The Use of Phase Change Materials for Thermal Management of Metal Hydride Reaction

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The integration of phase change composite improves energy storage of hydrogen in metal hydride system by absorbing and releasing the heat generated from the reaction.

Abstract

To meet the massive increase in energy demand, extensive research has been conducted over the past few decades on developing clean and sustainable energy storage methods. Hydrogen is considered as one of the most promising future energy carriers due to its high energy density and renewability, but it requires storage. Storing hydrogen using metal hydride offers several advantages, including stability, safety compactness and reversibility of the hydrogen absorption/desorption process. Thermal management during hydrogen storage using metal hydride is critically important since the reaction between the metal and hydrogen is highly exothermic. We are aiming to develop thermal storage systems based on composite phase change materials (CPCMs) that absorb the heat generated during hydrogen absorption and release it during desorption, in an effort to improve energy storage efficiency. Lightweight, shape-stable CPCMs are prepared by loading the selected organic phase change materials into expanded graphite and hydrophobic monolithic silica aerogel. The chemical structure, microstructure, thermal properties and leakage of CPCMs are investigated. These samples were subjected to variable power electrical heating to simulate the heat generated during hydrogen reaction, forming lanthanum hydride, according to its published reaction kinetics.

1. Introduction

To meet the massive increase in energy demand, extensive research has been conducted on sustainable energy generation and storage over the past few decades. Hydrogen has been considered as one of the most popular energy carriers and secondary sources of clean energy due to its high energy density of 120 MJ/kg, clean-burning nature and versatility [1]. Hydrogen energy storage is needed to store surplus energy generated from renewable resources such as geothermal, solar and wind, and release it again when needed [2].

Conventional physical-based hydrogen storage technologies (e.g., compressed hydrogen gas, cold-/cryo-compressed hydrogen and liquid hydrogen) require either tanks with heavy walls (to sustain pressure of 350–700 bar) or cryogenic temperature (<−252.87 °C, at 1 atm), both of which are costly to maintain [3]. Therefore, material-based storage technologies, particularly solid-state metal hydrides (MH) have gained significant attention and are developing rapidly, due to their competitive advantages, including stability, safety, high volumetric density, high storage capacity and the ability to store hydrogen at or near atmospheric conditions [4].

While novel metal hydride materials are being rapidly developed, increasing attention is being directed toward enhancing the performance of existing metal hydrides for engineering applications. It is well known that the hydrogenation reactions of metals forming metal hydride are accompanied by significant enthalpy changes. These changes require effective and efficient heat removal or addition to maintain thermodynamically favorable conditions [5]. Sanjay et al. studied the thermodynamics and kinetics of hydrogen absorption–desorption in highly crystalline LaNi₅, reporting that the enthalpy of the intermetallic was −40 ± 7 kJ mol⁻¹ of hydrogen for absorption [6]. These reversible exothermic metal hydride reactions require effective heat removal to drive the reaction forward as dictated by reaction equilibrium. In practical applications, metal hydride powder is often packed into a reactor, where heat transfer becomes more challenging. Therefore, well-designed thermal management is essential for improving the energy efficiency and cost effectiveness of metal-hydride-based storage systems.

Within the various thermal management strategies, latent heat storage using phase change materials (PCMs) is a promising technology, offering higher absorbing energy density than sensible heat storage and greater simplicity compared to thermochemical energy storage [7]. Proposed designs for incorporating PCMs into metal hydride storage range from simple jacketed tanks [8] through to reactors with a multitude of jacketed pipes [9]. The former are relatively simple structures but have low cross-sectional area for heat transfer between the PCM and metal hydride and the layers of each are thick. This means high thermal conductivity is required in both the PCM and the metal hydride for the storage to respond quickly to the loading and unloading of hydrogen. The jacketed pipe reactors are designed to allow lower thermal conductivity in each layer, but this comes at the cost of greater complexity.

The most common PCMs are solid–liquid type and can be catalogized into organic, inorganic (including liquid metal) and eutectic PCM materials. Organic PCMs, such as paraffin wax (C_nH_2n+2), are widely used in low- to medium-temperature thermal storage due to their wide range of phase change temperatures, high heat storage density, low supercooling and low cost. However, their poor thermal conductivity (0.1–1 W/m K) reduces the rate of heat transfer and fails to meet the heat dissipation requirements of some high heat flux chemical reactions [10]. In contrast, the room-temperature liquid metal phase change material gallium (Ga), which has a melting point of 303k and high thermal conductivity (~33 W/m K) [11], exhibits excellent heat transfer performance, as demonstrated in a gallium-based heat sink [12]. Although gallium has only moderate mass-based latent heat of melting, its high density gives it one of the highest volumetric energy densities among known PCMs. Unlike many organic PCMs, liquid-state gallium has a higher density than its solid form, resulting in reduced volume upon melting [13]. However, the drawback of gallium is its high cost relative to many organic PCMs, which significantly limits its large-scale application.

The tendency of PCMs to leak, particularly in their molten state, is a major concern in the application of solid–liquid PCMs for thermal energy storage system. To enhance thermal conductivity and reduce the leakage, various encapsulation techniques, such as in situ polymerization and impregnation, have been developed to produce core–shell structure PCMs or shape-stabilized PCMs [14,15,16]. Among these, compressed expanded graphite (CEG)-based PCM composites stand out due to their enhanced thermal conductivity and minimal leakage [17,18,19]. Recently, a new ultralight, highly porous 3D silica aerogel-based PCM with excellent flame retardancy, shape stability and chemical stability has attracted significant attention for the practical energy storage application. However, a major limitation of silica aerogel is its inherently low thermal conductivity (about 20~40 mW/m∙K) [20,21]. Khodadadi and Hosseinizadeh demonstrated that adding metal nanoparticles into PCMs can significantly enhance their thermal conductivity compared to the base PCMs. Similarly, Deng and co-workers fabricated graphite matrix–octadecane/gallium composites and reported that even a small amount of gallium loading can markedly improve the thermal conductivity of the composite PCMs [22].

Effective thermal management in metal hydride hydrogen storage reactors require composite phase change materials (CPCMs) that exhibit high phase change enthalpy, high thermal conductivity, and therefore, the design and fabrication of CPCMs must be carefully optimized to meet these functional requirements.

This study explores the potential for the low loading of gallium to counteract the insulating effect of an aerogel matrix material. In this study, compressed expanded graphite (CEG)-based and monolithic silica aerogel (Siagl)-based shape-stabilized composite PCMs were synthesized and evaluated for their suitability in thermal management within metal hydride hydrogen storage systems. The objective is to develop CPCMs capable of operating at temperatures of approximately 290–340 k, with the aim of enhancing the thermal efficiency of metal hydride-based reactors. The selection of an appropriate PCM is guided by several key criteria, including its phase transition (melting) temperatures, specific heat storage capacity, long-term thermal stability with freezing/melting cycling, and cost effectiveness. Based on these considerations, low-cost paraffin, a saturated hydrocarbon with good chemical compatibility under target reaction conditions and a melting point of about 38 °C was selected for use in this research. To improve the thermal conductivity of the PCM, a small amount of (~2 wt%) in situ synthesized gallium micro-/nanoparticles has been dispersed into the pure paraffin. The thermal physical properties of paraffin (PCM38) and gallium are illustrated in

Table 1. Thermal physical properties of paraffin (PCM38) and gallium [12].

Parameters	Paraffin (PCM38)	Gallium
Melting temperature (°C)	36–38	29.8
Latent heat (kJ/kg)	226	80
Volumetric latent heat (MJ/m3)	188	472
Liquid density (kg/L)	~0.77	6.095
Solid density (kg/L)	0.83	5.904
Specific heat (kJ/(kg·K))	2	0.4
Volume expansion coefficient (%)	7–8%	3.1
Thermal conductivity (W/(m·K))	0.24	29.28
Type	S-L	S-L

.

The thermo-physical properties of the fabricated CEG-based and aerogel-based CPCMs are tested, and their thermal management performance is evaluated using an electric heater with a varying power level to simulate the heat generated during the hydrogenation reactions of LaNi₅, a commercially available material that is easy to activate and can absorb hydrogen at target operating condition [23]. This study presents a novel approach that involves the theoretical prediction of transient heat generation during the metal hydride formation reaction, which is then applied in an experimental setup utilizing an electrically heated surface. The integration of a CPCM for surface cooling in this configuration, aimed at evaluating its thermal performance, has not been previously reported in the literature.

2. Materials and Methods

2.1. Materials

Paraffin (PCMCOOL-PCM38) was purchased from Shanghai Ru entropy New Energy Technology Co. Ltd., Shanghai, China. Gallium (≤100%) was provided by Sigma Aldrich. Compressed expanded graphite was supplied by Duranice Applied Materials (Dalian), Co., Ltd., Dalian, China. The hydrophobic monolithic silica aerogel was synthesized in-house according to the method described in the literature using methyltriethoxysilane [24].

2.2. Fabrication of CPCMs

In this work, high-porosity compressed expanded graphite (ρ_bulk = 170–200 kg/m³) and monolithic silica aerogel (ρ_bulk = 100–120 kg/m³) were used as matrix materials to host the PCMs. The matrix was submerged in a bath of PCM (either paraffin or paraffin with gallium) at 40 °C and 1 atm for a certain time (

Table 2. Impregnation of CEG or aerogel with paraffin and gallium-doped paraffin at 40 °C, 1 atm.

Sample ID	Matrix	PCM	Temp. (°C)	Time	PCM Loading Mass %
CPCM-CEG-0	CEG	paraffin	40	18 h	74–77
CPCM-CEG-1	CEG	Ga/paraffin	40	18 h	74–77
CPCM-Siagl-0	aerogel	paraffin	40	10 min	87–89
CPCM1-Siagl-1	aerogel	Ga/paraffin	40	10 min	87–89

), followed by placement into an oven at 50 °C for 10 min to remove excess PCM from the outer surface. The gallium/paraffin was prepared by directly dispersing gallium (2 wt%) into paraffin in a beaker at 40 °C for 10 min, followed by ultrasonication for 5 min. Figure 1 displays the morphology of the pure paraffin and gallium particles/paraffin. The gallium particles were coated and stabilized with a large amount of paraffin, preventing aggregation and potential exposure to air, which has important implications for enhancing their long-term stability (Figure 1b). The PCM percentage absorption was determined from the increase in mass of the matrix after immersion (Equation (1)). The percentage mass was used to calculate the theoretical latent heat of CPCMs.

$$\mathrm{\%}PCM={\displaystyle \frac{m_1-m_0}{m_1}}\times 100$$

(1)

where:

-

m₁ is the mass of CPCM

-

m₀ is the mass of 3D matrix material

2.3. Characterization

2.3.1. Thermal Properties of CPCMs

The morphologies and microstructures of aerogel, CEG and CPCMs were analyzed using scanning electron microscope (JEOL JSM-6500 and JEOL JSM-6610LA), and energy dispersive spectroscopy (EDS) mapping was used to perform the element distribution and compositions of samples.

The thermal properties of the samples were measured using differential scanning calorimetry (DSC, equipment model: NETZSCH DSC 3500, NETZSCH Analyzing & Testing, Selb, Germany). The tests were performed at a ramping rate of 0.5 °C/min over a temperature range of −10 °C to 70 °C under a flow of nitrogen gas. An isothermal hold of 5 min was implemented at the end of each heating and cooling stage to allow samples to reach thermal equilibrium.

The thermal stability of synthesized aerogel-based CPCMs was evaluated by repetitive thermal cycling in water using a programmable circulating bath (equipment model: AP07H200-A12E, PolyScience, Touhy Avenue Niles, IL, USA). The mass and the thermophysical properties (melting and latent heat) of samples were measured for 100 heating and cooling cycles. During the measurement, the samples were enclosed in thin plastic bags (inset, Figure 2). Each cycle was performed over a temperature range of 20–50 °C, with heating rate 15 °C/min from 20 °C to 50 °C, followed by a 30 min isothermal hold at 50 °C. The thermal stability after cycling was assessed by comparing the DSC results obtained before and after 100 cycles, following the DSC testing procedure described above but measured at a ramping rate of 3 °C/min.

2.3.2. Thermal Performance Testing Experimental Design and Setup

A thermal performance apparatus was developed to conduct two types of experiments: measuring material response to a constant heat flow and simulating one-dimensional heat flow through a CPCM in response to a short burst of heat from hydrogenation of a hydrogen storage alloy (Figure 3). The thermal source in the apparatus is a small flat cartridge heater (2 cm × 2 cm), controlled by a programmable DC power supply. A 5 mm thick CPCM sample of the same dimensions was placed directly on the top of the heater and heated from the bottom side. All other sides were insulated to approximate one-dimensional heat flow through the CPCM. This is to align with a practical configuration; the CPCM would be cylindrical so heat would only flow in the radial direction.

The change in temperature with time at three points along the heat flow direction were measured using K-type thermocouple (0.5 mm in diameter) fixed in the CPCMs and recorded by a data logger and computer. The measuring points in the sample were at three locations: TB (between the heater and CPCM), TM (in the middle of the CPCM) and TT (on top of the CPCM).

The thermal response of full-size CPCMs was tested by providing a propagation heat flux (at a constant applied voltage of 3.3 V) to the CPCMs.

To simulate the heat generated during the metal alloy hydrogenation (LaNi₅), the electrical input to the cartridge heater varied with time from peak to almost zero. This has been done based on the thermodynamics and kinetics of the hydrogen reaction to form LaNi₅, as reported in the literature [25,26,27], and as follows:

The reaction forming LaNi₅ can be described by Equation (2):

$$({\displaystyle \frac{1}{3.1}})LaN{i}_5+H_2\rightleftharpoons ({\displaystyle \frac{1}{3.1}})LaN{i}_5{H}_{6.2}$$

(2)

If the partial pressure of hydrogen remains almost constant—assuming an excess or continuous supply of hydrogen in the metal hydride reactor—the reaction kinetics can be primarily governed by temperature-dependent factors. Under these conditions, the reaction may be assumed first order with respect to the non-reacted metal concentration according to the Johnson–Mehl–Avrami (JMA) model for LaNi₅ [26]. Assuming the reaction is first order with respect to LaNi₅ concentration and constant hydrogen concentration, then

$${\displaystyle \frac{d\left[x\right]}{dt}}=-k\left[x\right]$$

(3)

where:

-

k is the rate constant, S⁻¹

-

[x] the concentration of LaNi₅

-

t is the reaction time, S

Integrating both sides of the Equation (3),

$${\int }_{{\left[x\right]}_0}^{{\left[x\right]}_t}{\displaystyle \frac{1}{\left[x\right]}}dx=-{\int }_{t_0}^{t}kdt ⟹\mathit{ln}{\left[x\right]}_t=ln[x{]}_0-kt⟹{\left[x\right]}_t=[x{]}_0{e}^{-kt}$$

(4)

where:

[x]₀—is the initial concentration of LaNi₅

[x]_t—is the concentration of LaNi₅ after given time t

Following the Arrhenius relationship between the reaction rate and temperature,

$$k=A{e}^{({-}^{E_a}{/}_{RT})}$$

(5)

where:

T—is assumed as 311 K to match the temperature of the PCM.

A—is the pre-exponential factor for LaNi₅, equal to 1856 S⁻¹

E_a—is the activation energy, 30.1 kJ/mol H₂ at near 1 atm.

So,

$$k=1856 S^{-1}\times \mathit{exp}(-{\displaystyle \frac{30.1\times 1000 \mathrm{J}·{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}}{8.314 {\mathrm{J}·\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}·k^{-1}\times 311 K}})=0.0163{ S}^{-1}$$

(6)

Therefore,

$${\left[x\right]}_t=[x{]}_0{e}^{-0.0163t} or {\displaystyle \frac{d\left[x\right]}{dt}}=-0.0163[x{]}_0{e}^{-0.0163t}$$

(7)

The enthalpy of reaction (2) is −99.5 ± 0.3 kJ∙mol⁻¹ $(∆{rH}_m^0$) [27], hence the heat released from the reaction per second is

$$\begin{array}{l}Q=99.5 {\mathrm{k}\mathrm{J}·\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}\times 0.0163{ S}^{-1}{\left[x\right]}_0{e}^{-0.0163t}\\ Q=1.622[x{]}_0{e}^{-0.0163t}\end{array}$$

(8)

If we select arbitrary initial concentration [x]₀ = 6.17 M, we obtain the following equation for the rate of heat generated:

$$Q=10e^{(-0.0163t)}\mathrm{watt}$$

(9)

Then, the heat generated from LaNi₅ as obtained from theoretical calculation is illustrated in Figure 4a. Figure 4a shows the calculated heat generated from the reaction, which decreases exponentially in a short time. The experimental DC power is set up accordingly (Figure 4b).

3. Results and Discussion

3.1. Structure Characterization

The morphology of porous CEG and aerogel matrix and their based composite with paraffin is shown in Figure 5. Both matrices are highly porous with large open pores (Figure 5a,d). The CEG displays a web-like structure, while aerogel exhibits a coral-reef-like fibrous structure. The morphologies of the CEG-based PCMs (Figure 5b,c) and aerogel-based PCMs (Figure 5e,f) show that the compressed expanded graphite and aerogel matrices retain their structure on impregnation with paraffin.

The SEM image and corresponding EDS mapping (Figure 6) of a cross-section of the aerogel-based gallium–paraffin CPCM reveal the presence of gallium particles, exhibiting a broad size distribution ranging from the nanoscale to the microscale, uniformly distributed throughout the aerogel coral structure (Figure S1).

3.2. Thermal Properties

The DSC melting curves of pure paraffin, Ga/paraffin mixture and composite PCMs are shown in Figure 7. The thermal properties derived from these curves are shown in

Table 3. Melting point and latent heat of paraffin, Ga/paraffin mixture and composites.

Name	Onset (°C)	End Set (°C)	M.P. (°C)	ΔH (J/g)	PCM Loading Mass %	Theoretical Latent Heat (J/g) 1
Paraffin	37.1	39.2	38.7	227	-	-
Ga/Paraffin	37	39	38.3	230	-	-
CPCM-CEG-0	36.3	38.4	38.1	170	76.3	173
CPCM-CEG-1	36.2	38.8	38.3	175	76.2	175
CPCM-Siagl-0	36.2	40.6	39.5	197	88.1	199
CPCM-Siagl-1	36.3	39.9	38.6	195	87.7	200

¹ Theoretical latent hear was calculated using the latent heat values of pure paraffin and Ga/paraffin mixture.

.

The melting curves for the two PCM materials tested (Figure 7a) show a negligible difference between pure paraffin and the paraffin mixture with gallium. There is a very slight reduction in melting point for the gallium mixture. While this is consistent with gallium having a lower melting temperature than the paraffin, the difference is within the measurement error of the instrument, so for the purposes of analysis they are assumed to be the same.

The melting curves for the CPCMs (Figure 7b) show a broadening of the peak and slight shift in melting point with the aerogel-based composites. This may be attributed to the insulating effect of the aerogel matrix slowing the transfer of heat into the sample. This is similar to the peak shift seen in DSC measurement of PCMs when the heating rate is too high [28,29]. Generally, however, in the CPCMs the phase change temperature remains quite similar to the phase change temperature as pure paraffin, as there is a major peak at approximately 38 °C.

The latent heat for the samples, in Table 3, shows that the CPCMs have a lower latent heat than pure PCM, and that the compressed expanded graphite CPCM has a lower latent heat than the aerogel CPCM. This is consistent with there being a lower quantity (by weight percentage) of PCM in the CPCMs and the compressed expanded graphite CPCM having the lowest weight percentage PCM. The effect of gallium on the latent heat is inconclusive and this is likely due to the very low concentration used and potential variability in loading.

3.3. CPCMs Thermal Cycling Stability

The thermal cycling stability of aerogel CPCM with (CPCM-Siagl-1) and without gallium (CPCM-Siagl-0) was assessed by comparing their melting points and latent heat values before and after 100 thermal cycles using DSC. The results in Figure 8 show that the melting point and latent heats remained nearly the same. This indicates that the CPCMs are thermally stable over repeated phase change cycles. The SEM images, corresponding EDS spectrum and elemental mapping of a cross section of the aerogel-based paraffin–gallium composite after cycling test (Figures S2 and S3) revealed no notable structural deformation. Furthermore, the atomic ratio of oxygen to silicon in the EDS spectrum is approximately 2:1, consistent with the stoichiometry of silica. Elemental mapping of gallium and oxygen further suggests that gallium oxidation during cycling is minimal, indicating good overall stability of the composite.

3.4. Thermal Response of CPCMs to Surface Temperature Increase

The results from testing the CPCMs in the thermal performance apparatus are shown in Figure 9. These results show how the temperature of the samples changes during constant heat input. The temperature profiles (Figure 9a) for the compressed expanded graphite CPCM (CPCM-CEG-0 and CPCM-CEG-1) show the heating process can be divided into three distinct stages: solid sensible heating, latent heat storage and liquid sensible heating. During the solid sensible heat storage stage (before 12 min), the temperatures at all measurement points increase rapidly, attributed to the high thermal conductivity of CEG-based composites, consistent with findings reported in previous studies [18]. Notably, sample CPCM-CEG-1, which incorporated gallium/paraffin, exhibits a narrow temperature gradient across the sample (from bottom to top) compared to CPCM-CEG-0, which contained only base paraffin (CPCM-CEG-0). This suggests the addition of gallium has enhanced the thermal conductivity of the PCM impregnated in the graphite. During phase change (at about 12–70 min), the temperature at all the measurement points remains nearly constant (plateaus) in both samples. Following the completion of the melting process (after 70 min), a sharp rise in temperature is observed at all measured points.

The temperature curves (Figure 9b) recorded for the aerogel-based composites (CPCM-Siagl-0 and CPCM-Siagl-1) are different than those for compressed expanded graphite. Significant temperature gradients are observed in both aerogel-based samples in the latent heat storage stage, indicating limited internal heat transfer. Although the CPCM incorporating gallium (CPCM-Siagl-1) exhibits a slightly narrower temperature gradient compared to CPCM-Siagl-0, the gradient remains large due to the inherently low thermal conductivity and slow heat transfer characteristics of the silica aerogel matrix.

3.5. Simulated Behaviour of CPCMs in Response to Heating from Hydrogen Absorption in LaNi₅

The temperature profiles in Figure 10 show the response of the CPCMs to the simulated heat pulse from hydrogenation of a hydrogen storage alloy. The temperatures at the heater (TB), in the middle (TM) and top (TT) of the CEG matrix (Figure 10a), with no PCM, rapidly reached their peak value of approximately 100 °C, 90 °C and 80 °C, respectively, within 120 s. The small temperature gradient observed across the matrix is consistent with the high thermal conductivity of compressed expanded graphite. On cessation of the power input, a rapid temperature decline was observed, indicative of the material’s limited heat absorption capacity. By contrast, under the same experimental conditions, the aerogel matrix, with no PCM, exhibited a significantly different thermal response (Figure 10b). The temperatures at TB, TM and TT reached their peak values of approximately 120 °C, 80 °C and 40 °C, respectively, after 150 s. The large temperature gradient highlights the inherently lower thermal conductivity of the aerogel matrix, which hinders efficient heat transfer through the material. A similar rapid temperature decline was observed on termination of the heat flux, indicating that the aerogel matrix also possesses low heat capacity.

In comparison to the temperature profile of the pure matrices (Figure 10a,b), a thermal lag was observed in the CPCMs (Figure 10c,d). This is attributed to the large latent heat storage capacity of the PCM, which absorbs heat during the phase change process. This means PCM moderates temperature fluctuations and extends the duration of heat absorption, contributing to improved thermal regulation.

Furthermore, the results in Figure 10c,d show that CPCMs incorporating gallium/paraffin exhibited higher heat absorption compared to their corresponding CPCMs with paraffin only. The maximum temperature (surface temperature of the metal hydride) was reduced by approximately 6 °C and 12 °C for CEG-based CPCMs and aerogel-based CPCMs, respectively, with and without the addition of gallium particles. This enhancement in thermal performance can be attributed to the improvement in thermal conductivity resulting from the incorporation of gallium. A comparison of Figure 10c,d reveals that the CEG-based CPCMs reached their peak temperature within 120 s, which is shorter than the approximately 150 s required by the aerogel-based CPCMs. This suggests that CEG-based composite exhibits a more rapid heat absorption capability and that the addition of gallium in the aerogel matrix lowers the peak temperature close to that of the compressed expanded graphite matrix due to its enhancement of the thermal conductivity of the aerogel.

The gradual decay in the temperature profile later in the experiments is due to heat loss through the uninsulated top of the CPCM. A practical system would have insulation and possibly trace heating to prevent heat loss to the environment as heat leakage from the CPCM would impact the reversibility of the system by causing the over-cooling of the metal hydride during desorption of hydrogen.

4. Conclusions

This study compiles existing data on the reaction kinetics between hydrogen and lanthanum nickel, as well as the thermodynamics of the reaction, to calculate the transient heat generated. The predicted heat release was experimentally validated through transient electrical heating. This work introduces an approach that eliminates the need to perform the chemical reaction itself, while effectively demonstrating the capability of a composite phase change material (CPCM) to maintain a low surface temperature.

The study explored the potential for the low loading of gallium in PCM to counteract the negative impact of the insulating properties of an aerogel matrix. For comparison, CPCMs were fabricated by impregnating silica aerogel and compressed expanded graphite matrix materials with paraffin PCM and a paraffin PCM gallium mixture (2 wt%).

SEM revealed effective absorption of PCM within the porous structure of both matrices. SEM/EDS mapping further confirmed the distribution and presence of gallium particles within the paraffin phase of the CPCMs. The hydrophobic aerogel absorbed more of the PCM and did so faster than the compressed expanded graphite.

Thermal cycling tests demonstrated that the CPCMs fabricated were thermally stable over 100 repeated phase change cycles. The compressed expanded graphite based CPCMs showed the best thermal performance, which is expected due to the higher thermal conductivity of the matrix material. The addition of gallium to the aerogel-based CPCM, however, did bring thermal performance close to that of the graphite-based CPCM without gallium in the simulated heating from hydrogenation of a hydrogen storage alloy. This effect from a low loading of gallium offers potential to make hydrophobic aerogels suitable as a matrix material for CPCMs and suitable for practical thermal energy management applications in metal hydride reactors.