Materials Today Energy
In situ measurement technologies on solid-state hydrogen storage materials: a review
Graphical abstract
Recent advances of in situ measurement technologies on hydrogen storage materials are comprehensively reviewed. In situ measurement technologies show tremendous merits compared with the ex situ ones.
Introduction
Hydrogen is a clean and sustainable energy carrier for future society [1,2]. As it is primarily derived from water, hydrogen can perfectly address the issues of sustainability, environmental emissions, and energy security [3]. Hydrogen storage and transport are one of the key technologies needed for the widespread use of hydrogen energy. Although the commercialized fuel cell vehicles are equipped with high-pressure tanks with hydrogen pressure values as high as 350–700 bar, more compact and safe hydrogen storage technology is greatly needed. In this regard, solid-state hydrides hold the potential for onboard hydrogen storage [[4], [5], [6]]. The key for onboard hydrogen storage is the use of materials with high gravimetric and volumetric hydrogen densities, while reversibility and moderate hydrogenation/dehydrogenation conditions are also required [7]. Taking the DOE 2025 hydrogen storage targets for an example [8], the gravimetric and volumetric hydrogen densities of the system are set as 5.5 wt% and 40 g/l, respectively, indicating that the material-based hydrogen densities should be above 11 wt% and 79 g/l for a storage material with an enthalpy change of approximately 30 kJ/mol H for hydrogen absorption/desorption [9]. These targets are still challenging to achieve, and no existing material can well satisfy them. Therefore, it is important to continue the efforts to explore more compact and efficient hydrogen storage materials. In fact, the study of solid-state hydrogen storage materials is currently one of the most popular topics in the materials science and energy science fields.
In the solid-state hydrogen storage materials family, metal hydrides and complex hydrides are two promising branches with high hydrogen storage densities and/or good hydrogenation/dehydrogenation reversibility [10]. The study of metal hydrides dates back to the late 1960s, when AB5-type alloys were occasionally developed [11]. Since then, Mg-based alloys [12,13], the TiFe hydrogen storage alloy [14], and many others have been developed. The hydrogen storage reversibility and the kinetics of metal hydrides are good; however, their hydrogen storage capacities are generally lower than 5.0 wt.%. In the past two decades, complex hydrides with hydrogen storage capacities above 10.0 wt.%, such as LiBH4, LiNH2, LiAH4, etc., have attracted abundant attention [15]. However, the hydrogen storage reversibility and the kinetics of the complex hydrides are not satisfactory. To overcome the drawbacks of metal hydrides and complex hydrides, the strategies of nanostructuring, alloying, and catalyzing via numerous technical routes have been proposed [10,16], such as high energy ball milling [[17], [18], [19]], high-pressure torsion [20,21], thin film formation [22,23], melt-spinning [[24], [25], [26]], etc. Herein, some important research topics involving metal hydrides and complex hydrides are summarized as follows:
- 1)
Developing novel metals hydrides, complex hydrides or their composites with high hydrogen storage capacities and moderate thermodynamics;
- 2)
Improving thermodynamic properties, e.g., increasing the PCI (Pressure-Composition Isotherm) plateau of Mg-based materials;
- 3)
Improving the hydrogenation and dehydrogenation kinetics of materials;
- 4)
Understanding the relation between hydrogen storage properties and microstructures; and
- 5)
Revealing the catalysis mechanism of catalyzed hydrogen storage materials.
It is important to note that for metal hydrides, it is important to understand the relation between phase structures and their behaviors as a function of the hydrogen content and the roles of vacancies, dislocations and boundaries, while for complex hydrides, it is essential to understand the intermediate products of the hydrogenation and dehydrogenation reactions, the thermal stability and the conditions for formation [27]. The studies on solid-state hydrogen storage materials generally involve the microstructures (including the phase structure, particle and crystalline size, and surface morphology, etc.), thermal/structural stability, chemical compositions, bonding, etc. A lab X-ray diffraction (XRD) machine is generally applied to characterize the phase structures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are usually performed to observe the surface morphologies and crystallite structures. Neutron diffraction (ND) can provide information regarding the hydrides/deuterides structures and hydrogen diffusion dynamics, although it is not always accessible for most researchers [28].
The commonly used characterization technologies are the so-called ex situ methods, which means that the samples would be taken out after or during the hydrogen storage measurement and then be properly treated for the next characterization experiments. There are some obvious disadvantages for the ex situ measurements. First, both metal hydrides and complex hydrides are very oxygen (O2) and water (H2O) sensitive. They are easily contaminated by air. Therefore, the results obtained from these ex situ measurements may not always contain accurate data, as oxidation or moisture absorption could occur during the sample transfer. Second, the sample handling is usually performed in an inert gas glove-box. Both pumping treatments before entering the glove-box and sample handling under an inert gas atmosphere could lead to dehydrogenation for some hydrides with very high PCI plateaus at room temperature. Third, the same intermediate phases are metastable or unstable, and they are quite easily decompose to more stable phases; therefore, they may not be detected by the ex situ measurements. Finally, the sample handling process for measurement is sometimes time-consuming and sophisticated.
To make up for these drawbacks, in situ observation methods have been invented and developed in recent decades, which show significant merits compared with those of the ex situ ones. To date, in situ measurement methods, such as in situ XRD, in situ ND, in situ Raman, in situ SEM/TEM, in situ NMR, etc., have been reported as efficient experimental methods for the study of hydrogen storage materials. Table 1 summarizes some typical in situ research on metal hydrides and complex hydrides.
In this review, the recent advances in the in situ measurement technologies for solid-state hydrogen storage materials have been summarized and reviewed, mainly focusing on metal hydrides and complex hydrides. The working principle together with the devices used for the in situ experimental methods are briefly introduced. Afterwards, both the classic and recent advances in the in situ measurement technologies for metal hydrides and complex hydrides are comprehensively summarized and reviewed. In addition to highlighting the tremendous merits of in situ methods and the studies in the field of hydrogen storage materials, the remaining challenges and the directions of emerging research are discussed.
Section snippets
In situ lab X-ray diffraction (XRD)
X-ray diffraction (XRD) is a commonly used method that is based on the scattering of X-rays by periodical ordered atoms in crystalline materials. XRD experiments provide diffraction patterns to show the crystallographic structure information. The in situ XRD method can be used to monitor the evolution of phase structures during the hydrogenation, dehydrogenation or oxidation reactions of hydrides in high-pressure hydrogen, vacuum, air, or inert gas atmospheres up to certain temperatures.
In situ X-ray absorption spectroscopy (XAS)
The X-ray absorption spectroscopy (XAS) technique can be applied to investigate the local geometric structure and electronic structure of hydrogen storage materials associated with the catalytic effect on the hydrogenation or dehydrogenation properties [41,43]. The absorption spectra contain two main regions, each of which reveals very specific and different information. On the one side, the X-ray near-edge spectroscopic (XANES) part of the spectra provides precise information on the chemical
In situ neutron diffraction (ND)
Neutron diffraction is a powerful technology to identify the occupation sites of hydrogen atoms, which are difficult to accurately detect with X-rays because X-rays scatter from the electron density [46,50,101]. The incoherent cross section of the hydrogen nucleus is 80.26 barn, which is more than 10 times larger than that of most other common chemical elements [102].
The ND method has been applied in studying AB5-type hydrogen storage alloys since the 1990s. Particularly, in situ ND studies
In situ electron microscopy
It is well known that the microstructures of materials have strong relations with their properties. To elucidate the hydrogenation and dehydrogenation mechanisms of materials, electron microscopy can help to efficiently achieve tremendous microstructure information, which includes the particle and powder size, phase and composition determination, and the morphology and structure of the materials at the micro- or nanoscale [108]. A special sample holder may be used to transfer the samples from
In situ Raman spectroscopy
Raman spectroscopy has many merits for the study of hydrogen storage materials, particularly borohydrides and ammonia borane (NH3BH3). In particular, the amorphous phases of some decomposed complex hydrides could be well characterized by Raman spectroscopy, such as CaB6 in the decomposition of Ca(BH4)2, MgB2 in the decomposition of Mg(BH4)2, and lithium dodecaborane (Li2B12H12) and boron in the decomposition of LiBH4 [64].
Xie [123] et al. reported the pressure-induced structural transformations
In situ nuclear magnetic resonance (NMR)
Nuclear magnetic resonance (NMR) is an efficient tool to study the diffusive motion, relaxation, activation energy of atoms [71]. 1H, 7Li, 11B, and 27Al containing hydrides, such as LiBH4, LiAlH4, MgH2, are particularly suitable for the NMR study.
Stowe [70] et al. performed in situ solid-state 11B MAS-NMR studies on the thermal decomposition of ammonia borane. The decomposition of ammonia borane can be described by an induction, nucleation and growth mechanistic pathway. Little hydrogen is
Remarks and outlook
In summary, recent advances of in situ measurement technologies for hydrogen storage materials have been comprehensively summarized and reviewed in this work. We mainly focus on six in situ measurement technologies, including in situ XRD and synchrotron XRD, in situ XAS, in situ ND, in situ SEM/TEM, in situ Raman spectroscopy, and in situ NMR, although there are more in situ techniques also showing merits for the study on hydrogen storage materials, such as the in situ coincidence Doppler
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank the financial support from National Natural Science Foundation of China (No. 51601090), Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515011985), Guangzhou Science and Technology Association Young Talent Lifting Project (No. X20200301071) and the Fundamental Research Funds for the Central Universities (No. 21619415).
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