Preparation, characterization and oxygen reduction catalytic activities of nanocomposites of Co(ii)/montmorillonite containing polypyrrole, polyaniline or poly(ethylenedioxythiophene)

Abstract

Electronically conducting composites of cobalt(II)/conducting polymer/montmorillonite are synthesized through ion exchange of montmorillonite (MMT) followed by oxidative polymerization of the monomers such as aniline, pyrrole or ethylenedioxythiophene within the interlayer spaces of the clay particles using Co(III) as the oxidant. In these syntheses, bentonite clay is purified to obtain pure MMT containing Na⁺ interlayer cations. Then, cobalt(III) ions are exchanged for Na⁺ ions present within the interlayer spaces. Monomers (aniline, pyrrole or 3,4-ethylenedioxythiophene) are then added to the ion-exchanged MMT dispersions. Co(III) is then reduced to Co(II) while oxidatively polymerizing aniline, pyrrole or 3,4-ethylenedioxythiophene monomers to polyaniline (PANI), polypyrrole (PPY) and poly(3,4-ethylenedioxythiphene) (PEDOT) respectively, thus forming Co(II)/MMT/PANI, Co(II)/MMT/PPY and Co(II)/MMT/PEDOT composite materials. These composites are characterized using XRD, XRF, FTIR, XPS, SEM and TGA analyses. These analyses show that in Co(II)/MMT/PANI and Co(II)/MMT/PPY composites the respective polymer and Co(II) ions are intercalated within the interlayer spaces of MMT while Co(II)/MMT/PEDOT shows exfoliated MMT platelets in the polymer/Co(II) matrix. The materials are also characterized for their electrical conductivities through DC conductivity measurements and AC impedance analyses and Co(II)/MMT/PANI, Co(II)/MMT/PPY and Co(II)/MMT/PEDOT have conductivities of 0.38 S m⁻¹, 0.78 S m⁻¹ and 0.32 S m⁻¹ respectively. Electrochemical data depict that all three composites are good catalysts for the oxygen reduction half-reaction, and particularly Co(II)/MMT/PPY and Co(II)/MMT/PEDOT show good catalytic properties comparable to those of conventional and expensive C/Pt catalysts that are currently used in fuel cells. As such, very low-cost oxygen reduction catalysts for fuel cell applications can be realized with these clay/polymer/Co(II) systems.