Flowchart of the coating preparation procedure. Route A: wet chemical route; Route B: electrodeposition. Thick coatings can be obtained by reduplicative coating / sintering operation for Route A, or by prolonging the electrodeposition time with the subsequent one-step thermal sintering for Route B. 

Contexts in source publication

Context 1

... the surface bridging O atoms (O br ) determine its catalytic reactivity. Therefore, electrocatalyst coatings should be prepared with increased exposure of active sites to the reactant species. This can be achieved by using nanoscale catalysts [31], by creating porous structure and by controlling the coating microstructure and surface morphologies [15], as discussed below. RuO 2 -based DSA electrodes are also involved in many technological applications, such as H 2 O electrolysis, electro-organic synthesis and electrochemical oxidation [32– 34]. In addition, RuO 2 is the key catalytic component for the Deacon process. A de- tailed discussion for the recent development of stable RuO 2 -based Deacon catalysts by Sumitomo and Bayer MaterialScience can be found in the literatures [3,4,10]. A com- prehensive review paper covering the versatile role of RuO 2 as oxidation catalysts in the fields of heterogeneous catalysis as well as electrocatalysis and the atomic scale understanding of physicochemical properties of RuO 2 has been published recently by Over [12]. Wet chemical routes such as thermal decomposition and sol-gel methods are most widely used techniques. An oxide coating is deposited onto a substrate by wet coating of precursor solutions such as brushing, dipping and subsequent thermal treatment. The catalyst particles are thus immobilized onto the conductive substrate. The electrochemical synthesis of metal oxides is achieved by passing an electric current between the electrodes. The synthesis takes place at the electrode / electrolyte interface. An overview by Therese and Kamath compared the technical features of various electrodeposition processes such as anodic deposition, cathodic deposition and their applications in the synthesis of oxide materials [35]. The reaction mechanisms involved in several preparation routes of thermal decomposition, sol-gel and cathodic electrodeposition in the fabrication of RuO 2 -based oxide coatings are summarized in this section. DSA are fabricated traditionally by thermal decomposition technique, in which oxide is obtained by thermally assisted breaking of chemical bonds of the precursor salts (Ru − Cl bonds for RuCl 3 , for instance) and the subsequent formation of M − O bonds in O 2 -containing atmosphere [36]. Structural change from the chloride precursor salts to their respective oxides during the thermal decomposition processes have been investigated in detail for RuCl and IrCl by extended X-ray absorption fine structure [37]. The coordination environment (from M Cl to M O) and coordination numbers of the Ru and Ir cations change with the variation of sintering temperature. The structure change occurs at 250–300 ◦ C and the final RuO 2 , IrO 2 rutile structure is formed at about 450 ◦ C. In the preparation of mixed oxide, due to the difference in the temperature for structure change among different precursor salts ( i. e. , the different kinetics of thermal decomposition of various precursors), some salts can be converted to oxide faster than others and this will result in a heterogeneous microstructure with mixed clusters of individual composition rather than solid solution [38]. This could ex- plain the formation of mixed phase for the thermal decomposition prepared commercial Ru 0 . 3 Ti 0 . 7 O 2 coatings (Table 2 in Sect. 5) [15]. In addition, the actual chemical composition of mixed oxide may deviate from its nominal composition during the thermal decomposition in case that some precursors will evaporate at high temperature rather than decompose [39]. For instance, distinct off-stoichiometry was observed commonly for the SnO 2 -containing oxide coatings prepared by conventional thermal decomposition route [40]. This fact can restrict the ability of thermal decomposition route to explore the optimized combination of noble metal oxides and valve metal oxides. We found that the sol-gel Ru 0 . 3 Sn 0 . 7 O 2 coating shows superior performance to the thermal decomposition Ru 0 . 3 Ti 0 . 7 O 2 coating [15], as discussed in Sect. 6. Sol-gel technique is superior to thermal decomposition technique to obtain coatings with improved stability [41]. In general, sol-gel is particularly suitable to fabricate mixed oxides because molecular level homogeneity of M − O − M networks can be obtained through the controlled hydrolysis (M − OH is formed by ligand exchange reaction) and condensation reactions (the formation of M − O − M with the release of H 2 O, ethanol etc .) [42]. For the preparation of mixed oxides, chelating agents can be used to control the competitive hydrolysis and condensation reaction rates of different precursors [43]. The synthesis of pure phase oxides with the formation of solid solution can be easily achieved. For the wet chemical routes, due to the molar concentration of the precursor solution (the solubility of precursors in solvent) is limited to assure a stable and homoge- neous solution, multiple coating / thermal treatment cycles are needed (Fig. 3, route A) to gain a thick oxide coating (a few micrometers in thickness) to meet the industrial demands of durability for years. It has been reported that the active surface area of electrode coatings increases with coating thickness until the coating surface reaches to a constant roughness [44]. Further increase in thickness can only prolong its service life. As an alternative, for electrodeposited coatings the coating thickness is dependent on the duration of deposition. Only one-step thermal sintering after electrodeposition is necessary to finish the preparation procedure (Fig. 3, route B). Note that the resistivity of the deposits is a factor limiting the development of thick coatings during electrodeposition. Chu et al. [45] reported that under galvanostatic control, the operating voltage keeps increasing during the electrodeposition due to the low conductive nature of the deposits in the preparation of mixed RuO 2 − TiO 2 oxide. To prepare RuO 2 − TiO 2 oxides with the electrodeposition route, the corresponding precursor salts need to be mixed and dissolved into electrochemical bath. Under galvanostatic or potentiostatic control, quasi-simultaneous electrodeposition of metal ions, M n + , or their complexes occur at the working electrode (Ti substrate) surface as thin film. For cathodic electrodeposition, it is considered that the electrochemi- cally produced OH (2H 2 O 2e H 2 2OH , E 0 83 V SHE) increases the local pH value and this is responsible for the formation of oxides or hydroxides (M n + + n ( OH ) − → M ( OH ) n ↓ ) [45]. Intermediate species such as Ti ( O 2 )( OH ) ( n 4 − − 2 n ) + , TiO 3 ( H 2 O ) x , Ru, Ru ( OH ) 4 , ( RuTi ) O x (OH) y , RuO 2 · x H 2 O may form depending on the preparation conditions and parameters [45–47]. These intermediates will be converted into oxide by the subsequent calcination under O 2 -containing atmosphere. Due to the difference in the redox potential among various M n + , the competitive deposition could happen and this will results in a heterogeneous microstructure with mixed phases [47]. Zhitomirsky et al. reported that the electrodeposition of Ru and Ti species follows independent mechanisms [48], which is also responsible for the structural heterogeneity. In addition, parasitic process such as the H 2 or O 2 evolution may happen, which has been exploited to prepare materials with special morphologies using H 2 or O 2 bubbles as dynamic templates [49]. RuO 2 -based electrocatalysts are used as coating form supported onto a metallic Ti substrate. The coating morphology can affect significantly the electrode performance such as the available active surface area, electrode deactiviation due to the passivation of the Ti-substrate [15] and also Cl 2 gas bubble evolution behavior [16]. DSA has a typical mud-crack surface morphology, as shown in Fig. 4a. The cracks are formed during thermal treatment due to the development of tensile stress [50] (Fig. 4c). The crack gaps may accommodate electrolyte and therefore may offer more available inner surface area. However, the penetration of electrolyte through the gaps may attack the underlying Ti substrate and result in the formation of an insulating TiO interlayer between the oxide coating and the substrate (Fig. 4b). The increase in the ohmic resistance of the oxide film can result in the loss of electrode performance before the complete electrochemical dissolution of the active Ru species, which will cause an ineffective use of Ru. In this section, two different strategies to control the electrode coating surface morphologies and thus to improve the coating performance were described. One way is to fabricate crack-free sol-gel coatings (Fig. 5), which are proven to be effective to avoid the direct contact of electrolyte with Ti-substrate. They are promising to prolong the electrode service life and to utilize Ru more effectively [51]. Another way to modify the RuO 2 − TiO 2 coating morphology is practiced by the electrodeposition route. A novel surface structure with Ru-containing spheres sitting on the top of the mud-crack layer was obtained (Fig. 6), which has the capability of increasing the outer active surface area (thus increasing the utilization of Ru) and improving electroactivity with reduced Ru content [47]. The electrode / electrolyte interface processes for the coatings with tailored surface morphologies were characterized by the in situ technique of electrochemical cyclic voltammetry (CV). The electrochemical contact of electrolyte with the oxide coating matrix could provide valuable information about the electrochemically accessible active surface, which is proportional to the integration area of the anodic branches in CV curves ( i. e. , voltammetric charge, q ). Furthermore, this could help es- timating the inner (such as inner gaps, pores) and outer surface area by changing the potential sweep rates ( ν ), since the electrochemical response rates of the inner / outer surface are ν ...

Context 2

... component for the Deacon process. A de- tailed discussion for the recent development of stable RuO 2 -based Deacon catalysts by Sumitomo and Bayer MaterialScience can be found in the literatures [3,4,10]. A com- prehensive review paper covering the versatile role of RuO 2 as oxidation catalysts in the fields of heterogeneous catalysis as well as electrocatalysis and the atomic scale understanding of physicochemical properties of RuO 2 has been published recently by Over [12]. Wet chemical routes such as thermal decomposition and sol-gel methods are most widely used techniques. An oxide coating is deposited onto a substrate by wet coating of precursor solutions such as brushing, dipping and subsequent thermal treatment. The catalyst particles are thus immobilized onto the conductive substrate. The electrochemical synthesis of metal oxides is achieved by passing an electric current between the electrodes. The synthesis takes place at the electrode / electrolyte interface. An overview by Therese and Kamath compared the technical features of various electrodeposition processes such as anodic deposition, cathodic deposition and their applications in the synthesis of oxide materials [35]. The reaction mechanisms involved in several preparation routes of thermal decomposition, sol-gel and cathodic electrodeposition in the fabrication of RuO 2 -based oxide coatings are summarized in this section. DSA are fabricated traditionally by thermal decomposition technique, in which oxide is obtained by thermally assisted breaking of chemical bonds of the precursor salts (Ru − Cl bonds for RuCl 3 , for instance) and the subsequent formation of M − O bonds in O 2 -containing atmosphere [36]. Structural change from the chloride precursor salts to their respective oxides during the thermal decomposition processes have been investigated in detail for RuCl and IrCl by extended X-ray absorption fine structure [37]. The coordination environment (from M Cl to M O) and coordination numbers of the Ru and Ir cations change with the variation of sintering temperature. The structure change occurs at 250–300 ◦ C and the final RuO 2 , IrO 2 rutile structure is formed at about 450 ◦ C. In the preparation of mixed oxide, due to the difference in the temperature for structure change among different precursor salts ( i. e. , the different kinetics of thermal decomposition of various precursors), some salts can be converted to oxide faster than others and this will result in a heterogeneous microstructure with mixed clusters of individual composition rather than solid solution [38]. This could ex- plain the formation of mixed phase for the thermal decomposition prepared commercial Ru 0 . 3 Ti 0 . 7 O 2 coatings (Table 2 in Sect. 5) [15]. In addition, the actual chemical composition of mixed oxide may deviate from its nominal composition during the thermal decomposition in case that some precursors will evaporate at high temperature rather than decompose [39]. For instance, distinct off-stoichiometry was observed commonly for the SnO 2 -containing oxide coatings prepared by conventional thermal decomposition route [40]. This fact can restrict the ability of thermal decomposition route to explore the optimized combination of noble metal oxides and valve metal oxides. We found that the sol-gel Ru 0 . 3 Sn 0 . 7 O 2 coating shows superior performance to the thermal decomposition Ru 0 . 3 Ti 0 . 7 O 2 coating [15], as discussed in Sect. 6. Sol-gel technique is superior to thermal decomposition technique to obtain coatings with improved stability [41]. In general, sol-gel is particularly suitable to fabricate mixed oxides because molecular level homogeneity of M − O − M networks can be obtained through the controlled hydrolysis (M − OH is formed by ligand exchange reaction) and condensation reactions (the formation of M − O − M with the release of H 2 O, ethanol etc .) [42]. For the preparation of mixed oxides, chelating agents can be used to control the competitive hydrolysis and condensation reaction rates of different precursors [43]. The synthesis of pure phase oxides with the formation of solid solution can be easily achieved. For the wet chemical routes, due to the molar concentration of the precursor solution (the solubility of precursors in solvent) is limited to assure a stable and homoge- neous solution, multiple coating / thermal treatment cycles are needed (Fig. 3, route A) to gain a thick oxide coating (a few micrometers in thickness) to meet the industrial demands of durability for years. It has been reported that the active surface area of electrode coatings increases with coating thickness until the coating surface reaches to a constant roughness [44]. Further increase in thickness can only prolong its service life. As an alternative, for electrodeposited coatings the coating thickness is dependent on the duration of deposition. Only one-step thermal sintering after electrodeposition is necessary to finish the preparation procedure (Fig. 3, route B). Note that the resistivity of the deposits is a factor limiting the development of thick coatings during electrodeposition. Chu et al. [45] reported that under galvanostatic control, the operating voltage keeps increasing during the electrodeposition due to the low conductive nature of the deposits in the preparation of mixed RuO 2 − TiO 2 oxide. To prepare RuO 2 − TiO 2 oxides with the electrodeposition route, the corresponding precursor salts need to be mixed and dissolved into electrochemical bath. Under galvanostatic or potentiostatic control, quasi-simultaneous electrodeposition of metal ions, M n + , or their complexes occur at the working electrode (Ti substrate) surface as thin film. For cathodic electrodeposition, it is considered that the electrochemi- cally produced OH (2H 2 O 2e H 2 2OH , E 0 83 V SHE) increases the local pH value and this is responsible for the formation of oxides or hydroxides (M n + + n ( OH ) − → M ( OH ) n ↓ ) [45]. Intermediate species such as Ti ( O 2 )( OH ) ( n 4 − − 2 n ) + , TiO 3 ( H 2 O ) x , Ru, Ru ( OH ) 4 , ( RuTi ) O x (OH) y , RuO 2 · x H 2 O may form depending on the preparation conditions and parameters [45–47]. These intermediates will be converted into oxide by the subsequent calcination under O 2 -containing atmosphere. Due to the difference in the redox potential among various M n + , the competitive deposition could happen and this will results in a heterogeneous microstructure with mixed phases [47]. Zhitomirsky et al. reported that the electrodeposition of Ru and Ti species follows independent mechanisms [48], which is also responsible for the structural heterogeneity. In addition, parasitic process such as the H 2 or O 2 evolution may happen, which has been exploited to prepare materials with special morphologies using H 2 or O 2 bubbles as dynamic templates [49]. RuO 2 -based electrocatalysts are used as coating form supported onto a metallic Ti substrate. The coating morphology can affect significantly the electrode performance such as the available active surface area, electrode deactiviation due to the passivation of the Ti-substrate [15] and also Cl 2 gas bubble evolution behavior [16]. DSA has a typical mud-crack surface morphology, as shown in Fig. 4a. The cracks are formed during thermal treatment due to the development of tensile stress [50] (Fig. 4c). The crack gaps may accommodate electrolyte and therefore may offer more available inner surface area. However, the penetration of electrolyte through the gaps may attack the underlying Ti substrate and result in the formation of an insulating TiO interlayer between the oxide coating and the substrate (Fig. 4b). The increase in the ohmic resistance of the oxide film can result in the loss of electrode performance before the complete electrochemical dissolution of the active Ru species, which will cause an ineffective use of Ru. In this section, two different strategies to control the electrode coating surface morphologies and thus to improve the coating performance were described. One way is to fabricate crack-free sol-gel coatings (Fig. 5), which are proven to be effective to avoid the direct contact of electrolyte with Ti-substrate. They are promising to prolong the electrode service life and to utilize Ru more effectively [51]. Another way to modify the RuO 2 − TiO 2 coating morphology is practiced by the electrodeposition route. A novel surface structure with Ru-containing spheres sitting on the top of the mud-crack layer was obtained (Fig. 6), which has the capability of increasing the outer active surface area (thus increasing the utilization of Ru) and improving electroactivity with reduced Ru content [47]. The electrode / electrolyte interface processes for the coatings with tailored surface morphologies were characterized by the in situ technique of electrochemical cyclic voltammetry (CV). The electrochemical contact of electrolyte with the oxide coating matrix could provide valuable information about the electrochemically accessible active surface, which is proportional to the integration area of the anodic branches in CV curves ( i. e. , voltammetric charge, q ). Furthermore, this could help es- timating the inner (such as inner gaps, pores) and outer surface area by changing the potential sweep rates ( ν ), since the electrochemical response rates of the inner / outer surface are ν -dependent [52]. Thus, the penetration character of oxide coatings for electrolyte can be evaluated by using CV measurements. For the preparation of crack-free sol-gel coatings, repetitive sol-gel dipping- withdrawing / sintering cycles were performed [51]. In this case, the tensile stress was relaxed through the plastic deformation for each single thin layer during thermal treatment [50]. By applying thin layers with diluted coating solutions, no cracks were formed, as observed in Fig. 5c. Thick crack-free coatings can be obtained by increasing the wet-coating / sintering cycle ...