Ionic Conductivity of Fe(II)-, Co(II)- and Ni(II)-Based Metallo-Supramolecular Polymers with a One- or Three-Dimensional Structure

Novel materials with high ionic conductivity are essential for the next generation energy-related requirements such as fuel cells, solar cells and batteries. Apart from the high ionic conductivity the materials are also required to have an extremely stable and rigid structure for their use in diverse conditions. Metallo-supramolecular polymers are normally synthesized by a self-assembly cum complexation route involving metal ions and ditopic organic ligands. They show various electrochemical and photophysical properties, which primarily rely on the electronic interaction between the metal ions and the ligands. In this work we report Fe(II)-, Co(II)- and Ni(II)-based metallo-supramolecular polymers, polyFe, polyCo and polyNi having a linear polymer structure and 3d-polyFe, 3d-polyCo and 3d-polyNi with a 3-dimensional cross-linked network type structure. These polymers were synthesized by the complexation of metal ions with bis(terpyridyl)benzene and tris(terpyridine) respectively. These polymers showed high humidity-responsive ionic conductivity. The cross-linked polymers show a better ionic conduction property judged by a lower activation energy and impedance than their linear analogous. Introduction of cross-linking in any polymer would also bring mechanical strength to the polymer, which is required for various applications.

                In order to measure the ionic conductance of the polymer we have prepared a pellet of the polymer powder, which was kept inside a humidity cum temperature control chamber and Nyquist plots were used to determine the ionic conductance of the polymer. The ionic conductivity of the polyFe at room temperature and 98% RH was calculated to be 0.32 mS/cm, using the information of the pellet thickness and dimensions. Similarly, the ionic conductivities of polyCo, and polyNi were determined to be 0.05 and 0.02 mS/cm respectively. On the other hand the ionic conductivity was obtained to be 5.7, 4.8 and 1.4 mS/cm for 3d-polyFe, 3d-polyCo and 3d-polyNi, respectively. These values are much larger than their corresponding linear polymers.

A marked difference in different polymers was found that in the 3-dimensional polymers the log σ vs. %RH plot appears to be arc shape than being a straight line one as seen in the linear polymers. Further the activation energies in 3-dimensional polymers were also found to be lower than the corresponding linear ones. This indicates the presence of a large amount of charge carrying species in the 3-dimensional polymers.

In conclusion, the superior humidity dependent ionic conduction in 3-dimensional polymers than corresponding linear polymers was observed suggesting a greater number of counter anions in the polymer structure. The 3-dimensional metallo-supramolecular polymers having higher mechanical strength and better ion transport properties are natural choice for device development and we plan to do more studies with these polymers in future.

 

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