Preparation and Characterization of Manganese Dioxide Electrodes For

J. Ind. Eng. Chem., Vol. 13, No. 4, (2007) 545-551

Preparation and Characterization of Manganese Dioxide Electrodes for Highly Selective Oxygen Evolution During Diluted Chloride Solution Electrolysis Hwan Young Song*, Nikolay B. Kondrikov**, Valey G. Kuryavy**, Young Hwan Kim*, † and Young Soo Kang*

*Department of Chemistry, Pukyong National University 599-1 Daeyeon-3-dong, Nam-gu, Busan 608-737, Korea **Department of Chemistry, Far Eastern State University, Vladivostok 690014, Russia

Received September 12, 2006; Accepted March 6, 2007

Abstract: Electrolysis of aqueous chloride solutions led to oxygen evolution reaction when using a prepared highly selective electrode. The evolution efficiency reached over 90 %. The anode electrode was prepared by modification of a conventional dimensionally stable anode (DSA) through surface covering with manganese dioxide. The result of this modification was an unusual change in the behavior of chlorine-hydrogen oxygen-hydrogen cells under diluted chloride solution electrolysis. This system satisfies the kinetic conditions of the chlorine evolution reaction (CER). The efficiency and physical properties of the electrode were characterized using cyclic voltammetry and partial polarization curves. The surface structure and morphology of the electrode were characterized using SEM and STM. In the absence of chlorine adsorption, the electrode surface containing MnO2 particles had an even distribution of MnO2 particles on the surface and particularities of its morphology. This morphology resulted in the low selectivity of this electrode for the chlorine evolution reaction.

Keywords: oxygen evolution reaction, manganese dioxide electrode, chloride solution electrolysis

Introduction no longer high, active and stable electrocatalysis for the 1) oxygen evolution reaction (OER) from solutions having An ultimate goal in electrocatalysis is optimization of specific compositions should be investigated to de- the surface properties of electrodes through judicious termine whether the reaction is continued actively under choice of electrode materials. Electrocatalysis has bene- severe conditions [4]. Significant interest in environ- fited from improvements in the properties of electrode mental and energetic problems has focused on hydrogen materials and better understanding of the structural de- production by seawater electrolysis using different kinds sign of the electrode surface [1,2]. Determining the se- of ocean energy, such as thermal, stream, wave, and solar lectivity of electrode materials for chemical reactions is energy, without chlorine evolution, which usually occurs one of the most important problems in electrocatalysis on conventional electrode materials, e.g., the hypochlor- [3]; e.g., for the general selectivity of electrocatalysis in ite production reaction [5-7]. the electrochemical oxidation of concentrated aqueous The most promising way to prevent the undesirable NaCl solution on the chlorine evolution reaction (CER). chlorine evolution process is to create electrode materials This behavior is usually connected with the selectivity of with high selectivity to the OER and to inhibit or sup- the electrode material for the CER, which is used widely press the CER based on the electrocatalytic approach. in industrial processes. While practical interest in im- This approach is related to modification of the electrode proving the activity of anodes for chlorine evolution is surfaces. Such electrodes have been created by modification of ruthenium-titanium oxide anodes and other types of dimensionally stable anodes (DSA) with a se- † To whom all correspondence should be addressed. lective form of manganese dioxide [8]. (e-mail: [email protected]) 546 Hwan Young Song, Nikolay B. Kondrikov, Valey G. Kuryavy, Young Hwan Kim, and Young Soo Kang

The anodic reaction for chlorine evolution has been per- electrode surface mediate the final heterogeneous chem- formed at oxide anodes (magnetite anodes), but, since the ical oxidation or reduction of the target substrate. The beginning of this century, carbon anodes, particulary converted surface groups are continuously recovered by Acheson graphite anodes, have been used. Graphite is electrochemical oxidation or reduction. Homogeneous not stable, but is oxidized to CO CO2, and is even worse, redox catalysis, which is also labeled as “mediated elec- also chlorinated to perchlorinated compounds (chlorine trochemical conversion”, makes use of the initial oxida- better). In 1968, Henry Beer invented the dimensionally tion or reduction of a soluble redox couple, for instance, stable anode, which consisted of a supporting titanium Mn (II)/Mn (III), which reacts in homogeneous reactions anode covered with a catalytic layer consisting mainly of with a soluble substrate. Heterogeneous and homoge- a mixture of TiO2 and RuO2. Due to the joint efforts of neous redox catalysises are frequently used in the field of Beer and the De Nora Company, the dimensionally sta- organo-electrosynthesis, where well-known chemical re- ble chlorine anode revolutionized the technology of dox methods are adopted for electrochemical practice in chloroalkali electrolysis. In particular, the exchange of closing loops by introducing continuous electrochemical amalgam technology by membrane technology, which recuperation of the redox reactants [9]. Among the vari- initially was enforced in Japan by legislation, profited ous metal oxides, manganese dioxide has a low over- from the already-existing technology of anodic chlorine potential for oxygen evolution [10,11]. The manganese evolution at DSAs. The generally accepted mechanism of dioxide layer on the electrode surface should be prepared electrocatalysis for anodic chlorine evolution by ruthe- by thermal decomposition of manganese nitrate, β-mod- nium dioxide at RuO2-coated titanium electrodes in- ification of MnO2 at the first, and by electrodeposition volves a change in the valency of the surface groups of technique from manganese sulfate solution, γ-modifi- RuO2. From the resulting pentavalent [Ru(V)] ruthenium cation of MnO2 at the second, and finally electro- oxide chlorides, chlorine is released by a chemical re- deposition from manganese chloride solution, amorphous action that returns the ruthenium to its original oxidation modification of MnO2 (so-called δ-modification). The state, written schematically as the following equations: last modification affects the highly selective properties related to oxygen evolution from the electrolysis of di- - - RuO2 + Cl ⇄ RuO2Cl + e luted chloride solution. In this present study, a highly selective MnO2 electrode was prepared by electro- RuO2Cl → 2 RuO2 + Cl2 deposition of manganese chloride solution and special modification of MnO2; the surface structure of the MnO2 The anodic chlorine evolution reactions through this electrode was studied using SEM and STM to elucidate type of electrocatalysis at carbon anodes and RuO2-acti- the microstructure of these mixed oxides and to provide vated titanium anodes occur at relatively low over- the basis necessary for interpretation of more extensive potential, even at the highest current density. Comparing electrochemical investigations. The selectivity of the the catalytic activity of different anodic electrocatalysts MnO2 electrode for oxygen evolution reaction was char- for oxygen evolution from caustic alkaline solutions, acterized using cyclic voltammetry and electrolysis RuO2 is certainly one of the most efficient catalysts. reactions. Unfortunately, the stability of the electrode under the electrolysis conditions is a critical problem. Its dis- solution occurs in a matter of hours in the alkaline Experimental electrolyte. Therefore, this study was concerned with the improvement of the electrode stability by mixing it with A titanium plate (99.99 %, 0.05-mm thick, 50 × 50 mm) TiO2 and SnO2 and finally coating with MnO2 on the sur- was purchased from Aldrich Chem. Co. RuCl3 (99.9 %), face of the pretreated electrode. Heterogeneous electro- TiCl4 (99.995 +%), and SnCl2 (99.999 %) were pur- catalysis of the anodic evolution of oxygen or chlorine is chased from Aldrich Chem. Co. and used without any only a special case of heterogeneous redox catalysis, further purification. Other reagents used in this study which is based on the initial oxidation of appropriate re- were of ACS reagent grade and used without any further dox systems that undergo rapid and easy electrochemical purification. The water used has a resistivity of 18.2 MΩ. conversion or recuperation followed by a secondary The solvents used in this study were of analytical grade chemical redox reaction with a substrate. This reaction and used without any further purification. technique, known as redox catalysis, has been used for Manganese dioxide plating was performed by electro- many years in two completely different procedures: het- deposition on the titanium plate, on which intermediate erogeneous redox catalysis and mediated electrochemical ruthenium dioxide and titanium dioxide were fabricated conversion. using a method described previously [3]. Sublayers were In heterogeneous redox catalysis, surface groups at the Preparation and Characterization of Manganese Dioxide Electrodes for Highly Selective Oxygen Evolution During Diluted Chloride Solution Electrolysis 547

Figure 1. STM images of a three-dimensional projection (360 × 360 nm) (a), two-dimensional projection (360 × 360 nm) (b), two-dimensional projection (72 × 72 nm) (c), and two-dimensional projection (22 × 22 nm) (d) of an electrodeposited MnO2 layer on a RuO2-TiO2 electrode, and a two-dimensional projection of selective RuO2-TiO2 electrode (180 × 180 nm) (e). needed to avoid an insulating manganese dioxide layer was measured by iodometric titration of chlorine and hy- on titanium during electrolysis in diluted chloride sol- pochlorite using commonly accepted methods in an un- utions. For this electrodeposition, the titanium plate was divided cell at 25 oC. Chlorine and hypochlorite were etched in concentrated HCl solution and then in 1 M converted into iodine by added iodide; it was titrated us- o aqueous oxalic acid solution at 90 C for 1 h. The RuO2- ing 0.01 M thiosulphate solution. In addition, the surface TiO2 intermediate covering was formed by immersing structure of the electrode was examined using an SEM the titanium plate into a solution of RuCl3 (30 mol%) and from Hitachi (Japan, S-2400) and an STM from Digital TM TiCl4 (70 mol%) to form the salt layer, drying at 60∼80 Instruments (U.S.A., Multimode SPM). The properties oC for 10 min for the formation of a layered covering, and chlorine evolution efficiency of the electrode were and thermal decomposition of the mixture covering at studied using cyclic voltammetry, gas chromatography, 380∼450 oC for 2 h in an oxidizing atmosphere of air and full and partial polarization curve methods. The sur- [8]. The preparation of RuO2-TiO2-SnO2 was performed face morphology of the electrode (STM images) was following the same procedure, but using a solution of monitored under ambient conditions at a current volume RuCl3 (25 mol%), TiCl4 (50 mol%), and SnCl2 (25 of 2 nA and a voltage between a tungsten needle (zoned) mol%). The same procedure was repeated three times for and the surface of the sample of 998 mV. Analysis of the the formation of three layers. elements of the electrode plating was studied using an The manganese dioxide layer was anodically deposited electron probe microanalysis (EPMA) system from on the RuO2-TiO2 intermediate covering of the titanium Shimatzu (Japan, EPMA-1600). plate at a constant current density of 0.1∼0.2 Acm-2 in a solution of aqueous MnCl2 (from 0.0016 to 0.238 M) and 0.274 M HCl at pH 0.0∼2.0 for 20∼120 min. A simple Results and Discussion type of glass cell without separation of the anode and cathode compartments was used for the anodic deposi- Elemental analysis of the prepared electrode was per- tion. A platinum-coated titanium plate was used as a formed using EPMA to determine the composed ele- counter electrode. The oxygen evolution efficiency was ments as Mn, Ti, Ru, Sn, and oxygen on each electrode. investigated by electrolysis at current densities from 0.1 The contents were 0.231(Mn), 0.432 (Ti), 0.224 (Ru), to 0.5 Acm-2 in 0.5 M NaCl solution. The oxygen evolu- 0.211 (Sn), and 0.998 (O). The surface morphology and tion efficiency was determined as the difference between roughness of the electrode were studied with STM and the total charge passed and the charge passed for chlorine SEM. STM images of the MnO2 layers on the RuO2-TiO2 evolution. The change for the CER during electrolysis electrode, which were prepared on the titanium plate 548 Hwan Young Song, Nikolay B. Kondrikov, Valey G. Kuryavy, Young Hwan Kim, and Young Soo Kang

Figure 2. SEM images of (a) the surface layer of a selective MnO2 layer deposited on the sublayer of an RuO2-TiO2-SnO2 electrode from 0.238 M MnCl2 and (b) the surface layer of a selective MnO2 layer deposited on the sublayer of an RuO2-TiO2 electrode from 0.238 M MnCl2 solution and (c) the surface of the RuO2-TiO2 electrode. Size bar, 100 mm.

were obtained in different sizes of dimensional projec- of MnO2 deposited on the sublayer of the RuO2-TiO2- tions, as shown in Figures 1(a), (b), (c), and (d); an STM SnO2 electrode from the MnCl2 solution, the sublayer of image of a two-dimensional projection of a selective the RuO2-TiO2 electrode, and the surface layer of the se- RuO2-TiO2 electrode (180 × 180 nm) is shown in Figure lective MnO2 layer deposited on the sublayer of RuO2- 1(e). The three-dimensional projection STM images of TiO2 electrode from the MnCl2 solution. The morphol- the MnO2 layer on the RuO2-TiO2 electrode in Figure ogy of the surface of the selective MnO2 layer deposited 1(a) shows that the electrode surface was formed evenly on the sublayer of the RuO2-TiO2-SnO2 electrode from with the MnO2 layer on the surface of the RuO2-TiO2 0.238 M MnCl2 solution possessed a smaller crack size; electrode. This image also shows that most parts of the the stability of this electrode system was better than electrode surface consisted of similar forms of blocks, of those of the electrodes obtained from the MnO2 layer de- which structures were developed from more shallow posited on the sublayer of the RuO2-TiO2 electrode and structures of equal forms. Therefore, the surface structure on the RuO2-TiO2 electrode itself from 0.238 M MnCl2 of the films was observed to be a homogeneously similar solution and on the same electrode surface from more di- structure for characterizing the fractal organization of the luted MnCl2 solution. Therefore, the highest selectivity substances [12]. There are three exposed levels of fractal for the oxygen evolution reaction and suppressing the systems for oxide ruthenium-titanium anodes (ORTA): chlorine evolution reaction was achieved from mod- the largest hills (300 nm sizes), second-level hills (30 ification of the electrode surface by changing the sub- nm), and first-level separate hills (clusters of 1 nm). A layers and electrodeposition conditions. The electrode of conducting percolating system of ORTA was formed MnO2 deposited on the surface of the RuO2-TiO2 or from nanoclusters with sizes of 1 nm, packing the vol- RuO2-TiO2-SnO2 electrode showed almost full suppre- ume of films by fractal method with fulfillment of at ssion of the undesired chlorine evolution reaction on the least three hierarchical levels. These results are clearly electrode surface. This result will be explained and dis- shown in Figures 1(a)∼(d). Therefore, the morphology cussed later; it is related to characteristics such as the of the manganese dioxide covering deposited from man- electrocatalytic properties and surface structure of the ganese chloride solution also depended on the morphol- electrodes. The highly selective properties of anodes de- ogy and character of the intermediate sublayers formed pend on many factors. Particularly, intermediate layers before electrodeposition. Especially, Figure 1(e) shows on the titanium support influence the selective properties an STM image of the manganese dioxide covering elec- of anodes. Usually, RuO2-TiO2 or RuO2-TiO2-SnO2 cov- trodeposited on the sublayer ORTA from the thermal de- erings are used as the intermediate layer for preventing o composition of RuCl3 + TiCl3 at 450 C for 10 min. The titanium oxidation. However, ruthenium-based ternary electrocatalytic properties of highly selective anodes de- oxides containing tin-oxide are an appropriate inter- pend on many factors. Particularly, the intermediate lay- mediate layer for electrocatalytic anodes for the selective ers on the titanium supports influenced the selective oxygen evolution reaction by suppressing the chlorine properties of the anodes. Usually, an RuO2-TiO2 elec- evolution reaction of seawater [13]. Chloroalkali electrode covering is used as the intermediate layer for pre- trolysis has been commercially performed for more than venting titanium oxidation. However, Ru-based ternary 100 years [9]. oxides containing tin-oxide have also been proved to be Figure 3(a) shows the current efficiency of the chlorine appropriate for the manufacture of the intermediate layer evolution reaction versus the electrolysis time in 0.5 M [9,13]. Figure 2 shows SEM images of the surface layers NaCl solution with electrodes prepared from different Preparation and Characterization of Manganese Dioxide Electrodes for Highly Selective Oxygen Evolution During Diluted Chloride Solution Electrolysis 549

(a) (b) Figure 3. (a) Current efficiency of the chlorine evolution reaction versus electrolysis time in 0.5 M NaCl solution on different compositions of selective MnO2 electrodes prepared from various concentrations of MnCl2 (■: 0.238 M; △: 0.0238 M; ▲: 0.00238 M; ○: 0.000238 M) on the sublayers of RuO2-TiO2 and (●: 0.238 M) on the sublayer of RuO2-TiO2-SnO2. (b) Current efficiency of the chlorine evolution reaction of selective MnO2 electrodes prepared from 0.238 MnCl2 solution on the sublayer of RuO2-TiO2 plotted versus the electrodeposition time.

increased extraordinarily, particularly when the covering was formed on the titanium substrate with an intermediate sublayer of Ru-Ti-Sn-oxides. The effects of the concentration of manganese chloride, the current density, and the electrodeposition time and temperature were studied and optimal conditions for selective electrode formation were determined. The process of coating and the electrodeposition time can lower the evolution of active chlorine. The characteristics and morphology of the selective coatings depend on the concentration of manganese chloride solutions as well as the nature and composition of the sublayers formed before electrodeposition covering. Figure 4 shows partial polarization curves of the chlor- Figure 4. Partial polarization curves of (1) the chlorine evolu- ine evolution reaction in 0.5 M NaCl solution on the tion reaction in 0.5 M NaCl solution on the electrode modified electrode modified with a selective MnO2 layer on RuO2- with a selective MnO2 layer on RuO2-TiO2, (2) the oxygen evo- TiO2, the oxygen evolution reaction on the unmodified lution reaction on the unmodified RuO2-TiO2 electrode, (3) the RuO2-TiO2 electrode, the chlorine evolution reaction on chlorine evolution reaction on the unmodified RuO2-TiO2 electrode, and (4) the oxygen evolution reaction on the electrode the unmodified RuO2-TiO2 electrode, and the oxygen modified with a selective MnO2 layer on RuO2-TiO2. evolution reaction on the electrode modified with a selective MnO2 layer on RuO2-TiO2. The partial polar- concentrations of MnCl2 on the different sublayers. ization curves for the chlorine and oxygen evolution re- Figure 3(b) shows the current efficiency of the chlorine actions are very different; the limiting currents of the evolution reaction of a selective MnO2 electrode pre- chlorine evolution reaction with the modified selective pared from 0.28 M MnCl2 solution on a sublayer of forms of MnO2 electrode were almost three times lower RuO2-TiO2 versus the electrodeposition time. As shown than those of the other modified electrode prepared in the in Figure 3(a), the current efficiency for the chlorine evo- proper concentration of MnCl2. The selectivity of the lution reaction decreased with increasing concentration oxygen evolution reaction in 0.5 M NaCl solution of manganese chloride solution. Therefore, the selectivity (imitating seawater) reached over 90 %. This result was of the oxygen evolution reaction under these conditions also proved by independent iodometric titration analysis 550 Hwan Young Song, Nikolay B. Kondrikov, Valey G. Kuryavy, Young Hwan Kim, and Young Soo Kang

As far as the chlorine volume increased and chlorine evolved in the anodic semicycle of the cyclic voltammo- gram, the peak current in the cathodic semicycle increased to some limit (curves 1 and 1'). The presence of current peaks can be explained by removing adsorbed chlorine-containing particles from the ORTA surface. Manganese dioxide anodes differ from ORTA anodes in the point that near the equilibrium potential, peaks of current are not observed (curves 2 and 3). This finding indicates that the adsorbed chlorine-containing products or semiproducts of chlorine electrolysis did not accumu- late on the surface of the dimensionally stable anode. From these results, it is concluded that the adsorption of chlorine atoms is the main cause of the exceptionally high selectivity of the ORTA electrode for the chlorine evolution reaction and the critical parameter. From the standpoint of thermodynamic electrolysis of water, chlorine solutions should lead to oxygen evolution rather than that of chlorine because the equilibrium potential of oxygen evolution is lower than that of chlorine. The standard electrode potentials of the two reactions are as follows:

- - Cl2 (g) + 2 e = 2 Cl Eo = 1.3583 V

Figure 5. Cyclic voltammograms of 0.5 M NaCl solution with + - 4H + O2 (g) + 4 e = 2 H2OEo = 1.2229 V the RuO2-TiO2 electrode by different chlorine volumes evolved in the anodic semicycle (1, 1'), with the RuO2-TiO2 electrode formed pyrolytically (2), and the galvanic MnO2 electrode (3) Local acidification near the anode layer and other kinetic at a scan rate of 0.1 V/s. conditions lead to the predominance of the chlorine evolution reaction, especially at oxide ruthenium anodes, of the gas composition using gas chromatography. The over a wide range of current densities [13]. In the kinetic activation energy of the oxygen evolution reaction for se- conditions of the chlorine and oxygen evolution re- lective modification of MnO2 was two times lower than actions, the relative polarization curve positions of these -3 -3 -2 that for β-MnO2 prepared by the pyrolytic method and two exchange currents, jo = 6 × 10 and, 6 × 10 Acm , ca. 10∼12 kJ/M lower in 0.5 M NaCl solution. The rea- correspondingly, are likely to be less important than the son for the selective reaction on the modified surface of relative values of the Tafel slopes. the dimensionally stable anodes by the amorphous manganese dioxide film is not clearly understood. A possible interpretation is that the electrode functions as a diffusive Acknowledgment barrier for chlorine ions while the active center of its surface remains for the oxygen evolution reaction [14]. It is This research was supported financially by the Brain assumed that the selective properties of amorphous films Korea 21 project in 2006. of MnO2 are the critical factor for blocking chlorine dif- fusion and the passage of oxygen. Figure 5 shows the cyclic voltammograms of the 0.5 M NaCl solution with the References RuO2-TiO2 electrode by different chlorine volumes evolved in the anodic semicycle (1, 1'), with the 1. S. Trasati, Electrochim. Acta, 45, 2377 (2000). RuO2-TiO2 electrode prepared pyrolytically (2), and the 2. (a) J. C. Shim, H. R. Rim, D. Y. Yoo, J. I. Park, J. galvanic MnO2 electrode (3) at scan rate of 0.1 V/s. The W. Kim, and J. S. Lee, J. Ind. Eng. Chem., 5, 177 cyclic voltammograms of the 0.5 M NaCl solution show (1999). (b) D. Kim and Y. Kim, J. Ind. Eng. Chem., significant differences between the oxide ruthenium-tita- 11, 579 (2005). nium anodes (ORTA) and the manganese dioxide anodes 3. N. B. Knodrikov, E. Y. Kiseliov, I. E. Ilyin, E. V. (MDA). From the reverse motion of the cyclic voltam- Shchitovskaya, V. P. Berdiugina, I. V. Postnova, O. mogram peak current near the equilibrium potential with O. Klimenko, and V. V. Belinsky, Electrokhimiya, ORTA, the chlorine ionization reaction is well observed. 26, 540 (1993). Preparation and Characterization of Manganese Dioxide Electrodes for Highly Selective Oxygen Evolution During Diluted Chloride Solution Electrolysis 551

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