Effect of gravity conditions on heat transfer and flow fluctuations of liquid hydrogen in a non-isothermal horizontal circular tube

Liquid hydrogen phase transition is a common phenomenon in space missions for space vehicles using low temperature liquid hydrogen as propellant. In this study, a numerical model with coupled RANS solver and VOF/Level-set method was used to simulate the liquid hydrogen phase transition in a non-isothermal horizontal circular tube under different gravity conditions (1g-10^?4 g). The gas phase hydrogen produced by evaporation of liquid hydrogen was calculated by Lee model. The statistics of the overall volume, heat flux, mass flow rate, mean velocity of gas phase hydrogen was carried out. The data results shown that the flow fluctuations were strongest under the gravity acceleration of 10^?1 g relative to other gravity conditions. The average bubble volume at 10^?1 g was the smallest, which was 11.58% smaller than that at 10^?3 g condition. The intermittent contact with the tube wall, which leaded to intermittent long bubble and flow resistance, was the main reason.     

Fluid thermal stratification in a non-isothermal liquid hydrogen tank under sloshing excitation

To the safe space operation of cryogenic storage tank, it is significant to study fluid thermal stratification under external heat leaks. In the present paper, a numerical model is established to investigate the thermal performance in a cryogenic liquid hydrogen tank under sloshing excitation. The interface phase change and the external convection heat transfer are considered. To realize fluid sloshing, the dynamic mesh coupled the volume of fluid (VOF) method is used to predict the interface fluctuations. A sinusoidal excitation is implemented via customized user-defined function (UDF) and applied on tank wall. The grid sensitivity study and the experimental validation of the numerical mode are made. It turns out that the present numerical model can be used to simulate the unsteady process in a non-isothermal sloshing tank. Variations of tank pressure, liquid and vapor mass, fluid temperature and thermal stratification are numerically investigated respectively. The results show that the sinusoidal excitation has caused large influence on thermal performance in liquid hydrogen tank. Some valuable conclusions are arrived, which is important to the depth understanding of the non-isothermal performance in a sloshing liquid hydrogen tank and may supply some technique reference for the methods of sloshing suppression.     

Effect of gravity on cryogenic boiling heat transfer during tube quenching

Cryogenic forced convective boiling under terrestrial and microgravity conditions for the development of cryogenic fluid management on orbit is studied. The experiments are conducted in a low mass velocity region (100–300 kg/m² s) that is easily influenced by gravity, and fluid behavior observations and heat transfer measurements are performed simultaneously. These experiments aim at understanding the effect of gravitational acceleration on the relation between the flow behavior and thermal characteristics during the quenching of the tube by a cryogenic fluid. The heat transfer increases under microgravity conditions, and results from an increase in the quench front velocity.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

The design and optimization of a cryogenic compressed hydrogen refueling process

Cryo-compressed hydrogen storage has excellent volume and mass hydrogen storage density, which is the most likely way to meet the storage requirements proposed by United States Department of Energy（DOE）. This paper contributes to propose and analyze a new cryogenic compressed hydrogen refueling station. The new type of low temperature and high-pressure hydrogenation station system can effectively reduce the problems such as too high liquefaction work when using liquid hydrogen as the gas source, the need to heat and regenerate to release hydrogen, and the damage of thermal stress on the storage tank during the filling process, so as to reduce the release of hydrogen and ensure the non-destructive filling of hydrogen. This paper focuses on the study of precooling process in filling. By establishing a heat transfer model, the dynamic trend of tank temperature with time in the precooling process of low-temperature and high-pressure hydrogen storage tank under constant pressure is studied. Two analysis methods are used to provide theoretical basis for the selection of inlet diameter of hydrogen storage tank. Through comparative analysis of the advantages and disadvantages of the two analysis methods, it is concluded that the analysis method of constant mass flow is more suitable for the selection in practical applications. According to it, the recommended diameter of the storage tank at the initial temperature of 300 K, 200 K and 100 K is selected, which are all 15 mm. It is further proved that the calculation method can meet the different storage tank states of hydrogen fuel cell vehicles when selecting the pipe diameter.     

Research of heat transfer processes during pre-launch chilldown of PS consumption lines of upper-stage LV

The paper deals with heat transfer processes during pre-launch chilldown of consumption lines of the upper stage propulsion systems (PS) by a cryogenic component. Propulsion systems operating on high-efficiency cryogenic propellant components may be used at the first as well as at the upper stages of launch vehicles to deliver spacecrafts into final orbits. Cryogenic PS produce high specific impulse being environmentally friendly, which may provide a growing interest towards their application. Low temperature of cryogenic propellant components may pose considerable challenges when chilling down, fueling and storing components as well as draining fuel tanks and starting PS, especially under zero gravity conditions in long-term space flights. Complex experimental facilities (EF) along with test methods as well as physical and mathematical models for calculating nonsteady multiple-phase heat transfer processes with intense phase transformations are being developed to overcome these challenges. The methods for mathematical modelling of the chilldown and cryogenic component flow processes have been proposed considering mass - and energy-conservation equations in relation to supply lines, tank gas blanket and tank volume filled with cryogen liquid. Closing dependency ratios have been specified according to results of bench and flight testing of PS to solve equations for heat transfer and friction in the cryogenic supply system components of test stand (TS) and PS.Also, the paper studies heat transfer processes during pre-launch chilldown of consumption lines (CL) of 12KRB LV PS with the lox/liquid hydrogen engine KVD-1 considering results of bench and flight testing of the block.Findings on chilldown and fueling of the 12KRB upper stage may be used to verify calculation models while designing and developing the KVTK advanced upper stage with the lox/liquid hydrogen engine RD0146 of the Angara-A5V heavy class launch vehicle.     

Finite element simulation of heat and mass transfer in activated carbon hydrogen storage tank

A mathematical model of heat and mass transfer in activated carbon (AC) tank for hydrogen storage is proposed based on a set of partial differential equations (PDEs) controlling the balances or conservations of mass, momentum and energy in the tank. These PDEs are numerically solved by means of the finite element method using Comsol Multiphysics^TM. The objective of this paper is to establish a correct set of PDEs describing the physical system and appropriate parameters for simulating the hydrogen storage process. In this paper, we establish an axisymmetric model of hydrogen storage by adsorption on activated carbon, considering heat and mass transfer of hydrogen in storage tank during the charging process at room temperature (295 K) and the pressure of 10 MPa. To simulate the hydrogen storage process accurately, the heat capacity of adsorbed phase, the contact thermal resistance between the AC bed and the steel wall and the inertial resistance of high speed charging hydrogen gas are all taken into account in the model. The governing equations describing the hydrogen storage process by adsorption are solved to obtain the pressure changes, temperature distributions and adsorption dynamics in the storage tank. The pressure reaches a maximum value of 10 MPa at about 240 s. A small downward trend appears in the later stage of the charging process, which lasts 700 s. The temperature distribution is highest in the center of the tank. The temperature history exhibits a rapid increase initially, followed by a steady decline. A modified Dubinin–Astakhov (D–A) model is used to represent the hydrogen adsorption isotherms. The highest hydrogen uptake is 10 mol H₂/kg AC, at the entrance of hydrogen storage tank, where the temperature is lowest. The adsorption distribution at a given time is mainly determined by the temperature distribution, because the pressure is almost uniform in the tank. The adsorption history, however, is dominated by the pressure history because the pressure change is much larger than temperature change during the charging process of hydrogen storage.     

Thermodynamic performance on the pressurized discharge process from a cryogenic fuel storage tank

Due to excellent performance, cryogenic propellants, such as liquid hydrogen and liquid oxygen, are widely used in aerospace engineering. However, low storage temperature and low kinetic viscosity bring a lot of technique issues for high efficient thermal management on cryogen. An actual cryogenic fuel storage tank is selected as the research object, and a two-dimensional axial symmetrical computational model is established to study the pressurized discharge process, by adopting the volume of fluid (VOF) model. Both external environment heat leakage and the heat exchange occurring between the liquid and vapor are considered. Compared to the experimental results, the relative error is limited in 20.0%. Based on the developed numerical model, the temperature variation and heat flux through the insulation and tank wall, the pressurized discharge performance and the fluid temperature distribution are analyzed. The results show that during the pressurized discharge process, the lowest temperature appears in the inner side of the foam, and the external heat invasion does not absolutely penetrate into the tank. The vapor mass experiences fluctuating variations, and the vapor is always in condensation. In the first 200s, the temperature of the outflow fluid keeps constant, and then increases gradually. Under the present initial setting, the violent boiling phenomenon does not form during the whole process. The present study is significant to the depth understanding on the pressurized discharge of cryogenic fuels.     

Hydrogen physisorption in high SSA microporous materials - A comparison between AX-21_33 and MOF-177 at cryogenic conditions

The two most promising materials for a hydrogen cryo-adsorption tank, activated carbon AX-21_33 and metal-organic framework MOF-177, have been investigated in the pressure range up to 2 MPa and at temperatures from 77 K to 125 K and at room temperature. The total hydrogen storage, including adsorbed hydrogen and gaseous hydrogen, has been determined for both samples. The results were evaluated with respect to the operating conditions of a tank system at cryogenic conditions, assuming a maximum tank pressure of 2 MPa and a minimum back pressure for the hydrogen consumer of 0.2 MPa. AX-21_33 shows a usable capacity of 3.5 wt.% in the case of isothermal operation at 77 K and 5.6 wt.%, if the tank is loaded at 77 K and the temperature is increased by 40 K during unloading. Under the same conditions, MOF-177 has a usable capacity of 6.1 wt.% and 7.4 wt.%, respectively. The results show that the heat of adsorption has a high impact on the amount of hydrogen remaining in a tank after unloading and that the heat management plays a crucial role for the design of a cryogenic tank system.     

Simulation of hydrogen storage tank packed with metal-organic framework

Hydrogen adsorption in high surface metal-organic framework (MOF) has generated significant interest over the past decade. We studied hydrogen storage processes of MOF-5 hydrogen storage systems with adsorbents of both the MOF-5 powder (0.13 g/cm³) and its compacted tablet (0.30 g/cm³). The charge–discharge cycles of the two MOF-5 adsorbents were simulated and compared with activated carbon. The physical model is based on mass, momentum and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. The adsorption process was modeled using a modified Dubinin–Astakov (D–A) adsorption isotherm and its associated variational heat of adsorption. The model was implemented by means of finite element analysis software Comsol Multiphysics™, and the system simulation platform Matlab/Simulink™. The thermal average temperature from Comsol simulation is used to fill the gap between the system model and the multi-dimensional models. The heat and mass transfer feature of the model was validated by the experiments of activated carbon, the simulated pressure and temperatures are in good agreement with the experimental results. The model was further validated by the metal-organic framework of Cu-BTC and is being extended its application to MOF-5 in this study. The maximum pressure in the powder MOF-5 tank is much higher than that in the activated carbon tank due to the lower adsorbent density of MOF-5 and resulting lower hydrogen adsorption. The maximum pressure in the compacted MOF-5 tank is a little bit lower than that in the activated carbon tank due to the higher adsorbent density and resulting higher hydrogen adsorption. The temperature swings during the charge–discharge cycle of both MOF-5 tanks are higher than that of the activated carbon tank. These are caused mainly by pressure work in the powder MOF-5 tank and by adsorption heat in the compacted MOF-5 tank. For both MOF-5 hydrogen storage systems, the lumped parameter models implemented by Simulink agree well with experimental pressures and with pressures and thermal average temperatures from Comsol simulation.     

Automotive hydrogen storage system using cryo-adsorption on activated carbon

An integrated model of a sorbent-based cryogenic compressed hydrogen system is used to assess the prospect of meeting the near-term targets of 36 kg-H₂/m³ volumetric and 4.5 wt% gravimetric capacity for hydrogen-fueled vehicles. The model includes the thermodynamics of H₂ sorption, heat transfer during adsorption and desorption, sorption dynamics, energetics of cryogenic tank cooling, and containment of H₂ in geodesically wound carbon fiber tanks. The results from the model show that recoverable hydrogen, rather than excess or absolute adsorption, is a determining measure of whether a sorbent is a good candidate material for on-board storage of H₂. A temperature swing is needed to recover >80% of the sorption capacity of the superactivated carbon sorbent at 100 K and 100 bar as the tank is depressurized to 3–8 bar. The storage pressure at which the system needs to operate in order to approach the system capacity targets has been determined and compared with the breakeven pressure above which the storage tank is more compact if H₂ is stored only as a cryo-compressed gas. The amount of liquid N₂ needed to cool the hydrogen dispensed to the vehicle to 100 K and to remove the heat of adsorption during refueling has been estimated. The electrical energy needed to produce the requisite liquid N₂ by air liquefaction is compared with the electrical energy needed to liquefy the same amount of H₂ at a central plant. The alternate option of adiabatically refueling the sorbent tank with liquid H₂ has been evaluated to determine the relationship between the storage temperature and the sustainable temperature swing. Finally, simulations have been run to estimate the increase in specific surface area and bulk density of medium needed to satisfy the system capacity targets with H₂ storage at 100 bar.     

Experimental study on the heat transfer characteristics of fluoride salt in the new conceptual passive heat removal system of molten salt reactor

A new conceptual design of a passive residual heat removal system (PRHRS) has been proposed for molten salt reactor. High‐temperature heat pipes are used in this new design to improve the system inherent safety and make the PRHRS more compact. An experimental system using fluoride salt FLiNaK has been constructed to validate and support the future design of PRHRS of molten salt reactors. In this research, tests on the natural convection heat transfer of FLiNaK in the drain tank with an inclined heat pipe inserted at different heights were performed. The temperature distribution of fluoride salt in the tank was analyzed. The height of heat pipe and the bulk temperature of FLiNaK have little influence on the normalized salt temperature distribution. However, with the height of heat pipe increasing, the temperature difference of molten salt decreases and heat transfer coefficient of natural convection increases. In addition, the empirical correlations of natural convection heat transfer between liquid FLiNaK and inclined heat pipe are obtained within the range of Rayleigh numbers from 3.97 × 10⁶ to 1.16 × 10⁷. The comparisons show that a good agreement with less than 5% deviation is obtained between the proposed correlations and the test data.     

The effects of micro-structured surfaces on multi-nozzle spray cooling

Experiments were conducted to investigate heat transfer characteristics of spray cooling with eight nozzles for micro-structured surfaces included cubic pin fins and straight pin fins of different sizes. Liquid volume flow rate ranged from 2.46 × 10⁻² m³/s/m² to 3.91 × 10⁻² m³/s/m² and the corresponded inlet pressures changed from 0.28 MPa to 0.6 MPa by keeping the inlet water temperature between 20.4 °C and 24.31 °C. And the input power of heat block varied from 180 W to 1080 W. The results show that the heat transfer performances of straight fins2 and straight fins3 are the best in single phase zone, but the cubic pin fins is better in two phase zone. Notably, the critical point between single phase zone and two phase zone shifts to left with the increasing of liquid volume flow rate. Moreover, with the liquid volume flow rate increasing, the heat transfer coefficient increases as well, but straight fins1 and polished surface are not sensitive to this change. For a deeper analysis of the heat transfer enhancement, a dimensionless number (DM) is created to characterize heat transfer performance of different microstructures in single phase heat transfer. We verified the dimensionless number using experimental results in this study and previous literature. Furthermore, the micro-structured surfaces have negligible effects on temperature distribution except for cubic pin fins.     

Modeling of adsorption storage of hydrogen on activated carbons

The storage of hydrogen on board vehicles is one of the most critical issues for the transition towards an hydrogen-based transportation system. An electric vehicle powered by a typical gasoline tank will require 3.1 kg of hydrogen (H₂) to achieve a range of 500 km. Compared to a typical gasoline tank, this would correspond to a hydrogen density of 65 kg/m³ (including the storage system) and 6.5 wt%. Presently, only liquid hydrogen (LH₂) systems with a density of 51 kg/m³ and 14 wt% is close to this target. However, LH₂ is costly and requires more complex refueling systems. The physical adsorption of hydrogen on activated carbon can reduce the pressure required to store compressed gases. Though an efficient adsorption-based storage system for vehicular use of natural gas can be achieved at room temperature, the application of this technology to hydrogen using activated carbon as the adsorbent requires its operation at cryogenic temperature. We present the results of a parametric and comparative study of adsorption and compressed gas storage of hydrogen as a function of temperature, pressure and adsorbent properties. In particular, the isothermal hydrogen storage and net storage densities for passive and active storage systems operating at 77, 150 and 293 K are compared and discussed.     

Investigation on no-vent filling process of liquid hydrogen tank under microgravity condition

In order to investigate the no-vent filling performance under microgravity, the computational fluid dynamic (CFD) method is introduced to the study, where a model aiming at filling a liquid hydrogen (LH₂) receiver tank is especially established. In this model, the solid and fluid regions are considered together to predict the coupled heat transfer process. The phase change effect during the filling process is also taken into account by embedding a pair of mass and heat transfer models into the CFD software FLUENT, one of which involves liquid flash driven by pressure difference between the fluid saturated pressure and the tank pressure, and the other one indicates and calculates the evaporation–condensation process driven by temperature difference between fluid and its saturated state. This CFD model, verified by experimental data, could accurately simulate the no-vent filling process with good flexibility. Moreover, no-vent filling processes under different gravities are comparatively analyzed and the effects of four factors including inlet configuration, inlet liquid temperature, initial wall temperature and inlet flow rate, are discussed, respectively. Main conclusions could be made as follows: 1) Compared to the situations in normal gravity, the no-vent filling in microgravity experiences a more adequate liquid–vapor mix, which results in a more steady pressure response and better filling performance. 2) Inlet configuration seems to have negligible effect on the no-vent filling performance under microgravity since liquid could easily reach the tank wall and then cause a sufficient fluid-wall contact under any inlet condition. 3) Higher initial tank wall temperature may directly cause a higher pressure rise in the beginning, while this effect on the final pressure is not significant. Sufficient precooling and reasonable inlet liquid subcooled degree are suggested to guarantee the reliability and efficiency of the no-vent fill under microgravity.     

Numerical study on particle distribution characteristics of slush hydrogen in a cryogenic tank

A numerical model considering phase change and heat transfer was established by the Euler-Euler two-fluid method to investigate the storage characteristics and two-phase flow field of slush hydrogen. Numerous numerical simulations were performed to discuss the effect of particle diameter (d_p = 0.02–0.5 mm), content of solid hydrogen (α_s = 10%–50%), and heat leakage (q = 50–200W·m⁻²) on the flow field. It was found that particle deposition could occur during the storage process, and there exist moving vortices with contrary directions under specific conditions. The sedimentation characteristics and vortex size are influenced by many factors including particle size, solid hydrogen content, and heat leakage. An increase in particle size could lead to the strengthening of precipitation and the expansion of the counterclockwise vortex region on the right side of the tank. And the increase in solid hydrogen content could result in more deposition and more collisions and friction between particles. Moreover, the increase in heat leakage could increase the area of the counterclockwise vortex. Numerical results of the deposition and flow field characteristics in the storage tank could clearly show the physical law of the slush hydrogen so that the uniform distribution of slush hydrogen could be promoted for efficient storage and application.     

Effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat

This paper reports the effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat. The unsteady model was set up, which has been applied to investigate the effects of particle sizes (1.77 mm–14.60 mm) on particle temperature, heat transfer quantity, overall coefficient of heat-transfer, etc. The heat transfer performance of waste heat recovery heat exchanger is improved when the particle size increases, which is conducive to increase hydrogen production. The particle temperature change rate, the specific enthalpy change rate, the moving velocity of the maximum heat release rate particle, the contribution rate of solid phases, the heat release rate and the overall coefficient of heat-transfer increase, but the effective time of heat transfer decreases. When the particle size increases from 1.77 mm to 14.60 mm, the solid phase average contribution rate increases from 89.43% to 94.03%, the overall coefficient of heat-transfer increases from 1.39 W m⁻² K⁻¹ to 13.41 W m⁻² K⁻¹, the heat release rate increases from 48.9% to 99.9% and the effective time of heat transfer reduces from 48 h to 6.7 h.     

Effects of tank heating on hydrogen release from metal hydride system in VoltaFCEV Fuel Cell Electric Vehicle

Metal hydride (MH) storage is known as a safe storage method because it does not require complex processes like high pressure or very low temperature. However, it is necessary to use a heat exchanger due to the endothermic and exothermic reactions occurring during the charging and discharging processes of the MH tanks. The performance of the MH is adversely affected by the lack of a heat exchanger or a suitable temperature range and it causes non-stable hydrogen supply to the fuel cell systems. In this study, effect of the tank surface temperature on hydrogen flow and hydrogen consumption performance were investigated for the MH hydrogen storage system of a hydrogen Fuel Cell Electric Vehicle (FCEV). Different temperature values were arranged using an external heat circulator device and a heat exchanger inside the MH tank. The fuel cell (FC) was operated at three different power levels (200 W, 400 W, and 600 W) and its performance was determined depending on the temperature and discharge flow rate of the MH tank. When the heat exchanger temperature (HET) was set to 40 °C, the discharge performance of the MH tank increased compared to lower temperatures. For example, when the FC power was set to 200 W and the HET of the system was at 40 °C, 1600 L hydrogen was supplied to the FC and 2000 Wh electrical energy was obtained. The results show that the amount of hydrogen supplied from the MH tank decreases significantly by increasing the flow rate in the system and rapid temperature changes occur in the MH tank.     

The Influence of the Magnitude of Gravitational Acceleration on Marangoni Convection About an Isolated Bubble under a Heated Wall

Thermocapillary or Marangoni convection is the liquid motion caused by surface tension variation in the presence of a temperature gradient along a gas–liquid or vapor–liquid interface. This work numerically investigates the effect of the magnitude of gravitational acceleration on the flow and temperature fields resulting from the presence of a hemispherical air bubble of constant radius of 1.0 mm, situated on a heated wall immersed in a liquid silicone oil layer of constant depth of 5.0 mm. The model is oriented such that the Marangoni and gravitational forces act to oppose one another. To elucidate the effect of gravity on Marangoni flow and heat transfer, the simulations were carried out for a silicone oil of Prandtl number 83, at a Marangoni number of 915. The gravity levels tested were 0g, 0.01g, 0.1g, 0.25g, 0.5g, 0.75g, and 1g, where g represents the earth gravitational acceleration of 9.81 m/s ² . The influence of the magnitude of gravitational acceleration on the velocity profile along the bubble interface and on the location of maximum velocity was analyzed. It was found that the gravity level affects the velocity profile by influencing the interfacial temperature gradient, but that the location of maximum velocity was almost independent of gravity level. The increase in heat flux on the wall to which the bubble is attached was calculated and it has been determined that local heat transfer enhancement of up to nearly 1.7 times that of the conduction only case can be achieved for the parameter range tested. Furthermore, local enhancement was observed to occur up to a distance of seven bubble radii for the zero-gravity case, but increased gravity levels cause a reduction in the effective radius of enhancement. The influence of the Marangoni flow on the heat transfer for the opposite cooled wall has also been analyzed.     

Enhancement of H2 uptake due to morphological modulation in vanadium pentoxide foam

The high pressure H₂ sorption isotherms for vanadium pentoxide foam (VOF) were obtained at a liquid nitrogen temperature. The enhancement of hydrogen storage capacity occurred in as-prepared VOF (∼1.0 wt%) in contrast to that in pristine vanadium pentoxide (∼0.2 wt%). The maximum capacity of hydrogen storage (∼2.0 wt%) was achieved by thermal annealing at T_a = 623 K. The enhancement of hydrogen storage in VOF is attributed to the morphological modulation by thermal annealing.