Hierarchical carbon coated vertically aligned α-MoO₃ nanoblades anode materials for supercapacitor application

Highlights

• Vertically aligned α-MoO₃ nanoblades grown over FTO via hydrothermal synthesis.

• Various reducing atmospheres (N₂, H₂ and Vacuum) treated with α-MoO₃ nanoblades to manipulate electrical conductivity.

• Binder-free carbon coated vertically aligned α-MoO₃ nanoblades electrodes revealed good electrochemical performance.

• Carbon coated α-MoO₃ nanoblades proved as appreciable negative electrodes different from conventional electrodes.

Abstract

Molybdenum oxide (MoO₃) is an appropriate electrode material for vast applications such as gas sensing, catalyst, and energy storage devices. Its high oxidation states provide the possibility for ion intercalation and de-intercalation to the electrode material, which is truly advantageous for supercapacitor application. Herein, we report about reduced vertically aligned α-MoO₃ nanoblades on conducting substrates via a post-treatment in reduced gas environments (H₂, N₂ and vacuum) to tune their conductivity by introducing oxygen vacancies. These α-MoO₃ nanoblades were further carbonized through decomposition of glucose as a carbohydrate material to obtain binder-free carbon coated vertically aligned α-MoO₃ electrode. As a binder-free supercapacitor negative electrode, the vacuum treated α-MoO₃ electrode showed the highest specific capacitance (39.8 mF cm⁻²), as compared to that achieved by N₂ and H₂ treated samples (29.2 mF cm⁻² and 24.5 mF cm⁻², respectively). Besides, the vacuum annealed electrode also maintained around 76.4% of its initial specific capacitance value after 10,000 cycles indicating a more stable electrochemical performance of such electrode. An asymmetric device 3D-MnO₂//α-MoO₃ was assembled and it attained maximum specific capacitance value of 37.5 mF/cm² at current density value 1.5 mA cm^− 2 and maximum energy density value of 16.875 µWh cm^-2 at power density value of 675 µW cm^-2. The results demonstrate that the proposed hybrid synthesis approach is very promising for preparation of binder-free materials for high-performance supercapacitors.

Introduction

Recent development in the portable electronic appliances and hybrid electric automobiles led to a considerable attention to search for high performing, eco-friendly energy storage devices such as supercapacitors [1], [2]. Supercapacitors are a crossing bridge devices between conventional capacitors and rechargeable batteries owing to their high specific power (>500 W kg^–1), long-term cyclic stability (>10⁵ cycles) and stability in comparison to batteries [3], [4]. Electrical double layer capacitors (EDLCs; non-Faradaic) and pseudocapacitors (Faradaic) are the two prime charge storage mechanisms of supercapacitors. Carbon based materials are extensively used in EDLCs, which represent charge accumulation at the interface between the electrode and electrolyte. In contrast, in pseudocapacitors, Faradaic redox reactions happen at the electrode surface [5]. Combining fast and the reversible Faradaic process along with non-Faradaic electric double layer formation increases the storage capacity of the pseudocapacitors, as charging is not limited by bulk ion diffusion as in batteries [6].

Transition metal oxides (TMO) or hydroxides and conductive polymers are the widely dominated pseudocapacitive electrode materials. However, the poor electrical conductivities and slow Faradaic reaction kinetics hinder their performance at high current densities [7]. Research in the field of negative electrodes for supercapacitor application is primarily focused on carbon allotropes such as reduced graphene oxide, carbon onions, and graphene oxide etc. Therefore, designing a non-carbon type supercapacitor negative electrode is of vital importance now-a-days.

Molybdenum oxide (MoO₃) is considered as a suitable TMO for vast applications such as gas sensing [8], [9], catalyst [10], and energy storage devices e.g., pseudocapacitors [1]. MoO₃ has a tuneable chemical structure which determines its electronic [11] and optical properties [12]. It exists in different phases; the most common ones are the thermodynamically stable orthorhombic α-phase (space group pbmn), the metastable monoclinic β-phase (space group p₂₁/m), and the metastable hexagonal h-phase (space group pb₃/m) [13]. Among these, only α-MoO₃ as a broad band gap (≈3.2 eV) n-type material has a desired layer structure, which facilitates the synthesis of different 2D and 1D nanostructures offering large surface area. The high surface area improves intercalation process ability in electrochemical devices [14], [15]. The α-MoO₃ crystal consists of bi-layers, which are formed from linked distorted MoO₆ octahedral with a corner-share along a-axis and edge-sharing cross zigzag along c-axis. The lamellar formation occurs, when double bi-layers connect through Van der Waals bonds through b-axis. Notably, α-MoO₃ has various coordination numbers ranging from 0 to 8 and oxidation states (2 + to 6 +) that determine its conductivity [15], [16].

α-MoO₃ has a higher theoretical specific capacity (1111 mAh g⁻¹) than graphite (gravimetric capacity of 372 mAh g⁻¹) [17]. The main limitation in achieving this theoretical specific capacitance using α-MoO₃, is its low electrical conductivity. In addition, conventional α-MoO₃ nanostructures lead to a drop in device performance over time during charge-discharge cycles [18]. Different strategies can be employed to overcome the inferior device performance and instability. For instance, conductivity of MoO₃ can be tuned by controlling oxygen deficiency in the crystal, which in turn, also controls its stoichiometry. The stoichiometric α-MoO₃ structure is considered as an insulator, whereas the sub-stoichiometric structure is a semiconductor material, leading to enhanced electrochemical activity [1]. Moreover, morphological engineering such as interconnected arrays of metal oxides nanostructures grown on conductive substrates can also enhance charge/ion transport and electrochemical activity (thus a higher electrochemical performance) [4]. Furthermore, hybridization of α-MoO₃ with carbon material (the so-called carbonization) can enhance electrochemical performance by improving thermal and electrical conductivities and also can improve electrochemical stability [1], [2], [15]. Besides, heat treatment of α-MoO₃ under inert and H₂ atmosphere can tune their electronic properties and improve their electrical conductivity [19], [20], [18]. Based on this concept, in this work the α-MoO₃ nanoblades are post treated with various atmospheric annealing (H₂, N₂ and Vacuum) and their electrochemical performance are studied.

In this study, we developed vertically aligned α-MoO₃ nanoblades on a conducting substrate via hydrothermal reaction. These α-MoO₃ nanoblades were reduced in various reducing atmospheres to manipulate electrical conductivity and subsequently carbonized to improve electrochemical performance. A conventional supercapacitor design employing a nanostructure often involves a binder material for electrode preparation. Our study reports a binder-free hybrid electrode preparation approach, which is largely ignored in the literature. The hierarchal structure is achieved by decomposing a carbohydrate, e.g., glucose on the α-MoO₃ nanoblades. These binder-free hybrid electrodes, when employed as a supercapacitor electrode, showed a maximum specific capacitance of 39.8 mF cm⁻². The negative electrode also showed an impressive electrochemical stability of 76.4% for 10,000 charge discharge cycles. Further, the assembled asymmetric device 3D-MnO₂//α-MoO₃ delivered a maximum specific capacitance, energy density and power density of 37.5 mFcm^-2, 16.875 µWh cm^-2, and 675 µW cm^-2, respectively.