Revealing the Roles of Heat Transfer, Thermal Dynamics, and Reaction Kinetics in Hydrogenation/Dehydrogenation Processes for Mg-Based Metal Hydride Hydrogen Storage

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Hydrogen in its gaseous and liquid forms requires specially designed tanks of specific materials. Mg and its alloys are among the best materials for constructing these tanks. In this work, the thermodynamic and kinetic characteristics of the hydrogenation and dehydrogenation processes are investigated and analyzed in detail, and effects of initial conditions on the thermochemical hydrogen storage reactor are discussed. The key issue to be solved for hydrogen storage materials is dual tuning the thermodynamic and kinetics. More references published in 2025 and 2024, such as ‘Journal of Materials Science & Technology, 178(2024): 90-99’ should be summarized for the development of Mg based hydrogen energy and the key issue for hydrogen storage materials via dual tuning the thermodynamic and kinetics.

Elevating hydrogen charging pressure (0.6→1.0 MPa) induced a nonlinear increase in equilibrium temperatures (327→342°C), concurrently shortening temperature rise duration and amplifying curve curvature, highlighting its dual regulation on reaction kinetics and heat transfer efficiency. The relationship between the hydrogen charging pressure and equilibrium temperatures should be further discussed. Meanwhile, reducing initial temperatures (300→260°C at 0.8 MPa) shortened plateau duration and cooldown time by 41% (178→105 min), lowered average temperature peaks, yet accelerated reaction rates with nonlinear kinetic characteristics, demonstrating coupling mechanisms between thermal gradients and reaction progression. The relationship between the thermodynamic and kinetics should be further discussed.

It is recommended that authors add some recently published references on hydrogen storage, hydrogen compression materials, especially in 2025 and 2024. A one-step approach towards hydrogen generation, storage, transportation and application without hydrogen compression step as the new method for Li (Na) BH4 regeneration via using Magnesium alloys or hydrides as the new topics for hydrogen-energy process chain and hydrogen economics published in ‘Adv. Energy Mater., 2017, 1700299’ and ‘Angew. Chem. Int. Ed., 59(2020)8623–8629’ was suggested to be summarized.

Author Response

Comment1: Hydrogen in its gaseous and liquid forms requires specially designed tanks of specific materials. Mg and its alloys are among the best materials for constructing these tanks. In this work, the thermodynamic and kinetic characteristics of the hydrogenation and dehydrogenation processes are investigated and analyzed in detail, and effects of initial conditions on the thermochemical hydrogen storage reactor are discussed. The key issue to be solved for hydrogen storage materials is dual tuning the thermodynamic and kinetics. More references published in 2025 and 2024, such as ‘Journal of Materials Science & Technology, 178(2024): 90-99’ should be summarized for the development of Mg based hydrogen energy and the key issue for hydrogen storage materials via dual tuning the thermodynamic and kinetics.

Response1: I think there is a real shortage in the background section of the article and have improved on this section, adopted the reviewer's recommended article and searched for more other articles. A review of solid-state hydrogen storage materials has been extended with new research aimed at increasing hydrogen storage material features.

The revised objective now reads: line78-88; line93-109

Solid hydrogen storage materials mainly comprise four types: (1) Metal hydrides including Mg-based materials (e.g., MgH₂ with 7.6 wt% capacity but requiring >300°C operation, improvable via catalysts like Ni/Fe or nano structuring) and rare-earth materials (e.g., LaNi₅ with 1.4 wt% capacity but moderate operating conditions); (2) Com-plex hydrides such as NaAlH4 and LiBH4 (theoretical capacity up to 18 wt% but suffering from high temperatures and poor kinetics, requiring catalyst doping or nano-confinement) [20]; (3) Metal-organic frameworks (MOFs) exhibiting excellent low-temperature performance due to high surface area and tunable porosity, though with limited room-temperature capacity; and (4) Carbon-based materials including activated carbon, carbon nanotubes, and graphene that store hydrogen through phys-ical adsorption, offering low-cost solutions primarily for cryogenic applications.

However, it suffers from sluggish kinetics, as the hydrogenation/dehydrogenation processes require overcoming high activation energies, resulting in limited reaction rates. Existing studies have improved the material's performance through strategies including catalyst addition/alloying, synergistic effects of composite hydrides, and nano structuring/ball milling processes. Huot et al. [22] demonstrated that while ball milling reduced the specific surface area to one-tenth of its original value, it significantly enhanced the reaction kinetics. Shen et al. [23] proposed a dual-regulation strategy by incorporating LiBH4 (for thermodynamic tuning to enable reversible hydrogen storage and reaction enthalpy modification) and YNi5 alloy (for kinetic enhancement through in-situ formation of MgNi₃B₂ and YH₃ catalysts to facilitate hydrogen diffusion and optimize interfacial structure). This approach achieved remarkable performance in MgH₂−0.04LiBH₄−0.01YNi₅ composites, demonstrating 7 wt.% hydrogen release within 10 minutes at 300°C with 90.3% capacity retention after 110 cycles. Zhiqiang Lan et al. [24] reported that their Ni-N-C ternary nanocomposites could be fully hydrogenated in just 60 minutes at 100°C, with DSC tests revealing substantially lowered dehydrogenation energy barriers. Remarkably, the material maintained 99.5% of its capacity through 100 cycles.

 

[20] He, T., Pachfule, P., Wu, H., Xu, Q., & Chen, P. (2016). Hydrogen carriers. Nature Reviews Materials, 1(12), 1-17. https://doi.org/10.1038/natrevmats.2016.59

 

[22] Huot, J., Liang, G., Boily, S., Van Neste, A., & Schulz, R. (1999). Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride. Journal of Alloys and Compounds, 293-295, 495-500. https://doi.org/10.1016/s0925-8388(99)00474-0

 

[23] Shen, S., Liao, W., Cao, Z., Liu, J., Wang, H., & Ouyang, L. (2024). Enhanced hydrogen storage properties of MgH₂ with the co-addition of LiBH₄ and YNi₅ alloy. Journal of Materials Science & Technology, 178, 90-99. https://doi.org/10.1016/j.jmst.2023.08.039

 

[24] Lan, Z., Liu, Z., Liang, H., Shi, W., Zhao, R., Li, R., Fan, Y., Liu, H., & Guo, J. (2024). Introducing Ni-N-C ternary nanocomposite as an active material to enhance the hydrogen storage properties of MgH2. Journal of Material Science and Technology, 196, 12-24. https://doi.org/10.1016/j.jmst.2024.01.055

 

 

Comment2: Elevating hydrogen charging pressure (0.6→1.0 MPa) induced a nonlinear increase in equilibrium temperatures (327→342°C), concurrently shortening temperature rise duration and amplifying curve curvature, highlighting its dual regulation on reaction kinetics and heat transfer efficiency. The relationship between the hydrogen charging pressure and equilibrium temperatures should be further discussed. Meanwhile, reducing initial temperatures (300→260°C at 0.8 MPa) shortened plateau duration and cooldown time by 41% (178→105 min), lowered average temperature peaks, yet accelerated reaction rates with nonlinear kinetic characteristics, demonstrating coupling mechanisms between thermal gradients and reaction progression. The relationship between the thermodynamic and kinetics should be further discussed.

Response2: The reviewers' remarks were quite constructive, and the improvements they advised were incorporated into the article. The dual thermodynamic and kinetic modulation of the hydrogen charging pressure change is investigated further, and the relationship between the hydrogen charging pressure and the equilibrium temperature is demonstrated. The relationship between hydrogen charging pressure and equilibrium temperature is investigated further as the beginning temperature is varied.

The revised objective now reads: line771-789; line824-854

Thermodynamically, according to the van't Hoff equation, the pressure increase from 0.6 MPa to 1.0 MPa increases the equilibrium temperature nonlinearly. At higher pressures, the increased collision frequency of hydrogen molecules induces more energy to overcome the activation energy instead of being converted into a temperature increase, resulting in a gradual decrease in the increase of the equilibrium temperature with the increase in pressure. Kinetically, at low pressure (0.6 MPa), the reaction rate is slower, and thermal diffusion dominates the temperature distribution with a gentle curve slope. At high pressure (1.0 MPa), the initial driving force is increased by about 67%. The rate of heat production at the beginning of the reaction is much higher than the rate of thermal diffusion, resulting in a steep temperature profile. When the equilibrium temperature is reached in the center region, the heat dissipation is gradually dominated, and the slope of the profile tends to slow down. Under high pressure, a more significant thermal plateau is formed in the center region, and the temperature in the edge region falls back rapidly due to the efficient heat dissipation from the alumi-num shell, exacerbating the nonlinear characteristics of the curve. It is worth noting that elevated pressure directly enhances the collision frequency and accelerates the surface adsorption and chemical bond formation, but too high pressure may lead to the expansion of MgH2 particles and the decrease of porosity, which hinders the hydrogen diffusion, resulting in the late reaction rate being limited by internal diffusion rather than pressure-driven force.

The reduction of the initial temperature significantly impacts the reaction process through the synergistic effect of thermodynamics and kinetics. Fourier's law shows that the widening temperature difference between the system and the environment enhances the thermal conduction drive. It increases the heat dissipation efficiency by 41% (cooling time from 178→105 min), which leads to the rapid export of reaction heat and shortens the high-temperature plateau period. Despite the increase in apparent activation energy by the low temperature, the enhanced thermal gradient instead promotes hydrogen diffusion, resulting in the peak reaction rate occurring 20 min earlier and a 15% increase in the average rate, suggesting that the promotion of kinetics by heat transfer outweighs the suppression of activation energy by the cooling down. The initial temperature reduction caused the temperature in the edge region of the reactor to drop closer to ambient temperature more quickly, resulting in a significant radial thermal gradient. The gradient drives the reaction interface from the edge to the center, similar to a "heat wave" propagation, with the edge region completing the reaction first due to rapid heat dissipation and the center region lagging due to heat accumulation. Under low-temperature conditions, the exothermic heat at the beginning of the reaction is rapidly exported by the high thermal conductivity gradient, thus avoiding the reaction rate decay due to overheating in the center region. This "heat dissipation while reacting" mode maintains a continuous pressure difference, keeping the reaction rate high at low temperatures. Conversely, the mid-term rate slowdown is caused by diffusion limitation and activation energy effects, which highlights the nonlinear coupling of thermodynamics and kinetics. The strong radial thermal gradient drives the reaction interface from the edge to the center as a "heat wave" - the edge region is the first to complete the reaction due to rapid heat dissipation. In contrast, the center region lags due to heat accumulation. This "heat dissipation while reacting" mode avoids rate decay due to overheating in the center and keeps the overall reaction rate high by maintaining a constant pressure difference. Notably, the rate decline in the middle and late stages of the reaction results from diffusion limitation and competing activation energy effects. The current model does not consider the thermal conductivity changes caused by the low-temperature phase transition (α→βMgH₂) and the interfacial hindrance of the magnesium oxide impurity layer, which may lead to the deviation of the actual working conditions from the theoretical predictions.

 

 

Comment3: It is recommended that authors add some recently published references on hydrogen storage, hydrogen compression materials, especially in 2025 and 2024. A one-step approach towards hydrogen generation, storage, transportation and application without hydrogen compression step as the new method for Li (Na) BH4 regeneration via using Magnesium alloys or hydrides as the new topics for hydrogen-energy process chain and hydrogen economics published in ‘Adv. Energy Mater., 2017, 1700299’ and ‘Angew. Chem. Int. Ed., 59(2020)8623–8629’ was suggested to be summarized.

Response3: Thank you for your valuable suggestions, which have been added to the article.

The revised objective now reads: line118-126

Beyond thermal management optimization, low-cost regeneration of hydrogen storage materials is equally crucial, as it indirectly reduces overall system energy consumption by minimizing raw material usage. Yongyang Zhu et al. [27] developed a room-temperature ball milling process that regenerates NaBH₄ with 78.9% yield by treating NaBH₄ hydrolysis products with CO₂, followed by ball milling with magnesium powder. This method eliminates the need for high temperature/pressure and expensive reagents, enabling hydrogen storage material recycling and facilitating practical applications of solid-state hydrogen storage technology.

[27] Zhu, Y., Ouyang, L., Zhong, H., Liu, J., Wang, H., Shao, H., Huang, Z., & Zhu, M. (2020). Closing the Loop for Hydrogen Storage: Facile Regeneration of NaBH4 from its Hydrolytic Product. Angewandte Chemie International Edition, 59(22), 8623-8629. https://doi.org/10.1002/anie.201915988

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear,

This is a very interesting work, synthetizing few years of investigation on modeling heat transfert inside a tank or a bench test for metal hydride storage system.

Main interest lies on the demonstration that models are quite well appropriate nowadays to describe thermal phenomena and to have prediction of temperature evolution occuring both during absorption/desorption of H2. Could we go a bit further now and simulate evolution of system according to time and utilization (including degragation for instance)?

I thing bibliography could have been extended.

Thanks for attention

Author Response

Comment1: Main interest lies on the demonstration that models are quite well appropriate nowadays to describe thermal phenomena and to have prediction of temperature evolution occurring both during absorption/desorption of H2. Could we go a bit further now and simulate evolution of system according to time and utilization (including degradation for instance)?

Response1: Your recommendations are helpful, and precisely predicting the pattern of change in system performance over long lengths of time due to material degradation and structural fatigue is critical. However, simple assumptions and static parameters in present models are insufficient; physiological systems that change over time must be considered, and multi-field coupled models established. Future research must focus on creating new models that take into account the following key factors: the decrease in reactivity with increasing number of cycles, leading to a gradual increase in activation energy and a steady decrease in frequency factor; the deterioration of the material's thermophysical properties, such as the decrease in porosity, leading to a decrease in thermal conductivity; and the cumulative effect of structural stresses due to volume expansion. Modeling these dynamic changes allows for more precise prediction of system performance progression. This work provides a foundational quasi-model for thermodynamic and kinetic analysis in future research. Future research must incorporate additional physical models, such as material degradation mechanisms and multi-physics field dynamic coupling, into the current system to give more thorough theoretical support for the optimal design and lifetime evaluation of hydrogen storage reactors. The publication includes additional information on this point. Thank you for the suggestions.

The revised objective now reads: line915-930

To accurately simulate the system's performance evolution over extended use, influenced by material degradation and structural fatigue, the current model with simplistic assumptions and static parameters is inadequate. It is essential to incorporate a time-dependent physical mechanism and develop a multi-field coupled model. Future research must focus on creating new models that concurrently consider the following critical factors: the decline in reactivity with the number of cycles, resulting in a progressive rise in activation energy and a steady reduction in the frequency factor; the deterioration of the material's thermophysical properties, such as the decrease in thermal conductivity due to reduced porosity; and the cumulative impact of structural stresses caused by volume expansion. The simulation of these dynamic changes enables a more precise forecast of the system's performance evolution.  This paper presents a foundational paradigm for thermodynamic and kinetic analysis in future research. The subsequent study must integrate novel physical models, including material degradation mechanisms and multi-physics field dynamic coupling, into the current system to furnish more comprehensive theoretical support for the optimal design and life evalu-ation of hydrogen storage reactors.

 

 

 

Comment2: I thing bibliography could have been extended.

Response2: Thank you for your suggestion, I have added the bibliography.

The revised objective now reads: line1006-1066

 

[20] He, T., Pachfule, P., Wu, H., Xu, Q., & Chen, P. (2016). Hydrogen carriers. Nature Reviews Materials, 1(12), 1-17. https://doi.org/10.1038/natrevmats.2016.59

 

[22] Huot, J., Liang, G., Boily, S., Van Neste, A., & Schulz, R. (1999). Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride. Journal of Alloys and Compounds, 293-295, 495-500. https://doi.org/10.1016/s0925-8388(99)00474-0

 

[23] Shen, S., Liao, W., Cao, Z., Liu, J., Wang, H., & Ouyang, L. (2024). Enhanced hydrogen storage properties of MgH₂ with the co-addition of LiBH₄ and YNi₅ alloy. Journal of Materials Science & Technology, 178, 90-99. https://doi.org/10.1016/j.jmst.2023.08.039

 

[24] Lan, Z., Liu, Z., Liang, H., Shi, W., Zhao, R., Li, R., Fan, Y., Liu, H., & Guo, J. (2024). Introducing Ni-N-C ternary nanocomposite as an active material to enhance the hydrogen storage properties of MgH₂. Journal of Material Science and Technology, 196, 12-24. https://doi.org/10.1016/j.jmst.2024.01.055

 

[25] Xu, H., Shi, T., Xu, H., & Yu, G. (2025). Regulation and optimization of solid-state hydrogen storage process in thermochemical reactors with metal hydride by using gradient metal foams. Applied Thermal Engineering, 264, 125464. https://doi.org/10.1016/j.applthermaleng.2025.125464

 

[26] Tong, L., Xiao, J., Yang, T., Bénard, P., & Chahine, R. (2018). Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger. International Journal of Hydrogen Energy, 44(30), 15907-15916. https://doi.org/10.1016/j.ijhydene.2018.07.102

 

[27] Zhu, Y., Ouyang, L., Zhong, H., Liu, J., Wang, H., Shao, H., Huang, Z., & Zhu, M. (2020). Closing the Loop for Hydrogen Storage: Facile Regeneration of NaBH4 from its Hydrolytic Product. Angewandte Chemie International Edition, 59(22), 8623-8629. https://doi.org/10.1002/anie.201915988

 

[34] El Mghari, H., Huot, J., & Xiao, J. S. (2019). Analysis of hydrogen storage performance of metal hydride reactor with phase change materials. International Journal of Hydrogen Energy, 44(49), 27834-27851. https://doi.org/10.1016/j.ijhydene.2019.09.090.

 

[36] Kuznetsov, A., & Vafai, K. (1995). Analytical comparison and criteria for heat and mass transfer models in metal hydride packed beds. International Journal of Heat and Mass Transfer, 38(15), 2873-2884. https://doi.org/10.1016/0017-9310(94)00331-o

 

[37] Jemni, A., Nasrallah, S. B., & Lamloumi, J. (1999). Experimental and theoretical study of ametal-hydrogen reactor. International Journal of Hydrogen Energy, 24(7), 631-644. https://doi.org/10.1016/S0360-3199(98)00117-7

 

[38] Mayer, U., Groll, M., & Supper, W. (1987). Heat and mass transfer in metal hydride reaction beds: Experimental and theoretical results. Journal of the Less Common Metals, 131(1-2), 235-244. https://doi.org/10.1016/0022-5088(87)90523-6

 

[39] Aldas, K., Mat, M. D., & Kaplan, Y. (2002). A three-dimensional mathematical model for absorption in a metal hydride bed. International Journal of Hydrogen Energy, 27(10), 1049-1056. https://doi.org/10.1016/S0360-3199(02)00010-1

 

[43] Chaise, A., Marty, P., de Rango, P., & Fruchart, D. (2009). A simple criterion for estimating the effect of pressure gradients during hydrogen absorption in a hydride reactor. International Journal of Heat and Mass Transfer, 52(19-20), 4564-4572. https://doi.org/10.1016/j.ijheatmasstransfer.2009.03.052

 

[44] Zhao, Y., Zhao, C. Y., Markides, C. N., Wang, H., & Li, W. (2020). Medium-and high-temperature latent and thermochemical heat storage using metals and metallic compounds as heat storage media: A technical review. Applied Energy, 280, 115950. https://doi.org/10.1016/j.apenergy.2020.115950

 

[45] Li, H., Chen, Y., Leng, L., & Hu, Y. (2021). Thermochemical energy storage of concentrated solar power by novel Y2O3-doped CaO pellets. Energy & Fuels, 35(15), 12610-12618. https://doi.org/10.1021/acs.energyfuels.1c01270

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The following article “Revealing the Roles of Heat Transfer, Thermal Dynamics and Reaction Kinetics in Hydrogenation/Dehydrogenation Processes for Mg-Based Metal Hydride Hydrogen Storage” is written in good manners however following revision are required.

1. The written English needs improving to be clearer, correct and more flowing.
2. Industrial processing and scale-up considerations?
3. Novelty needs to be stated.
4. Please improve the citations 2024-2025.

Comments for author File: Comments.pdf

Author Response

Comment1: The written English needs improving to be clearer, correct and more flowing.

Response1: Thanks to your helpful recommendations, we've encouraged native English speakers from the United States to assist us improve our articles.

The revised objective now reads: complete text

 

 

Comment2: Industrial processing and scale-up considerations?

Response2: Thank you for the suggestion; including this material will make the article more thorough. I've included 4.5 as a distinct part on topics to consider in real engineering applications.  The topic of increasing the volume has also been raised.

The revised objective now reads: line861-911

Magnesium hydride serves as an effective hydrogen storage medium with significant potential applications, and McPhy Energy has commercialized the ball mill pro-duction process. Future advancements in larger reactors must prioritize the improvement of thermal management, namely through the creation of integrated thermal storage devices to effectively harness the precious heat generated during the absorption of magnesium hydride. To improve the applicability of this research in actual engineering, the effects of reactor structural characteristics (such as wall thickness), ma-terial qualities, and control systems on hydrogen storage performance are particularly significant. The mechanism of performance evolution in materials and systems after prolonged recycling should be investigated.

 The thickness of the reactor wall influences the heat transfer efficiency and structural integrity of the hydrogen storage system. From the perspective of heat transfer, an increase in wall thickness markedly diminishes heat transfer efficiency. Fourier's law states that the heat flow density is reduced by half as the wall thickness increases from 5 mm to 10 mm, leading to a corresponding reduction in the heat flow density of the aluminum shell at a temperature differential of 300°C. The thermal re-action time constant of the hydrogen storage system escalates with temperature. The thermal response time constant exhibits a square relationship with wall thickness; thus, when wall thickness is doubled, the thermal response time increases by a factor of four. For instance, a 5 mm aluminum shell has a response time of 100 seconds, which ex-tends to 400 seconds at 10 mm thickness, leading to considerable temperature response hysteresis. Thick walls further intensify the radial temperature gradient, resulting in a smaller temperature differential between the edge and the center at a 5 mm wall thickness compared to 10 mm. This phenomenon increases response inhomogeneity, evident in the reaction completing at the edge of the center with a lower conversion rate. Moreover, augmented wall thickness reduces the effective hydrogen storage ca-pacity, diminishing by 9.5% when the wall thickness of the 100 mm OD reactor is elevated from 5 mm to 10 mm. The wall thickness directly influences the pressure-bearing capability in mechanical design. The aluminum shell, with a wall thickness of 5 mm, can withstand a pressure of 10MPa at 300°C, satisfying the safety criterion of a 1.0 MPa operating condition. However, as the temperature escalates to 350 °C, the tank, due to thermal expansion and contraction resulting from a rapid increase, may approach the aluminum's bearing limit. A significant concern is that the MgH₂ phase transition will undergo expansion and contraction, generating mechanical loads that induce radial stresses potentially exceeding the aluminum shell's capacity. To address this issue, engineers incorporated a layer of "cushioning pads" within the tank's inner wall, specifically a porous flexible liner. Another factor to consider is fatigue damage to the tank material; routine inspections of stress-prone locations can enhance operational safety. The engineering implementation of the control method mostly relies on thermal management technology. The article's background section succinctly outlines current research initiatives in thermal management, highlighting the coupled thermal storage system as a promising approach. This includes the hydrochloric acid thermal storage system, metal hydroxide thermal storage system, metal oxide thermal storage system, and phase change material (PCM) thermal storage system. These thermal storage technologies are thermochemical, which offer the benefits of high thermal density and minimal energy loss [44]. Li et al. [45] examined the impact of incorporating Y₂O₃ as a metallic framework into CaO, revealing that the composites could sustain a high thermal density of up to 1956 kJ/kg after 100 cycles with a Y₂O₃ content of 35 wt%. The team additionally pelletized the composites. Despite the granulation process compromising a portion of the material's structure and diminishing thermal storage efficacy, the mechanical strength of the material was markedly enhanced. Experiments demonstrated that the pelletized material experienced a mass loss of merely 4.28 wt% after 7,000 rotations in a brittleness tester.

 

Comment3: Novelty needs to be stated.

Response3: Regarding your recommendation, we have made changes to the article.

The revised objective now reads: line912-915

This paper presents a model that effectively characterizes the thermal phenom-ena and temperature variation patterns of the magnesium hydride hydrogen storage system during hydrogen absorption and discharge, with its innovation evident in the integrated analysis of thermodynamic equilibrium and kinetic processes.

 

 

Comment4: Please improve the citations 2024-2025.

Response4: Thank you for your suggestion, I have added the citations 2024-2025.

The revised objective now reads:

 

[23] Shen, S., Liao, W., Cao, Z., Liu, J., Wang, H., & Ouyang, L. (2024). Enhanced hydrogen storage properties of MgH₂ with the co-addition of LiBH₄ and YNi₅ alloy. Journal of Materials Science & Technology, 178, 90-99. https://doi.org/10.1016/j.jmst.2023.08.039

 

[24] Lan, Z., Liu, Z., Liang, H., Shi, W., Zhao, R., Li, R., Fan, Y., Liu, H., & Guo, J. (2024). Introducing Ni-N-C ternary nanocomposite as an active material to enhance the hydrogen storage properties of MgH₂. Journal of Material Science and Technology, 196, 12-24. https://doi.org/10.1016/j.jmst.2024.01.055

 

[25] Xu, H., Shi, T., Xu, H., & Yu, G. (2025). Regulation and optimization of solid-state hydrogen storage process in thermochemical reactors with metal hydride by using gradient metal foams. Applied Thermal Engineering, 264, 125464. https://doi.org/10.1016/j.applthermaleng.2025.125464

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Manuscript "Revealing the Roles of Heat Transfer, Thermal Dynamics and Reaction Kinetics in Hydrogenation/Dehydrogenation Processes for Mg-Based Metal Hydride Hydrogen Storage".

Majors:

1) Section 2.1. and Figure 1. From the figure and the text description, it can be concluded that the inner tank is completely impermeable. There are arrows at the bottom that indicate the inlet and outlet. But it is not clear how the input and outlet of the components are physically implemented. This is a model with assumptions. But it is necessary to describe how the input and outlet of hydrogen is organized in a real tank and in the model.

2) Lines 195-196. The authors write that they ignore the movement of the hydrogen flow. At the same time, equation (5) contains a convective component and a velocity variable. Where is the error here? It is also necessary to substantiate the assumption of the absence of gas movement.

3) In the problem statement, the authors write about the expansion of the volume and different values ​​of pressure at the boundaries during hydrogenation and dehydrogenation. In the results section, there are a lot of graphs and fields with temperature and hydrogen concentration. However, there is not a single figure with graphs and pressure fields. This is also an interesting value that affects the strength of the reactor.

4) Figure 3 shows different boundaries 2, 3 and 4. The conditions at boundary 4 differ from the conditions at boundaries 2 and 3. As I understand it, hydrogen enters and exits through boundary 4. From figures 13 and 17, the differences in these boundaries are not noticeable. Near boundary 2 and boundary 4, there are almost identical changes in temperature and hydrogen concentration. Although the upper boundary is impenetrable, and the lower one acts as an inlet and outlet. How can this picture be explained? Perhaps the reactor model is too simplified?

The manuscript cannot be accepted for publication in its current form. The manuscript can be re-examined after revision.

Author Response

Comment1: Section 2.1. and Figure 1. From the figure and the text description, it can be concluded that the inner tank is completely impermeable. There are arrows at the bottom that indicate the inlet and outlet. But it is not clear how the input and outlet of the components are physically implemented. This is a model with assumptions. But it is necessary to describe how the input and outlet of hydrogen is organized in a real tank and in the model.

Response1: Thank you for offering a thoughtful assessment, which has been amended and depicted in Figure 1.

The revised objective now reads: line181-183

As shown in Fig. 1, heat is carried away by conduction through the vessel walls, the vessel shell, and natural convection with the surrounding air, and H2 flows in and out through the pass-through.

 

Figure 1. Geometric design of hydrogen storage.

 

 

Comment2: Lines 195-196. The authors write that they ignore the movement of the hydrogen flow. At the same time, equation (5) contains a convective component and a velocity variable. Where is the error here? It is also necessary to substantiate the assumption of the absence of gas movement.

Response2: Your review was really detailed. Equation (5) is the more thorough energy equation; however, because we ignored the flow of hydrogen in our simulations, including the convective component and velocity variables in equation (5) would have created uncertainty, which has now been addressed. And the premise that there is no gas motion has been verified. Suggestions are accepted.

The revised objective now reads: line186-198; line250-255; line269-280

Many studies have been conducted on mathematically modeling a metal hydride hydrogen reactor, and scholars have used different methods to provide an in-depth analysis of the reactor's heat and mass transfer processes. Some simplify the system by considering the main physical phenomena during hydrogenation. Kuznetsov and Vafai [36] presented theoretical criteria for judging the model's validity without need-ing experimental tests. The models involved assumptions such as local thermal equi-librium, steady-state, and frontal model approximation. Jemni et al. [37] further showed that the pressure gradient had a negligible effect on reactor performance, supporting the assumption of uniform pressure. Mayer et al. [38] created a two-dimensional mathematical model to study the hydrogenation and dehydrogenation process, neglecting convective effects. The simulation results are in better agreement with the experimental data. Aldas et al. [39] performed three-dimensional simulations and showed that neglecting the gas flow only affects the temperature distribution, while other parameters (e.g., pressure) are little affected.

The possibility of avoiding the calculation of hydrogen flow in hydride storage tanks during adsorption has been investigated [43]. From this work, three conditions have to be satisfied to neglect the influence of the gas flow: 1) convective heat transfer is negligible compared to thermal diffusion; 2) sensible energy required to change the temperature in the reactor is negligible compared to the heat of reaction; 3) pressure changes caused by hydrogen flow have a negligible effect on the rate of reaction.

In our case, the axial height of the tank () is 0.07 m, and the tank radius () is 0.035 m. We estimate a characteristic velocity of V =3 × 10⁻⁴ m/s from the mean mass flow rate. This gives a Peclet number (Pe) of 0.2. Although this value is close to unity, heat convection remains negligible compared to conduction. Table 2 lists the main parameters of the calculation. Calculations give C > 1000 and N < 10-3, showing that thermal diffusion dominates the reaction kinetics and controls the hydrogenation process, with pressure changes having a negligible effect on hydrogenation. Based on these findings, our numerical simulations can neglect the hydrogen flow. Consequently, the pressure distribution is treated as spatially uniform while remaining time-dependent. The pressure evolution within the powder bed is calculated through a dedicated subroutine that accounts for: 1) hydrogen inflow through the filter, 2) hydrogen consumed during the reaction, and 3) hydrogen occupying the void spaces, maintaining mass balance throughout the process.

 

[36] Kuznetsov, A., & Vafai, K. (1995). Analytical comparison and criteria for heat and mass transfer models in metal hydride packed beds. International Journal of Heat and Mass Transfer, 38(15), 2873-2884. https://doi.org/10.1016/0017-9310(94)00331-o

 

[37] Jemni, A., Nasrallah, S. B., & Lamloumi, J. (1999). Experimental and theoretical study of ametal-hydrogen reactor. International Journal of Hydrogen Energy, 24(7), 631-644. https://doi.org/10.1016/S0360-3199(98)00117-7

 

[38] Mayer, U., Groll, M., & Supper, W. (1987). Heat and mass transfer in metal hydride reaction beds: Experimental and theoretical results. Journal of the Less Common Metals, 131(1-2), 235-244. https://doi.org/10.1016/0022-5088(87)90523-6

 

[39] Aldas, K., Mat, M. D., & Kaplan, Y. (2002). A three-dimensional mathematical model for absorption in a metal hydride bed. International Journal of Hydrogen Energy, 27(10), 1049-1056. https://doi.org/10.1016/S0360-3199(02)00010-1

 

[43] Chaise, A., Marty, P., de Rango, P., & Fruchart, D. (2009). A simple criterion for estimating the effect of pressure gradients during hydrogen absorption in a hydride reactor. International Journal of Heat and Mass Transfer, 52(19-20), 4564-4572. https://doi.org/10.1016/j.ijheatmasstransfer.2009.03.052

 

 

Comment3: In the problem statement, the authors write about the expansion of the volume and different values ​​of pressure at the boundaries during hydrogenation and dehydrogenation. In the results section, there are a lot of graphs and fields with temperature and hydrogen concentration. However, there is not a single figure with graphs and pressure fields. This is also an interesting value that affects the strength of the reactor.

Response3: Section 4.3 has been included as a result of your helpful suggestion to depict pressure fluctuations during hydrogenation and dehydrogenation.

The revised objective now reads: line410-422

In the hydrogenation process, the interpolation function is employed to expedite the increase of hydrogen filling pressure, which then decelerates to attain the target pressure value. Conversely, in the dehydrogenation process, the system stabilizes under an initial constant pressure, and upon triggering the reaction, it swiftly diverges from the equilibrium state.  The pressure curve exhibits a pronounced decline. Subsequently, as the dehydrogenation reaction advances, magnesium hydride (MgH₂) persistently decomposes to generate hydrogen, which is progressively expelled from the system, leading to a reduction in the hydrogen quantity and a gradual decline in pressure. The pressure diagrams for the hydrogenation and dehydrogenation processes are illustrated in Fig. 5.

 

Figure 5. Variations in pressure during hydrogenation and dehydrogenation processes.

 

 

 

Comment4: Figure 3 shows different boundaries 2, 3 and 4. The conditions at boundary 4 differ from the conditions at boundaries 2 and 3. As I understand it, hydrogen enters and exits through boundary 4. From figures 13 and 17, the differences in these boundaries are not noticeable. Near boundary 2 and boundary 4, there are almost identical changes in temperature and hydrogen concentration. Although the upper boundary is impenetrable, and the lower one acts as an inlet and outlet. How can this picture be explained? Perhaps the reactor model is too simplified?

Response4: We sincerely appreciate the reviewer’s insightful comments regarding the boundary effects in our model. The reviewer raised a valid point about the indistinct differences near Boundary 2 and Boundary 4, which is indeed due to the small size of the model and the influence of the volume effect. The article line345-353 addresses this issue with additional clarification, our primary focus was on the overall thermo-dynamic behavior and kinetic processes within the hydrogen storage tank, rather than the localized flow characteristics near the boundaries. This simplified approach allows us to maintain computational accuracy while significantly improving efficiency, which aligns with the main objectives of this study. We fully acknowledge that boundary effects could play a more prominent role in certain scenarios, and we agree that further investigation into these effects could provide additional insights. However, given the scope of this study, we believe that the current simplification is justified. In future work, we plan to explore the boundary effects in greater detail, especially for larger-scale systems or applications where localized flow characteristics are critical. Once again, we thank the reviewer for their valuable feedback, which will undoubtedly help improve our research.

The revised objective now reads: line345-353

This study employs a simplified geometric model that is considerably smaller than the actual hydrogen storage system. In the modeling procedure, boundary 4 is established as a uniform boundary condition for the hydrogen inflow and output. This results from the model's reduced size, and the constraints of the volume effect diminish the impact of inlet localization. This study primarily focuses on the comprehensive thermodynamic behavior and kinetic processes within the hydrogen storage tank, rather than the localized flow characteristics at the boundaries. This streamlined approach can significantly enhance computational efficiency while maintaining accuracy, aligning with the primary aim of this research.

Author Response File: Author Response.docx

Reviewer 5 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for the great work delivered. Kindly address the comments attached and re-submit. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The manuscript is generally understandable but contains several grammatical errors and spelling mistakes (e.g., “mornitoring” instead of “monitoring”). A thorough language revision is recommended to improve clarity, consistency, and professionalism.

Author Response

Technical Comments

Comment1: The paper assumes fixed values for activation energy and pre-exponential factors (e.g.,

130 kJ/mol, k0=10). A sensitivity study on how these values influence simulation accuracy would strengthen the kinetic credibility.

Response1: Thank you for your valuable suggestion, the difficulties you identified were thoroughly considered, and the paper provides a detailed explanation of how the two parameters are evaluated.

The revised objective now reads: line317-323

Plotting ln(k)—ln ((/) -1) vs. 1/T for kinetics analysis, measured at 1 MPa and 0.2 MPa, allows us to determine  and  during absorption. Results showed that =132kJ/mol and =9.89s-1 when the pressure is 1 Mpa, and at a pressure of 0.2 Mpa,=128kJ/mol and =1.1410s-1.According to the conclusions of studies, the activation energy  and the frequency factor  of the hydrogenation reaction are almost insensitive to the pressure change, and in order to simplify the numerical calculations, the central values of =130kJ/mol and =1010s-1 are used.

 

Comment2: The assumption of negligible hydrogen flow and constant material properties simplifies the system. A brief discussion of the potential impacts on accuracy (particularly at high reaction rates or temperature gradients) would enhance transparency.

Response2: Thank you for your useful insight; the idea of bypassing the computation of hydrogen flow in hydride storage tanks has been studied, and the effect has been demonstrated to be insignificant when specific criteria are met. This section is explained and calculated in depth in the article.

The revised objective now reads: line250-255; line269-280

The possibility of avoiding the calculation of hydrogen flow in hydride storage tanks during adsorption has been investigated [43]. From this work, three conditions have to be satisfied to neglect the influence of the gas flow: 1) convective heat transfer is negligible compared to thermal diffusion; 2) sensible energy required to change the temperature in the reactor is negligible compared to the heat of reaction; 3) pressure changes caused by hydrogen flow have a negligible effect on the rate of reaction.

In our case, the axial height of the tank () is 0.07 m, and the tank radius () is 0.035 m. We estimate a characteristic velocity of V =3 × 10⁻⁴ m/s from the mean mass flow rate. This gives a Peclet number (Pe) of 0.2. Although this value is close to unity, heat convection remains negligible compared to conduction. Table 2 lists the main parameters of the calculation. Calculations give C > 1000 and N < 10-3, showing that thermal diffusion dominates the reaction kinetics and controls the hydrogenation process, with pressure changes having a negligible effect on hydrogenation. Based on these findings, our numerical simulations can neglect the hydrogen flow. Consequently, the pressure distribution is treated as spatially uniform while remaining time-dependent. The pressure evolution within the powder bed is calculated through a dedicated subroutine that accounts for: 1) hydrogen inflow through the filter, 2) hydrogen consumed during the reaction, and 3) hydrogen occupying the void spaces, maintaining mass balance throughout the process.

 

[43] Chaise, A., Marty, P., de Rango, P., & Fruchart, D. (2009). A simple criterion for estimating the effect of pressure gradients during hydrogen absorption in a hydride reactor. International Journal of Heat and Mass Transfer, 52(19-20), 4564-4572. https://doi.org/10.1016/j.ijheatmasstransfer.2009.03.052

 

 

Comment3: The paper should clarify how uniform temperature is maintained during dehydrogenation, particularly in cases where external heating is required.

Response3: Thank you for your useful feedback; the proposed adjustments have been implemented into the manuscript.

The revised objective now reads: line353-356

In the simulation of the dehydrogenation process, it is presumed that the temperature remains constant throughout. Optimal temperature conditions can be achieved by incorporating adjustable heating coils within the shell to supply the requisite heat for hydrogen desorption.

 

Comment4: The numerical model does not account for possible hysteresis effects between hydrogenation and dehydrogenation. Kindly elaborate on that.

Response4: 

We genuinely value the reviewer’s astute observation concerning the hysteresis effects associated with hydrogenation and dehydrogenation. The observed pressure hysteresis arises from intrinsic thermodynamic and kinetic disparities in these processes: hydrogenation necessitates surmounting significant activation energy barriers (approximately 130 kJ/mol for MgH₂), causing actual equilibrium pressures to surpass theoretical values, whereas dehydrogenation is impeded by product-layer diffusion, resulting in diminished actual equilibrium pressures. This generates unique absorption/desorption pathways that produce hysteresis loops in PCT curves, which our existing model, predicated on a singular van’t Hoff equilibrium curve, fails to explicitly represent. This simplification corresponds with established methodologies in previous research (e.g., [32,42]) that concentrate on first-principle thermo-kinetic coupling effects during discrete cycles, wherein hysteresis is subordinate to transient heat and mass transport. Although the model successfully fulfills its primary objective of examining bulk reaction kinetics and thermal behavior, we recognize the significance of hysteresis in multi-cycle activities. In subsequent research, we will integrate Van’t Hoff equations and kinetic correction factors to mitigate diffusion constraints. We appreciate the reviewer for this insightful recommendation, which augments the model's applicability in practical contexts.

 

 

 

Comment5: The mesh-independence study could benefit from a quantified error analysis.

Response5: Incorporating your invaluable recommendations, the mesh-independence validation section of the article has been revised.

The revised objective now reads: line369-385

In this study, a Multiphysics field numerical model is developed to simulate the hydrogen addition and release processes in a metal hydride hydrogen storage reactor, including the physics fields of reaction kinetics, fluid flow, heat transfer in porous media, domain ordinary differential equations and differential algebraic equations. To determine the grid division of the model, it is necessary to verify the effect of the number of grid divisions on the experimental results. The physical field control grid in the COMSOL Multiphysics software is selected, and simulations are performed for the three divisions: regular (number of meshes is 30536), ultrafine (number of meshes is 58529), refinement (number of meshes is 108817). The reaction progress at the monitoring point (2.5, 3.5) is selected as the evaluation index. From Fig. 4(a), it can be seen that the number of meshes changes from 30536 to 58529, and the curve changes are more obvious, which indicates that the simulation accuracy using the regular mesh is not high enough. When the number of meshes is changed from 58529 to 108817, the curves overlap, the calculation results no longer change significantly, and the average deviation of the two calculation results is 0.0025, which satisfies the requirement of calculation accuracy. The ultrafine mesh size of 58529 is finally chosen to simulate the hydrogen loading process of conventional metal hydrides.

 

 

 

Comment6: The use of a 2D axisymmetric model is practical, but a brief justification of its representativeness for 3D phenomena is needed.

Response6: We sincerely appreciate the reviewer’s valuable suggestion regarding the use of a 2D axisymmetric model. In response to this comment, we have added a 3D schematic diagram in the revised manuscript (Fig. 1) to clarify that the 2D cross-section represents a typical unit of the 3D phenomenon, maintaining full geometric and physical representativeness. This approach is consistent with widely adopted practices in similar studies (e.g., [34]), where 2D axisymmetric models have been effectively employed to analyze symmetric systems while significantly reducing computational costs. The 2D simplification retains the essential thermo-kinetic characteristics of the 3D system, as demonstrated by the agreement between our simulation results and experimental data (Fig.4(a)). We thank the reviewer for this constructive feedback, which has helped improve the clarity of our methodology.

The revised objective now reads: line174-177

The MgH₂ hydrogen storage reactor is designed as a three-dimensional cylindrical structure exhibiting symmetry. To streamline the calculations, the longitudinal two-dimensional cross-section is utilized as a unit for analysis. A comparable approach to research methodology was employed by Hafsa El Mghari et al [34].

[34] El Mghari, H., Huot, J., & Xiao, J. S. (2019). Analysis of hydrogen storage performance of metal hydride reactor with phase change materials. International Journal of Hydrogen Energy, 44(49), 27834-27851. https://doi.org/10.1016/j.ijhydene.2019.09.090.

 

 

Comment7: The boundary condition implementation for natural convection and external heat transfer lacks detail.

Response7: We express our gratitude to the reviewers for their astute observations regarding the boundary conditions for natural convection and external heat transfer. This work examines the internal thermodynamic and kinetic properties of the MgH₂ bed, which is the fundamental mechanism of the hydrogen storage system. The exterior heat exchange is reduced to standard natural convection boundary conditions, in alignment with common passive cooling strategies. This simplification enables us to concentrate our computational resources on addressing critical internal processes while preserving enough accuracy. We acknowledge that enhanced characterization may be beneficial for research focused on system-environment interactions, and this will be taken into account in further investigations. We appreciate your helpful recommendations that enhance the transparency of our efforts.

 

 

 

Comment8: No mention is made of possible phase change behavior or structural transformations within Mg/MgH2 during repeated cycling. Kindly address this point in the manuscript.

Response8: Thank you for your astute recommendation, which has been integrated into the text.

The revised objective now reads: line892-896; line912-930

A significant concern is that the MgH2 phase transition will undergo expansion and contraction, generating mechanical loads that induce radial stresses potentially exceeding the aluminum shell's capacity. To address this issue, engineers incorporated a layer of "cushioning pads" within the tank's inner wall, specifically a porous flexible liner.

This paper presents a model that effectively characterizes the thermal phenomena and temperature variation patterns of the magnesium hydride hydrogen storage system during hydrogen absorption and discharge, with its innovation evident in the integrated analysis of thermodynamic equilibrium and kinetic processes. To accurately simulate the system's performance evolution over extended use, influenced by material degradation and structural fatigue, the current model with simplistic assumptions and static parameters is inadequate. It is essential to incorporate a time-dependent physical mechanism and develop a multi-field coupled model. Future research must focus on creating new models that concurrently consider the following critical factors: the decline in reactivity with the number of cycles, resulting in a progressive rise in activation energy and a steady reduction in the frequency factor; the deterioration of the material's thermophysical properties, such as the decrease in thermal conductivity due to reduced porosity; and the cumulative impact of structural stresses caused by volume expansion. The simulation of these dynamic changes enables a more precise fore-cast of the system's performance evolution.  This paper presents a foundational paradigm for thermodynamic and kinetic analysis in future research. The subsequent study must integrate novel physical models, including material degradation mechanisms and multi-physics field dynamic coupling, into the current system to furnish more comprehensive theoretical support for the optimal design and life evaluation of hydrogen storage reactors.

 

General Comments

Comment1: Several instances of grammatical errors and non-standard terms (e.g., “mornitoring point,” “curve curvature,” “thermokinetic”) require correction for clarity and professionalism.

Response1: Thank you for your meticulous review; the content has been amended.

 

 

Comment2: A separate section discussing potential design implications (e.g., reactor wall thickness, material selection, control strategies) would enhance the engineering relevance of the findings.

Response2: Section 4.5 has been incorporated into the text to address this aspect, as per your recommendation.

The revised objective now reads: line871-899

The thickness of the reactor wall influences the heat transfer efficiency and structural integrity of the hydrogen storage system. From the perspective of heat transfer, an increase in wall thickness markedly diminishes heat transfer efficiency. Fourier's law states that the heat flow density is reduced by half as the wall thickness increases from 5 mm to 10 mm, leading to a corresponding reduction in the heat flow density of the aluminum shell at a temperature differential of 300°C. The thermal re-action time constant of the hydrogen storage system escalates with temperature. The thermal response time constant exhibits a square relationship with wall thickness; thus, when wall thickness is doubled, the thermal response time increases by a factor of four. For instance, a 5 mm aluminum shell has a response time of 100 seconds, which ex-tends to 400 seconds at 10 mm thickness, leading to considerable temperature response hysteresis. Thick walls further intensify the radial temperature gradient, resulting in a smaller temperature differential between the edge and the center at a 5 mm wall thickness compared to 10 mm. This phenomenon increases response inhomogeneity, evident in the reaction completing at the edge of the center with a lower conversion rate. Moreover, augmented wall thickness reduces the effective hydrogen storage capacity, diminishing by 9.5% when the wall thickness of the 100 mm OD reactor is elevated from 5 mm to 10 mm. The wall thickness directly influences the pressure-bearing capability in mechanical design. The aluminum shell, with a wall thickness of 5 mm, can withstand a pressure of 10MPa at 300°C, satisfying the safety criterion of a 1.0 MPa operating condition. However, as the temperature escalates to 350 °C, the tank, due to thermal expansion and contraction resulting from a rapid increase, may approach the aluminum's bearing limit. A significant concern is that the MgH₂ phase transition will undergo expansion and contraction, generating mechanical loads that induce radial stresses potentially exceeding the aluminum shell's capacity. To address this issue, engineers incorporated a layer of "cushioning pads" within the tank's inner wall, specifically a porous flexible liner. Another factor to consider is fatigue damage to the tank material; routine inspections of stress-prone locations can enhance operational safety. The engineering implementation of the control method mostly relies on thermal management technology.

 

 

Comment3: Consider citing additional recent works to enrich the literature background, particularly regarding Mg-based hydride simulations and heat management strategies in storage reactors:

A.M. Sadeq, Hydrogen: The Carrier of Future Energy,1st ed., Aug.2023[Online]. DOl: 10.13980/RG.2.2.33942.88992.ISBN: 979-8-9907836-2-1.

2024. Wang, B Han, Z. Zeng, S. Wen, F.Xu, and Y. Du, "Understanding the dehydrogenationproperties of Mg (0001)/MgH (110) interface from first principles," Materials Reports: Energy, vol. 4, no.1, p.100254, Feb. 2024. [Online]. Available: https://doi.org/10.1016/j.matre.2024.100254.

Response3: Due to your recommendation, a party has been incorporated into the text regarding heat management measures in magnesium-based hydride simulation and storage reactors.

The revised objective now reads: line109-126; line899-911

Thermal management represents another critical factor affecting metal hydride performance. Tao Shi et al. [25] demonstrated that aluminum foam-metal hydride composite reactors significantly improve thermal conductivity to enhance heat transfer. Long et al. [26] found that double-spiral coil reactors outperformed combined spiral-vertical tube designs during hydrogenation in comparative studies of different reactor configurations. These approaches share a common requirement for either dissipating reaction heat or supplying external desorption heat, which inevitably reduces overall energy efficiency. To optimize heat utilization, a more promising solution involves integrating thermal energy storage modules, such as carbonate-based systems. Beyond thermal management optimization, low-cost regeneration of hydrogen storage materials is equally crucial, as it indirectly reduces overall system energy consumption by minimizing raw material usage. Yongyang Zhu et al. [27] developed a room-temperature ball milling process that regenerates NaBH₄ with 78.9% yield by treating NaBH₄ hydrolysis products with CO₂, followed by ball milling with magnesium powder. This method eliminates the need for high temperature/pressure and expensive reagents, enabling hydrogen storage material recycling and facilitating practical applications of solid-state hydrogen storage technology.

The article's background section succinctly outlines current research initiatives in thermal management, highlighting the coupled thermal storage system as a promising approach. This includes the hydrochloric acid thermal storage system, metal hydroxide thermal storage system, metal oxide thermal storage system, and phase change material (PCM) thermal storage system. These thermal storage technologies are thermochemical, which offer the benefits of high thermal density and minimal energy loss [44]. Li et al. [45] examined the impact of incorporating Y₂O₃ as a metallic framework into CaO, revealing that the composites could sustain a high thermal density of up to 1956 kJ/kg after 100 cycles with a Y₂O₃ content of 35 wt%. The team additionally pelletized the composites. Despite the granulation process compromising a portion of the material's structure and diminishing thermal storage efficacy, the mechanical strength of the material was markedly enhanced. Experiments demonstrated that the pelletized material experienced a mass loss of merely 4.28 wt% after 7,000 rotations in a brittleness tester.

 

[25] Xu, H., Shi, T., Xu, H., & Yu, G. (2025). Regulation and optimization of solid-state hydrogen storage process in thermochemical reactors with metal hydride by using gradient metal foams. Applied Thermal Engineering, 264, 125464. https://doi.org/10.1016/j.applthermaleng.2025.125464

 

[26] Tong, L., Xiao, J., Yang, T., Bénard, P., & Chahine, R. (2018). Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger. International Journal of Hydrogen Energy, 44(30), 15907-15916. https://doi.org/10.1016/j.ijhydene.2018.07.102

 

[27] Zhu, Y., Ouyang, L., Zhong, H., Liu, J., Wang, H., Shao, H., Huang, Z., & Zhu, M. (2020). Closing the Loop for Hydrogen Storage: Facile Regeneration of NaBH4 from its Hydrolytic Product. Angewandte Chemie International Edition, 59(22), 8623-8629. https://doi.org/10.1002/anie.201915988

 

[44] Zhao, Y., Zhao, C. Y., Markides, C. N., Wang, H., & Li, W. (2020). Medium-and high-temperature latent and thermochemical heat storage using metals and metallic compounds as heat storage media: A technical review. Applied Energy, 280, 115950. https://doi.org/10.1016/j.apenergy.2020.115950

 

[45] Li, H., Chen, Y., Leng, L., & Hu, Y. (2021). Thermochemical energy storage of concentrated solar power by novel Y2O3-doped CaO pellets. Energy & Fuels, 35(15), 12610-12618. https://doi.org/10.1021/acs.energyfuels.1c01270

 

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

In the added figure 5, the temperature scale should be removed. The graphs describe only the pressure.

Author Response

Comment1: In the added figure 5, the temperature scale should be removed. The graphs describe only the pressure.

Response1: Thanks for your valuable suggestion, I have fixed Fig. 4.

The revised objective now reads: line420-421

Author Response File: Author Response.docx