Edition Chemistry's Constant Companion Fr Ee Ac Ce Ss** to Th E M Er Ck Ind Ex on Lin E Bimetallic Molybdenum Tungsten Oxynitride: Structure and Electrochemical Properties

PAPER Alexey M. Glushenkov et al. Bimetallic molybdenum tungsten oxynitride: structure and electrochemical properties With over 1 million copies sold, The Merck Index is the definitive reference work for scientists and professionals looking for authoritative information on chemicals, drugs and biologicals. The fully revised and updated 15 th edition is now available from the Royal Society of Chemistry (RSC). Every book purchase includes one-year's FREE ACCESS ** to The Merck Index Online. a Mesoporous molybdenum tungsten oxynitride was synthesised by the temperature-programmed reduction of the bimetallic oxide precursor in ammonia and its electrochemical properties were investigated in 1 M H 2 SO 4 aqueous electrolyte. The reaction product is a single-phase material, in which molybdenum and tungsten are distributed throughout the sample, with mesoporous morphology. The maximum of the pore size distribution is located at approximately 4 nm. Nearly rectangular voltammograms with the presence of small redox peaks were detected by cyclic voltammetry measurements in the acidic aqueous electrolyte, indicating properties relevant to electrochemical supercapacitors. The capacitance of 124 F g À1 was measured by galvanostatic charge–discharge and 43% of the initial capacitance can be retained upon the 400-fold increase in the current density from 0.05 to 20 A g À1. The electrochemical properties and the rate capability of the synthesised material are compared with those of monometallic oxynitrides of molybdenum and tungsten. A symmetric cell assembled with molybdenum tungsten oxynitride electrodes is also evaluated.


Introduction
Transition metal nitrides and oxynitrides possess a unique set of physico-chemical characteristics including high conductivity, resistance to corrosion and catalytic activity. 1][16][17][18][19][20][21][22][23][24][25][26][27][28] More specically, the electrocatalytic activity of transition metal nitrides such as TiN 5 or MoN 6 has made them promising candidates for replacing noble metals catalysts for the oxygen reduction reaction.3][4] Studies have conrmed that DSSCs with counter electrodes consisting of Mo 2 N, W 2 N or VN have reached 91%, 83% and 92%, respectively, of the photovoltaic performance of the DSSCs using Pt as a counter electrode. 2-4Furthermore, transition metal nitrides have been successfully applied in electrochemical energy storage devices.For example, vanadium nitride and chromium nitride thin lms used as anode materials in lithium ion batteries have reached reversible capacity values of approximately 800 mA h g À1 for VN 7 and 1200 mA h g À1 for CrN 8 through a conversion reaction mechanism, while Co 3 N and Fe 3 N thin lms attained reversible capacity values uctuating between 324 and 420 mA h g À1 for Co 3 N and between 323 and 440 mA h g À1 for Fe 3 N, respectively. 9urthermore, bulk CrN nanoparticles have demonstrated a reversible capacity of approximately 500 mA h g À1 , 10 while bulk CoN nanoparticles and CoN nanoake lms have reached a reversibly capacity of 660 mA h g À1 and 990 mA h g À1 , respectively. 11,12Additionally, a reversible capacity of 400 mA h g À1 has been reported for (Ni 0.33 Co 0.67 )N nanoparticles. 13mong applications in energy conversion and storage devices, the use of transition metal nitrides and oxynitrides in supercapacitor electrodes has been promising due to their high intrinsic conductivity and their resistance to corrosion. 14itrides and oxynitrides of vanadium, [14][15][16][17][18] tungsten, [19][20][21] molybdenum [22][23][24]27 and titanium 25,26,28 have demonstrated pseudocapacitive behaviour, with typical capacitance values between 30 and 350 F g À1 in aqueous electrolytes. Rae capability of the active materials is an important performance criterion, as supercapacitors used in the high power applications such as the automotive sector, smart grids or power backup systems are required to deliver energy in the pulse form and work at high discharge currents.29,30 Unlike many transition metal oxides, nitrides and oxynitrides possess high conductivities and therefore good rate capabilities. For examle, W 0.75 (N,O) has demonstrated about 40% of the capacitance retention upon the increase of the current load from 0.05 to 10 A g À1 , 21 while VN has shown 79% of its initial capacitance at 0.05 A g À1 , at a current load of 1 A g À1 .17 TiN-VN core-shell mesoporous bres retained 64% of the initial capacitance aer increasing the current from 2 to 10 A g À1 .28 It has been previously observed that a partial substitution of one transition metal by another in the crystal lattice of nitrides or oxynitrides could alter their physico-chemical properties.[31][32][33] For instance, bimetallic compounds such as cobalt molybdenum nitride 31 or vanadium molybdenum oxynitride 33 have demonstrated superior catalytic properties when compared to the corresponding monometallic compounds.However, for supercapacitor electrode materials, studies have been almost exclusively focusing on the monometallic nitride compounds so far, and only limited information exists on the electrochemical performance of bimetallic transition metal nitrides and oxynitrides.Experimental data have been reported on the mixed g-Mo 2 N and Co 3 Mo 3 N "composite material" demonstrating higher capacitance values than those of the pure g-Mo 2 N compound.24 Meanwhile, the electrochemical performance of the pure bimetallic compound has not been studied.It is, in our view, interesting to study how the introduction of the second metal into the crystal lattice of nitrides and oxynitrides inuences their electrochemical performance in supercapacitors.
In this article, a bimetallic molybdenum tungsten oxynitride is characterised as a supercapacitor electrode material.The oxynitride is synthesised by a temperature-programmed reduction (TPR) of the oxide precursor in NH 3 .The synthesised material is extensively characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), low temperature N 2 adsorption and transmission electron microscopy (TEM).Electrochemical properties of the material are studied in 1 M H 2 SO 4 electrolyte by performing cyclic voltammetry and galvanostatic charge-discharge measurements in the three-electrode conguration as well as in the symmetric cell, emulating an accepted design of an electrochemical capacitor.Furthermore, samples are analysed by electrochemical impedance spectroscopy (EIS).The electrochemical performance of molybdenum tungsten oxynitride is compared to those of the monometallic molybdenum oxynitride and tungsten oxynitride synthesised from the oxides of the corresponding metals by a temperature-programmed reduction under similar experimental conditions.

Synthesis
Molybdenum tungsten oxynitride was synthesised by a solid state route similar to the method described by C. C. Yu and S. T. Oyama. 3213.5 g of WO 3 (Sigma-Aldrich, 95410) and MoO 3 (Sigma-Aldrich, 267856) mixture in a 1 : 1 ratio was placed inside a planetary ball mill (Fritsch Pulverisette 5) and milled for 50 hours in order to achieve a high homogeneity of the mixture.Steel balls with 20 mm diameter were used with the 1 : 20 powder to ball ratio and the rotor speed was set to 150 rpm.The milled powder was subsequently pressed into pellets with 13 mm diameter dye under the pressure load of 8 tonnes.The pellets were annealed in an alumina crucible inside a horizontal tube furnace with open ends of the tube (Tetlow Kilns & Furnaces Pty Ltd, Australia) at 785 C for 6 hours and naturally cooled down to room temperature.The annealed pellets were crushed into powder again with an agate mortar in order to proceed to the temperature-programmed reduction process.Temperature-programmed reduction of the mixed oxide powder was performed in the same tube furnace (Tetlow Kilns & Furnaces Pty Ltd, Australia) under ammonia ow.First, the furnace was heated to approximately 700 C with the heating rate of 5 C min À1 , kept at that temperature for 3 hours and cooled down to room temperature naturally.The NH 3 ow was set to 0.4 l min À1 throughout the experiment.Finally, the sample was passivated with a special gas mixture (Ar with 0.1% O 2 ) for 1 hour at a ow rate of 0.5 l min À1 prior to the exposure to air.In order to compare the electrochemical performance of the bimetallic oxynitride to those of the monometallic compounds, tungsten and molybdenum oxynitrides were synthesised by the temperature-programmed reduction of the corresponding oxides.5][36][37] For simplicity, the synthesised materials are denoted as MoW(N,O), W(N,O) and Mo(N,O).

Characterisation
The morphology of the synthesised material was examined by scanning electron microscopy (SEM, Carl Zeiss SUPRA 55VP).Surface area and porosimetry measurements were performed using a Micrometrics Tristar 3000 analyser.From the low temperature N 2 adsorption isotherm, pore size distribution and surface area were calculated by Barrett-Joiner-Helenda (BJH) and Branauer-Emmett-Teller (BET) methods, respectively.A PANalytical X'Pert PRO diffractometer with Cu K-alpha radiation (l ¼ 0.15418 nm) was used to measure the X-ray diffraction pattern of the material.
The transmission electron microscopy (TEM) examination was carried out in an FEI Titan instrument operating at 300 kV accelerating voltage.The TEM instrument is equipped with a scanning unit (STEM), a Gatan Imaging Filter (GIF), and an energy dispersive X-ray (EDX) spectrometer.For energy-ltered TEM (EFTEM) imaging, a three-window method was employed to acquire tungsten, nitrogen, and oxygen elemental distributions.

Electrochemical testing
For the electrochemical characterisation of the synthesised material, electrode slurry was prepared by mixing molybdenum tungsten oxynitride, carbon nanopowder (Sigma-Aldrich, #699632) and poly(vinylidene)uoride (PVDF, Sigma-Aldrich) in a 90 : 5 : 5 ratio.NMP (N-methyl-2-pyrrolidone, anhydrous, 99.5%, Sigma-Aldrich) was used as a solvent.Electrode slurries were coated onto titanium foils and dried in a vacuum at 90 C overnight in a conventional oven.The weight of the active material on the dried electrodes was between 2 and 4 mg cm À2 .Electrodes were assembled into three-electrode cells using a Pt wire as a counter electrode and Ag/AgCl as a reference electrode.Symmetric cells were prepared by immersing two nearly identical electrodes separated by a microporous polyethene lm (MTI Corp., USA) into the electrolyte.Cells were lled with 1 M H 2 SO 4 electrolyte under vacuum and characterised by galvanostatic charge-discharge as well as cyclic voltammetry measurements using a Solartron Analytical 1470E potentiostat/ galvanostat.The impedance spectroscopy was performed in the symmetric cell conguration at the open circuit potential (OCP) from 50 kHz to 0.05 Hz using an Ivium-n-stat electrochemical analyser.The amplitude of the modulation signal was set to 10 mV.Experimental data were tted with ZView soware.

Characterisation
The XRD pattern of the synthesised compound (Fig. 1) shows ve diffraction peaks located at 37.4 , 43.3 , 63.0 , 75.9 and 80.0 , matching the diffraction lines of MoWN 2.1 O 2.4 (Powder Diffraction File #: 50-0134).It is in agreement with the assumption that the dominant phase in the sample is molybdenum tungsten oxynitride with a cubic structure. 32The broadening of the diffraction peaks indicates a small crystalline size and a possible high surface area of molybdenum tungsten oxynitride, consistent with the previous reports. 32Indeed, the surface area of 72.6 m 2 g À1 is measured by the BET method.
Low temperature nitrogen adsorption-desorption isotherm (Fig. 2a) demonstrates a hysteresis and belongs to type IV, indicating that the material is mesoporous.BJH measurements show a narrow pore size distribution, with the maximum located at around 4 nm (Fig. 2b) and virtually no pores larger than 12 nm are present in the sample.
The low magnication SEM image (Fig. 3a) shows that particle sizes of the synthesised powder do not exceed 1 mm, while the high resolution image (Fig. 3b) indicates that particles are porous, with the visible pore sizes less than 10 nm, consistent with the low temperature nitrogen adsorption measurements.In order to characterise the structure of molybdenum tungsten oxynitride in more detail, the synthesised material is investigated by TEM.A bright eld TEM images conrms the nanocrystalline structure of the synthesised material (Fig. 3c and e).The electron diffraction pattern (Fig. 3d) shows ve diffraction rings, in agreement with the XRD pattern (Fig. 1).EFTEM maps of W, Mo, N and O (Fig. 3f) taken from the area depicted in Fig. 3e show that the elements are distributed throughout the sample.

Electrochemical properties
It follows from the analysis of the selected specic capacitance values reported in the literature for the monometallic nitrides and oxynitrides 14,[19][20][21][22][23]27 of molybdenum and tungsten (Table 1) that the electrochemical properties of these phases vary considerably depending on the precursor and synthesis conditions.
For example, the capacitance of g-Mo 2 N may vary in a wide range between 70 and 380 F g À1 . 14,22,23,27For consistency, here we compare the electrochemical properties of molybdenum tungsten oxynitride with those of molybdenum and tungsten oxynitrides synthesised from their corresponding oxide powders under similar nitridation conditions (TPR in ammonia with a maximum temperature of 700 C).The detailed characterisation of these Mo(N,O) and W(N,O) samples is presented elsewhere. 37yclic voltammetry (CV) measurements in the three-electrode cell conguration (Fig. 4a) indicate that the operating potential window of molybdenum tungsten oxynitride is between À0.2 and 0.55 V vs. Ag/AgCl in 1 M H 2 SO 4 aqueous electrolyte.The CV curves have nearly rectangular shapes at various scan rates, with small reversible redox peaks present in cyclic voltammograms, typical for pseudocapacitive materials.
It could be observed that the operating potential window of the bimetallic oxynitride as well as the shape of the CV curves in 1 M H 2 SO 4 electrolyte differ from both molybdenum and  tungsten monometallic compounds synthesised from the pure oxide precursors (Fig. 4b).In fact, tungsten oxynitride produced from the WO 3 precursor has an ideal rectangular CV curve in the voltage window between À0.4 and 0.5 V vs. Ag/AgCl with no visible redox peaks, while molybdenum oxynitride synthesised from the MoO 3 precursor operates between À0.5 and 0.5 V vs. Ag/AgCl and possesses several redox peaks as well as a less rectangular cyclic voltammogram (Fig. 4b).
The shape of the galvanostatic charge and discharge curves (Fig. 5a) is close to triangular.This shape is preserved when increasing the current load from 0.5 to 10 A g À1 , although an IR drop is observed at high currents.43% of the capacitance is retained upon increasing the current density from 0.05 to 20 A g À1 .The maximum reversible specic capacitance value measured at 0.05 A g À1 in 1 M H 2 SO 4 electrolyte is 124 F g À1 (172 mF cm À2 ).This value is orders of magnitude higher when compared to the typical 20-50 mF cm À2 , characteristic of the electrical double layer charge storage mechanism, 38 indicating the presence of pseudocapacitive processes. 39Cyclic stability measurements (Fig. 6) reveal that molybdenum tungsten oxynitride retains 46% of its initial capacitance aer 5000 galvanostatic charge and discharge cycles at a current load of 5 A g À1 .
It can be noted that, at the same charge-discharge current densities, the bimetallic compound possesses higher specic capacitance values as well as a better rate capability when   compared to both oxynitrides synthesised from MoO 3 or WO 3 (Fig. 5b).At a high current load of 10 A g À1 , MoW(O,N) possesses a specic capacitance of 67 F g À1 , twice the corresponding value for tungsten oxynitride, 21 while the capacitance of molybdenum oxynitride quickly fades and reaches only 2 F g À1 at a current density of 10 A g À1 .In order to investigate the difference between the electrochemical performances of the samples further, the electrochemical impedance spectroscopy data were collected for molybdenum tungsten oxynitride electrodes and compared to the corresponding data measured for the monometallic compounds.Nyquist plots of the impedance spectra are presented in Fig. 7a-c.4][45] The proposed equivalent circuit ts the experimental data well in the high-frequency region (Fig. 7d).
The equivalent circuit contains the following variables: the bulk resistance (R b ), distributed double-layer capacitance within the pores (CPE dl ), charge transfer resistance (R ct ) and the diffusion impedance in the porous electrode (CPE W ). Equivalent circuit data are tted with ZView soware and are summarised in Table 2.The impedance of the CPE element is dened as: 40,43 (1) where Y 0 is admittance, j is the imaginary unit, u is the modulation frequency and a is a dimensionless coefficient.When a ¼ 1, the CPE behaves as an ideal capacitor while at   Table 2 Fitted impedance data modelled with a Randles-type equivalent circuit (Fig. 7d)  The charge transfer resistance R ct represents the charge transfer resistance at the electrode-electrolyte interface.1][42][43] The lowest value of R ct for molybdenum tungsten oxynitride electrodes could be attributed to their superior electronic conductivity in respect to those of both monometallic compounds.

Symmetric cell
The assembled supercapacitor devices could be classied into several categories according to the cell design and active materials used. 30,46,47In this work, two molybdenum tungsten oxynitride electrodes were tested in the symmetric conguration as an approximation of a real device.
The cyclic voltammetry curve demonstrates an ideal rectangular shape in the potential range between 0 and 0.75 V in 1 M H 2 SO 4 electrolyte at the scan rates of 1, 5, 10 and 20 mV s À1 (Fig. 8a).Interestingly, no redox peaks can be observed in the cyclic voltammograms of the symmetric device.Galvanostatic charge and discharge curves form nearly ideal triangular shapes (Fig. 8b) at the current densities of 0.5, 1, 5, and 10 A g À1 , with a small IR drop being observed at 10 A g À1 .The maximum specic capacitance value, measured at a current load of 0.05 A g À1 , is 23 F g À1 and approximately 50% of this value is maintained when increasing the current density from 0.05 to 20 A g À1 (Fig. 8c).

Conclusions
Bimetallic molybdenum tungsten oxynitride has been synthesised by a TPR in ammonia and extensively characterised by XRD, low-temperature N 2 adsorption, SEM and TEM.XRD and TEM analyses indicate that the synthesised material consists of a molybdenum tungsten oxynitride phase and no obvious unreacted oxide is present in the sample.It has been concluded that the material has mesoporous morphology with the maximum of the pore size distribution at around 4 nm.Mo, W, N and O elements are distributed throughout the sample.Electrochemical characteristics of MoW(N,O) in 1 M H 2 SO 4 aqueous electrolyte differ from both molybdenum and tungsten monometallic oxynitrides synthesised under similar experimental conditions.The material demonstrates a capacitance of 124 F g À1 at a current density of 0.05 A g À1 in galvanostatic charge-discharge experiments.43% of this capacitance can be retained upon the 400-fold increase in the current density from 0.05 to 20 A g À1 .The symmetric cell with two molybdenum tungsten oxynitride electrodes exhibits a capacitance of 23 F g À1 and is able to operate up to a high current of 20 A g À1 .

Fig. 3
Fig. 3 Electron microscopy characterisation: SEM images (a and b); bright field TEM image and the electron diffraction pattern (c and d); a TEM image and the corresponding EFTEM maps depicting the distribution of W, Mo, N and O elements (e and f).

Fig. 4
Fig. 4 Electrochemical characterisation of MoW(N,O) as a supercapacitor electrode material in 1 M H 2 SO 4 electrolyte.Cyclic voltammetry measurements (third cycle) at various scan rates (a) and CV of the bimetallic oxynitride compared to the cyclic voltammograms of the monometallic compounds at a scan rate of 5 mV s À1 ; the current is normalised with the weight of the active material (b).

Fig. 5
Fig. 5 Galvanostatic charge and discharge curves of molybdenum tungsten oxynitride (a) and its rate capability in 1 M H 2 SO 4 electrolyte (b).Rate capabilities of molybdenum and tungsten oxynitrides are given for comparison in (b).

Fig. 6
Fig. 6 Cyclic stability of molybdenum tungsten oxynitride in 1 M H 2 SO 4 electrolyte cycled at a current load of 5 A g À1 : initial capacitance retention and galvanostatic charge and discharge curves (inset).

Fig. 7
Fig. 7 Nyquist plots of the impedance spectra of molybdenum tungsten (a), tungsten (b) and molybdenum (c) oxynitrides and the equivalent circuit used to fit the experimental data (d).

Table 1
Selected specific capacitance values reported in the literature for the monometallic molybdenum and tungsten oxynitrides CPE demonstrates the transmission line behaviour and represents the mass transport impedance within the pores, with a characteristic 45 phase angle.