J. Cent. South Univ. (2015) 22: 8−13 DOI: 10.1007/s11771-015-2488-8

Composition change and capacitance properties of ruthenium oxide thin film

LIU Hong(刘泓)1, GAN Wei-ping(甘卫平)2, LIU Zhong-wu(刘仲武)1, ZHENG Feng(郑峰)2

1. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510000, China; 2. School of Materials Science and Engineering, Central South University, Changsha 410083, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2015

Abstract: RuO2·nH2O film was deposited on tantalum foils by electrodeposition and heat treatment using RuCl3·3H2O as precursor. Surface morphology, composition change and cyclic voltammetry from precursor to amorphous and crystalline RuO2·nH2O films were studied by X-ray diffractometer, Fourier transformation infrared spectrometer, differential thermal analyzer, scanning electron microscope and electrochemical analyzer, respectively. The results show that the precursor was transformed gradually from amorphous to crystalline phase with temperature. When heat treated at 300 °C for 2 h, RuO2·nH2O electrode surface gains mass of 2.5 mg/cm2 with specific capacitance of 782 F/g. Besides, it is found that the specific capacitance of the film decreased by roughly 20% with voltage scan rate increasing from 5 to 250 mV/s. Key words: ruthenium oxide; thin film; heat treatment; composition change; electrochemical capacitor 1 Introduction

Electrochemical capacitors (ECs) have attracted great interests in energy storage components with high power density. They become increasingly important as power demand of portable devices boosts and more and more people prefer to drive electric vehicles and use renewable energy sources. In addition to fast faradic reactions occurring on or near a solid electrode surface at appropriate potential, energy storage mechanisms for EC capacitors include separation of charges at interface between solid electrode and electrolyte. The former is called faradic pseudo-capacitor (FPC), and the latter is electric double-layer capacitor (EDLC).

Carbon supercapacitors are typical EDLCs without faradic charge stored at carbon/electrolyte double-layer. Of low cost, excellent cycle life, and wide potential window in non-aqueous solvents, high surface-area carbons are used as the electrode of EDLCs [1−3]. Intensive research has been devoted to carbon as a supercapacitor electrode. Despite of their high specific capacitance (up to 250 F/g), carbonaceous materials suffer from low specific energy density. In contrast, metal oxides, such as RuO2·nH2O [4−6], MnO2 [7−10], NiO [11−12], CoO2 [13], are current under evaluation for their charge-storage behaviors. Among them, amorphous hydrous form of RuO2·nH2O has been found to be one of the best candidates for supercapacitor application due to

its high specific capacitance, high specific energy density (compared with conventional carbon materials), better electrochemical stability (compared with polymer materials), high electrochemical reversibility, and long cycling life. However, the drawback is its high cost and scarcity of precious metal Ru. Lattice water inside RuO2·nH2O decreases with temperature. JOW and ZHENG [14] reported a specific capacitance of 760 F/g for amorphous ruthenium oxide prepared by sol−gel process involving mixing of aqueous solutions of NaOH and RuCl3·3H2O. At higher temperatures, water vaporization and oxide crystallization lead to rapid loss of capacitance. A specific capacitance of 1170 F/g has been reported for Ru oxide using carbon fiber paper support [15]. However, the estimation of this specific capacitance must involve considerable uncertainty due to small contribution of carbon fiber to the total mass of composite materials. HU and CHANG [16] deposited RuO2·nH2O on titanium substrate directly through potential-cycling with relatively low specific capacitance. They attributed this to the existence of Ru metal. Without Ru, the specific capacitance of RuO2·nH2O film reached 552 F/g [17]. The improvement in capacitance is strongly dependent on preparation method and structure of RuO2·nH2O. There are many fabrication techniques, such as electrostatic spray deposition [18−19], sol−gel [20], solid-state reaction of K2CO3 and RuO2·nH2O [21−22], oxidative synthesis [6, 23], and incipient wetness method [24] have been developed. All of them

Foundation item: Project(S2013040015492) supported by the Natural Science Foundation of Guangdong Province, China; Project(2007AA03Z240)

supported by Hi-tech Research and Development Program of China Received date: 2013−10−28; Accepted date: 2014−04−01 Corresponding author: LIU Hong, PhD; Tel: +86−20−22236906; E-mail: liuhongphd@163.com

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are aimed at synthesizing amorphous/crystalline RuO2·nH2O with hopes to achieve good capacitive performance, e.g., specific capacitance, cycle stability, power characteristics.

Of low-cost apparatus, ease control of thickness and friendly deposition atmosphere [19, 25−26], electro- deposition method has gained much attention as the choice of deposition of various metal oxides and organoceramic materials. Electrodeposition is capable of forming uniform thin films on substrates of complicated shapes, impregnating porous substrates and depositing on selected areas of the substrates. It has been used to prepare thin and thick films of metal oxides and ceramic materials. The main drawback however associating with electrodeposition method is such that only can conducting substrates be used.

In this work, the RuO2·nH2O thin films are prepared preparation of through cathodic electrodeposition deposited RuCl3·3H2O precursor following by calcination. Composition change of film from RuCl3·3H2O to RuO2 is studied. The electrochemical behavior of precursor and phase structure of film as a function of temperature is investigated. The electrochemical properties of RuO2·nH2O film are also studied as a function of temperature. 2 Experimental

Hydrous ruthenium oxide (RuO2·nH2O) was electrodeposited directly onto commercial 99.9% tantalum substrate (d34 mm×0.1 mm). Prior to deposition, the substrate was first mechanically polished with emery, then degreased with acetone and deionized (DI) water in an ultrasonic bath for 15 min and etched for 30 min in alkali liquor at 80 °C. It was rinsed again with DI water and then pickled for 40 s in a solution consisting of H2SO4 (1.84 g/cm3), HNO3 (1.40 g/cm3), HF (40%) and water in volume ratio of 3:2:4:10. After pickling, the substrate was rinsed with DI water.

The electrolyte was a mixed water solution containing ruthenium chloride (3 mmol/L), NaNO3 (0.1 mol/L), tantalum chloride (5 mol/L) and HNO3 (10 mmol/L) with initial pH of 2.10. Tantalum sheet was placed in plating solution with platinum plate as counter electrode. Cathodic electrodeposition was performed with slow stirring on hot plate at 25 °C. Galvanostatic current density was maintained at 3 mV/cm2 for 0.5 h. After drying at room temperature in air, the film was dip- coated in ruthenium chloride (0.5 mol/L) isopropanol solution and heated at 10 °C/min to 100, 150, 200, 300, 400, 500 °C for 2 h, respectively.

The surface morphology of the deposits was examined by scanning electron microscope (SEM, Sirion

200) operating at 15 kV. Diffraction patterns were obtained from an X-ray diffractometer (XRD, D-MAX2500) using Cu target (Cu Kα=1.5418 Å) at 2θ speed of 4 (°)/min. Thermal data of oxides were obtained by thermogravimetric/differential scanning calorimetry (TG/DSC, NETZSCH STA 449C, German) performed in flowing air at heating rate of 10 °C/min from room temperature to 600 °C. Fourier transform infrared (FTIR) spectra in reflectance mode (FTIR-ATR) were collected by a Nicolet 380 (Thermo Fisher Scientific Inc., USA) with 40 scans at resolution of 4 cm−1.

Electrochemical properties of RuO2·nH2O electrode film were examined by means of electrochemical analysis system, CHI 660C at 25 °C in a three- compartment cell. An Hg/Hg2SO4 electrode was used as reference and platinum plate as counter electrode. A Luggin capillary was used to minimize errors due to IR drop in the electrolyte. All solutions used in this work were prepared with cm DI water18 MΩ produced by a reagent water system (Exceed KL-UP-Ⅱ, China). In addition, the electrolyte (0.5 mol/L H2SO4) used for electrochemical characterization was degassed with purified nitrogen before measurement for 25 min. Nitrogen was passed over the solution during each measurement. 3 Results and discussion 3.1 Electrochemical behavior

The mechanism of RuCl3·3H2O deposition is different from metal electroplating. The following reactions could take place to produce Ru(OH)(3−δ)+ δCl(3−δ) and H+ in RuCl3·3H2O solution:

3 2 2RuCl 3H O+( 3)H Oc 3 2RuCl H Oc (1)

3 2 2RuCl H O+ H Oc 3 2Ru(OH) Cl H O+c +H + Cl (2)

During deposition from an aqueous bath, following

reaction is considered to take place on surface of cathode:

2 22H O+2e H +2OH (3)

Note that reaction (3) could increase pH value of the

solution. But in reality the pH decreased rapidly during initial stage of electrodeposition as shown in Fig. 1. This is caused by increase of cation concentration due to adsorption of Ru(OH)(3−δ)+δ and H+ ions on cathode. The pH value however increases gradually once the amount of [H+] consumed in reaction (2) is less than [OH−] generated by reaction (3). As a result, reaction (2) is speeded up and moved toward right, resulting in continuous generation of micelle Ru(OH)(3−δ)+δ. Micelle could be adsorbed onto tantalum substrate and

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form initial film when electrostatic repulsion and van der Waals force reach a balance. Under influence of galvanostatic field, film grows continuously. Micelle Ru(OH)(3−δ)+δ could precipitate if pH is larger than isoelectric point of colloid. In order to prevent micelle from precipitation, we need to control pH. The suitable pH is 1.8−3.0 based on our experiments. The temperature in deposition bath was varied from 20 to 30 °C, with 25 °C to be the optimal.

Fig. 1 Change of pH with deposition time at 30 °C 3.2 Morphological characterization

Once precursor of Ru(OH)(3−δ)+δCl3·3H2O grown on Ta substrate on a large scale by electrochemical method, the binding force between the film and tantalum foils was weak. Heat treatment is necessary to transform Ru(OH)(3−δ)+δCl3·3H2O into RuO2·nH2O and improve its adhesion and increase the stability of capacitance. But, in our early work, it was found that specific capacitance of the film decreased when lattice water inside RuO2·nH2O was vaporized with temperature [27−28]. Morphology of the film before and after heat treatment was studied by field emission scanning electron microscopy (FESEM). Precursor Ru(OH)(3−δ)+ δCl3·3H2O film used for investigation was of flocculation, crack-free and relatively dense. However, with surface mass of 5.4 mg/cm2 (Fig. 2(a)), its adhesion was poor and delaminated in solution. RuO2·nH2O was transformed from precursor Ru(OH)(3−δ)+δCl3·3H2O with surface mass of 2.5 mg/cm2 after dip-coating in ruthenium chloride (0.5 mol/L) isopropanol solution and heated at 300 °C for 2 h at 10 °C/min (Fig. 2(b)). The film thickness was measured to be about 4.13 μm. In addition, the RuO2·nH2O film has higher density of of and larger binding force with substrate. 3.3 Composition change

We found heat treatment increasing electronic conductivity, utilization and cycle life of RuO2·nH2O electrodes for supercapacitors. Especially, the cycle life

Fig. 2 FESEM photographs of films: (a) Dried in air; (b) After heat treatment at 300 °C for 2 h of RuO2·nH2O increased significantly. The crystalline structure of RuO2 was characterized using an X-ray diffractometer as shown in Fig. 3. No peak appears for sample annealed at 100 °C. At 150 °C, there are no peaks of RuO2 and Ru except some small ones below 20° (2θ) which may be assigned to carbohydrate. At 200 °C, peaks corresponding to anhydrous RuO2 start to form and their intensity increase with temperature [14]. From 300 to 500 °C, the peak intensity of RuO2 increases while its width is reduced. Intensity of Ru decreases with temperature from 200 °C up to 500 °C in our testing conditions. These results indicate that Ru(OH)(3−δ)+ δCl3·3H2O film transformed from amorphous RuO2·nH2O to crystalline with temperature. And the film

Fig. 3 XRD patterns of films with different function of temperatures

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treated at 300 °C for 2 h has high binding force and good overall electrochemical properties.

FTIR spectroscopy was used to measure the behavior of functional groups as a function of temperature (Fig. 4). Presence of amorphous ruthenium oxide layer is confirmed by FTIR absorption bands at 599.76 and 500.31 cm−1. The stronger peaks at 3138.50 and 3412.77 cm−1 are assigned to asymmetrical and symmetrical stretching vibration of H2O inside RuO2·nH2O, respectively. The water content of RuO2·nH2O at 100 °C is larger than that at 300 °C. There are no peaks of isopropanol and carboxylic acid. The peak at 1617.44 and 1400.65 cm−1 result from stretching vibration of carboxyl ( — COO−). The carboxyl is monodentate ligand with ruthenium since the difference of wavenumber is more than 200 cm−1. The former is higher and wider associating to asymmetrical stretching vibration of —COO−, while the latter is lower and narrower relating to symmetrical stretching vibration of — COO−. The relative intensity of — COO− peaks changes with temperature increasing from 100 to 300 °C while their positions remain unchanged. In addition, the peaks of water and carboxyl become lower, while those of Ru−O get higher at 400 °C. So, we conclude that one part of isopropanol vaporizes while the rest is oxidized to carboxylate. The water content of RuO2·nH2O decreases gradually with temperature.

Fig. 4 Infrared spectra of films with different temperatures

TG analysis (TGA) was performed in order to estimate hydration number (n in RuO2·nH2O) and chemical compositions of the film. Figure 5 shows thermal profiles of the film in atmosphere environment. The initial mass loss from 25 to 150 °C is responsible for the evaporation of isopropanol. The RuO2·nH2O formation takes place at 200 °C as evidenced by XRD. A gradual mass loss of 6.9% from 200 to 400 °C is observed for the decrease of water content. At the same time, the amorphous RuO2·nH2O transforms into RuO2 crystal as shown by XRD. Calculated n value of RuO2·nH2O decreases from 2.2 to 1.2 with temperature

increasing from 200 to 300 °C. From DTA curve in Fig. 5, there are three endothermic reactions occurring from room temperature to 300 °C. The first peak at 126.8 °C is due to the evaporation of isopropanol. Its molar heat of vaporization, ΔvapH (isopropanol, l), is calculated to be 41.4 kJ/mol, corresponding to a total mass loss of 31% as TGA curve shown in Fig. 5. The second peak associating with 120.84 kJ/mol (heat of absorption, RuO2·nH2O), centered at 157.1 °C, is attributed to following reaction:

3 2 2 2 2Ru(OH) Cl H O+1/4O RuO H O + c n

2(3 )HCl +( + 3/2)H Oc n (4)

The third peak, centered at 195.0 °C, results in coalescence and growth of RuO2·nH2O crystal as demonstrated by XRD, as the beginning of following reaction:

2 2 2 2RuO H O RuO + H On n (5) There is an exothermic reaction taking place at

413.3 °C, which may be oxidation of carboxylate.

Fig. 5 TG and DTA curves of film under flow of air 3.4 Electrochemical characteristics

As a proton condenser, surface sites in RuO2·nH2O occur reversible redox with simultaneous exchange of protons in solution according to following reaction [29]:

+RuO (OH) + H + ex y +RuO (OH)x y (6)

The electrochemical reaction results in capacitive, nearly featureless voltammograms for hydrous surface of RuO2·nH2O in aqueous electrolyte between H2 and O2 evolution potentials [29]. Cyclic voltammograms of RuO2·nH2O film treated at 300 °C for 2 h with scan rate of 10−110 mV/s in 0.5 mol/L H2SO4 electrolyte are shown in Fig. 6. The potential ranges from −0.6 to 0.6 V. All cyclic voltammograms exhibit a rectangular shape, associated with pure capacitive behavior. The specific capacitance (cs) was determined by integration of enclosed area of anodic and cathodic curves in cyclic voltammograms between −0.6 to 0.6 V (versus a

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reversible Hg/Hg2SO4 electrode) [21]. The reversibility of redox phenomena involved is found to be mirror image, which can be judged by relative area over and under the abscissa and the shape of the curve. In addition, multiple-peak characteristics of surface redox reactions can be observed, with typical features of amorphous ruthenium oxide hydrate [30]. The features are attributed to faradic reactions involved in proton-dependent oxidation reduction of RuO2·nH2O. Redox waves are then visible as indicated in Fig. 6.

Fig. 6 Cyclic voltammograms of films at scan rates of 10, 30, 50, 70, 90, 110 mV/s

Figure 7 shows the dependence of the first and second peaks of the cyclic voltammograms of the film treated at 300 °C for 2 h at different scan rates. The first peak current increases slowly with scan rate, while the second one increases quickly and proportional to scan rate. We believe that the valence state of Ru corresponding to the first peak is not suitable for fast charge/discharge in H2SO4 electrolyte, and that corresponding to the second peak is representative of good fast charge/discharge. In addition, there are arguments for increasing ohmic drops in bulk electrolyte with scan rate [31]. Based on this work, we believe these are the likely reasons for the observation that the specific

Fig. 7 Specific capacitances and peak currents of cyclic voltammograms of films as a function of scan rate

capacitance decreases with scan rate. For example, the specific capacitance was 782 F/g at 10 mV/s, and it decreased to 625 F/g (roughly 20% of its original value) at 250 mV/s, as showed in Fig. 7.

Figure 8 shows the specific capacitance of RuO2·nH2O films heat treated at different temperatures as a function of cycle numbers. Below 300 °C, cs decreases rapidly in 40 cycles at initial stage and approaches to certain value after 100 cycles prior to stable state. At 300 °C, cs drops a little bit in first 10 cycles then remains stable. The redox reactions in H2SO4 are highly reversible and stable upon extensive cycling. Above 300 °C, there is no measurable drop in cs as a function of cycle numbers except its values decreasing with temperature. After 120 cycles, the cs values of films heat treated in 100, 150, 200, 300, 400, 500 °C retain 39%, 33%, 72%, 98%, 99% and 100% of their original values, respectively. According to Eq. (6), H+ can exchange with amorphous RuO2·nH2O, but it is difficult to exchange with crystal RuO2. As a result, the specific capacitance of crystal RuO2 is smaller. We believe that the small cs in H2SO4 can be greatly boosted by using amorphous RuO2·nH2O electrode. The film prepared at low temperature contains amorphous RuO2·nH2O, but its binding force on tantalum is weak. When treated at 300 °C for 2 h, the film showed good overall properties.

Fig. 8 Specific capacitances of films heat treated at different temperatures as a function of cycle numbers 4 Conclusions

Physical properties and electrochemical characteristics of RuO2·nH2O film formed by cathodic electrodeposition and heat treatment were systematically studied. We found Ru(OH)(3−δ)+ δCl3·3H2O film is crack-free and relatively dense when deposited at 25 °C with pH 3.0. Ru(OH)(3−δ)+ δCl3·3H2O transforms to crystal RuO2 with evaporation of isopropanol and formation of amorphous RuO2·nH2O, RuO2·nH2O film has Density higher and larger binding force with substrate after being heat treated at 300 °C for 2 h. Composition of film is changed with temperature. The

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specific capacitance of RuO2·nH2O is strongly dependent on its phase structure. High specific capacitance can be obtained from use of amorphous RuO2·nH2O. The specific capacitance of the film heat treated at 300 °C for 2 h is 782 F/g at scan rate of 10 mV/s. In addition, RuO2·nH2O is stable in H2SO4 electrolyte and has a mirror-like cyclic voltammogram behavior. But crystal RuO2 has a smaller specific capacitance. Taking these properties into consideration, we think RuO2·nH2O is a promised material for EC capacitors. References [1] HULICOVA-JURCAKOVA D, SEREDYCH M, LU G Q,

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(Edited by YANG Hua)