two phases is manifested by a constant potential of approximately +50 mV compared to the reversible hydrogen electrode. This potential is independent of the amount of hydrogen absorbed over a wide range. This property has been utilized in the construction of a palladium/hydrogen reference electrode. The main feature of such electrode is an absence of non-stop bubbling of molecular hydrogen through the solution as it is absolutely necessary for the standard hydrogen electrode.