# In situ neutron diffraction of NaAlD4/carbon black composites during decomposition/deuteration cycles and the effect of carbon on phase segregation

Rasmus Palm, Kenneth Tuul, Frank Elson, Elisabetta Nocerino, Ola K. Forslund, Thomas C. Hansen, Jaan Aruväli, Martin Månsson

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Rasmus Palm, Kenneth Tuul, Frank Elson, Elisabetta Nocerino, Ola K. Forslund, et al. In situ neutron diffraction of NaAlD4/carbon black composites during decomposition/deuteration cycles and the effect of carbon on phase segregation. International Journal of Hydrogen Energy, 2022, 47 (80), pp.34195-34204. 10.1016/j.ijhydene.2022.08.019. hal-05520025

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# In situ neutron diffraction of NaAlD₄/carbon black composites during decomposition/deuteration cycles and the effect of carbon on phase segregation

<figure><img src="image_3.png" /></figure>

Rasmus Palm<sup>a, *</sup>, Kenneth Tuul<sup>b</sup>, Frank Elson<sup>a</sup>, Elisabetta Nocerino<sup>a</sup>, Ola K. Forslund<sup>a</sup>, Thomas C. Hansen<sup>c</sup>, Jaan Aruväli<sup>d</sup>, Martin Månsson<sup>a</sup>

## HIGHLIGHTS

* NaAlD₄ confined in a mesoporous carbon black.

* In situ neutron powder diffraction during dedeuteration/deuteration cycles.

* Crystalline composition determination at all experimental instances.

* D₂ release at highly lowered temperatures from small-particulate/amorphous phase.

* Full regeneration of Na₃AlD₆ from NaD at low deuterium pressures (≤20 bar).

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Article history:**

Received 28 April 2022

Received in revised form

19 July 2022

Accepted 2 August 2022

Available online 26 August 2022

**Keywords:**

Solid-state H₂ storage

Complex metal hydride

## ABSTRACT

The influence on the decomposition and reforming of the hydrogen storage material NaAlH₄ by adding relatively low amounts of mesoporous carbon black is investigated with in situ diffraction. A 60:40 NaAlH₄/carbon black composite is prepared via ball milling and characterised ex situ via X-ray diffraction, gas adsorption, temperature-programmed decomposition, and dehydrogenation/hydrogenation cycling methods. The prepared composite is deuterated, and the crystalline phase composition is determined with in situ neutron powder diffraction method during multiple decomposition/deuteration cycles. Changes in the crystalline phase composition start slightly below the melting temperature of the pristine alanate, whereas the release of deuterium starts at considerably lower temperatures. The decomposition of Na₃AlD₆ to NaD is almost completely reversible at the

<footnote><sup>a</sup> Department of Applied Physics, KTH Royal Institute of Technology, SE-106 91 Stockholm, Sweden</footnote>

<footnote><sup>b</sup> Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia</footnote>

<footnote><sup>c</sup> Institut Laue-Langevin, 71 avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France</footnote>

<footnote><sup>d</sup> Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia</footnote>

<footnote>* Corresponding author.</footnote>
In situ neutron powder diffraction
Phase composition

applied low deuterium pressures of ≥2 MPa. Thus, the strong effect of even low concentrations of a mesoporous carbon black on the capability to store H₂ reversibly is showcased and analysed in-depth.

© 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## Introduction

The decomposition and reforming mechanism of NaAlH₄ has been studied extensively for use as a solid-state H₂ storage material [1–8]. In case of bulk NaAlH₄, decomposition undergoes in three steps (Eqs. (1)–(3)) with increasing temperature (T) [1]:

$$3\text{NaAlH}_4 \rightarrow \text{Na}_3\text{AlH}_6 + 2\text{Al} + 3\text{H}_2\uparrow \quad (1)$$

$$\text{Na}_3\text{AlH}_6 \rightarrow 3\text{NaH} + \text{Al} + 1.5\text{H}_2\uparrow \quad (2)$$

$$3\text{NaH} \rightarrow 3\text{Na} + 1.5\text{H}_2\uparrow \quad (3)$$

As the decomposition of NaAlH₄ is kinetically hindered, H₂ release of any practical consideration starts after the melting of NaAlH₄ at T = 456 K, even though H₂ release from surface layer decomposition, according to Eq. (1), is possible at remarkably lower T values [1–3]. The second (Eq. (2)) and third decomposition steps (Eq. (3)) start around T = 500 K and T = 550 K, respectively, in the case of bulk NaAlH₄ [1,4,5].

The beforementioned temperatures, required for H₂ release, are too high for practical applications, e.g., for H₂ in automobile applications [9–11]. In addition, the segregation of decomposition products, especially Al, into separate phases during H₂ release limits the recovery of initial NaAlH₄ through hydrogenation and, thus, cyclability during many H₂ release and uptake cycles [12]. To decrease the temperature of H₂ release and hydrogenation and to improve the cyclability different strategies have been applied, e.g., catalyst-doping [13–17], nanosizing and ball milling [13], nanoconfinement [4,5,18–24] and combinations of such methods [25–28].

The decomposition temperature of NaAlH₄ has been shown to be strongly linked to the particle size, where sub 10 nm particles start releasing H₂ already at ambient temperature [19]. In addition to the particle size, the addition of a scaffolding material has been shown to decrease the decomposition temperature considerably [4,18–20,24,25]. Nanoconfinement of NaAlH₄ in different carbon materials has resulted in remarkable results. Solution impregnation has already achieved decomposition at ambient temperatures [4,19], with hydrogenation possible at mild conditions of 398–423 K and 3.5–5.5 MPa of H₂ gas pressure [19]. However, these results were achieved with very low and impractical NaAlH₄ loadings of ≤10 wt%. Melt infiltration has achieved similar results with higher loadings, indicating a more efficient and practical approach. Structural analysis also showed that when NaAlH₄ is confined to pores 2–15 nm in size, the decomposition skips the Na₃AlH₆ phase altogether [24].

Ball milling has offered the best results, being a simple method for nanostructuring with the possibility of using high NaAlH₄ loadings. A lowering of (de)hydrogenation temperatures and pressures (1.98 MPa H₂ and 413 K) similar to solution and melt impregnation was achieved with ball milling and with only 23.1 wt% of carbon additive. However, with ball milling, it was discovered that although reversibility was enhanced, it was mainly attributed to the Na₃AlH₆ ↔ NaH transition (Eq. (2)) [25].

In addition to nanostructuring with the use of different scaffold materials, different mixtures of hydrides have been investigated for the improvement of hydrogen storage properties [29–32]. For example, the small additions of graphene nanosheets and single wall carbon nanotubes have been shown to improve the hydrogen storage properties and cyclability of MgH₂–NaAlH₄ composites [29]. TiF₃ additives have been shown to improve the dehydrogenation onset temperatures of various NaAlH₄ composites with other hydrides, e.g. MgBH₄ and MgH₂ [30,31].

In situ electron microscopy studies of NaAlH₄ and other complex metal hydrides have been shown to yield invaluable understanding of the decomposition and hydrogen release process [33,34]. With the use of in situ environmental scanning electron microscopy it has been shown that in case of TiCl₃ and Nb₂O₅-catalyzed NaAlH₄ and LiAlH₄, respectively, the morphological change during heating is suppressed in comparison to uncatalyzed alanates [33]. With the use of in situ transmission electron microscopy the formation of pores in NaAlH₄ during decomposition and the two-step decomposition process to NaH has been shown in-depth [34].

X-ray and neutron scattering techniques can be used to obtain information about the crystalline structure and dynamics of nanostructured NaAlH₄ during cycling. Sensitivity to X-rays is dependent on the number of electrons, making hydrogen (H) virtually invisible. However, neutrons do not have proportionality relating to the number of nucleons, and H and its isotopes are readily distinguishable [35]. Although there is a plethora of studies investigating in situ (de)hydrogenation of bulk and catalyzed NaAlH₄ via various X-ray and neutron techniques [2,36–41], there is a serious lack of in situ studies of confined NaAlH₄ systems, especially regarding diffraction. Incoherent inelastic neutron scattering has revealed information about chemical interactions between nanoconfined NaAlH₄ and the scaffold material [42]. Synchrotron X-ray diffraction (XRD) studies have shown that with multi-walled carbon nanotubes, the dehydrogenation pathway is substantially altered, as there is even formation of Al₄C₃, but this is not seen with activated carbon [43]. This supports the idea that in addition to the structure, also the chemical composition of the chosen scaffold material plays a vital role via reactions, which might be caused by possible
contaminants and processing routines [43,44]. Small-angle X-ray scattering (SAXS) and XRD studies have revealed that nanoparticles preferentially form within more accessible mesopores and there are clear changes in the diffraction pattern of NaAlH₄ due to nanostructuring. Post dehydrogenation SAXS clearly demonstrates the change in pore filling [44]. Small-angle neutron scattering has revealed that NaAlD₄ particles that are nanoconfined in porous scaffolds maintain their size distribution and have remarkably stabilized morphology [45]. Finally, a synchrotron powder XRD and neutron powder diffraction (NPD) study analysed the rehydrogenation of NaAlH₄ with Ti additive. Rietveld refinement accurately determined the composition of crystalline phases during the reaction process. They confirmed that Al initially reacts with NaH to form Na₃AlH₆ with the onset of NaAlH₄/NaAlD₄ formation occurring before the total consumption of NaH/NaD. This concurrent process is in contrast with the sequential decomposition reactions [39].

In this study, the effect of the confinement of NaAlD₄ in a mesoporous carbon black scaffold material on the decomposition/deuteration temperature, pressure, and crystalline phase formation is investigated with in situ NPD. Using in situ neutron diffraction method on confined NaAlD₄ composites novel in-depth information about the influence of the scaffold material on the hydrogen storage properties and decomposition mechanism of complex metal hydrides is obtained. For this, ball milling is used to prepare a 60 wt% NaAlH₄/carbon black composite material, which is characterised *ex situ* with XRD, gas adsorption, temperature-programmed decomposition, and dehydrogenation/hydrogenation cycling. The 60 wt% composition was chosen based on the H₂ release properties, post-synthesis phase composition and H₂ yield obtained by the aforementioned characterisation of a series of composites with NaAlH₄ in NaAlH₄/carbon black wt% from 50 wt% to 90 wt%. The prepared composite is deuterated for the *in situ* NPD measurements because deuterium (D) has over three times higher coherent neutron scattering cross-section in comparison to H [46]. From these experiments, the effect of a relatively low amount of carbon black additive on the decomposition and deuteration process route of NaAlD₄ is determined and analysed.

## Materials and methods

### Synthesis of NaAlD₄/carbon composites

Commercial bulk NaAlH₄ (90%, Sigma-Aldrich) was purified by recrystallisation. For recrystallisation, NaAlH₄ was dissolved in tetrahydrofuran (anhydrous, ≥ 99.9%, Sigma-Aldrich) yielding a 0.05 gNaAlH₄ ml⁻¹THF solution. The prepared solution was filtrated through a glass microfiber filter (GF/B, Whatman) and dried at 323 K and under a partial vacuum of 26 kPa for 2 h and then at 295 K and a vacuum of 0.1 kPa for 15 h. Carbon black (CB) Vulcan XC 72 (Cabot, USA) was used as the scaffold material and degassed under 10 kPa and 423 K for at least 24 h before use. 60:40 recrystallised NaAlH₄:CB by weight was ball-milled using a PULVERISETTE 6 classic line planetary mono mill (Fritsch, Germany) with a rotation speed of 400 rpm for 40 min, allowing for 10 min cooling breaks every 5 min. 25 ZrO₂

10 mm diameter balls were used as the grinding media in a 75:1 ball to powder mass ratio and the system was hermetically sealed with a clamp during milling.

Deuteration of the milled samples was performed with an iSorb HP1 (Quantachrome Instruments, USA) high-pressure gas sorption system. 1.64 g of sample was held under vacuum at 443 K for 24 h and then D₂ gas pressure of 16 MPa was applied for 72 h. Deuteration of the samples was performed because of the considerably higher coherent neutron scattering cross-section of D, which contributes to the diffraction signal of interest, and much lower incoherent scattering cross-section, which reduces the background signal, in comparison to H [46]. Based on the D vs H occupancies obtained from refinement of NPD data, a ≥90% replacement of H with D was achieved. The deuterated sample will be designated as NaAlD₄/CB and the pre-deuterated sample as NaAlH₄/CB. All sample preparation, manipulation, and storage steps were performed under Ar atmosphere.

### Neutron powder diffraction

In situ NPD measurements were performed on the D20 instrument at ILL, France [47]. Incident neutrons with a wavelength of 1.54 Å were obtained from the (115) reflection of a Ge monochromator at 90°. Cylindrical sample holders with inner diameter of 6 mm were used. Diffraction patterns of empty sample holders were measured in the T range from 300 K to 450 K under vacuum conditions, in order to subtract the background and to assign the Bragg peaks caused by the sample cell. The NaAlD₄/CB sample was prepared in an inert gas-filled glovebox. The sample and sample stick gas line was hermetically sealed for transportation from the glovebox to the instrument and during the connection to the gas handling system. Four heating, D₂ pressurisation (with multiple concurrent dosing steps), and cooling cycles were performed (Table S1). Diffractograms were obtained over 10 min and 5 min in the case of isothermal conditions and temperature ramp conditions, respectively. The raw scattering data is available in the ILL data repository [48].

The diffraction pattern of the empty sample cell was subtracted from all measurements, yielding a pattern consisting of NaAlD₄/CB and the incoherent background signal caused by the applied D₂ pressure (pD₂) (Fig. S1a). As the equilibrated temperatures were not equivalent between the applied routine and the empty sample cell measurements, the signal subtraction was not perfect and four 2θ regions were excluded from further analysis, between 44 and 45.5°, between 63 and 66.5°, between 98 and 100°, and between 115 and 118° (Fig. S1a). Data reduction and initial analysis were performed with OriginPro 2021b (OriginLab Corporation, Northampton, MA, USA) software package.

Topas software package was used for the applied refinement routines (Fig. S2) [49]. Sequential fitting routines were applied for the batch refinement of multiple diffractograms in one go. H-based equivalent crystalline structures obtained from the Materials project [50] and Refs. [2,6] were taken as a basis for refinement of NaAlD₄, Na₃AlD₆, NaD, and Al structures and are presented in Table S2 with the corresponding space groups. The H and D isotope occupancy (occ) was fitted based on the assumption that occH + occD = 1 for each site.
Neutron scattering from the isotopes, H and D, could be separated based on the divergent coherent neutron scattering cross-section (1.758 b for H vs 5.592 b for D) and the ratio of coherent/incoherent neutron scattering cross-sections (0.02 for H vs 2.73 for D) [46]. The hydrogenated initial and refined deuterated crystalline structures are presented in Table S2. The spherical harmonics preferred orientation correction was applied to the refinement of the carbon crystal structure. This was done to account for the anisotropic crystalline ordering, i.e., different length scales of crystalline ordering for intra- and inter-layer graphitic structures.

Initially, refinement was performed with all possible crystalline phases present. Certain crystalline phases were eliminated based on the high uncertainty of the refined wt% in comparison to the obtained value and based on the divergence of lattice parameters of the refined structure in comparison to the original structure. The goodness of fits (GoF) for all refinements was below 1.65 (Fig. S1b). The weighted profile R-

factors were between 6.8 and 13.5 and the expected R-factors were between 4.1 and 10.8, yielding the aforementioned GoF values.

The amount of $D_2$ released during the decomposition/deuteration cycles applied during the NPD measurements was calculated from the in situ pressure readings. The free volumes in the gas handling system used for the calculation were determined beforehand with He and the temperatures of different zones were measured in situ during the experiment. The calculations for the amount of $D_2$ are brought in detail in the supplementary information (SI).

Temperature-programmed decomposition

The amount of released $H_2$ vs T was determined from measurements with AutoChem 2950HP (Micromeritics, USA) chemisorption analyzer. The amount of $H_2$ was calculated from the change in thermal conductivity caused by $H_2$

<figure><img src="image_5.png" /><figcaption>Fig. 1 – a) $N_2$ sorption isotherms adsorption (full line) and desorption (dashed line) for the used carbon black (0% $NaAlH_4$), $NaAlH_4/CB$ (60% $NaAlH_4$), and bulk $NaAlH_4$ (100% $NaAlH_4$), The $S_{BET}$ and $V_{tot}$ values calculated from the $N_2$ adsorption data are tabulated in the figure inset. b) XRD diffractograms of $NaAlH_4/CB$ before and after the application of 5 (de)hydrogenation cycles, where the pristine materials intensities have been shifted up for better readability. c) $H_2$ release curves of $NaAlH_4/CB$ before (black line) and after cycling (red dotted and dashed line) and of bulk $NaAlH_4$ (blue dashed line). d) Molar ratios of released $H_2$ per $NaAlH_4$ during each dehydrogenation cycle, where the theoretical maximal amount of $H_2$ from Eq. (1)–2 are brought as dashed lines. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
addition to the constant carrier gas flow of 50 ml min⁻¹ N₂ (6.0, Linde). The sample was transported hermetically sealed to the instrument and was supported on quartz wool inside the sample holder. Measurements were performed on ~10 mg of sample. Before the measurements, the sample was flushed with Ar and the temperature stabilized at 273 K. Before and after the temperature ramp routine, the thermal conductivity signal was measured for 2 h for stabilisation of the system and background subtraction. Measurements were performed in the T range from 273 K to 873 K with a T ramp rate of 2 K min⁻¹. The H₂ evolution data reduction and analysis were performed with the OriginPro Version 2021b (OriginLab Corporation, Northampton, MA, USA) software package.

## Dehydrogenation/hydrogenation cycling

Five dehydrogenation and hydrogenation cycles, at 463 K and near 0 kPa and at 463 K and under H₂ pressure of 6 MPa, respectively, were performed with an iSorb HP1 (Anton Paar, Austria) high-pressure gas sorption system. Each dehydrogenation step was equilibrated for 4 h and each hydrogenation step was equilibrated for at least 10 h. The amounts of H₂ uptake and release during hydrogenation and dehydrogenation cycles, respectively, were determined from the pressure change at the respective conditions. As the last step, hydrogenation conditions were applied, and characterisation of the post-cycled composite was performed.

## X-ray diffraction

XRD measurements were done on a D8 Advance diffractometer (Bruker, Germany) with Cu Kα radiation of wavelength 1.5406 Å. Measurements at 2θ from 13° to 80° with a total step time of 166 s and a step size of 0.0128° were performed using a LynxEye detector. The powder sample was sealed in an X-ray transparent airtight specimen holder under Ar atmosphere. The airtight specimen holder caused a wide scattering peak at 2θ = 20.5°.

## Gas adsorption

N₂ gas adsorption measurements were performed on an ASAP 2020 (Micromeritics, USA) gas sorption system at 77 K. The pure carbon black sample was degassed at 573 K and under a vacuum of at least 1.3 Pa for 12 h. Bulk NaAlH₄ and the NaAlH₄/CB composite were degassed at ambient temperature and under a vacuum of at least 3.3 Pa for 24 h, where the milder temperature conditions were applied to withhold decomposition and melting of the confined NaAlH₄. The specific surface area, S<sub>BET</sub>, and total pore volume, V<sub>tot</sub>, were calculated based on the BET theory in the relative pressure, *p*/*p*°, range from 0.05 to 0.2 and from the amount of adsorbed N₂ at *p*/*p*° = 0.95, respectively.

## Results and discussion

### Characterization and ex situ hydrogenation/dehydrogenation cycling

Characterisation of the pristine alanate and ball-milled composites, with the ex situ cycling, was performed to determine

the efficiency of ball milling for the formation of confined composite. Based on the results from N₂ adsorption, ball milling caused a considerable decrease in the specific surface area and pore volume of the prepared NaAlH₄/CB composite in comparison to the carbon black supporting material (Fig. 1a). In addition, the shape of the N₂ adsorption isotherm of the ball-milled NaAlH₄/CB composite is closest to the Type II isotherm, which is indicative of a nonporous or macroporous material [51]. Whereas, the shape of the N₂ adsorption isotherm of the pure CB is closest to the Type IV isotherm, typical for mesoporous materials, indicating filling of the CB porosity with NaAlH₄ during the ball milling process [51]. The NaAlH₄/CB XRD diffractograms (Fig. 1b) exhibit crystalline NaAlH₄ and Al Bragg peaks indicating that some decomposition of the pristine alanate was caused by the ball milling process. H₂ release from NaAlH₄/CB starts at near-ambient temperatures (Fig. 1c), and clear H₂ release maxima are observed at 439 K, 450 K, and 549 K, corresponding most likely to the three main decomposition steps of bulk NaAlH₄ (Eqs. (1)–(3)). Purely ball milling of the NaAlH₄ with the CB

<figure><img src="image_6.png" /></figure>

Fig. 2 – Heatmaps of diffractograms (intensities in arbitrary units from blue to red), from which the scattering signal from the empty sample holder has been subtracted, obtained at a) the 1st cycle decomposition and b) the 3rd cycle decomposition and regeneration. The timescale presented on the y-axis is the experimental time equivalent to the one presented in Fig. 3. The wide Bragg peaks noted with * are from NaAlD₄, ♦ from Na₃AlD₆, ♣ from NaD, and ♠ from Al. In the case of two notations on top of each other, the Bragg peaks are overlapping. (For interpretation of the references to color/ colour in this figure legend, the reader is referred to the Web version of this article.)
decreased the maximum H₂ release temperature from each decomposition step by around 50 K. The strong effect of ball milling and carbon addition has been shown previously in many publications [25] and, in the case of this work, the considerable decrease in the temperature at which the release of H₂ starts confirms a successful sample confinement routine. Based on theoretical calculations it has been shown that local temperature increases of up to 1000 K can be caused by ball milling [52]. Thus, the ball milling likely caused partial melting and decomposition of the pristine hydride and the filling of the intra-particulate pores, resulting in a lowered pore volume and specific surface area of the composite material. Based on the presence of crystalline phases, detectable by XRD, and that most of the H₂ is released above 423 K, the NaAlH₄ particles are of considerable size. Nevertheless, the decreased T of H₂ release and porosity values in comparison to bulk NaAlH₄ indicates strong intermixing and covering of carbon black particles with NaAlH₄ during ball milling, which brings out the effect of the scaffolding carbon black.

During the first dehydrogenation cycle at 463 K over 4 wt% of H₂ (over 1.1 mol·H₂ mol·NaAlH₄⁻¹) is released from the NaAlH₄ in the composite (Fig. 1d). This amount of released H₂ is possible if the two foremost decomposition steps (Eqs. (1) and (2)) are involved to some degree. During further cycles,

around 1.3 wt% (0.35 mol·H₂ mol·NaAlH₄⁻¹) of H₂ is released, indicative of partial rehydrogenation at the applied conditions. Based on post-cycling XRD (Fig. 1b), Na₃AlH₆ and Al phases are present, exhibiting intense Bragg peaks, in addition to low-intensity Bragg peaks from NaAlH₄. The amount of released H₂ from the post-cycled NaAlH₄/CB decreased 51% in comparison to pristine NaAlH₄/CB (Fig. 1c). Most of the decrease in the amount of released H₂ can be attributed to the H₂ release in the T range from 423 K to 473 K, which is characteristic of H₂ release from large-particulate and/or bulk NaAlH₄. The H₂ release starting from near-ambient temperatures is mostly retained during cycling. Thus, based on post-cycling XRD and TPD H₂ release measurements, both first two decomposition steps are reversible to some degree at the applied cycling conditions where the decrease in the amount of reversibly stored H₂ is mostly from the large particulate/bulk phase.

## In situ neutron powder diffraction (NPD)

In situ NPD measurements were performed to determine the crystalline phase composition at different temperature and deuterium gas pressure conditions. Example diffractograms obtained during the decomposition in the 1st and 3rd (de)

<figure><img src="image_7.png" /><figcaption>Fig. 3 – a) D₂ pressure (p<sub>D2</sub>) and temperature in the sample cell vs experimental time and b) mass ratios of crystalline phases in the sample vs experimental time. The initial mass ratio of the carbon (Fig. S3) had been subtracted and mass ratios are normalized so that the sum of all presented mass ratios equals unity.</figcaption></figure>
deuteration cycles are shown in Fig. 2a and b, respectively, as heatmaps. The carbon black additive causes wide Bragg peaks at all experimental conditions noted as CB (Fig. 2). In addition, some of the Bragg peaks from different crystalline phases are overlapping (Fig. 2a) and after the first (de)deuteration cycle the Bragg peak intensities decrease considerably, making the determination of crystalline composition difficult. Thus, refinement of in situ NPD data was performed at all experimental T and D₂ gas pressure conditions applied (Fig. 3a) to obtain quantitative crystalline phase compositions. From the refinement of NPD data, mass ratios of carbon scaffold (Fig. S3), NaAlD₄, Na₃AlD₆, NaD, and Al (Fig. 3b) were calculated. The mass ratio of the carbon black scaffold, obtained from refinement, was around 40% throughout the whole experimental routine (Fig. S3), where it decreased in conjunction with the decomposition of NaAlD₄ into Na₃AlD₆ and Al and the decomposition of Na₃AlD₆ into NaD and Al (Eqs. (1) and (2)). Thus, an amorphous and/or nano-crystalline deuterated alanate phase was present in the pristine material, which sintered into larger particles and crystallites upon decomposition at the higher temperatures applied during heating and, thus, decreasing the ratio of the scattering signal from the carbon phase.

For the comparison of NaAlD₄ and its decomposition products, mass ratios at all experimental time-steps were

calculated. The mass ratio contribution of the carbon black was removed and the sum of all other contributions was assumed to be unity (Fig. 2a). Up to T = 420 K, the NaAlD₄ fraction is over 90% of the crystalline phase and around 20% of the NaAlD₄ decomposes at 420 K in a relatively quick manner, in 30 min. The NaAlD₄ crystalline phase disappears completely upon heating to T = 450 K, where the melting temperature of bulk NaAlH₄ is 456 K [8], and does not form again during subsequent deuteration cycles (Fig. 3). The polymorphic transition or melting temperatures of bulk Na₃AlH₆, NaH, and Al are 525 K, 905 K, and 933 K [7,53], respectively, and, thus, the influence of phase transitions on the mass ratios, obtained from refinement, can be disregarded in case of these compounds.

With the disappearance of the NaAlD₄ crystalline phase, starting at T = 420 K, the mass ratios of Na₃AlD₆ and Al start to increase (Fig. 3b and S4a), corresponding to the 1st decomposition step (Eq. (1)). The mass ratios of NaD and Na₃AlD₆ start to increase and decrease, respectively, only upon cooling to T = 430 K from T = 450 K. Thus, even with the use of a scaffolding carbon additive, the 2nd decomposition step (Eq. (2)) is hindered by the slow kinetics of the 1st decomposition step (Eq. (1)), which is exemplified by further decomposition/deuteration cycles discussed later. Upon the application of pD₂ > 2 MPa at T = 430 K (Fig. 3 and Fig. S4a), the NaD phase

<figure><img src="image_8.png" /><figcaption>Fig. 4 – a) Temperature (T, red line, left axis) and pressure (p<sub>D2</sub>, blue line, right axis) dependence during the first three decomposition cycles, where the temperature axis has equivalent values for all three cycles. b) Amount of released D<sub>2</sub>, calculated from the pressure increase, during the first three decomposition cycles, where the T applied during decomposition and the theoretical maximum amount of releasable D<sub>2</sub> from the first decomposition reactions (Eqs. (1) and (2)) are brought in the figure. c) Crystalline phase molar ratios calculated from the results obtained from the refinement of NPD data. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
decreases relatively quickly, as the majority of the phase disappeared in $t \le 30$ min, and is accompanied by the increase in $Na_3AlD_6$ phase. This corresponds to the deuteration of NaD according to the reverse reaction of Eq. (2) and as only the crystalline Al phase is determinable in addition to $Na_3AlD_6$, where $Na_3AlD_6$ mass ratio equilibrates at 0.61 under the mild applied $p_{D2} > 2$ MPa, indicating a reversible 2nd decomposition step (Eq. (2)). The reversibility of the second decomposition step (Eq. (2)) is also exhibited during the 3rd decomposition/deuteration cycle, where crystalline NaD forms upon heating to $T = 450$ K and disappears when $p_{D2}$ is applied (Fig. 3 and Fig. S4d). The reversibility of the second decomposition step (Eq. (2)) under relatively low applied $p_{H2}$ (2.8 MPa) has been shown previously in case of $TiCl_3$ additive [17].

Upon heating to $T = 420$ K and dosing with $p_{D2} > 4$ MPa, no change was determined in the crystalline composition (Fig. 3 and Fig. S4b). Nevertheless, a pressure increase of 30 kPa (Fig. 4a), corresponding to 5.5 wt% ($0.11 \text{ mol}_{\text{D2}} \text{ mol}_{\text{NaAlD4}}^{-1}$) of $D_2$ released from the total theoretical maximum $D_2$ present in the pristine $NaAlD_4$ (Fig. 4b), is exhibited upon heating to $T = 420$ K. Thus, the presence of CB supports the decomposition of $NaAlD_4$ at a $T$ considerably lower than that of bulk $NaAlD_4$ and without a notable change in the crystalline composition. This is mostly caused by the decomposition of amorphous and/or small particulate $NaAlD_4$ or $Na_3AlD_6$ in the $NaAlD_4/CB$ material, which is not determinable by NPD. The formed Al, produced as a decomposition product from small particulate/amorphous $NaAlD_4$ and/or $Na_3AlD_6$ (Eqs (1) and (2)), is separated and confined to a considerable degree as no increase in the ratio of crystalline Al is determined (Fig. 3b and Fig. S4b) during the second heating cycle. Identical release of $D_2$ at $T \le 420$ K, without a change in the crystalline phase composition, can be seen during the 3rd decomposition cycle (Fig. 4a and Fig. S4c) and the consistency of the crystalline phase during the 4th cycle, even though vacuum was applied during the 4th cycle (Fig. 3 and Fig. S4c).

Molar ratios of the crystalline phases determined by refinement of NPD data are presented in Fig. 4c. Based on the NPD data refinement only the first two decomposition steps are evoked at the applied $T$ and $p$ conditions. Thus, the total crystalline phase stoichiometry can be summed up based on Eq. (1) and Eq. (2):

$$3\text{NaAlD}_4 \leftrightarrow \text{Na}_3\text{AlD}_6 + 2\text{Al} + 3\text{D}_2 \leftrightarrow 3\text{NaD} + 3\text{Al} + 4.5\text{D}_2 \quad (4)$$

The presence of Al, $n_{Al}/n_{sum}$ of around $0.1 \text{ mol}_{\text{Al}} \text{ mol}^{-1}$, in the pristine $NaAlD_4/CB$, is most likely caused by the preliminary decomposition of $NaAlH_4$ and $NaAlD_4$ during the ball milling and deuteration routines applied, respectively, during material preparation. As the crystalline presence of neither $Na_3AlD_6$ nor NaD is determined in the pristine $NaAlD_4/CB$, other decomposition products must be of amorphous and/or too small particulate origin to be detectable by NPD. During the 4th applied cycle, molar ratios of NaD and Al reached 0.45 and 0.55, respectively, with no additional detectable crystalline phases present (Fig. 4c). Based on the equivalent molar ratios of NaD and Al after the 2nd decomposition step (Eq. (4)) and considering the $0.1 \text{ mol}_{\text{Al}} \text{ mol}^{-1}$ Al content present in the pristine $NaAlD_4/CB$, full decomposition of $NaAlD_4$ into NaD at $T = 450$ K is ascertained.

## Conclusion

With the use of in situ NPD measurements the influence of the mesoporous carbon scaffold on the confinement of $NaAlD_4$ was investigated during four decomposition/deuteration cycles. With these in situ measurements a repeatable low temperature, $T \le 420$ K, release of $D_2$ is shown to take place without notable changes to the crystalline phase composition. This is attributed to the amorphous and/or small particulate $NaAlD_4$ and/or $Na_3AlD_6$, which is confined on/in the CB surface. The Al formed from the decomposition of this confined hydride phase is also confined and unable to segregate into detectable crystalline Al phase. Based on the crystalline composition of the sample and the amount of released $D_2$ during the cycling experiment, the decomposition step of $Na_3AlD_6$ into NaD is shown to be reversible even under low $p_{D2}$ of around 2 MPa. Thus, the catalytic/confining effect of the mesoporous carbon for the improved cyclability at mild conditions is considerable. Even in the case of carbon scaffold supported $NaAlD_4$, the segregation of a separate crystalline Al phase is clearly detected. The Al phase segregation is a considerable hindrance to the deuteration/hydrogenation of decomposition products into $NaAlD_4$ required for high capacity cyclable $H_2$ storage. This could be achieved with better confinement of the decomposition products by using well-optimised scaffold materials and improved confinement routines. Thus, this work presents clear hindrances and possibilities for the further development and use of $NaAlH_4$ confined in a mesoporous carbon scaffold material as a solid-state low-pressure $H_2$ storage material.

## 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.

## Acknowledgements

The authors wish to express their gratitude for the beamtime as well as the great support offered by the staff of Institut Laue-Langevin (ILL). This work was supported by the Estonian Research Council Grants (PUTJD957), by the EU through the European Regional Development Fund (Centers of Excellence, TK141 "Advanced materials and high-technology devices for energy recuperation systems"), as well as by the Swedish Research Council through a neutron project grant (Dnr. 2016-06955 and Dnr. 2021-06157). E.N. is fully funded by the Swedish Foundation for Strategic Research (SSF) within the Swedish national graduate school in neutron scattering (SwedNess).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2022.08.019.
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