The Palladium-Hydrogen electrode (Abbreviation: Pd/H₂) is one of the common reference electrodes used in electrochemical study. 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.

Two phases can coexist in palladium when hydrogen is absorbed:

• alpha-phase at hydrogen concentration <0.3 atom percent
• beta-phase at hydrogen concentration corresponds to the non-stoichiometric formula PdH_0.6
  

The electrochemical behaviour of a palladium electrode in equilibrium with H₃O⁺ ions in solution parallels the behaviour of palladium with molecular hydrogen
1/2H₂ = H_ads = H_abs
Thus the equlibrium is controlled in one case by the partial pressure of fugacity of molecular hydrogen and in other case - by activity of H⁺-ions in solution.

When palladium is electrochemically charged by hydrogen, the existence of two phases is manifested by a constant potential of aprox. +50 mV vs 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 a absence of non-stop bubbling of molecular hydrogen through the solution as it is absolutely necessary for the standard hydrogen electrode.

Partially cited from:

• E.Gileadi, E.Kirowa-Eisner, J.Penciner Experiment in interfacial electrochemistry. Institute of Chemistry, Tel-Aviv Univ., part 2, pp. 247-249