Advanced Characterization of Solid-State Hydrogen Storage Materials via AMI

Abstract                                                                        

Hydrogen's high gravimetric energy density (120 MJ/kg) positions it as a critical energy carrier for the transition to clean energy systems. However, its extremely low density at ambient conditions significantly limits its volumetric energy density, complicating storage and transportation. Among various hydrogen storage strategies, solid-state hydrogen storage has emerged as the most promising due to its safety, efficiency, and volumetric advantages. This application note presents a comprehensive analysis of hydrogen storage materials—including Mg-based hydrides, rare earth alloys, carbon-supported systems, and MOFs—evaluated using AMI advanced sorption instrumentation.

Introduction

The volumetric energy density of hydrogen is limited by its low density at ambient conditions—0.0824 kg/m³ compared to 1.184 kg/m³ for air. Methane and gasoline have volumetric energy densities of approximately 0.04 MJ/L and 32 MJ/L, respectively. Hydrogen’s flammability, diffusivity, and explosion risks further challenge its storage [1].

Three primary hydrogen storage methods are commonly used:

• Compressed gas
• Cryogenic liquid
• Solid-state materials

Solid-state hydrogen storage stands out for safety, energy density, and moderate operating conditions, relying on physical adsorption or reversible chemical bonding. The development of high-performance materials is now central to advancing this technology.

Material Classes and Storage Mechanisms

1. Magnesium-Based Hydrogen Storage Materials

Magnesium hydride (MgH₂) offers a theoretical hydrogen capacity of ~7.6 wt% and excellent stability under ambient conditions. However, its high desorption enthalpy (ΔH = 76 kJ/mol) and poor kinetics limit practical use. Agglomeration during cycling also reduces reversibility.

Approaches to Improve MgH₂ Performance:

• Nanoconfinement: Chen et al. [2] encapsulated Mg-V nanoparticles in a 1 nm carbon shell, achieving 6.6 wt% H₂ storage and excellent reversibility (5.2 wt%) at 200–300°C with reduced activation energy.
• Ultrasound-assisted synthesis: Zhang and Liu [2] produced 4–5 nm MgH₂ particles with reversible capacity up to 6.7 wt% at 30°C and 99% retention after 50 cycles.

2. Alloying with Transition and Rare Earth Metals

Alloying Mg with elements like Ni, La, Ce, or Pr forms metastable phases, reducing reaction temperatures and improving kinetics.

• MgNi/Graphene composites (Samantaraya et al. [3]) achieved 5.4 wt% H₂ at 3 MPa due to high dispersion and surface area.
• Mg₃RE alloys (Ouyang et al. [4]) enhanced dehydrogenation rates due to amorphous structure formation and stable cycle behavior.

Figure: Adsorption Kinetics at Different Temperatures & Cyclic Adsorption Performance

3. Carbon-Supported Magnesium Systems

Carbon materials improve dispersion, prevent agglomeration, and enhance hydrogen kinetics via electron transfer and surface defects.

• Mortazavi et al. [5] increased hydrogen storage from 0.7 to 1.5 wt% with CNTs.

4. LaNi₅ and Its Derivatives

LaNi₅ offers:

• High hydrogen capacity (~1:1 H/M ratio)
• Rapid kinetics
• Mild operation conditions (2–10 atm, room temperature)
• Stable multi-cycle performance

Zhu et al. [7] and Liu et al. [8] showed LaNi₅₋ₓCoₓ alloys maintain structure and capacity over 1000 cycles. Substituting La with Pr, Ce, or Gd improves equilibrium pressure and kinetics. Neto et al. [9] demonstrated improved absorption kinetics in PEI-LaNi₅ composite films at 40°C/20 bar.

Figure: Effects of Different Annealing Conditions on Hydrogen Absorption Kinetics

5. Metal-Organic Frameworks (MOFs)

MOFs combine high porosity, surface area, and tunable pore chemistry for physisorption-based H₂ storage.

• Musyoka et al. [9] enhanced Zr-MOF with rGO, increasing capacity from 1.4 to 1.8 wt%.

Experimental Evaluation Using AMI

1. Volumetric Sorption via RuboSorp MPA

Using the RuboSorp MPA, AMI’s high-pressure volumetric gas sorption analyzer, LaNi₅ was tested at room temperature under pressures up to 3 MPa. Results showed rapid hydrogen uptake at low pressures (to 1.35 wt%), saturating at six hydrogen atoms per unit cell—consistent with theoretical expectations. An observable hysteresis loop indicated structural changes in LaNi₅ during hydrogen cycling.

Figure: Pressure vs. hydrogen uptake curve for LaNi₅ using RuboSorp MPA

2. Gravimetric Sorption via RuboSorp MSB

Using the RuboSorp MSB, a high-precision magnetic suspension balance system, real-time weight changes during hydrogen adsorption were recorded. The MSB provides higher accuracy than traditional volumetric systems and enables visualization of subtle structural changes through its unique high-resolution volume acquisition capabilities.

     Figure: Gravimetric hydrogen uptake curve recorded by RuboSorp MSB

3. Thermal Desorption via AMI-300 and AMI-400TPx

Hydrogen desorption kinetics of commercial MgH₂ were evaluated using the AMI-300 and AMI-400TPx. TCD-based thermal desorption testing revealed that faster heating rates increased the hydrogen evolution temperature (421–435°C) and slightly reduced total hydrogen release (7.12 → 6.98 wt%). These results illustrate the importance of thermal uniformity and diffusion efficiency during dynamic desorption.

Figure: Desorption peaks of MgH₂ at varying heating rates

Conclusion

Solid-state hydrogen storage materials—especially Mg-based hydrides, LaNi₅ alloys, and MOFs—offer a viable route to safe, efficient hydrogen storage. Key factors such as thermodynamic stability, kinetic barriers, nanostructuring, and composite design strongly affect performance. Using AMI’s RuboSorp MPA, RuboSorp MSB, and the AMI-300 and AMI-400TPx systems enables precise evaluation of these effects. These instruments provide a comprehensive toolkit to support the development and commercialization of next-generation hydrogen storage materials.

References

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