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)

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