Palladium-hydrogen electrode

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 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

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

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.

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

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.

• Dynamic hydrogen electrode
• Reversible hydrogen electrode

Electrochimica Acta