CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO₂ for high performance asymmetric supercapacitors

Highlights

• Mesoporous MnO₂ with a specific surface area of 523 m² g⁻¹ is prepared.

• PPy nanofilm is CVD-deposited on the surface of mesoporous MnO₂.

• The as-prepared composite maintains the micrographs and pore/channel structure.

• The sample shows a high specific capacitance of 320 F g⁻¹ and a long cyclic stability.

• An assembled ASC exhibits a high energy density of 38.6 Wh kg⁻¹ at 0.5 A g⁻¹.

Abstract

High-porosity MnO₂ with a mesoporous structure is synthesized through a facile redox reaction, and the polypyrrole (PPy) nanofilms are grown on the synthesized mesoporous MnO₂ by chemical vapor deposition to form a 3D nanocomposite structure. The as-prepared high-porosity MnO₂/PPy with a nanocomposite structure presents a high specific capacitance of 320 F g⁻¹ and a 91.4% capacitance retention after 5000 charge–discharge cycles. MnO₂/PPy and N-doped active carbon are employed as a positive electrode and a negative electrode, respectively, to assemble an asymmetric supercapacitor (ASC), which exhibits a high energy density of 38.6 Wh kg⁻¹ at 0.5 A g⁻¹ and long-term cyclic stability (90.6% capacity retention after 5000 cycles). Such high-performance capacitive behavior is attributed to the high porosity, mesoporous structure, and improved electrical conductivity of the fabricated ASC, which is promising for various supercapacitor applications.

Introduction

Pseudocapacitors, with an energy storage mechanism based on reversible surface Faradaic reactions, have been widely demonstrated to provide much higher energy densities than double-layer energy-storage devices [1], [2], [3], [4]. As promising alternative electrode materials of RuO₂, transition metal oxides have been widely studied because of their good capacitive behaviors, low toxicity, and low cost [5], [6], [7], [8]. Among these transition metal oxides, manganese oxide (MnO₂) is widely considered an attractive material for next-generation supercapacitors because of its high specific capacitance, which is approximately 1110 F g⁻¹ based on the calculation of Faradaic law [9], and well-defined electrochemical redox activity [10], [11].

In recent years, the design and fabrication of nanomaterials with mesoporous structures have captured growing interest in their practical application in electrochemical supercapacitors. Apparently, the high surface area and fast electrical pathways of these nanomaterials are beneficial to faradic redox reactions [12], [13], [14]. Many forms have been suggested for the development of nanostructured manganese oxides with high surface area, such as nanorod arrays [15], nanoflowers [16], two-dimensional ordered nanopore arrays [17], nanosheets [18], and well-ordered whisker-like arrays [19].

Although manganese oxides are considered the most attractive electrode material, a key challenge in their practical application to energy storage systems is their low electrical conductivity (10⁻³–10⁻² S cm⁻¹) [20], which limits fast electron transport and deteriorates rate capabilities for high power energy storage devices. To enhance the electroconductibility of MnO_x-based electrode materials, many effective approaches have been proposed using conductive grapheme [21], [22], carbon spheres [23] and carbon nanotubes [24] as supporting materials. However, the high cost possibly restricts their practical applications. Manganese oxides modified by conductive polymers, such as polyaniline [25], polypyrrole (PPy) [26], and poly(3,4-ethylenedioxythiophene) [27], have been widely investigated. Conductive polymers offer high electrical conductivity and possess pseudocapacitance properties themselves [28], thereby significantly improving the capacitance of manganese oxide-based materials.

Various methods have been employed to fabricate PPy/manganese oxide composites, such as the redox process including pyrrole monomer and KMnO₄ in an acid solution [29], MnO₂ coating by PPy derived from the reaction between pyrrole monomer and ammonium persulfate [30], and electrodeposition from MnSO₄ and pyrrole monomer systems [31]. However, MnO₂ molecules are prone to react with pyrrole monomers; as a result, the nanostructured morphology of MnO₂ is destroyed. In this work, we report a novel and facile synthesis route for PPy chemical vapor deposition (CVD)-grown on manganese oxide with a high porosity and mesoporous structure. The synthesized MnO₂/PPy is used as a supercapacitor electrode material. High-porosity manganese oxide is first synthesized through a redox reaction between KMnO₄ and maleic acid/hexadecyltrimethyl-ammonium bromide, followed by the CVD method to uniformly deposit PPy nanofilms on mesoporous manganese oxides to form unique hierarchical mesoporous architectures.

In a supercapacitor, the relationship of the stored energy (E) and the potential window (V) can be described as: E = 1/2CV², and many previous references [32], [33], [34], [35] have confirmed that the potential window of asymmetric supercapacitors (ASCs) in an aqueous solution can be extended beyond the thermodynamic limit of water. ASCs usually employ metal oxides/hydroxides as the positive electrode and carbon-based materials as the negative electrode [36], [37], [38]. A novel N-doped mesoporous carbon derived from PPy with an excellent electroconductibility and a high specific surface is an ideal negative material [39], [40]. In the present work, the similar route employed in a previous study [39] is employed to prepare N-doped mesoporous carbon, which is used as the negative electrode of an ASC to evaluate the electrochemical properties of PPy-modified mesoporous manganese oxide (PMMO).