Mechanisms of Ion Conduction in Polyelectrolyte Multilayers and Complexes

Abstract

This paper reviews the progress made in understanding of the mechanisms of ion conduction in polyelectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). The basis are experimental conductivity data obtained by impedance spectroscopy as a function of relative humidity and temperature, respectively. Mechanically stable thin films of PEM have interesting perspectives as ion conductors, however, being prepared by self-assembly, their stoichiometry and content of ionic charge carriers is unknown. Therefore PEC act as a model material with a variable stoichiometry and known ion content.Employing poly(sodium 4-styrene sulfonate) (NaPSS) and poly(diallyldimethyl ammoniumchloride) (PDADMAC), we present conductivity spectra of dried polyelectrolyte complexes of type x NaPSS · (1 − x)PDADMAC as a function of temperature and composition, respectively. The dependence of the dc conductivity is discussed along with scaling properties of the spectra. The results show that the conductivity is always determined by the sodium ions, even in PEC with an excess of PDADMAC. The ion dynamics and transport mechanisms are, however, different in PDADMAC-rich than in NaPSS-rich PEC.PEM of different polyionic compounds are investigated in dependence on relative humidity. A general law of an exponential increase of the dc conductivity with relative humidity is found. Absolute values of the conductivity and the strength of the humidity dependence are different for different polyion materials, however, they do not depend on the type of small counterion employed in layer formation. Therefore, it is concluded that in hydrated PEM, protons are the dominant charge carriers.For both PEM and PEC we show that the MIGRATION concept developed by Funke and co-workers can be used for describing the experimental spectra over wide ranges in frequency. This implies that forward-backward hopping motions of small ions play a vital role in solid polyelectrolyte materials. Apart from these potentially successful hops, localized motions of charged particles are found to influence the conductivity spectra as well.

dcProton conductivity at low-frequency in Nafion conductivity spectrum probed by time-resolved SAXS measurements and impedance spectroscopy

“…While several authors agree that the high‐ f conductivity is mainly due to the proton migration in the aqueous solution via structural diffusion, few reports evidenced the presence of protonic charges residing in a distinct chemical environment, which exhibits a lower mobility than the free counterions . This slower charge displacements were attributed to the motion of protonic charges from site‐to‐site within the polymer structure via ion‐hopping . Thus, the low‐ f dc conduction can be a result of the long‐range displacements of protons via ion‐hopping along a percolation pathway in the polymer phase upon the applied electric field.…”

The solvation and ion condensation properties for sulfonated polyelectrolytes in different solvents—a computational study

“…The technological benefit for an improvement of proton as well as lithium ionic conductivities as typical dissociated counterions due to favorable solvent-solute interactions and possible increased dissociation rates is of great importance. Recent publications have reported on the importance of the humidity for ionic conductivities [31,32], as well as the interesting behavior of ions in polyelectrolyte multilayers and polyelectrolyte complexes [33]. These results clearly reveal that a full understanding can be only achieved by novel theoretical insights into the underlying mechanism.…”

Electrochemical nanoarchitectonics and layer-by-layer assembly: From basics to future

“…They also used (LiCF 3 SO 3 ) as anion with delocalized charge. The group of Schönhoff performed systematic investigations of ionic conduction in PSS/PolyDADMAC multilayers in the dry state and at various levels of hydration observing that ionic conductivity increased exponentially with the degree of hydration [61]. In hydrated states, the conductivity was dominated by protons.…”

“…In hydrated states, the conductivity was dominated by protons. In the dry state, when using X + Cl − salts where X is an alkali metal cation, the conductivity was dominated by these cations even with an excess of polycation [61]. This behavior was explained as being due to a micro-phase separation in the film into neutral and chargerich regions with no communication between the charged areas.…”