Abstract
The effect of surface etching by H2C2O4 and H2SO4 on Ti substrates on the electrochemical properties and surface morphologies of titanium oxide anode was investigated through different etching methods. The scanning electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy were used to analyze the specimen structure. The electrocatalytic activity and the electrochemical stability of the specimens were also evaluated by the electrochemical workstation and accelerated lifetime tests, respectively. Results show that the dual acid etching can achieve denser and more homogeneous surface with better catalytic stability. In addition, the influence mechanism of pretreatment on the lifetime of Ti anodes was discussed. The catalytic activity and stability of IrO2-Ta2O5/Ti anodes are strongly dependent on the sequence of acid etching and the surface structure of anodes, and thereby the relationship between the pretreatment methods and the anode performance is established. The dual acid etching can achieve a Ti surface with moderate roughness, therefore improving the coating adhesion. The titanium hydride formed through the dual acid-treatment is transformed into the rutile with barely changed surface morphology, which is conducive to the electrons transport. Therefore, the coating adhesion is enhanced and the accelerated lifetime is prolonged.
Science Press
In the production of copper foil for lithium-ion battery, reducing the electrolytic cell voltage and power consumption is important to decrease the production cost. The oxygen evolution reaction (OER) occurs on the anode of electrolytic cell, and the over-potential of OER is the main cause of the high cell voltage. Compared with other anodes, such as PbO2 and graphite, the titanium anodes of size-stability type with precious metal coating show better performance[1-3]. These dimensionally stable anodes (DSAs) have better properties in electrocatalytic activity and service life than the traditional anodes do, such as graphite and lead alloys. Among them, the IrTa-coated Ti-based anode (IrO2-Ta2O5/Ti) has been rapidly developed in the past decades due to its high stability and extreme durability under aggressive operation conditions. However, due to the high current density required for electrolysis of copper foil, the IrO2-Ta2O5/Ti anode is easily in contact with the oxygen, leading to the oxidation of Ti matrix and the abnormal failure of anode[4,5]. It is found that Ti substrate is usually oxidized during heating, and thus a TiO2 film layer with poor conductive is formed on the Ti substrate surface, which results in high cell voltage and short service life[6]. Based on these results, the coating process of the precious metal anode needs to be further improved to reduce the possibility of abnormal anode failure and to prolong the service life of the anode.
As reported in Ref.[7-10], the surface layer structures of Ti substrates etched by hot HCl, HF, and boiling oxalic acid solutions are modified to achieve better adhesion between the substrate and IrO2-Ta2O5 coating layer. The relative content of TiH2 on the Ti surface is independently determined by X-ray diffraction (XRD)[11-14]. The surface analysis of the etched anodes shows that TiH2 on the surface of Ti substrate is conductive to the improvement of electrocatalytic activity of the anodes.
Therefore, in this research, the Ti substrate was firstly treated by oxalic acid and sulfuric acid etching in order to reduce or even eliminate the negative effect of TiO2 film layer which is generated during the heating process. After that, the IrO2-Ta2O5-coated anodes were prepared by thermal decomposition method. Furthermore, the morphology, phase component, and electrochemical properties of these anodes were examined to distinguish the influence of the pretreated Ti substrate on the IrO2-Ta2O5-coated anodes.
The Ti sheet (TA1) was purchased from Xi'an Baotai Co., Ltd, and it was cut by laser cutting into the specimens with dimension of 60 mm×80 mm×1 mm. The coating solution was obtained by mixing H2IrCl6, Ta2Cl5, and HCl in n-butanol solution. The electrochemical test was performed in 0.5 mol/L H2SO4. The accelerated lifetime test was conducted in 1 mol/L H2SO4. All the chemical reagents were at analytical grade without further purification.
In order to remove the oxide layer and to obtain a larger surface area of the Ti flakes, all the Ti flakes were sandblasted. Ti sheets were ultrasonically cleaned with deionized water. Then the plates were etched in 10wt% H2SO4, 10wt% H2C2O4, or 10wt% HCl solution at 90 °C for 2 h to form a rough surface. Finally, the pretreated Ti substrate was placed in ethanol solution at room temperature to prevent oxidation.
The metal oxide coating was prepared by conventional pyrolysis method. The mixed solution of H2IrCl6 and Ta2Cl5 was diluted by the n-butanol solution with the molar ratio of 7:3. After stirring for 30 min, the precursor was obtained. Then, the pretreated Ti sheets were painted with the precursor solution through a brush. After drying at 60 °C for 3 min, the solvent evaporated. Then the plates were heated in a muffle furnace at 500 °C for 15 min. The painting process was re-peated for 15 times, and the heating duration was 1 h for the final painting process. These Ti sheets were air-cooled to room temperature for further use in electrochemical test.
The CS2350H electrochemical workstation was used for electrochemical tests. The working electrode was the coated anode with active area of 1.4 cm2. The reference electrode was the saturated calomel electrode (SCE), and the Pt-sheet of 4 cm2 was used as the auxiliary electrode. To ensure the consistency of experiment results, all electrochemical tests were conducted on the same workstation under the same test conditions.
2.1 Morphology characterization
After polishing by 1500# sandpaper, the Ti sheets without oxide scale were obtained. Then the pretreatment of acid etching was conducted immediately. These Ti sheets were etched by different acid solutions for different durations, as shown in Table 1.
Table 1
Pretreatment parameters of different Ti specimens
| Specimen | Pretreatment |
|---|
|
TA1 |
10wt% H2C2O4/2 h |
|
TA2 |
10wt% H2SO4/2 h |
|
TA3 |
10wt% H2SO4/1 h+10wt% H2C2O4/1 h |
|
TA4 |
10wt% H2C2O4/1 h+10wt% H2SO4/1 h |
|
TA5 |
20wt% HCl/2 h |
With increasing the reaction temperature, small gas bubbles are formed on the surface of Ti substrate during the acid-etching process. When the Ti substrate is treated by the oxalic acid, the yellow-brown titanium oxalate is generated, and the solution color is gradually darkened with prolonging the reaction. In contrast, the sulfuric acid reacts violently with Ti, and the solution gradually becomes dark purple with the reaction proceeding. The Ti sheet surface after acid etching is grayish white. The surface morphologies of the etched Ti sheets were analyzed by scanning electron microscope (SEM), as shown in Fig.1b~1f. The surface morphology of the Ti sheet before etching is shown in Fig.1a. The Ti sheet surface etched by oxalic acid is scattered with corrosion pits of different sizes, and there are jagged structures in the pits, as shown in Fig.1b. As shown in Fig.1c, the corrosion pits on Ti sheet surface treated by sulfuric acid are round with different sizes. The depth of these pits is greater than that of the pits caused by oxalic acid, which is consistent with the higher surface roughness caused by the sulfuric acid treatment[15,16]. The morphologies of TA3 and TA4 specimens show scale-like corrosion structure and circular corrosion pits. It can be seen that the surface morphology of TA3 specimen consists of three-dimensional honeycomb structures with uniform corrosion depth, and no deep corrosion pits appear. This phenomenon is similar to that of TA5 specimen, as shown in Fig.1f. It is generally believed that the corrosion of Ti sheet by hydrochloric acid is shallow[13], due to the formation of chlorine gas in the reaction of hydrochloric acid with Ti matrix, resulting in the new dense oxide film on the Ti surface at the beginning of reaction. As a result, the deposition of H2 and H+ on the Ti matrix surface is hindered. It is well-known that the hydrogen produced during the reaction can be absorbed by Ti substrate because of its strong hydrogen absorption ability in the initial stage of acid etching. With the hydrogen adsorption amount increasing, TiH2 with various crystal phases is precipitated at the grain boundary[17], where the crack likely occurs due to the phase transformation stress between the brittle titanium hydride and the matrix. The TiH2 formed through dual acid-treatment is transformed into the rutile without obvious changes of the surface morphology, which is beneficial to the electrons transport. This phenomenon leads to the more violent hydrogen-absorbing corrosion along the crack and then the formation of more corrosion pits. In these corrosion pits, some of the hydrogen ions are used as depolarizing agent to reduce to hydrogen, and the others produce the unstable black-gray titanium hydride. Due to the corrosion pit and corrosion crack, Ti4+ accumulates in the corrosion pit and reacts with water to form dense oxide passivation film[18]. When the hydrogen reduction process is hindered, the hydrogen ions spill out from the corrosion pits, and thereby more titanium hydride is formed outside the corrosion pit, which gradually increases the corrosion area and finally forms the ichthyoid pattern.
Fig.1 SEM morphologies of Ti sheets after different pretreatments: (a) after polishing; (b) TA1; (c) TA2; (d) TA3; (e) TA4; (f) TA5
The valence states of the elements on the Ti substrate surfaces etched by different solutions were studied by X-ray photoelectron spectroscopy (XPS). As shown in Fig.2a, the high-resolution spectra of TA3 specimen show a typical fluctuation curve of titanium. The peaks at 453.8 and 458.6 eV correspond to the spin orbital peaks of Ti 2p3/2 and Ti 2p1/2, respectively. The peak at 458.8 eV corresponds to Ti4+, which is consistent with the characteristic peak of TiO2. This result indicates that Ti exists mainly in the form of TiO2. Meanwhile, the peak of Ti 2p3/2 can be divided into a high binding energy peak (456.0 eV) and a low binding energy peak (453.8 eV), which correspond to Ti2+ and Ti0, respectively[19]. The appearance of Ti2+ characteristic peaks indicates the existence of TiH2 compound on the Ti surface, which is consistent with the results in Ref.[19,20]. The peak at 464.3 eV is considered as a satellite peak of one of these peaks. The XPS spectra of Ti sheets etched by different acid solutions are shown in Fig.2b. It is clear that the XPS spectrum of TA3 specimen is similar to that of TA1 specimen, suggesting that Ti0, Ti2+, and Ti4+ exist on both the TA1 and TA3 specimen surfaces. Meanwhile, the TA2 and TA4 specimens show smaller peaks of Ti0 and Ti2+, compared with those of TA3 specimen, indicating that there is less TiH2 phase on the surface of the Ti sheets[19]. In addition, the trend of electron loss can be observed from the spectrum of TA2 specimen.
Fig.2 XPS spectra of TA3 specimen (a) and Ti sheets after different pretreatments (b)
2.2 Coating characterization
The phase composition and crystal structure of the IrO2-Ta2O5 coating on Ti substrate were investigated by XRD, as shown in Fig.3. The spectra of coatings on different Ti substrates are in good agreement with those of IrO2 (PDF #86-0330) and Ti (PDF #44-1294). The sintering temperature of the anodes is 500 °C, which is far below the crystallization temperature (800 °C) of Ta2O5. Therefore, the characteristic peaks of Ta2O5 cannot be observed. Meanwhile, due to the thin coating thickness, the diffraction peaks of Ti substrate appear. The obvious characteristic peaks of IrO2 in all the specimens indicate that the synthesized material is well crystallized and has a pure phase. On the contrary, the diffraction peaks of tantalum oxide at the (200) and (211) crystal planes are weak, suggesting the existence of amorphous tantalum oxide phase.
Fig.3 XRD patterns of IrO2-Ta2O5/Ti specimens processed by different pretreatments
2.3 Electrochemical performance
The electrochemical properties of the specimens were tested in 0.5 mol/L H2SO4 solution. Fig.4a shows the linear sweep voltammetry (LSV) curves of the specimens prepared by different pretreatments. Fig.4b shows the overpotential of all the specimens at current density of 50 mA‧cm-2. It can be seen that the overpotential of TA1 specimen is 320 mV, which is higher than that of TA4 (258 mV), TA3 (278 mV), TA5 (287 mV), and TA2 (250 mV) specimens. The Tafel plots in Fig.4c show the similar slopes of all specimens in the range of 90~95 mV/dec. In this research, there is no significant change due to the same IrO2-Ta2O5 coating on different Ti substrates. However, it should be considered that the Tafel slope can also exhibit the small contribution of the difference in specific surface areas. Fig.4d shows the change in current density with the variation of scanning rate. It is well-known that the electrochemical active surface area (ECSA) of an electrocatalyst can be represented by a double layer capacitance (2Cdl). The calculated ECSA of TA3 specimen is 74.3 mF‧cm-2, which is much larger than that of TA1 specimen (61.5 mF‧cm-2). The ECSA of TA5 specimen is 71.7 mF‧cm-2, which is similar to that of TA3 specimen. This phenomenon infers that the pretreatment increases the active area of Ti anode, and the high ECSA value guarantees the high catalytic activity of TA3 specimen[20]. The integral area of the cyclic voltammetry (CV) curves is another important factor to evaluate the catalytic effective area. According to CV curves of all the specimens, TA1 specimen shows the smallest catalytic active area (Fig.4e), which can be attributed to the shallow etching pits caused by oxalic acid, and therefore results in less loading of IrO2-Ta2O5 coating on the surface of Ti substrate. As shown in Fig.4f, the impedance spectra of the specimens can represent the kinetic effects of the reactions during the catalytic reaction. The fitting model of electrochemical impedance spectrum (EIS) is also shown in Fig.4f, where RS denotes the solution resistance, Q describes the internal cracking of the coating, and Rct represents the electrocatalytic activity of oxidation evolution of the electrode. The value of Rct is negatively correlated with the chlorine evolution activity of the electrode. As shown in Fig.4f, TA2, TA3, and TA4 specimens all exhibit lower charge transfer resistance Rct, compared with that of TA1 specimen, which is consistent with their better catalytic activity. This may be ascribed to the better adhesion between the coating layer and the Ti substrate for TA2~TA4 specimens, and more active sites in these anodes.
Fig.4 OER electrocatalytic performance including LSV curves (a), overpotential (b), Tafel plots (c), ECSA plots (d), cyclic voltametric curves (e), and Nyquist plots (f) of IrO2-Ta2O5/Ti specimens processed by different pretreatments (RHE represent the reversible hydrogen electrode)
The service life was investigated through the acceleration life test for shorter experiment period. The H2SO4 solution of 10wt% was used. The solution was heated to 40 °C through water bath heating, the current density was 4 A‧cm-2, and Ti plate was used as the cathode plate. It can be seen form Fig.5 that the TA3 specimen exhibits the best stability and the longest service life among all these specimens. Although the catalytic activity of TA5 specimen is not the best, its lifetime is relatively long under harsh conditions. However, the TA2 and TA4 specimens show shorter service life. The difference in these properties is related to the TiH2 phase on the surface of Ti substrate after pretreatment[10]. The TiH2 existence increases the binding force between the substrate and the oxide layer, and decreases the resistance between the substrate and the coating layer. In brief, the acid etching and heat treatment jointly improve the coating adhesion and accelerated lifetime.
Fig.5 Accelerated lifetime results of IrO2-Ta2O5/Ti anodes processed by different pretreatments
1) The IrO2-Ta2O5/Ti anode treated by sulfuric acid followed by oxalic acid shows the best electrocatalytic activity and the longest service life. The anodes with etched surface of uniform honeycomb structures have higher catalytic activity for oxygen reduction.
2) The sulfuric acid etching shows the more obvious corrosion effect on Ti sheets than the oxalic acid etching or hydrochloric acid etching does.
3) More TiH2 can be observed on the surface of Ti sheet treated by oxalic acid, thereby improving the bonding force between the substrate and oxide coating. The TiH2 formed through dual acid-treatment is transformed into the rutile without obvious changes of the surface morphology, which is beneficial to the electrons transport. The acid etching and heat treatment jointly improve the coating adhesion and accelerated lifetime.
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