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IrO2RuO2SiO2 anodes for oxygen evolution reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
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Crystallization behavior-dependent electrocatalytic activity and stability of Ti/IrO2eRuO2eSiO2 anodes for oxygen evolution reaction Bao Liu a, Chengyan Wang a,b,*, Yongqiang Chen a,b, Baozhong Ma a,b,**, Jialiang Zhang a,b a
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China b Beijing Key Laboratory of Rare and Precious Metals Green Recycling and Extraction, University of Science and Technology Beijing, Beijing, 100083, PR China
article info
abstract
Article history:
IrO2eRuO2eSiO2 ternary oxide coatings were fabricated on Ti substrate using a sol-gel
Received 27 August 2018
route. The impacts of calcination conditions (e.g., temperature and time) on the elec-
Received in revised form
trocatalytic activity and stability of the anodes were explored. It was found that the
6 November 2018
calcination temperature and time significantly impact the electrocatalytic properties of
Accepted 10 November 2018
the anode for the oxygen evolution reaction (OER). This can result from the properties of
Available online 4 December 2018
the surface (e.g., defects, crystallinity and crystallite size) as well as the preferred orientation of the active components. The amount of the defects of the coatings de-
Keywords:
creases with the increase of the calcination temperature and time. Besides, the crys-
Coating
tallinity and crystallite size increase with the increase of calcination temperature and
Electrocatalytic
time. The amorphous oxide coating can be observed from the sample calcined at 350 C
Crystallization behavior
for 15 min, while this coating can be crystallized when the calcination time is 60 min.
Oxygen evolution reaction
The coatings calcined in the temperature range between 350 and 450 C show preferred (101) planes of IrO2 and RuO2 crystallites, whereas the coatings calcined at the temperatures higher than 450 C show the (211) orientation. The increase of calcination time does not change the preferred orientation of IrO2 and RuO2. The calculated voltammetric charges suggest that the active surface area of the prepared coating is dominated by the “outer” active surface area over the entire calcination temperature and time ranges. Given the electrocatalytic activity and stability of all investigated anodes, the anode calcined at 450 C for 15 min is considered the most suitable for applications. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. ** Corresponding author. Beijing Key Laboratory of Rare and Precious Metals Green Recycling and Extraction, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail addresses: [email protected] (C. Wang), [email protected] (B. Ma). https://doi.org/10.1016/j.ijhydene.2018.11.068 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction The efficiencies of water electrolysis and metal electrowinning are significantly limited by the sluggish kinetics of the oxygen evolution reaction (OER) [1e9]. Identification of an efficient, long-lived and low-cost anode catalyst that improves the OER by lowering the overpotential has aroused huge interest in the past decades. Ruthenium oxide (RuO2) is shown as the most active catalyst for OER [10,11]; yet its application is limited since it is less stable in the sulfuric acid solution [12]. Iridium oxide is also an active catalyst toward OER, and its service life is nearly 20 times longer than that of RuO2 [13e16]. Yet application of IrO2 is limited by its low abundance and high cost. To increase the electrocatalytic properties and decrease the cost, scholars have generally mixed the active noble metal oxides with other non-active and non-noble stabilizers [17e19]. Among the mixed oxide coatings, the 70 mol% IrO2-30 mol%Ta2O5 composition is considered as the most suitable coating for OER [20]. Silica, one of the most abundant oxides on the earth, has been broadly applied in industry. Yet the application of silica for fabricating high-performance electrodes is underestimated. Silica has the properties of high corrosion resistance in an acidic environment and homogeneous dispersion when used with other components, making it a promising stabilizer for active electrocatalytic anodes. Mushiake et al. [21] explored the impact of silica on the electrocatalytic properties of IrO2-based anodes. They found that the electrocatalytic stability of the IrO2-based anodes is significantly increased by the addition of silica. Zhang et al. [22] reported an improvement of the electrocatalytic activity of the IrO2-based anodes by the doping of silica. The improved activity can be attributed to the increased active surface area. It is concluded that the addition of silica into an active coating contributes to both activity and stability. Ti-supported noble metallic oxide electrodes are often prepared using the thermal decomposition method. The microstructure and electrocatalytic properties of the electrodes are significantly impacted by the thermal decomposition process, especially by the temperature and time of calcination. Xu et al. [23] explored the impact of the calcination temperature on the catalytic properties of the Ti/IrO2e Ta2O5 electrodes. As they reported, the crystallinity and crystallite size of the active component IrO2 rely on the calcination temperature. The amorphous IrO2, formed at a low calcination temperature, shows a higher catalytic activity than the crystalline IrO2. Wang et al. [24] explored the impact of the calcination temperature on the electrochemical activity of the Ti/IrO2eSiO2 electrodes. They found that the catalytic activity of the Ti/IrO2eSiO2 electrodes decreases with the increase of calcination temperature, while the stability increases. In the present work, we investigated the impacts of the calcination temperature and the calcination time on the crystallization behavior, microstructure, electrocatalytic activity and stability of the Ti/IrO2eRuO2eSiO2 anode. Next, the dependence of the preferable IrO2 and RuO2 orientations on calcination temperature and time was clarified. Subsequently, the impact of the preferred orientation of IrO2 and RuO2 on the
electrocatalytic properties was discussed. Finally, the relationships between the service lifetime and the calcination conditions were investigated by performing accelerated life tests (ALT).
Experiment Electrode preparation Ti/IrO2eRuO2eSiO2 electrodes were prepared with the precursor solutions mixing H2IrCl6, RuCl3 and tetraethoxysilane (TEOS). Before being used, the Ti plates were degreased in an alkaline solution, etched in 10% oxalic acid and rinsed with deionized water. To obtain a gel, the precursor solutions were prepared by mixing H2IrCl6, RuCl3 and tetraethoxysilane (TEOS). To form a sol solution, the resultant gel was dissolved in the mixed solvent of 1:1 (v/v) n-butyl alcohol and isopropanol. The concentrations of the RuCl3, H2IrCl6 and TEOS in the sol solution reached 0.018, 0.042 and 0.14 mol L1, respectively. The obtained precursor solution was brushed onto the pretreated Ti plates repeatedly. After each painting step, the samples were first dried at 120 C for 15 min and then calcined at a fixed temperature between 350 and 600 C for 15 and 60 min, respectively. The samples were repeatedly painted, dried and calcined 25e30 times until the oxide coating reached 1.5 mg cm1. Finally, the prepared anodes were calcined at a fixed temperature for 60 min.
Mineralogical and microstructure characterization The composition of the prepared sample was analyzed using X-ray diffraction (XRD). XRD patterns were collected by an Xray diffractometer under Cu-Ka radiation at 100 mA and 40 kV. The 2q was ranged from 10 to 90 , and the scanning rate was 10 min1. Under field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) measurements, respectively, the microstructure and chemical composition were determined.
Electrochemical measurements Using anodic polarization, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, we characterized the electrocatalytic properties of the prepared samples. All the measurements were performed in 0.5 mol L1 sulfuric acid solution. The prepared anode, a platinum plate and a saturated calomel electrode (SCE) served as the working electrode, the counter electrode and the reference electrode, respectively. Anodic polarization measurements were performed at a scan rate of 5 mV s1. CV curves were recorded between 0 and 1.2 V vs. SCE at sweep rates from 5 to 100 mV s1. To correct the IR drop and delve into the electrochemical behavior of the anodes, EIS measurements were performed. The frequency ranged from 0.01 Hz to 100 kHz and the AC amplitude of 5 mV rms1 were employed in the EIS measurement. To evaluate the service life, an accelerated life test (ALT) was performed at the constant current density of 2 A cm2. When the voltage increased to 5 V relative to its initial value, the service life of the anode was determined.
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Results and discussion XRD analysis The XRD patterns of the samples calcined for 15 min are shown in Fig. 1a. This figure shows that the peaks corresponding to IrO2, RuO2 and metallic Ti are well-defined in the temperature range between 400 and 600 C. The shift of the IrO2 and RuO2 peaks reveals the generation of the IrO2eRuO2 solid solution [20]. The ionic radii of Ru4þ and Ir4þ are very similar. Moreover, IrO2 and RuO2 both show rutile-type crystal structures. This binary oxide system can produce a solid solution [25]. The intensity of IrO2 and RuO2 increase with the rise of the calcination temperature, suggesting that the degree of crystallinity of the coating increases. The high crystallinity degree of the coating contributes to the active surface area more, which helps to improve the electrocatalytic activity of the coating for OER. The diffraction peaks of SiO2 are not detected in the XRD patterns. It can be assumed that SiO2 is present as an amorphous phase in the coatings. In the sample calcined at 350 C for 15 min, only the peaks corresponding to metallic Ti can be observed, while no peaks corresponding to IrO2 and RuO2 are detected. This suggests that IrO2 and RuO2 are present as amorphous phases in the ternary oxide coating after calcination at 350 C for 15 min. The crystallization of the active components would occur if the free energy of the solid is equal to or less than that of the liquid. Besides, the crystallization of the active components may require a considerable amount of time at a low calcination temperature. The calcination time acts as a critical parameter for the crystallization of the coating. To further explore the crystallization behavior of the coatings, we performed XRD measurements for the samples calcined for 60 min. The intensity of IrO2 and RuO2 peaks of the samples calcined for 60 min are higher than those of the samples calcined at the same temperature for 15 min, as shown in Fig. 1(b). The coating shows a crystalline state after calcination at 350 C for 60 min. This suggests that the crystallinity degree of the coating increases with the increase of calcination time. The peaks corresponding to titanium oxides are detected in the XRD patterns of the samples calcined at the temperatures higher than 450 C for 60 min. The detected titanium oxides can result from the oxidation of the Ti substrates, which may deteriorate the electrocatalytic properties of the anodes. The
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diffraction peaks of SiO2 can not be found in any of the XRD patterns of the samples calcined for 60 min, suggesting that the crystallization of SiO2 can not occur in the temperature range between 350 and 600 C. Miller indexes of IrO2 and RuO2 were detected in the investigated samples, viz. (110), (101) and (211). The texture coefficients of the different planes in rutile IrO2 and RuO2, TC (h k l), were calculated by Eq. (1) [26]: TCðhklÞ ¼
1 IðhklÞ 1 X IðhklÞ I0 ðhklÞ n I0 ðhklÞ
(1)
where I(h k l) denotes the measured intensity of the (h k l) plane; I0(h k l) is the standard intensity of the standard XRD patterns data; n is the total number of reflections. The calculated values of TC(h k l) for IrO2 and RuO2 are shown in Fig. 2. The same orientation is shown to be preferred for the rutiletype IrO2 and RuO2. The (101) plane is preferred as the growth orientation of IrO2 and RuO2 in the calcination temperature ranged from 350 to 450 C. Yet in the temperature ranged from 500 to 600 C, the (211) plane is the preferred orientation. The rise in the calcination time does not vary the preferred orientation of IrO2 and RuO2. As the results of Hu et al. suggested [27], the (101) rutile-type IrO2 shows better electrocatalytic activity for OER since the IreO bond is shorter for the (101) plane than for the other facets. The Ti/IrO2e RuO2eSiO2 anodes calcined at low temperatures may show higher electrocatalytic activity than the anodes calcined at high temperatures.
FESEM analysis The FESEM images of the prepared electrodes obtained at low magnification are shown in Fig. 3. As shown in Fig. 3, considerable amounts of cracks can be observed on the coatings calcined at 350 and 400 C for 15 min, but they are rarely found on the coatings calcined at high temperatures or for 60 min. The amount of the cracks in the coatings decreases with the rise of calcination temperature and calcination time. The oxidation of the Ti substrate would occur due to the presence of pores and cracks in the coating, leading to the failure of the anode in the OER process [28]. The samples calcined at high temperatures or for a long time may show higher stability for OER due to fewer cracks in the coating. For the samples calcined at the temperatures higher than 400 C
Fig. 1 e XRD patterns of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
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Fig. 2 e Calculated texture coefficient (a) IrO2 calcined for 15 min, (b) RuO2 calcined for 15 min, (c) IrO2 calcined for 60 min, and (d) RuO2 calcined for 60 min.
for 15 min or for 60 min, the coatings with the dispersion of fine crystals can be observed. Yet the fine crystals are rarely observed for the samples calcined at 350 and 400 C for 15 min. As the EDS analysis suggests, the fine crystals are enriched in iridium and ruthenium, which can be considered as the IrO2e RuO2 solid solution. To further investigate the surface morphology of the coatings, FESEM analysis was conducted at high magnification, with the obtained images shown in Fig. 4. The active components are not crystallized and show an amorphous phase for the sample calcined at 350 C for 15 min, as shown in Fig. 4a. In addition, a small amount of spherical crystals can be observed on the coating calcined at 400 C for 15 min (Fig. 4b). The coatings with a large amount of dispersed acerose crystals can be observed for the samples calcined in the temperature ranged from 450 to 600 C for 15 min (Fig. 4cef). All the obtained coatings of the samples calcined for 60 min show crystalline state (Fig. 4gel). The crystallinity degree and the crystallite size of the coating calcined at 400 C for 60 min are obviously higher than those of the coating calcined at 400 C for 15 min. This suggests that the crystallization and the growth of IrO2 and RuO2 crystals require a considerable amount of time at 350 and 400 C. The crystallite sizes were evaluated based on the FESEM analysis results, as shown in Table 1. Examination of the data in Table 1 suggests that the rise of the calcination temperature and the calcination time lead to an increase of the crystallite size. The larger crystallite size contributes less to the active surface area of the coating that may decrease the electrocatalytic activity for OER.
Electrochemical active surface area The CV curves of the prepared samples in a sulfuric acid solution are shown in Fig. 5. The calcined anodes show typical voltammetric behavior representative of thermally prepared oxide coatings. The broad peaks observed in the potentials ranged from 0 to 1.2 V can be attributed to the oxidation state transitions of the noble metal, e.g., Ru(III)/Ru(IV), Ru(IV)/Ru(V), Ir(III)/Ir(IV) and Ir(IV)/Ir(V) [29,30]. The heterogeneity of the coatings broadens the peaks observed in the CV curves [31]. Regardless of the sample calcined at 400 C for 15 min, the voltammetric response of the electrode decreases with a rise of the calcination temperature, as shown in Fig. 5a. Increasing calcination time improves the voltammetric response of the coating calcined at 400 C, while it reduces the voltammetric response of the other samples (Fig. 5b). Since the compositions of the prepared anodes are basically the same, the differences in the voltammetric response can result from a variation in the active surface area. To estimate the active surface area of the anodes, we calculated the voltammetric charge, q*, using graphic integration. On the whole, at a high sweep rate, the reaction occurs at the more accessible part of the surface (“outer” active surface). Besides, at a low potential scan rate, the reaction takes place on both the “outer” and the “inner” active surfaces. The “inner” active surface is associated with the pores, cracks and defects of the coatings. The values of “outer” and “inner” surface voltammetric charges (qouter and qinner) were calculated using the following equations [22,32]:
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Fig. 3 e FESEM images of the Ti/IrO2eRuO2eSiO2 electrodes at low magnification ( £ 500).
1 q* ¼ 1 qinner þ qouter þ C1 v1=2
(2)
q* ¼ qouter þ C2 v1=2
(3)
where C1 and C2 are constants, and v denotes the sweep rate. The total voltammetric charge, qtotal, is defined as the sum of qouter and qinner. The voltammetric charge value is sweep rate dependent. Based on the voltammetric charge values of various sweep rates, the plots of 1/q* vs. v1/2 and q* vs. v1/2 can be obtained. These two plots enable the data to be extrapolated to v ¼ 0 and v ¼ ∞ to achieve the values of qtotal
and qouter, respectively, from which the value of qinner can be derived. The calculated voltammetric charge of the prepared samples is shown in Fig. 6. This Figure shows that the highest qtotal value can be obtained from the coating calcined at 350 C for 15 min, so this sample has the largest active surface area. As the results of the XRD and FESEM analyses suggest, iridium and ruthenium oxides are present as an amorphous phase after calcination at 350 C for 15 min. Due to the disorder of the amorphous oxide, the amorphous parts of the active components contribute more to the active surface area than the crystalline parts [23]. The qtotal values of the coatings calcined
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Fig. 4 e FESEM images of the Ti/IrO2eRuO2eSiO2 electrodes at high magnification ( £ 30000).
Table 1 e Crystallite size of the IrO2eRuO2eSiO2 ternary oxide coatings. Temperatures ( C)
350 400 450 500 550 600
Crystallite size (nm) Calcined for 15 min
Calcined for 60 min
e 30e40 50e60 70e110 80e110 100e120
20e40 55e80 60e90 110e140 125e200 230e500
in the temperature ranged from 450 to 550 C for 15 min are higher than those of the coatings calcined at 400 and 600 C for 15 min. The rise of calcination time increases the qtotal value of the coating calcined at 400 C, while it decreases the qtotal values of the other samples. This suggests that the active sites of the surface are determined by the crystallization behavior of the active components. More active sites are attributed by higher crystallinity degree, and the number of active sites also increases with finer components. The obtained qouter values are higher than the qinner values over the entire calcination temperature and calcination time range. This suggests that the active surface area of the coating is dominated by the “outer” active surface.
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0.020
0.020
0.015
o
o
450 C
o
500 C
o
(a)
-2
o
400 C
Current density (A cm )
-2
Current density (A cm )
o
350 C
o
550 C
600 C
0.010 0.005 0.000 -0.005 -0.010 0.0
0.2
0.4 0.6 0.8 Potential vs. SCE (V)
1.0
350 C o 500 C
0.015
o
450 C (b) o 600 C
0.010 0.005 0.000 -0.005 -0.010 0.0
1.2
o
400 C o 550 C
0.2
0.4 0.6 0.8 Potential vs. SCE (V)
1.0
1.2
Fig. 5 e CV curves of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
800
qtotal
1200
(a) -2
1000
qinner qouter
Charge (mC cm )
-2
Charge (mC cm )
1200
600 400 200
1000
qinner qouter
800
qtotal
(b)
600 400 200
0
0 350
400 450 500 550 o Calcination Temperature ( C)
600
350
400 450 500 550 o Calcination Temperature ( C)
600
Fig. 6 e Voltammetric charge calculated by integration and extrapolation of the CV curves (a) calcined for 15 min, (b) calcined for 60 min.
Anodic polarization curves The IR-corrected anodic polarization curves of the prepared electrodes in a sulfuric acid solution are shown in Fig. 7. Fig. 7a shows that the amorphous coating calcined at 350 C for 15 min has the best electrocatalytic activity for OER. Thanawala et al. [33] and Reier et al. [34] also suggested that the reactivity of the amorphous IrO2 coating is higher than that of the crystalline one, suggesting better catalytic activity for OER. Pfeifer et al. [35] investigated the electronic structure of iridium oxide electrodes. They observed that the enhanced activity of the amorphous iridium oxides compared to their crystalline counterparts is attributed to the electronic defects in the near-surface region of the anionic and cationic framework of the amorphous iridium oxides.
The electrocatalytic performance of the electrode is controlled by both the active surface area and the intrinsic catalytic nature [36,37]. Shan et al. [38] validated that the IrO2 nanoparticles show a better electrocatalytic activity than the bulk one due to the high active surface area. The performance of the IrO2eRuO2eSiO2 coating calcined at 450 C for 15 min is almost similar with that of the amorphous coating, which can be attributed to more and finer active components and the higher conductivity of the crystalline parts [39]. In addition, given the preferred orientation of the IrO2 and RuO2, the shorter length of the bond for IreO and RueO of (101) facet may to some extent improve the electrocatalytic activity. The electrocatalytic activity of the anodes calcined in the temperature range between 500 and 600 C for 15 min is slightly lower than that of the anode calcined at 450 C for 15 min. This
0.08
0.08
0.06
o
400 C o 550 C
o
450 C o 600 C
(a)
0.05 0.04 0.03 0.02 0.01 0.00 1.0
1.1
1.2 1.3 Potential vs. SCE (V)
o
-2
350 C o 500 C
Current density (A cm )
-2
Current density (A cm )
o
0.07
1.4
0.07 0.06
350 C o 500 C
o
400 C o 550 C
o 450 C (b) o 600 C
0.05 0.04 0.03 0.02 0.01 0.00 1.0
1.1
1.2 1.3 Potential vs. SCE (V)
1.4
1.5
Fig. 7 e IR-corrected anodic polarization curves of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
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o
350 C o 450 C o
550 C
0
5
o
400 C o 500 C
h ið1nÞ Q ¼ ðCdl Þn ðRs Þ1 þ ðRct Þ1
(a)
o
600 C
10 15 20 2 Re (Z) (Ω cm )
To further explore the electrochemical properties of the prepared samples, EIS measurements were performed. The obtained Nyquist diagrams and Bode plots are shown in Fig. 8. All impedance spectra show a depressed semicircle, as shown in Fig. 8a and b. This suggests that the electrochemical processes taking place on the ternary oxide coatings are controlled by the electrochemical reaction [40]. The Bode plots given in Fig. 8c and d show the inductive properties of the ternary oxide coatings due to the porosity of the active oxide layer. Similar results were commonly obtained for the anodes prepared using thermal decomposition method [22]. The obtained Nyquist diagrams and Bode plots were fitted using an equivalent electrical circuit (EEC), described as LRs(Rct, OCdl, O) (Rct, ICdl, I) [22]. Here, L is the inductance, Rs, Rct, O and Rct, I refer to the solution resistance, the charge transfer resistance at the “outer” and “inner” active surfaces, respectively. Cdl, O and Cdl, I are the double-layer capacitances of the “outer” surface/electrolyte and “inner” surface/electrolyte interfaces, respectively. Given the nonuniformity of the prepared coatings, we replaced constant phase element (CPE) with the pure capacitors (Cdl, O and Cdl, I), denoted by Qdl, O and Qdl, I, respectively [41]. When the CPE is coupled with the charge transfer resistance, the double layer capacitance can be evaluated using the following equation [42,43]:
2
20 18 16 14 12 10 8 6 4 2 0 -2
EIS measurements
-lm (Z) (Ω cm )
2
-lm (Z) (Ω cm )
can result from the increased crystallite size and the preferred orientation of the active components. It is noteworthy that the anode calcined at 400 C for 15 min exhibits the lowest activity for OER in comparison with the anode calcined at 350 and 450 C for 15 min. Increasing calcination temperature decreases the disorder of the amorphous oxides, and the low crystallinity of the coating calcined at 400 C for 15 min contributes less to the active surface area, resulting in the low electrocatalytic activity of this anode for OER. The electrocatalytic activity of the anodes calcined for 60 min first increases and then decreases with the rise of calcination temperature, as shown in Fig. 7b. The coating calcined at 450 C for 60 min shows the best electrocatalytic activity over the entire anodes calcined for 60 min, whereas it is still lower than that of the anodes calcined at 350 and 450 for 15 min. The rise of calcination time improves the electrocatalytic activity of the sample calcined at 400 C, while it decreases the activity of the other samples. This can be attributed to the crystallization behavior of the active components, impacting the active surface area of the coating for OER. Moreover, the oxidation of the Ti substrate can deteriorate the electrocatalytic activity of the anode, resulting in the increase in the overpotential for OER. Similar results were found by Reier et al. [34]. They illustrated that a Ti oxide interlayer can be formed and Ti oxide migrates into the Ir oxide layer at a high calcination temperature, leading to the decrease of the electrocatalytic activity for OER.
25
30
o
400 C
o
o
500 C o 600 C
350 C
5
(b)
o
450 C o 550 C
0
100
10 15 20 25 30 35 40 45 50 2 Re (Z) (Ω cm )
120 350 C
80
o
450 C o 550 C
60
o
400 C
o
(c)
o
500 C o 600 C
Phase angle (deg)
o
Phase angle (deg)
40 36 32 28 24 20 16 12 8 4 0
(4)
40 20 0
100
350 C
80
450 C o 550 C
o
o
400 C
(d)
o
500 C o 600 C
60 40 20 0
-20
-20 -2
-1
0
1 2 3 log (frequency / Hz)
4
5
-2
-1
0
1 2 3 log (frequency / Hz)
4
5
Fig. 8 e EIS plots of the Ti/IrO2eRuO2eSiO2 electrodes (a) Nyquist diagrams of the electrodes calcined for 15 min, (b) Nyquist diagrams of the electrodes calcined for 60 min, (c) Bode plots of the electrodes calcined for 15 min, and (d) Bode plots of the electrodes calcined for 60 min.
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Based on the obtained Qdl, O and Qdl, I values, the real Cdl, O and Cdl, I values were calculated using Eq. (4). Where, Cdl, total is defined as the sum of the Cdl, O and Cdl, I. Also, Rct, total is defined as the sum of Rct, O and Rct, I [22]. The values of the charge transfer resistance and the double-layer capacitance obtained by fitting of the EIS measurements are shown in Fig. 9. The lowest Rct, total value can be obtained from the anode calcined at 350 C for 15 min, as shown in Fig. 9a. This therefore shows the highest electrocatalytic activity for OER [40,44]. The Rct, total obtained from the anode calcined at 450 C for 15 min is lower than that of the anodes calcined in the temperatures ranged from 500 to 600 C for 15 min. This suggests that the (101) IrO2 and (101) RuO2 show a higher efficiency for the charge transfer than the (211) IrO2 and (211) RuO2. At the entire calcination temperatures, the obtained Rct, I values are higher than the Rct, O values, suggesting that the “outer” surface is more electrocatalytically active than the “inner” surface. The calculated voltammetric charge values suggest that the active surface area of the ternary oxide coatings is dominated by the “outer” active surface. The higher electrocatalytic activity of the “outer” surface can therefore result from its larger active surface area. Fig. 9b shows that the anode calcined at 350 C for 15 min has the highest Cdl, total value and therefore the largest active surface area [45], which is associated with the disordered structure of the amorphous oxide. In the crystalline samples, regardless of the anode calcined at 400 C, the Cdl, total value decreases with a rise of the calcination temperature. The decreased Cdl, total can be attributed to the increased crystallite size decreasing the active surface area of the coating. The low
value of the Cdl, total obtained from the anode calcined at 400 C is associated with the slight generation of the fine crystals. The obtained Cdl, O values are higher than the Cdl, I values over the entire calcination temperature range. This suggests that the active surface area of the prepared coatings is dominated by the “outer” active surface, which is consistent with the results of the voltammetric charge calculation. The Rct, total values of the samples calcined for 60 min first decrease and then increase with the rise of calcination temperature, as shown in Fig. 9c. The variation of the Cdl, total values presents a reverse tendency (Fig. 9d). Regardless of the anode calcined at 400 C, the charge transfer resistance value increases, and the double-layer capacitance value decreases with the rise of calcination time. This suggests that the electrocatalytic activity and active surface area of the coating are crystallization behavior dependent, which are impacted by the calcination conditions (e.g., temperature and time). The significant increase in the Rct, total values can be observed for the samples calcined at 550 and 600 C for 60 min in comparison with the samples calcined at the same temperatures for 15 min. This suggests that the oxidation of Ti substrate deteriorates the electrocatalytic activity of the anodes for OER.
Electrocatalytic stability The results of the accelerated life test are shown in Fig. 10. The service lifetime of the anodes is shown to increase with the rise of the calcination temperature. The growth becomes weak when the calcination temperature is higher than 500 C. The increase of calcination time can also slightly improve the
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Fig. 9 e Charge transfer resistance and double-layer capacitance of the Ti/IrO2eRuO2eSiO2 electrodes (a), (b) calcined for 15 min, (c), (d) calcined for 60 min.
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Fig. 10 e Accelerated lifetime of the Ti/IrO2eRuO2eSiO2 electrodes.
service lifetime of the anodes for OER. The lowest accelerated lifetime of nearly 10 h can be obtained from the anode calcined at 350 C for 15 min. The amorphous oxide coating shows low electrocatalytic stability for OER in a sulfuric acid solution. Similar results were reported by Wang et al. [24], suggesting that the amorphous IrO2 has low electrocatalytic stability in acidic media. The stability of nanostructured Ir oxide electrocatalysts was also investigated by Cherevko et al. [46]. They indicated that the stability of Ir oxide film increases with increasing calcination temperature. The increased stability of the Ir oxide film calcined at 450 C compared to the film calcined at 250 and 350 C is attributed to the formation of a crystalline IrO2 phase that is shown to exhibit a higher degree of lattice oxygen at the surface and a lower degree of surface hydroxylation. It can plausible to conclude that the electrocatalytic stability of the anode relies on the crystalline order and lattice oxygen of the coating. As illustrated in the literature [20,47], the service lifetimes of the Ti/IrO2eRuO2 anode and Ti/IrO2eRuO2eTa2O5 anode are 12e30 and 80e120 h, respectively, during the ALT in sulphuric acid solution. The Ti/IrO2eRuO2eSiO2 anodes calcined at the temperatures higher than 450 C show higher service lifetimes than these two anodes. Moreover, silica is a non-noble oxide. Doping silica into the coating decreases the cost for electrode production. We believe that the prepared Ti/IrO2eRuO2eSiO2 anodes with long-term stability and low cost are promising candidates for the OER applications. Anode deactivation consists of metal substrate passivation, film consumption, film detachment as well as mechanical damage [48,49]. According to the results of Cherevko et al. [50], Ir and Ru exhibit both transient dissolution during oxide formation/reduction and steady-state dissolution during the evolution reaction. Kotz et al. [51] and Gajic-Krstajic et al. [12] reported the deactivation mechanism of the Ti/IrO2 anode and the Ti/RuO2 anode, respectively. Their results suggested that the deactivation can result from the dissolution of the active components and the oxidation of the Ti substrate through the defects of the coating. The FESEM analysis of the prepared anode after ALT are shown in Fig. 11. The Ti substrate is shown to be exposed with the dissolution of the active coating. According to the results of the EDS analysis shown in Table 2, Ir, Ru and Si remain on the IrO2e
Fig. 11 e FESEM analysis of the Ti/IrO2eRuO2eSiO2 anode after ALT.
Table 2 e EDS analysis of the Ti/IrO2eRuO2eSiO2 anode before and after ALT, in at.%. Elements
Ti
Ir
Ru
Si
O
Before ALT After ALT
1.06 25.47
4.07 1.71
7.57 1.10
22.42 5.46
64.88 66.26
RuO2eSiO2 ternary oxide coating in a small amount after ALT. The at.% of Ti of the failed anode is much higher than that of the fresh anode. The atom ratio of (Ir þ Ru þ Si þ Ti):O of the failed anode is approximately 1:2. This indicates that the Ti substrate is oxidized during the electrolysis process. Similar results were obtained by Hoseinieh et al. [52]. They indicated that the growth of an insulating TiO2 layer is the major reason for the deactivation of the Ti/IrO2eRuO2eTiO2 anode. Consequently, the failure of the prepared Ti/IrO2eRuO2eSiO2 anode can be attributed to the dissolution of the active coating and the oxidation of the Ti substrate. The higher electrocatalytic stability of the anodes calcined at high temperature and for a long time can result from the fewer cracks and high crystallinity of the coating. Furthermore, the higher stability of the (211) IrO2 and (211) RuO2 can also extend the service lifetime of the anode for OER in a sulfuric acid solution.
Conclusion The impact of calcination temperature and calcination time on the crystallization, surface properties, electrocatalytic activity and stability were investigated. The prepared coating is composed of amorphous SiO2, rutile-type IrO2, RuO2 as well as IrO2eRuO2 solid solution. The amorphous oxide coating is observed from the sample calcined at 350 C for 15 min, while this coating would be crystallized when the calcination time is 60 min. The coatings calcined in the temperature ranged from 350 to 450 C show preferred (101) planes of IrO2 and RuO2 crystallites, whereas the coatings calcined at the temperatures higher than 450 C have (211) orientation. The rise of calcination time does not vary the preferred orientation of IrO2 and RuO2. The crystallinity and crystallite size increase with the rise of calcination temperature and calcination time, impacting the active surface area of the coating for OER. As the
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results of the calculated voltammetric charge suggest, the active surface area of the ternary oxide coating is dominated by the “outer” active surface, which is also confirmed by performing the EIS measurements. The rise of calcination temperature and calcination time extend the service lifetime of the anode for OER. The electrocatalytic activity and stability of the anode rely on the crystallization behavior of the active components, which are impacted by the calcination conditions. The largest active surface area is obtained for the anode calcined at 350 C for 15 min, thus showing the highest electrocatalytic activity for OER; yet the stability of this anode is low. Given the electrocatalytic activity and stability of all investigated anodes, the anode calcined at 450 C for 15 min is considered the most suitable for use in applications.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No.51674026), the Beijing Natural Science Foundation, China (No.2182040), the Beijing Science & Technology Program (Z171100002217063), the Fundamental Research Funds for the Central University (230201606500078), and the National Natural Science Foundation of China (No. U1302274).
references
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Crystallization behavior-dependent electrocatalytic activity and stability of Ti/IrO2eRuO2eSiO2 anodes for oxygen evolution reaction Bao Liu a, Chengyan Wang a,b,*, Yongqiang Chen a,b, Baozhong Ma a,b,**, Jialiang Zhang a,b a
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China b Beijing Key Laboratory of Rare and Precious Metals Green Recycling and Extraction, University of Science and Technology Beijing, Beijing, 100083, PR China
article info
abstract
Article history:
IrO2eRuO2eSiO2 ternary oxide coatings were fabricated on Ti substrate using a sol-gel
Received 27 August 2018
route. The impacts of calcination conditions (e.g., temperature and time) on the elec-
Received in revised form
trocatalytic activity and stability of the anodes were explored. It was found that the
6 November 2018
calcination temperature and time significantly impact the electrocatalytic properties of
Accepted 10 November 2018
the anode for the oxygen evolution reaction (OER). This can result from the properties of
Available online 4 December 2018
the surface (e.g., defects, crystallinity and crystallite size) as well as the preferred orientation of the active components. The amount of the defects of the coatings de-
Keywords:
creases with the increase of the calcination temperature and time. Besides, the crys-
Coating
tallinity and crystallite size increase with the increase of calcination temperature and
Electrocatalytic
time. The amorphous oxide coating can be observed from the sample calcined at 350 C
Crystallization behavior
for 15 min, while this coating can be crystallized when the calcination time is 60 min.
Oxygen evolution reaction
The coatings calcined in the temperature range between 350 and 450 C show preferred (101) planes of IrO2 and RuO2 crystallites, whereas the coatings calcined at the temperatures higher than 450 C show the (211) orientation. The increase of calcination time does not change the preferred orientation of IrO2 and RuO2. The calculated voltammetric charges suggest that the active surface area of the prepared coating is dominated by the “outer” active surface area over the entire calcination temperature and time ranges. Given the electrocatalytic activity and stability of all investigated anodes, the anode calcined at 450 C for 15 min is considered the most suitable for applications. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. ** Corresponding author. Beijing Key Laboratory of Rare and Precious Metals Green Recycling and Extraction, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail addresses: [email protected] (C. Wang), [email protected] (B. Ma). https://doi.org/10.1016/j.ijhydene.2018.11.068 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction The efficiencies of water electrolysis and metal electrowinning are significantly limited by the sluggish kinetics of the oxygen evolution reaction (OER) [1e9]. Identification of an efficient, long-lived and low-cost anode catalyst that improves the OER by lowering the overpotential has aroused huge interest in the past decades. Ruthenium oxide (RuO2) is shown as the most active catalyst for OER [10,11]; yet its application is limited since it is less stable in the sulfuric acid solution [12]. Iridium oxide is also an active catalyst toward OER, and its service life is nearly 20 times longer than that of RuO2 [13e16]. Yet application of IrO2 is limited by its low abundance and high cost. To increase the electrocatalytic properties and decrease the cost, scholars have generally mixed the active noble metal oxides with other non-active and non-noble stabilizers [17e19]. Among the mixed oxide coatings, the 70 mol% IrO2-30 mol%Ta2O5 composition is considered as the most suitable coating for OER [20]. Silica, one of the most abundant oxides on the earth, has been broadly applied in industry. Yet the application of silica for fabricating high-performance electrodes is underestimated. Silica has the properties of high corrosion resistance in an acidic environment and homogeneous dispersion when used with other components, making it a promising stabilizer for active electrocatalytic anodes. Mushiake et al. [21] explored the impact of silica on the electrocatalytic properties of IrO2-based anodes. They found that the electrocatalytic stability of the IrO2-based anodes is significantly increased by the addition of silica. Zhang et al. [22] reported an improvement of the electrocatalytic activity of the IrO2-based anodes by the doping of silica. The improved activity can be attributed to the increased active surface area. It is concluded that the addition of silica into an active coating contributes to both activity and stability. Ti-supported noble metallic oxide electrodes are often prepared using the thermal decomposition method. The microstructure and electrocatalytic properties of the electrodes are significantly impacted by the thermal decomposition process, especially by the temperature and time of calcination. Xu et al. [23] explored the impact of the calcination temperature on the catalytic properties of the Ti/IrO2e Ta2O5 electrodes. As they reported, the crystallinity and crystallite size of the active component IrO2 rely on the calcination temperature. The amorphous IrO2, formed at a low calcination temperature, shows a higher catalytic activity than the crystalline IrO2. Wang et al. [24] explored the impact of the calcination temperature on the electrochemical activity of the Ti/IrO2eSiO2 electrodes. They found that the catalytic activity of the Ti/IrO2eSiO2 electrodes decreases with the increase of calcination temperature, while the stability increases. In the present work, we investigated the impacts of the calcination temperature and the calcination time on the crystallization behavior, microstructure, electrocatalytic activity and stability of the Ti/IrO2eRuO2eSiO2 anode. Next, the dependence of the preferable IrO2 and RuO2 orientations on calcination temperature and time was clarified. Subsequently, the impact of the preferred orientation of IrO2 and RuO2 on the
electrocatalytic properties was discussed. Finally, the relationships between the service lifetime and the calcination conditions were investigated by performing accelerated life tests (ALT).
Experiment Electrode preparation Ti/IrO2eRuO2eSiO2 electrodes were prepared with the precursor solutions mixing H2IrCl6, RuCl3 and tetraethoxysilane (TEOS). Before being used, the Ti plates were degreased in an alkaline solution, etched in 10% oxalic acid and rinsed with deionized water. To obtain a gel, the precursor solutions were prepared by mixing H2IrCl6, RuCl3 and tetraethoxysilane (TEOS). To form a sol solution, the resultant gel was dissolved in the mixed solvent of 1:1 (v/v) n-butyl alcohol and isopropanol. The concentrations of the RuCl3, H2IrCl6 and TEOS in the sol solution reached 0.018, 0.042 and 0.14 mol L1, respectively. The obtained precursor solution was brushed onto the pretreated Ti plates repeatedly. After each painting step, the samples were first dried at 120 C for 15 min and then calcined at a fixed temperature between 350 and 600 C for 15 and 60 min, respectively. The samples were repeatedly painted, dried and calcined 25e30 times until the oxide coating reached 1.5 mg cm1. Finally, the prepared anodes were calcined at a fixed temperature for 60 min.
Mineralogical and microstructure characterization The composition of the prepared sample was analyzed using X-ray diffraction (XRD). XRD patterns were collected by an Xray diffractometer under Cu-Ka radiation at 100 mA and 40 kV. The 2q was ranged from 10 to 90 , and the scanning rate was 10 min1. Under field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) measurements, respectively, the microstructure and chemical composition were determined.
Electrochemical measurements Using anodic polarization, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, we characterized the electrocatalytic properties of the prepared samples. All the measurements were performed in 0.5 mol L1 sulfuric acid solution. The prepared anode, a platinum plate and a saturated calomel electrode (SCE) served as the working electrode, the counter electrode and the reference electrode, respectively. Anodic polarization measurements were performed at a scan rate of 5 mV s1. CV curves were recorded between 0 and 1.2 V vs. SCE at sweep rates from 5 to 100 mV s1. To correct the IR drop and delve into the electrochemical behavior of the anodes, EIS measurements were performed. The frequency ranged from 0.01 Hz to 100 kHz and the AC amplitude of 5 mV rms1 were employed in the EIS measurement. To evaluate the service life, an accelerated life test (ALT) was performed at the constant current density of 2 A cm2. When the voltage increased to 5 V relative to its initial value, the service life of the anode was determined.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
Results and discussion XRD analysis The XRD patterns of the samples calcined for 15 min are shown in Fig. 1a. This figure shows that the peaks corresponding to IrO2, RuO2 and metallic Ti are well-defined in the temperature range between 400 and 600 C. The shift of the IrO2 and RuO2 peaks reveals the generation of the IrO2eRuO2 solid solution [20]. The ionic radii of Ru4þ and Ir4þ are very similar. Moreover, IrO2 and RuO2 both show rutile-type crystal structures. This binary oxide system can produce a solid solution [25]. The intensity of IrO2 and RuO2 increase with the rise of the calcination temperature, suggesting that the degree of crystallinity of the coating increases. The high crystallinity degree of the coating contributes to the active surface area more, which helps to improve the electrocatalytic activity of the coating for OER. The diffraction peaks of SiO2 are not detected in the XRD patterns. It can be assumed that SiO2 is present as an amorphous phase in the coatings. In the sample calcined at 350 C for 15 min, only the peaks corresponding to metallic Ti can be observed, while no peaks corresponding to IrO2 and RuO2 are detected. This suggests that IrO2 and RuO2 are present as amorphous phases in the ternary oxide coating after calcination at 350 C for 15 min. The crystallization of the active components would occur if the free energy of the solid is equal to or less than that of the liquid. Besides, the crystallization of the active components may require a considerable amount of time at a low calcination temperature. The calcination time acts as a critical parameter for the crystallization of the coating. To further explore the crystallization behavior of the coatings, we performed XRD measurements for the samples calcined for 60 min. The intensity of IrO2 and RuO2 peaks of the samples calcined for 60 min are higher than those of the samples calcined at the same temperature for 15 min, as shown in Fig. 1(b). The coating shows a crystalline state after calcination at 350 C for 60 min. This suggests that the crystallinity degree of the coating increases with the increase of calcination time. The peaks corresponding to titanium oxides are detected in the XRD patterns of the samples calcined at the temperatures higher than 450 C for 60 min. The detected titanium oxides can result from the oxidation of the Ti substrates, which may deteriorate the electrocatalytic properties of the anodes. The
513
diffraction peaks of SiO2 can not be found in any of the XRD patterns of the samples calcined for 60 min, suggesting that the crystallization of SiO2 can not occur in the temperature range between 350 and 600 C. Miller indexes of IrO2 and RuO2 were detected in the investigated samples, viz. (110), (101) and (211). The texture coefficients of the different planes in rutile IrO2 and RuO2, TC (h k l), were calculated by Eq. (1) [26]: TCðhklÞ ¼
1 IðhklÞ 1 X IðhklÞ I0 ðhklÞ n I0 ðhklÞ
(1)
where I(h k l) denotes the measured intensity of the (h k l) plane; I0(h k l) is the standard intensity of the standard XRD patterns data; n is the total number of reflections. The calculated values of TC(h k l) for IrO2 and RuO2 are shown in Fig. 2. The same orientation is shown to be preferred for the rutiletype IrO2 and RuO2. The (101) plane is preferred as the growth orientation of IrO2 and RuO2 in the calcination temperature ranged from 350 to 450 C. Yet in the temperature ranged from 500 to 600 C, the (211) plane is the preferred orientation. The rise in the calcination time does not vary the preferred orientation of IrO2 and RuO2. As the results of Hu et al. suggested [27], the (101) rutile-type IrO2 shows better electrocatalytic activity for OER since the IreO bond is shorter for the (101) plane than for the other facets. The Ti/IrO2e RuO2eSiO2 anodes calcined at low temperatures may show higher electrocatalytic activity than the anodes calcined at high temperatures.
FESEM analysis The FESEM images of the prepared electrodes obtained at low magnification are shown in Fig. 3. As shown in Fig. 3, considerable amounts of cracks can be observed on the coatings calcined at 350 and 400 C for 15 min, but they are rarely found on the coatings calcined at high temperatures or for 60 min. The amount of the cracks in the coatings decreases with the rise of calcination temperature and calcination time. The oxidation of the Ti substrate would occur due to the presence of pores and cracks in the coating, leading to the failure of the anode in the OER process [28]. The samples calcined at high temperatures or for a long time may show higher stability for OER due to fewer cracks in the coating. For the samples calcined at the temperatures higher than 400 C
Fig. 1 e XRD patterns of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
514
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
Fig. 2 e Calculated texture coefficient (a) IrO2 calcined for 15 min, (b) RuO2 calcined for 15 min, (c) IrO2 calcined for 60 min, and (d) RuO2 calcined for 60 min.
for 15 min or for 60 min, the coatings with the dispersion of fine crystals can be observed. Yet the fine crystals are rarely observed for the samples calcined at 350 and 400 C for 15 min. As the EDS analysis suggests, the fine crystals are enriched in iridium and ruthenium, which can be considered as the IrO2e RuO2 solid solution. To further investigate the surface morphology of the coatings, FESEM analysis was conducted at high magnification, with the obtained images shown in Fig. 4. The active components are not crystallized and show an amorphous phase for the sample calcined at 350 C for 15 min, as shown in Fig. 4a. In addition, a small amount of spherical crystals can be observed on the coating calcined at 400 C for 15 min (Fig. 4b). The coatings with a large amount of dispersed acerose crystals can be observed for the samples calcined in the temperature ranged from 450 to 600 C for 15 min (Fig. 4cef). All the obtained coatings of the samples calcined for 60 min show crystalline state (Fig. 4gel). The crystallinity degree and the crystallite size of the coating calcined at 400 C for 60 min are obviously higher than those of the coating calcined at 400 C for 15 min. This suggests that the crystallization and the growth of IrO2 and RuO2 crystals require a considerable amount of time at 350 and 400 C. The crystallite sizes were evaluated based on the FESEM analysis results, as shown in Table 1. Examination of the data in Table 1 suggests that the rise of the calcination temperature and the calcination time lead to an increase of the crystallite size. The larger crystallite size contributes less to the active surface area of the coating that may decrease the electrocatalytic activity for OER.
Electrochemical active surface area The CV curves of the prepared samples in a sulfuric acid solution are shown in Fig. 5. The calcined anodes show typical voltammetric behavior representative of thermally prepared oxide coatings. The broad peaks observed in the potentials ranged from 0 to 1.2 V can be attributed to the oxidation state transitions of the noble metal, e.g., Ru(III)/Ru(IV), Ru(IV)/Ru(V), Ir(III)/Ir(IV) and Ir(IV)/Ir(V) [29,30]. The heterogeneity of the coatings broadens the peaks observed in the CV curves [31]. Regardless of the sample calcined at 400 C for 15 min, the voltammetric response of the electrode decreases with a rise of the calcination temperature, as shown in Fig. 5a. Increasing calcination time improves the voltammetric response of the coating calcined at 400 C, while it reduces the voltammetric response of the other samples (Fig. 5b). Since the compositions of the prepared anodes are basically the same, the differences in the voltammetric response can result from a variation in the active surface area. To estimate the active surface area of the anodes, we calculated the voltammetric charge, q*, using graphic integration. On the whole, at a high sweep rate, the reaction occurs at the more accessible part of the surface (“outer” active surface). Besides, at a low potential scan rate, the reaction takes place on both the “outer” and the “inner” active surfaces. The “inner” active surface is associated with the pores, cracks and defects of the coatings. The values of “outer” and “inner” surface voltammetric charges (qouter and qinner) were calculated using the following equations [22,32]:
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
515
Fig. 3 e FESEM images of the Ti/IrO2eRuO2eSiO2 electrodes at low magnification ( £ 500).
1 q* ¼ 1 qinner þ qouter þ C1 v1=2
(2)
q* ¼ qouter þ C2 v1=2
(3)
where C1 and C2 are constants, and v denotes the sweep rate. The total voltammetric charge, qtotal, is defined as the sum of qouter and qinner. The voltammetric charge value is sweep rate dependent. Based on the voltammetric charge values of various sweep rates, the plots of 1/q* vs. v1/2 and q* vs. v1/2 can be obtained. These two plots enable the data to be extrapolated to v ¼ 0 and v ¼ ∞ to achieve the values of qtotal
and qouter, respectively, from which the value of qinner can be derived. The calculated voltammetric charge of the prepared samples is shown in Fig. 6. This Figure shows that the highest qtotal value can be obtained from the coating calcined at 350 C for 15 min, so this sample has the largest active surface area. As the results of the XRD and FESEM analyses suggest, iridium and ruthenium oxides are present as an amorphous phase after calcination at 350 C for 15 min. Due to the disorder of the amorphous oxide, the amorphous parts of the active components contribute more to the active surface area than the crystalline parts [23]. The qtotal values of the coatings calcined
516
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
Fig. 4 e FESEM images of the Ti/IrO2eRuO2eSiO2 electrodes at high magnification ( £ 30000).
Table 1 e Crystallite size of the IrO2eRuO2eSiO2 ternary oxide coatings. Temperatures ( C)
350 400 450 500 550 600
Crystallite size (nm) Calcined for 15 min
Calcined for 60 min
e 30e40 50e60 70e110 80e110 100e120
20e40 55e80 60e90 110e140 125e200 230e500
in the temperature ranged from 450 to 550 C for 15 min are higher than those of the coatings calcined at 400 and 600 C for 15 min. The rise of calcination time increases the qtotal value of the coating calcined at 400 C, while it decreases the qtotal values of the other samples. This suggests that the active sites of the surface are determined by the crystallization behavior of the active components. More active sites are attributed by higher crystallinity degree, and the number of active sites also increases with finer components. The obtained qouter values are higher than the qinner values over the entire calcination temperature and calcination time range. This suggests that the active surface area of the coating is dominated by the “outer” active surface.
517
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
0.020
0.020
0.015
o
o
450 C
o
500 C
o
(a)
-2
o
400 C
Current density (A cm )
-2
Current density (A cm )
o
350 C
o
550 C
600 C
0.010 0.005 0.000 -0.005 -0.010 0.0
0.2
0.4 0.6 0.8 Potential vs. SCE (V)
1.0
350 C o 500 C
0.015
o
450 C (b) o 600 C
0.010 0.005 0.000 -0.005 -0.010 0.0
1.2
o
400 C o 550 C
0.2
0.4 0.6 0.8 Potential vs. SCE (V)
1.0
1.2
Fig. 5 e CV curves of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
800
qtotal
1200
(a) -2
1000
qinner qouter
Charge (mC cm )
-2
Charge (mC cm )
1200
600 400 200
1000
qinner qouter
800
qtotal
(b)
600 400 200
0
0 350
400 450 500 550 o Calcination Temperature ( C)
600
350
400 450 500 550 o Calcination Temperature ( C)
600
Fig. 6 e Voltammetric charge calculated by integration and extrapolation of the CV curves (a) calcined for 15 min, (b) calcined for 60 min.
Anodic polarization curves The IR-corrected anodic polarization curves of the prepared electrodes in a sulfuric acid solution are shown in Fig. 7. Fig. 7a shows that the amorphous coating calcined at 350 C for 15 min has the best electrocatalytic activity for OER. Thanawala et al. [33] and Reier et al. [34] also suggested that the reactivity of the amorphous IrO2 coating is higher than that of the crystalline one, suggesting better catalytic activity for OER. Pfeifer et al. [35] investigated the electronic structure of iridium oxide electrodes. They observed that the enhanced activity of the amorphous iridium oxides compared to their crystalline counterparts is attributed to the electronic defects in the near-surface region of the anionic and cationic framework of the amorphous iridium oxides.
The electrocatalytic performance of the electrode is controlled by both the active surface area and the intrinsic catalytic nature [36,37]. Shan et al. [38] validated that the IrO2 nanoparticles show a better electrocatalytic activity than the bulk one due to the high active surface area. The performance of the IrO2eRuO2eSiO2 coating calcined at 450 C for 15 min is almost similar with that of the amorphous coating, which can be attributed to more and finer active components and the higher conductivity of the crystalline parts [39]. In addition, given the preferred orientation of the IrO2 and RuO2, the shorter length of the bond for IreO and RueO of (101) facet may to some extent improve the electrocatalytic activity. The electrocatalytic activity of the anodes calcined in the temperature range between 500 and 600 C for 15 min is slightly lower than that of the anode calcined at 450 C for 15 min. This
0.08
0.08
0.06
o
400 C o 550 C
o
450 C o 600 C
(a)
0.05 0.04 0.03 0.02 0.01 0.00 1.0
1.1
1.2 1.3 Potential vs. SCE (V)
o
-2
350 C o 500 C
Current density (A cm )
-2
Current density (A cm )
o
0.07
1.4
0.07 0.06
350 C o 500 C
o
400 C o 550 C
o 450 C (b) o 600 C
0.05 0.04 0.03 0.02 0.01 0.00 1.0
1.1
1.2 1.3 Potential vs. SCE (V)
1.4
1.5
Fig. 7 e IR-corrected anodic polarization curves of the Ti/IrO2eRuO2eSiO2 electrodes (a) calcined for 15 min, (b) calcined for 60 min.
518
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
o
350 C o 450 C o
550 C
0
5
o
400 C o 500 C
h ið1nÞ Q ¼ ðCdl Þn ðRs Þ1 þ ðRct Þ1
(a)
o
600 C
10 15 20 2 Re (Z) (Ω cm )
To further explore the electrochemical properties of the prepared samples, EIS measurements were performed. The obtained Nyquist diagrams and Bode plots are shown in Fig. 8. All impedance spectra show a depressed semicircle, as shown in Fig. 8a and b. This suggests that the electrochemical processes taking place on the ternary oxide coatings are controlled by the electrochemical reaction [40]. The Bode plots given in Fig. 8c and d show the inductive properties of the ternary oxide coatings due to the porosity of the active oxide layer. Similar results were commonly obtained for the anodes prepared using thermal decomposition method [22]. The obtained Nyquist diagrams and Bode plots were fitted using an equivalent electrical circuit (EEC), described as LRs(Rct, OCdl, O) (Rct, ICdl, I) [22]. Here, L is the inductance, Rs, Rct, O and Rct, I refer to the solution resistance, the charge transfer resistance at the “outer” and “inner” active surfaces, respectively. Cdl, O and Cdl, I are the double-layer capacitances of the “outer” surface/electrolyte and “inner” surface/electrolyte interfaces, respectively. Given the nonuniformity of the prepared coatings, we replaced constant phase element (CPE) with the pure capacitors (Cdl, O and Cdl, I), denoted by Qdl, O and Qdl, I, respectively [41]. When the CPE is coupled with the charge transfer resistance, the double layer capacitance can be evaluated using the following equation [42,43]:
2
20 18 16 14 12 10 8 6 4 2 0 -2
EIS measurements
-lm (Z) (Ω cm )
2
-lm (Z) (Ω cm )
can result from the increased crystallite size and the preferred orientation of the active components. It is noteworthy that the anode calcined at 400 C for 15 min exhibits the lowest activity for OER in comparison with the anode calcined at 350 and 450 C for 15 min. Increasing calcination temperature decreases the disorder of the amorphous oxides, and the low crystallinity of the coating calcined at 400 C for 15 min contributes less to the active surface area, resulting in the low electrocatalytic activity of this anode for OER. The electrocatalytic activity of the anodes calcined for 60 min first increases and then decreases with the rise of calcination temperature, as shown in Fig. 7b. The coating calcined at 450 C for 60 min shows the best electrocatalytic activity over the entire anodes calcined for 60 min, whereas it is still lower than that of the anodes calcined at 350 and 450 for 15 min. The rise of calcination time improves the electrocatalytic activity of the sample calcined at 400 C, while it decreases the activity of the other samples. This can be attributed to the crystallization behavior of the active components, impacting the active surface area of the coating for OER. Moreover, the oxidation of the Ti substrate can deteriorate the electrocatalytic activity of the anode, resulting in the increase in the overpotential for OER. Similar results were found by Reier et al. [34]. They illustrated that a Ti oxide interlayer can be formed and Ti oxide migrates into the Ir oxide layer at a high calcination temperature, leading to the decrease of the electrocatalytic activity for OER.
25
30
o
400 C
o
o
500 C o 600 C
350 C
5
(b)
o
450 C o 550 C
0
100
10 15 20 25 30 35 40 45 50 2 Re (Z) (Ω cm )
120 350 C
80
o
450 C o 550 C
60
o
400 C
o
(c)
o
500 C o 600 C
Phase angle (deg)
o
Phase angle (deg)
40 36 32 28 24 20 16 12 8 4 0
(4)
40 20 0
100
350 C
80
450 C o 550 C
o
o
400 C
(d)
o
500 C o 600 C
60 40 20 0
-20
-20 -2
-1
0
1 2 3 log (frequency / Hz)
4
5
-2
-1
0
1 2 3 log (frequency / Hz)
4
5
Fig. 8 e EIS plots of the Ti/IrO2eRuO2eSiO2 electrodes (a) Nyquist diagrams of the electrodes calcined for 15 min, (b) Nyquist diagrams of the electrodes calcined for 60 min, (c) Bode plots of the electrodes calcined for 15 min, and (d) Bode plots of the electrodes calcined for 60 min.
519
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
Based on the obtained Qdl, O and Qdl, I values, the real Cdl, O and Cdl, I values were calculated using Eq. (4). Where, Cdl, total is defined as the sum of the Cdl, O and Cdl, I. Also, Rct, total is defined as the sum of Rct, O and Rct, I [22]. The values of the charge transfer resistance and the double-layer capacitance obtained by fitting of the EIS measurements are shown in Fig. 9. The lowest Rct, total value can be obtained from the anode calcined at 350 C for 15 min, as shown in Fig. 9a. This therefore shows the highest electrocatalytic activity for OER [40,44]. The Rct, total obtained from the anode calcined at 450 C for 15 min is lower than that of the anodes calcined in the temperatures ranged from 500 to 600 C for 15 min. This suggests that the (101) IrO2 and (101) RuO2 show a higher efficiency for the charge transfer than the (211) IrO2 and (211) RuO2. At the entire calcination temperatures, the obtained Rct, I values are higher than the Rct, O values, suggesting that the “outer” surface is more electrocatalytically active than the “inner” surface. The calculated voltammetric charge values suggest that the active surface area of the ternary oxide coatings is dominated by the “outer” active surface. The higher electrocatalytic activity of the “outer” surface can therefore result from its larger active surface area. Fig. 9b shows that the anode calcined at 350 C for 15 min has the highest Cdl, total value and therefore the largest active surface area [45], which is associated with the disordered structure of the amorphous oxide. In the crystalline samples, regardless of the anode calcined at 400 C, the Cdl, total value decreases with a rise of the calcination temperature. The decreased Cdl, total can be attributed to the increased crystallite size decreasing the active surface area of the coating. The low
value of the Cdl, total obtained from the anode calcined at 400 C is associated with the slight generation of the fine crystals. The obtained Cdl, O values are higher than the Cdl, I values over the entire calcination temperature range. This suggests that the active surface area of the prepared coatings is dominated by the “outer” active surface, which is consistent with the results of the voltammetric charge calculation. The Rct, total values of the samples calcined for 60 min first decrease and then increase with the rise of calcination temperature, as shown in Fig. 9c. The variation of the Cdl, total values presents a reverse tendency (Fig. 9d). Regardless of the anode calcined at 400 C, the charge transfer resistance value increases, and the double-layer capacitance value decreases with the rise of calcination time. This suggests that the electrocatalytic activity and active surface area of the coating are crystallization behavior dependent, which are impacted by the calcination conditions (e.g., temperature and time). The significant increase in the Rct, total values can be observed for the samples calcined at 550 and 600 C for 60 min in comparison with the samples calcined at the same temperatures for 15 min. This suggests that the oxidation of Ti substrate deteriorates the electrocatalytic activity of the anodes for OER.
Electrocatalytic stability The results of the accelerated life test are shown in Fig. 10. The service lifetime of the anodes is shown to increase with the rise of the calcination temperature. The growth becomes weak when the calcination temperature is higher than 500 C. The increase of calcination time can also slightly improve the
0.30 Rct, O
30
Rct, I
25
Rct, total
(a)
(b) -2
35
Capacitance (F cm )
2
Resistance (Ω cm )
40
20 15 10 0
Cdl, I Cdl, total
0.20 0.15 0.10
0.00 350
400 450 500 550 o Calcination Temperature ( C)
600
Rct, O
(c)
350
400 450 500 550 o Calcination Temperature ( C)
600
0.20 -2
50
Rct, I
40
Capacitance (F cm )
60 2
Cdl, O
0.05
5
Resistance (Ω cm )
0.25
Rct, total
30 20 10 0
(d) 0.16
Cdl, O Cdl, I Cdl, total
0.12 0.08 0.04 0.00
350
400 450 500 550 o Calcination Temperature ( C)
600
350
400 450 500 550 o Calcination Temperature ( C)
600
Fig. 9 e Charge transfer resistance and double-layer capacitance of the Ti/IrO2eRuO2eSiO2 electrodes (a), (b) calcined for 15 min, (c), (d) calcined for 60 min.
520
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
Accelerated lifetime (h)
350 Calcined for 15 min Calcined for 60 min
300 250 200 150 100 50 0 350
400 450 500 550 o Calcination Temperature ( C)
600
Fig. 10 e Accelerated lifetime of the Ti/IrO2eRuO2eSiO2 electrodes.
service lifetime of the anodes for OER. The lowest accelerated lifetime of nearly 10 h can be obtained from the anode calcined at 350 C for 15 min. The amorphous oxide coating shows low electrocatalytic stability for OER in a sulfuric acid solution. Similar results were reported by Wang et al. [24], suggesting that the amorphous IrO2 has low electrocatalytic stability in acidic media. The stability of nanostructured Ir oxide electrocatalysts was also investigated by Cherevko et al. [46]. They indicated that the stability of Ir oxide film increases with increasing calcination temperature. The increased stability of the Ir oxide film calcined at 450 C compared to the film calcined at 250 and 350 C is attributed to the formation of a crystalline IrO2 phase that is shown to exhibit a higher degree of lattice oxygen at the surface and a lower degree of surface hydroxylation. It can plausible to conclude that the electrocatalytic stability of the anode relies on the crystalline order and lattice oxygen of the coating. As illustrated in the literature [20,47], the service lifetimes of the Ti/IrO2eRuO2 anode and Ti/IrO2eRuO2eTa2O5 anode are 12e30 and 80e120 h, respectively, during the ALT in sulphuric acid solution. The Ti/IrO2eRuO2eSiO2 anodes calcined at the temperatures higher than 450 C show higher service lifetimes than these two anodes. Moreover, silica is a non-noble oxide. Doping silica into the coating decreases the cost for electrode production. We believe that the prepared Ti/IrO2eRuO2eSiO2 anodes with long-term stability and low cost are promising candidates for the OER applications. Anode deactivation consists of metal substrate passivation, film consumption, film detachment as well as mechanical damage [48,49]. According to the results of Cherevko et al. [50], Ir and Ru exhibit both transient dissolution during oxide formation/reduction and steady-state dissolution during the evolution reaction. Kotz et al. [51] and Gajic-Krstajic et al. [12] reported the deactivation mechanism of the Ti/IrO2 anode and the Ti/RuO2 anode, respectively. Their results suggested that the deactivation can result from the dissolution of the active components and the oxidation of the Ti substrate through the defects of the coating. The FESEM analysis of the prepared anode after ALT are shown in Fig. 11. The Ti substrate is shown to be exposed with the dissolution of the active coating. According to the results of the EDS analysis shown in Table 2, Ir, Ru and Si remain on the IrO2e
Fig. 11 e FESEM analysis of the Ti/IrO2eRuO2eSiO2 anode after ALT.
Table 2 e EDS analysis of the Ti/IrO2eRuO2eSiO2 anode before and after ALT, in at.%. Elements
Ti
Ir
Ru
Si
O
Before ALT After ALT
1.06 25.47
4.07 1.71
7.57 1.10
22.42 5.46
64.88 66.26
RuO2eSiO2 ternary oxide coating in a small amount after ALT. The at.% of Ti of the failed anode is much higher than that of the fresh anode. The atom ratio of (Ir þ Ru þ Si þ Ti):O of the failed anode is approximately 1:2. This indicates that the Ti substrate is oxidized during the electrolysis process. Similar results were obtained by Hoseinieh et al. [52]. They indicated that the growth of an insulating TiO2 layer is the major reason for the deactivation of the Ti/IrO2eRuO2eTiO2 anode. Consequently, the failure of the prepared Ti/IrO2eRuO2eSiO2 anode can be attributed to the dissolution of the active coating and the oxidation of the Ti substrate. The higher electrocatalytic stability of the anodes calcined at high temperature and for a long time can result from the fewer cracks and high crystallinity of the coating. Furthermore, the higher stability of the (211) IrO2 and (211) RuO2 can also extend the service lifetime of the anode for OER in a sulfuric acid solution.
Conclusion The impact of calcination temperature and calcination time on the crystallization, surface properties, electrocatalytic activity and stability were investigated. The prepared coating is composed of amorphous SiO2, rutile-type IrO2, RuO2 as well as IrO2eRuO2 solid solution. The amorphous oxide coating is observed from the sample calcined at 350 C for 15 min, while this coating would be crystallized when the calcination time is 60 min. The coatings calcined in the temperature ranged from 350 to 450 C show preferred (101) planes of IrO2 and RuO2 crystallites, whereas the coatings calcined at the temperatures higher than 450 C have (211) orientation. The rise of calcination time does not vary the preferred orientation of IrO2 and RuO2. The crystallinity and crystallite size increase with the rise of calcination temperature and calcination time, impacting the active surface area of the coating for OER. As the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 5 1 1 e5 2 2
results of the calculated voltammetric charge suggest, the active surface area of the ternary oxide coating is dominated by the “outer” active surface, which is also confirmed by performing the EIS measurements. The rise of calcination temperature and calcination time extend the service lifetime of the anode for OER. The electrocatalytic activity and stability of the anode rely on the crystallization behavior of the active components, which are impacted by the calcination conditions. The largest active surface area is obtained for the anode calcined at 350 C for 15 min, thus showing the highest electrocatalytic activity for OER; yet the stability of this anode is low. Given the electrocatalytic activity and stability of all investigated anodes, the anode calcined at 450 C for 15 min is considered the most suitable for use in applications.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No.51674026), the Beijing Natural Science Foundation, China (No.2182040), the Beijing Science & Technology Program (Z171100002217063), the Fundamental Research Funds for the Central University (230201606500078), and the National Natural Science Foundation of China (No. U1302274).
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