Synergistic effect of chitin nanofibers and polyacrylamide on electrochemical performance of their ternary composite with polypyrrole

Abstract

Development of simple methods for preparation of polymeric electrode materials with nanofibrous network structure is a perspective way toward cheap supercapacitors with high specific capacitance and energy density. In this work one-pot synthesis of electroactive ternary composite based on polypyrrole, polyacrylamide and chitin nanofibers with beneficial morphology was elaborated. Ternary system demonstrates better electrochemical performance in comparison with both polypyrrole–polyacrylamide and polypyrrole–chitin binary composites. Possible mechanism of synergistic effect of simultaneous influence of polyacrylamide and chitin nanofibers on the formation of composite's structure is discussed. The highest attained specific capacitance of electroactive polypyrrole in ternary composite reached 249 F/g at 0.5 A/g and 150 F/g at 32 A/g. Symmetrical supercapacitor was assembled using the elaborated electrode material. High specific capacitance 89 F/g and good cycling stability with capacitance retention of 90% after 3000 cycles at 2 A/g were measured.

Introduction

Growing number of various portable electronic gadgets increase the interest of researches to the developing of new generations of power sources. Among energy storage devices the supercapacitors based on nanostructured materials [1] attract considerable attention due to their high power density and cycling stability [2], [3], [4]. The conducting polymers such as polyaniline and polypyrrole (PPy) are widely investigated as components of electrodes in supercapacitors because they are cheap and demonstrate high electrical conductivity, ability to undergo ox–red reactions and sustainability [5], [6], [7], [8], [9], [10]. At the same time, low compatibility with water electrolyte and agglomerated morphology of conducting polymers limit their electrochemical performance. There are two main opportunities to increase accessibility of electrode's volume for electrolyte. The first is preparation of material which has an interconnected system of pores [11], [12] either by coating of suitable porous template with conducting polymer [13] or by the formation of the porous structure during random packing of 1D nanoparticles (nanofibers or nanotubes) [14]. The second is the synthesis of electroconducting polymer in the form of hydrogel [15], [16], [17]. These approaches allow to increase the mobility of ions inside the electrode and, to accelerate electrochemical charge–discharge processes and, consequently, to raise the power density and capacitance of supercapacitor. One of the most sustainable and promising porous substrate for preparation of composite electrode for supercapacitors is a bacterial cellulose membranes [18]. This material was also used for preparation of ternary composite electrodes with PPy and inorganic red-ox active materials such as CuO [19], CuS [20] or CoS [21] with improved electrochemical performance.

Formation of porous structure at drying of dispersion of polymeric nanofibers is the simplest and easy scalable way to attain the desired morphology of the electrode. Polymerization of pyrrole in the presence of hard inorganic (metal oxides [22], [23], carbon nanotubes [24], [25], [26]) or soft organic (hexadecyltrimethylammonium bromide [27], methyl orange [28]) templates is used for this purpose. In order to increase sustainability of supercapacitors the attention to the natural and renewable 1D templates such as cellulose nanofibers [29], [30], [31] significantly increased. This was the reason for using in the present work of chitin nanofibers (CN) as template for polymerization of pyrrole. CN, as a perspective renewable, natural and biodegradable material, is already used in cosmetic and biomedical applications [32], [33], but was never studied in supercapacitors.

At the same time, addition of water soluble polymers to the media, where the synthesis of PPy proceeds, is considered as a useful method to improve the compatibility of conducting polymer with water and to increase the specific surface area of the material. The systems based on PPy and polyacrylic acid [34], chitosan [35], polyacrylamide [36] and some others [37] were investigated in literature. However, profitable influence of this approach on the electrochemical performance of PPy in supercapacitor application was demonstrated only in Ref. [38] for phosphorylated chitosan. In this work we use polyacrylamide (PAAm) because it is cheap and effective in stabilization of the PPy particles in water. Additionally, materials on the base of acrylate monomers [39], [40] are often used for preparation of hydrogel electrolytes due to their high permeability for electrolytes. At the same time, the addition of non-electroactive component can reduce the mass normalized storage ability of the material. So the influence of PAAm content on the electrochemical properties needs to be investigated.

Thus the aim of this work was to join valuable impacts of the 1D template and water soluble polymer on the electrochemical performance and morphology of polypyrrole in a ternary polypyrrole-polyacrylamide-chitin nanofibers composite (PPy–PAAm–CN). This innovative composite, applied in supercapacitors, has been investigated to know the influence of its composition on the morphology and electrochemical properties of the system.