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ISSN : 1225-7672(Print)
ISSN : 2287-822X(Online)
Journal of the Korean Society of Water and Wastewater Vol.31 No.5 pp.397-407
DOI : https://doi.org/10.11001/jksww.2017.31.5.397

Sonoelectrodeposition of RuO2 electrodes for high chlorine evolution efficiencies

Tran Le Luu1, Choonsoo Kim2, Jeyong Yoon2*
1Department of Mechatronics & Sensor Systems Technology, Vietnamese German University, Le Lai Street, Hoa Phu Ward, Thu Dau Mot City, Binh Duong Province, Viet Nam
2School of Chemical and Biological Engineering, College of Engineering, Institute of Chemical Process, Asian Institute for Energy, Environment & Sustainability (AIEES), Seoul National University (SNU), Gwanak-gu, Daehak-dong, Seoul 151-742, Korea
Corresponding author : Jeyong Yoon (jeyong@snu.ac.kr)
July 25, 2017 September 28, 2017 October 11, 2017

Abstract

A dimensionally stable anode based on the RuO2 electrocatalyst is an important electrode for generating chlorine. The RuO2 is well-known as an electrode material with high electrocatalytic performance and stability. In this study, sonoelectrodeposition is proposed to synthesize the RuO2 electrodes. The electrode obtained by this novel process shows better electrocatalytic properties and stability for generating chlorine compared to the conventional one. The high roughness and outer surface area of the RuO2 electrode from a new fabrication process leads to increase in the chlorine generation rate. This enhanced performance is attributed to the accelerated mass transport rate of the chloride ions from electrolyte to electrode surface. In addition, the electrode with sonodeposition method showed higher stability than the conventional one, which might be explained by the mass coverage enhancement. The effect of sonodeposition time was also investigated, and the electrode with longer deposition time showed higher electrocatalytic performance and stability.


초음파 전기증착법을 활용한 고효율 염소 발생용 루테늄 옥사이드 전극

트 란 루 레1, 김 춘수2, 윤 제용2*
1베트남 독일 대학교 메카트로닉스-센서 시스템 학과
2서울대학교 화학생물공학부 화학공정 신기술 연구소 & 아시아에너지환경지속가능발전 연구소

초록


    Ministry of Environment
    E617-00211-0608-0

    1.Introduction

    The chlor-alkali industry produces annually around 70 million tons of chlorine in the world, which is one of the most widely used electrochemical technologies. The chloralkali process is closely linked to energy and is the second largest consumer product in the electrolytic industry of about 240 billion kWh annually (Srinivasan et al., 2006). The Ruthenium oxide(RuO2) is a well-known electrode material for its excellent electrocatalytic features as a Dimensionally Stable Anode (DSA) (Luu et al., 2015a; Trasatti, 1984; Luu et al, 2015b). Roughly, 10-15% of the annual production of Ru is used for the preparation of DSA-type electrodes (Srinivasan et al., 2006).

    The RuO2 electrode is environmentally benign because it reduces the cost, time, and energy consumption and increases the chlorine evolution efficiency as well as its stability (Katerina et al, 2009). Various methods of synthesizing RuO2 electrodes have been developed such as sol–gel (Panic et al., 2005), thermal decomposition (Burrows et al., 1978), polyol (Terezo et al., 2002), Adams fusion (Ribeiro et al., 2008), reactive sputtering (Chou et al., 2013), and chemical vapor deposition (Han et al., 2010).

    In these methods, the desired catalyst loading can be achieved by several coating applications and calcination steps. Especially, the high temperature vapor-phase processes in reactive or chemical vapor deposition are expensive, and they are limited by their low yield. The electrodeposition method has been proven to be simple, versatile, one-step, film thickness controllable, and cost effective for electrode preparation (Vukovic et al., 1989; Metikos-Hukovic et al., 2006; Hu et al., 2009; Jowa et al., 2007; Tsuji et al., 2011; Burke et al., 1976). The charge transfer and the diffusion of supersaturated ions on the electrode surface, which are major mechanisms of crystal formation in electrodeposition, have been determined by this method (Macherzynskia et al., 2013).

    Electrodeposition of RuO2 based electrodes proceeds through wet chemical precipitation induced by a negative electrode-generating base (Zhitomirsky et al., 1997; Zheng et al., 2008; Zheng et al., 2008; Trieu et al., 2012). As the outer surface is increased, the chlorine over electro potential decreases, and an electro potential reduction in noble RuO2 can be achieved by the electrodeposition method.

    The electrodeposition route for the preparation of the RuO2 electrode is related to a local cathodic reaction to increase the pH near the electrode surface. The main cathodic reaction to produce OH is initiated from the decomposition of water under hydrogen evolution as follows (Zheng et al., 2008; Trieu et al., 2012):(1)(2)

    O 2  + 2H 2 O + 4e - 4OH -
    (1)

    2H 2 O + 4e - H 2 + 2 OH -
    (2)

    The above reactions consume H2O, generate OH, and then react with Ru ions to form Ru oxide. Hydroxide, or peroxide colloidal particles deposit on cathodic substrates. Ruthenium hydroxide and peroxide deposits can be converted into RuO2 by thermal treatment. The assumed reactions are represented as follows:(3)

    Ru 3+ + OH - RuO x ( OH ) y RuO 2
    (3)

    In addition, the cathodic decomposition of water competes with the cathodic electrodeposition of the Ru metal:(4)

    Ru 4+ + 4e - Ru o
    (4)

    In recent decades, the use of sonoelectrodeposition has increased rapidly in the production of various nanomaterials (Garcia et al., 2010a; Garcia et al., 2010b; Hyde et al., 2002; Walker et al., 1997; Mallik et al., 2011; Floaate et al., 2002; Garcia et al., 2002; Saez et al., 2004; Zheng et al., 2008; Lecina et al., 2012; Niu et al., 2012; Pollet et al., 2008; Pollet et al., 2011). It has been shown that the effect of high intensity sonication in electrochemical processes leads to both chemical and physical effects. Sonication decreases the thickness of the diffusion layer which consequently increases the limiting current. The increased limiting current affects the cavitation, micro- and macro-streaming, electrolyte concentration on the electrode surface, and the reaction rate (Garcia et al., 2010a; Garcia et al. 2010b; Hyde et al. 2002; Walker et al., 1997).

    In this study, the cathodic sonoelectrochemical deposition method is proposed for RuO2 synthesis. The properties of the electrodes synthesized by the sonoelectrodeposition method are compared with conventional ones synthesized by mechanical stirring electrodeposition.

    2.Experimental

    2.1.Electrode preparation

    RuO2 nanoparticles were deposited onto commercial Ti substrates by cathodic electrodeposition under sonication or mechanical stirring followed by calcination (Burke et al., 1976; (Macherzynskia et al., 2013; Zhitomirsky et al., 1997; Zheng et al., 2008). Ti foils (dimensions, 30 × 20 × 0.25 mm, purity 99.7%, Aldrich-Sigma, USA) were used as the substrate materials. The contaminants of the foil were removed by degreasing in acetone, and then, it was etched in boiling concentrated HCl at 86˚C for 1 h to produce a gray surface with uniform roughness. A Pt electrode (Samsung DSA, Korea) was used as the counter electrode.

    The electrochemical bath was prepared by dissolving 5 mM RuCl3 and 20 mM NaNO3 as the supporting electrolyte (99.9 %, Aldrich) in deionized water at room temperature (25˚C). Cathodic deposition was performed at a constant current density of 50 mA/cm2 controlled by a power source (Unicorn Tech, Korea) with ultrasonic treatment at 20 kHz (Branson, USA) or mechanical agitation at 200 rpm for different deposition times. Next, the electrode was calcined at 450˚C for 1 hr to allow the removal and formation of the functional metal oxide. The backside of the electrode is covered with epoxy to prevent exposure to the electrolyte.

    2.2.Microstructure characterization

    The amount of RuO2 nanoparticles deposited on the electrode surface was measured by the weight difference between the electrodes before and after the electrodeposition using a balance (Ohaus E02140, USA; Zhitomirsky et al., 1997).

    Microstructures on the electrode surfaces were characterized with a field emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL Co., Japan) and a transmission electron microscope (TEM, JEOL 2000EXII, Japan). The TEM samples were prepared by scraping off the coating using a sharp knife and dispersing the powders in isopropyl alcohol. High resolution X-ray diffraction patterns were obtained to study the crystal structure of the RuO2 electrodes (Bruker-AXS, Germany).

    2.3.Chlorine evolution and electrochemical measurement

    Chlorine was evolved by electrolysis in a two-electrode system with the following conditions: an acidic NaCl solution (0.1 M & pH 6) and a current density of 16.7 mA/cm2. The aqueous active chlorine concentration was determined by the DPD (N, N-diethyl-p-phenylenediamine) colorimetric method (Klug et al., 2001; Jeong et al., 2009; Ardizzone et al., 1990). The experiment was repeated in triplicated, and the average value with standard deviation was reported.

    The electrochemical characterizations of the RuO2 electrodes with cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were carried out in a conventional single compartment cell with three electrodes using a computer-controlled potentiostat (PARSTAT 2273A, Princeton Applied Research, USA) (Jeong et al., 2009). The volume of the electrolyte solution in the cell was 150 mL. RuO2/Ti, Pt (Samsung Chemicals, Korea), and Ag/AgCl (in saturated KCl) were used as the working electrode (anode), the counter electrode (cathode), and the reference electrode, respectively.

    The CV was measured in a 0.5 M H2SO4 solution as the electrolyte with the potential ranging between 0-1 V and the scan rate varying from 5-320 mV/s. The voltammetric charge q obtained by integration of the voltammetric curve is a measurement for the size of electrochemically active surface area, which is accessible by the electrolyte.

    The inner and outer surface areas were identified by plotting and extrapolating the voltammetric charges according to infinitely low (0) and fast (∞) scan rates, which was reported elsewhere (see more in the supporting information). The pseudo-capacitive reaction, which consists of coupled redox transitions involving proton exchange with the solution at a broad reversible peak around 0.6 V vs. Ag/AgCl, can be described as (APHA et al., 2005):(1)

    RuO x (OH) y  + zH +  + ze - RuO x-z (OH) y+z
    (1)

    LSV measurements were done in an electrolyte containing 5 M NaCl + 0.01 M HCl (pH 2) which is a favorable condition for chlorine evolution (Terezo et al., 2002). The stability of the prepared RuO2 electrodes was examined using the accelerated stability test (AST) with a considerably higher current density, and an electrolyte solution that is more dilute than those usually used in industrial electrochemical conditions. It provides information on the electrode stability and lifetime (via the electrode potential - time dependence at a constant current density) (Ribeiro et al., 2004; Panic et al., 1999; Aromaa et al., 2006). The experiments were performed galvanostatically at a current density of 1 A/cm2 in a solution of 0.5 M NaCl, pH 2 at room temperature (25˚C). The potential of RuO2 electrode was recorded during electrolysis.

    3.Results and discussion

    3.1.Chlorine evolution properties of RuO2 electrode by sonoelectrodeposition method

    As shown in Fig. 1 (a), the efficiency of the chlorine evolution at Sono-10’ produced by sonoelectrodeposition increases to 17.1% compared with Convention-15’ produced by conventional stirring electrodeposition.

    The chlorine evolution efficiency increases up to 17.1% in the case of the sonoelectrodeposition RuO2 electrode (171 mg/L) compared to the one made by the conventional stirring electrodeposition (146 mg/L) which was fabricated with the same deposition time of 15 min. Moreover, the chlorine concentration of the RuO2 electrode produced by sonoelectrodeposition after 10 min is also higher than that prepared conventionally for 15 min.

    As shown in Fig 1 (a), the increases in sonoelectrodeposition time induced higher chlorine evolution performance. Also, the higher current density in LSV (Fig 1. (b)) was observed at the electrode fabricated with longer sonoelectrodeposition time, which supports the result in Fig 1 (a). The current density of each method begins to increase steadily above 1.2 V, which indicates that chlorine generation reaction occurred above 1.2 V. The current densities extracted from the linear sweep voltammograms in the region of chlorine evolution is higher in the electrodes made by sonoelectrodeposition than in the ones produced by conventional stirring electrodeposition. The efficiency of chlorine generation is related to the active surface area, which contributes to the electrochemical reaction.

    Clearly, sonoelectrodeposition has a positive role on improving the electrocatalytic performances of RuO2 electrodes for chlorine evolution compared to the conventional method.

    3.2.Surface characterization

    Fig. 2 shows the SEM (a) and TEM images (b) of the RuO2 surfaces made by sonoelectrodeposition and conventional deposition. As shown in Fig. 2 (a), the morphologies of these electrodes show different morphology. The electrode made by sonoelectrodeposition has higher roughness compared to the conventional method. The electrode made by sonochemical deposition contains various structures including big hemispheres, fine grains, and mushroom-like structures, while just a few pebble-like small spheres are deposited with the conventional method. This high roughness with sonoelectrochemical method could enhance the mass transport of chloride ions from electrolyte to the substrate, which is expected to be the main reason for increased electrocatalytic performance with sonoelectrochemical method in section 3.1.

    As the sonoelectrodeposition time increases, the roughness and surface coverage increase, and the uniformity of RuO2 increases. It means that the sonication strongly affects the surface morphology of the RuO2 electrode. Sonication removed all the hydrogen gas evolved from the surfaces, and it enhances the deposition process with well covered deposits (reference Supporting Information S1). The sonication wave makes the diffusion layer thinner and reduces the depletion of the electroactive surface near the substrate to form a hemisphere in the existing nucleus.

    Fig. 2 (b) shows the difference in the crystal size of the RuO2 nanoparticles produced by sonoelectrodeposition and conventional stirring electrodeposition. The water molecules in hydrous form are removed due to the annealing temperature. As a result, tiny sized RuO2 crystals with 7-10 nm in diameter while that of the conventional method is 20-25 nm with high agglomerates. The nanoparticles made by the sonoelectrodeposition method are smaller in size and more uniform and have a finer distribution than the nanoparticles made by the conventional method.

    From these observations, we postulated that the implosion of cavitation bubbles on the surfaces of the substrate, the acoustic streams enabling deagglomeration, and the activation of nucleation sites of RuO2 all lead to a fine dispersion of RuO2 nanoparticles on the surface. On the other hand, there is no difference between the crystal size of the electrode made by sonoelectrodeposition after 10 and 15 min. It means that the sonoelectrodeposition time does not affect the crystal size of the RuO2 electrodes (Pollet et al., 2011).

    Fig. 3 shows the diffraction patterns of the RuO2 electrodes made by sonoelectrodeposition and conventional stirring electrodeposition after 15 min. The typical peaks of the rutile RuO2 metal oxide easily detected.

    The XRD spectra suggest that the oxide nanoparticles in all cases are fine and have a polycrystalline structure, which is desirable in terms of electrode stability. The peak for Ru metal is shown for the electrode made by conventional stirring electrodeposition, but it is not present for the electrode produced by the sonoelectrodeposition method. An electrode containing Ru metal is undesirable because it makes the electrode sensitive to corrosion during electrolysis. The Ru metal deposition is suppressed in sonoelectrochemical method, owing to the favored H2 generation in sonic ation.

    Overall, the applied sonication has a significant positive influence on the resulting phase formation of RuO2 electrodes.

    3.3.Active surface area of RuO2 electrode by sonoelectrodeposition method

    Fig. 4 shows the extrapolated voltammetric charges (a) and cyclic voltammograms (b) from a set of RuO2 electrodes made by sonoelectrodeposition and conventional stirring electrodeposition. As shown in Fig. 4 (a), the voltammetric charges (active surface areas) decrease with the increase in the scan rate. It indicates that it is difficult for the electrolyte to penetrate the inner surface of the electrode through structures such as micro pores, micro cracks, and grain boundaries.

    The total and outer voltammetric charges for the 15 min sonoelectrodeposition electrode were 32.5 mC/cm2 and 23.9 mC/cm2 while the values for the conventional electrode were 22.2 mC/cm2 and 14.2 mC/cm2, respectively. The higher voltammetric charges of the electrodes due to the sonoelectrodeposition method can be attributed to the higher roughness factor, hemispheres, mushroom-like morphology, and decreased crystal sizes.

    The total and outer voltammetric charges increased as the sonoelectrodeposition time was increased from 10 to 15 min. This result is due to the increased roughness factor and the amount of deposited RuO2. The voltammetric charges of the electrodes due to sonoelectrodeposition after 10 min were also higher than those for the conventional electrode after 15 min. This result indicates that the outer active surface area of the RuO2 electrode is more important than the total active surface area for the chlorine electrocatalytic activity (Burke et al., 1979; Trasatti et al., 1987; Chen et al., 2012a). The outer surface area is the main working part for chlorine evolution. The inner surface is blocked by adherent chlorine gas bubbles and becomes partially inactive (Chen et al., 2012b; Zeradjanina et al., 2012).

    Fig. 4 (b) shows the cyclic voltammetry curves of sonoelectrodeposited RuO2 electrodes. The current densities in the cyclic voltammetry are enhanced on the sonoelectrodeposited RuO2 compared to the conventional one. It means that the electrodes made by sonoelectrodeposition have better electrochemical properties. The rectangular shape of the cyclic voltammetry of the RuO2 electrodes remains unchanged with the scan rate (Data not shown). The current densities extracted from the cyclic voltammograms in Fig. 4 (b) are very similar to the voltammetric charges shown in Fig. 4 (a) of these RuO2 electrodes.

    3.4.Sonoelectrodeposited amount of RuO2

    The electrodeposition experiments revealed the formation of black deposits on all the Ti substrates. The amount of RuO2 nanoparticles deposited on the electrode surface can be measured by the film thickness or the increase in the weight of the electrode after and before deposition. An accurate measurement of the RuO2 film thickness was not possible due to the rough morphology of the film. Therefore, the deposited weight (mg/cm2) of the RuO2 film was measured instead of the thickness (Zhitomirsky et al., 1997).

    Fig. 5 shows the weights of the RuO2 films made by sonoelectrodeposition and conventional stirring electrodeposition for different deposition times. It was observed that the deposited weight was increased at longer sonoelectrodeposition time. However, the deposited weight did not significantly increase at the sonoelectrodeposition time above 15min, which might be caused by a lack of reactive species in the electrolyte.

    The weight of the RuO2 nanoparticles deposited by sonication for 10 min was 0.20 mg/cm2. This value is close to the weight of the RuO2 nanoparticles produced by the conventional stirring method for 15 min, which was 0.21 mg/cm2. On the other hand, the weight of the RuO2 nanoparticles deposited by sonication for 15 min was 0.30 mg/cm2. It is higher than that for the conventional one. This result means that sonoelectrodeposition produces a larger amount of RuO2 nanoparticles at the same amount of time. This might be caused by the removal of the H2 bubbles, which enhances the mass transfer and the deposition process rates (Walker et al., 1997; Mallik et al., 2011; Pollet et al., 2011).

    3.5.Accelerated stability test (AST) of RuO2 electrode by sonoelectrodeposition method

    Considering the industrial application of chlorine generating electrode, the stability should be considered. Therefore, the electrode lifetime was measured by stability test, which is defined by the time at which the potential of an electrode suddenly escalates under galvanostatic conditions in simultaneous oxygen and chlorine evolution reactions.

    Fig. 6 shows the time dependencies of the electrode potential and the appropriate differential curves for the RuO2 electrodes prepared by sonoelectrodeposition and conventional stirring electrodeposition. The potential seems to be slightly decreased over the initial period, and this result is related to the activation process. The life time of the RuO2 electrodes made by the sonoelectrodeposition method (90 and 100 min) are longer than that for the electrode made by conventional stirring electrodeposition (60 min) under the same electrolysis condition.

    The electrode stability increased as the sonoelectrodeposition time was increased from 10 to 15 min. This phenomenon can be explained by the mass coverage enhancement.

    In general, the failure of the RuO2 electrode is explained by three mechanisms, which are the growth of an insulating TiO2 layer at the coating (a substrate interface that becomes less doped with the catalytic oxide), the removal of the catalytic material by intense gas production, and the dissolution of Ru during long-term electrolysis (Ribeiro et al., 2004; Panic et al., 1999; Aromaa et al., 2006).

    The higher stability of electrode with sonoelectrodeposition method might be ascribable to the morphology of the surface. The RuO2 layer produced by this method is denser than the layer fabricated by conventional one, which makes lower penetration rate of electrolyte into Ti substrate. Thus, it enables the formation of a less non-conductive intermediate TiO2 layer compared to the conventional electrode. Overall, the stability of the electrodes made by sonoelectrodeposition is higher than that made by conventional stirring electrodeposition.

    4.Conclusions

    This study reports on the sonoelectrodeposition of RuO2 electrodes for chlorine generation. The electrode obtained by the sonoelectrodeposition method shows strong improvement in the electrocatalyst efficiency and stability for chlorine generation compared to the conventional stirring electrodeposition method.

    Sonoelectrodeposition increases the external surface area of the RuO2 electrode. In addition, this technique provides RuO2 electrodes with broad hemispheres and mushroom-like and compact structures. It also produces a large amount of RuO2 nanoparticles uniformly with a smaller size. Furthermore, the electrodes produced by the sonoelectrodeposition method do not contain undesirable Ru metal. Finally, the most important effect is the large increase of mass transfer to reduce the diffusion layer.

    Acknowledgements

    This research was supported by Korea Ministry of Environment as “Global Top Project (E617-00211-0608-0)" and a grant (code 17IFIP-B065893-05) from the Industrial Facilities & Infrastructure Research Program funded by the Ministry of Land, Infrastructure and Transport of Korea government.

    Figure

    JKSWW-31-397_F1.gif

    Chlorine concentrations (a) and LSV (b) of the RuO2 electrodes prepared by sonoelectrodeposition and conventional stirring electrodeposition. Sono-x’ and convention-x’: where x indicates the deposition time (min). (Experimental conditions: (a) 0.1 M NaCl, pH 6, 10 min; (b) 5 M NaCl, pH 2).

    JKSWW-31-397_F2.gif

    SEM (a) and TEM (b) images of the RuO2 electrodes made by sonoelectrodeposition and conventional stirring electrodeposition. Sono-x’ and convention-x’: where x indicates the deposition time (min).

    JKSWW-31-397_F3.gif

    XRD spectra of the RuO2 electrodes made by sonoelectrodeposition and conventional stirring electrodeposition. Sono-x’ and convention-x’: where x indicates the deposition time (min).

    JKSWW-31-397_F4.gif

    Voltammetric charges (a) and cyclic voltammograms at 320 mV/s (b) in 0.5 M H2SO4 for RuO2 electrodes made by sonoelectrodeposition and conventional stirring electrodeposition. Sono-x’ and Convention-x’: where x indicates the deposition time (min).

    JKSWW-31-397_F5.gif

    Weights of RuO2 deposits made by sonoelectrodeposition and conventional stirring electrodeposition for different deposition times with a constant current of 50 mA/cm2. Sono-x’ and Convention-x’: where x indicates the deposition time (min).

    JKSWW-31-397_F6.gif

    The AST of the RuO2 electrodes prepared by sonoelectrodeposition and conventional stirring electrodeposition. Sono-x’ and Convention-x’: x indicates the deposition time (min). (Experimental conditions: 0.5 M NaCl; pH 2; 25oC; 1 A/cm2).

    JKSWW-31-397_S1.gif

    Cathodic polarization curves of RuCl3 aqueous solution under sonoelectrodeposition and conventional stirring electrodeposition.

    Table

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