Electrochemical studies of Ir coating deposition from NaCl-KCl-CsCl molten salts

Surface & Coatings Technology 322 (2017) 76–85

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Electrochemical studies of Ir coating deposition from NaCl-KCl-CsCl molten salts Yongle Huang, Shuxin Bai ⁎, Hong Zhang, Yicong Ye, Li'’an Zhu College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China

a r t i c l e

i n f o

Article history: Received 20 November 2016 Revised 10 May 2017 Accepted in revised form 11 May 2017 Available online 12 May 2017 Keywords: Electrochemistry Microstructure Iridium Melts Chronopotentiometry Electrochemical impedance spectrum

a b s t r a c t In this paper, the electrochemical behavior of Ir3+ ions in melting NaCl-KCl-CsCl-IrCl3 and the major kinetic parameters were investigated by transient and steady electrochemical techniques from 823 K to 913 K. Effects of the electrochemistry of Ir3+ ions on the microstructure of Ir coatings were also discussed. The electrochemical reduction of Ir3+ ions from the melts was the diffusion-controlled and irreversible reaction with exchange of three electrons at 823–883 K and converted to be a quasi-reversible one with the temperature increasing to 913 K. The major kinetic parameters of Ir3+/Ir couples, the diffusion coefficient D0 and exchange current density j0, respectively increased from 2.4 × 10−6 cm2/s to 9.2 × 10−6 cm2/s and from 5 mA/cm2 to 16 mA/cm2 with the temperature. With the temperature and applied cathodic potential decreasing, the microstructure of Ir coatings converted from the coarse and preferential oriented columnar grain structure to the fine and unoriented dispersive grain structure. © 2017 Published by Elsevier B.V.

1. Introduction Metal iridium (Ir) is considered as one of the most promising oxidation resistance materials at elevated temperatures due to its high melting point and low oxygen permeability and good mechanical properties [1–4]. Ir and its alloys thus have been widely used as the oxidation resistance coatings for high temperature structural applications, such as satellites liquid rocket engines and leading edges of hypersonic aircraft [5– 7]. Ir coatings have been prepared by versatile approaches [8–12], among which electrodeposition was the most efficient and highquality one for fabricating Ir coatings on complex-shaped components [13–18]. And nontoxic ternary NaCl-KCl-CsCl melts were widely used as electrolytes in the Ir electrodeposition [19]. Dense and continuous Ir coatings on various substrates have been prepared successfully from the NaCl-KCl-CsCl melts by Zhu and Huang [20–30]. According to them, the morphology of Ir coatings is decided by the deposition conditions, including temperature, current density and concentration of Ir3 + ions. Moreover, according to Saltykova N. A. [31,32], while the electrochemical reduction of Ir3 + ions in ternary melts is totally irreversible at 773–873 K, it will be quasi-reversible or reversible when the temperature is higher than 873 K. Kuznetsov [33] revealed that as the melt basicity increases, number of electrode processing stage decreased and a transition from the reversible to irreversible process of ⁎ Corresponding author. E-mail address: [email protected] (S. Bai).

http://dx.doi.org/10.1016/j.surfcoat.2017.05.032 0257-8972/© 2017 Published by Elsevier B.V.

electrochemical reaction occurred. However, although many efforts have been paid to the Ir electrodeposition, the electrochemical behavior of Ir3 + ions in melting NaCl-KCl-CsCl and corresponding electrochemical mechanism were rarely reported. It was unfavorable for the further improvement of Ir coating. In this work, the electrochemistry and major kinetics parameters of Ir3+ ions in the NaCl-KCl-CsCl melts were studied. The influences of deposition parameters on the microstructure of Ir coatings and the corresponding electrochemical mechanism were also discussed, which may provide more insights for better understanding of electrodeposition of Ir coatings from the melting chlorides.

2. Experimental 2.1. Chemicals All operations of chemicals were carried out under the inert argon atmosphere. After being dried under vacuum at 423 K for 24 h, equal mole anhydrous NaCl, KCl, CsCl (analytical grade, Alpha Chemical Reagent Co., Ltd) were mixed and used as the supporting electrolytes. The temperature deviation of melts, which was measured using a nickel-chromium thermocouple packed by a closed-end quartz tube, was maintained within ± 2 K. After being completely melted, the melts were pre-electrolyzed for several hours to remove impurities. Ir3 + ions was introduced into the bath through anhydrous IrCl3 powder (purity: 99.99%, Shanxi Kaida Chemical Engineering Co., Ltd).

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2.2. Electrochemical electrodes and apparatus Ag/AgCl (1 wt%AgCl) and Ir3 +/Ir were used as the reference electrodes (RE) in different experiments. The former was made of the silver wire (Φ1.0 mm, 99.99%, China New Metal Materials Technology Co., Ltd), which was immersed into the melting NaCl-KCl-CsCl with 1.0 wt.% AgCl contained in a thin-walled quartz tube with an capillary pore. The latter was made of the pure metallic Ir strip (2.0 × 2.5 × 100 mm, 99.95%, China New Metal Materials Technology Co., Ltd), which was immersed directly into the solution of NaCl-KCl-CsCl molten salts containing IrCl3. Platinum wires (Φ1.0 mm, 99.99%, China New Metal Materials Technology Co., Ltd) and Ir strips sealed in corundum tube with Pyrex glass, which had been polished by abrasive papers, ultrasonically cleaned in deionized water and dried in vacuum at 423 K successively, were used as the working electrodes (WEs). Counter electrodes were made up of graphite rods (Φ10 × 100 mm SGL Carbon Co, Ltd.), which were polished and cleaned in acetone prior to each experiment. 2.3. Electrochemical measurements All electrochemical measurements were performed using a CHI660E potentiostat-galvanostat electrochemical workstation controlled by a computer with the CHI version 14.01 software package. Transient and steady electrochemical techniques, i.e. cyclic voltammetry (CV), liner sweep voltammetry (LSV), chronopotentiometry (CP) and electrochemical impedance spectrum (EIS) were measured to explore the electrochemical reduction of Ir3 + ions in the melts. In order to confirm reproducibility of the experiments, electrode cleaning process was performed and electrochemical operation was stopped for 20 min to reach the temperature and composition equilibrium of the melts. 2.4. Electrodeposition of Ir coatings on Re coated graphite substrate Graphite rods (Φ10 × 50 mm, SGL Carbon Co, Ltd.) whose surface was polished by abrasive papers and ultrasonically cleaned in acetone for 15 min, were used as substrates. Re layers were prepared on the graphite substrates as the transition layers between the substrates and Ir coatings. Re layers on graphite substrates were prepared by chemical vapor deposition (CVD) with the device reported in previous work [21,22]. Graphite substrates were treated at 1573 K for 10 min under argon atmosphere, to remove entrapped gases and impurity. Rhenium layers with the thickness of 20–25 μm were prepared by the thermal decomposition of ReCl5, with the chlorination temperature of 973–1173 K, deposition temperature of 1273–1573 K, flow rate of chlorine (purity: 99.999%) of 50–100 mL/min, flow rate of argon (purity: 99.999%) of 100–500 mL/min. Ir coatings on Re coated graphite substrates (Φ10mm × 60 mm) were prepared by electrodeposition, which were carried out in the equal mole NaCl-KCl-CsCl ternary electrolyte containing 0.20 mol/L IrCl3. The cylinder graphite crucible (Φ90 mm × 130 mm) which was degassed at 1773 K for 1 h in a vacuum furnace before used and stored

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the electrolyte salts was used as the anode. While the disk cathode was made up of the Re coated graphite rods inserting in the quartz tubes with 10 μm inside diameter. Pure Ir strips immersed into the melts used as REs. The parameters of Ir electrodeposition was listed in Table 1. The morphology of Ir coating was observed by scanning electron microscope (SEM, Hitachi S-4800). The energy dispersion spectroscopy (EDS) was performed by GSM-6360LV to determine the composition of coating. 3. Results and discussions 3.1. Electrochemical reduction of IrCl3 Fig. 1 shows a family of CVs with the scan rate ranging from 0.05 V/s to 0.40 V/s obtained on the Pt electrode at 883 K in the NaCl-KCl-CsCl melts containing 0.2 mol/L IrCl3. A couple of redox current peaks corresponding to the electrochemical deposition/dissolution of Ir are appeared at about − 0.20 V and +0.30 V when the scan rate is 0.05 V/s. While the peaks of cathodic and anodic current are dumpy. There is another anodic peak when the potential increases to about +0.40 V which should correspond to the anodic reaction of Cl− ions in the melts, because only the Cl− ions in the melts can lost its electrons to form anodic current peak in the CVs. The CVs at different scan rates were considered as the solution regarding to reversibility of the electrochemical reaction though application of peak-voltammetry theories. As shown in Fig. 1, as the scan rates increases from 0.05 V/s to 0.40 V/s, the cathodic peaks shifts negatively and the anodic peaks shifts positively accordingly. According to peak-voltammetry theories [34], the cathodic peak potential Ep is a function of the scan rate v for the quasi-reversible or irreversible reaction, while it is independent of the scan rate v for the reversible reaction. That is to say, electrochemical reduction of Ir3+ ions should be a quasireversible or irreversible reaction. The relationships between cathodic peak current ip, cathodic peak potential Epc and scan rate v are displayed in Fig. 2a and b, respectively. As can be seen, ip linearly increases with square root of the scan rate v1/2, while Epc is linearly dependent of lnv1/2. According to Matsud and Hubbard [35,36] [37], the electrochemical reduction of Ir3+ ions is an diffusion-controlled irreversible reaction, for which the peak potential Epc and ln v1/2 has a relationship as [38]: Epc ¼ b−

RT ln v1=2 αF

ð1Þ

Table 1 electrodeposition parameters of Ir coating on Re coated graphite. Current or Groups potential 1 2 3 4 5 6 7 8

−20 mA/cm2

−0.03 V −0.10 V −0.15 V −0.20 V

Temperature (K) 823 853 883 913 853

Others electrolyte: NaCl-KCl-CsCl CIr3+: 0.20 mol/L substrate: Re coated graphite (1.13 cm2)

Fig. 1. A family of CVs on the Pt cathode at 883 K in NaCl-KCl-CsCl molten salts containing 0.2 mol/L IrCl3: − 0.90 V (vs Ir/Ir3+) ~ + 0.50 V (vs Ir/Ir3+), scanning rate 0.05 V/s– 0.40 V/s.

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Fig. 2. Variation of the cathodic peak current density of Ir3+ ions reduction with the square root of scan rate (a); evolutions of the peak potentials of Ir3+ ions reduction as a function the scan rate (b).

where, b is a constant independent of the scan rate v; F is the Faraday's constant (96,485 C/mol); v is the scan rate (V/s); R is the gas constant (8.314 J·mol−1·K−1); T is the absolute temperature (883 K in this work); α is the transfer coefficient, which can be calculated as 0.45 according to data shown in Fig. 2b. Moreover, the linear relationship of the cathodic peak current ip with square root of scan rate v1/2 can be described as: 1=2 ip ¼ 0:4958  nFAC o D1=2 o v

  αnF 1=2 RT

ð2Þ

where, jp is the current density of cathodic peak (A); n is the transfer electrons number (n = 3); Co is the initial concentration of the active species (2 × 10−4 mol/cm3); A is the cathodic electrode surface (0.05 cm2). The diffusion coefficient Do of Ir3+ in melts thus can be calculated as 6.10 × 10−6 cm2/s based on Eq. (2) and the data shown in Fig. 2a. Fig. 3a and Fig. 3b are the SEM images of Pt electrode before and after electrodeposition at −0.40 V (v.s. Ir/Ir3+) for 60 s in NaCl-KCl-CsCl molten salts containing 0.2 mol/L IrCl3, respectively. Obviously, some protrusions, which can be deem as the deposits, are found on the Pt wire. These protrusions are affirmed to be pure metallic Ir by EDS, as shown

Fig. 3. Morphologies of the platinum wire before and after electrodeposited in the NaClKCl-CsCl melts containing 0.2 mol/L IrCl3 at −0.40 V (vs Ir/Ir3+) for 60 s: (a) before, (b) after, (c) EDS analysis.

in Fig. 3c. It means that the content of deposits is 100% Ir, agreeing with results of CV tests. To determine the two anodic reactions shown as two anodic peaks in Fig. 1, a pre-deposition of Ir3+ ions was carried out on the platinum electrode at −0.40 V for 10s and then followed by an anodic reaction of the pre-deposits. Several such pre-deposition/anodic polarization operations were carried out at various anodic potentials from + 0.10 V to

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Fig. 4. Anodic dissolution of pre-deposited Ir on the platinum electrode in NaCl-KCL-CsCl melts containing 0.20 mol/L IrCl3: +0.10 V (vs Ir/Ir3+) ~ +0.40 V (vs Ir/Ir3+), 853 K. Fig. 5. chronopotentiograms of Ir3+/Ir redox couples on a platinum electrode in molten NaCl-KCl-CsCl salts containing 0.2 mol/L IrCl3; T = 853 K.

+0.40 V, as shown in Fig. 4. Two anodic current plateaus are observed at all j–t curves of the anodic polarization stage. The current density of the first plateaus increases linearly as the applied anodic potential. Coulombs of the first plateaus at different potentials keep constant which is close to that of Ir pre-deposition at − 0.40 V for 10 s. It means that the first plateau is resulted from the anodic dissolution of the pre-deposited Ir. After the dissolution of Ir deposits on the Pt electrode, the anodic current decreases to the second plateau. When the applied anodic potential is lower than +0.20 V, the anodic current density is close to zero. It demonstrates the second anodic reaction on the Pt electrode is negligible under + 0.20 V. As the applied anodic potentials increases, the current density of the second plateau increases much more quickly. As discussed above, the second current plateau should correspond to the anodic reaction of the free Cl− ions in the melts. That is to say, the anodic dissolution of pre-deposited Ir is prior to the anodic reaction of Cl− ions on the Pt electrode. So far, the electrochemical mechanism during electrodeposition of Ir coatings from the NaCl-KCl-CsCl-IrCl3 melts is obtained and listed as following: When the anode is made of inert materials, such as platinum and graphite, Ir 3þ þ 3e− →Ir 1 − Cl þ e− → Cl2 2

ðcathodeÞ

ð3aÞ

ðanodeÞ

ð3bÞ

3 IrCl3 →Ir þ Cl ↑ 2 2

3.2. Reaction kinetics 3.2.1. Diffusion coefficients Fig. 5 illustrates the chronopotentiograms using different current intensities on the Pt electrode in NaCl-KCl-CsCl melts containing 0.2 mol/L IrCl3 at 853 K. The curves show a family of voltage plateaus at the potentials between 0.00 V and − 0.40 V, which correspond to the electrochemical reduction of Ir3+ ions from the melts. It is in agreement with the reduction potential in the CVs. After these voltage plateaus, the cathodic potential rapidly decreases to that of another cathodic reaction (maybe the deposition of Na+). The transition time τ for Ir reduction is determined by using the method described in Ref. [40]. Fig. 6 shows the j − τ−1/2 plots at four different temperatures exhibiting the linear relationship. This indicates that Sand's equation, derived for diffusion controlled electrochemical reactions, is applicable to the deposition of Ir on the Pt electrode. The diffusion coefficients of Ir3+ ions are calculated from slopes of the j − τ−1/2 plots per Sand's equation: j¼

1=2 nFD1=2 o π C o τ−1=2 2

ð5Þ

ð3Þ

When the anode is made of pure metallic Ir, Ir 3þ þ 3e− →Ir

ðcathodeÞ

ð4aÞ

Ir−3e− →Ir 3þ

ðanodeÞ

ð4bÞ

Ir ðanodeÞ→IrðcathodeÞ

ð4Þ

To sum up, it was feasible to electrodeposit Ir coating continuously by using pure Ir metal as anodes for supplement of Ir3 + ions to the melts simultaneously. This conclusion is agreeable with the report that Ir crucibles with the thickness of N3 mm is prepared successfully by electrodeposition from molten salts by using Ir as anodes [39].

Fig. 6. Linear relationship of j versus τ−1/2 for the chronopotentiometric data obtained at four different temperatures.

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Table 2 Diffusion coefficients of Ir3+ ions (0.2 mol/L) in NaCl-KCl-CsCl molten salts as various temperatures. T (K)

D0 (×10

−6

2

cm /s)

913

883

853

823

9.2

5.7

3.7

2.4

where j is the applied current density (A/cm2); Co is the bulk concentration of Ir3+ ion (2 × 10−4 mol/cm3); Do is the diffusion coefficient (cm2/s); F is the Faraday's constant (96,485 C/mol); n is the number of exchanged electrons (n = 3); and τ is the transition time (s). As shown in Table 2, the diffusion coefficients of Ir3+ ions range from 2.4 × 10−6 cm2/s to 9.2 × 10−6 cm2/s as the temperatures increase from 823 K to 913 K, which are further used to produce the Arrhenius plot, as shown in Fig. 7. Under the same conditions, the diffusion coefficient of Ir3 + ions at 883 K measured by CP and CV techniques is 6.10 × 10−6 cm2/s and 5.70 × 10−6 cm2/s respectively. The complex chemical reaction of Ir3+ ions in NaCl-KCl-CsCl melts and differences in the principles of CV and CP techniques can account for these discrepancies. The diffusion activation energy is measured to be about 94.8 ± 5.6 kJ/mol through the resulting Arrhenius slope. Actually, the diffusion coefficients Do of Ir3+ ions in NaCl-KCl-CsCl melts are close to that of other transition metal ions in the melting chlorides, such as lanthanides and actinides in LiCl-KCl [41–43], while the diffusion activation energy is almost twofold higher than that of other transition metal ions in melting chlorides. Two possible reasons should be considered. Firstly, the counter-polarization effects of Cs+ ions on the Ir-Cl complex ions is much smaller than that of K+ and Li+ ions. Secondly, when the compounds IrCl3 dissolves into the melting chlorides, the Ir3 + ions coordinates with four or six Cl− ions to form [IrCl6]3 − and [IrCl4]− in the melts. A structural transition of the Ir-Cl complexes from [IrCl6]3− to [IrCl4]− happened as the temperature increased from 823 K to 913 K. Decreasing in legend number of the Ir-Cl complexes increases the mobility of Ir3+ ions in the melts. “Termodynamic activation” and “structural activation” of the Ir-Cl complexes co-contributes to high diffusion activation energy for mobility of Ir3+ ions.

3.2.2. Exchange current density The exchange current density j0 is an important reaction kinetics parameter, relating to the reversibility of reactions and the nucleation and morphology of electrodeposits. The linear polarization method, utilizing the E-j data obtained at very low over-potentials, is employed to

Fig. 8. Linear polarization curves for 0.2 mol/L IrCl3 in molten NaCl-KCl-CsCl salts on the Ir electrode: ±15 mV (vs AgCl/Ag), 0.1 mV/s, 823 K–913 K.

calculate the exchange current density j0. According to Bulter-Volmer equation, when the over-potential is very low, the E-j curves have the following expression: i ¼ j0

nFA ∙η RT

ð6Þ

where i is the cathodic current (A); j0 is the exchange current density (A/cm2); n is the number of transfer electrons (n = 3); A is the surface area of Ir electrode (0.05 cm2); and η is the over-potentials (η = E − Eeq, E is the polarization potential). Fig. 8 shows the linear polarization data of i-E plots at η = ±15 mV on the Ir electrode in NaCl-KCl-CsCl melts at different temperatures, whose scan rate is set as 0.1 mV/s. Slopes of the fitting lines are used to calculate the exchange current densities j0 (Table 3). As shown in Fig. 9, the exchange current density j0 and temperature T satisfies the Arrhenius relationship and the activation energy is about 78.9 ± 2.5 kJ/mol according to slopes of the j0 − 1/T plots. The standard rate constant (k0) for charge transfer reaction can be calculated using the following equation [35]: k0 ¼

j0

ð7Þ

nFC 1−α o

where n is the number of transferred electrons; F is Faraday's constant (96,485 C/mol); Co is the concentration of IrCl3 (2 × 10−4 mol/cm3); j0 is the exchange current density; α is the transfer coefficient (≈0.45). Matsuda and Ayabe [35] have used the ratio of the mass transfer rate and the electron transfer rate for quantitatively judgment of the reversibility of electrochemical reactions. It demonstrated that the diagnostic parameter Λ does not change with the kinetic parameters, such as n, Do, T and ν, but the standard rate constant k0 did. For reversible, quasireversible and totally irreversible systems the range of Λ is given by Table 3 Values of the kinetic parameters for the Ir3+/Ir redox couples in a platinum electrode at various temperatures. T (K)

Fig. 7. The Arrhenius plot: the logarithm of the diffusion coefficient as a function of inverse of temperature.

j0 (A/cm2) k0 (×10−6 cm/s) Λ (×10−4)

913

883

853

823

0.016 3.9 12.3

0.012 2.9 9.2

0.008 2.0 6.1

0.005 1.2 3.8

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Fig. 9. The Arrhenius polt: the logarithm of the exchange current density as a function of inverse of temperature.

Fig. 11. X-ray diffractive spectrums of Ir coatings on Re coated graphite substrates by electrodeposition from melting chlorides containing IrCl3: −20 mA/cm2, 0.2 mol/L IrCl3, (a) 823 K, (b) 853 K, (c) 883 K, (d) 913 K.

the following equations respectively:

Reversible :

15 ≤Λ ¼

k0
1=2 Do nFv = RT Totally irreversible :

Quasi−reversible :

10−2ð1þαÞ ≤Λ ¼

k0
1=2 Do nFv = RT

Λ¼

k0
1=2 Do nFv = RT

≤10−2ð1þα Þ

≤15

Fig. 10. Fracture face of Ir coatings on Re coated graphite substrates by electrodeposition from melting chlorides containing IrCl3: −20 mA/cm2, 0.2 mol/L IrCl3, (a) 823 K, (b) 853 K, (c) 883 K, (d) 913 K.

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Fig. 12. A family of impedance spectrums for Ir3+/Ir redox reaction on the Re coated graphite substrate in NaCl-KCl-CsCl-IrCl3 melts at various temperatures under the rest potential: 0.2 mol/L IrCl3, 823 K–913 K.

In the present study, ν = 0.1 V/s, T = 823 K and Do = 2.4 × 10− 6 cm2/s, then Λ is calculated to be about 3.8 × 10−4 which is much smaller than ~10−3 (α = 0.45). According to Table 3, Ir electrodeposition is qualified as a totally irreversible reaction at 823 K–883 K. As the temperature increases to about 913 K, the Ir3 +/Ir redox reaction converts from irreversible to quasi-reversible, which is in good agreement with the previous results [31,32].

3.3. Effects of electrodeposition parameters on microstructure of Ir coatings As discussed above, the deposition temperature and applied cathodic potentials had great influences on the electrochemical mechanism of Ir3+ ions reduction from the melting chlorides. Changes in the electrochemistry of Ir3+ ions reduction further affected microstructure of the deposited Ir coatings. Relationships between deposition parameters, electrochemistry and microstructure of deposits had been concluded as the so-called “Theory of Microstructure Control (TMC)” [44]. Fig. 10 shows the fracture morphologies of Ir coatings prepared on Re coated graphite substrates using − 20 mA/cm2 in NaCl-KCl-CsCl melts containing 0.2 mol/L IrCl3 at different temperatures (823 K– 913 K) and Fig. 11 is the corresponding XRD of the Ir coatings. It testifies that the Ir coatings converts from the powdery grain structure without orientation (unoriented dispersion type, UD) to the coarse columnar grain structure with the (111) orientations (field-oriented texture type, FT) accompanying with decreasing in grain size when the deposition temperatures increases. As the temperature increases to 913 K, many pores are found at both inside and interface of Ir grains. As stated above, both the diffusion coefficient and exchange current density of Ir3+/Ir redox reactions increased quickly with the deposition temperatures, which resulted in decreasing of the needed over-potential for Ir electrodeposition. While the nucleation rate and nucleation density of Ir grains during electrodeposition positively relates to the cathodic over-potentials. Therefore, with increasing the temperature, nucleuses number of Ir grains decreases and a transition from fine grain without preferential orientation to coarse columnar grain with vertical orientation occurs to the deposited Ir coatings. When the temperature increases to 913 K, the electrochemical reduction of Ir3+ ions turns to be the quasi-reversible reaction. Increasing in the electrochemical reversibility means that equilibrium of Ir deposition/dissolution on the electrodes occurs at a higher exchange speed (larger exchange current i0). Even through using the lower over-potential, the growth rate of Ir grains is relatively faster and larger amounts of growth defects are left

behind, which would cause the deposited Ir coating to be porous [33, 45]. In this case, the deposition temperature should be lower than 913 K. Electrochemical impedance spectroscopy (EIS) is a useful technique for studying metal electrodeposition. Fig. 12 shows a family of impedance plots obtained on Re coated graphite electrode at various temperatures under the rest potential (η = 0.00 V). Distorted semicircular loops in the high frequency and lines with the slope closed to 45° in the low frequency consist of these electrochemical impedance spectrums. According to Adatom Model [46,47] of electrocrystallization, the electrochemical impedance of Ir3+/Ir redox reactions relates to the mass transfer, charge transfer, surface diffusion and lattice incorporation process. The high-frequency semicircular loop represents the electrical behavior due to the parallel combination of the capacitance at electrode/electrolyte interface and the electrons transfer resistance. Distortion of the semicircular loop at medium frequency is attributed to the surface diffusion and lattice incorporation process. The low-frequency line represents the mass transfer resistance of Ir3+ complexes. In Fig. 12, as the temperature increases from 823 K to 913 K, the mass transfer and charge transfer resistance of Ir3+ ions decreases remarkably, suggesting the decrease in the needed over-potential for Ir electrodeposition, which confirms the reaction kinetics of Ir3+/Ir redox couples above. Refinement of grains in Ir coating is beneficial to its oxidation resistance and mechanical property [24–26]. It means that the decrease of temperatures is preferred during Ir electrodeposition. However, as shown in Fig. 13, after electrolyzed at 823 K for 3 h, Ir grains at the surface of Ir coatings convert from fine and dense to coarse and dendritic. Changes are ascribed to the fact that the mass transfer rate is seriously affected by the low temperature which leads to quick formation of thick Ir3+ ions dilution layer near the electrode surface. Dendritic grains would gradually grow on the electrode surface with increasing the electrodeposition time. Because dendritic grain structure is harmful to compactness of Ir coating, it should be avoided as soon as possible. That is to say, the reasonable deposition temperature range should be 853 K–883 K. Fig. 14 shows the surface morphologies of Ir coatings prepared at various cathodic potentials in the NaCl-KCl-CsCl melts containing 0.2 mol/L IrCl3 at 853 K. In Fig. 14a, Ir coatings deposited at potential −0.03 V (vs Ir/Ir3+) consist of coarse but discontinuous and uncompact spherical grains. With the cathodic potential decreasing to about − 0.10 V (vs Ir/Ir3+), the Ir coatings turn to be continuous and dense with grain size about 10–15 μm. As the cathodic potentials decreasing continuously to −0.15 V and −0.20 V (vs Ir/Ir3+), the coatings maintain continuous and the grain size decreases to about 1–3 μm. Refinement of Ir grains should be attributed to continuous nucleation of Ir grains during electrodeposition. When the applied cathodic potential decreases, the over-potential of Ir electrochemical reduction increases accordingly. Higher over-potentials on the cathode promote the nucleation of Ir grains continuously during electrodeposition. At lower overpotential, both nucleation rate and nucleuses density of Ir grains on the electrode surface are quite low. The Ir coatings deposited at lower overpotentials is discontinuous with coarse grains. On the contrary, Ir coatings deposited at higher over-potentials turn to be compact and continuous with fine grains. Accordingly, low cathodic potential is preferred during Ir electrodeposition. While it is also limited by the mass transfer rate of Ir3+ ions from the intrinsic electrolyte to the electrode surface. As shown in Fig. 15, electrochemical impedance spectrums of Ir3+/Ir redox reaction at the cathodic potentials 0.00 V, − 0.03 V, −0.10 V and −0.20 V (vs Ir/ Ir3+) are obtained on Re coated graphite electrodes in the melts containing 0.2 mol/L Ir3+ ions at 853 K, which matches with the different regions of Ir electrodeposition according to the CVs. It demonstrates that both the mass transfer and charge transfer resistance increased quickly as the applied cathodic potentials decreasing. However, the impedance spectrums transform from the shape of semicircular loop + 45° line to a 45° line in the whole frequency range, which versifies that the electrodeposition of Ir3+ ions is totally diffusion-controlled at high over-potential. Therefore, to keep Ir coating compact, it is better

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Fig. 13. Fracture face of Ir coating by electrodeposition from melting chlorides containing IrCl3 at 823 K: −20 mA/cm2, 0.2 mol/LIrCl3, 10,800 s.

to control the cathodic potential or current density in suitable range during electrodeposition. 4. Conclusions This study was to elucidate the electrochemical deposition mechanism of Ir3 + ions in NaCl-KCl-CsCl molten salts. It showed that the

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electrochemical reduction of Ir3+/Ir was a diffusion-controlled irreversible reaction at the temperature range from 823 K to 883 K and turned to be a diffusion-controlled quasi-reversible one with the temperature up to 913 K. The diffusion coefficients was calculated to be 2.4–9.2 × 10−6 cm2/s by applying chronopotentiometry at 823 K–913 K. It showed dependence on temperature in compliance with the Arrhenius law. The diffusion activation energy of the Ir3+ ions in NaCl-KCl-CsCl molten salts was calculated to be 94.8 ± 5.6 kJ/mol. Increasing dependence on temperature for mobility of Ir3+ ions resulted from the increase of ions thermodynamic motion and transformation of the Ir-Cl complexes. The exchange current density of Ir3+/Ir was estimated by linear polarization method at 823 K–913 K and the obtained values were about 0.005–0.016 A/cm2. The exchange current density also showed dependence on temperature in compliance with the Arrhenius law with the activity energy about 78.9 ± 2.5 kJ/mol. Deposition parameters had great influence on microstructure of Ir coating prepared on Re coated graphite substrate. With the increase of temperature, the mass transfer and charge transfer rate increased accordingly, resulting in decrease of the needed applied over-potential of Ir electrodeposition. Thus nucleation rate of Ir grains decreased during electrodeposition and a transition from fine grains without orientation to coarse columnar grains with remarkable (111) orientations occurred. Specially, when the temperature increased to 913 K, due to a transition from irreversible reaction to quasi-reversible reaction during the Ir electrodeposition, the Ir coatings converted from dense to porous. Similarly, with the applied cathodic potentials decreasing, the cathode over-potential increased accordingly and Ir coatings turned to be compact and continuous with fine grains. However, in order to obtain

Fig. 14. morphologies of Ir coating prepared by electrodeposition at various cathodic potentials (vs Ir/Ir3+) from melting chlorides containing IrCl3: 853 K, 0.2 mol/L IrCl3, (a) −0.03 V, (b) −0.10 V, (c) −0.15 V, (d) −0.20 V.

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Fig. 15. electrochemical impedance spectrums of Ir3+/Ir redox reaction at various cathodic potentials (vs Ir/Ir3+) from melting chlorides containing IrCl3: 853 K, 0.2 mol/L IrCl3, (a) 0.00 V, (b) −0.03 V, (c) −0.10 V, (d) −0.20 V.

continuous and compact Ir coatings, both the temperature and cathodic potential should been set in suitable range due to the limit of mass transfer rate.

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