# Boosting hydrogen storage and release in MOF-5 / graphite hybrids via in situ synthesis

Laura Jimenez Lopez, Rafael Morales-Ospino, Jimena Castro-Gutiérrez, Haroon Osman Sumbhaniya, Giuseppe Sdanghi, Sergio García Dalí, Alain Celzard, Vanessa Fierro

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Laura Jimenez Lopez, Rafael Morales-Ospino, Jimena Castro-Gutiérrez, Haroon Osman Sumbhaniya, Giuseppe Sdanghi, et al.. Boosting hydrogen storage and release in MOF-5 / graphite hybrids via in situ synthesis. International Journal of Hydrogen Energy, 2025, 173, pp.151272. 10.1016/j.ijhydene.2025.151272. hal-05246631

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# Boosting hydrogen storage and release in MOF-5 / graphite hybrids via in situ synthesis

Laura Jimenez Lopez<sup>a</sup>, Rafael Morales-Ospino<sup>a, ID</sup>, Jimena Castro-Gutiérrez<sup>a, ID</sup>, Haroon Osman Sumbhaniya<sup>a, b</sup>, Giuseppe Sdanghi<sup>b</sup>, Sergio García Dalí<sup>a, ID</sup>, Alain Celzard<sup>a, c, *, ID</sup>, Vanessa Fierro<sup>a, ** ID</sup>

## ARTICLE INFO

Handling Editor: Umit Koylu

Keywords:

Metal-organic frameworks (MOFs)

Hydrogen storage

Adsorption

Exfoliated graphite

MOF hybrids

## ABSTRACT

This study focuses on the hybridization of MOF-5 with exfoliated graphite (EG) to improve hydrogen (H₂) storage and release performance. Using an in-situ synthesis approach, controlled amounts of EG, ranging from 1 to 10 wt %, were incorporated into MOF-5. Characterization showed that adding EG, particularly at 5 wt%, increased the micropore volume by 15%, which correlated with higher H₂ storage capacity. At 77 K and 40 bar, the 5 wt% EG hybrid achieved a gravimetric H₂ uptake of 5.3 wt%, a 14% increase over pristine MOF-5. Similarly, the 1 wt% EG hybrid reached 5.1 wt%, a 10% increase. Volumetric storage capacities also improved: the 5 wt% and 1 wt% EG hybrids reached 39.3 kg m⁻³ and 38.1 kg m⁻³, respectively, compared to 36.1 kg m⁻³ for unmodified MOF-5 and 31.0 kg m⁻³ for cryo-compressed H₂ in an empty vessel under the same conditions. Additionally, the 5 wt% EG hybrid exhibited improved H₂ release capacity at both 77 K and 160 K. These results demonstrate that MOF-5@EG hybrids offer enhanced H₂ storage properties, supporting their potential for use in high-density H₂ storage systems.

## 1. Introduction

Green hydrogen (H₂) is a promising energy carrier for sustainable development, especially when produced from renewable energy sources, as it produces only water when used [1,2]. However, its adoption is hindered by a very low volumetric energy density (0.0108 MJ L⁻¹ at 1 bar and 298 K), which is nearly 3000 times lower than that of gasoline (34.2 MJ L⁻¹), posing major storage challenges [3]. Physisorption in porous materials at cryogenic temperatures addresses this issue by enabling reversible H₂ uptake through weak van der Waals interactions, while maintaining the material’s structure [4,5]. To be effective, H₂ storage materials must combine high microporosity (pores <2 nm) and large surface area (>2000 m² g⁻¹) to allow enough H₂ adsorption on pore walls [6,7]. Among porous materials such as zeolites and activated carbons [8–11], metal-organic frameworks (MOFs) stand out as promising candidates for H₂ storage. MOFs are crystalline structures composed of metal ion clusters linked by organic connectors, creating

tunable 3D networks with high surface areas and well-defined pore sizes [12,13]. Their high porosity, structural flexibility and reversible H₂ adsorption make MOFs strong contenders for next-generation H₂ storage systems [14,15].

MOF-5 is a well-known zinc-based metal-organic framework composed of [Zn₄O] clusters with octahedral geometry, linked by 1,4-benzenedicarboxylate (BDC) units derived from terephthalic acid, forming the structure Zn₄(BDC)₃ [16]. This arrangement, shown in Fig. 1, creates a three-dimensional cubic framework with two distinct pore types: larger primary pores (~15 Å diameter) accessible through smaller windows (~8 Å), resulting in a hierarchical pore system favorable to H₂ physisorption [17]. MOF-5 exhibits high porosity, with surface areas typically exceeding 2500 m² g⁻¹ and pore volumes above 1.3 cm³ g⁻¹ [18]. Its H₂ storage capacity at 77 K varies widely (4.3–7.1 wt %), depending on synthesis and activation procedures [19–21]. Despite this high gravimetric H₂ uptake, MOF-5 has limited volumetric performance due to its low bulk density [22,23]. To address this, recent work

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, F-88000, Epinal, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, LEMTA, F-54500, Vandæuvre-les-Nancy, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), F-75231, Paris, France</footnote>

<footnote>* Corresponding author. Université de Lorraine, CNRS, IJL, F-88000, Epinal, France.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: alain.celzard@univ-lorraine.fr (A. Celzard), vanessa.fierro@univ-lorraine.fr (V. Fierro).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Schematic representation of the MOF-5 structure, with the cluster and linker forming the 3D framework.</figcaption></figure>

has focused on densification strategies, including pelletization with exfoliated graphite (EG) as a binder. EG refers to graphite that has been expanded and exfoliated into thin, layered structures by electrochemical processes. Compared to other carbon materials such as graphene oxide (GO) or carbon nanotubes, EG offers distinct advantages for pelletization: it combines high electrical conductivity and mechanical stability with outstanding thermal properties, while its layered morphology facilitates the formation of mechanically robust pellets with enhanced packing density. These features make EG a particularly suitable additive for improving the volumetric performance of MOF-5. In these *ex situ* methods, EG is combined with pre-synthesized MOF-5 to form compressed pellets, improving volumetric H₂ storage [24–26]. The EG obtained by this approach differs significantly from conventional GO or reduced graphene oxide (rGO) synthesized by Hummers’ method, as it retains high electrical/thermal conductivity and mechanical flexibility from the starting graphite [27,28].

Purewal et al. improved the H₂ storage performance and thermal conductivity of MOF-5 by employing an *ex situ* hybridization approach, where MOF-5 was combined with 10 wt% expanded natural graphite (ENG) after synthesis, followed by pelletization. This method resulted in a five-fold increase in thermal conductivity and a 23 % improvement in volumetric H₂ capacity compared with MOF-5 powder, while leading to only a modest 6 % reduction in total H₂ uptake [25]. Similarly, Palla et al. improved H₂ storage in MOF-5 by pelletizing the material to achieve a packing density greater than 0.5 g cm⁻³, which resulted in a 35 % increase in volumetric capacity over the powder form. They incorporated 5 wt% graphite, which served both as a binder to increase mechanical stability and as an enhancer of thermal conductivity, which increased threefold [26].

In contrast to previous studies based on simple physical mixtures, *in situ* hybridization of MOFs with carbon-based materials, where the MOF is synthesized directly within the carbon matrix, demonstrates superior effectiveness in preserving the material surface area and porous structure while significantly enhancing thermal conductivity compared to *ex situ* methods [29,30].

This study aims to combine the excellent H₂ adsorption properties of MOF-5 with the functional benefits of EG, which can reduce hydrophobicity, enhance thermal conductivity, and potentially serve as a binder in future densification or pelletization strategies. By integrating these complementary features, the resulting hybrids are expected to offer improved overall performance for practical H₂ storage applications. This study presents a systematic evaluation of MOF-5@EG hybrids synthesized via an *in-situ* method, and initially tested in powder form, in order to assess the intrinsic effect of EG integration on structural stability and textural properties. Through comprehensive characterization, including XRD, FTIR, TGA, SEM, N₂/Ar physisorption and water affinity tests, we establish structure-property-performance relationships critical for H₂ storage. H₂ uptake was measured under practical operating

conditions, ranging from cryogenic to near-ambient temperatures, i.e. from 77 to 273 K, and pressures up to 140 bar. Gravimetric and volumetric H₂ storage capacities together, as well as deliverable H₂ capacity, were systematically evaluated. This rigorous framework enables a direct performance benchmarking against pristine MOF-5, revealing how EG hybridization optimizes both the intrinsic properties of the material and the efficiency of H₂ storage and release at the system level. To the best of our knowledge, this is the first study to explore the *in situ* hybridization of MOF-5 with EG for H₂ storage, with a particular focus on enhancing volumetric storage capacities.

## 2. Experimental SECTION

### 2.1. Materials

All chemicals were obtained commercially and used without further purification: zinc acetate dihydrate (Zn(OAc)₂·2H₂O, Alfa Aesar, Ward Hill, MA, USA, >97 %), 1,4-benzenedicarboxylic acid (H₂BDC, Sigma-Aldrich (Merck KGaA, Darmstadt, Germany), >98 %), N,N-dimethylformamide (DMF, Sigma-Aldrich (Merck KGaA, Darmstadt, Germany), 99.8 %), sodium sulfate (Na₂SO₄, Chem-Lab NV, Zedelgem, Belgium, >99 %), HPLC-grade chloroform (CHCl₃, Carlo Erba Reagents, Val-de-Reuil, France), triethylamine (NEt₃, VWR Chemicals, Radnor, PA, USA), and pure graphite foil (Papyex I980 from Mersen, measuring approximately 40 × 25 × 0.5 mm³).

### 2.2. Synthesis

**Exfoliated graphite (EG) synthesis:** Anodic EG was synthesized by electrochemical exfoliation following established methods [31] using a two-electrode system where a platinum plate served as cathode and a flexible graphite foil (40 × 25 mm) served as anode, both immersed in 20 mL of a 1 M aqueous sodium sulfate (Na₂SO₄) electrolyte (Fig. 2-a). A constant voltage of 10 V was applied to induce vigorous electrochemical oxidation for 10 min, resulting in the progressive expansion of the graphite electrode. The resulting expanded material was then thoroughly washed with distilled water to remove residual electrolyte and oxidation byproducts. To further separate the expanded sheets, the

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Scheme of the synthesis of: a) exfoliated graphite (EG) by electrochemical exfoliation; b) MOF-5; and c) MOF-5@EG hybrids by *in situ* method.</figcaption></figure>
product was dispersed in DMF and subjected to ultrasonication for 2 h using an ultrasound bath. This post-processing treatment facilitated delamination of the carbon layers while minimizing structural damage.

**MOF-5 synthesis** (Fig. 2-b): The synthesis was adapted from the method reported by Tranchemontagne et al. [32] with minor modifications. A solution of terephthalic acid (0.46 g) and triethylamine (0.76 mL) in DMF (36 mL) was prepared. Separately, zinc acetate dihydrate (Zn(OAc)₂·2H₂O, 1.53 g) was dissolved in DMF (45 mL). The zinc acetate solution was added dropwise to the terephthalic acid mixture over 15 min under continuous stirring, resulting in the formation of a white precipitate. The reaction mixture was stirred for an additional 2.5 h at room temperature. The precipitate was isolated by centrifugation and subsequently washed by immersion in fresh DMF (50 mL) for 12 h. After a second centrifugation step, the solid was transferred to HPLC-grade chloroform (50 mL) for solvent exchange. The chloroform was replaced three times over seven days (at 2, 3, and 7 days) to ensure complete purification. Finally, the product was recovered by centrifugation, dried overnight at 393 K, and stored in a glovebox under an inert atmosphere.

**MOF-5@EG hybrids** (Fig. 2-c): The hybrid materials were synthesized analogously to the pristine compound, with the modification that an EG dispersion (10.6 mg mL⁻¹ in DMF) was introduced immediately after the addition of zinc acetate. The EG content was adjusted to achieve distinct weight percentages in the final composites: M5-EG 1 (1 wt% EG) incorporated 0.416 mL of the EG-DMF dispersion, M5-EG 5 (5 wt% EG) used 2.18 mL, and M5-EG 10 (10 wt% EG) required 4.38 mL. The reaction mixture was stirred for 2.5 h, after which isolation, washing, and solvent exchange procedures were performed according to the previously established synthesis method for pure MOF-5.

## 2.3. Physicochemical characterization

X-ray diffraction (XRD) analysis was performed to characterize the crystallinity of the adsorbents. Measurements were carried out on a Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5406 Å; 40 kV, 40 mA), with diffraction data collected across a 2θ range from 5° to 30°. Phase identification and pattern processing were conducted using DIFFRAC.EVA software (Bruker AXS). For comparative analysis, the theoretical XRD pattern of MOF-5 was generated with Mercury 3.0 (CCDC) software, employing its crystallographic information file (CIF; deposition code 938401) [33].

Fourier-transform infrared (FTIR) spectroscopy was used to identify the functional groups and bonding interactions within the MOF structure. Spectra were acquired in transmission mode in the mid-infrared region (650-4000 cm⁻¹) with a PerkinElmer Frontier 400 spectrometer. To minimize the effect of humidity, all samples were dried (398 K, 12 h) under vacuum prior to analysis. Spectral features were evaluated to verify the presence of characteristic MOF functional groups and identify potential coordination bonds.

Thermogravimetric analysis (TGA) was performed using a Netzsch STA 449F3 Jupiter microbalance to evaluate the thermal stability of the samples. Prior to measurements, all specimens were degassed under secondary vacuum (SmartVacPrep, Micromeritics) at 398 K for 12 h to remove residual moisture. Approximately 15–20 mg of each sample was loaded into an alumina crucible and subjected to a dynamic heating program from room temperature to 1173 K under continuous argon flow, with a controlled heating rate of 10 K min⁻¹. Data acquisition and derivative thermogravimetry (DTG) analysis were conducted using Proteus® Thermal Analysis software (Netzsch).

Optical characterization was carried out using a Nikon LV100ND Eclipse polarizing microscope equipped for both transmitted and reflected light illumination. The system's five-objective turret (5 × to 100 ×) enabled multiscale microstructural analysis, while an integrated 5 Mpx CCD camera provided high-resolution imaging for qualitative documentation.

Morphological and compositional characterization was performed

using a Zeiss GeminiSEM 360 field-emission microscope operating at an acceleration voltage of up to 20 kV. Samples were prepared by ultrasonic dispersion in absolute ethanol to ensure homogeneity, followed by dropcasting onto polished aluminum stubs. Elemental mapping was conducted by energy-dispersive X-ray spectroscopy, allowing spatially resolved identification of elements associated with the MOF/EG phase distribution. X-rays were generated during electron beam irradiation, and their characteristic energies were used to determine the local chemical composition at the sample surface.

Gas adsorption-desorption analyses were carried out on a Micromeritics 3Flex volumetric adsorption system using N₂ (77 K), and Ar (87 K) as probe molecules. Prior to measurements, approximately 100 mg of each sample was degassed under secondary vacuum at 398 K for 12 h using a SmartVacPrep unit (Micromeritics). The Brunauer-Emmett-Teller (BET) surface area (A<sub>BET</sub>, m² g⁻¹) was calculated from N₂ and Ar adsorption isotherms using Microactive® software (Micromeritics), following Rouquerol's consistency criteria [34] to ensure data validity. In addition, pore size distributions (PSDs) and surface areas were obtained by applying the two-dimensional non-local density functional theory (2D-NLDFT) model for zeolites (H-form) to the isotherm data using SAIEUS® software (Micromeritics). Although specific kernels for MOF-5@EG hybrids are not yet available, the 2D-NLDFT model for zeolites provided a best fit for Ar (87 K) adsorption isotherms in these materials. From the PSDs, key textural parameters were obtained, including the 2D-NLDFT-based surface area (S<sub>2D-NLDFT</sub>, m² g⁻¹), and a comprehensive pore volume breakdown. This comprised total pore volume (V<sub>tots</sub>, cm³ g⁻¹), micropore volume (V<sub>μ</sub>, cm³ g⁻¹: pore width w < 2 nm), ultramicropore volume (V<sub>μμ</sub>, cm³ g⁻¹: w < 0.7 nm), supermicropore volume (V<sub>μμ</sub>, cm³ g⁻¹: 0.7 < w < 2 nm), and mesopore volume (V<sub>meso</sub>, cm³ g⁻¹: 2 < w < 50 nm).

Water vapor adsorption isotherms at 293 K were measured using a Microtrac Bellsorp Max II automatic adsorption analyzer. Prior to analysis, approximately 100 mg of each sample was degassed under secondary vacuum at 398 K for 12 h using an ASAP 2420 system (Micromeritics), allowing complete removal of pre-adsorbed species. The apparent water affinity constant was calculated from the slope of the linear region of the adsorption isotherm at low relative pressures (p/ p° < 0.2), which corresponds to primary water binding sites.

The thermal properties of the powder samples were measured using a Hot Disk TPS 2500 apparatus, which uses the Transient Plane Source method. For the test, the sensor was placed between two layers of the same powder, enabling the properties to be measured directly without pressing the powder into a solid pellet, which can alter the results. Prior to analysis, all samples were degassed under secondary vacuum at 398 K for 12 h to remove any moisture that could affect the measurements. For the pure MOF and hybrid samples, a heating pulse of 10 mW for 5 s was used. For the pure EG sample, a stronger pulse of 200 mW for 1 s was used. These settings helped optimize the balance between signal resolution and minimal sample disturbance. This configuration made it possible to simultaneously determine thermal conductivity, diffusivity, and volumetric heat capacity by analyzing the transient temperature response.

The skeletal density (ρ<sub>skelet</sub>) of the samples was determined using a Micromeritics AccuPyc II 1340 helium pycnometer at room temperature. This analysis was performed after a 12-h degassing process under vacuum at 398 K to eliminate any pre-adsorbed species. The tapped density (ρ<sub>tap</sub>) was measured using a Quantachrome Autotap device. Approximately 4 mL of each sample was placed in a 5 mL graduated cylinder and subjected to 1500 mechanical taps until no change in volume was observed. The final volume was recorded, and the tapped density was obtained by dividing the sample mass by this final, tapped volume.

## 2.4. High-pressure hydrogen adsorption

H₂ storage performance was assessed by measuring adsorption
isotherms at 77, 160 and 273 K, with pressures ranging up to 140 bar. A Micromeritics HPVA II high-pressure volumetric adsorption analyzer was used, featuring precise temperature regulation thanks to a closed-cycle cryogenic refrigeration system. Before each experiment, ~1 g of sample was degassed under vacuum at 398 K for 12 h to eliminate residual adsorbates, ensuring consistent measurements.

### 2.4.1. Excess and total hydrogen uptake

The excess H₂ uptake ($n_{exc}$) was calculated using the Leachman equation of state (recommended by NIST) [35], with dead volume corrections based on the skeletal density of the samples ($\rho_{skel}$). The total uptake ($n_{tot}$) provides a more practical measure by including both excess H₂ and free gas in the void volume. The total H₂ uptake was calculated as follows [36]:

$$n_{tot} = n_{exc} + \left( \frac{1}{\rho_{tap}} - \frac{1}{\rho_{skel}} \right) \rho_{H_2} \quad (1)$$

where $\rho_{H2}$ is the density of H₂ gas at the given temperature and pressure. The relationship between $\rho_{tap}$ and $\rho_{skel}$ is critical for assessing volumetric storage efficiency in space-constrained applications.

### 2.4.2. Hydrogen release capacity

The release capacity, a key indicator of storage system performance [37–39], was determined as the difference between total uptake under charging and discharging conditions:

$$Release \ capacity = (n_{tot})_{storage} - (n_{tot})_{release} \quad (2)$$

Here, $(n_{tot})_{storage}$ refers to the total H₂ uptake under charging conditions, while $(n_{tot})_{release}$ corresponds to uptake under discharge conditions. Charging was conducted at 77 K and 100 bar, corresponding to the operational limits of Type I metal tanks [37]. Discharge conditions were selected based on guidelines from the Hydrogen Storage Engineering Center of Excellence [40], which designates 5 bar as the minimum pressure for efficient H₂ delivery to fuel cells or engines [38,41]. Two discharge scenarios were examined: cryogenic discharge (77 K, 5 bar) and moderate-temperature discharge (160 K, 5 bar). The dual-temperature discharge approach allowed a comparative assessment of H₂ recovery under different thermal conditions.

## 3. Results and discussion

### 3.1. Fundamental properties of the hybrid materials

Fig. 3-a shows the XRD patterns of MOF-5 material and its hybrids containing 1, 5 and 10 wt% EG prepared by the in-situ synthesis method. All samples display the main reflections of MOF-5 at 6.8° (200), 9.7° (220) and 13.7° (400), confirming the preservation of the crystal

structure after EG incorporation [42,43]. Pure EG normally shows a clear graphite band at 26.6° (see Fig. S1), but this peak does not appear in the hybrid samples. This means that the EG is well mixed within the MOF-5 and does not form separate graphite phases. However, a small peak at 8.8° is seen only in the samples containing EG. This suggests that a minor secondary phase may form, likely due to some early breakdown of MOF-5 when exposed to moisture during sample preparation [44]. Importantly, this does not affect the main structure of MOF-5, as the main diffraction peaks of the hybrid materials still match the simulated MOF-5 patterns. This shows that the crystal structure is preserved and that the synthesis method successfully produces the desired material, even with the addition of EG.

FTIR spectroscopy confirmed the structural integrity of MOF-5 after EG incorporation, as all hybrids exhibited vibrational modes identical to those of pristine MOF-5 (Fig. 3-b). The characteristic bands observed were: 1614 cm⁻¹ (H–O–H bending), 1397 cm⁻¹ (symmetric O–C–O stretching), 1508 cm⁻¹ (C=C aromatic stretching), and 1067, 1508, 825 and 747 cm⁻¹ (C–H deformations) [45–47]. Even after degassing the samples, peaks at 2900, 2987 and 3660 cm⁻¹ still appear, attributed to stretching vibrations of OH groups from adsorbed moisture [48,49]. The FTIR results confirm that EG can be added to MOF-5 without inducing chemical changes or damaging its structure.

The TGA profiles (Fig. 3-c) show a main decomposition step for all samples, in line without previous studies on MOF-5 materials [50–52]. A strong mass loss occurs above 790–810 K (as shown in Fig. S2), linked to the removal of –OH groups and bonded molecules, leading to structural decomposition. All samples showed exceptional thermal stability under argon, with decomposition starting above 790 K, which is consistent with previous research [50–52]. Notably, the absence of weight loss at lower temperatures confirms that the solvents were completely removed during activation. Pure MOF-5 and EG hybrid samples also show nearly identical decomposition patterns, proving that adding EG does not weaken the material’s heat resistance. Moreover, thermogravimetric analysis of pristine EG under an argon atmosphere (see Fig. S3) demonstrates that EG exhibits negligible mass loss up to 1000 K, indicating its high thermal stability and lack of significant degradation under the experimental conditions.

Fig. S4 shows optical microscopy images of the samples. Dry EG appears as agglomerated graphite sheets (Fig. S4-a), while pure MOF-5 is seen as a typical white powder (Fig. S4-b). The hybrids containing 1 and 10 wt% EG (Fig. S4-c and S4-d, respectively) show a darker MOF matrix. The images also suggest that MOF-5 may coat EG sheets, probably as a result of in situ synthesis, where EG acts as a site for MOF-5 crystal growth.

Fig. 4-a shows the SEM image of the M5-EG 1 hybrid, with elemental mapping for zinc (Fig. 4-b) and carbon (Fig. 4-c). The Zn signal confirms the presence of MOF-5 (where Zn is a key component), while the C mapping highlights the homogeneous distribution of EG. A similar

<figure><img src="image_5.png" /><figcaption>Fig. 3. a) X-ray diffraction patterns of MOF-5 and hybrids with different wt.% EG compared with the simulated MOF-5 pattern; b) FTIR spectra of MOF-5 and hybrids with different wt.% EG; c) Thermogravimetric profiles of pure MOF-5 and hybrids with different wt.% EG measured under argon.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** a) SEM image of M5-EG 1, with its b) Zn and c) C mapping images; d), e) and f): same as a), b), and c), respectively, but for M5-EG 5 and g), h) and i): same as a), b) and c), respectively, but for M5-EG 10.</figcaption></figure>

analysis of M5-EG 5 (Fig. 4-d to 4-f) and M5-EG 10 (Fig. 4-g to 4-i) shows a uniform distribution of Zn and C, supporting consistent integration. Both optical and SEM data reveal strong interactions between MOF-5 and EG, as overlapping Zn and C signals indicate a well-mixed composite with no phase separation. Oxygen mapping (Fig. S5-a, S5-b and S5-c) further shows a homogeneous distribution of O from MOF-5's organic linker on the hybrid surfaces. Closer examination of M5-EG 10 confirms that the EG is clearly observable, and distributed all over the sample (Fig. S6). The EG, marked in red, appears as large and irregular particles, while the MOF-5 exhibits a small and elongated structure. This suggests that during the in-situ synthesis, MOF-5 crystals nucleate and grow around and between the EG sheets, which act as a structural and functional matrix, facilitating integration without compromising the crystallinity of the MOF.

Fig. 5-a shows the N₂ adsorption-desorption isotherms for all samples. MOF-5 and its hybrids display Type I(a) isotherms without hysteresis, according to the IUPAC classification, which confirms their microporous texture [53]. However, a slight hysteresis loop is observed for M5-EG 10, suggesting the presence of a minor mesoporous contribution in this sample. The incorporation of EG affects the porosity of the hybrids: when a small amount of EG (1–5 wt%) is added, an increase in

adsorption capacity is observed in M5-EG 1 and M5-EG 5 hybrids. However, with a higher EG content (10 wt%), the N₂ uptake decreases, as seen in the isotherm of sample M5-EG 10. Additionally, the N₂ isotherm of pure EG is not shown, since it exhibits near-zero adsorption up to p/p° = 1, which translates into negligible surface area, consistent with its non-porous nature. Although EG is non-porous and does not contribute directly to the surface area, its layered morphology can promote the formation of interfacial voids and micro-gaps at the MOF-5@EG interfaces. These additional spaces may become accessible to gas molecules, slightly enhancing the overall porosity of the hybrid.

Ar adsorption-desorption isotherms at 87 K were used instead of N₂ physisorption because Ar provides more accurate results for MOF characterization. Indeed, although Ar and N₂ have molecular similar sizes (3.4 Å vs. 3.6 Å), the absence of argon polarity, which is beneficial for avoiding specific interactions between the MOF and the analysis gas, and the higher measurement temperature (87 K vs. 77 K for N₂) improve molecular diffusion and allow faster pore filling. As shown in Fig. 5-b, pristine MOF-5 and its MOF-5@EG hybrids display isotherm profiles similar to those obtained with N₂. However, when comparing BET areas calculated from Ar and N₂ (Fig. 5-c), minimal differences were observed and both gases provided the same overall trend in BET area values: M5-
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Textural characteristics of pure MOF-5 and hybrids: a) $N_2$ adsorption (solid symbols) - desorption (empty symbols) isotherms at 77 K; b) Ar adsorption (solid symbols) - desorption (empty symbols) isotherms at 87 K; c) BET areas obtained from $N_2$ (solid symbols) and Ar (empty symbols) isotherm data; d) Fit of the 2D-NLDFT cylindrical model for zeolite (H-form) to MOF-5 Ar isotherm; e) Differential and f) cumulative pore size distributions ($dV/dw$ and $V_{cum}$, respectively) as a function of pore width.</figcaption></figure>

EG 1 and M5-EG 5 have the highest and comparable areas around 2920–2955 $m^2 g^{-1}$, while pristine MOF-5 and M5-EG 10 have around 2890 and 2150 $m^2 g^{-1}$, respectively.

Currently, the development of 2D-NLDFT models for MOFs is limited, mainly because of the great structural diversity of these materials, which makes the characterization of hybrid materials more challenging. To obtain the textural properties of the samples, the 2D-NLDFT model designed for H-form zeolites with Ar (87 K) isotherms (Fig. 5-b) was used. This method was chosen because it best represents the textural properties of our samples, as shown by the strong agreement between our experimental isotherms and the model predictions (see the example of MOF-5 in Fig. 5-d). Analysis of the pore size distribution (PSD) (Fig. 5-e) shows that adding EG modifies the microporous structure of the hybrids. All samples remain predominantly microporous, with pore widths below 2 nm. While the mesoporous content is negligible for pure MOF, M5-EG 1 and M5-EG 5 (less than 3 % of total pore volume), M5-EG 10 exhibits a slightly higher mesoporous contribution (11.3 %), as seen by the small hysteresis loop observed in its isotherm. The cumulative pore volume ($V_{cum}$) as a function of pore width (Fig. 5-f) reveals that, in the ultramicroporous region (<0.7 nm), the hybrids have slightly lower pore volumes than pristine MOF-5. This trend continues in the 0.7–1.0 nm range, where all hybrids still show reduced pore volumes. However, for pores larger than 1.0 nm, M5-EG 1 and M5-EG 5 display increased porosity compared with MOF-5, while M5-EG 10 has the least developed pore structure in this range. The data suggest a beneficial upper limit for EG incorporation (~5 wt%), beyond which higher EG content (above 5 wt%) does not improve the hybrids' microporosity or surface area, and even reduce their textural properties to values below those of the pure MOF. A summary of surface areas (from 2D-NLDFT and BET method), total pore volumes and pore size distributions for all samples is provided in Table S1 of the Supplementary Information.

Despite its high porosity, practical use of MOF-5 is limited by its strong sensitivity to moisture. When exposed to humidity, its structure rapidly decomposes as water reacts with its zinc-based clusters ($Zn_4O$), resulting in a permanent pore collapse and a loss of surface area of over 60 % within a day [44,54]. To address this problem, a study was carried out into how the addition of different amounts of EG (1–10 wt%) affects the material’s attraction to water. If the hybrids have a lower affinity for water, this could mean slower degradation and better durability for real-world applications. Water vapor adsorption isotherms at 298 K, shown in Fig. 6-a, indicate that MOF-5 has a relatively low water affinity, with moderate uptake (<20 $cm^3 g^{-1}$ STP) at low relative pressures ($p/p^° < 0.1$). To quantify the materials’ affinity for water, the slopes of the water adsorption isotherms up to a relative pressure of 0.2 were calculated, focusing on the linear region of the isotherm (in the $p/p^°$ range from 0.05 to 0.2, Fig. 6-b). The results reveal a non-linear dependence of water affinity on EG content in MOF-5 hybrids. Pure EG, which is highly hydrophobic, displays the lowest slope (14 $cm^3 g^{-1}$). Incorporating 1 wt% EG reduces the slope by 28 % (to 46 $cm^3 g^{-1}$), compared with MOF-5 (67 $cm^3 g^{-1}$) and, with 5 wt% EG, the reduction reaches 69 % (23 $cm^3 g^{-1}$). However, by increasing the EG content to 10 wt%, the slope to return to values similar to those of pure MOF-5 (70 $cm^3 g^{-1}$ vs. 67 $cm^3 g^{-1}$ for pure MOF-5). This trend suggests that EG can effectively protect MOF-5 pores from water at optimum concentrations (≤5 wt%), whereas higher EG loadings may promote its aggregation, resulting in higher water uptake, specially at relative pressures higher than 0.3, which corresponds to cluster formation and growth with eventual filling of micro- and mesopores, if any, at higher relative pressures [55,56]. Given that graphitic surfaces interact only weakly with water molecules, the presence of oxygen-containing functional groups at the edges of the graphenic layers, especially those not in intimate contact with the MOF due to aggregation, may explain the
<figure><img src="image_8.png" /><figcaption>Fig. 6. Water affinity: a) water adsorption isotherms at 293 K for pure MOF-5, EG and hybrids with different wt.% EG; b) zoom in on the water affinity region; c) thermal conductivity of samples measured in powder form (298 K and 25 % relative humidity).</figcaption></figure>

higher water uptake observed [57,58].

Fig. 6-c shows thermal conductivity measurements for MOF-5 and its hybrid materials. The pure MOF-5 powder has a thermal conductivity of 0.063 W m⁻¹ K⁻¹, in agreement with previously reported values [59, 60]. This low thermal conductivity highlights the poor heat transfer ability of MOF-5, which is a typical feature of porous, crystalline MOF structures [61]. The incorporation of EG into MOF-5 did not result in the

linear improvement in thermal conductivity predicted by theoretical models (dashed line in Fig. 6-c). Instead, the powdered hybrids showed only marginal improvements over pure MOF-5 (0.063 W m⁻¹ K⁻¹), with values of 0.079 W m⁻¹ K⁻¹ (1 wt% EG), 0.081 W m⁻¹ K⁻¹ (5 wt% EG) and 0.149 W m⁻¹ K⁻¹ (10 wt% EG). Although the 10 wt% EG sample exhibited a clearer increase, its conductivity remained orders of magnitude lower than that of pure EG, indicating the incomplete

<figure><img src="image_9.png" /><figcaption>Fig. 7. High-pressure hydrogen adsorption measurements: gravimetric excess H₂ uptake (nexc, wt.%) at a) 77 K, b) 160 K and c) at 273 K; d) total volumetric H₂ uptake (ntot, kg m⁻³) at 77 K; e) compressed and adsorbed contributions to stored H₂ (ntot, kg m⁻³) at 77 K and 100 bar; f) release capacities H₂ (kg m⁻³) (charging conditions: 77 K, 100 bar; discharging conditions: 77 K, 5 bar vs 160 K, 5 bar).</figcaption></figure>
formation of a continuous conductive network.

This non-linear behavior is probably due to the difficulties inherent
in measuring powder-based composites as well as to the high porosity of
MOF-5, which likely disrupted heat transfer pathways. The modest in-
crease at 10 wt% EG suggests a percolation threshold, where EG begins
to form interconnected pathways, but where particle boundaries and air
gaps in the uncompacted powder still limit overall conductivity. These
results highlight the limitations of simple powder mixing to improve the
thermal performance of porous MOF composites. To improve heat
transfer without compromising porosity, strategies such as controlled
pelletization or binder-assisted compaction may be necessary to opti-
zize particle connectivity and minimize interfacial resistance [23,24,
62].

## 3.2. Evaluation of H₂ adsorption capacity at high pressure

Fig. 7-a shows the excess ($n_{exc}$, wt.%) H₂ isotherms for all samples at 77 K and up to 140 bar. At 77 K and 40 bar, the M5-EG 5 hybrid achieves the highest capacity at 5.3 wt%, followed by M5-EG 1 at 5.1 wt%, representing a 14% and 10% improvement over pure MOF-5 (4.6 wt%). In contrast, M5-EG 10 exhibits comparable performance, with a maximum capacity of 4.5 wt%, i.e. a 2% reduction compared with MOF-5. This trend aligns with the enhanced surface area and pore structure observed in hybrids with ≤5 wt% EG, confirming that optimal H₂ storage occurs when incorporating 5 wt% EG.

Fig. 7-b shows the H₂ adsorption isotherms at 160 K, revealing trends distinct from those observed at 77 K. At 160 K and 100 bar, M5-EG 5 and M5-EG 1 maintain their superior performance with an uptake of 1.7 wt% each, while M5-EG 10 reverses its previous trend, outperforming pristine MOF-5 (1.5 wt% vs. 1.4 wt%). At 273 K and 100 bar (Fig. 7-c), the hierarchy persists, but capacities converge: MOF-5 and M5-EG 10 show almost identical uptake (0.43 wt% vs. 0.45 wt%).

To evaluate the stability of the hybrid materials under high-pressure adsorption, cyclic H₂ adsorption-desorption measurements at 77 K were conducted on the optimum composite, M5-EG 5. These tests (Fig. S7) confirmed its structural stability, with nearly identical uptake over three adsorption-desorption cycles. Total H₂ uptake ($n_{tot}$, wt.%, Fig. S8) follows $n_{exc}$ trends, with M5-EG 5 leading at 39.3 kg m⁻³ (77 K, 100 bar). Gravimetric (wt.%) and volumetric (kg m⁻³) capacities align due to similar packing densities ($\rho_{tap}$, inset of Fig. 7-d). The $\rho_{tap}$ and $\rho_{skel}$ values used to calculate $n_{tot}$ are provided in Table S1. Table S2 summarizes the H₂ storage parameters for MOF-5 and its hybrid materials, including $n_{exc}$ and $n_{tot}$ in gravimetric (wt.%) and volumetric (kg m⁻³) units. In these systems, H₂ is stored in both adsorbed and compressed phase. $n_{exc}$ represents the additional amount of gas stored within the material's porous framework compared with the amount of H₂ that would be compressed in the same volume of adsorbed phase, while the second term in Equation (1) takes compressed H₂ into account. At 77 K and 100 bar (Fig. 7-e), all materials outperform an empty tank (31.0 kg m⁻³). The best-performing material, M5-EG 5, achieves 39.3 kg m⁻³ (+27 %), followed by M5-EG 1 (38.1 kg m⁻³, +23 %), pristine MOF-5 (36.1 kg m⁻³, +16 %), and M5-EG 10 (34.0 kg m⁻³, +10 %). This confirms that adding 1–5 wt% EG improves H₂ storage, with 5 wt% being optimal. The observed increase in H₂ adsorption capacity arises from the combined effect of the enhanced surface area and the relationship between the materials' tap density and skeletal density, as described by Equation (1). This relationship accounts for the hybrid's improved performance in both gravimetric and volumetric H₂ storage capacities.

Two distinct H₂ release scenarios were evaluated, as summarized in Table S3: (1) isothermal discharge at the charging temperature (77 K→77 K) and (2) temperature-assisted discharge with heating to 160 K (77 K→160 K). Under isothermal discharge conditions, at 77 K and 5 bar (Fig. 7-f), both M5-EG 1 and M5-EG 5 hybrids outperform the empty tank (29.5 kg m⁻³), achieving higher release capacities of 2 % (30.0 kg m⁻³) and 6 % (31.1 kg m⁻³), respectively. In contrast, pristine MOF-5 (28.6 kg m⁻³) and M5-EG 10 (26.9 kg m⁻³) exhibit lower release

capacities, corresponding to decreases of 3 % and 9 %, respectively, relative to the empty tank. Remarkably, under temperature-assisted discharge conditions at 160 K and 5 bar, all porous materials outperform the empty tank (30.4 kg m⁻³). The M5-EG 5 composite exhibits the most pronounced improvement, with a 25 % increase in release capacity (38.1 kg m⁻³) under these discharge conditions.

The H₂ release efficiency, which is defined as the percentage of H₂ released relative to the total stored capacity, is given in Table S3. Notably, raising the discharge temperature to 160 K results in higher efficiencies of 96–97 % for all porous materials. By contrast, isothermal discharge at 77 K yields significantly lower efficiencies (78–79 %). These findings clearly establish that discharge temperature is a critical operational parameter influencing H₂ release performance in adsorbent-based storage systems.

To contextualize the results of this study, Fig. 8-a compares the H₂ adsorption performance of our MOF-5 hybrids with that of previously reported materials under identical conditions (1–3 bar, 77 K). Notably, our materials rank among the top three systems in terms of surface area, while simultaneously achieving the highest $n_{exc}$ (wt.%) H₂ uptake values documented in the literature for these pressure-temperature conditions. This dual superiority in terms of surface area and gravimetric capacity underscores the structural optimization achieved through our hybridization strategy.

The enhancement effect of hybridization was further evaluated by plotting the $n_{exc}$ of each MOF hybrid against its pristine MOF counterpart (Fig. 8-b). All data points consistently lie above the diagonal (y = x) for which there would be no improvement, demonstrating consistent gains in H₂ adsorption capacity across the series. This trend is independent of the parent MOF's baseline performance, confirming that hybridization confers universal advantages.

## 4. Conclusions

The combination of MOF-5 with exfoliated graphite (EG) produced hybrid materials with improved textural properties. The hybrid with 5 wt% EG stood out, showing a 15 % increase in micropore volume while retaining high surface area, which are key factors for improved hydrogen (H₂) storage. Determination of H₂ uptake, both excess and total, revealed consistent improvements at 77 K and 40 bar, with the 5 wt% EG hybrid storing 14 % more H₂ by weight (5.3 wt%) and 9 % more by volume (39.3 kg m⁻³) than pure MOF-5. Importantly, this hybrid released H₂ more efficiently than all other materials, even at 77 K, proving its usefulness for real-world storage systems. Repeated charge-discharge cycling tests confirmed the material's durability, with no loss in performance over three cycles. By balancing enhanced storage capacity, reliable H₂ release and long-term stability, MOF-5/EG hybrids emerge as strong candidates for advancing cryogenic H₂ storage technology. Future investigations should focus on enhancing the volumetric H₂ storage capacity of these materials via controlled densification or pelletization approaches, which are also expected to improve interparticle connectivity and, consequently, the effective thermal conductivity of the composites.

### CRediT authorship contribution statement

**Laura Jimenez Lopez:** Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Rafael Morales-Ospino:** Writing – review & editing, Supervision, Investigation, Formal analysis, Data curation. **Jimena Castro-Gutiérrez:** Writing – review & editing, Investigation. **Haroon Osman Sumbhaniya:** Writing – review & editing, Investigation. **Giuseppe Sdanghi:** Writing – review & editing. **Sergio García Dalí:** Writing – review & editing, Methodology, Investigation. **Alain Celzard:** Writing – review & editing, Supervision. **Vanessa Fierro:** Writing – review & editing, Validation, Supervision, Resources, Methodology, Funding acquisition, Formal analysis, Conceptualization.
<figure><img src="image_10.png" /><figcaption>Fig. 8. a) H₂ storage capacities of MOF hybrids as a function of A<sub>BET</sub>; b) n<sub>exc</sub> of MOF hybrids as a function of n<sub>exc</sub> of pristine MOF. (□ from Ref. [63]), (■ from Ref. [64]), (□ from Ref. [65]), (□ from Ref. [66]), (□ from Ref. [67]) and (□ from Ref. [68]).</figcaption></figure>

## Funding

This work was made possible thanks to (i) SOLHYD project (ANR-22-PEHY-0007) funded by the Agence Nationale de la Recherche, (ii) FRCR HyPE project funded by Région Grand-Est, and (iii) TALiSMAN and TALiSMAN2 projects funded by FEDER.

## 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 gratefully acknowledge Mr. Philippe Gadonneix for his invaluable dedication and outstanding support in the laboratory.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2025.151272.

## ABREVIATIONS

(n<sub>total</sub>)<sub>release</sub> Total H₂ uptake at discharging conditions

(n<sub>total</sub>)<sub>storage</sub> Total H₂ uptake at charging conditions

2D-NLDFT Two-dimensional non-local density functional theory

A<sub>BET</sub> Brunauer-Emmett-Teller surface area

CCDC Cambridge Crystallographic Data Centre

DTG Thermogravimetric derivative

FTIR Fourier-transform infrared spectroscopy

H₂BDC 1,4-benzenedicarboxylate

MOF Metal organic framework

n<sub>exc</sub> Excess hydrogen uptake measured

n<sub>tot</sub> Compressed H₂ + adsorbed H₂

PSDs Pore size distributions

S<sub>2D-NLDF</sub> 2D-NLDFT based surface area

SEM Scanning electron microscope

TGA Thermogravimetric analysis

V<sub>meso</sub> Mesopore volume

V<sub>μ</sub> Supermicropore volume

V<sub>tot</sub> Total pore volume

V<sub>μ</sub> Ultramicropore volume

V<sub>μ</sub> Micropore volume

XRD X-ray diffraction

ρ<sub>H2</sub> H₂ density

ρ<sub>skel</sub> Skeletal density

ρ<sub>tap</sub>: Tapped density

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