1196
J. Phys. Chem. C 2009, 113, 1196–1203
Hydrogen Absorption in Nanocrystalline Cubic Ti-Based Compounds M. E. Bonneau,† M. Blouin,† C. Chabanier,† L. Roue´,† R. Schulz,‡ and D. Guay*,† INRS-E´nergie, Mate´riaux et Te´le´communications, 1650 BouleVard Lionel Boulet, C. P. 1020, Varennes, Que´bec J3X 1S2, Canada, and Institut de recherche d’Hydro-Que´bec, Expertise sciences des mate´riaux, 1800 BouleVard Lionel Boulet, Varennes, Que´bec J3X 1S1, Canada ReceiVed: July 25, 2008; ReVised Manuscript ReceiVed: NoVember 7, 2008
Hydrogen absorption in a series of compounds having the same crystallographic structure but different chemical compositions was studied. To this end, high-energy ball milling was used to prepare cubic nanocrystalline Ti-based compounds of general formula Ti-Ru-M, with M ) Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. After extensive milling, the most important phase (>95 wt %) has a simple cubic structure (CsCl). Hydrogen absorption in these compounds was assessed by potential step and current efficiency measurements, as well as by in situ electrochemical X-ray diffraction experiments. In the case of potential step experiments, severe degradation of nanocrystalline Ti-Ru-V, Ti-Ru-Cr, and Ti-Ru-Ni electrodes was observed. This is because these measurements were obtained after a succession of hydrogen absorption/hydrogen desorption cycles, that emphasize the mechanism responsible for the degradation of the electrodes. This is not the case with current efficiency measurements, and an estimation of the amount of H atom absorbed was obtained for all electrode materials. It was shown that as much as 0.7 H atom per unit cell can be absorbed for M ) V. In situ electrochemical X-ray diffraction patterns were recorded while the electrodes were negatively polarized. The change of the lattice parameter, ∆a, and the change of the volume of the unit cell, ∆V, were measured as a function of M. For M belonging to the fifth period, it was shown that the nature of M does not have any marked influence on the stability of the electrode, all nanocrystalline Ti-Ru-M materials absorbing only small amount of hydrogen. This assertion does not hold true in the case of M coming from the fourth period. In that case, the amount of hydrogen absorbed is correlated to the unit cell volume of the material. Introduction Electrolytic hydrogen liberation at the cathode is one of the most frequently occurring reactions in industrial cells. In some instances, hydrogen liberation is the wanted reaction, as it is the case in water electrolysis. In some other instances, the hydrogen liberated is a byproduct, as it is the case in the chloralkali process and in chlorate production, which are among the most intensive electrochemical processes. Finding new and improved electrode materials that could lower the cathodic overpotential for hydrogen evolution is thus of considerable importance from a technological point of view to increase the energy efficiency of electrochemical processes. New and improved electrode materials face several requirements to be technologically relevant and find their way into an industrial process. These are (a) low H2 evolution overpotential, (b) stability upon short periods of current reversal, (c) chemical stability under open circuit conditions, (d) extended lifetimes (several years ideally), (e) ease of preparation, and (f) low cost. This list is not exhaustive, and several requirements will be added to meet a specific application. The first step in hydrogen evolution is hydrogen discharge giving rise to atomic hydrogen atoms being adsorbed at the surface of the electrode. Then, atomic hydrogen can be either further reduced or recombined to form molecular hydrogen. Atomic hydrogen can also migrate inside the electrode material, leading to hydrogen absorption. Hydrogen absorption in an * To whom correspondence should be addressed. E-mail address:
[email protected]. † INRS-E´nergie, Mate´riaux et Te´le´communications. ‡ Institut de recherche d’Hydro-Que´bec.
electrode will provoke an increase of the lattice parameter of the material or a modification of the structure (phase change) of the material. Thus, the amount of hydrogen absorbed and the nature of the electrode material will have a very strong impact on the long-term stability of the electrode. This is especially important in the case of an industrial application since alternating periods of operation (hydrogen evolution) and shut down (open circuit conditions) might lead to successive hydrogen absorption and hydrogen desorption cycles that could enhance and fasten the degradation of the electrode. Therefore, in the search for new and improved electrode materials, it is of the utmost importance to develop strategies to minimize hydrogen absorption while maintaining a high electrocatalytic activity for hydrogen evolution. It was shown that the use of activated cathodes made of nanocrystalline Ti-Ru-Fe-O yields to a 250-300 mV lowering of the cathodic overpotential in chlorate electrolysis conditions.1-3 This translates to a 10% improvement of the electrical efficiency of the process compared with the steel cathodes used nowadays. The nanocrystalline compound is obtained through high-energy ball milling of the starting Ti, Ru, Fe2O3, and Fe powders. The active phase is formed after a few hours of milling and is made of a cubic phase (B2 phase, CsCl). In its ordered form, the Ti atoms are located on the 1a (0,0,0) site, while Ru or Fe atoms are located on the 1b (1/2,1/ 2,1/2) site of the unit cell. As suggested by a series of electrochemical and X-ray diffraction measurements, the tendency of the Ti-Ru-Fe-O nanocrystalline alloys to absorb hydrogen depends on a delicate balance among the atomic ratios of Ti/Fe, Ti/O, and Ti/Ru.3-8 Thus, besides being the catalytic element of the Ti-Ru-Fe-O electrode,2 ruthenium plays an
10.1021/jp8066266 CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
H Absorption in Nanocrystalline Cubic Ti Compounds important role in the electrode’s long-term stability by reducing hydrogen absorption in the electrode material. All attempts to reduce further the Ru content, which is the most expensive element of the alloy, should also consider the effect of the substituting element (M) on the structure of the material (B2 phase), on the hydrogen absorption properties, and on the longterm stability of the electrode. Pure titanium has a hexagonal structure. However, an inspection of Ti-M binary phase diagrams reveals that the addition of a small amount of an extraneous element, M, to Ti changes its structure from hexagonal to cubic. This is the case for M ) Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. Thus, it was expected that the addition of M to the milled powder, with M chosen from the preceding list, would yield to the formation of a nanocrystalline compound made of a cubic phase in which all the elements of the initial powder mixture would be dissolved. Therefore, with M chosen from the preceding list, it is expected that all compounds prepared by mechanical alloying are going to have the same crystallographic structure, therefore allow the comparison of the influence of M on the hydrogen absorption properties of the electrodes. In the following, a series of Ti-Ru-M compounds were prepared by high-energy ball milling, with M ) Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. After milling, all compounds are mostly made of a single-phase (B2 structure) with a lattice parameter, a, that varies with M. The hydrogen absorption properties of these compounds were measured by three different methods involving both electrochemical methods (potential step and current efficiency measurements) and in situ electrochemical X-ray diffraction experiments that give the expansion of the lattice parameter as a result of hydrogen absorption in the compounds. Experimental Procedures Electrode Preparation. All compounds were prepared by high-energy ball milling using a SPEX-8000 mill and a steel crucible of 55 mL capacity containing two 11 mm diameter and one 14 mm diameter steel balls. Typically, 5 g of Ti, Ru, and M metallic powders were introduced into the crucible, where M ) Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. The stoichiometry of the initial mixture was fixed at 2Ti + 1Ru + 1M. In each case, pure metals (>99.9%, Alfa Aesar) were used as starting materials, and the crucible was sealed under Ar. The milling time was set to 40 h. All electrochemical measurements were realized using diskshaped electrodes (2 cm2) made by cold pressing (10 000 kg cm-2, during 10 minutes) 0.3 g of milled powder deposited on 1.0 g of a Ti powder bed. For electrochemical in situ X-ray diffraction measurements, 10 wt % of WC powder was added to the milled powder as an internal standard before making the electrode. Electrochemical Measurements. An electrical connection to the electrode was made by using a copper wire that was attached to the back of the electrode using silver-loaded epoxy. The electrode and its electrical connection were introduced into a glass tube and both the back and the side of the electrode were glued to the glass tube. All measurements were carried out in 1 M NaOH solution at room temperature. The reference electrode was a saturated calomel electrode (SCE), and a 20 cm2 nickel plate was used as counter electrode. All experiments were done in a two-compartments cell separated by an anionic Membrion membrane. A Luggin capillary was positioned close to the electrode surface to reduce the ohmic drop. Before each experiment, nitrogen was bubbled into the electrolyte to remove
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1197 dissolved oxygen. All measurements were obtained with a Solartron multistat 1480 controlled by a Corrware program. Stationary polarization curves were recorded in 1 M NaOH electrolyte on freshly prepared electrodes. The data points were collected every 1 mV, from -0.4 to -1.3 V before reversing the sweep direction, and then from -1.3 to -0.4 V. The waiting time at each potential was 10 s. Current efficiency measurements were obtained using a liquid displacement technique. In these measurements, the cathodic compartment was hermetically closed, and the hydrogen gas released during hydrogen evolution was collected in a 1 mL burette hanging upside down in a cylinder filled with water. The time needed to fill the 1 mL burette was recorded as a function of the electrolysis time. The burette was emptied between each measurement throughout the experiment. Physico-chemical characterization. Powder X-ray diffraction (XRD) measurements of the various samples were performed on a Bruker D8 Advance diffractometer equipped with a so-called Go¨bel mirror (weighted Cu KR1 and Cu KR2 at 1.5418 Å). All powder XRD measurements were done using a scintillation detector in the Bragg-Brentano (θ - 2θ) mode. Electrochemical in situ XRD measurements were realized using a specially designed electrochemical cell. The cell design and its operation are described in detail elsewhere.9 In brief, the electrochemical in situ XRD measurements were performed in the following sequence. First, the surface of the electrode was brought to the reference plane of the diffractometer by moving the electrode in the upward direction until half of the intensity of the direct beam was cut. Afterwards, an XRD pattern of the pristine electrode was recorded. Then, the piston was moved in the downward direction, the Mylar membrane was fixed to the cell, and the electrolyte was introduced into the cell. The electrode was cathodically polarized to -2.1 V vs SCE, and hydrogen gas evolution was observed. The gas was evacuated from the cell, and the electrolyte was replenished by using two external peristaltic pumps. After 2 h, the electrode was moved in the upward direction while keeping the electrode potential constant. There was a drop of the electrode current as the electrode surface came in close proximity to the Mylar window, indicating that diffusion of reacting species and products was severely hindered. The surface of the electrode was adjusted to the reference plane of the diffractometer by moving the whole electrochemical cell until the direct beam intensity was cut by half. The in situ XRD pattern of the electrode was then recorded using a 2-D detector (GADDS software) whose distance to the sample (31 cm) was optimized to achieve maximum resolution. In this mode, data can be collected over a range of 2θ values not exceeding ∼19°. Data were collected by locating the center of the detector at 2θ ) 42° (with θinc ) 21°), which is the 2θ region where the (110) most intense diffraction peak of the cubic phase occurs, and afterwards at 2θ ) 62° where the (200) peak is found. Structure refinement was performed according to the Rietveld method10 using GSAS and EXPGUI softwares.11,12 As shown elsewhere,13 powder diffraction data obtained using multilayer optics (Go¨bel mirror) are similar enough to traditional optics that a common Rietveld analysis package can model the data with reasonable accuracy. This is especially the case when the main concern is the position of the diffraction peaks to determine the change in the lattice parameter and volume of the unit cell upon exposure to hydrogen. The value of the long-range order parameter, S, is obtained from the relation S ) (xTi - FTi)/(1 FTi), with FTi ) 0.5 in the case of Ti2RuM compounds. In this
1198 J. Phys. Chem. C, Vol. 113, No. 4, 2009
Figure 1. Powder XRD patterns of 2Ti + 1Ru + 1M compounds prepared by high-energy ball milling. From A to I, M ) Rh, Pd, V, Ru, Ag, Al, Ni, Fe, and Cr. All patterns show the characteristic diffraction peaks of a cubic phase (1) and, in some cases, those of free (hexagonal) Ru (3).
Bonneau et al.
Figure 2. Powder XRD pattern of Ti-Ru-Ni (2:1:1) after 40 hours of milling. The dots are the experimental data, while the superimposed full line is the simulated pattern obtained from Rietveld analysis. Also shown at the bottom is the difference curve between the experimental data points and the simulated curve.
equation, xTi is the fraction of Ti sites occupied by Ti atoms.14 The value of xTi is given by the minimum of the curve giving the variation of wRp/Rp with xTi, and where wRp/Rp is a measure of the adequacy of the fit. Results and Discussion Structural Characterization. The powder XRD patterns of all compounds synthesized in this study are shown in Figure 1. All patterns display the characteristic diffraction peaks of a cubic structure (filled symbols), and the 2θ peak positions vary with the nature of M in Ti2RuM. Also, in some cases, the diffraction peaks of hexagonal Ru are observed (open symbols). This is most particularly the case for M ) Ag (curve E) and Ni (curve G). The structural parameters of the various phases were extracted from a Rietveld analysis of the powder XRD patterns. A typical example of a Rietveld refinement analysis is shown in Figure 2. In this figure, the dots are the experimental data and the continuous curve is the fit obtained from the analysis. The difference curve is shown at the bottom of the graph. As seen in Figure 2, the fit between the curve and the data points is excellent. All powder XRD patterns were analyzed according to the same procedure. The experimental data were fitted using only two phases (a cubic phase and Ru), except in the case of M ) Ag, where the introduction of a third phase (Ag) was mandatory to obtained a good fit. The results of the Rietveld analysis are shown in Table 1. After 40 h of milling, as Table 1 shows, most of the compounds are made almost exclusively of a cubic phase with little unalloyed Ru. This assertion holds true for M ) Rh, Ru, Pd, V, Cr, and Fe, for which the weight fraction of unalloyed Ru is zero. For M ) Al and Ni, the weight fraction of unalloyed Ru reaches 1.3 and 5.0 wt %, respectively. In the case of M ) Ag, the weight fraction of unalloyed Ru reaches 9.1 wt %, and there are also some traces of unalloyed Ag (4.4 wt %). In the case of M ) Rh, Ru, Pd, Al, Fe, and Ni, the most important phase has a simple cubic structure (CsCl). The value
Figure 3. Quasi-potentiostatic polarization curve of nanocrystalline Ti2RuPd in NaOH (1 M) at 25°C.
of the long-range order parameter, S, varies according to M, being as small as 0.6 for M ) Rh and Pd and as high as 1.0 for M ) Al. In the case of M ) V, Ag, and Cr, the predominant phase has a centered cubic structure. The lattice parameter of the cubic phases varies according to M, being as small as 3.017 Å for M ) Rh and as large as 3.170 Å for M ) Ag. In all cases, the crystallite size of the cubic phase was smaller than 50 nm. Since most compounds are made of a nanocrystalline cubic phase, they will be referred to as nanocrystalline Ti2RuM in the upcoming discussion. In the following, we will look at the hydrogen absorption properties of these compounds and assess how these properties vary with the chemical composition of the materials (M in Ti2RuM). Hydrogen Absorption Measurements. Hydrogen absorption in nanocrystalline Ti2RuM was first assessed by potential step
H Absorption in Nanocrystalline Cubic Ti Compounds
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1199
TABLE 1: Structural Parameters of 2Ti + 1Ru + 1M Compounds Milled during 40 h site occupancy M
phase
a (Å)
Rh
Ti2RuRh
3.017
100
Ru
Ti2Ru2
3.049
100
Pd
Ti2RuPd
3.056
100
V
β-Ti
3.065
100
β-Ti
3.170
Ru Ag
2.709 4.115
Al
Ti2RuAl
3.096
Ru
2.698
Cr
β-Ti
3.051
Ag
Fe
Ti2RuFe
3.021
Ni
Ti2RuNi
3.052
Ru
2.707
c (Å)
weight fraction (wt %)
Im3jm (0,0,0) P63/mmc (1/3,2/3,1/4) Fm3jm 1a(0,0,0)
85.6 4.287
9.1 4.4
Pm3m 1a (0,0,0,) 1b (1/2,1/2,1/2) P63/mmc (1/3,2/3,1/4) Im3jm 1a (0,0,0)
98.7 4.273
1.3 100
Pm3jm 1a (0,0,0,) 1b (1/2,1/2,1/2) Pm3jm 1a (0,0,0,) 1b (1/2,1/2,1/2) P63/mmc (1/3,2/3,1/4)
100
95.0 4.284
structure j Pm3m 1a (0,0,0,) 1b (1/2,1/2,1/2) Pm3jm la(0,0,0) 1b (1/2,1/2,1/2) Pm3jm 1a (0,0,0,) 1b (1/2,1/2,1/2) Im3jm 1a (0,0,0)
5.0
Ti
Ru
M
order parameter, S
wRp/Rp
0.80 0.20
0.10 0.40
0.10 0.40
0.6
1.25
0.90 0.10
0.10 0.90
0.10 0.90
0.8
1.28
0.80 0.20
0.10 0.40
0.10 0.40
0.6
1.25
0.50
0.25
0.25
0.60
0.20 1.00
1.27 1.28
1.00 1.00 0.00
0.00 0.50 1.00
0.00 0.50
1.0
0.50
0.25
0.25
0.90 0.10
0.05 0.45
0.05 0.45
0.8
0.90 0.10
0.05 0.45 1.00
0.05 0.45
0.8
1.28 1.27 1.26
1.28
TABLE 2: Electrochemical Data Eeq (V vs SCE) M
negative sweep ((0.03)
positive sweep ((0.01)
Qox (C)
Rh Ru Pd V Ag Al Cr Fe Ni
-0.78 -0.62 -0.80 -0.77 -0.98 -0.94 -1.08 -0.95 -0.79
-1.08 -1.07 -1.10 -1.07 -1.07 -1.06 -1.10 -1.08 -1.07
11.0 15.0 15.3 16.3 17.9 26.7 35.1 42.0 43.7
a
electrode stabilitya O O O b O O b O b
Qabs (C)
Qabs 1/2 (s)
a ( 0.007 (Å)
V ( 0.2 (Å3)
∆a/a (%)
∆V/V (%)
H atom per unit cell
20.0 24.5 47.2 180.1 33.9 73.4 164.2 48.5 45.0
22 66 69 674 144 412 734 165 109
3.014 3.046 3.044 3.060 3.095 3.103 3.047 3.017 3.049
27.4 28.3 28.2 28.7 29.7 29.9 28.3 27.5 28.4
0.08 0.16 0.09 1.91 0.18 0.39 1.89 0.32 1.33
0.23 0.47 0.26 5.83 0.54 1.17 5.79 0.95 4.05
0.10 0.13 0.24 0.77 0.18 0.28 0.70 0.21 0.20
O indicates stable; b indicates loss of material.
measurements. In these measurements, the oxidation charge, Qox, is obtained from the current transient recorded upon stepping the electrode potential from a value (Ei) where both hydrogen evolution and hydrogen absorption can occur to a more positive value (Ef) inducing hydrogen desorption from the electrode. By carefully selecting Ef, Qox values can be associated with the oxidation of hydrogen previously absorbed in the electrode material during the period where Ei was applied. As shown elsewhere, the interpretation of these data depends critically on the choice of Ef since oxidation of the electrode material could also occur in addition to the oxidation of absorbed hydrogen if Ef is made too positive.7 Therefore, before going into the details of the potential step experiments, it is critical to assess the electrode potential at which significant oxidation of the electrode material occurs. To this end, stationary polarization curves were recorded in 1 M NaOH electrolyte on freshly prepared Ti2RuM electrodes. A typical example of stationary polarization curves is shown in Figure 3 for Ti2RuPd. As seen in that figure, a large hysteresis exists between the current recorded in both sweep directions,
with two different equilibrium potentials. The equilibrium potential observed at -1.10 V on the positive sweep, Eeq(+), corresponds to the H2/H2O equilibrium potential. On the negative sweep, a different equilibrium potential, Eeq(-), is observed at -0.80 V, most probably associated with the reduction of the oxide/hydroxide film present at the surface of the electrode. As discussed in detail elsewhere,7 the current observed in a positive sweep between the two equilibrium potentials must be associated with the oxidation of hydrogen originating from the decomposition of the hydride previously formed on the cathodic sweep. In a potential step experiment, the oxidation charge recorded between Ec and Ea should give a measure of the amount of hydrogen absorbed in the electrode material, as long as Ea is negative with respect to Eeq(-). Similar measurements were made on freshly prepared Ti2RuM electrodes and the results are summarized in Table 2, where the Eeq(+) and Eeq(-) values are listed for the various M. In some cases, measurements were made on duplicate samples, and the uncertainties indicated in Table 2 correspond to the maximum deviation between two different measurements. All Eeq(+) values
1200 J. Phys. Chem. C, Vol. 113, No. 4, 2009
Figure 4. Potential step experiments on Ti2RuPd performed in NaOH (1 M) at 25°C: (A) current transient with Ei ) -1.3 V and Ef ) -0.96 V; (B) oxidation charge, Qox, as a function of the final potential, Ef. The Qox values are obtained by the integration of current transients such as those in panel A.
are between -1.06 and -1.10 V, with a mean value of -1.08 V, consistent with the fact that it corresponds to the equilibrium potential for the H2/H2O reaction. At the opposite end, the Eeq(-) values vary from -0.62 V for Ti2RuRu to -1.08 V for Ti2RuCr. Since this value is associated with the reduction of oxides and hydroxides present at the surface of the electrode, it is not surprising that it varies over ∼400 mV according to the composition of the electrode. An in depth study of the influence of M on the behavior of the oxide/hydroxide layer formed at the electrode surface and of the corrosion resistance properties of Ti2RuM is underway. In the context of this work, it is sufficient to realize that the current recorded on a positive sweep between the two equilibrium potentials must be associated with the oxidation of hydrogen absorbed in the electrode and not with the oxidation of the electrode material itself. Hydrogen absorption into the various electrodes was thus measured by potential step measurements, using freshly prepared electrodes in each case. All electrodes were first activated at -1.3 V during 1 h. At this electrode potential, a strong hydrogen discharge occurs. Following that initial activation step, potential step measurements were obtained by recording the current transient upon stepping the electrode potential from a fixed initial electrode potential, Ei, to a more positive final value, Ef, that was varied from -1.2 to - 0.8 V by steps of 0.02 V. In each case, Ei was applied during 10 min, while the current transient was recorded during 30 min. After that period, the anodic current is negligibly small (
