IrO2 Coated on RuO2 as Efficient and Stable Electroactive

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IrO Coated on RuO as Efficient and Stable Electroactive Nanocatalysts for Electrochemical Water Splitting Thomas Audichon, Teko W. Napporn, Kouakou Boniface Kokoh, Christine Canaff, Cláudia Morais, and Clement Comminges J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11868 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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The Journal of Physical Chemistry

IrO2 Coated on RuO2 as Efficient and Stable Electroactive Nanocatalysts for Electrochemical Water Splitting Thomas Audichon*, Teko W. Napporn, K. Boniface Kokoh, Christine Canaff, Cláudia Morais and Clément Comminges* Université de Poitiers, IC2MP UMR CNRS 7285, “Equipe SAMCat”, Département de Chimie, 4 rue Michel Brunet – B27, TSA 51106, 86073 Poitiers cedex 09, France

Corresponding author. Tel.: +33 5 49 45 36 28 - Fax: +33 5 49 45 35 80. E-mail address: [email protected] (C. Comminges), [email protected] (T. Audichon)

1 Environment ACS Paragon Plus


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Abstract: With the aim of obtaining a highly stable and active catalyst for Oxygen Evolution Reaction (OER) a core-shell like IrO2@RuO2 material was synthesized by using a surface modification / precipitation method in ethanol medium. The comparison of this catalyst with pure RuO2 and pure IrO2 showed that the obtained mixed oxide catalyst displayed the highest amount of active sites as well as a good accessibility for water. Moreover, this catalyst was shown to be highly stable towards repetitive redox cycling. Polarization curves of the three catalysts showed that the IrO2@RuO2 was the most active for the OER owing to the large number and high accessibility of active sites. These catalytic benefic effects are attributed to an intimate contact between the two oxides in the IrO2-covered RuO2 nanocatalyst that combines the RuO2 intrinsic activity and the IrO2 stability. The present study contributes therefore to the rational design of efficient and stable electrocatalysts for water splitting in acidic media.


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1. Introduction The world consumption of fossil fuels as energetic vector increases drastically since 1850

1

and the concomitant CO2 emissions are contributing to the raising of the greenhouse

effect 2. In this context, it is important to diversify the energy sources in order to promote energetic systems based on eco-friendly resources and also to avoid the emission of harmful by-products. Utilization of hydrogen as an energetic vector is one of the most promising solutions since it is potentially non-polluting 3. However, it is necessary to produce clean hydrogen in order to develop and promote this vector. Different H2 chemical production processes exist 4; and this paper focuses on the technology related to hydrogen production by from water electrolysis in acidic media. The proton exchange membrane water electrolyzer (PEMWE) is one of the electrochemical systems used to split the water into oxygen (O2) and hydrogen (H2)

5-6

. The hydrogen evolution reaction (HER) occurs at the cathode with

negligible overpotential when platinum based catalysts supported on different carbon substrates are used

7-9

. Contrariwise, due to the complex reaction involving four electrons

transfer, the oxygen evolution reaction occurs with largest overpotential which results in an efficiency loss

10

. Higher efficiency for OER is obtained when catalyst composed of noble

metal are used and more particularly under their oxide form

11

. Indeed, oxide materials are

more stable than the corresponding metals as they cannot be further oxidized under high potential conditions. Ruthenium and iridium oxide based materials are known to be the most active electrocatalysts for the OER as well as a high electronic conduction conferred by their rutile crystallographic structure 10, 12. . However, IrO2 oxide is much more stable than RuO2 whereas the opposite is observed for the corresponding electrocatalytic activities13. Many strategies concerning the synthesis of these anode materials by different chemical or physical ways were reported like bulk oxides 14, thin layers 15-16, nano oxides 17-18 and supported nano oxides19-20.



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The main challenge is therefore to improve the long-term stability as well as to decrease the oxide loading in order to minimize the cost of noble metal catalyst without affecting the catalytic properties. Recent study on the design of Ru-Ir oxides catalysts

21

revealed that the

activity and catalytic efficiency are directly correlated to the composition and stability of the nano-catalyst surface for OER. Therefore, a solution to achieve the aforementioned goals might be to coat RuO2 with an IrO2 protective layer. In our previous works

22-24

, we presented a new synthesis path to obtain ruthenium-

iridium mixed nano-oxides using reasonable amounts of solvents, reactants and surfactants. By this way, highly electroactive materials were obtained. In the present work, we report a similar synthesis method in order to coat commercial ruthenium oxide with iridium oxide and finally obtain a core-shell likestructure. The metallic molar composition of 75 % Ru and 25 % Ir was chosen in agreement with our previous studies

23

. The synthesized materials were

characterized by X ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetric (TGA) analyses. The activity of the prepared catalysts was assessed by means of cyclic voltammetry, polarization and electrochemical impedance spectroscopy measurements in 0.5 M H2SO4. Finally, the intrinsic activity as well as stability of this new catalyst was compared to that of RuO2 and IrO2 nanocatalysts.

2. Experimental Iridium (III) chloride hydrate (IrCl3, xH2O, 99.99 % metal basis) was purchased from Alfa Aesar. Ruthenium (IV) oxide hydrate (RuO2, xH2O), ethanol absolute (CH3CH2OH > 99.8 %) and Nafion solution (5 wt%. in aliphatic alcohol) were obtained from Sigma Aldrich. Ammonia solution (32 % pure solution in water) was purchased from VWR. All chemical reagents were of analytical grade and used without further purification except for commercial RuO2 oxide which was heat treated in order to recover its crystalline structure.


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2.1 Oxides synthesis The goal of this work was to stabilize RuO2 catalyst by surface modification with the stable electrocatalyst IrO2. . The synthesis path of the RuO2 covered by IrO2 nanoparticles is described in Scheme 1 and was inspired of the Ma et al. study 25. Briefly, commercial RuO2 hydrate was used as a support IrO2 coating was prepared by precipitation in ethanol medium as described in our previous works 22-23.

Scheme 1: The synthesis procedure of the iridium (IV) oxide coated on commercial ruthenium (IV) oxide nanoparticles 25. Firstly, the ruthenium (IV) oxide support was grinded and dispersed in 20 mL of absolute ethanol in an ultrasonic bath during two hours. Then, iridium salt precursor (IrCl3, x H2O) was added in order to obtain a nominal molar ratio of 0.25 and 0.75 for iridium and ruthenium, respectively. Absolute ethanol was then added so as to obtain a concentration of 0.025 mol L-1 in metallic iridium. After that, the solution was homogenized in an ultrasonic bath for two hours. Ammonia was next added dropwise to the solution under vigorous stirring leading to the growth of suspended particles. The pH value was maintained at 12 for two hours under stirring. The particles were then separated from the translucent supernatant by decantation, and the resulting precipitate was recovered and heated at 60 °C in order to remove traces of solvent. Afterwards, the mixture was calcined in an oven under an air atmosphere to dehydrate the intermediate species and form the oxides. The temperature was increased gradually to 400 °C by applying successively 1 h steps at 250, 350 and 400 °C with heating



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rate of 2 °C min-1. This method allows the obtaining of small particle size free from organic carbon. In the following, this sample will be designated as IrO2@RuO2. To observe the effect of IrO2 coating, the same heat treatment was realized on commercial RuO2 hydrate powder. Moreover, the same synthesis path was used to elaborate pure unsupported IrO2. 2.2 Electrochemical measurements The electrochemical measurements (voltammetry, steady-state polarization, and electrochemical impedance spectroscopy (EIS)) were performed in a three-electrode electrochemical cell. A reversible hydrogen electrode (RHE) and a glassy carbon slab were used as reference and counter electrodes, respectively. The RHE reference was connected to the cell via a Luggin capillary and referred all the potentials. The solution of 0.5 mol L-1 H2SO4 (Merck, Suprapur) used as electrolyte was prepared with ultrapure water produced and purified with Millipore-Milli-Q system (18.2 MΩ cm at 20 °C). A gold slab of 0.55 cm2 geometric area was used as working electrode substrate. A cyclic voltammogram of this gold substrate was recorded in the supporting electrolyte prior to each measurement to verify the cleanliness of the cell. Comparison of the results obtained herein with previous work

22

was

made by keeping the similar composition and procedure of obtaining the catalytic ink. The electrocatalytic powder (4 mg) was dispersed in a solution containing ultrapure water (725 µL) and a Nafion® solution (114 µL) and sonicated for 10 min. Then, for each electrochemical experiment, the anode was prepared with 42 µL of catalytic ink deposited onto the substrate which corresponds to 0.38 mg of catalyst per cm2. The ink was dried under a low nitrogen flow (U quality, supplied by Air Liquide). All the measurements were performed using an Autolab potensiostat/galvanostat (PGSTAT302) with Nova 3.8 Software®. The EIS experiments were performed at different potential values. Fifty


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frequencies were scanned between 50 kHz and 10 mHz with an amplitude of 10 mV. Zview Software was used to fit the EIS data with representative equivalent electrical circuit (EEC).

2.3 Physical and physicochemical characterizations The catalytic powders were characterized by various physical methods. The material structures were determined by X-Ray Diffraction (XRD) on an Empyrean diffractometer of PANalytical. This apparatus is equipped with a copper tube (monochromatic) powered at 45 kV and 40 mA and an Xcelerator detector. The measurements were performed in scanning mode between 20 and 140 ° (2θ) with a step of 0.05 ° and an accumulation time of 720 seconds per step. The powder samples were deposited on a silicon wafer after crushing and sifting. Thermogravimetric analysis (TGA) was performed with TA Instruments SDT Q600 apparatus. The measurements were done by heating an amount of catalytic powder between 5 and 10 mg in alumina crucible under air atmosphere (flow, 100 mL min-1) from 25 to 750 °C with a heating rate of 5 °C min-1. The surface morphology of the materials was examined by transmission electron microscopy (TEM). TEM images were acquired on a JEOL 2100UHR (200 kV) electron microscope equipped with LaB6 filament. TEM grids were prepared by placing one drop of the particle in ethanol solution on a copper grid and by evaporating the solution in the open atmosphere. Energy dispersive X-ray spectrometry analysis (EDS) also allowed estimating the material composition at particle scale. The catalyst compositions were determined by X-ray fluorescence spectroscopy on a S4 Explorer of Bruker AXS. This apparatus is equipped with a rhodium tube source powered at 50 kV and 20 mA and three crystals detector (LiF200, PET and XS-55). Analyses were performed on at least 100 mg of synthesized powders, non-pelleted under inert atmosphere. The surface material compositions were evaluated through X-ray photoelectron spectroscopy (XPS) measurements. Analyses were realized on a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al



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Kα source (1486.6 eV) operating at 150W (10mA, 15kV). Instrument base pressure was 9 x 10-8 Pa. The catalyst powder was pressed in copper holder of 3 mm diameter and introduced in the preparation chamber after outgassed overnight. High-resolution spectra were recorded using an analysis area of 300µm x 700 µm and 20 eV pass energy. This corresponds to Ag 3d5/2 FWHM of 0.55 eV. Data were acquired with 0.1 eV steps. The binding energy was calibrated using O 1s (O-Ru) binding energy fixed at 529.4 eV as an internal reference because C1s overlaps with Ru3d and could not be used as a reference. Spectra were fitted with CasaXPS software (version 2.3.17). Shirley background has been chosen, Ru3d and Ir4f peaks fitting were performed using asymmetric Gaussian-Lorentzian profile functions.

3. Results and discussions 3.1 Physicochemical characterizations of the materials 3.1.1 Crystallographic structure Commercial and synthesized materials were analysed by X-ray diffraction. Figure 1 shows the XRD patterns of the four materials. Commercial ruthenium dioxide was analysed before and after heat treatment, as well as synthesized pure IrO2 and IrO2@RuO2. XRD pattern of commercial RuO2 reveals the absence of well defined diffraction peaks, only a shoulder at 35 ° is observed, which implies that the material is poorly crystalline. This result could be explained by the small particle size and/or the powder hydration

26

. As it can be

observed in Figure 1, when annealing the RuO2 powders at 400 °C (using the temperature program described in section 2.1), radical change of the XRD pattern is observed. The appearance of crystallographic peaks indicates a modification of the material structure, moreover all the observed peaks are characteristic of the rutile structure of ruthenium dioxide 27

. For pure IrO2 and IrO2@RuO2, the diffractogramms are similar since the peaks attributed

to IrO2 and RuO2 oxides appear closely to the same diffraction angles, as ruthenium and


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iridium oxides have the same structure and similar lattice parameters. However, a low peak shift is observed between pure IrO2 and RuO2, 34.56 ° and 35.12 ° respectively for the peak (011). Contrariwise, IrO2@RuO2 does not modify the diffraction angles, thus the position is similar to the one obtained for commercial RuO2 after heat treatment (35.13 °). No extra diffraction peaks corresponding to metallic phases were detected; indicating that only pure oxides with a high degree of crystallinity were obtained.

Figure 1: XRD patterns of commercial RuO2 hydrate (▬),RuO2 after heat-treatment at 400 °C (▬), IrO2 (▬) and IrO2@RuO2 (▬) catalyst powders.

However for the IrO2 catalyst; the peaks broadness is higher compared to RuO2 after heat treatment which indicates that the IrO2 material is composed of smaller particles. For all the crystalline samples, the diffractograms have the prominent peak at 35 ° which is attributed to (011) plane. Based on the full width at half maximum (FWHM) of XRD peak (011), the average crystallite sizes have been calculated using the Scherrer equation (Table 1).



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Table 1: Physicochemical parameters obtained for RuO2 hydrate, RuO2 after heat treatment, IrO2 and IrO2@RuO2 catalyst. Mean Lattice parameters X-ray fluorescence crystallite size RuO2/IrO2 phase spectroscopy of RuO2/IrO2 Catalyst a=b c Molar percentage phase (nm) (Å) (Å) Ru Ir n.d. n.d. n.d. 100 0 RuO2 commercial 11.3 4.489 3.101 100 0 RuO2 calcinated 5.7 4.491 3.110 85.4 14.6 IrO2@RuO2 4.4 4.535 3.158 0 100 IrO2 n.d. : not determined

The obtained results are 4.4 nm and 11.3 nm for IrO2 and RuO2, respectively. The value is higher for RuO2 probably due to the heat treatment and the crystallization temperature of RuO2 which is lower than that of IrO2. However, no sintering effect is observed as this value is in agreement with our previous work

22

. IrO2 coated on RuO2

decreases drastically the average crystallite size which becomes closer to that of pure IrO2. The lattice parameters were also calculated from the Bragg relation with the position of peaks (110) and (002). The results are summarized in Table 1. For pure oxides, the values are in agreement with the JCPDS database. When coating IrO2 on the surface of RuO2, the lattice parameters increase slightly compared to pure RuO2 indicating that only one mixed material solution is obtained.

3.1.2. Thermogravimetric analysis The TGA curves obtained for commercial ruthenium (IV) oxide before and after heat treatment and IrO2@RuO2 are plotted in Figure 2. Since the materials are non-supported and have an oxide structure; in the temperature range studied (25 to 700 °C) the weight loss should be negligible and represent only the impurities adsorbed on the powders surface. When increasing the temperature from 25 to 700 °C, the total weight loss for RuO2 after heat treatment and IrO2@RuO2 represent less than 2 % of the sample mass. These losses occur


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over the entire temperature range, and could be attributed to the impurities or a low water adsorption on the surface of the oxide particles

28

. After an appropriate heat treatment, the

oxide materials synthesized are stable. Contrariwise, the TGA curves for the commercial ruthenium dioxide shows a total weight loss of 24.2 %. As often described in the literature 26, 28

, two distinct temperature region appears. The first one corresponds to the temperature range

of 25 - 100 °C, and the mass loss is assigned to desorption of the physically adsorbed water on the material surface. While for the second range between 100-350 °C, the loss could be assigned to the desorption of the water inserted in the oxide lattice. So the commercial ruthenium dioxide is highly hydrated. 13 % of its total mass represents the adsorbed water whereas 11.2 % of the mass is assigned to the water molecules bound to the oxide lattice. These results could explain the non-crystalline structure observed on XRD patterns.

Figure 2: TG analysis of commercial RuO2 hydrate (▬), commercial RuO2 after heat treatement (▬), and IrO2@RuO2 catalyst (▬).

3.1.3 Morphology TEM measurements were undertaken to analyze the morphology and the mean particle size of the commercial ruthenium (IV) oxide powders before and after heat treatment. TEM images at different magnifications are presented in Figure 3. The two samples are composed 11 Environment ACS Paragon Plus


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of aggregated particles with heterogeneous sizes as often observed for unsupported oxide catalysts. However, it can be seen that the morphologies are different before and after calcination. Figures 3a and 3b show that the particles observed display large size and seem to be an agglomeration of small particles of hydrous RuO2. Moreover, it can be clearly observed in Figure 3c the partially crystalline nature of the surface material in accordance with the XRD analysis. Contrariwise, after the heat treatment, the RuO2 particles are clearly defined and crystalline. Heterogeneous particles are aggregated (Figures 3d and 3e) and heterogeneous in size. The particles size is ranging between 10 and 50 nm. During the heat treatment, water molecules are removed from the crystalline lattice leading to well defined nano-crystalline structure of RuO2 (Figure 3f).

Figure 3: TEM images of RuO2 hydrate at different magnification (a-c) and commercial RuO2 after heat treatment (d-f). TEM measurements coupled with EDS analysis were also realized for IrO2@RuO2 samples in order to analyze their morphology, mean particle size and atomic composition. The global aspect of the particles is similar to that observed for commercial ruthenium (IV) oxide sample (Figures 4a, 4b and 4c). Large size agglomerated particles are always present; however the surface is better defined and reveals a crystalline structure. The apparent particles size on the 12 Environment ACS Paragon Plus

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surface is very small, in agreement with the decreased average mean crystallite size measured by XRD. Moreover, EDS analysis showed that the two elements are present in the same aggregates (Figure 4d). The EDS measurement was performed in two different regions of the same agglomerated particles in order to observe the evolution of the metallic composition. At the surface, ruthenium and iridium elements are present in almost equal amounts (blue and green in Figure 4d), whereas Ru is the major element in the core of the agglomerate (red in Figure 4d). These results let us to conclude that a thin layer (nm) of IrO2 covers the core RuO2 surface. Although during the synthesis a heat treatment was performed, the IrO2 coating on RuO2 did not modify the global morphology and structure of the RuO2 core structure.

Figure 4: TEM images of IrO2@RuO2 catalyst at different magnification (a-c) and coupling with EDS analysis (d).

3.1.4 Chemical composition The atomic metal compositions of the mixed oxides were determined by using X-ray Fluorescence spectroscopy analysis on ca. 100 mg of powder sample. The metallic molar percentages were measured taking into account the atomic molar mass of the metallic elements, so as to compare them with the nominal composition of 75 mol% for Ru and 25 mol% for Ir. The obtained values are 85.4 and 14.6 for ruthenium and iridium, respectively, 13 Environment ACS Paragon Plus


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and are summarized in Table 1. These values are in accordance with the result obtained for the core agglomerate by EDS analysis (Figure 4d). The difference between nominal and experimental compositions could originate from the synthesis path. It is likely that only a portion of the iridium chloride precursor was grafted onto the surface of the RuO2 support or the hydration of the oxide support did not permit a grafting of all precursors. In both cases, iridium excess did not precipitate in the form of hydroxide species coating the support, and was recovered with the supernatant. XPS analysis was carried out so as to determine the atomic metal surface composition of the IrO2@RuO2 material. The obtained XPS spectra (Figure 5) revealed characteristic shapes of Ru 3d and Ir 4f core level peaks of RuO2 and IrO2 oxides respectively 29-30. For the Ru 3d core level, the two main peaks located at 280.5 and 284.7 eV are attributed to the primary 5/2 and 3/2 spin-orbit components, respectively. For Ir 4f core level, 7/2 and 5/2 components are located at 61.4 and 64.4 eV respectively. In addition to the dominant spinorbit doublets two satellites (282.4 and 286.6 eV for Ru 3d and 63.2 and 66.2 eV for Ir 4f) are attributed to final-state screening effects (green curves Figure 5). However, a contribution to these peaks of intermediate species closely related to, respectively, RuO2 and IrO2 with an additional hydroxyl group cannot be excluded 31. Results from XPS spectra fitting performed using asymmetric (for Ru 3d and Ir 4f) and symmetric (for C1s) Gaussian-Lorentzian lineshapes on Shirley backgrounds are also included in Figure 5. Since the samples were exposed to air, the carbon contamination has been taken into account, particularly in the Ru 3d fitting by including different C 1s components (Figure 5a) at 284.4 eV (graphitic carbon), 286.1 eV (-C-O), 287.4 eV (-C=O) and 288.9 eV (-COO). The obtained results reveal a Ru:Ir surface atomic ratio of nearly 1:1. Taking into account the atomic compositions measured by X-ray Fluorescence spectroscopy (85.4% Ru


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and 14.6% Ir) this clearly indicates that iridium is mostly present at the surface of the IrO2@RuO2 material. This 1:1 ratio is also in agreement with EDS results.

Figure 5: XPS spectra of the Ru 3d (a) and Ir 4f (b) for IrO2@RuO2 catalyst, fitted with Gaussian-Lorentzian curves on Shirley backgrounds. 3.2 Electrochemical characterizations Cyclic voltammetry measurements were used to characterize the behaviour of the different oxides in H2SO4 supporting electrolyte in the potential range 0.05–1.2 V versus RHE. All currents were normalized to the geometric area of the working electrode, as shown Figure 6. The CV shape of the commercial RuO2 hydrate in the insert in Figure 6 is characteristic of non-crystalline and hydrous ruthenium dioxide

32

. Contrariwise, the CV of

the RuO2 obtained after heat-treatment is characteristic of crystalline RuO2, with the presence of oxidation state changes, Ru(III)/Ru(IV) and Ru(IV)/Ru(VI) during the potential scan at 0.6 and 0.8 V vs. RHE, respectively

33

. The cathodic peak at low potentials (below 0.3 V vs.

RHE) was attributed to the hydrogen absorption in the oxide lattice

34-35

. For hydrous RuO2,

only a large peak is present at 0.7 V vs. RHE, with a low shoulder during the anodic scan certainly correlated to the same redox transitions. Moreover, current densities in this potential range are higher for hydrous than crystalline RuO2, since the double layer formation at the interface catalyst / electrolyte is more consequent 36. For RuO2 hydrate, below 0.4 V vs RHE,



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the weak cathodic current was attributed to the water in the oxide lattice which created a potential barrier for the electron transfer process in the material

32

. IrO2 voltammogram

exhibits a characteristic shape with the presence of the reversible peak corresponding to Ir(III) / Ir(IV) redox transition at 0.9 V vs. RHE 37. For the IrO2@RuO2 material, although the IrO2 is at the interface electrolyte/oxide catalyst, the CV shape is quite close to the one obtained for pure RuO2, indicating that the material behaves mostly like RuO2 despite a slightly higher capacitance induced by IrO2. Therefore, the iridium oxide coating does not affect much the double layer formation induced by the RuO2 support. Furthermore, the current density values are in the same order of magnitude than those obtained with IrO2, which is in agreement with low and comparable mean crystallite sizes.

Figure 6: Voltammograms of RuO2 hydrate (▬), RuO2 after heat treatment (▬), IrO2 (▬) and IrO2@RuO2 catalyst (▬) based electrodes in 0.5 mol L-1 H2SO4 electrolyte recorded at 20 mVs-1 and 25 °C. From CV measurements, an evaluation of the charge accumulation at the interface electrolyte/ catalyst for each crystalline material was carried out through capacitance measurements. In order to avoid the faradic contribution, the specific capacitances (C in F g-1) were measured


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by integrating the CV curves between 0.4 and 1.1 V vs RHE at 10 and 100 mV s-1, using Equation 1.

C=

1

E2

vm( E 2 − E1 ) E∫1

i( E )dE

(1)

where v is the scan rate (V.s-1), m is the mass in mg of oxide catalyst deposited on the E2

working electrode, E1 and E2 are limits potential of the integration curves and

∫ i(E)dE

the

E1

integration of the voltammetric curve. The values obtained are summarized in Table 2.

Table 2: Electrochemical parameters obtained for RuO2 after heat treatment; IrO2 and IrO2@RuO2 electrocatalysts. Total charge (q*Total)

Most accessible charge (q*Outer)

1

(mC)

(mC)

17.70 35.44 32.86

4.16 7.13 7.19

3.28 5.39 4.59

-1

Capacitance (F g ) Catalyst 10 mV s-1 RuO2 calcinated IrO2@RuO2 IrO2

20.74 41.06 39.98

100 mV s-

Accessibility q*Outer / q*Total

0.79 0.76 0.63

For the three studied catalysts, when increasing the scan rate, the capacitance values decrease, indicating that the double layer formation is more consistent in quasi-stationary mode 26, 38. In fact, the diffusion of the active species can occur even in the porosity and thereby all the oxide active sites contribute to the double layer formation. Contrariwise, for high scan rate, the charge accumulation occurs only on the surface active sites. The values for commercial RuO2 after heat treatment (20.74 and 17.70 F g-1 respectively at 10 and 100 mV s-1) are lower than those obtained for IrO2 synthesized by hydrolysis in ethanol medium (39.98 and 32.86 F g-1). The capacitive properties of the oxide materials seem therefore to be correlated with the mean crystallite size. The smaller the crystallite size, the higher the number of active sites in



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the catalytic layer and thus the capacitance values are higher for lower mean crystallite sizes. However, this increase in capacitance can also be correlated to the higher oxidation state of Ir in the oxide 39. Capacitance values obtained for IrO2@RuO2, 41.06 and 35.44 F g-1 at 10 and 100 mV s-1, respectively, are slightly higher than those measured for pure iridium oxide, although the mean crystallite size is slightly lower for the latter. To improve the charge accumulation, it is important to increase the number of actives sites but also their accessibilities. For IrO2@RuO2, the presence of well defined small particles on the catalyst surface could confer a high concentration of active site at the interface electrolyte/catalyst with high oxidation state changes and therefore improved capacitive properties. Globally, the oxide coated on the RuO2 support brings a beneficial effect on the surface OH groups accumulation compared to the pure oxides, certainly due to the morphology and the structure of this material. The voltammetric charges (q*) determined by integrating the CV curves are considered to be proportional to the active surface area or to the number of active sites of the catalytic layer

40-41

. The method established to calculate the q* is well described in our previous work

22

. Briefly, it consists in averaging the anodic and cathodic charges measured between 0.4 and

1.1 V vs RHE. Based on diffusion species phenomena at the interface electrode/electrolyte depending on the scan rate, Ardizzone and co-workers 40 established two relations (Equation 2 and 3) to determine the total charges (q*Total) and the most accessible charges (q*Outer) when the scan rate values tends to 0 and ∞,respectively 42.

1 v

(2)

1 1 = + C2 v * q* qTotal

(3)

* q* = qOuter + C1


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where C1 and C2 are constants, v is the scan rate and q* are the average charges calculated for different scan rates included between 7 and 500 mV s-1. The q*Total and q*Outer values for crystalline oxides samples are obtained by linear part extrapolations of the curves presented in Figures 7a and 7b and are summarized in Table 2. The total charges for commercial ruthenium oxide after heat treatment are the lowest (4.16 mC) whereas the values obtained for pure iridium oxide and IrO2@RuO2 support are quite similar, 7.19 and 7.13 mC respectively. The supposition established from capacitance measurements is therefore confirmed, when decreasing the mean crystallite size, the number of active sites increases and promote the charge accumulation properties of the catalyst. The highest value for the most accessible charge was obtained for the IrO2@RuO2 material (5.39 mC), followed by those obtained for pure iridium oxide (4.59 mC) and then for pure ruthenium oxide (3.28 mC). The IrO2@RuO2 has allowed obtaining a higher number of accessible active sites in the catalytic layer, which is probably due to the higher concentration of active sites on the surface of the mixed oxide..

Figure 7: Scan rate dependency of voltammetric charges: extrapolation of the total charges (q*Total) (a); extrapolation of the most accessible charges (q*Outer) (b) for RuO2 after heat treatment (blue), IrO2 (red) and IrO2@RuO2 catalyst (green).

Moreover, from the obtained charge values, the active sites accessibility was evaluated by the ratio q*Outer / q*Total (Table 2) 16. The highest active sites accessibility was then obtained with the commercial RuO2 after heat treatment (0.79) whereas the lowest one was measured for 19 Environment ACS Paragon Plus


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pure iridium oxide (0.63). For the coated catalyst, the obtained value is slightly inferior to the one obtained with pure RuO2 (0.76). Synergetic effect between the RuO2 core and the IrO2 shell appears since the coated material properties are quite similar to those of pure RuO2 whereas its surface is predominantly composed of IrO2.

3.2.2 Stability test Repetitive CV measurement was achieved in order to assess the oxides electrochemical activity and stability 33, 43. As shown in Figure 8, one thousand voltammetric cycles were carried out between 0.3 and 1.2 V vs RHE in 0.5 mol L-1 H2SO4 at 20 mV s-1 with the aim of evaluating catalysts ageing during a long-term test through their current density evolution. The voltammogram shape changes significantly during the long-term cycling for the commercial hydrous ruthenium oxide. The material seems to be stable during the first 100 cycles. Afterwards, the shape and the current densities of the anodic and cathodic peaks change drastically indicating a crystallographic modification of the RuO2 hydrate (Figure 8a). Globally, the commercial material is unstable in acidic media under potential cycling.


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Figure 8: Cyclic voltammograms of RuO2 hydrate (a), RuO2 after heat treatment (b), IrO2 (c) and IrO2@RuO2 (d) electrocatalysts in 0.5 mol L-1 H2SO4 electrolyte during repetitive potential cycles at 20 mVs-1 and 25°C. For the three crystalline samples, no modification of the CV shape appears during the one thousand cycles of the long term test. This observation indicates that no alteration of the particles structure which composed the catalytic layer occurs during the test. However, the current densities decrease during the first cycles and finally tend to stabilize. The fact that the cathodic and anodic current densities do not evolve drastically and no extra peaks appear reveals a high stability of the oxide material obtained after heat treatment under variable transient conditions. The corresponding voltammetric cycles overlap for the two pure oxides RuO2 and IrO2 from the 100th cycle, whereas for IrO2@RuO2, the overlapping begins as soon as the 50th cycle is reached (Figures 8b-d). The evolution of the current during the test could arise from a slight loss of active sites in the catalytic layer which could contribute to a small catalytic performance decrease. For different cycles of this stability test, the charge values (q*) were measured in order to quantify the active-sites losses and the degradation of the electrocatalytic performances (Figure 9a). In good agreement with the CV observations, charges decrease in a first activation time (first hundred cycles). For the pure oxides the decrease takes place until the 200th cycle, whereas for the coated catalyst, it is less significant and occurs only for the first hundred cycles. Moreover, the IrO2 charge value tends to reach the value obtained for IrO2@RuO2, the latter having the most stable behaviour over the thousand cycles. As the charges are considered to be proportional to the number of active sites; the ratio q*/q*Initial as a function of the number of cycles may be used to evaluate the active sites loss (Figure 9b).



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Figure 9: Charge (q*) evolution (a) and q*/q*Initial ratio (b) for RuO2 after heat treatment (blue), IrO2 (red) and IrO2@RuO2 catalyst (green) electrocatalysts during stability test measurement. The remaining active sites after 1000 cycles for IrO2 and RuO2 are 87.8 % and 90.8 % of the initial charges, respectively. The highest value of 96.7 % was obtained for IrO2@RuO2. As the CV shapes remain the same along the durability test, the charge ratio evolution could be attributed to degradation of the catalytic layers, due to a slight erosion of catalyst at the interface with the electrolyte

44

. It seems that coating on the oxide support improves the

stability of the catalytic material in acidic media, which might be probably due to a direct interaction and an intimate contact between the two oxides.

3.2.3 OER measurement To evaluate the electrocatalytic properties of the oxide materials toward the OER as well as their behaviour during this reaction, anodic polarisation measurements were performed. As shown in Figure 10a, current was first normalized to the geometric surface area of the working electrode in order to compare the catalytic performances of the oxide materials, since the same masses were deposited on the electrode. The curves are presented


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with no ohmic drop correction; consequently, the Tafel slopes were determined from EIS data (see Section 3.2.4). For the commercial RuO2 hydrate (insert Figure 10a), the obtained curve is different from those obtained for the crystalline samples. The current density increases around 1.3 V vs RHE to decrease drastically at 1.5 V vs RHE. After that a plateau at low current density is observed prior to another slight increase. Similar results were reported by Devadas et al

26

, however the reactions and mechanisms which evolve during the OER for

hydrous ruthenium oxide are still under debate. Due to the experimental observation of bubbling, both anodic contributions can be attributed to the water oxidation. Contrariwise, the passivation step between those two active zones cannot be clearly elucidated. However, it can be postulated that this dramatic change in activity could originate from oxide material instability and be related to crystallographic hydration. For the three crystalline samples, only the behaviour at low overpotentials was measured. The upper imposed potential was 1.5 V vs RHE to avoid the disturbance induced by O2 bubbling, whereas it was of 1.8 V vs RHE for commercial RuO2 hydrate. Compared to hydrous material, the OER onset is observed at a slightly higher potential, around 1.35 V vs RHE. As often described in the literature 42, 45, IrO2 electrocatalytic efficiency for OER is inferior to the one obtained for pure ruthenium oxide. Despite the fact that the major element present at the material surface is iridium oxide, the highest current density value at 1.5 V vs RHE (10.8 mA cm-2) was obtained for IrO2@RuO2. The proximity between the two metallic elements and the low particles size at the interface catalyst/electrolyte must contribute to this performance.



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Figure 10 : Polarization measurements of RuO2 hydrate (▬), RuO2 after heat treatment (▬), IrO2 (▬) and IrO2@RuO2 catalyst (▬) electrodes recorded at 5 mV s-1. Current normalized to the electrode surface area (a) and to the most accessible charges (q*Outer) (b). Supporting electrolyte: 0.5 mol L-1 H2SO4 at 25 °C. The polarization curves obtained for the crystalline samples were also normalized to the most accessible charges (Figure 10b) so as to evaluate only the electrocatalytic activity per active site involved in the OER. Using this normalisation does not modify the order of the catalysts performances. These results indicate that, even if iridium is added in the catalyst composition of the coated material, the electrocatalytic properties and intrinsic efficiency of active sites are enhanced. This is certainly due to a synergistic effect between the two metallic elements induced by their proximity.


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3.2.4 EIS and Tafel measurements Electrochemical impedance spectroscopy measurements were carried out at different anodic potentials in order to complete the analysis of the catalyst properties during the OER., One measurement was realised each 25 mV between 1.4 and 1.5 V vs RHE. Figure 11 shows the Nyquist plots obtained at 1.5 V vs RHE for the three crystalline samples as an example.

Figure 11 : Nyquist plots for mono and coated oxides electrodes recorded at 1.5 V vs. RHE during the OER at 25 °C (○ and □ represent 90 and 820 mHz, respectively). Solid curves are the fitting of experimental impedance data by using the equivalent circuit shown.

As it can be seen, a slightly depressed semicircle is obtained for all oxide catalysts EIS spectra; which is characteristic of a charge-transfer process during the OER. At low frequencies (below 100 mHz), a noisy signal attributed to the bubbling effect is observed. This frequency range was not used for the EIS analysis and fitting. The characteristic EEC for a simple redox reaction, represented by one capacitive loop (RΩ (Rct Cdl)), as shown in the insert in Figure 11 was used to fit the Nyquist plots

46-47

. RΩ is the cell resistance including

the connections, the electrolyte, and oxide deposit resistance; Rct is the charge-transfer



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resistance for the OER; and Cdl is the double-layer capacitance. To take into account of the catalyst surface roughness and heterogeneity, the double layer capacitance was replaced in the EEC by a constant phase element (CPE) so-called QCPE(dl). Values of QCPE(dl) were finally converted to capacitance values using the Brug relation 48-49. Table 3 summarizes the Rct, and n values obtained by fitting as well as calculated Brug capacitances (CBrug).

Table 3: Parameters obtained by fitting EIS experimental spectra recorded at various potentials for RuO2 after heat treatment, IrO2 and IrO2@RuO2 electrocatalysts in 0.5 mol L-1 H2SO4 at 25 °C. Potential Rct CBrug Catalysts n (V vs. RHE) (Ω cm2) (F g-1) 562.1 0.96 34.15 1.400 120.7 0.95 38.82 1.425 RuO2 (calcinated) 33.9 0.93 45.71 1.450 12.5 0.89 49.63 1.475 6.3 0.85 43.85 1.500 229.6 0.93 45.75 1.400 55.7 0.91 48.67 1.425 IrO2@RuO2 17.7 0.88 49.98 1.450 8.3 0.81 42.41 1.475 4.9 0.75 33.33 1.500 906.4 0.85 24.53 1.400 462.8 0.84 23.96 1.425 IrO2 187.1 0.83 23.37 1.450 63.1 0.81 22.56 1.475 21.4 0.79 20.56 1.500 The decreasing n value when increasing the applied potential can be explained by the bubble effects occurring during the acquisition, particularly at high overpotentials which clog the pores and increase the interface roughness inducing additional surface heterogeneities. As it is shown in Figure 11, the lowest charge transfer resistance Rct is obtained for IrO2@RuO2 what clearly demonstrates the highest kinetic for the OER. In accordance to the polarisation measurements (Figure 10a), IrO2 has the highest Rct value whereas the one obtained for RuO2 after heat treatment is intermediate. This observation is valid for the entire range of potential investigated (Table 3). 26 Environment ACS Paragon Plus

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As the n values rapidly decreases with overpotential due to the formation of bubbles, capacitance values were estimated using the Brug relation. As long as n > 0.88, the Brug capacitances (CBrug) increase. However, when n is lower than 0.88, CBrug decreases sharply. The CBrug value is influenced by the catalyst surface state (which is reflected by the n value). and its evolution could be related to the oxygen evolution occurring on the electrode surface. The generated bubbles lead to a lower accessibility or to a temporary diminution of the number of active sites

49

. The presence of RuO2 in the IrO2@RuO2 catalyst increases the

charge storage properties of the catalyst under electrode polarisation in the OER potential range (at potential value lower than 1.450 V vs RHE). Afterwards, the “O2 bubbling” diminishes this ability, certainly due to the presence of the IrO2 at the interface electrode/electrolyte. However, the RuO2 presence in the catalytic layer seems effective to maintain capacitance values higher than the one obtained with pure IrO2. OER mechanism is often considered to be complex due to the four electronic transfers and also the different intermediate species which evolve during the multi-step reaction. Several mechanisms were established with different rate-determining steps and were indexed by Matsumoto et al

10

. One of them was suggested for the use of electrochemical path on

oxide catalyst and it is composed of the following three steps. The first one is the water adsorption on the active site (S) which conduces to a hydroxyl intermediate specie formation with an electron transfer (R1). S + H2O → S-OHads + H+ + e-

(R1)

The second step consists in the second charge transfer reaction and the deprotonation of the hydroxyl species to form the second intermediate species (R2). S-OHads → S-Oads + H+ + e-


(R2)


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However, an alternative step can take place in parallel due to catalytic layer composition and the bond strength of the intermediate species to the active site leading to an oxide path step (R2’) 50-52. 2 S-OHads → S-Oads + M + H2O

(R2’)

The last step is the oxygen desorption occurring by combination of intermediate species present on two adjacent active sites (R3). 2 S-Oads → 2 S+ O2

(R3)

For this mechanism, Tafel slopes values were established and are 120 mV dec-1 for R1, 40 mV dec-1 for R2 (this value can slightly differ with the intervention of the R2' step) and finally 30 mV dec-1 for R3 50, 53. In order to avoid the ohmic drop contribution in the OER kinetics, Tafel slopes values were estimated from Rct values obtained from EIS data by plotting E vs. log (Rct-1)

54-55

(Figure 12). The Tafel slopes values are obtained by linear fitting of the plots and are of 50.0; 57.8; 60.2 mV dec-1 for RuO2 after heat treatment; IrO2@RuO2 and pure IrO2, respectively. Certainly due to the compactness of the catalytic layer 56, the value for pure ruthenium oxides is slightly superior to those obtained generally in the literature (40 mV dec-1) 19, 57. However, this value is inferior to the one obtained for pure IrO2 and the well-established difference in Tafel slopes between these two oxides is preserved

45, 58

. For the coated catalyst, the value

obtained is intermediate due to the presence of the two oxides. Although the molar IrO2 content is lower, the result tends significantly to the pure iridium oxide material value which could be directly correlated to its presence at the surface of catalytic layer.


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Figure 12: Tafel plots based on EIS data for RuO2 after heat treatment, IrO2 and IrO2@RuO2 electrocatalysts during the OER at 25 °C. Globally the Tafel slope values are slightly higher than the value established for the step R2. This difference could arise from the catalytic layer properties as mentioned by Lodi et al 56. However, De Faria et al 50 explained that intermediate values could also be correlated to a mixed kinetic control of the OER dependent on the metallic elements present. Thereby, Antolini

51

in his review explained that for IrO2 based catalysts, the first step of the OER

mechanism can dissociate in two parallel reactions, as follow. S + H2O → S-OH*ads + H+ + e-

(R1’)

S-OH*ads → S-OHads

(R1”)

The intermediates species S−OH*ads and S−OHads have the same chemical structure but their energy states are different due to a different bond strength to the active site. In this way, the Tafel slope value of these two simultaneous steps is of 60 mV dec-1. In our case, for the two samples composed of IrO2 the rate determining steps are the combination of R1’and R1”, whereas it is the step R2 for pure RuO2.



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4. Conclusion A core-shell like structure, IrO2@RuO2, was synthesized by using a surface modification / precipitation method in ethanol medium. Small size crystalline particles were obtained and TEM EDS analyses showed that crystalline RuO2 core particles are well covered by IrO2. X-Ray Fluorescence spectroscopy revealed a global atomic composition of 85.4% Ru and 14.6% Ir whereas X-Ray Photoemission Spectroscopy proved that iridium is mostly present at the catalyst surface. This was additionally confirmed by the TEM EDS analyses. Voltammetric analysis showed that the IrO2@RuO2 catalyst displayed mainly the RuO2 signature despite the presence of IrO2 at the surface. The comparison of this catalyst with pure RuO2 and pure IrO2 in terms of charges showed that the mixed oxide catalyst displayed the highest amount of active sites as well as a good accessibility among the three catalysts. Moreover, this catalyst was shown to be highly stable towards repetitive redox cycling as it retained 96.7 % of its initial charges after one thousand cycles. Polarization curves of the three catalysts showed that the IrO2@RuO2 was the most active for the OER owing to the large number and high accessibility of active sites. Moreover, the determination of Tafel slopes has evidenced that the OER mechanism was unchanged on the IrO2@RuO2 catalyst. These activity and stability enhancements compared to pure IrO2 or RuO2 are attributed to an intimate contact between the two oxides that boosts the well known synergistic effect between RuO2 and IrO2. Taking the benefit of the RuO2 intrinsic activity as well as of the IrO2 stability, this study contributes to the rational design of efficient and stable electrocatalysts for water splitting in acidic media.

Acknowledgements The authors acknowledge financial support from the French National Research Agency ANRAITOILES from the PROGELEC program.


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IrO2 Coated on RuO2 as Efficient and Stable Electroactive

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