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1.1. STATEOFTHEARTELECTRODES 7 structure, with sizes ranging from 10 nm to 30 nm[37–39]. The particles likely consist of a solid solution of RuO2 and TiO2, meaning that Ti and Ru cations are mixed on the atomic scale[40–42]. On the micrometer scale, the coating has a structure similar to that of dry mud, with large cracks separating more dense areas (the “cracked mud” or “mud crack” structure)[38, 39, 43]. These cracks have been found to facilitate chlorine bubble detachment, resulting in improved activity[44]. The nanoscale structure of the particles, specifically regarding whether any pre- ferred arrangement of Ru and Ti cations exists in the surface layer of these parti- cles, is still unknown. 1.1.2 Cathodes for HER Today, steel or Ti cathodes are used for HER in chlorate cells[5], while Ni or RuO2-coated cathodes are used in chlor-alkali cells[3, 12]. Ni cathodes are also used in industrially-sized water electrolysis plants. In both chlor-alkali and water electrolysis, the cathode is exposed to highly concentrated caustic solutions at a high temperature, as this accelerates the kinetics of the process and maximizes the electrical conductivity of the electrolyte[12, 13]. Under such conditions, Ni com- bines an acceptable activity with a sufficient stability. Nevertheless, Ni electrodes exhibit overpotentials of several 100 mV. This overpotential can be reduced signif- icantly by measures that increase the surface area of the electrode (e.g. in so-called Raney Ni). This does not change the per-site activity of the electrode, but improves the activity by exposing a larger number of active sites[13]. Furthermore, RuO2 deposited on Ni combines acceptable stability with a lower overpotential than that of smooth Ni, making it an alternative to Raney Ni[3]. The metal with the highest activity for HER is Pt[45, 46], but its limited stability in alkaline conditions and high cost prevents is use in industrial cells. However, an al- ternative process for water electrolysis is based on cells using polymer electrolyte membranes (PEM) similar to those used in fuel cells[12, 47]. In this case, the cathode material can be graphite, usually with Pt deposited on the graphite. Here it is also possible to apply other activated cathode materials, e.g. sulphides, oxides or alloys of several elements (see Trasatti [13]), that have insufficient stability in alkaline solutions. MoS2 deposited on graphite is one such material. While it has been known that a variety of sulphides, including MoS2, are active for HER[13], an increased interest in this material probably originated in the theoretical study of Hinnemann et al. [48], which indicated that edge sites in MoS2 should exhibit a similar electrocatalytic activity as Pt. This might enable electrolytic hydrogen pro- duction at lower overpotentials than those of Ni without use of noble metals. The correlation between the activity of MoS2 and the edge length was experimentally validated by Jaramillo et al. [49]. The details of the mechanism of HER on this material is discussed in one part of the present thesis[20]. However, MoS2 has of yet not been studied in a PEM electrolyzer, and electrolysis at industrially relevantPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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