Abstract
Flexible energy storage devices and supercapacitors in particular have become very attractive due to the growing demand for wearable consumer devices. To obtain supercapacitors with improved performance, it is useful to resort to hybrid electrodes, usually nanocomposites, that combine the excellent charge transport properties and high surface area of nanostructured carbon with the electrochemical activity of suitable metal oxides or conjugated polymers. In this work, electrochemically active conducting inks are developed starting from commercially available polypyrrole and graphene nanoplatelets blended with dodecylbenzenesulfonic acid. Films prepared by applying the developed inks are characterized by means of Raman measurements, Fourier Transform Infrared (FTIR) analysis, and Atomic Force Microscopy (AFM) investigations. Planar supercapacitor prototypes with an active area below ten mm2 are then prepared by applying the inks onto transparency sheets, separated by an ion-permeable nafion layer impregnated with lithium hexafluorophospate, and characterized by means of electrical measurements. According to the experimental results, the devices show both pseudocapacitive and electric double layer behavior, resulting in areal capacitance that, when obtained from about 100 mF⋅cm−2 in the sample with polypyrrole-based electrodes, increases by a factor of about 3 when using electrodes deposited from inks containing polypyrrole and graphene nanoplateles.
Highlights
Over the past few decades, there has been an increasing research interest in the field of energy storage systems and, among them, a great deal of attention has been focused on supercapacitors [1,2,3,4]: devices that combine a high power density and a fast charging rate with a simple design that enables them to be flexible, stretchable, and bendable [5,6,7], qualities that make them suited for wearable electronic applications [8,9,10]
Following one of the most popular choices in the scientific literature, the use of graphene and polypyrrole, in this paper, we show that electrochemically active films based on either polypyrrole, or graphene, or a mixture of the two can be applied using refillable pens filled with water inks consisting of commercially available unsoluble polypyrrole powder, graphene platelets, and dodecylbenzenesulfonic acid
Due to its ability to undergo electrically driven reversible transitions between its oxidized and reduced forms, polypyrrole is by far the most popular conjugated polymer that can be used as a component in electrodes for electrochemical energy storage devices [22,23,24]
Summary
Supercapacitors, which consist of a couple of electrodes impregnated with an electrolyte and spaced by an ion-permeable separator, have performances, electric behavior, and even working principles that change depending on the nature of the electrodes, of the medium interposed between the electrodes, and of the electrolyte. The choice of the electrode is a key element, as the main energy storage mechanism of any supercapacitor depends on the structural, morphological, charge-transport, and chemical properties of the electrode material it uses. When a couple of conducting electrodes impregnated by an electrolyte are bridged by a porous medium that allows ion diffusion, depending on the valence electron density of the states of the electrodes, mobile cations or anions accumulate at each of the electrode–electrolyte interfaces.
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