Effect of α-Fe2O3 nanoparticles on the mechanism of charge storage in polypyrrole-based hydrogel

Abstract

A hybrid hydrogel based on polypyrrole and hydrothermally prepared α-Fe2O3 nanoparticles was synthesized via in situ chemical polymerization of pyrrole using sodium trimetaphosphate as a crosslinker. Wide-angle X-ray diffraction study confirmed the presence of α-Fe2O3 in the prepared material. Mapping of the elemental composition using energy dispersive X-ray spectroscopy showed the uniform distribution of the inorganic particles inside the polypyrrole matrix. The effect of α-Fe2O3 on the structure of the hybrid hydrogel and on the mechanism of charge storage was studied with scanning electron microscopy, cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy. The specific capacitance was found to increase from 250 F g−1 for the polypyrrole hydrogel up to 509 F g−1 for the α-Fe2O3-doped hydrogel at the current density of 0.2 A g−1. The hematite incorporation also affected the morphology of the hydrogel leading to a slight increase in the double-layer capacitance accompanied with a strong increase in the pseudocapacitance: from 239 F g−1 up to 486 F g−1. The initial polypyrrole hydrogel and the hybrid hydrogel demonstrated a capacitance retention of about 75% and 79% after 3000 charge–discharge cycles at the current density of 4 A g−1, respectively.