High-performance supercapacitors based on S-doped polyaniline nanotubes decorated with Ni(OH)₂ nanosponge and onion-like carbons derived from used car tyres

Abstract

High performance supercapacitors are designed based on hierarchical Ni(OH)₂ nanosponges anchored on sulfur-enriched polyaniline (PANI) nanotubes and waste tyre-derived onion-like carbons (OLC) as counter electrode materials. The rational strategy of grafting mesoporous thin sheets of Ni(OH)₂ on PANI produces unique composite nanoarchitectures that shows superior potential in supercapacitors validated by higher electrochemical stability. The designed asymmetric supercapacitor configuration with OLC as anode exhibits excellent specific capacitance of 622 F g⁻¹ at higher current density of 2 A g⁻¹ with an exceptional capacity retention greater than 97% achieved upon 10,000 continuous charge-discharge cycles. The asymmetric device delivers the remarkable energy and power density of 70 Wh kg⁻¹ and 136 kW kg⁻¹, respectively, whereas the symmetric device delivers the maximum energy density of 23 Wh kg⁻¹ and power density of 292 kW kg⁻¹. Further, it is demonstrated that Ni(OH)₂@PANI composite-based all solid-state flexible asymmetric supercapacitor construction with OLC exhibits high specific capacitance value of 166 F g⁻¹ at a higher current density of 5 A g⁻¹. The prolonged cycle stability may be attributed to the synergistic effect of 3D-nanosponge-like Ni(OH)₂ on PANI nanotubes surface, stabilizing the volume changes upon cycling. The OLC derived from the pyrolysis of waste-tyres offers high energy density and better rate capability.

Introduction

There is growing interest in supercapacitors especially due to their intermediate properties between those of batteries and conventional electrical double-layer capacitors (EDLCs), which store charges at the electrode/electrolyte interface [1]. On the other hand, pseudocapacitors are a subclass of supercapacitors which store charge via Faradaic reactions near the electrode surface, deliver energy and power instantly and possess specific capacitance which exceeds that of carbon-based EDLCs by far [2,3]. They are important for a number of application areas, such as portable, flexible and wearable electronics, power saving units, regenerative braking systems and electrical vehicles (EVs) [[2], [3], [4], [5]]. Recent attempts have focused on increasing cycling stability by combining Faradaic and non-Faradaic processes, which would thus efficiently sustain cycling stability upon long-term charge-discharge processes [6,7]. In this regard, the integration of transition metal oxides and metal hydroxides with carbonaceous materials [8] or conducting polymers [9] has shown promising results. Materials such as Ni(OH)₂@Graphite foam [10], Ni(OH)₂@MWCNT [11], CuO@GC [12], rGONF/Ni(OH)₂ [13], NiCo₂O₄-PANI-G [14], Ni,Co–OH/rGO [15], MnO₂@rGO/PANI [16], and CNT-MnO₂-PANI [17] have shown potential as high-performance hybrid supercapacitor electrodes, delivering higher energy and power densities.

The development of durable and efficient hybrid pseudocapacitors has encountered a number of challenges. Volume expansion during cycling leads to undesirable deterioration of the electrodes and low durability [18,19]. Among the conducting polymers, polyaniline (PANI) shows fast redox rate, desirable electrical conductivity, low cost, eco-friendliness and ease of processing into different nanostructures [[20], [21], [22], [23], [24], [25], [26], [27]], but also suffers from large volume changes upon repeated charging, structural damage and poor durability. Promising metal hydroxides with high theoretical capacitance like Ni(OH)₂ (∼2082 F g⁻¹) suffer from low electrical conductivity (10⁻¹⁵ S cm⁻¹) [28,29], large volume expansion and poor electronic conductivity, which constraints their performance [7,30]. Incorporating carbon and carbon-related materials has been shown to promote structural stability and long cycling efficiency [13,[31], [32], [33], [34], [35], [36], [37], [38], [39]]. However, the fabrication of these electrodes still remains a challenge due to weak bonding between Ni(OH)₂ and graphene during direct growth of Ni(OH)₂@graphene, which results in delamination of electrode material upon repeated cycling [33]. Therefore, there is clearly a need to fabricate electrodes with an architecture that facilitates not only high reaction kinetics and conductivity but also mechanical robustness during repeated charge/discharge cycles.

In the present study we designed a rational combination of 1D and 3D nano-architectures consisting of sulfur-integrated PANI nanotubes which are decorated with sponge-like Ni(OH)₂. This nano-architectural design was chosen to increase the surface area and mechanical stability of Ni(OH)₂@PANI, whilst still enabling rapid electrochemical reaction kinetics through space confinement electron and ion transfer with short diffusion length. Moreover, the heteroatoms anchored on the PANI nanotubes surface simultaneously enhanced both, oxidation stability and conductivity.

Onion-like carbons (OLCs), a carbon structure of concentric graphene shells have emerged as high-performance supercapacitor electrode materials due to distinctive structural features like large accessible surface for electrolyte ion adsorption, high energy density and high rate capability [[40], [41], [42]]. OLCs are commonly made from nanodiamonds, which makes these expensive as they require more than 1700 °C for their production. In the present study, an alternative approach was used to prepare highly electrically-conductive OLCs from waste-tyres by pyrolysis followed by Hummer’s method and lower temperature (∼800 °C), increasing cost-effectiveness of OLCs. As anode material, the waste-tyre derived OLCs outperformed the commonly used graphitic nanostructures.