VN thin films as electrode materials for electrochemical capacitors
Introduction
New advances in electronic technologies come together with the need of high energy and power storage devices that can be manufactured in a compact or miniaturized way without sacrifying their performance. Electrochemical capacitors (ECs) fulfill the above requirements. Although they have a limited energy density compared to batteries, they possess a better reversibility, extended cycle life and faster energy delivery [1], [2]. Moreover, they can store more energy than conventional electrolytic capacitors. Recently ECs have also been designed as microdevices [3]. The main applications for ECs are in hybrid or electric vehicles, portable electronic equipments and memory back-up devices among others [1], [2], [3], [4], [5], [6]. There are two types of ECs according to their charge storage mechanism; (1) electrical double layer capacitors (EDLC) that store charge at the electrode-electrolyte interface by electrostatic forces and (2) pseudocapacitors which involve fast and reversible redox reactions occurring at the electrode surface. Due to the redox processes involved in pseudocapacitors, they can store more energy than EDLC, but usually at the expense of power density and cycle life [1], [2], [5], [6].
The major aim in the field of electrochemical capacitors (ECs) is to improve the energy density of the devices with only negligible impact on the power density and on the cycling ability of the device. Carbon materials that are used as electrodes in EDLC are restricted by their capacitance thus limiting their energy density. This is the main reason why so many studies are currently dedicated to the development of pseudocapacitors using metal oxides or conducting polymers. During the past decade transition metal oxides were extensively studied as electrode materials for ECs. MnO2 [5], [6], [7], [8], [9], [10], V2O5 [11], [12], Fe3O4 [12], [13], [14], [15], among others, have been proposed as alternative electrode materials for ECs due to the possible multiple oxidation states of the metal cation, accessible within a potential range of about 1 V allowing the surface redox reactions in aqueous electrolytes to take place. They are easily prepared in several polymorphs and with different micro and nanostructures at low cost [12], [16], [17]. However, none of them have better performance than the electronically conductive RuO2•xH2O (720 Fg−1) but its costs severely limits practical applications [2], [18], [19].
Recently, metal nitrides such as MoxN [20], [21], TiN [22], VN [23], WN [24] and RuN [25] have been studied, and impressive specific capacitance values up to 1340 Fg−1 at 2 mVs−1 and 554 Fg−1 at 100 mVs−1 were reported for VN [23]. According to Choi et al. the high specific capacitance of VN is due to its high electronic conductivity (σpowder = 8.82 × 103 Ω−1 m−1) coupled with fast and reversible redox reactions at the surface of the material [23]. The successive redox reactions arise due to the partially oxidized surface of VN (VNxOy), in the presence of OH− ions from the electrolyte, together with electrical double layer capacitance to a less extent.
The inner core of the VN allows the fast electron transport enabling charge transfer at very fast rates. The following reaction was proposed to illustrate the complete process [23], [26].VNxOy + OH− z VNxOy||OH− + (1-z) VNxOy-OH + (1-z) e−
Where VNxOy||OH− represents the electrical double layer formation and VNxOy-OH is correlated to the reduction of vanadium cations initially present in the compound. However, specific capacitance values of VN of only 161 Fg−1 at 30 mVs−1 and 186 Fg−1 at 1 Ag−1 respectively were reported for different electrodes [27], [28], [29]. Such difference between these values and those reported by Choi [23] can be partially assigned to the difference in the amount of oxygen and different oxidation states of vanadium cations at the surface of the electrode [23], [27], [28], [29]. However, all these reports suggest that in order to improve the specific capacitance of metal nitrides it is necessary to control the surface composition. Thus, appropriate synthesis methods are needed in order to control the surface composition which is a key parameter for enhancing the electrochemical properties of transition metal nitrides based electrodes and to emphasize their potential applications in ECs.
Vanadium nitride has been prepared by nitridation or ammonolysis of metals, metal halides and metal oxides although the use of cyanamide, urea and melamine as sources of nitrogen is also reported [28], [29], [30], [31], [32], [33], [34], [35]. It has also been obtained by solid-state metathesis reaction using metal oxides as a precursors and metal azides as a source of nitrogen. Such methods have also been used in order to prepare VN coated carbon nanotubes [36], [37], [38]. Alternative approaches are using TiN nanotubes as substrate for the preparation of core/shell TiN/VN [39], [40]. In all these approaches high temperatures (> 500 °C) with long annealing time are required [41], [42].
The main drawbacks of these methodologies are two; (1) it is difficult to control the amount of oxygen on the vanadium nitride surface and (2) they require high temperatures to obtain the metal nitrides which prohibits the deposition on substrates like silicon, aluminum, glasses and as a consequence the application of these materials in the fabrication of micro-devices. To overcome these limitations physical methods have also been used like pulsed laser deposition and D.C. reactive magnetron sputtering. These methods allow the deposition of complex materials even at room temperature for short periods of time (from minutes to hours) [43], [44]. It is also possible to prepare highly crystalline materials, to induce the growth of crystals in a preferential direction and to control the thickness of the materials to a few nanometers [43].
VN is a potential candidate for ECs not only for its high capacitance but for its metallic electronic conductivity (σ ≈ 106 Ω−1 m−1), high melting point, high density, excellent chemical stability (resistance to acidic and alkaline media) and low cost. However, the use of binders and conductive additives in composite electrodes are detrimental to the study of intrinsic properties of the material itself. Therefore, VN thin film electrodes present several advantages compared with other kind of electrodes. There is no need to add additives to increase the electronic conductivity, nor binder to improve the mechanical stability. The use of thin film electrode enables the study of the intrinsic properties of the material. Additionally, due to its high electronic conductivity, VN thin film electrode can be used both as the current collector and as the electrochemically active material (working electrode). VN thin film electrode can also be easily integrated in micro-devices. There are already some reports on the use of VN electrode in asymmetric microdevices [45], [46], [47]
Herein, we report the preparation of VN thin films on glass substrates by D.C. reactive magnetron sputtering for ECs. Cyclic voltammetry and galvanostatic cycling have been used to determine the performance of VN based electrodes. The effect of thickness on the electrochemical properties of the thin films has been evaluated. Moreover, micro-devices were prepared with a symmetrical design and their performance in 1 M KOH solution and in PVA-KOH polymer gel electrolyte are reported.
Section snippets
Preparation of VN thin films
The VN samples were prepared by D.C. reactive sputtering and deposited on soda lime glass substrates. The deposition was performed at room temperature. A vanadium disk of 99.9% purity was used as cathode. Prior to the deposition the chamber was evacuated to a pressure of 10−3 Pa. The substrates were cleaned in situ with argon sputter etching for 10 min. A target to substrate distance of 50 mm was used. A gas mixture of Ar/N2 with flows fixed at 30 sccm, and 2.5 sccm, respectively leading to a total
Structural Characterization
The XRD patterns of VN thin films with different thickness (25 nm, 66 nm, 209 nm, 270 nm, 480 nm and 594 nm) prepared by D.C. reactive sputtering are shown in Fig. 1. The main peak observed at 37.5° 2θ is ascribed to the (111) planes of a cubic crystal system (Fm3 m). The presence of this single peak indicates a preferential growth of the thin films along <111> direction. However, the intensity of this peak decreases with decreasing thickness and also tends to be wider due to smaller crystallite
Conclusions
VN thin films with different thickness were tested as electrode materials for electrochemical supercapacitors. The charge storage mechanism involves reversible redox reactions even at high scan rates (200 mVs−1) which are not common for pseudocapacitors in KOH while in standar organic electrolyte NEt4BF4, electric double layer arise as charge storage mechanism. The bulk volume only acted as an electronic conductor allowing fast reponse to potential changes while in the active volume the charge
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