Electrochemical Hydrogen Insertion in Substoichiometric Titanium

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Electrochemical Hydrogen Insertion in Substoichiometric Titanium Carbide TiC : Influence of Carbon Vacancy Ordering 0.6

Julien Nguyen, Nicolas Glandut, Cédric Jaoul, and Pierre Lefort Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402043x • Publication Date (Web): 16 Aug 2013 Downloaded from http://pubs.acs.org on August 21, 2013

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Electrochemical Hydrogen Insertion in SubstoichiometricTitanium Carbide TiC0.6: Influence of Carbon Vacancy Ordering

Julien NGUYEN, Nicolas GLANDUT* , C´edric JAOUL, and Pierre LEFORT

SPCTS, UMR 7315, CNRS, University of Limoges, European Ceramics Center, 12 Rue Atlantis, 87068 Limoges, France

*

Corresponding author. E-mail address: [email protected] Fax: +33 5 87 50 23 07

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Abstract Substoichiometric titanium carbide of formula TiC0.6 with different degrees of carbon vacancy ordering have been synthesized first, by reactive sintering at high temperature (2100 °C), and then, by annealing at low temperature (730 °C) for different durations. The effect of annealing on the structure of the carbide and its capacity to insert electrochemically hydrogen has been investigated. The XRD study reveals two phases transitions in the carbide during annealing. First, annealing of disordered TiC0.6 (space group Fm-3m) for 40 h leads to a trigonal superstructure of space group R-3m thanks to ordering of carbon vacancies. After the longest annealing time of 120 h, the structure of the carbide becomes cubic anew, but with a space group Fd-3m. The electrochemical hydrogen insertion in these different types of TiC0.6, as studied by cyclic voltammetry, strongly depends on the crystalline structure of the carbide. The maximum storage capacity is obtained for 40 h, corresponding to the R-3m ordered phase. Because the R-3m structure consists of alternately empty and full (111) carbon atomic planes, we show that the hydrogen insertion and diffusion in the material is rendered possible by the presence of vacant (111) planes. This behaviour is very similar to cation-ordered manganese-nickel spinel vis-à-vis of lithium ion insertion. Keywords: electrochemical hydrogen storage, substoichiometric titanium carbide, annealing treatment, long-range ordered vacancies


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1 Introduction There is nowadays a growing need for the development of cheap and effective energy storage and conversion materials for high-power applications. Indeed, the scenario that relies on a widespread and massive use of devices like lithium-ion batteries and fuel cells is incompatible with technologies based on expensive, noble and rare elements. That is why more and more recent studies do not focus on original chemical compositions, but on the tailoring of structures and shapes of already existing materials. For example, first principles calculations have been performed to predict the appropriate morphology of LiCoO2 in order to enhance rate capability.1 Also, concerning LiMn1.5 Ni0.5O4 , the “3-1” ordering of manganese and nickel cations is now known to affect substantially its electronic conductivity and its electrochemical characteristics.2-4 Another example, in the field of solid oxide fuel cells, can be cited: lanthanum silicate apatite-type electrolyte are being developed with a high c-axis orientation, in order to enhance ionic conductivity thanks to oxide ion migration into channels parallel to the c-axis.5 In the present work, we wish to extend this kind of approach to the field of hydrogen storage materials, by studying the effect of annealing the titanium carbide, on its properties of electrochemical hydrogen insertion. Indeed this material is a ceramic mainly known for its refractarity (melting point ca. 3067 °C), its high hardness (HV = 28 GPa) and its metallic conductivity (σ ≈ 106 Ω-1 m-1).6 But, in a recent electrochemical study7, it has been shown also that hydrogen can be inserted in the substoichiometric carbide TiC0.6, but not in TiC0.9, this latter being closer to the stoichiometry. Although a very recent study by Ding et al.8 confirmed theoretically by first-principles calculations that hydrogen storage in substoichiometric titanium carbides is possible, the real, experimental origin of such a behaviour has not been clearly explained. It was considered to be due to the presence of long-range ordered carbon vacancies in TiC0.6 that do not exist in TiC0.9, allowing hydrogen to diffuse along vacant (111) planes of TiC0.6, which was not possible in TiC0.9. Annealing of titanium carbide is well-known to be able to induce structural changes of this material9-15. According to these different studies of TiCx, with x around 0.6, three phases exist9-15: (i) a cubic, disordered structure, with Fm-3m space group, where the carbon vacancies are randomly distributed; (ii) a trigonal, ordered structure, with R-3m space group, which is a cubic structure compressed along its [111] diagonal, and that consists in alternately empty and full (111) carbon atomic planes; and (iii) a cubic, ordered structure, with Fd-3m space group, characterized by alternation of the (111) carbon planes in which one third and two thirds of the total number of sites are empty. An important distinction of the X-ray diffraction patterns of cubic carbides from 3 ACS Paragon Plus Environment

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that of trigonal carbide, is the presence of splitting of several cubic parent lines, due to the different crystal symmetry. Such a feature makes easy to distinguish these phases from each other, by considering the only X-ray diffraction patterns, the trigonal phase being the one where peaks splittings are observable, and the two cubic phases having different lattice parameters. On the basis of this original identification mode, it seemed possible to search if thermal treatments, which change the crystalline structure of TiC0.6, could also modify its ability to insert hydrogen electrochemically. It is the aim of the present work.

2 Experimental 2.1 Powders, sintering and annealings Titanium metal powder (purity 99.95 %, grain size ≤ 2 µm, main impurity: metallic Nb 50 ppm) was purchased from Neyco, France, and titanium carbide powder (purity 99.5 %, grain size ≤ 2 µm, main impurity: Si 0.01 %) from Cerac, USA. Both were used as received, without further purification. Figure 1 show that the morphology of the grains was angular with a size of 5-10 µm for Ti powder, and a size of 3-5 µm for TiC powder, accompanied of fine particles. Powders were mixed together in ligroin (petroleum ether, VWR), in a 30 kHz-ultrasonic bath (Mécasonic, France) for 3 minutes. Solvent evaporation and drying lasted overnight, under an extractive hood. The mixtures contained 34.8 wt% of titanium and 65.2 wt% of carbide, in order to form the substoichiometric carbide following: 0.6 TiC + 0.4 Ti → TiC0.6

(1)

The powder mixtures were cold-pressed into pellets (weight ca. 1.2 g, diameter = 10 mm) under a pressure of 1900 kg cm−2 during 30 s and sintered by reactive sintering in a pure argon atmosphere to prevent oxidation (Alphagaz 1, Air Liquide, France), and using a Nabertherm furnace (model VHT 08/22 GR, Germany) equipped with graphite resistor. The temperature program used for this synthesis is given in Figure 2a. It consisted in a first ramp from ambient to 1600 °C, followed by a first hold at 1600 °C, of duration 2 h. This first hold was necessary to avoid the melting of unreacted Ti metal (Ti melting point = 1668 °C). Then, a second ramp was programmed in order to homogenise the carbide with a hold at 2100 °C, which lasted 3.5 h. Then, the samples were cooled to ambient. All the ramp rates, for heat-up and for cool-down, were set at 10 °C/min. The annealing conditions are reported in Figure 2b. It consisted in several successive ACS Paragon Plus Environment

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cycles, each one being composed of a 10h-hold at 730 °C under vacuum (ca. 0.5 Pa), preceded and followed by heating and cooling at 10 °C/min. These annealings have been performed in the same furnace than above. A cumulative annealing time, τ, has been defined in a simple way: after the first cycle, τ equals 10 h, after the second one, τ = 20 h, and so on.

2.2 Characterization Structural characterization has been performed using a D8 Brüker X-ray diffractometer, with filtered Cu Kα1 radiation, equipped with a Ge (111) incident beam monochromator, within the 2θ = 15 – 110° range, with steps of 0.0150, with an exposure time of 1 s, and the chamber was maintained thermostatically at a steady temperature of 20 °C. Peakoc software16 has been employed for the fitting of XRD patterns. Morphology of the samples was observed thanks to an optical microscope (Nikon Eclipse LV100) and to a scanning electron microscope, SEM (Philips XL30, Netherlands), with in-situ Energy Dispersive Spectroscopy, EDS, (Oxford InstrumentsINCA, UK). Density of the sintered samples was determined using the helium pycnometry (Micromeritics AccuPyc II 1340). Figure 3a presents a micrograph of fracture of the sintered pieces, which shows a good homogeneity of the material, with an intergranular porosity, estimated at ca. 5 % by pycnometry and a grain size about 50 µm. The corresponding X-ray pattern given in Figure 3b confirms that the carbide was monophased, and its peaks were indexed with the ICDD-PDF cards No. 00-321383 of the cubic phase TiC (Fm-3m, NaCl type). (200), (220), (222) and (400) preferred orientations are also observed. The lattice parameter a was calculated to be 0.43163 ± 1×10-5 nm, which corresponds to the formula TiC0.60517, i.e. very close to the aimed substoichiometry of TiC0.6. EDS analysis performed on the surface is reported on Figure 3c. It confirms the absence of oxygen, whose peak at 0.5 keV cannot be seen, and not any other element than carbon and titanium was detected.

2.3 Electrochemical apparatus Electrochemical measurements were carried out at room temperature, i.e., 25 °C, in a standard three-electrode cell. An Autolab PGSTAT30 potentiostat was used, controlled by GPES and FRA 4.9 softwares (EcoChemie, Netherlands). The solution was a 1 M H2SO4 aqueous solution (Aldrich and Millipore MilliQ+ water). It was deaerated by bubbling pure argon gas (Alphagaz 1, Air Liquide, France), prior to measurements, to remove dissolved oxygen, and argon gas was continuously blown on the solution surface during the experiments. The reference ACS Paragon Plus Environment

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electrode was a K2SO4 saturated Hg/Hg2SO4 electrode. Potentials were recalculated vs. SHE (standard hydrogen electrode, i.e., + 0.65 V were added). The counter-electrode was a platinum disk (10 mm in diameter) and the working electrode, WE, consisted in the TiC0.6 pellets in a Teflon holder, with an effective surface area of 0.126 cm2. The WE was mounted on a rotating disk electrode system EDI 101 (Radiometer-Analytical, France). After each cycle of 10 hours of annealing, samples are carefully repolished with diamond suspension of grain size 1 µm. Doing this, several tens of microns are removed. Hence, the surface that is probed electrochemically is characteristic of bulk TiCx .

3 Results 3.1

Influence of annealing on the carbide microstructure Figure 4a presents the carbides microstructure observed after polishing and immersion in

aqua regia during 5 min, in order to reveal the grains boundaries. The microstructure appeared as rather homogeneous, with polygonal grains and a mean grain size of 41 µm, as determined by using Matrox Inspector software. After 120 hours of annealing at 730 °C, the microstructure, revealed in the same manner, exhibited no change, as it can be seen in Figure 4b. This result is not surprising insofar as the annealing temperature is noticeably lower than Tamman’s temperature of the carbide, which could be evaluated at least around 1200 °C: no appreciable grain boundaries moving can be observed at this temperature.18 It has been also verified that the microstructure remained unchanged for the others annealing duration.

3.2 Changes of lattice in the carbide during annealing For each annealing time, XRD patterns were measured. They showed widening of peaks (111) and (220), and splitting of the peaks (331) and (420), which evidences changes inside the carbide lattice during annealing. This is illustrated on the XRD patterns of Figure 5, which focuses around the (331) reflection (i.e., 2θ ≈ 102°), for different cumulative annealing times τ. The experimental data (dots) and the fits (lines) are given on the left. On the right one can see the fitted subpeaks, as determined by using the Peakoc software. On Figure 5 (left), are also given ACS Paragon Plus Environment

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below each pattern, the residual plots, which show that the fits are of very good quality. Peak (331) widens for τ = 10 h, then splits for the annealing durations between 20 and 90 h. For these last annealing times, the XRD patterns identified the trigonal variety (R-3m) of the carbide corresponding to the ICDD-PDF cards No. 01-070-9259 of TiC0.5, on the basis of Tashmetov et al. experiments and calculations.19 At τ = 120 h, the splitting of the (331) reflection vanished and the structure becomes cubic anew.

3.3 Electrochemical results Figure 6 presents the cyclic voltammograms, CVs, of TiC0.6 for two different annealing times, τ = 0 h and 40 h. It was recorded for a potential scan rate v = 10 mV s-1, in the range -0.55 to 0.2 V/SHE. Before annealing (curve i), two typical regions can be described. First, a ca. zero current region between -0.4 and 0.2 V/SHE, which is the double layer zone, showing no faradaic reactivity. The second one, below ca. -0.55 V, is the region of the hydrogen evolution reaction, HER, that is: 2 H+ + 2e- → H2

(2)

After an annealing time of 40 h, curve ii of Figure 6 shows that the HER starts at higher potential (~ -0.35 V/SHE). But, above all, the CV presents a totally different shape in the region around 0 V/SHE. These two new waves in reduction and oxidation are characteristic of hydrogen absorption and extraction20-25, according to the following one-step, quasi-reversible redox reaction: H+ + where

+ e- ↔ H

(3)

is a carbon vacancy of the carbide lattice located at the surface of the electrode, and H an

hydrogen atom inserted inside this vacancy. Notice that it is well known that the formation of vacancies is only possible in the carbon sublattice, i.e., no titanium vacancies exist in this material.6 The anodic and cathodic peak current densities, noted jpa and jpc on Figure 6 reach similar absolute values of 0.40 and 0.42 mA cm-2, respectively, indicating that hydrogen insertion is close to be reversible from the chemical point of view, i.e., the same quantity of hydrogen enters and leaves the carbide at each electrochemical cycle. In other words, the CVs obtained at different annealing times τ, show that the area under each wave, which is roughly proportional to the quantity of absorbed and extracted hydrogen, changed drastically according to τ, the maximum observed being given in Figure 6 (τ = 40 h). A decrease of jpa and jpc is observed for longer times. In order to quantify this effect, the anodic and cathodic current densities, jpa and jpc have been measured for different annealing times, and the results are provided in Figure 7. For short annealing times, i.e., between τ = 0 and 40 h, the peak current density in reduction jpc (hydrogen insertion) decreases with τ while the peak current density in oxidation jpa (hydrogen extraction) ACS Paragon Plus Environment

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increases symmetrically. On the contrary, from τ = 50 h to 120 h, jpa decreases (jpc increases). Lastly, the influence of the potential scan rate, v, has been tested for 5 different values of 0.5, 1, 2.5, 5 and 10 mV s-1 and for a sample annealed during 40 h. The corresponding CVs are displayed on Figure 8a, and the values of jpa and jpc versus the square root of v are plotted on Figure 8b. This latter shows a linear dependence of the peak currents with √v, for low v’s.

4 Discussion The whole results establish two clear relations, the first one between the annealing and the crystalline structure of the carbide TiC0.60, and the second one between the annealing of the samples and their ability to insert hydrogen. As far as annealing did not change any characteristics of the carbide except crystallographic (in particular its microstructure remains always the same), it is clear that insertion of hydrogen has to be related to the different mode of crystallization of the carbide.

4.1 Hydrogen insertion and crystalline structure of TiC0.60 Some previous studies have established that the substoichiometric titanium carbide may present several modes of crystallization according to the conditions of its synthesis, composition and annealing9-15, and the transitions between them appeared as rather complex. The composition chosen here, and way of synthesis followed in the present work, without annealing, is known for giving an f.c.c. structure of NaCl type (space group Fm-3m), with vacancies in the carbon sublattice distributed randomly, which was actually found here. Now, annealing under the conditions of the present work (730 °C, long durations), have never been extensively studied. Nevertheless, the transition cubic Fm-3m → trigonal R-3m obtained by annealing has already be mentioned several times, the trigonal phase being only transitory and metastable, the final stable structure being a second cubic phase type f.c.c. with a space group Fd-3m.26 This last phase and the trigonal one differ from the original one by a long-range ordering of the vacancies in the carbon sublattice. The progressive transition cubic Fm-3m → trigonal R-3m has clearly been found here, and it can be easily identified by following the splitting of the peaks (331) and (420) as a function of the annealing time. It can be assumed that the splitting reached its maximum when the cubic phase Fm-3m was quasi-completely converted into the trigonal phase. Under these conditions, Figure 9 presents the difference of d-spacings between peaks (0 2 10) and (0 1 14), ∆d, formed by the splitting of peak (331), as a function of the annealing duration τ. It appears that the maximum in ∆d, i.e., the maximum content of the trigonal phase R-3m in the carbide TiC0.60, was reached for an annealing time of ca. 40h, which is precisely the annealing time corresponding to the easiest ACS Paragon Plus Environment

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storage of hydrogen, as shown in Figure 6. From this observation, it was inferred that the hydrogen insertion was related to the presence of the ordered trigonal phase of the titanium carbide.

Concerning the final stable f.c.c. phase obtained after the longest annealing time (120h), it can be admitted that it crystallized with the Fd-3m space group. On the basis of the simple XRD study presented above, one may consider that this is difficult to establish certainly. Indeed, the basic lattice parameters of Fm-3m and Fd-3m structures are reputed to be very close, almost indistinguishable (see ICDD-PDF cards No. 00-032-1383 and 04-007-1462). Nevertheless, the accuracy of XRD allows to differentiate clearly the lattice parameter of the non-annealed carbide (τ = 0 h), which is 0.43163 ± 1 × 10 –5

(a = 0.43181 ± 1 × 10

–5

nm, from that of the carbide annealed after τ = 120 h

nm). This latter value, slightly but significantly greater than the former, 12,15

allows to consider, in accordance with previous works

, that the samples annealed during 120h

crystallized with the Fd-3m space group, with a superstructure double compared to the Fm-3m basic cubic structure. Moreover, the fact that the 120-h annealed samples were able to retain hydrogen, while those before annealing were not, is consistent with the difference between the two structures: the non-ordered one (initial Fm-3m) gives no way for hydrogen penetration, while the final Fd-3m does (with its carbon planes partially empty). Lastly, about the mechanism of the transition from trigonal R-3m to cubic Fd-3m after annealing for 120h, it is probably due to the 27

migration of the carbon vacancies, because of their segregation to the surface .

Table 1 summarizes the lattice parameters, as calculated for τ = 0, 40 and 120 h, and for the different structures observed. Now it remains to explain why the carbide of trigonal R-3m structure inserts hydrogen by far better than the other crystallized variety. The trigonal carbide directly derives from the cubic (Fm3m) via a gathering of the carbon vacancies in one plane (111) while the next plane of the same family is entirely filled by the carbon atoms.10,12,14,15 Hence, the (111) empty planes may constitute excellent paths for the atomic diffusion of hydrogen, allowing these atoms to penetrate easily deep inside the carbide structure. This provides a consistent explanation for the particular behaviour of the trigonal titanium carbide, the other crystalline varieties of titanium carbide that do not possess this characteristic being not able to insert hydrogen as well. Lastly, it can be noticed that the model implying that one (111) plane over two is empty suppose the mean composition TiC0.5 for the carbide. For higher carbon contents, e.g. here, with TiC0,6, it must be admitted that the empty ACS Paragon Plus Environment

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planes (111) contain yet some carbon atoms.

4.2 Mechanism of the electrochemical insertion of hydrogen Several well-known steps can limit kinetically the transport of hydrogen: diffusion in the solution near the electrode surface, surface phenomena, or diffusion inside the solid. The first step can be detected classically by the test of the rotating electrode. Figure 10 displays the CVs of ordered, R-3m TiC0.6 (τ = 40 h), obtained at a scan rate of 5 mV s-1. The dots are for a quiescent solution, i.e., an immobile electrode, and the line is for a RDE rotating at 2000 rpm. No difference can be found between these two shapes of CV. It indicates that there is no mass transport limitation in solution, and that the H+ concentration at the electrode surface is constant during the whole CV, and equal to the bulk solution concentration. On this basis, and assuming that hydrogen fills entirely the empty carbon sites of TiC0.6, it is possible to determine the maximum hydrogen concentration, c0, which is also equal to the vacancies concentration before reaction. In order to simplify this calculation, the carbide TiC0.60 is considered as cubic, with a lattice parameter a = 0.4311 nm, each lattice containing 4 Ti, 4 × 0.6 = 2.4 C and 4 × 0.4 = 1.6 vacancy. Under these conditions, the maximum content of hydrogen is: c0 = 1.6 / (Na × a3) = 3.32 × 104 mol m-3

(4)

Na representing Avogadro’s constant. Now, it is possible to verify that the diffusion of hydrogen in the carbide is the rate-limiting step, by considering that the peak current densities jpa and jpc are related to the square root of scan rate √v, as indicated by the Randles-Sevcik equation, written under semi-infinite linear diffusion conditions: jpa or c = ± 0.4463 F c0 √(v D F/RT)

(5)

In this equation, D is the hydrogen diffusion coefficient in the carbide, F is the Faraday constant, R the molar gas constant, and T the temperature. At 298 K, F/RT = 38.92 V-1. It is worth noticing that Equation 5 is only valid for fully reversible (Nernstian) and for diluted systems. Here, Figure 8a seems to indicate that the reaction is quasi-reversible, with a difference between peak potentials, ∆Ep, close to 110 mV. Nevertheless, for a first estimation of D, Equation 5 can be considered as sufficient, especially when one knows that the CVs have not been corrected from background current and from uncompensated resistance, Ru.28


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The peak current densities jpa and jpc considered here were directly taken from the CVs of Figure 8a, for samples annealed during 40 h. Their representation vs. the square root of v, is reported on Figure 8b. At low scan rates (v < 5 mV s-1) a linear dependence can be observed,

indicating a diffusion-controlled process.28 As far as the protons transport in aqueous solution has been proven to be not limitative, therefore this limiting diffusion step is the one of hydrogen inside the solid TiC0.6. The drift observed at the highest scan rates can be explained by two reasons. First, when v is rapid only a few amount of hydrogen is inserted into the material, making its diffusion easier, and logically the diffusion coefficient appears higher. Secondly, the background current and the uncompensated resistance distort the voltammetric signal, which may induce a non linear shape for the jp vs. √v representation. 28 The slopes of linear parts of the curves of Figure 11 allow to determine the value of the diffusion coefficient: D ≈ 1.2 × 10-13 cm2 s-1

(6)

This value is close to 4.2 × 10-13 cm2 s-1, which is the value of diffusion coefficient found for hydrogen electrochemical insertion into titanium monoxide, TiO, a ceramic material similar to titanium carbide.29

5 Conclusion Substoichiometric carbide TiC0.6 is able to insert electrochemically hydrogen, but only for certain of its crystallographic varieties, the best being the trigonal structure. Thanks to a systematic study of the effect of annealing on the crystalline structure of TiC0.6, this work offered the opportunity to confirm that the trigonal variety of this carbide can be regarded as a metastable and transitory phase in the transition cubic Fm-3m → cubic Fd-3m at 730 °C, easy to obtain almost pure after a 40-h annealing. The penetration of hydrogen inside the carbide is highly facilitated by the presence of empty (111) carbon planes, in the trigonal carbide TiC0.6. Nevertheless, the limiting step of hydrogen insertion is the diffusion inside the solid, with a diffusion coefficient by far lower than that of other materials candidates for hydrogen storage20-25, even for the trigonal variety of this carbide. However, this work constitutes an excellent example of the effect of tailoring the structure of a material, inducing major changes in its behaviour.

Acknowledgments The authors gratefully acknowledge the Région Limousin for J.N.’s Ph.D. scholarship.


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References (1) Kramer, D.; Ceder, G. Tailoring the Morphology of LiCoO2: A first Principles Study. Chem. Mater. 2009, 21, 3799-3809. (2) Kunduraci, M.; Al-Sharab, J. F.; Amatucci, G.G. High-Power Nanostructured LiMn2-xNixO4 High-Voltage Lithium-Ion Battery Electrode Materials: Electrochemical Impact of Electronic Conductivity and Morphology. Chem. Mater. 2006, 18, 3585-3592. (3) Lee, E.-S.; Nam, K.-W.; Hu, E.; Manthiram, A. Influence of Cation Ordering and Lattice Distortion on the Charge-Discharge Behavior of LiMn1.5Ni0.5O4 Spinel between 5.0 and 2.0 V. Chem. Mater. 2012, 24, 3610-3620. (4) Cabana, J.; Casas-Cabanas, M.; Omenya, F.O.; Chernova, N. A.; Zeng, D.; Whittingham, M.S.; Grey, C. P. Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi0.5Mn1.5O4. Chem. Mater. 2012, 24, 2952-2964. (5) Fukuda, K.; Asaka, T.; Hamaguchi, R.; Suzuki, T.; Oka, H.; Berghout, A.; Béchade, E.; Masson, O.; Julien, I.; Champion, E.; Thomas, P. Oxide-Ion Conductivity of Highly c-AxisOriented Apatite-Type Lanthanum Silicate Polycrystal Formed by Reactive Diffusion between La2SiO5 and La2Si2O7. Chem. Mater. 2011, 23, 5474-5483. (6) Pierson, H.O. Handbook of Refractory Carbides and Nitrides; Noyes Publications: Westwood, NJ, 1996. (7) Gringoz, A.; Glandut, N.; Valette, S. Electrochemical hydrogen storage in TiC0.6, not in TiC0.9. Electrochem. Commun. 2009, 11, 2044-2047. (8) Ding, H.; Fan, X.; Li, C.; Liu, X.; Jiang, D.; Wang, C. First-principles study of hydrogen storage in non-stoichiometric TiCx. J. Alloys Compd 2013, 551, 67-71. (9) Goretzki, H. Neutron Diffraction Studies on Titanium-Carbon and Zirconium-Carbon Alloys. Phys.Stat.Sol 1967, 20, K141-K143.


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(10) Parthe, E.; Yvon, K. On the Crystal Chemistry of the Close Packed Transition Metal Carbides. II. A Proposal for the Notation of the Different Crystal Structures. Acta Cryst. 1970, B26, 153-163. (11) Billigham, J.; Bell, P.S.; Lewis, M.H. Vacancy Short-Range Order in Substoiehiometric Transition Metal Carbides and Nitrides with the NaCI Structure. I. Electron Diffraction Studies of Short-Range Ordered Compounds. Acta Crystallogr. 1972, A28, 602-606. (12) De Novion, C. H.; Landesman, J. P. Order and disorder in transition metal carbides and nitrides: experimental and theoretical aspects. Pure Appl. Chem. 1985, 57, 1391-1402. (13) Lorenzelli, N.; Caudron, R.; Landesman, J.P.; De Novion, C.H. Influence of the ordering of carbon vacancies on the electronic properties of TiC0.625. Solid State Comm. 1986, 59, 765-769. (14) Gusev, A.I.; Rempel, A.A.; Magerl, A.J. Disorder and Order in Strongly Non-stoichiometric Compounds, Transition Metal Carbides, Nitrides and Oxides; Springer-Verlag: Berlin, 2001. (15) Gusev, A. I. Phase equilibria, phases and chemical compounds in the Ti-C system. Russ. Chem. Rev. 2002, 71, 439-463. (16) Masson, O. Peakoc profile fitting program, Version 1.0, 2008. (17) Storms, E.K. The Refractory Carbides; Academic Press: New York, 1967. (18) Flaherty, D.W.; May, R.A.; Berglund, S.P.; Stevenson, K.J.; Mullins, C.B. Low Temperature Synthesis and Characterization of Nanocrystalline Titanium Carbide with Tunable Porous Architectures. Chem. Mater. 2010, 22, 319-329. (19) Tashmetov, M.Yu.; Em, V.T.; Lee, C.H.; Shim, H.S.; Choi, Y.N.; Lee, J.S. Neutron diffraction study of the ordered structures of nonstoichiometric titanium carbide. Physica B 2002, 311, 318-325. (20) Lukaszewski, M.; Kusmierczyk, K.; Kotowski, J.;Siwek, H.; Czerwinski, A. Electrosorption of hydrogen into palladium-gold alloys. J. Solid State Electrochem 2003, 7, 6976.


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(21) Adams, B.D.; Ostrom, D.A.; Chen, A. Hydrogen Electrosorption into Pd-Cd Nanostructures. Langmuir 2010, 26, 7632-7637. (22) Khaldi, C.; Mathlouthi, H.; Lamloumi, J.; Percheron-Guégan, A. Electrochemical study of cobalt-free AB5-type hydrogen storage alloys. Int. J. Hydrogen Energy 2004, 29, 307-311. (23) Birry, L; Lasia, A. Effect of crystal violet on the kinetics of H sorption into Pd. Electrochim. Acta 2006, 51, 3356-3364. (24)

Raju,

M.;

Ananth,

M.V.;

Vijayaraghavan,

L.

Electrochemical

properties

of

MmNi3.03Si0.85Co0.60Mn0.31Al0.08 hydrogen storage alloys in alkaline electrolytes-A cyclic voltammetric study at different temperatures. Electrochim. Acta 2009, 54, 1368-1374. (25) Yuan, X.; Xu, N. Determination of hydrogen diffusion coefficient in metal hydride electrode by cyclic voltammetry. J. Alloys Compd 2001, 316, 113-117. (26) Em, V.T.; Tashmetov, M.Yu. The Structure of the Ordered Phase in Rocksalt Type Titanium Carbide, Carbidenitride, and Carbidehydride. Phys. Stat. Sol 1996, 198, 571-575. (27) Ding, H.; Wang, J.; Li, C.; Nie, J.; Liu, X. Study of the surface segregation of carbon vacancies in TiCx, Solid State Commun. 2012, 152, (2012) 185-188. (28) Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (29) Vondrak, J. Electrochemical absorption of hydrogen in transition metal oxides—II. Sorption of hydrogen in titanium monoxide TiO. Electrochim. Acta 1987, 32, 163-170.


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Table and Figure Captions: Table 1: Lattice parameters determined from X-ray diffraction, calculated using TOPAS software (version 3, Bruker Corporation, 2005) for the three cumulative annealing times 0, 40 and 120 h. For 12

–5

R-3m, hexagonal axes are used . Uncertainty: ± 1 × 10

–5

nm; except for c: ± 3 × 10

nm.

Figure 1: SEM images of the starting powders: (a) Ti, and (b) TiC. Figure 2: (a) Temperature program used for the solid-state reactive sintering of disordered, Fm-3m substoichiometric titanium carbide, TiC0.6 (b) Cyclic temperature program used for the

annealing

and ordering of TiC0.6 at 730 °C. The cumulative annealing time, τ, is defined as the number of cycles × the duration of the hold (i.e., 10 h). Figure 3: (a) SEM image of a fractured cross section of TiC0.6 (in secondary electron detection mode) (b) X-ray diffraction pattern of disordered, Fm-3m TiC0.6 (c) EDS analysis. (a) and (c) are characteristic of all TiC0.6 samples, ordered or not. Figure 4: Microstructures of (a) disordered TiC0.6 and (b) annealed for 120h. Figure 5: (Left) XRD patterns of the (331) fcc reflexion of TiC0.6, for different cumulative annealing times, τ. Dots: experimental data. Lines: Best-fit results. Lines below: difference curve between data and fit. (Right) Subpeaks for each fit on the left. Figure 6: Cyclic voltammograms, CVs, of TiC0.6 for (curve i) as-quenched sample, i.e., disordered Fm-3m structure, and (curve ii) after τ = 40 h of annealing at 730 °C. Scan rate: 10 mV s-1. Figure 7: Peak current densities in oxidation and reduction, jpa and jpc, for hydrogen insertion and extraction vs τ, the annealing time. The error bar, corresponding to a 95 % confidence interval, given for τ = 40 h, is valid for all samples. Figure 8: (a) CVs of TiC0.6 annealed at τ = 40 h, for several potential scan rates, v: (i) 0.5, (ii) 1, (iii) 2.5, (iv) 5, and (v) 10 mV s-1 (b) Randles-Sevcik plots, i.e., jpa and jpc vs. √v, and linear fits at low scan rates.


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Figure 9: Difference of d-spacings between (0 2 10) and (0 1 14) peaks, from the (331) fcc reflexion splitting, vs τ, the annealing time. Figure 10: CVs of TiC0.6 annealed at τ = 40 h, for (dots) a fixed electrode and (line) a RDE rotating at 2000 rpm. Scan rate: 5 mV s-1.


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τ/h

Space groups

Lattice parameters / nm

0

Fm-3m

a = 0.43163

40

R-3m

a = 0.30566 c = 1.49369

120

Fd-3m

a = 0.43181

Table 1


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(a)

(b)

Figure 1


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(a)

(b)

Figure 2


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(a)

(b)

(c)

Figure 3 ACS Paragon Plus Environment

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(a)

(b)

Figure 4


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Figure 5


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Figure 6


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Figure 7


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(b)

Figure 8


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Figure 9


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Figure 10


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Table of Contents Graphic:


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Electrochemical Hydrogen Insertion in Substoichiometric Titanium

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