Metallo-Supramolecular Polymers As Humidity Responsive Ionic Conductors

Materials with high ionic conductivity in the solid-state are important for energy-related applications such as fuel cells, solar cells and batteries. Here we report the synthesis of novel metallo-supramolecular polymers and their ionic conductivity response in the solid-state. The metallo-supramolecular polymers are composed of Ni(II) ions and bis(phenanthroline) ligands (Figure). The ionic conductivity of the polymer films was found to be heavily dependent on the ligand structure and percentage relative humidity (%RH). The elimination of a bulky hydrophobic spacer from the ligand structure considerably enhanced the ionic conductivity of the polymer film. Besides this, a four orders of magnitude increase in the ionic conductivity of the polymer film was observed with increasing humidity from 25 to 98 %RH.

Bis(1,10-phenanthroline) ligands with/without a bulky fluorene spacer were prepared and used for the metallo-supramolecular polymer synthesis. The 1:1 complexation with Ni(II) ions was confirmed using the UV-vis spectrophotometric titration experiments. This yielded the polymers containing a bulky spacer group (hydrophobic polymer) and without the spacer (hydrophilic polymer).

As for the investigation of the ionic conductivity, all the experiments were carried out in a closed humidity chamber. After keeping the electrode in the test humidity chamber, the temperature of the chamber was kept higher than the dew point of the chamber in order to prevent the formation of water droplets on the film surface. In order to measure the ionic conductivity of the polymer films, the Nyquist plots of the polymer films were obtained by the ac impedance measurement at 98 %RH and 25 ^oC. The conductivities of the hydrophobic and hydrophilic polymer films were calculated to be 0.75 x 10^-3 and 1.44 x 10^-6 Scm^-1 respectively. In addition we also studied the variation of the conductivity with relative humidity to get the insight of the conduction mechanism. A slope (dlogs/d%RH) value of 0.046 was obtained for the hydrophilic polymer film. A range of slope around 0.07±0.03 has been described in the literature for ion conducting organic polymers. The conductivity study of the polymers at various temperatures yielded the activation energy to be 0.43 eV, which indicates the likelihood of proton conduction by the Grotthuss mechanism in the polymer film. Additionally, the I-Vcurves of the polymer films, measured at several relative humidity values predicted the oxidation state of Ni ions in the film. With enhancing the humidity, the redox current peaks appeared at ±1.8 V. The magnitude of the peak increases with elevating humidity. The current peaks show the redox change of Ni ions between Ni(II) and Ni(I).

In conclusion, the decrease of the hydrophobicity in the ligand structure enhanced the ionic conductivity of the polymer film: the polymer without a hydrophobic spacer showed 500 times as high ionic conductivity as the polymers with the spacer. Since the hydrophobicity of the polymer is tuned by the ligand modification, they can be used to examine the relationship between the hydrophobicity and the ionic conductivity systematically.

Figure. The structures of metallo-supramolecular polymers: (a) the hydrophobic and (b) hydrophilic polymer.

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