Plasma-sprayed nanocrystalline Ti–Ru–Fe–O coatings for the electrocatalysis of hydrogen evolution reaction

Journal of Alloys and Compounds 345 (2002) 228–237

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Plasma-sprayed nanocrystalline Ti–Ru–Fe–O coatings for the electrocatalysis of hydrogen evolution reaction E. Irissou a , M. Blouin a , L. Roue´ a , J. Huot b , R. Schulz b , D. Guay a , * a

´ , Canada J3 X 1 S2 INRS-Energie et Materiaux, 1650 Blvd Lionel-Boulet, C.P. 1020, Varennes, Quebec b ´ , Canada J3 X 1 S1 IREQ , 1800 Blvd Lionel-Boulet, C.P. 1000, Varennes, Quebec Received 17 January 2002; received in revised form 22 February 2002; accepted 22 February 2002

Abstract Nanocrystalline Ti–Ru–Fe–O (2-1-1-2) was prepared by mechanical alloying in a ZOZ attritor. Vacuum plasma spray (VPS) was then used to deposit coatings of this material on a substrate. Energy dispersive X-ray fluorescence and X-ray diffraction analysis was used to follow the change in the chemical composition and crystalline structure of the powder upon deposition by VPS. Nanocrystalline Ti–Ru–Fe–O (2-1-1-2) prepared by the ZOZ attritor contains more than 50 wt.% of hexagonal Fe 2 Ti and a smaller amount (,10 wt.%) of a cubic phase. Iron contamination coming from the attrition of the milling tools yields [Fe] |38 wt.%, almost twice as much as the nominal composition of the powder. When it is used for VPS, reaction between Fe 2 Ti and Ru results in the formation of several cubic ˚ This reflects a change in the Ru content on the 1a (1 / 2, 1 / 2, 1 / 2) site of the phases with lattice parameters ranging from 2.96 to 3.02 A. cubic lattice. The deposition process also results in the formation of Ti 2 O 3 . This phase is present in excess at the surface of the coating but can be efficiently dissolved through etching in an acid solution. The cathodic overpotential for hydrogen evolution of such activated coatings in typical chlorate electrolysis conditions is h250 5 2550 mV.  2002 Elsevier Science B.V. All rights reserved. Keywords: Nanostructured materials; Coating materials; Electrode materials; Vapour deposition; Electrochemical reactions

1. Introduction Since the introduction of the dimensionally stable anode (DSA ) in the chlorate industry some 25 years ago [1], almost no further energy savings can be expected from the development of new electrodes for the oxidation of chlorine ions. On the other hand, the overpotential of the steel cathodes used in the electrolysis of sodium chlorate is more than one order of magnitude larger than that of DSA anodes. The cathodic overpotential constitutes the main energy losses in the electrochemical step of the electrolysis of sodium chlorate, and important energy savings and cost reduction could be gained by lowering the cathodic overpotential. During the last few years, we showed that considerable improvement (decrease) in the cathodic overpotential in typical chlorate electrolysis conditions could be reached by using nanocrystalline Ti–Ru–Fe–O materials [2–4]. When prepared by high-energy ball milling, these materials *Corresponding author. E-mail address: [email protected] (D. Guay).

produce a reduction in the cathodic overpotential by |0.3 V, which amounts to power savings close to 10%. Moreover, these nanocrystalline materials do not show any sign of deterioration in long-term electrolysis tests [5,6], or in more drastic experiments where electrodes are repeatedly switched from hydrogen discharge to open-circuit conditions [5,7]. Nanocrystalline materials prepared from high-energy ball milling are in a powdered form and small dimension electrodes can be efficiently prepared by cold pressing the powder. However, this preparation technique is not well suited for making electrodes with practical industrial dimensions. Clearly, a more efficient way of preparing large dimension electrodes from nanocrystalline powder must first be developed if these materials are to find any practical applications. Thermal spraying of powdered materials has been used for many years to produce thin coatings on metal substrates. More recently, it has been demonstrated that nanocrystalline coatings could be obtained by thermal spraying of nanocrystalline powders [8,9]. Typically, the deposited material consists of nanocrystalline Ni [10–14],

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00403-6

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Inconel 718 [10,12,15], Co–Cr [16], 316-stainless steel [17,18], Cr 3 C 2 –NiCr [19,20], WC- and TiC-based cermets [21–26], Fe [27] and TiO 2 [28,29]. Several plasma-spraying techniques have been used to deposit these materials, including high velocity oxygen-fuel thermal spraying, and both air and vacuum plasma spray. In most cases, coatings were made with the objective of studying their physical and mechanical properties. The electrochemical properties of nanocrystalline coatings prepared by thermal spray have only been studied on a few occasions [14,28]. In the following, we will describe how vacuum plasma spray can be used to prepare activated cathodes for hydrogen evolution from nanocrystalline Ti–Ru–Fe–O powder. Energy dispersive X-ray fluorescence and X-ray diffraction analysis will be used to follow the change occurring in the composition and the crystalline structure of the material when it is deposited on a substrate. The effect of these changes on the electrochemical properties will be assessed.

2. Experimental Nanocrystalline Ti–Ru–Fe–O (2-1-1-2) powder was prepared by high-energy ball milling using a ZOZ attritor apparatus. Typically, Ti, TiO, Ru and Fe 2 O 3 (500 g) were mixed and introduced with 10 kg of steel balls (diameter of 5 mm each) in an 8-l crucible. The milling time was set to 20 h. For comparison, a 5-g batch of the same material was prepared in a laboratory SPEX 8000 shaker mill [2,3,5]. Much of the material prepared in the ZOZ attritor has particle sizes smaller than the optimum 25–75-mm range for plasma spray. Therefore, an agglomeration process was developed to increase the average particle size. This process involves the mixing of the powder with polyvinyl alcohol (PVA) polymer. In a first step, PVA is dissolved with acetic acid and mixed with the nanocrystalline powder. Then, acetic acid is evaporated (90 8C for 12 h), leading to the formation of very large particles. The resulting material is then processed through a shatter box miller to reduce the particles size to the desired dimensions. Particles with size in the range 25–75 mm are then sieved and used in the plasma spray process. During the deposition, PVA is evaporated. The plasma spray parameters were adjusted to optimize the density of the coatings and their adherence to the substrates. These parameters were the gas compositions, the powder feeding rates and the substrate–torch distance. Both Fe and Ti substrates were used. In the case of Ti substrate, the use of an intermediate layer made of Ti / TiH 2 was necessary to achieve good adhesion. The crystalline structure of the powders and of the coatings was characterized by X-ray diffraction (XRD) in the Bragg-Brentano u / 2u configuration. The X-ray diffraction histograms were taken on a Philips X-PERT and on a Siemens D-500 diffractometer, using Cu Ka radiation.

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Quantitative analysis of these histograms was performed by the Rietveld refinement method using GSAS software [30]. Scanning electron microscopy (SEM) micrographs were obtained using HITACHI S-4700 instrument operated at 15 kV. Chemical analysis was performed by energy dispersive X-ray spectroscopy (EDX). The electrochemical characterization of the electrodes was performed in a two-compartment cell. A 54-cm 2 DSA was used as counter electrode and a standard saturated calomel electrode (SCE, Hg / Hg 2 Cl 2 / KCl saturated) as reference electrode. A Luggin capillary located close to the working electrode was used. The composition of the electrolyte was similar to that used in the chlorate industry (NaClO 3 : 550 g / l, NaCl: 110 g / l, NaClO: 1 g / l, pH 6.5, adjusted with NaOH and HCl). All the measurements were performed at 70 8C. The value of the reversible potential (based on an activity coefficient of unity) for the hydrogen evolution reaction under these conditions is 20.681 V versus SCE. The activity of the electrodes was determined in the galvanostatic mode at an applied current density of 250 mA cm 22 using a potentiostat-galvanostat (EG&G, model 273A) controlled by CorrWare software. All the electrode potential values have been corrected for the ohmic drop determined by impedance spectroscopy measurements, using a Solartron Frequency Response Analyzer (SI 1255).

3. Results and discussion

3.1. Powdered materials The X-ray diffraction histogram of nanocrystalline Ti– Ru–Fe–O (2-1-1-2) prepared in a ZOZ attritor is shown in Fig. 1, curve A. It shows the characteristic diffraction peaks of Ru, along with some less intense peaks belonging to a simple cubic structure. For comparison, the X-ray diffraction histogram of the material obtained by performing the milling in a SPEX 8000 shaker mill [2,3,31] is also displayed in Fig. 1, curve B. In contrast to the previous curve, the X-ray diffraction peaks of Ti 2 RuFe (a simple cubic structure) dominate the histogram. The reasons for this discrepancy will become clear later on. A Rietveld refinement analysis of the X-ray diffraction histograms was performed to determine the phase composition of the samples. A typical example is shown in Fig. 2 for nanocrystalline Ti–Ru–Fe–O (2-1-1-2) prepared in a ZOZ attritor. In this figure, the open circles are the experimental data and the full line is the fitted curve. The residual curve, which is the difference between the experimental data and the fitted curve, is shown at the bottom. The agreement between the experimental data and the fitting curve is excellent. The parameters extracted from the refinement of the nanocrystalline powders prepared with the ZOZ and the SPEX are given in Table 1. Up to nine different phases

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Fig. 1. X-ray diffraction histograms of nanocrystalline Ti–Ru–Fe–O (2-1-1-2) prepared (A) in a ZOZ attritor and (B) in a SPEX 8000 shaker mill. Curve (C) is the histogram of the coating obtained by vacuum plasma spraying of powder A. The position of the diffraction peaks of (1) Ti 2 O 3 , (2) Fe 2 Ti, (3) Ru and (4) Ti 2 RuFe (simple cubic structure) are indicated at the bottom.

were needed to achieve a good fit. Before discussing the structural results, it was first checked that these parameters are consistent with the actual composition of the material. To do so, the elemental composition (Ti, Fe and Ru) of the samples, based on the data of Table 1, was calculated and checked against that obtained from an EDX analysis. These results are shown in Table 2. In both cases, there is a very good agreement between the two sets of data. This shows that the structural parameters extracted from the Rietveld analysis are consistent with the actual elemental composition of the samples. Also shown in Table 2 is the nominal elemental composition of the powder. It is interesting to note that the amount of Fe in the powder prepared with the ZOZ attritor is almost twice as large as that expected from the nominal composition. The imbalance between Fe on one side and Ti and Ru on the other side is less important for the SPEX. This imbalance could originate from the attrition of the milling tools and / or a preferential sticking of Ru and Ti on the milling tools of the ZOZ. As it will be seen later on, this excess Fe has a

Fig. 2. Rietveld refinement analysis of the X-ray diffraction pattern of nanocrystalline Ti–Ru–Fe–O (2-1-1-2) prepared in a ZOZ attritor: experimental data (open symbol), fitted curve (full line). The bottom curve shows the difference between the experimental and the fitted curve.

marked influence on the phase composition of the powder prepared with the ZOZ. From an inspection of the structural parameters of the ZOZ and the SPEX sample, the following observations can be made. (i) Hexagonal Ru, Ru(Fe) (Fe dissolved in hexagonal Ru) and Fe 2 Ti are the most prominent phases of the ZOZ sample. All together, they amount to |60 wt.% of the ZOZ sample. These phases are almost totally absent from the SPEX sample. (ii) The cubic phase concentration in the ZOZ sample is |9 wt.%, much less than in the SPEX sample (77 wt.%). (iii) For both samples, TiO amounts to |15 wt.% of the sample and there is only a slight trace of Ti 2 O 3 . (iv) In both cases also, the crystallite size is close to 10 nm. As exemplified by items (i) and (ii) above, the phase compositions of the ZOZ and SPEX samples differ. To understand the origin of these differences, let us consider first the Ti–Fe binary phase diagram [32] and then the

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Table 1 Structural parameters of nanocrystalline Ti–Ru–Fe–O (2-1-1-2) powders and coating Phase identification

Space group

Lattice parameters

Cubic phase Ru Ru(Fe)a Ti 2 O 3 TiO Fe 2 Ti Ti(O)a b-Ti a-Fe(Ti)a

] Pm3m P6 3 /mmc P6 3 /mmc ] R3c ] Fm3m P6 3 /mmc P6 /mmm ] Im3m ] Im3m

˚ a (A)

ZOZ ˚ c (A)

2.98(1)–3.02(7) 2.69(8) 2.54(7) 5.11(7) 4.17(7) 4.78(3) 5.0(3) 3.21(6) 2.94(5)

4.28(0) 3.69(7) 13.7(7) 7.88(3) 2.9(0)

SPEX

Coating

Phase concentration (wt.%)

Crystallite size (nm)

Phase concentration (wt.%)

Crystallite size (nm)

Phase concentration (wt.%)

Crystallite size (nm)

8.9 28.4 8.3 – 13.5 22.9 2.8 4.3 10.9

8 12 5 – 11 8 12 9 8

77.2 – – – 16.9 1.6 – – 4.2

6 – – – 8 7 – – 7

59.3 – 5.0 16.3 13.2 – 1.4 – 4.8

20–37 – 12 79 10 – 39 – 25

a This notation indicates that the element in parenthesis is dissolved in the preceding element. For example Ru(Fe) means that Fe is dissolved in the hexagonal phase of Ru, thereby modifying its lattice parameters.

effective Ti and Fe atomic concentration. The equilibrium Ti–Fe phase diagram shows four distinct regions. The maximum equilibrium solid solutions are 21% Fe in b-Ti at 1085 8C and 10% Ti in a-Fe at 1290 8C. Both of them decrease at room temperature. Intermetallic b.c.c. FeTi and Fe 2 Ti with a hexagonal structure are stable at room temperature. Therefore, based on the Fe–Ti equilibrium phase diagram, the formation of b.c.c. FeTi and hexagonal Fe 2 Ti is not unexpected. Indeed, the occurrence of these phases has already been evidenced in mechanically alloyed Fe–Ti mixture prepared by high-energy ball milling [33,34]. The other element that must be considered to understand the phase composition difference between the ZOZ and the SPEX samples is the effective Ti and Fe atomic concentrations. In nanocrystalline Ti–Ru–Fe–O compounds, Ti reacts readily with O to form stable titanium oxide phases [6,31,35,36]. This reduces the amount of Ti atoms that can effectively participate in the formation of an intermetallic compound. For the sake of the discussion, let us call Ti effec those Ti atoms that are not bound to O atoms. So, a simple calculation based on the results of Table 1 yields Ti effec , while Fe effec is directly deduced from the EDX results of Table 2. In the ZOZ and the SPEX samples, the relative atomic percents of Ti effec compared to Fe effec are 36.3 and 59.0%, respectively. So, in the ZOZ sample, the atomic percent of Ti that can effectively form an alloy with Fe is close to that expected

for the formation of hexagonal Fe 2 Ti. In the SPEX sample, this value falls in the range where a cubic phase is expected. It is thus postulated that the phase composition difference between the ZOZ and the SPEX samples occurs mainly as a result of difference in the chemical composition of the two samples. The occurrence of Ru as a distinct phase in the ZOZ sample must thus reflect the lower solubility of Ru in Fe 2 Ti compared to b.c.c. FeTi.

3.2. As-deposited coating The X-ray diffraction pattern of the coating made by plasma spraying nanocrystalline Ti–Ru–Fe–O (2-1-1-2) is displayed in Fig. 1, curve C. This histogram shows a marked difference from that of the powder used to prepare the coating (curve A). For example, the characteristic diffraction peaks of Ru are not observed. Instead, the histogram is dominated by the diffraction peaks of a simple cubic phase, along with the characteristic diffraction peaks of Ti 2 O 3 . It is interesting to note that curve C has more in common with the histogram of the powder prepared with the SPEX 8000 shaker mill than it has with the histogram of the powder used to prepare the coating. Several coatings were prepared by varying the experimental conditions during the deposition process. All the X-ray diffraction histograms of these coatings were similar to that displayed as curve C of Fig. 1, indicating that they all possess an identical structure and phase composition.

Table 2 Elemental composition of nanocrystalline Ti–Ru–Fe–O (2-1-1-2) powders and coating ZOZ

Rietveld analysis EDX Nominal

SPEX

Coating

Ti (wt.%)

Ru (wt.%)

Fe (wt.%)

Ti (wt.%)

Ru (wt.%)

Fe (wt.%)

Ti (wt.%)

Ru (wt.%)

Fe (wt.%)

29.1 29.0 37.9

34.8 32.2 40.0

36.1 38.8 22.1

40.9 38.7 37.9

38.6 40.1 40.0

16.2 21.2 22.1

50.7 46.5 37.9

21.3 21.9 40.0

28.0 31.6 22.1

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Also, a Rietveld refinement analysis of the X-ray diffraction histogram of the coating was performed and the results are given in Table 1. As seen previously, there is an excellent agreement between the elemental composition of the coating calculated from the structural parameters extracted from the Rietveld analysis and that obtained by EDX (Table 2). It is interesting to note that there is more Ti in the coating than in the powder used to prepare it. As will be shown later, this is due to a thick overlayer of titanium oxide found at the surface of the coating. The most striking result of Table 1 is the difference between the amount of Ru and Fe 2 Ti found in the ZOZ sample and the coating. While these phases account for more than 50 wt.% of the ZOZ sample, they are not present in the coating obtained when this powder is vacuum plasma-sprayed. Instead, the amount of cubic phase increases from 8.9 to 59.3 wt.%. In its fully ordered form, this cubic phase has Ti sitting on the 1a (0, 0, 0) site of the unit cell, and either Ru or Fe located on the 1b (1 / 2, 1 / 2, 1 / 2) site. The lattice parameter of that cubic phase varies between 2.96 (FeTi) ˚ (TiRu), depending on the relative proportion of and 3.06 A Fe and Ru on the 1b site. This variation of the lattice parameter reflects the fact that the covalent radius of Fe is 0.117 nm compared to 0.124 nm for Ru. In the deposited coating, several cubic phases are found, with lattice ˚ This is parameter values ranging from 2.98 to 3.02 A. evident in Fig. 3, where the X-ray diffraction histogram of the coating is recorded at high 2u value. In that region, the fine structure of that (310) diffraction peak is revealed, showing that there are up to four distinct cubic phases contributing to the diffraction intensity. Indeed, the lattice parameters of the cubic phase are 2.964, 2.981, 3.005 and 3.027. The positions of the corresponding (310) diffraction peak are indicated at the bottom of Fig. 3. The fact that several lattice parameter values are found for the cubic phase indicates that the composition of that phase is not homogeneous throughout the coating. Assuming a linear variation of the lattice parameter with the Ru content of the unit cell, the stoichiometry of the cubic phase can be determined. This is done in Fig. 4, where the Ru (1 / 2, 1 / 2, 1 / 2) site occupancy is plotted against the lattice parameter. According to that graph, the Ru (1 / 2, 1 / 2, 1 / 2) site occupancy is 4, 21 45 and 67% for the cubic phases shown in Fig. 3. All these results show that a reaction is taking place during the vacuum deposition process between the compounds initially present in the nanocrystalline powder. Indeed, the dramatic increase in the amount of cubic phase, along with the disappearance of the hexagonal phase of Ru and Fe 2 Ti, points to the fact that these latter phases must have reacted together to form the former phase. Most probably the characteristic layered structure of the milled powder, along with the small crystallite size reached at the end of the milling process, both contribute to favor the reaction between Ru and Fe 2 Ti. Various local inhomo-

Fig. 3. X-ray diffraction histogram of the Ti–Ru–Fe–O (2-1-1-2) coating in the high 2u value region. The vertical bars at the bottom of the graph indicate the position of the (310) diffraction peak of the cubic phases contributing to the diffraction intensity in that region.

geneities in the starting powdered material or in the deposition process might lead to the observed variation in the composition of the cubic phases. The Rietveld analysis of the plasma deposited coating also shows that there is a significant proportion of Ti 2 O 3 (16.3 wt.%) and TiO (13.2 wt.%). While TiO is already present in the starting nanocrystalline powder, Ti 2 O 3 must have been formed during the deposition process. The distribution of these oxide phases throughout the thickness of the coating, and their influence on the electrochemical activities of the coating for the hydrogen evolution reaction, is a matter of interest. A depth profile analysis of the phase distribution was performed by alternatively polishing and recording the X-ray diffraction histogram of as-deposited coating. The results are shown in Fig. 5 for a restricted range of 2u value centered on the main (110) diffraction peak of the cubic phase. The characteristic diffraction peaks of Ti 2 O 3 are clearly discernable in the as-deposited coating (curve A). Upon removing the top layer of the coating (curve B), the intensity of the Ti 2 O 3 diffraction peaks decreases by a factor of 2. This indicates that there is an excess of Ti 2 O 3 at the surface of the coating. Upon further polishing (curves C and D), the intensity of these peaks does not vary, indicating that the Ti 2 O 3 phase concentration is

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Fig. 4. Ru site occupancy (in %) on the (1 / 2, 1 / 2, 1 / 2) site of the cubic phase as a function of the lattice parameter.

constant throughout the bulk of the coating. For the longer polishing time (curves D and E), the diffraction peaks of the Ti substrate start to appear. Since the diffraction peaks of Ti 2 O 3 are observed through the entire thickness of the coating, this oxide phase must be continuously formed throughout the whole deposition process. The fact that the diffraction peaks of that phase are more prominent at the surface of the coating (curve A) must be related to the surface being in contact with the atmosphere of the deposition chamber during a long period of time.

3.3. Activated coating It was found that the electrocatalytic activity for hydrogen evolution of the as-deposited coating was quite poor. For example, the cathodic overpotential at 2250 mA cm 22 (h250 ) recorded in typical chlorate electrolysis conditions (NaClO 3 : 550 g l 21 , NaCl: 110 g l 21 , NaClO: 1 g l 21 , pH 6.5, adjusted with NaOH and HCl) is roughly 2850 to 2900 mV. These values are 250–300 mV more anodic than the typical overpotential values recorded on a cold pressed disk electrode made from the nanocrystalline Ti–Ru–Fe– O (2-1-1-2) powder used to prepare the coating. Therefore, an activation step was devised before the coating could be used as efficient cathode for hydrogen evolution. The activation step consists of the dissolution of the superficial titanium oxide layer, performed by etching in concentrated acid solution (HF 10% at 25 8C). The evolu-

Fig. 5. Depth profile X-ray diffraction histogram of as-deposited nanocrystalline Ti–Ru–Fe–O (2-1-1-2) coating: (A) as-deposited and after polishing for (B) 30, (C) 60, (D) 120 and (E) 150 s. The position of the diffraction peaks of (1) Ti 2 O 3 , (2) Ru, (3) Ti 2 RuFe and (4) Ti are indicated at the bottom.

tion of the elemental surface composition, as determined by EDX, against the etching time is shown in Fig. 6. There is a steady decrease of the Ti surface concentration, from |50 to 20 at.%. At the same time, the Ru and Fe surface concentration increases from 16 and 32 at.% to 35 and 46 at.%, respectively. This behavior is consistent with the preferential dissolution of Ti-based species. Depth profile X-ray diffraction analysis of the activated coating is displayed in Fig. 7. After etching (curve B), the intensities of the characteristic diffraction peaks of Ti 2 O 3 are considerably reduced compared to that of the asdeposited coating (curve A). This indicates that the etching procedure is efficient in removing the titanium oxide phase present at the surface of the coating, which is consistent with the results previously shown in Fig. 6. It is also interesting to note that, apart from that, the X-ray diffraction histogram of the etched coating closely resembles that of the as-deposited film. In particular, the shape and position of the (110) diffraction peak of the cubic phases found in the layer does not change, meaning that the etching procedure does not affect the other phases present in the coating. The intensity of the diffraction peaks of Ti 2 O 3 increases

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Fig. 6. Evolution of the surface elemental composition of nanocrystalline Ti–Ru–Fe–O (2-1-1-2) coating as a function of the etching time in HF 10% at 25 8C.

as the polishing operation is resumed for longer periods of time (curves C–E of Fig. 7). It indicates that dissolution of Ti 2 O 3 is not complete and has not extended throughout the bulk of the coating. This would suggest that, although an increase of the effective surface area can be expected since one of the phases of the coating has been dissolved, this effect must nevertheless be limited. It also suggests that careful optimization of the etching process could possibly lead to the dissolution of all Ti 2 O 3 , therefore leading to a very porous coating. The previous assertion is confirmed by scanning electron micrographs of the coating taken before and after etching (Fig. 8). The morphology of the as-deposited coating has an irregular surface characteristic of vacuum plasma sprayed coating. The morphology of the surface after etching is quite similar to that of the as-deposited coating. In particular, there is no drastic change in the surface porosity of the coating.

3.4. Electrochemical activity The electrocatalytic activity for the hydrogen evolution reaction of the activated coating was tested in typical

Fig. 7. Depth profile X-ray diffraction histogram of etched (HCl at 70 8C during 150 min) nanocrystalline Ti–Ru–Fe–O (2-1-1-2) coating: (A) as-deposited, (B) as-etched, and after polishing for (C) 30, (D) 60 and (E) 90 s. The position of the diffraction peaks of (1) Ti 2 O 3 , (2) Ru, (3) Ti 2 RuFe and (4) Ti are indicated at the bottom.

chlorate electrolysis conditions. Two types of measurements were realized. In the first one, the electrode was galvanostatically polarized ( j5 2250 mA cm 22 ) and the electrode potential was recorded as a function of time. These results are depicted in the insert of Fig. 9. The overpotential starts at 2550 mV and increases (gets more cathodic) to 2575 mV over the next 3 h of electrolysis. From then, h250 value stays constant. More interesting are the results of the test performed by alternatively switching the electrode from open-circuit to cathodic polarization conditions ( j5 2250 mA cm 22 ). As shown elsewhere, these operating conditions are far more drastic than a test performed under constant current density [37,38]. Indeed, this type of measurement is really an accelerated ageing test since it amplifies the degradation process that normally occurs on a much longer time scale in a continuous electrolysis test. Thus, the electrode was switched from galvanostatic polarization to open-circuit conditions every 10 min. The results of this test are shown in Fig. 9, where h250 values are shown with respect to the number of cycles. At the beginning of the test, the h250 values are close to 2500 mV, which is |350 mV less cathodic than the same value measured on the steel

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Fig. 8. Scanning electron microscopy micrographs of plasma sprayed Ti–Ru–Fe–O (2-1-1-2): (A) as-deposited and (B) after etching in HF 10% at 25 8C for 40 min.

cathodes that are commonly used in the chlorate industry. Upon cycling between hydrogen discharge and open-circuit conditions, there is a steady decrease of the h250 value

(the overpotential becomes more cathodic). After 50 cycles, the h250 value is 2600 mV, which is still 250 mV lower than the value observed on steel cathodes. As far as

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measured in the same conditions. These results demonstrate that plasma spray is a viable deposition technique for the production of large-scale electrodes of nanocrystalline materials.

Acknowledgements The Natural Sciences and Engineering Research Council ´ (NSERC) of Canada and Hydro-Quebec have financially supported this work.

References

Fig. 9. Variation of the cathodic overpotential against the number of cycles. Each cycle consists of successive hydrogen discharge (10 min at 22 j5 2250 mA cm ) and open-circuit (10 min) conditions. The electrode was rinsed with hot water after the 50th cycle. The h250 values are recorded during hydrogen discharge. The insert shows the variation of h250 against time in a continuous electrolysis test.

we can tell from the analysis of the coating following these measurements, there is no physical degradation of the coating. It is interesting to note that the electrochemical activity of the coating can be regained by simply washing it in hot water. If this is done, h250 value goes back to its original value and the slow decrease of the cathodic overpotential resumes. This effect is related to the precipitation of NaClO 3 and NaCl salts into the open structure of the coating. It is interesting to note that this effect was not observed previously with pressed powder electrode [37,38].

4. Conclusion It was shown that an adherent coating of nanocrystalline Ti–Ru–Fe–O material can be prepared by plasma spray deposition with crystallite size in the range 10–20 nm. When used as activated cathode for hydrogen evolution in typical chlorate electrolysis conditions, these coatings exhibit a h250 value close to 2550 mV. This is |250–300 mV lower than the cathodic overpotential of steel cathode

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