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

Abstract

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 σdc and its dependence on humidity are extracted. The humidity dependence of σdc follows a general law of log(σdc) = aRH + b, which is valid for all systems. Absolute values of σdc 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 σdc is negligible.

Transport Mechanisms of Ions in Graft-Copolymer Based Salt-in-Polymer Electrolytes

“…The correlation times characterizing local ion dynamics reflect the complexation of the cations by the oligoether side chains of the polymer. 7 Li and 19 F diffusion coefficients and their activation energies are rather similar, suggesting the formation of ion pairs and clusters with similar activation barriers for cation and/or anion long-range transport. Activation energies of local reorientations are generally significantly smaller than activation energies of long range diffusion.…”

“…Selective RF inserts optimized for the 1 H, 19 F and 7 Li Larmor frequencies, respectively, were employed. The temperature was calibrated by a GMH 3710 controller with a Pt100 thermocouple (Greisinger electronics, Germany) and is controlled with a precision of 0.25 K. Samples for NMR measurements were placed in 5 mm glass NMR tubes (Wilmad), which were flushed with nitrogen and finally flame-sealed.…”

“…This material was also the first polyphosphazene showing ionic conduction if a lithium salt is dissolved [16]. In the meantime polyphosphazenes have covered more application areas, for example as constituents in ion conducting self-assembled polymeric films [18,19].…”

“…Key questions in salt-in-polymer materials concern the relative contributions of cation and anion transport to the effective total conductivity, the extent of correlation between cation and anion transport as well as between ion transport and segmental motion of the polymer and their corresponding temperature dependences [7]. NMR techniques represent an excellent approach to study these questions, since each of the system's constituents, namely cations, anions and polymer, can be studied in an element-selective way utilizing the 7 Li, 19 F and 1 H isotopes. In the present contribution, we describe the local mobilities of all the species on the basis of temperature dependent spinlattice relaxation times T 1 .…”

“…Polymer architectures are based on polysiloxane or polyphosphazene backbones with one or two side chains per monomer, respectively. NMR methods provide information about molecular dynamics on different length scales: The mechanisms governing local dynamics and long range mass transport are studied on the basis of temperature dependent spin-lattice relaxation rates and pulsed field gradient diffusion measurements for 7 Li, 19 F and 1 H, respectively. The correlation times characterizing local ion dynamics reflect the complexation of the cations by the oligoether side chains of the polymer.…”