Influence of Nanoconfinement on the Hydrogen Release Processes from Sodium Alanate

Abstract

Sodium alanate (NaAlH₄) is a prospective H₂ storage material for stationary and mobile applications, as NaAlH₄ contains 7.4 wt% of H₂, and it is possible to do multiple H₂ release and accumulation cycles. Nanoconfinement is a potential solution to enhance the H₂ release properties of NaAlH₄. To optimize the supporting material and the synthesis method used for the nanoconfinement of NaAlH₄, a better understanding of the influence of nanoconfinement on the H₂ release processes is necessary. Thus, the H₂ release from bulk, purely nanoconfined, and intermediate NaAlH₄ is measured at different temperature ramp rates, and the characteristic parameters for each hydrogen release process are determined. Activation energies for each process are determined using the Kissinger method, and the effect of nanoconfinement on the activation energies is analysed. The impact of nanoconfinement on the H₂ release processes from NaAlH₄ and the limitations of each process in case of bulk and nanoconfined NaAlH₄ are presented and discussed. Nanoconfinement of NaAlH₄ decreases activation energies of the initial reversible H₂ release steps to between 30 and 45 kJ mol⁻¹ and increased the activation energy of the last irreversible H₂ release step to over 210 kJ mol⁻¹.

1. Introduction

Hydrogen is a potential energy carrier, as H₂ can be produced directly from water through electrolysis, H₂ can be directly converted into electrical energy with up to 60% efficiency with fuel cells, and the only by-product during conversion into electricity is water [1,2]. Thus, using H₂ to accumulate cheap electricity from renewable sources during high winds and/or intense solar radiation would help balance the power grid and be a viable alternative to electric battery cars. One of the hindrances to the application of a hydrogen-based energy economy is the lack of a cheap and safe hydrogen storage technology. The current commercial standard for hydrogen storage in mobile applications is storing hydrogen in pressurized tanks of up to 700 bar [3]. As the compression of H₂ to such pressures, p, consumes at least 10% [4] of the stored energy, such high pressures have inherent hazards, and even then, the H₂ energy density is only 1.31 kWh L⁻¹ at 25 °C, alternative H₂ storage methods are of high interest.

The storage of hydrogen in a chemically bound form inside complex metal hydrides offers the advantage of high H₂ density, storage at ambient temperature, T, and p_H2 = 1 bar, and potential cyclability through the application of p_H2 [5]. One of the most promising complex metal hydrides for H₂ storage is sodium alanate (NaAlH₄), which incorporates 7.4 wt% of H₂. The release of H₂ from the bulk phase can be described through three decomposition steps (Equations (1)–(3)). The thermodynamical equilibrium T of decomposition (i.e., the release of H₂) for the first two decomposition reactions (Equations (1) and (2)) of NaAlH₄ are at comparatively low temperatures (80 and 150 °C, respectively) [6]. Alas, the release of H₂ from the bulk phase is kinetically limited in the solid phase, and thus, the first H₂ release step from bulk NaAlH₄ starts at 183 °C (Equation (1)) with the melting of NaAlH₄. H₂ is released (from Equation (1) to Equation (3)) over a wide T range (Figure 1 and Reference [7]). The third H₂ release step (Equation (3)) is of no use for H₂ storage as the reverse reaction has not been achieved at p_H2 and T feasible for current gas storage systems. Thus, the main problems of a NaAlH₄-based H₂ storage system are the high temperature of H₂ release caused by the kinetic limitation, the irreversibility of the third decomposition step (Equation (3)), and the segregation of decomposition products (Al, NaH, and Na₃AlH₆) into separate phases, which decreases the cycling lifetime of NaAlH₄-based H₂ storage systems.

3NaAlH₄(l) → Na₃AlH₆(s) + 2Al(s) + 3H₂ ↑ T_start = 183 °C

(1)

Na₃AlH₆(s) → 3NaH(s) + Al(s) + 1½H₂ ↑ T_start = 235 °C

(2)

3NaH(s) → 3Na(l) + 1½H₂ ↑ T_start = 270 °C

(3)

The T of H₂ release from NaAlH₄ can be lowered through nanoscaling, where the higher surface area of sub-micrometer particles promotes the release of H₂ from NaAlH₄ at a T closer to that of thermodynamical equilibrium through the elimination of kinetical limitation present in case of bulk material, i.e., diffusion of H₂ inside the bulk phase. There are two main methods for nanoscaling NaAlH₄: reducing the average particle size through ball-milling or nanoconfining nanoscale particles unto a supporting porous material [7,8,9,10,11]. Whilst ball-milling is relatively easy to perform, and the T of H₂ release is considerably lowered during multiple hydrogenation/dehydrogenation cycles, the nanoscale particles agglomerate together at increased T, and the improved H₂ release properties are lost over multiple dehydrogenation/hydrogenation cycles [12]. By using a supporting porous material for nanoconfinement of NaAlH₄, the starting T of H₂ release is lowered to ambient conditions [8,13]. In addition, the agglomeration of NaAlH₄ and the segregation of decomposition products during dehydrogenation into separate phases is limited by the confinement of NaAlH₄, e.g., during dehydrogenation of bulk NaAlH_4, particles of Al form in the micrometer size. In contrast, the size of Al particles formed during the dehydrogenation of confined NaAlH₄ is lowered to tens of nanometers [13,14]. Nanoscale NaAlH₄ does not release H₂ according to Equations (1)–(3), where the direct decomposition of NaAlH₄ to NaH has been shown by Gao et al. [7], and the H₂ release curves are inherently different, whereas the H₂ released from different decomposition steps (Equations (1)–3)) from nanoconfined NaAlH₄ are visually inseparable [7,8,13].

To fine-tune the H₂ release properties of nanoconfined NaAlH_4, it is essential to understand the energetical barrier associated with each H₂ release step. It has been shown by Baldé et al. [8] that the decrease in NaAlH₄ particle size decreases the activation energy, E_a, of the decomposition process and is dependent on the particle size, where the E_a decreases from 116 kJ mol⁻¹, in the case of bulk material, to 58 kJ mol⁻¹, in the case of nanoparticles from 2 to 10 nm.

In this work, the influence of nanoconfinement on the H₂ release processes from NaAlH₄ is presented in depth. Bulk NaAlH₄, NaAlH₄ deposited as bulk material on a carbon support material, and completely nanoconfined NaAlH₄ materials are synthesized, and H₂ release curves at constant T ramp rates, β = dT/dt, are measured and analyzed. By comparing E_a values of H₂ release from different phases of NaAlH₄, i.e., bulk, deposited as bulk on a supporting material, and nanoconfined, a more complete picture of the effect of nanoconfinement on the H₂ release processes from NaAlH₄ is obtained and presented.

2. Materials and Methods

2.1. Synthesis of NaAlH₄/Carbon Composites

NaAlH₄ is deposited on a high surface area, specific surface area = 1840 m² g⁻¹, microporous, most pores with widths under 2 nm, and high porosity, pore volume = 0.82 cm³ g⁻¹, dry carbon material RP-20 (Kuraray, Japan) through the solution impregnation method. Tetrahydrofuran, THF (anhydrous, ≥99.9%, Sigma-Aldrich, Germany), is used as the solvent, and a 0.05 g_NaAlH4 mL_THF⁻¹ solution is used for all samples. Bulk NaAlH₄ (90%, Sigma-Aldrich, Germany) was dissolved and filtrated through a glass microfiber filter (GF/B, Whatman, UK) to remove any insoluble impurities. Samples with 5, 60, and 100 mass% of deposited NaAlH₄ are synthesized to yield the nanoconfined, bulk-deposited, and bulk NaAlH₄, respectively, and are denominated as such. In the case of the bulk NaAlH₄ sample, recrystallization is performed at identical conditions to get rid of any impurities, but without the carbon support material. All syntheses, sample storage, and sample preparation for further analysis were performed in an Ar-filled (5.0, Linde) glovebox (MBraun LABmaster sp, Germany). A full description of the synthesis process and the characterization of the synthesized materials is provided in [13].

2.2. Temperature Controlled Decomposition

The H₂ release curves at constant β were measured with the Autochem 2950HP (Micromeritics, USA) chemisorption analyzer. The amount of released H₂ was determined with a thermal conductivity detector, where a 50 mL min⁻¹ N₂ (6.0, Linde) gas flow was used as the carrier gas. β from 0.5 °C min⁻¹ to 10 °C min⁻¹ were applied. A full description of the sample preparation and measurement routine is brought in [13]. Data reduction, data analysis, and H₂ release peak fitting were performed with the OriginPro Version 2016 (OriginLab Corporation, Northampton, MA, USA) software.

3. Results

3.1. Hydrogen Release from NaAlH₄/Carbon Composites

The bulk, bulk-deposited, and nanoconfined NaAlH₄ exhibit distinct H₂ release curve shapes (Figure 1) in the case of all β used. In the case of the bulk NaAlH₄, a low amount of H₂ is released at ~170 °C, which is just under the T for melting of bulk NaAlH₄. The release of H₂ at ~170 °C is most likely the release of H₂ from the surface layer and defect sites, for which the melting T is lower, and thus, H₂ is released before the melting of the bulk phase. Starting from T = 200 °C, H₂ is released from the bulk of the material through the decomposition steps Equations (1) and (2), and at T > 280 °C H₂ is released from the last decomposition step, Equation (3). At T > 375 °C, all of the H₂ has been released as the material has been completely decomposed.

In the case of bulk-deposited NaAlH_4, a very low amount of H₂ is already released, starting at T < 90 °C. The T at which the initial H₂ release starts depends strongly on the β applied and is released at an almost constant rate. This H₂ release step is caused by the small amount of NaAlH₄ deposited in an amorphous state (highly polycrystalline structure) and/or as separate nanosized particles during the recrystallization process. Starting from T = 165 °C, a high amount of H₂ is released over a narrow T range and is followed by a high amount of released H₂ over a wide T range up to ~200 °C, after which a low amount of H₂ is released at a constant rate up to T ~ 375 °C. These three steps correspond to NaAlH₄ decomposition: Equation (1) from the surface, Equations (1) and (2) from bulk, and Equation (3), respectively. For a better deconvolution of the H₂ release from different decomposition steps and processes, the whole H₂ release curve has been fitted with distribution functions (presented in the next section).

In the case of nanoconfined NaAlH₄ H₂ release starts already at ambient conditions, T < 23 °C in case of all applied β. H₂ is mainly released at T < 200 °C, with the shape of the H₂ release curve strongly dependent on the applied β. In addition, a very low amount of H₂ is released at T > 400 °C, which is most likely H₂ released from the decomposition of NaH. Thus, deconvolution of the H₂ release peaks through fitting is necessary to understand the decomposition processes of nanoconfined NaAlH₄.

The amount of H₂ released, compared to the theoretical amount of H₂ in the NaAlH₄ of the sample, yields the H₂ content efficiency of the investigated materials. The average H₂ content efficiencies are ~100%, ~66%, and ~40% for bulk, bulk-deposited, and nanoconfined, respectively. Thus, the amount of recovered H₂ from NaAlH₄ decreases with the deposition process, especially with the nanoconfinement of NaAlH₄. The decrease in H₂ content efficiency is likely caused by the decomposition of NaAlH₄ during synthesis, where the step-by-step addition of NaAlH₄ in THF solution to the carbon support promotes the decomposition of NaAlH₄, especially at the initial addition steps where NaAlH₄ is deposited as nanosized particles. In addition, as the H₂ release from nanoconfined NaAlH₄ begins at ambient conditions, it is very likely that some of the nanoconfined NaAlH₄ decomposes during storage at ambient conditions, i.e., at 21 °C and in Ar gas environment.

3.2. Modelling of Decomposition Processes

To better understand the decomposition processes of NaAlH₄ in different phases, e.g., bulk, bulk-deposited, and nanoconfined, the H₂ release curves were fitted with a combination of gaussian and bigaussian peak functions (Figure 2). The bigaussian, i.e., an asymmetric gaussian profile distribution peak function which has two independent peak widths, w₁ and w₂, at x < x_c (Equation (4)) and x ≥ x_c (Equation (5)), respectively, where x_c is the position of the peak center

y = y₀ + H × exp(−0.5 × (x − x_c/w₁)²) if (x < x_c)

(4)

y = y₀ + H × exp(−0.5 × (x − x_c/w₂)²) if (x ≥ x_c)

(5)

where y₀ is the baseline and H is the peak height.

The bigaussian profile of the H₂ release is caused by the kinetic limitation of some decomposition steps, where the increase in the H₂ release amount with the initial rise in T is quick. After reaching maximum H₂ release from the process at temperature T_max, the H₂ release process continues over a wide T range as H₂ release from the decomposition process is kinetically hindered.

From the fitting of the H₂ release curves, T_max is obtained for each decomposition process. The range of obtained T_max, determined from H₂ release curves measured at different β, is presented in

Table 1. Hydrogen release peaks and Kissinger method fitting results.

Material	Most likely H2 Release step	Temperature Range of Tmax/°C	Ea/kJ mol−1	Additional Comments
Bulk	Equation (1)	167–172	–	Low amount, most likely only surface layer, no clear dependence of Tmax on β
Bulk	Equation (1)	230–243	270 ± 10	Over a wide T range, from the whole phase
Bulk	Equation (2)	240–264	150 ± 60	Surface layer, low amount
Bulk	Equation (2)	245–279	120 ± 20	From the whole phase
Bulk	Equation (3)	308–337	150 ± 30	From the whole phase
Bulk-deposited	Equation (1)	129–165	–	Over a wide T range, no clear dependence of Tmax on β. Most likely decomposition of nano- and surface layer.
Bulk-deposited	Equation (1)	165–174	400 ± 50	H2 release over a very narrow T range
Bulk-deposited	Equation (1)	174–190	170 ± 20	From the whole phase
Bulk-deposited	Equation (2)	180–227	80 ± 10	From the whole phase
Bulk-deposited	Equation (3)	263–325	90 ± 20	From the whole phase
Nanoconfined	Equation (1)	51–57	70 ± 40	Only present at high β
Nanoconfined	Equations (1) and (2)	41–94	30 ± 5	Overlap strongly, both over a wide T range
Nanoconfined	Equations (1) and (2)	102–180	45 ± 5
Nanoconfined	Equation (3)	407–452	210 ± 30	Low amount

with the corresponding designations for each process. It is possible to calculate the E_a of a first-order reaction using the Kissinger method [8,15]

ln(β/T_max²) = ln(Z × R/E_a)− E_a/(R × T_max),

(6)

where R is the ideal gas constant, and Z is the Arrhenius pre-exponential factor. By plotting ln(β/T_max²) vs. 1/T_max, the slope will yield −E_a/R, and thus, E_a can be calculated. The E_a values of the deconvoluted H₂ release processes have been calculated from the slope of the Kissinger plot (Figure 3, Table 1).

In the case of bulk NaAlH₄, all the activation energies are relatively high, >100 kJ mol⁻¹, as the H₂ release processes are kinetically hindered by the diffusion of H₂ out of the bulk phase and by the thermal conductivity of the bulk phase, where the H₂ release processes from NaAlH₄ are endothermic [16], and thus preventively cooling the surrounding material upon decomposition. Even though the temperature of initial H₂ release from the first decomposition step depends on the applied constant β, the low amount of H₂ released hinders proper fitting. Thus, the obtained T_max does not remarkably depend on applied constant β. Therefore, the E_a could not be calculated and is most likely caused by the release of H₂ from the surface of NaAlH₄ particles over a wide range of T values under the melting T of bulk NaAlH₄. The highest E_a from the bulk material is for the second peak, which corresponds to Equation (1) reaction step from the bulk of the material. This H₂ release step is over a wide T range, as the H₂ diffusion length can vary widely in large NaAlH₄ particles, and thus H₂ release from inside the NaAlH₄ particles occurs at remarkably higher T values. Simultaneously with H₂ release from the Equation (1) reaction step, H₂ release from the Equation (2) reaction step starts. The H₂ release from the Equation (2) reaction step is discernably taking place in two distinct stages, as H₂ from the surface is released first, and then the H₂ from inside the particles is released. H₂ release from the last irreversible reaction step, Equation (3), is clearly separatable and has a relatively high E_a, 150 kJ mol⁻¹, as the decomposition products from preliminary H₂ release steps limit the kinetics of H₂ release from this step.

A similar case can be seen for bulk-deposited, but the T_max of all H₂ release steps are lowered, and the E_a values of the last two steps are also lower. This is most likely caused by the partial confinement of the decomposition products inside the porous carbon structure, enhancing the kinetics of the H₂ release step corresponding to Equation (2). This restructuring and the disappearance of clear crystalline phases upon the dehydrogenation of bulk-deposited NaAlH₄ has been shown before [10,13].

In the case of nanoconfined NaAlH₄, three separate H₂ release steps are identified under one continuous H₂ release peak, starting from ambient T and continuing up to 200 °C, where the H₂ release step starting with T_max from 51 to 57 °C is discernible only at a high applied constant β. All these H₂ release steps have relatively low E_a values, compared to bulk and bulk-deposited, are overlapping, and do not clearly correspond to a discrete H₂ release reaction. Baldé et al. have shown that the T_max of H₂ release decreases remarkably with the decrease in the nanoparticle size [8], and Gao et al. have shown that nanosized NaAlH₄ decomposes directly into NaH [7]. Thus, the indiscernibility of the H₂ release steps of nanoconfined NaAlH₄ is very likely caused by a distribution of differently sized NaAlH₄ nanoparticles, which decompose at different T values directly into NaH, going through the Equations (1) and (2) reactions in one step. In addition, the decomposition of the nanoconfined NaAlH₄ at ambient conditions, based on H₂ content efficiency, makes the assignment of the H₂ release peaks a nontrivial task. Regardless, the calculated E_a values, from 30 to 45 kJ mol⁻¹, of H₂ release steps discernible at all applied constant β from nanoconfined NaAlH₄ are notably lower than the value of 58 kJ mol⁻¹ achieved for 2–10 nm NaAlH₄ particles [8]. The H₂ release step detected at the highest T, most likely from the Equation (3) reaction step, releases a low amount of H_2, and the T_max of this step is 100 °C higher than that of bulk and bulk-deposited materials and has an increased E_a. Thus, it is possible that nanoconfined NaAlH₄ does not decompose completely to Na and Al under the investigated conditions, as, for some reason, nanosized NaH is more stable than bulk NaH. This would improve the cyclability and usability of nanoconfined-NaAlH₄-based H₂ storage systems, as the decomposition of NaH is irreversible and, thus, unwelcome. The increased stability of the nanoconfined NaH phase must be investigated further.

The addition of fitting the H₂ release curves to deconvolute different decomposition steps before applying the Kissinger method has clearly shown that various activation barriers are present even in the case of nanoconfinement. This increases our understanding of the fundamental processes involved in releasing H₂ from NaAlH₄, especially when nanoconfined, which may prove vital when improving and optimizing such materials for H₂ storage applications.

The achieved activation energies for the reversible steps are considerably lower than most activation energies of mixed hydride [17,18,19], doped hydride [20,21,22,23,24,25], and nanoconfined hydride [9,26,27] systems described in the literature. Those include systems that utilize NaAlH₄ [9,19,24,25,27], but also other metal and complex hydrides [17,18,20,21,22,23,26]. Most activation energies for bulk hydrides are well over 100 kJ mol⁻¹. The achieved improved energies in the referred papers are mostly larger, by at least a factor of two, than the ones presented in this paper, with relatively few coming into the range of 30–45 kJ mol⁻¹ [22,27]. This is remarkable, taking into account the simplicity of the system and its synthesis. Doping involves adding transition metals or compounds, which often adds a synthesis step of varying complexity. Mixtures may have side reactions, which are complex to analyze and may result in unwanted products or irreversible alloys [17,18,19]. Using a scaffold material for confinement improves cycling stability [13], which is essential for practical applications. In this work, the results have been achieved using a simple commercial microporous carbon, which offers lower E_a values than laboratory-made materials [26,27] and potentially higher loadings than, e.g., metal-organic frameworks [9]. Doping, ball-milling, and melt-infiltration, which even occurs naturally upon cycling above the melting temperature of NaAlH₄, is expected to improve the material’s hydrogen storage properties even further.

4. Conclusions

The deconvolution of H₂ release processes through curve-fitting procedures to determine T_max when applying the Kissinger method significantly improved the amount and quality of the information received about the H₂ release processes in bulk, bulk-deposited, and nanoconfined NaAlH₄. The coating of porous carbon particles with a thick layer of NaAlH₄, bulk-deposited, is shown to decrease the T_max of all H₂ release steps, whereas only the E_a values of the last steps are lowered. This indicates that whilst a supporting material reduces the T at which H₂ is released, the H₂ release processes are still limited by the H₂ diffusion out of the bulk phase. In the case of nanoconfined NaAlH₄, H₂ release starts at ambient T, and the E_a of H₂ release processes is remarkably lowered in comparison to bulk and bulk-deposited NaAlH₄. None of the H₂ release steps are limited by H₂ diffusion in the case of nanoconfined NaAlH₄. Even though the nanoconfined NaAlH₄ did not have a discrete particle size, based on the discernability of multiple H₂ release steps, very low E_a values from 30 to 45 kJ mol⁻¹ are determined. These E_a values are a significant improvement on the vast majority of previously used techniques for nanoconfinement. The H₂ release at ambient T and the very low E_a of the nanoconfined NaAlH₄, in addition to limiting the final, irreversible decomposition step, highlights the possible utility of nanoconfinement for the improvement of NaAlH₄-based H₂ storage materials, where the critical problem is synthesizing nanoconfined composites with a high total H₂ yield per mass and/or volume.