Humidity-Dependent DC Conductivity of Polyelectrolyte Multilayers: Protons or Other Small Ions as Charge Carriers?

A range of different combinations of polyelectrolytes is employed to form multilayers by layer-by-layer assembly, which are investigated by impedance spectroscopy. In particular, the alkali counterion employed in the layer formation is varied. Impedance spectra of different multilayer systems are qualitatively similar, and they are changing in a similar way with relative humidity (RH). From the spectra, the dc conductivity σ dc and its dependence on humidity are extracted. The humidity dependence of σ dc follows a general law of log(σ dc ) = a RH + b, which is valid for all systems. Absolute values of σ dc and slopes a depend on the type of polyelectrolytes employed but are independent of the type of alkali counterion involved. On the basis of these data, we discuss the contribution of different small ionic species, i.e., anions, alkali cations, or protons, to the conductivity and conclude that the differences between different polymer systems as well as the humidity dependence are consistent with the conduction of protons or hydronium ions, while the contribution of other cations or anions to σ dc is negligible.