First-principles study of hydrogen storage in non-stoichiometric TiCx

doi:10.1016/j.jallcom.2012.10.067

Journal of Alloys and Compounds

Volume 551, 25 February 2013, Pages 67-71

https://doi.org/10.1016/j.jallcom.2012.10.067 Get rights and content

Abstract

In this work, the first principles calculation has been performed to study the hydrogen storage in non-stoichiometric TiC_x. It is found that hydrogen absorption in stoichiometric TiC is energetically unfavorable, while it is favorable in non-stoichiometric TiC_x. This indicates that the existence of carbon vacancies is essential for hydrogenation storage in TiC_x. At the same time, multiple hydrogen occupancy of the vacancy has been confirmed and it is calculated that as many as four hydrogen atoms can be trapped by a carbon vacancy. These absorbed hydrogen atoms tend to uniformly distribute around the vacancy. However, it is also found that the diffusion of hydrogen atoms in TiC_x is difficult, especially in TiC_x with high x.

Highlights

► The absorption of hydrogen in non-stoichiometric TiC_x is thermally favorable. ► As many as four hydrogen atoms can be trapped by a carbon vacancy. ► The diffusion of hydrogen in TiC_x is difficult, especially in TiC_x with high x.

Introduction

Hydrogen storage materials have been widely studied due to the needs of hydrogen economy in finding new renewable sources of energy [1], [2], [3], [4]. In many cases, transition metals play a key role in the storage of hydrogen. Among them, Ti is well known for absorbing large quantities of hydrogen, which leads to the formation of titanium dihydride, TiH₂ [5], [6]. In addition, its carbide, TiC, is also studied for the hydrogen storage. But it is usually used as the auxiliary material to improve hydrogenation properties of others, such as MgH₂ [7], [8] and LiAlH₄ [9]. And few works have been considered TiC itself to be the potential hydrogen storage material. Recently, Gringoz et al. [10] has found that although hydrogen can not be absorbed in stoichiometric TiC or non-stoichiometric TiC_x with a high x, such as TiC_0.9, it can be electrochemically loaded, at room temperature, in the TiC_x with a small x, such as TiC_0.6, and a capacity of 2.9 wt.% can be easily reached, corresponding to the formula TiC_0.6H_1.6. And comparing with TiH₂, it is considered that TiC_xH_y is more reliable and stable. This indicates that TiC_x may be a promising hydrogen storage material.

It is known that TiC is experimentally never found to be fully stoichiometric, but contains a lot of carbon vacancies while keeping the same crystallographic structure. The concentration of vacancies in metals is usually less than one percent even at pre-melting temperatures. In contrast, up to one-half of carbon lattice sites may be vacant in TiC_x [11], [12], [13]. And it is considered that the hydrogen storage of TiC_x is closely related to these carbon vacancies. The existence of carbon vacancies in TiC_x can not only influence the absorption of hydrogen atoms, but also influence their diffusion in the crystal lattice. But, for the time being, there have been no investigation about the relationship between the carbon vacancies in TiC_x and its hydrogenation properties.

Therefore, in this work, the first-principles calculation was performed to study hydrogen storage in non-stoichiometric TiC_x. The vacancy–hydrogen interactions as well as multiple hydrogen occupancy of the vacancy were investigated. The relatively stable occupied sites for hydrogen atoms were identified. Furthermore, the diffusion of hydrogen in TiC_x was also calculated. Our calculations are expected to reveal the mechanism of hydrogen absorption in TiC_x and provide some explanations for the experimental results related to its hydrogen storage behavior.

Section snippets

Method of calculations

The calculations are based on the density-functional theory and performed with the program package CASTEP in Materials Studio of Accelrys Inc. The local density approximation (LDA) was utilized for structure optimization and energy calculation [14]. The supercell containing 32 lattice sites are used in the calculation. The plane-wave cut off energy of 350 eV was employed, which assured a tolerances of energy and displacement convergence of 5.0 × 10⁻⁶ eV/atom and 5.0 × 10⁻⁴ Å, respectively. The grids

The absorption of hydrogen in TiC and TiC_x

It is known that TiC is a typical faceted crystal with a NaCl type-structure and carbon atoms are located in the octahedron interstitial sites. Therefore, if hydrogen atoms can be absorbed in stoichiometric TiC, it is considered that the most possible occupied sites should be tetrahedral sites formed by Ti atoms, as shown in Fig. 1a.

The formation energy of a single hydrogen atom in the tetrahedral site has been calculated by Eq. (1) and it is about +0.97 eV. This result demonstrates that the

Conclusions

In conclusion, the first principles calculations have been performed to study the hydrogen behaviors in TiC_x. It is found that the absorption of hydrogen atoms in stoichiometric TiC is energetically unfavorable, while it is favorable in non-stoichiometric TiC_x. This indicates that the existence of carbon vacancies is essential for hydrogenation storage in TiC_x. At the same time, the calculated results on the interaction between hydrogen and vacancy confirm that the multiple hydrogen occupancy

Acknowledgments

This work was supported by a grant from National Nature Science Fund of China (No. 51071097) and a grant from the Fundamental Research Funds for the Central Universities (No. 11QG67).

References (20)

G.Y. Koh et al.
Int. J. Hydrogen Energy
(2012)
J. Andrews et al.
Int. J. Hydrogen Energy
(2012)
J. Cermak et al.
J. Power Sources
(2012)
T. Liu et al.
J. Power Sources
(2011)
V.I. Trefilov et al.
Int. J. Hydrogen Energy
(1996)
H. Shao et al.
Int. J. Hydrogen Energy
(2011)
M.Q. Fan et al.
Energy
(2010)
A. Gringoz et al.
Electrochem. Commun.
(2009)
W.S. Williams
Mater. Sci. Eng. A
(1988)
M.W. Barsoum
Prog. Solid State Chem.
(2000)

There are more references available in the full text version of this article.

Cited by (48)

Hydrogen trapping in mixed carbonitrides
2024, Acta Materialia
Transition metal carbides with NaCl-structure have long been in focus for the design of hydrogen embrittlement (HE) resistant high strength steels. While previous studies extensively investigated the pure transition metal precipitates, modern steels often contain multiple carbide forming elements at once, leading to the formation of mixed carbonitride precipitates. Since only little is known about the influence of precipitate chemistry on hydrogen trapping, we performed systematic density functional theory (DFT) calculations of disordered structures in order to investigate hydrogen trapping within the ${(Ti}_{x} V_{1-x} {)(C}_{y} N_{1-y})$ -precipitate system. Addressing possible trapping positions at bulk and interface non-metal vacancies, the coherent interface between ${(Ti}_{x} V_{1-x} {)(C}_{y} N_{1-y})$ and bcc iron, the strained iron lattice and misfit dislocations, we found that carbon vacancies in TiC, both at the interface and in the bulk, are the deepest hydrogen traps with segregation energies exceeding −1.1 eV. For all investigated positions, a dependence of the hydrogen trapping capability on the precipitate composition has been found, with non-metal vacancies being strongly affected, whereas misfit dislocations and the coherent interface exhibit comparably smaller changes to their hydrogen trapping behavior. With few exceptions, TiC offers the strongest hydrogen traps, while alloying with V or N decreases the hydrogen trapping capability of ${(Ti}_{x} V_{1-x} {)(C}_{y} N_{1-y})$ -precipitates. This study provides a detailed investigation of a wide range of hydrogen trapping sites related to mixed carbonitride precipitates within the ${(Ti}_{x} V_{1-x} {)(C}_{y} N_{1-y})$ -system. Our results extend the existing knowledge on hydrogen trapping at pure stoichiometric carbides and nitrides towards alloyed precipitates that help to improve current steel design strategies focusing on the hydrogen trap design.
First - Principles study of hydrogen - Carbide interaction in bcc Fe
2024, International Journal of Hydrogen Energy
Rapid developments in the field of hydrogen energy have prompted the need for safe and efficient hydrogen transportation and storage. Steels form the backbone of the current energy infrastructure and thus offer a fast and cost-effective solution. Their excellent mechanical properties are attributed to the underlying microstructure which comprises of finely dispersed nano-precipitates. However, one major factor restricting their application is their susceptibility to Hydrogen Embrittlement (HE). In the past decade, experimental and theoretical works have been carried out to understand if the nano-sized carbides can aid in reducing the susceptibility to HE along with providing strengthening. Within this ab-inito study, we investigated the effectiveness of fully coherent nano-carbides (i.e. TiC, VC and NbC) to limit the diffusible hydrogen content in bcc Fe. Our study revealed that the interplay between hydrogen and carbon vacancies, local atomic environment at interface as well as elastic strain fields at the interface can lead to significantly increased hydrogen solubilities. While in TiC, the deepest traps were found to be in the bulk of carbides, in VC and NbC, the elastic strain fields around the interface led to the strongest trapping. Further, the formation of a two-hydrogen-vacancy complex was found to be favourable in VC. Finally, the migration barriers for hydrogen trapping in bulk TiC as well as across the Fe/TiC coherent interface indicate that these deep traps in the form of carbon vacancies are fairly accessible.
Hydrogen diffusion and storage in substoichiometric TiC
2024, International Journal of Hydrogen Energy
In this paper we investigate the nature of hydrogen diffusion in substoichiometric TiC using simulations. Specifically, we examine how well connected the carbon structural vacancies are in TiC_x using percolation theory and how many structural carbon vacancies are connected to the surface of TiC_x. This provides direct insight into the number of sites available for hydrogen storage and the interconnected nature of the carbon vacancy network. We also compute the binding energies and migration energies of hydrogen in TiC_x using density functional theory. This is conducted over a range of carbon concentrations from nearly stoichiometric TiC to Ti₂C and includes the differences in vacancy ordering. We find that the migration energy of hydrogen in TiC_x generally decreases with carbon loss and is associated with the tetrahedral interstices near them but that the binding energy of hydrogen to TiC_x is independent of carbon concentration. Furthermore, we show that this migration path can occur through the tetrahedral interstices, which is influenced by local carbon concentration. These results also demonstrate the vacancy ordered $R \bar{3} m$ Ti₂C structure does not possess any more superior ability to store or transport hydrogen than the $F d \bar{3} m$ Ti₂C structure.
Current state-of-the-art of hydrogen trapping by carbides: From theory to experiment
2024, International Journal of Hydrogen Energy
This study reviews the interactions of hydrogen with carbides. Often, the introduction of well-chosen carbides is suggested as a strategy in the development of steels with higher resistance against hydrogen embrittlement. However, despite numerous experimental studies published in literature, many uncertainties and discrepancies still exist regarding their potential beneficial effect. This could be related to differences in carbide/matrix interface character, chemical composition, carbide distribution or even charging and/or mechanical loading conditions, which all affect the hydrogen trapping characteristics and thus the hydrogen distribution in the steel specimens. Consequently, advanced insights in the fundamental aspects of the hydrogen trapping by the different sites provided by carbides are of high importance. In this paper, the fundamental insights obtained by theoretical calculations is reviewed and linked to experimental observations reported in literature, as such alleviating some of the reported discrepancies and revealing new insights in the hydrogen-carbide interactions.
Enhancing the hydrogen storage properties of (Ti, M)C<inf>1−x</inf> materials (M = W, Mg, Ni, and Al) depends on the carbon vacancies: Potential for the recycling of scraps
2024, International Journal of Refractory Metals and Hard Materials
New energy storage systems and recycling schemes are required to solve the increasing environmental and energy problems. Owing to the increasing cost of raw materials, scraps need to be recycled. Carbides can be used as hydrogen storage materials; however, their mechanical properties remain unclear. To fabricate high-quality carbides from scraps as potential candidates for hydrogen storage materials, it is necessary to understand the mechanical properties of the carbides when absorbing hydrogen atoms. In this study, we evaluated the thermodynamic stabilities, mechanical properties, and electronic structures of (Ti, M)C_1-x materials (M = W, Mg, Ni, and Al, $x = 0 - 1$ . The effects of the carbon vacancies were examined using density functional theory calculations. Based on the results, the TiC_0.625 composition was stable at ambient temperature and pressure. Titanium carbides have shown potential for use in hydrogen storage applications at $0 \leq x \leq 0.375$ , as the hardness decreases rapidly when $x \geq 0.375$ . In addition, the results indicate that (Ti, Al)C should mainly be used in the target hydrogen storage materials at a composition below $x = 0.25$ . These findings support the design of hydrogen storage materials based on recycled carbides.
Hydrogen trapping of carbides during high temperature gaseous hydrogenation
2023, International Journal of Hydrogen Energy
This study evaluates the hydrogen absorption and trapping in carbide containing steels from a very low hydrogen partial pressure atmosphere at elevated temperatures. Two generic titanium-containing Fe–C steels, differing in carbon and titanium content, are studied. The steels are subjected to a quench and temper treatment, where tempering is performed in a dilute hydrogen gas atmosphere. Detailed microstructural analysis via SEM and TEM together with characterization via thermal desorption spectroscopy and hot extraction are performed to analyze the hydrogen trapping ability of the different types of carbides.
A significant amount of hydrogen is introduced in the Fe–C–Ti alloys during tempering. Moreover, the hydrogen appears to be strongly trapped. This could be related to trapping at the carbon-vacancies inside the TiC, and more specifically the large undissolved carbides. The binding energy is estimated to range between 80 kJ/mol and 90 kJ/mol and appears to be independent of the undissolved carbide size.

View all citing articles on Scopus

View full text

First-principles study of hydrogen storage in non-stoichiometric TiCx

Abstract

Highlights

Introduction

Section snippets

Method of calculations

The absorption of hydrogen in TiC and TiCx

Conclusions

Acknowledgments

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

J. Power Sources

J. Power Sources

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Energy

Electrochem. Commun.

Mater. Sci. Eng. A

Prog. Solid State Chem.

First-principles study of hydrogen storage in non-stoichiometric TiC_x

The absorption of hydrogen in TiC and TiC_x