Development of Advanced RuO2-based Electrocatalysts for Electrochemical Chlorine Production

Abstract

Chlorine is produced by the electrolysis of brine solution (chlor-alkali industry) and now become one of the most important chemical in the chemical industry, pharmaceuticals, and wastewater treatment [1-3]. Chlor-alkali process represents one of the most energy- and resource-intensive technological applications of electrocatalysis. The growth rate of chlor-alkali industry in the world through has increased more than 350% since last 5 decades. The demand for Cl2 supply reaches about 65 Mtons in 2010 worldwide, and the overall energy consumption is 1.5×1011 kWh/year, while the most efficient membrane process consumes around 2600-2800 kWh/Cl2 ton [4]. The total energy consumption in the chlor-alkali process is proportional to the total cell voltage, including thermodynamic potential of anodic and cathodic reactions, electrode overpotential, ohmic drop from the electrolyte, membrane and bubble effect, etc. From a practical point of view, a possible efficiency improvement of the electrolytic chlorine production and reduce the energy consumption become a critical issue for the sustainable development with multilateral significance. This way provides highly environmentally benign electrical devices to address the problems of climate change, the impending exhaustion of fossils fuels [5]. Dimensionally Stable Anodes (DSA®) are now become the most popular electrode in chlor-alkali industry. These electrodes were discovered 50 years ago and are still described as one of the most striking and greatest technological breakthrough of the history of electrochemistry in the last century [6-10]. RuO2 is the main component in DSA® because of a very high catalytic activity and incredible stability for chlorine and oxygen evolutions [11, 12]. Most commonly, the active catalyst layer consists of RuO2 or mix metal oxide RuO2 + TiO2, RuO2 + IrO2 + TiO2 supported with metallic Ti as substrate material [13, 14]. It is critically needed to boost the energy efficiency in industrial processes by using selection electrocatalysts with improved performance [4, 5]. Thus it is a key for chemists to develop and evaluate new catalytic materials and accordingly new preparation routes to meet the continuous expansion of industrial requirements. Since their microstructures, phase compositions and surface morphologies are inhomogeneous, the electrocatalytic activities of these coatings are strongly dependent on the preparation conditions [13]. The thermal decomposition methods conducted with various preparation parameters such as types of solvents, precursors, and calcination times have led to the development of the RuO2 electrode for the chlorine evolution [14]. The enhanced electro-catalytic activity of RuO2 electrodes is attributed to the increased outer surface area, which is an easily accessible region for the electrolyte, and has become a significant factor in chlorine evolution [14-18]. Nevertheless, it has not fully been investigated on how these fabrication parameters affect to the chlorine evolution efficiency in the RuO2 electrode prepared by thermal decomposition method. In spite of remarkable electrocatalytic effects, there are remains a drawback in the conventional nanograins or nanospheres with zero dimensional effortlessly agglomerate under the fabrication or reaction conditions, which leads to low catalytic activity as well as the mass transport limitation [19]. The improvement electrocatalytic activity can be achieved by using the special nanostructure morphologies of RuO2-based electrodes which can increase the surface area as well as mass transfer, and so on favor the chlorine evolution. Especially one, two and three dimensional nanostructured materials such as nanorods, nanosheets or macroporous morphologies have been shown to exhibit unique properties such as high active surface area, fast mass transfer, charge/discharge process and high stability [20-22]. Previous studies suggested that attainment nanostructures of RuO2 nanorod or nanosheet electrodes have been obtained mostly through vacuum sputtering techniques [23-25] and chemical vapor deposition [26-28] in the form of thin films. In these methods, the high temperature (1500°C) vapor-phase processes are expensive and also limited by their low yield. In particular, the promising nanostructure materials as nanorod, nanosheet, macroporous RuO2-based electrodes which intend to have a controlled pore size without collapse of the wall structure can be simply prepared by employing soft templates based on surfactant has not been investigated so far for chlorine evolution. This process is easy, reliable, versatile, low cost and applicable to a wide variety of electrodes. In addition, many conventional methods were used for fabrication RuO2-based electrode as thermal decomposition [14], sol-gel [29], polyol [30], Adams fusion [31], electrodeposition [5], chemical vapour deposition [26-28], reactive sputtering [23-25]. These methods still having some drawback as low efficiency, by-product forming, high cost, require high temperature, high electrical energy and not environmental friendly. As a solution, using novel green fabrication methods as sonoelectrodeposition and microwave-assisted sol-gel which reduces the time, cost, energy consumption, and so on increase the chlorine electrocatalyst activity or stability should be examined. By these ways, the increasing in the utilization of active Ru species and reducing the use of Ru noble metal can be achieved.