3.2. Phase Characterization of Coatings
The same Sn-Sb-Ru-CoOx metal oxide coating was prepared on the two pretreated titanium sheets, respectively, and their morphologies are shown in
Figure 3. White granular crystals are distributed on the surface of the two kinds of titanium substrates, and the edges of the layered structure of the porous titanium sheet are wrapped by the crystals. From the enlarged view of “Area 1” in
Figure 3a,b, it is evident that the crystals on the porous titanium sheet are clearer and more uniform. Judging from the SEM micrograph of the coating on “Area 2”, the coating formed inside the porous titanium sheet is sized using nanometers, with a particle size of 22–64 nm, and an average value of about 40 nm; however, the coating on the flat titanium sheet still had crack defects, therefore, the denseness of the porous titanium sheet coating is significantly better than that of the flat titanium sheet. In addition, the pore structure greatly increases the specific surface area and the hydrophilicity of the substrate, which corresponds with the rapid penetration of the coating liquid during painting. The weight gain after sintering is 5–6 times the weight of the coating on the ordinary titanium sheet.
The different intensities of the XRD diffraction peaks of Ti (89-4893#), in
Figure 4, on the same crystal plane are caused by the differences between the types of titanium flakes. The porous titanium sheet shows more grains of a certain orientation due to its loose structure; therefore, the intensity of the diffraction peaks, which correspond to the crystal plane, is very high. The diffraction peaks of RuO
2 (88-0286#) were detected in the two kinds of titanium coatings. The diffraction peaks of RuO
2 were relatively weak due to the very strong diffraction peaks of the porous titanium substrate; however, in this case, the intensity of the diffraction peak of RuO
2 is similar to that of the flat titanium sheet, and only the (222) crystal plane is detected in the porous titanium substrate coating. It can be determined that the RuO
2 content in the porous titanium substrate coating is higher than that of the flat titanium sheet. Sn mainly appears in the form of Ti/Sn mixed oxides, such as (Ti
0.6Sn
0.4)O
2 (70-4406#); this is manifested in the strong diffraction peaks that are superimposed on the (002) crystal plane of the titanium matrix at approximately 38° and 82° (400) of the characteristic peaks of the crystal planes. It was indicated that Sn acts as an intermediate layer element that maintains a good chemical bond with the Ti substrate, which further increases and improves the adhesion of the coating on the substrate as a whole, and it prolongs the electrode’s service life. The individual oxide diffraction peaks of Co and Sb were not detected. The corresponding peaks of the mixed oxide, represented by CoSb
2O
6 (84-2602#), exist at around 28° and 63°. This causes the surface morphology of the coating to become a spherically stacked arrangement, which effectively releases the tension on the surface of the coating.
3.3. Electrochemical Catalytic Performance of Coated Electrodes
In
Figure 5, the original EIS curves of the coatings on the two titanium substrates, and the curves after fitting using the equivalent model, are shown.
Table 2 lists the parameters in the simulated circuit.
Among them:
Rs denotes the solution resistance;
Qf describes the internal cracking of the coating;
Rf is the resistance of the electrode itself;
Qdl is the number of active points on the surface of the electrode, and its value was directly proportional to the number of active points loaded on the electrode surface;
Rct represents the electrocatalytic activity of CER (chlorine evolution reaction) on the electrode, and its value is negatively correlated with the chlorine evolution activity of the electrode [
16].
The impedance arc of the flat titanium sheet electrode is about 2.3 times higher than that of the porous titanium sheet electrode. The impedance arc shows that the electron transfer process on the surface of the porous titanium sheet coating is blocked less frequently during the reaction, and the number of electron transfers per unit of time is larger. Similarly, the Qdl of the porous titanium flake coating is 50 times higher than that of the flat titanium flake coating, and the surface has unusually abundant active sites and better chlorine evolution performance. Ignoring the resistance of the copper wire connected to the test electrode, it was found that the impedance value of the porous titanium substrate electrode is smaller. This is due to the better bonding force between the active coating and the titanium substrate, and the tight bonding between the substrate and coating is, to a certain extent, reduced by interfacial resistance.
Qdl is coupled with the charge transfer resistance, and the double layer capacitance,
Cdl, can be evaluated using Formula (1):
where
n accounts for the deviation from ideal behavior. For perfect capacitors,
n = 1.
Cdl can be used as a relative measure of the surface area of the electrode [
16].
Table 2 lists the
Cdl and roughness factors (
RF) of the anodes. The
RF may be calculated using Formula (2):
where
C*, an assumed reference value for the capacitance, is proposed to be 20 Μf cm
−2 for smooth Hg electrodes. The
RF values that are normally observed in anodic oxide coatings frequently occur due to their characteristic morphology.
In the cyclic voltammetry curve, the capacitance area is usually defined as the electrochemically active surface area of the coated electrode in the chlorate electrolyte. Theoretically, the electrode with the lowest chlorine evolution potential should have more active sites and higher chlorine evolution activity. The capacitance area of the CV curve shown in
Figure 6 is 1.018 × 10
−4 (flat plate) and 1.707 × 10
−3 (porous plate), respectively. Such high activity is mainly attributed to the layered superposition of its coatings. During the sintering process of the coating, when the experimental temperature is higher than the phase transition temperature, the chloride in the coating liquid is converted into oxides, which subsequently shrinks in the pores. The large area of contact between the coating and the substrate provides strong adhesion. During the cooling process of the second coating, when the temperature is lower than the phase transition temperature, the coating liquid will change and shrink into oxides. The corrosion pits of the metal oxide on the flat titanium sheet expand and squeeze each other; although, if it can improve the bonding force of the substrate, it will produce micro-cracks due to extrusion stress. There is enough space for oxide growth in the pores of the porous titanium sheet. Even if microcracks are formed, a large area can be covered by the coating liquid. In fact, this enhances the bonding force between each layer of coating.
As shown in
Figure 7a, at a current density of 500 A/m
2, the chlorine evolution potentials of the prepared coated anodes are 1.296 V (flat plate) and 1.176 V (porous plate), respectively. Corresponding to the constant current polarization curve in
Figure 6b, the working potentials of the two electrodes are very stable, with only slight fluctuations. When comparing the chlorine evolution potentials of these anodes, it is found that the potential of the porous titanium substrate is 121 mV lower than that of the flat titanium, which is a very large improvement. In the solution, the diffusion resistance of ions on the electrode surface is closely related to the distribution of active substances. In a sense, it profits from its porous characteristic, in that it is more hydrophilic. Moreover, the electric field distribution is more uniform on the surface with larger specific surface area; therefore, the large local ion concentration difference caused by tip discharge is effectively avoided. Thus, when porous titanium is used as the matrix, the diffusion resistance is smaller, which is conducive to the catalytic reaction.
3.4. Accelerated Life Test and Corrosion Resistance of the Electrodes
The electrode potential is recorded regularly during the accelerated life test, as shown in
Figure 8, and the potential change interval is shown in
Table 3. In the initial phases, the electrode potential will rise briefly and then drop slightly for a period of time, which is called the activation phase. Then the electrode potential floats in a small range for a long time, and the chlorine evolution is stable. Before the electrode is about to fail, the potential of the flat titanium sheet rises rapidly in a short period time, which eventually leads to failure. Here, the difference is that the porous titanium substrate electrode has three plateau periods in the failure stage, at 6 V, 7.5 V, and 9 V, respectively. After the potential rises to this plateau, the electrode will work stably for a period of time. A rise in voltage indicates an increase in electrode resistance, which may be due to the formation of a non-conductive oxide; however, the remaining active sites can still meet the demand for oxygen evolution activity, and moreover, non-conductive oxides are not continuously generated, thus, the electrode voltage remains stable for a period time. When the dissolution of the active material and the exposure of the substrate gradually increases, the electrode voltage rises again. In this experiment, after the abovementioned process was repeated three times (platforms I, II, and III), the electrode finally failed. This also shows that the failure process of the electrode is divided into multiple stages. The lower electrode potential corresponds with a lower cell voltage in the actual production process, which is more efficient and it saves more energy.
SEM and EDS analysis were performed on the failed electrodes.
Figure 9e is a digital photo of the disassembled electrode. Compared with the closed surface, the color of the working surface changed from dark gray to light gray, which is almost similar to its state before painting and sintering, thus indicating that the coating in this area has been dissolved and separated. It is consistent with the results of the morphology characteristics that are shown in
Figure 9a–d, as a result of using SEM. A significant number of the white granular RuO
2 crystals in the coating are lost, thus exposing the coating–substrate bonding surface. Furthermore, the bonding surface also becomes blurred due to the corrosion of corrosive ions. We also found that the boundary between the closed domain and the working domain is not an established as closed straight line, which may be related to the surface tension of the solution. The fine pores in the matrix show the migration path of the solution, which produces capillary action, and causes the solution to diffuse upward in the matrix.
The coating element composition of the electrode surface layer, a deeper layer, and an even deeper layer, after the accelerated corrosion failure, was detected by EDS. As shown in
Figure 10a, it was found that the surface area is almost composed of Ti and O, and only 0.307 wt.% of Ru remains. This implies that the surface coating was almost dissolved, and a part of the Ti substrate was exposed before it became oxide. The element S was detected due to the residual SO
42− in the sulfuric acid system. The reaction initially occurred on the surface of the electrode, and the active material in the coating was slowly drained under the action of the applied voltage. According to thermodynamic calculations [
17], RuO
2 will be oxidized to RuO
4 when the polar potential is higher than 1.157 V (40 °C, vs. SCE):
Subsequently, RuO
4 will further decompose:
The porous titanium sheet has a larger specific surface area than the flat titanium sheet, which provides more bonding sites and at the same time enhances the bonding force between the substrate and the coating. Meanwhile, the coating is flake-like with repeated superimposed and strengthened effects. It is because of this kind of electrode surface, with a large specific surface area, under the same apparent current density, that its actual current density is lower, and thus, the corrosion rate of RuO2 as a result of its surface coating is relatively low. Furthermore, it produces better electrochemical activity. With the prolongation of the electrolysis time, the active material on the surface layer is continuously decomposed and is moved away, and the remaining active sites on the surface of the flat titanium sheet are not enough to maintain the full activity of the chlorine evolution; thus, the electrode becomes invalid.
The dissolution of the coating inside the porous structure is slightly better than that of the surface. The content of Ti is about 20 wt.% less than that of the surface layer, and the proportion of O is increased by 20 wt.%. There is also about 1.537 wt.% Ru and a trace amount of Co. This shows that the oxygen evolution process mainly occurs at this interface. It is known that Sn and Sb are the intermediate layer elements that form compounds with the Ti matrix to enhance the bonding force [
18]; however, the Sn and Sb components in the coating solution were not detected, which indicates that the coating peels off after being corroded [
19]. When the electrode is working in the solution, the electrolyte will penetrate into the tortoise crack due to the siphoning effect, and O
2 will continue to be precipitated when the oxygen evolution reaction occurs [
20]. The precipitated gas will then produce tensile stress on both sides of the crack, and the resulting updraft will also have a scouring effect on the active coating and cause the coating to peel off [
21]. Due to the long-term operation of the electrochemical reaction, the bubbles experienced repeated generation, growth, and rupture processes; this meant that both sides of the crack and the coating surface were subjected to periodic impacts. When the peeling of the active coating reaches a certain limit, the coated electrode slowly loses its activity and becomes ineffective [
22,
23]. The advantages of the porous titanium sheet are as follows: firstly, because the coating layer is repeatedly superimposed during the sintering and oxidation process, the early peeling does not have a great impact on the electrode performance; secondly, the combination of the coating and the porous substrate is stronger than that of the flat titanium sheet, which further delays the failure of the coating, as a result of the peeling.
The amount of Sn, Sb, and Ti in the intermediate layer that could be detected in the deeper part of the porous sheet was also reduced compared with the shallow detection point, and the amount of Ru rose to 6.781 wt.%, thus indicating that the coating in the deeper part has not been destroyed. There were more S elements, thus indicating that corrosion occurred previously. The nano-particle-like coating inside the porous titanium sheet can effectively resist being filled by ions in the solution, and it had a certain blocking effect on the infiltration damage of corrosive ions and media, thus making the infiltration path tortuous, and prompting the corrosion speed to be reduced; this will extend the service life of the electrode. It is difficult to change from initial electronic conductivity to ionic conductivity, thus avoiding the change of the conductive mechanism of the coating and the resulted increase of potential; however, due to the dissolution of the active material in the outer layer, the Ti matrix in the deeper part was exposed and oxidized to non-conductive Ti oxide, therefore, the test cell voltage reached 10 V, and the electrode was considered to have failed. In the accelerated life test, this still greatly prolonged the life of the electrode to 8h 11min, before the electrode failed.
Combined with the test results, we propose the failure mechanism of the coating on the porous titanium sheet (
Figure 11): the dissolution of the active material in the outer layer, the corrosion and peeling from the middle bonding layer, and the exposure of the Ti matrix and the formation of oxides. The penetration corrosion of SO
42− ions and H+ ions, as well as the continuous precipitation and oxidation effect of O
2, are the main causes of corrosion.