https://doi.org/10.15255/KUI.2015.009
Published: Kem. Ind. 65 (3-4) (2016) 127−136
Paper reference number: KUI-09/2015
Paper type: Original scientific paper
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Characterisation of Pseudocapacitive Properties of Chemically Prepared MnO2 and MnO2/Polypyrrole Composite

N. Šešelj, D. Sačer and M. Kraljić Roković

Abstract

This article describes the synthesis of MnO₂ and MnO₂/polypyrrole composites. Composite1 was prepared by chemical reaction of pyrrole and potassium permanganate resulting in a product composed of polypyrrole and MnO₂. Composite2 was prepared by pyrrole polymerisation in the presence of MnO₂, using FeCl₃ as oxidant. In order to carry out the electrochemical experiments the samples were: 1) directly precipitated during the synthesis on Pt or carbon cloth supports, and 2) synthesized and subsequently applied on Pt support with the addition of binder and activated carbon, resulting in MnO₂ electrode, Composite1 electrode and Composite2 electrode. It has been shown that it is possible to precipitate the sample directly at Pt or carbon cloth support during the chemical synthesis, thus providing a fast and easy procedure of material characterisation. The pseudocapacitive properties of the samples were determined in NaCl solution (concentration 0.5 mol dm⁻³) using the cyclic voltammetry method. Good pseudocapacitive properties were obtained within the potential window from 0 V to 0.6 V vs. saturated calomel electrode for all tested electrodes. The characteristic of good capacitive response is constant current in a wide potential range. However, during polarisation at potentials more negative than 0.0 V, structural changes and the loss of pseudocapacitive properties occur for MnO₂ and Composite1 electrodes in contrast to Composite2, which was stable throughout the potential window. Structural changes and the loss of pseudocapacitive properties are evident from irreversible current peaks in the cyclic voltammogram. The MnO₂ sample was additionally tested by means of electrochemical impedance spectroscopy and it was found that for the electrode polarized at potential more negative than 0.0 V the resistance increased and the pseudocapacitive Mn⁴⁺/Mn³⁺ redox reaction rate slowed down. This phenomenon was explained by water intercalation within the material during polarisation at potentials < 0.0 V. In the case of Composite2 electrode, it seems that polypyrrole provided stability of MnO₂ within the wide potential window, which resulted in good capacitive response. Furthermore, from the morphological properties of the samples, it was established that Composite1 contained two separate phases, which was ascribed to the independent growth of MnO₂ and polypyrrole, while Composite2 was homogeneous and the MnO₂ particles were uniformly covered by polypyrrole layer that enabled stability of Composite2 electrode. The MnO₂ electrode with the sample subsequently applied on the support showed different behaviour compared to MnO₂ electrode with the sample directly precipitated on the support. This was the consequence of higher resistance caused by higher thickness of the sample subsequently applied on the support. Due to the higher resistance, typical reversible capacitive behaviour was not registered. However, better response was obtained for Composite2 electrode with the sample subsequently applied on the support because of the presence of polypyrrole that improved conductivity of the material. For the same reason, the highest value of specific capacitance has been registered for the Composite2 electrode (23 F g⁻¹ − 31 F g⁻¹).

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Keywords

cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, supercapacitor