IrO2-ZrO2 anodes for oxygen evolution reaction

Accepted Manuscript Surface morphology and electrochemical properties of RuO2-doped Ti/IrO2-ZrO2 anodes for oxygen evolution reaction Bao Liu, Shuo Wang, Chengyan Wang, Yongqiang Chen, Baozhong Ma, Jialiang Zhang PII:

S0925-8388(18)34320-2

DOI:

https://doi.org/10.1016/j.jallcom.2018.11.191

Reference:

JALCOM 48417

To appear in:

Journal of Alloys and Compounds

Received Date: 21 June 2018 Revised Date:

8 November 2018

Accepted Date: 15 November 2018

Please cite this article as: B. Liu, S. Wang, C. Wang, Y. Chen, B. Ma, J. Zhang, Surface morphology and electrochemical properties of RuO2-doped Ti/IrO2-ZrO2 anodes for oxygen evolution reaction, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.191. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Surface morphology and electrochemical properties of

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RuO2-doped Ti/IrO2-ZrO2 anodes for oxygen evolution

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reaction

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Bao Liua, Shuo Wanga, Chengyan Wanga, b, *, Yongqiang Chena, b, Baozhong Maa, b, **, Jialiang Zhanga, b

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a

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Technology Beijing, Beijing 100083, P. R. China

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b

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School of Metallurgical and Ecological Engineering, University of Science and

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Beijing Key Laboratory of Rare and Precious Metals Green Recycling and

Extraction, University of Science and Technology Beijing, Beijing 100083, P. R.

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China

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* and ** To whom correspondence should be addressed (Chengyan Wang and

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Baozhong Ma);

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E-mail: [email protected] (Chengyan Wang);

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[email protected] (Baozhong Ma);

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ACCEPTED MANUSCRIPT Abstract: RuO2-doped IrO2-ZrO2 ternary oxide coatings with different Ru/Ir/Zr mole ratios were

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synthesized using a sol-gel route. The surface morphology was determined by X-ray diffraction

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(XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy

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(FESEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS) analysis.

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The electrochemical behavior was investigated using linear sweep voltammetry (LSV), cyclic

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voltammetry (CV), electrochemical impedance spectroscopy (EIS) and accelerated life test (ALT).

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The results showed that the compositions of the coatings are rutile IrO2 and RuO2. ZrO2 is present

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as an amorphous phase in the coatings. The coating with Ru content of 21 mol% shows a compact

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structure with needle-like crystals dispersed. With the increase in Ru content, the porous and

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cracked structure of the ternary oxide coating of increased active surface area is obtained.

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Increasing Ru content improves the electrocatalytic activity, but worsens the stability of the

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coating in OER. The coating with Ru content of 21 mol% shows good electrocatalytic activity and

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the longest service lifetime in OER.

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Key words: Anodes; Oxygen evolution reaction; Surface morphology; Electrochemical properties

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1. Introduction

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To support a sustainable economic growth, the energy utilization efficiency has

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become more and more important. The sluggish kinetics of oxygen evolution reaction

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(OER) at the anode is regarded as the major efficiency loss in electrowinning and

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water electrolysis process [1-5]. Identification of an efficient, long-lived and low-cost

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anode that facilitates the OER by lowering the overvoltage has attracted intense

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interest. Currently, ruthenium- and iridium-based oxides are regarded as the most

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efficient electrocatalysts for OER [6-9]. However, ruthenium-based oxides suffer from

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instability and exhibit a low service lifetime in sulfuric acidic solution [10, 11].

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Iridium-based oxides present a longer service lifetime than the ruthenium-based

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oxides in sulfuric acidic solution, but the low abundance and the high cost limit its

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application [12]. The balance between the electrocatalytic activity, stability and the

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cost is the primary challenge for exploring the proper electrocatalysts for OER.

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Mixed metal oxide anodes (MMO), which are the active noble metal components

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dispersed in the non-noble components, such as IrO2-SnO2, IrO2-MnO2, RuO2-ZrO2,

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IrO2-Ta2O5 and IrO2-RuO2-Ta2O5, have been intensively investigated [13-18]. These 2

ACCEPTED MANUSCRIPT anodes present a high resistance to erosion and exhibit an excellent electrocatalytic

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activity in a variety of environments and suitability for OER applications. Among the

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non-noble and non-active oxides, zirconium dioxide (ZrO2) shows high corrosion

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resistance and homogeneous dispersion properties when used along with other

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components, which are proper stabilizers for MMO fabrication [19-21]. The presence

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of ZrO2 in the ruthenium- and iridium-based oxide coatings can enhance their service

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lifetime during the OER process [22].

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Ruthenium oxide exhibits a good electrocatalytic activity, and iridium oxide

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shows a long lifetime in sulfuric acidic solution. The ionic radii of Ru4+ is 0.076 nm,

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which is very similar to the ionic radii of Ir4+ (0.077 nm). Moreover, the crystal

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structure of RuO2 and IrO2 is of rutile-type. These two oxides can form a solid

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solution [23, 24]. The improvement of the electrocatalytic performance and the trade

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off between activity and stability by the formation of the IrO2-RuO2 solid solution

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have been intensively researched [25, 26]. Those investigations indicate that the

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electrocatalytic activity of IrO2 is improved and the stability of RuO2 is enhanced by

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the formation of the IrO2-RuO2 solid solution.

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Ti/IrO2-ZrO2 anodes are generally prepared by thermal decomposition method

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using the precursors mixed with H2IrCl6 and ZrOCl2 [27]. The prepared Ti/IrO2-ZrO2

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anode shows high stability, but low electrocatalytic activity in an OER application.

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RuO2 shows better electrocatalytic activity and lower cost than that of IrO2 [28-30].

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Adding RuO2 to the IrO2-ZrO2 binary oxide coating is recommended as it will reduce

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the cost of production and may achieve more favorable electrochemical properties.

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Panic et al. [31] illustrated that the active surface area of the sol-gel prepared anode is

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larger than the anode produced by thermal decomposition method. Zhang et al. [14]

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prepared the Ti/IrO2-SiO2 anode using a sol-gel route. The obtained IrO2-SiO2 binary

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oxide coating showed a large active surface area that improved the electrocatalytic

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activity of the anode in OER. To our knowledge, few studies focused on the

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electrocatalytic properties of the Ti/IrO2-RuO2-ZrO2 anodes prepared using a sol-gel

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route in an OER application. Moreover, the effects of RuO2 content on the

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electrocatalytic activity and stability of the Ti/IrO2-RuO2-ZrO2 anodes were still not

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fully understood. In this study, RuO2-doped Ti/IrO2-ZrO2 anodes were synthesized by a sol-gel

3

route using the precursors mixed with chloroiridic acid (H2IrCl6), ruthenium chloride

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(RuCl3) and zirconium-n-butoxide (C16H36O4Zr). The effects of Ru content on the

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surface morphology and electrochemical behavior were investigated by physical

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characterization and electrochemical analysis.

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2. Experimental procedure

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2.1 Electrode preparation

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Titanium plates, which were degreased in an alkaline solution, chemical etched

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by oxalic acid and rinsed with deionized water, were used as substrates. HCl was used

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for dissolving ruthenium chloride (RuCl3, solid). The precursor solutions used for

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electrode fabrication were firstly prepared by mixing chloroiridic acid (H2IrCl6,

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liquid), dissolved ruthenium chloride (RuCl3, liquid) and zirconium-n-butoxide

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(C16H36O4Zr, liquid) to get a gel. After that, the prepared gel was dissolved into a 1:1

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(v/v) n-butyl alcohol and iso-propanol mixed solvent to form a painting precursor

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with total metal concentration of 0.2 mol L-1. The Ir/Ru/Zr mole ratios in the obtained

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precursor solutions were 49:21:30, 35:35:30 and 21:49:30, respectively. The

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pretreated Ti substrate was painted with the resultant precursor solution, followed by

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drying at 120 oC for 15 min and heating at 450 oC for 15 min. The painting, drying

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and heating steps were repeated 20-25 times. The total loading of the oxides was

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about 1.5 mg cm-2. Finally, the obtained samples were heated at 450 oC for an hour.

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2.2 Surface characterization

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The crystalline phases of the samples were identified by X-ray diffraction (XRD)

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analysis. XRD patterns were collected using an X-ray diffractometer (SmartLab,

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Rigaku) with Cu-Kα radiation at 100 mA and 40 kV. The 2θ range was from 10 to 90o,

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and the scanning rate was 10o min-1. The valence state was examined by high

27

resolution X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi)

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using Mg-Kα and hv = 1253.6 eV. The surface morphology was determined by field

29

emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM)

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analysis. The FESEM images were collected on a ZEISS SUPRA55, and the AFM 4

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figures were obtained using Bruker Multimode 8. The chemical composition was

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measured using energy dispersive spectroscopy (EDS, Thermo-NS7).

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2.3 Electrochemical analysis The electrochemical experiments were conducted in a three-electrode system,

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using a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as

6

the reference electrode. The geometric area of the working electrode was 1.0 cm2. The

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electrolyte used for electrochemical measurements was a 0.5 mol L-1 H2SO4 solution.

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Anodic polarization measurements were performed at a scan rate of 5 mV s-1. Cyclic

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voltammetry (CV) curves were recorded between 0 and 1.2 V vs. SCE at sweep rates

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from 5 to 100 mV s-1. In order to correct the IR drop and explore the electrocatalytic

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properties of the electrode, electrochemical impedance spectroscopy (EIS)

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measurements were carried out. The frequency range was from 0.01 Hz to 100 kHz.

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The imposed AC amplitude was 5 mV rms-1. To evaluate the stability of the electrode,

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accelerated life tests (ALT) were performed. The constant current density used for

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ALT was 2 A cm-2. Deactivation of the electrode was considered when the cell voltage

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increased by 5 V relative to its initial value.

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3. Results and discussion

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3.1 Phase analysis

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Fig. 1 shows the XRD patterns collected for the anodes with different Ir:Ru:Zr

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mole ratios. As observed, the peaks corresponding to IrO2, RuO2 and metallic Ti are

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well-defined. The crystal structure of IrO2 and RuO2 are both rutile phase. No

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diffraction peaks corresponding to ZrO2 was found in any of the patterns, suggesting

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that ZrO2 is present as an amorphous phase in the prepared ternary oxide coatings.

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Similar results were obtained by Benedetti et al. [32]. They illustrated that the

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metastable tetragonal form of ZrO2 can be generated only when the zirconium

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concentration is higher than 70 mol%. The shift of the rutile IrO2 and RuO2 peaks was

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observed in the XRD patterns. Moreover, the lattice constants, a and c, of the rutile

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phases decreased with increasing Ru content. This suggests that solid solution phases

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with rutile structure were formed during the calcination process [33, 34]. As

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mentioned above, IrO2 and RuO2 can form the solid solution phase of IrO2-RuO2 [24].

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ACCEPTED MANUSCRIPT According to the studies of Shao et al. [35] and Bose et al. [16], some of the Ir4+ and

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Ru4+ can be replaced by Zr4+ to form solid solution phases of IrO2-ZrO2 and

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RuO2-ZrO2, respectively. The intensity of the diffraction peaks relative to the rutile

4

phase increased, while the full width at half maximum (FWHM) decreased with

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increasing Ru content. This indicates that the crystallinity degree and the crystallite

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size of the ternary oxide coating increased with the increase of Ru content [36].

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♦ Ti ● IrO2 ▼ RuO2

5000

♦

▼

●

(c)

3000 2000

♦▼

●♦

▼

4000

●

♦ ♦

♦

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Intensity (counts)

7000 6000

(b)

1000 (a) 0 20

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50 60 o 2θ ( )

80

90

4+

Ru

Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

460

Intensity (counts)

3+

Ir

Ir:Ru:Zr=49:21:30 4+

Ir

Ir:Ru:Zr=35:35:30

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(c)

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Ir 4f

Ir:Ru:Zr=21:49:30

465 470 475 480 485 Binding energy (eV)

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Intensity (counts)

Ru 3p

Ir:Ru:Zr=49:21:30

450

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Fig. 1 XRD patterns collected for the prepared samples with different Ir:Ru:Zr mole ratios (a) Ir:Ru:Zr=49:21:30, (b) Ir:Ru:Zr=35:35:30, (c) Ir:Ru:Zr=21:49:30 (a)

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Intensity (conuts)

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62 64 66 68 Binding energy (eV)

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Zr 3d 4+

Zr

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30 176

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180 182 184 186 Binding energy (eV)

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Fig. 2 XPS spectra of the prepared samples with different Ir:Ru:Zr mole ratios (a) Ru 3p, (b) Ir 4f, and (c) Zr 3d

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To further explore the chemical properties of the coating, XPS analysis were

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performed. The Ru 3p peak instead of Ru 3d was analyzed to avoid the overlapping

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between Ru 3d and C 1s peaks. Fig. 2(a) shows the XPS spectra of Ru 3p. As shown 6

ACCEPTED MANUSCRIPT in Fig. 2 (a), the peaks observed at 484.88 and 463.06 eV are attributed to Ru 3p1/2

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and Ru 3p3/2, respectively, corresponding to Ru4+ [37]. The Ir 4f peak split into two

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main peaks and two satellite peaks as shown in Fig. 2(b). The main peaks at 64.65 and

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61.79 eV are related to Ir 4f5/2 and Ir 4f7/2 of Ir3+, while the satellite peaks observed at

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65.54 and 62.61 eV are associated with Ir4+ [38]. This indicates that Ir existed in two

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oxidation state of Ir3+ and Ir4+ in the coating that consistent with the results reported in

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the literature [38, 39]. As observed in Fig. 2(c), two symmetrical peaks at 184.41 for

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Zr 3d3/2 and 182.06 for Zr 3d5/2, showing the Zr4+ form [39]. There is a small shift of

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peak positions of Ir, Ru and Zr with increasing Ru content. This indicates that solid

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solution phases were formed during the calcination process, in agreement with the

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results of XRD analysis [25].

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3.2 Surface morphology

crack

flat areas

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(f) (e) (d)

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Fig. 3 FESEM images of the samples before ALT (a), (b) Ir:Ru:Zr=49:21:30, (c)-(e) Ir:Ru:Zr=35:35:30, (f)-(h) Ir:Ru:Zr=21:49:30

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Fig. 3 presents the obtained FESEM images of the prepared samples. As shown

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in Fig. 3(a) and (b), a compact structure with the dispersion of a large amount of

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ACCEPTED MANUSCRIPT needle-like crystals was observed from the sample with Ru content of 21 mol%. The

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dimensions of the crystals ranged from 40 to 70 nm. Fig. 3(c)-(h) show the surface

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morphology of the coatings with Ru content of 35 mol% and 49 mol%. It can be seen

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that a large amount of pores and cracks could be observed in the coatings. The amount

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of the pores and cracks increased with increasing Ru content. The coatings with Ru

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content of 35 mol% and 49 mol% showed cracked structures combined with flat areas.

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The crystals could be formed both inside cracks and on the flat areas. The dimensions

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of the crystals formed inside cracks were 40-60 nm, while those generated on the flat

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areas were 65-120 nm.

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Fig. 4 exhibits the EDS mapping of the sample with Ir:Ru:Zr=49:21:30. EDS

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mapping analysis showed that elemental Ir, Ru, Zr and O were homogeneously

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dispersed over the surface. The electrocatalytic activity can be significantly increased

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by the uniform dispersion of IrO2 and RuO2 [34, 40]. ZrO2 is a metallic oxide, which

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shows high corrosion resistance in sulfuric acidic solution. The homogeneous

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dispersion of ZrO2 may effectively prevent the active components from dissolving

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into the electrolyte that enhanced the stability of the anode for OER.

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Fig.4 EDS mapping of the sample with Ir:Ru:Zr=49:21:30

3.3 AFM analysis

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The three-dimensional AFM figures as observed in Fig. 5 showed a nonuniform

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surface morphology. Some pores and cracks could be observed in the coatings. The

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amount of pores and cracks increased with increasing Ru content. To evaluate the 8

ACCEPTED MANUSCRIPT roughness of the coatings, the average surface roughness parameter, Ra, were

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calculated and shown in Table 1. As observed, the Ra values of the coatings with Ru

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content of 21 mol%, 35 mol% and 49 mol% were 49.4, 53.1 and 63.8 nm, respectively.

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That can be attributed to the porous and cracked structure of the coating and the

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crystallization behavior of the active components. The higher roughness can provide a

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larger active surface area for OER.

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Fig. 5 AFM 3D surface micrographs of the prepared coatings with different Ir:Ru:Zr mole ratios (a) Ir:Ru:Zr=49:21:30, (b) Ir:Ru:Zr=35:35:30, (c) Ir:Ru:Zr=21:49:30 Table 1 Average surface roughness (Ra) of the prepared samples

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53.1

63.8

3.4 Cyclic voltammetry characterization Fig. 6 shows the CV curves conducted on the prepared electrodes. The CV

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curves obtained for the ternary oxide coatings are similar to that obtained for the

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ruthenium- and iridium-based oxide coatings [14, 17, 41]. The CV curves showed two

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broad peaks in the potential regions of 0.4-0.6 V vs. SCE and 0.9-1.1 V vs. SCE. They

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were likely to be the result of the redox transitions of Ru(IV)/Ru(III) and Ir(IV)/Ir(III),

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respectively [42-44]. The redox transition reactions can be generally described as:

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MOx (OH )y + δH + + δe − ↔ MOx −δ (OH ) y +δ

(1) 9

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where M stands for Ru and Ir. The broad nature of the peaks observed in the CV

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curves is typical of thermally prepared oxide coatings [24, 38, 39]. 0.003 0.002

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

0.001 0.000 -0.001 -0.002 0.0

0.4

0.6

0.8

Potential vs. SCE (V)

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1.0

1.2

Fig. 6 CV curves of the prepared samples with different Ir:Ru:Zr mole ratios measured at a scanning rate of 20 mV s-1

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Current density (A cm )

0.004

To evaluate the number of active sites of the coatings, the values of voltammetric

7

charge (q*) were calculated by graphic integration. As illustrated in the literature [28,

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45], the electrochemical behavior of the electrode is dependent on the sweep rate. The

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reaction (1) occurs at the most accessible active sites at a high sweep rate, whereas it

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takes place at all active sites at a enough low sweep rate. Taking into account these

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phenomenon, the total active surface area was divided into an “outer” and an “inner”

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surface, and represented by qouter and qinner, respectively. The qtotal values stands for the

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all active sites of the coating, and were calculated as the sum of qouter and qinner. The

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detailed method for the calculation of qouter and qinner can be found in the study of

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Ardizzone et al.[46].

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Table 2 shows the calculated voltammetric charges of the prepared anodes. The

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values of qouter, qinner and qtotal all increased with increasing Ru content in the coating.

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According to the results of FESEM and AFM analysis, the amount of pores and

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cracks in the coating increased with increasing Ru content. The increased number of

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“inner” active sites is related to the porous and cracked structure of the coatings. The

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voltammetric charge values were also influenced by the crystallization behavior of the

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coatings. ZrO2 can not be crystallized at the temperature of 450 oC [39]. As illustrated

23

by Roginskaya and Morozova [47], the rutile-type phases contained in the

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ruthenium-based oxide coatings calcined at 450 oC is 70%, while the iridium-based

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oxide coatings prepared at the same calcination temperature consist by 35%. The

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crystallization of the rutile RuO2 is easier than IrO2 at 450 oC. According to the results

2

of XRD analysis, the crystallinity degree of the ternary oxide coating increased with

3

increasing Ru content. More active sites are contributing to voltammetric charge due

4

to higher crystallinity degree.

-2

JDL (mA cm )

Ir:Ru:Si=49:21:30 Ir:Ru:Si=35:35:30 Ir:Ru:Si=21:49:30

3.0 2.5 2.0 1.5 1.0 0.5 0.0

10 20 30 40 50 60 70 80 90 100 110 -1 v (mV s )

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Fig. 7 Non-Faradaic current density obtained from the CV curves at 0.20 V vs. SCE as a function of scan rate

8

To quantitatively estimate the active surface area of the coatings,

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electrochemically active surface area (ECSA) measurements were performed. The

10

accessible ECSA for an electrochemical reaction can be estimated by analyzing the

11

electrochemical double-layer capacitance of the catalytic surface [48]. The

12

electrochemical capacitance was determined by measuring the non-Faradaic

13

capacitive current associated with double-layer charging from the scan-rate

14

dependence CV curves. The CV curves were recorded in a narrow potential range

15

with different sweep rates. Under the assumption that the double layer charging is the

16

only process, a straight line could be obtained from the JDL vs. v plot. The capacitive

17

current density，JDL, is equal to the product of the scan rate, v, and the electrochemical

18

capacitance, CDL, as given by equation (2) [49, 50]:

19

JDL = CDL · v

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In the present work, the linear charging region of the CV curves from 0.145 to 0.255

21

V vs. SCE was linked to the electrochemical capacitance in the non-Faradaic region.

22

Fig. 7 shows the JDL vs. v plot measured at 0.20 V vs. SCE. The CDL values of the

23

samples with different mole ratios of Ir:Ru:Zr were determined by the slope of these

24

curves. Based on the obtained CDL values, the ECSA of the coatings was then

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calculated using equation (3) [51]:

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ACCEPTED MANUSCRIPT ECSA = CDL / Ce

(3)

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where Ce (0.035 mF cm-2) is the specific capacitance of a metal electrode in H2SO4

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solution [49]. The ECSA values for the coatings with Ru content of 21 mol%, 35

4

mol% and 49 mol% were 325, 562 and 782 cm2, respectively. The calculated ECSA

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values show the same trend with the Ra values and the voltammetric charge values.

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Based on the results of the voltammetric charges calculation, the electrochemical

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porosity factor (φ) was evaluated using equation (4) [52-54]:

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φ = qinner / qtotal

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As shown in Table 2, the φ values increased from 0.359 to 0.472 with increasing Ru

10

content. The φ value is related to the porous and cracked structure of the coatings. As

11

observed in the collected FESEM images and AFM figures, the number of pores and

12

cracks existed in the coating increased with increasing Ru content that leads to the

13

increase in φ values. The lowest value of φ is obtained for the sample with Ru content

14

of 21 mol%.

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49:21:30 35:35:30 21:49:30

qinner (mC cm-2)

84 91 103

47 63 92

qtotal (mC cm-2)

φ

131 154 195

0.359 0.409 0.472

0.5 (a) 0.4

-1

i / qtotal (A C )

0.06

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

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Current density (A cm )

0.08

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qouter (mC cm-2)

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(4)

0.04

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

(b)

0.3 0.2

0.02

0.1

0.00 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45

0.0 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45

Potential vs. SCE (V)

Potential vs. SCE (V)

Fig. 8 Polarization curves (a) and charge-normalized polarization curves (b) of the prepared samples with different Ir:Ru:Zr mole ratios

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Fig. 8(a) shows the IR-corrected anodic polarization curves of the prepared

21

samples. The potentials for the coatings with Ru content of 21 mol%, 35 mol% and 49

22

mol% were 1.350, 1.335 and 1.314 V vs. SCE, respectively, at the current density of 12

ACCEPTED MANUSCRIPT 50 mA cm-2. The electrocatalytic activity of the anodes increased with increasing Ru

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content. The electrocatalytic activity is related to both the intrinsic catalytic nature and

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the active surface area. We can not differentiate the real electrocatalytic effect of

4

RuO2 from the apparent activity as shown in Fig. 8(a). Since qtotal represents the

5

whole number of surface active sites, a ratio of current, i, and total voltammetric

6

charge, qtotal, was used to normalize the activity per active site [14, 55]. Fig. 8(b)

7

shows the charge-normalized polarization curves. As shown in Fig. 8(b), the

8

normalized electrocatalytic activity increased with increasing Ru content. RuO2 shows

9

better electrocatalytic activity for OER than IrO2 [28-30]. Increasing Ru content in the

10

coating can improve the electrocatalytic activity and decrease the overvoltage. The

11

increase in the normalized activity is smaller than the apparent activity with

12

increasing Ru content. This indicates that the apparent activity is mainly impacted by

13

the active surface area of the coating. As mentioned above, the amount of pores and

14

cracks in the coating increased with increasing Ru content. The larger amount of

15

pores and cracks provides more active sites, resulting in the improvement of the

16

electrocatalytic activity.

17

3.6 EIS measurements before ALT

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

8

2

-lm (Z) (Ω cm )

(a)

EP

6 4 2

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0

0

2

4

6

8

60 Phase angle (deg)

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10

18

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(b)

50 40

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

30 20 10 0 -10 -20

10

12

14

16

18

-2

-1

0

1

2

3

4

5

log (frequency / Hz)

2

Re (Z) (Ω cm )

19 20 21

Fig. 9 EIS plots of the prepared samples with different Ir:Ru:Zr mole ratios before ALT (a) Nyquist diagrams, (b) Bode plots, obtained at 1.25 V vs. SCE with the imposed AC amplitude of 5 mV rms-1

22

The surface structure and the electrochemical behavior were also investigated by

23

EIS measurements. The impedance spectra of the prepared samples are shown in Fig.

24

9. As observed in the Nyquist diagrams (Fig. 9(a)), all impedance spectra exhibited a

25

well-formed semicircle. The obtained semicircle is related to the charge-transfer

26

process at the electrode/electrolyte interface [56]. An inductive behavior was observed 13

ACCEPTED MANUSCRIPT 1

in the Bode plots in the high frequency (Fig. 9(b)). The inductive behavior was

2

commonly registered on the thermally-prepared active oxide coating, and was

3

believed being caused by the porous structure [57]. The equivalent electrical circuit (EEC) described as LRs(RpCp)(RctCdl) was used

5

to fit the measured spectra [58]. Here, L stands for the inductance, Rs represents the

6

solution resistance, (RpCp) combination is the response of porous structure of the

7

oxide coating, and (RctCdl) combination describes the electrochemical process [56].

8

Considering the porous and cracked structure of the coatings, the pure capacitors (Cp

9

and Cdl) were replaced by the constant phase element and represented by Qp and Qdl,

10

respectively [59]. Based on the obtained Qp and Qdl values, the values of Cp and Cdl

11

were calculated [14].

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The equivalent circuit parameters are shown in Table 3. The Cp values and the Rp

13

values were found to increase with the increase in Ru content. The Cp and Rp values

14

observed in the high frequency domain of the impedance spectra are related to the

15

processes (e.g., reaction (1) and OER) occurring in the more internal, porous part of

16

the oxide layer [57, 60, 61]. These processes are very fast in easily accessible regions

17

(external surface), whereas they create a slow diffusion controlled process in regions

18

with difficult access (pores and cracks) [62]. According to the studies of Alves et al.

19

and Da silva et al. [57, 60], a more compact structure showed a smaller Rp value. The

20

increased Cp and Rp values indicates that the porosity degree of the ternary oxide

21

coating increased with increasing Ru content. Similar results were obtained by Hou et

22

al. [58]. The obtained Cp and Rp values are well consistent with the results of the

23

microstructural characterizations. The values of the double-layer capacitance, Cdl,

24

which can be used as a relative measure of the active sites of the electrode, showed a

25

rising trend as the Ru content in the ternary oxide coating increased. This suggests

26

that more active sites were obtained from the anode with higher Ru content. The

27

increased values of Cdl can be attributed to the porous and cracked structure of the

28

coating and the crystallization behavior of the active components. Similar results were

29

obtained by voltammetric charges calculation. Both the voltammetric charge and the

30

double-layer capacitance are directly proportional to the number of surface active

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14

ACCEPTED MANUSCRIPT sites. According to the study of Alves et al. [60], the voltammetric charge and the

2

double-layer capacitance present a linear relationship with a slope of ~ 1.3, indicating

3

the high relativity of both these two parameters in evaluating the electrochemically

4

active surface area of the oxide electrodes. The values of the charge transfer resistance,

5

Rct, of the anode with Ru content of 21 mol%, 35 mol% and 49 mol% were 9.038,

6

5.613 and 3.065 Ω cm2, respectively. The Rct values decreased with increasing Ru

7

content, indicating the improvement of the electrocatalytic activity. The improved

8

activity is attributed to the better electrocatalytic activity of RuO2 than that of IrO2 for

9

OER. Both Cdl and Rct values are dependant on the real surface area of the coating. As

10

illustrated in the AFM analysis, the roughness of the coating increased with increasing

11

Ru content. The high roughness implies a large real surface area of the coating. The

12

Cdl values increased, while the Rct values decreased with the increase in the real

13

surface area of the coating.

SC

Table 3 Equivalent circuit parameters of the prepared samples with different Ir:Ru:Zr mole ratios before ALT, as calculated from the EIS data Ir:Ru:Zr

49:21:30

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1.081×10 1.746 106.2 0.938 1.834 49.5 0.862 5.613

21:49:30

-6

9.590×10-7 1.596 194.5 0.887 1.914 65.7 0.877 3.065

3.7 Electrocatalytic stability

AC C

17

1.029×10 1.833 63.5 0.898 1.528 18.4 0.945 9.038

35:35:30

-6

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L (H) Rs (Ω cm2) Cp (mF cm-2) np Rp (Ω cm2) Cdl (mF cm-2) ndl Rct (Ω cm2) 16

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Table 4 Accelerated service lifetime of the prepared samples in sulfuric acid solution Samples

Service lifetime (h)

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

264 161 42

18

The electrocatalytic stability of an electrode is essential to its application and is,

19

to some extent, much more important than the electrocatalytic activity. The obtained

20

accelerated service lifetimes of the prepared samples are shown in Table 4. As

21

observed, the values of the service lifetime (SL) decreased with increasing Ru content.

22

The decreased SL can be attributed to the low electrocatalytic stability of RuO2 for 15

ACCEPTED MANUSCRIPT 1

OER in sulfuric acidic solution and the porous and cracked structure of the ternary

2

oxide coating. The highest SL of 264 h was obtained for the anode with Ru content of

3

21 mol%, which is about ten times longer than the Ti/IrO2-RuO2 anode with Ru

4

content of 30 mol% [63]. Fig. 10 shows FESEM images of the coatings after ALT. It can be seen that a

6

small amount of the active components remained on the IrO2-RuO2-ZrO2 ternary

7

oxide coating after ALT. The Ti substrate was exposed with the dissolution of the

8

active components. To further explore the corrosion mechanism of the anodes, EIS

9

measurements were performed. Fig. 11 exhibits the EIS plots of the anodes after ALT.

10

The obtained EIS data was fitted using the EEC of LRs(RpCp)(RctCdl). The pure

11

capacitors (Cp and Cdl) were replaced by Qp and Qdl, respectively. As shown in Fig. 11,

12

the experimental data are well fitted with the simulated data. Table 5 shows the

13

equivalent circuit parameters of the coating after ALT. The Rp and Cp values of the

14

failed anodes are much smaller than that of the fresh anodes, indicating that the

15

surface of the coating became smooth after ALT [24]. The Rct values of the anode

16

increased to 137.4-169.3 Ω cm2, while the Cdl values decreased to 1.8-4.2 mF cm-2

17

after ALT. This suggests that the electrocatalytic activity and the active surface area of

18

the coating significantly decreased after ALT.

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(b) (c) (a)

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Ti substrate

50µm 50µm

19 20 21

Fig. 10 FESEM images of the samples after ALT (a) Ir:Ru:Zr=49:21:30, (b) Ir:Ru:Zr=35:35:30, (c) Ir:Ru:Zr=21:49:30 (a)

120 Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

100 80 60 40 20

80

Ir:Ru:Zr=49:21:30 Ir:Ru:Zr=35:35:30 Ir:Ru:Zr=21:49:30

60 40 20 0

0

-20 0

22

(b)

100 Phase angle (deg)

2

-lm (Z) (Ω cm )

160

140 120

20

40

60 80 100 120 140 160 180 2 Re (Z) (Ω cm )

-2

16

-1

0 1 2 3 log (frequency / Hz)

4

5

ACCEPTED MANUSCRIPT Fig. 11 EIS plots of the prepared samples with different Ir:Ru:Zr mole ratios after ALT (a) Nyquist diagrams, (b) Bode plots, obtained at 1.25 V vs. SCE with the imposed AC amplitude of 5 mV rms-1 Table 5 Equivalent circuit parameters of the prepared samples with different Ir:Ru:Zr mole ratios after ALT, as calculated from the EIS data 49:21:30

L (H) Rs (Ω cm2) Cp (mF cm-2) np Rp (Ω cm2) Cdl (mF cm-2) ndl Rct (Ω cm2) 6

1.054×10 1.994 5.6 0.861 1.031 4.2 0.914 137.4

35:35:30

-6

1.190×10 1.970 3.1 0.918 0.853 1.8 0.853 169.3

21:49:30

-6

1.154×10-6 2.006 1.1 0.903 0.815 3.3 0.913 163.3

RI PT

Ir:Ru:Zr

SC

1 2 3 4 5

Anode failure can be attributed to the mechanical damage as a result of bubble production,

dissolution,

and

Ti

substrate

8

Gajic-Krstajic et al. [64] proposed a pathway for OER on RuO2 as follows:

9

RuO2 + H2O → RuO2-OH + H+ + e-

(5)

10

RuO2-OH → RuO3 + H+ + e-

(6)

11

2RuO3 → 2RuO2 +O2

12

The dissolution of the active component RuO2 was described through the following

13

elementary steps:

14

RuO3 + H2O → RuO3-OH + H+ + e-

(8)

15

RuO3-OH → RuO4 + H+ + e-

(9)

16

RuO4 shows a low stability and can be dissolved in the electrolyte. According to the

17

study of Moradi et al. [26], the OER occurred on IrO2 can be described as:

18

Ir(OH)3 → IrO(OH)2 + H+ + e-

(10)

19

IrO(OH)2 → IrO2(OH) + H+ + e-

(11)

20

IrO2(OH) → IrO3 + H+ + e-

(12)

21

2IrO3 + 2H2O → O2 + 2IrO(OH)2

(13)

22

The formed IrO3 suffers from instability in sulfuric acidic solution and would be

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active

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passivation.

(7)

ACCEPTED MANUSCRIPT 1

transformed to IrO42- dissolved ions:

2

IrO3 + H2O → IrO42- + 2H+

3

Larger Ir and Ru losses indicate a smaller amount of active components left in the

4

coating. The anode is failed when the reduced amount of the active components on

5

the coating can no longer support the OER. In addition, the Ti substrate would be

6

oxidized and formed the insulating TiO2 layer, resulting in the failure of the anode

7

during the OER process. Therefore, the possible reason for the deactivation of the

8

anode was the combined effects of the active components dissolution and the Ti

9

substrate passivation during the OER process.

RI PT

SC

10

(14)

A relationship between the SL and the current density was used to evaluate the actual SL of the electrode, and described as equation (15) [65]:

12

SL ∝

13

where m is a parameter ranges from 1.4 to 2.0. In this study, m was identified as 1.4 to

14

obtain a lower actual SL for better evaluation of the SL in practical application. The

15

actual SL in application under a current density of 50 mA cm-2 was calculated by

16

(20001.4/501.4)×SL. The obtained values illustrated that the actual SL of the anodes

17

with Ru contents of 21 mol%, 35 mol% and 49 mol% can be predicted to be about 5

18

yrs, 3 yrs and 1 yr, respectively, in application under a current density of 50 mA cm-2

19

in 0.5 mol L-1 sulfuric acid solution. As illustrate in the literature [24, 66, 67], Ti/IrO2

20

(70mol%)-Ta2O5 (30mol%) electrode is by far the best anode for oxygen evolution.

21

The prepared Ti/IrO2-RuO2-ZrO2 anode with Ru content of 21 mol% showed a

22

comparable OER activity and stability with the Ti/IrO2-Ta2O5 electrode. Moreover,

23

the metal components of IrO2 and Ta2O5 were replaced by RuO2 and ZrO2 that

24

decreased the cost for electrode preparation.

25

4. Conclusion

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(15)

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1 im

26

RuO2-doped IrO2-ZrO2 ternary oxide coatings with three Ru/Ir/Zr mole ratios

27

were synthesized on Ti substrate by a sol-gel route using the precursors mixed with

28

H2IrCl6, RuCl3 and C16H36O4Zr. Physical characterizations and electrochemical

29

measurements were conducted to explore the surface morphology and electrocatalytic 18

ACCEPTED MANUSCRIPT properties of the prepared electrodes. It was found that the compositions of the

2

coatings are amorphous ZrO2, and rutile IrO2 and RuO2. Solid solution phases with

3

rutile structure were formed during the calcination process. The coating with Ru

4

content of 21 mol% showed a compact structure with the dispersion of a large amount

5

of needle-like crystals. The porous and cracked structure was found in the coatings

6

with Ru content of 35 mol% and 49 mol%. The porosity degree increased with the

7

increase in Ru content. The porous and cracked structure contributes more to the

8

active sites for OER that improved the electrocatalytic activity. Increasing Ru content

9

improved the electrocatalytic activity, however, decreased the stability of the coating

10

for OER. The coating with Ru content of 21 mol% shows a certain electrocatalytic

11

activity and the longest service lifetime for OER, which is most suitable for OER

12

applications.

13

Acknowledgments

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This work was financially supported by the National Natural Science Foundation

15

of China (No. 51674026), the Beijing Natural Science Foundation, China (No.

16

2182040), the Beijing Science & Technology Program (No. Z171100002217063), the

17

Fundamental Research Funds for the Central University (No. 230201606500078), and

18

the National Natural Science Foundation of China (No. U1302274).

19

References

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

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Highlights RuO2-doped Ti/IrO2-ZrO2 anodes were prepared using a sol-gel route Compositions and nanostructured morphology of coatings were detected Effects of Ru content on crystallization and microstructure were determined Effects of Ru content on electrocatalytic properties were investigated