α-MnO2 nanorod/boron nitride nanoplatelet composites for high-performance nanoscale dielectric pseudocapacitor applications

https://doi.org/10.1016/j.jiec.2019.06.009Get rights and content

Highlights

  • A new type of NDPC with high specific capacitance is introduced for the first time.

  • The NDPC is composed of dielectric BNNPs and α-MnO2 nanorod composites.

  • This nanocomposite can deliver ca. >72% of the theoretical capacitance of α-MnO2.

  • A new energy storage mechanism of intercalation/deintercalation of K+ is presented.

Abstract

We demonstrate the synthesis of a composite of α-MnO2 nanorods and dielectric boron nitride nanoplatelets(BNNPs) as an electrode material for application in nanoscale dielectric pseudocapacitors. The optimize nanocomposite delivers a significantly high specific capacitance (890 F/g at 0.41 A/g), which is >72% of the theoretical specific capacitance of α-MnO2 at a mass loading of ca. 6 wt%. Pure BNNPs exhibit negligible charge storage capacity with a specific capacitance of 1.30 F/g at 0.024 A/g. The BNNPs increase the amount of K+ insertion/extraction and the conductivity of α-MnO2 nanorods by lowering the charge transfer resistance at the electrode-electrolyte interface. This is due to the electrical polarization of dielectric BNNPs during charging and discharging, which increases the rate and amount of K+ insertion or extraction induce by electrostatic force. The nanocomposite shows good capacity retention (94.12% after 2000 cycles) with high energy and power density. This research opens up a new avenue for the development of new types of nanoscale dielectric pseudocapacitors with high capacitance by exploring other suitable metal-oxides and nanoscale dielectric material composites.

Introduction

Conventional rechargeable energy storage devices, including capacitors and batteries, are widely used in portable electronics, computing systems, and electronic vehicles [1], [2]. Hence, it is important to obtain high specific power and specific energy density to meet future energy demands. Capacitors are being considered as a potential energy storage device due to their low-cost, long-cycling life, good reversibility, and quick charging [3], [4], [5]. Capacitors are generally classified as conventional (electrostatic or dielectric) and electrochemical (e.g., electrochemical pseudocapacitors or double layer capacitors (EDLCs)) [6], [7]. Electrochemical capacitors with high specific capacitance (i.e., supercapacitors) have been widely investigated and applied in various technologies. They exhibit intermediate energy density between that of a conventional dielectric capacitor and batteries [8], [9]. Dielectric capacitors are made from two conductive electrodes (usually bulk metal) separated by a solid dielectric material and exhibit low energy storage capacity [10]. This is due to the limited surface charges at the conductive electrodes.

Recently, nanoscale dielectric capacitors (NDC) have attracted significant interest due to the enhanced surface charges and charge storage capacity in conductive electrodes [6], [10], [11], [12], [13], [14], [15]. This is due to the high surface area of nanomaterials. Nanoscale conductive electrodes in an ideal NDC can deliver and store charge efficiently. Dielectric layers between nanoscale conductive electrodes enhance the capacitance without affecting the dimensions of structures [4], [12]. Metallic carbon nanotubes, graphene, and metallic nanowires can behave as ideal charge-holding plates when they are placed on either side of dielectric materials [10], [11], [13], [16]. For example, an NDC based on CNT/anodic aluminum oxide (AAO)/CNTs can deliver energy density of 2 Wh/kg [11].

To date, only metallic nanostructures have been explored to develop NDCs. EDLCs have widely been developed using carbon-based conducting nanostructures (e.g., activated carbon, functionalized CNTs, and graphene), carbon derived from a metal–organic framework, and transition metal carbides and nitrides [17], [18], [19]. Transition metal oxides have also been widely used for the development of pseudocapacitors (e.g., TiO2, MnO2, and RuO2) [20]. Nanomaterials for both EDLCs and pseudocapacitors can deliver high specific capacitances. For example, in EDLCs, carbon materials under optimal conditions can deliver a capacitance of ca. 150 F/g, and that of graphene is in the range of 100–250 F/g [2], [21], [22]. In contrast, in pseudocapacitors, ultrathin MnO2 and RuO2 can deliver maximum specific capacitances of >1000 and 720 F/g, respectively [23], [24], [25]. These capacitance values are far below the theoretical capacitances of the corresponding materials. This is mainly due to the low conductivity of electrode materials and kinetic limitation of the intercalation/de-intercalation of ions. The current strategy is focused on the incorporation of nanoscale dielectric hexagonal boron nitride nanoplatelets (BNNPs) into pseudocapacitive α-MnO2 to overcome the kinetic limitation of ion intercalation/deintercalation. The electrical conductivity is also enhanced to increase the charge storage capacity in nanoscale dielectric pseudocapacitors (NDPCs).

MnO2 has different crystallographic phases (e.g., α-, β-, δ-, γ-, and ε-MnO2) due to the different ways of interlinking of MnO6 octahedra. This allows the formation of tunnel structures with varying sizes [7]. These tunnels enable the insertion and extraction of ions from the electrolyte within them. Subsequently, the Mn4+ ions are electrochemically reduced and oxidized, and the charges are balanced [26]. The net amount of ions that can be inserted and extracted in MnO2 is significantly dependent on the tunnel sizes and the rate of the redox reaction of Mn4+ to Mn3+ [7], [26]. Among the different phases of MnO2, α-MnO2 with 2 × 2 tunnels has one of the largest tunnel sizes [26], [27]. Thus, α-MnO2 can store more foreign cations than others with a reversible Faradaic reaction, which is why it has the highest specific capacitance among all known phases. For a complete one-electron transfer process between Mn3+ and Mn4+, a theoretical capacitance of 1233 F/g can be obtained over a potential window of 0.9 V [28]. However, the reported experimental capacitance values for MnO2 powder and thin film remain only 100 and 700 F/g [29]. This might be due to the low electron conduction and limited ion conduction in MnO2. The capacitance value is also strongly dependent on the quality of the MnO2 crystal and the charge percolation through the MnO2 and the current collector. Thus, it is crucial to control the spacing between the MnO2 at the nanoscale by incorporating dielectric materials to obtain high capacitance. Layer-by-layer, mixed, or 3D combinations of MnO2 and dielectric nanostructures might offer a number of options in constructing novel NDPCs with diverse functions. The introduction of dielectric into MnO2 nanocrystals is anticipated to increase the rate and amount of ion insertion/extraction induced by dielectric materials.

Two-dimensional hexagonal boron nitride (BN) consisting of sp2 bonded boron and nitrogen atoms has attracted intense interest due to its several advantages, including excellent mechanical strength, high thermal conductivity, and chemical and thermal stability [30], [31]. More importantly, BN exhibits high electric field strength (ca. 15 V for 11-nm-thick BN) with the dielectric constants of 2.49 for a 3-layer-thick BN sheet [32]. This indicates that BN is a reliable insulator than can be used to obtain higher capacitance with high cycling stability in NDPCs.

In this research, we prepared the composites of α-MnO2 nanorods and boron nitride nanoplatelets (α-MnO2 NRs/BNNPs) by a low-cost and scalable in-situ hydrothermal method at room temperature, as shown in Scheme 1. The mass loading of α-MnO2 NRs in the composites was varied from 2 to 6 wt% by controlling the amount of KMnO4 precursor. The composites were used for the development of NDPCs. The incorporation of BNNPs into the α-MnO2 NRs can significantly increase the capacitance, which is close to the theoretical value for MnO2.

Section snippets

Chemicals and reagents

All chemicals and reagents used in this research were of analytical grade and used without further purification. Boron nitride (BN) power (˜1 μm, 98%), potassium permanganate (KMnO4, 99%), H2SO4 (99.999%), and KCl were obtained from Sigma-Aldrich (St Louis, MO, USA). Ultrapure water (18 MΩ cm) was attained from a Millipore Milli-Q Biocell A10 water-purifying system (Merck, Darmstadt, Germany) and used throughout the experiments.

Instrumentation

The crystallographic phase of samples was analyzed by an X-ray

Results and discussion

X-ray diffraction (XRD) and Raman analyses were performed to investigate the crystallographic phase structure of BNNPs and the α-MnO2 NRs/BNNP composite with the α-MnO2 NRs of 6 wt% (Fig. 1). The XRD pattern of the BN powder, BNNPs, and α-MnO2 NRs/BNNP composites were measured in the 2θ range of 10–50° (Fig. 1a). All samples exhibited the characteristic diffraction peaks of BN corresponding to the (002), (100), and (101) planes of the hexagonal phase of BN (JCPDS card 34-0421) [33]. The

Conclusions

In summary, we have successfully synthesized α-MnO2 NRs/BNNP composites with varying mass loading percentage (wt%) of α-MnO2 NRs by a simple and low-cost hydrothermal method at room temperature. The α-MnO2 NRs/BNNP nanocomposite was used as an electrode material to develop a new type of capacitor (NDPCs). The specific capacitance of MnO2 was substantially enhanced due to the incorporation of BNNPs. The optimized electrodes showed the specific capacitance of 890 F/g at 0.41 A/g, which is >72% of

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

This work was funded by grant NRF-2018R1A2B3001246 of the National Research Foundation of Korea.

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