Nanoporous carbon materials are considered promising hydrogen storage carriers due to their cost efficiency, low density, and high porosity. These materials and their precursors are naturally abundant. The different forms of nanoporous carbon materials include carbon nanofibers (CNFs), carbon nanotubes (CNTs), carbide-derived carbons, and activated carbon (AC).

These various forms of nanoporous carbon materials display a broad diversity in their synthetic approaches and material structures. They offer varying functionalities, surface areas, pore sizes, and compositions for hydrogen storage.

This article compares the selected nanoporous carbon materials and their hydrogen storage properties.

Carbon nanotubes (CNTs)

Storing hydrogen in carbon nanotubes (CNTs) has been deeply researched. CNTs are materials with diameters in the nanometer range. They refer to multiwall carbon nanotubes (MWCNTs) and single-wall carbon nanotubes (SWCNTs), which consist of nested single-wall carbon nanotubes.

This kind of nanostructure allows carbon nanotubes to store hydrogen within the tube structure or in their microscopic pores. Early studies estimated the capacity of carbon nanotubes to be around 5 to 10 wt%.

However, recent experimental studies indicate that the capacity of hydrogen storage for MWCNTs and SWCNTs can achieve between 4.5 and 8 wt% at 77 K. However, at ambient temperature and pressure, a moderate capacity of about 1 wt% can be achieved.

In high-pressure environments at room temperature, the capacity for hydrogen storage of MWCNTs can reach 6.3 wt% at 148 bar, 4.0 wt% at 100 bar, and 2.0 wt% at 40 bar. Similar to activated carbon, metal-doping can effectively enhance the storage capability of CNTs.

MWCNTs doped with lithium can offer hydrogen uptake up to 20 wt% at 1 bar and room temperature. Under ambient conditions, MWCNTs doped with potassium can achieve up to 14 wt% hydrogen uptakes.

Studies have also shown that defects such as topological distortion, heteroatoms (P, N, and B) substitution, and pentagon-heptagon pair can enhance the hydrogen storage capacities and adsorption binding energies of SWCNTs.

Carbon nanofibers (CNFs)

Carbon nanofibers exhibit excellent mechanical properties and a high surface area. These types of nanoporous carbon materials can be synthesized via templating methods, chemical vapor deposition, and electrospinning techniques.

These synthesis methods are simple and desirable for mass production. This makes CNFs a promising candidate due to commercial availability and low cost. Early studies showed that the capacity for hydrogen storage of CNFs is between 0.7 wt% and 6.54 wt% at around 100 bar and room temperature.

The large deviations in the hydrogen storage capacity can be attributed to the methods of CNFs synthesis. However, studies have shown that producing CNFs via chemical activation treatment is more favorable due to the resulting controllable pore sizes and increased surface area.

Researchers at the Korea Research Institute of Chemical Technology, Chungnam National University, and Inha University studied nickel-doped CNFs produced via metal doping and reported improved uptake of hydrogen of 2.2 wt% at 100 bar and 298 K. Other studies have reported chemical treatment by using phosphoric acid, zinc chloride, carbonate salts, and hydroxide salts.

Activated carbon (AC)

Activated carbon is seen as a promising candidate for gas storage. This is due to its availability for chemical modification, commercial availability, and extremely low cost. In general, AC displays a high porous degree, presenting a surface area exceeding 3000 m2g–1.

Normally, hydrogen physical adsorption in carbon materials follows the Langmuir isotherm model. This indicates monolayer surface adsorption. Activated carbon’s high surface area improves the capacity of physical adsorption capacity, especially at high pressure and cryogenic temperature.

However, as a result of the thermal instability of the absorption interaction, it is vital to modify AC to raise the adsorption heat between AC and hydrogen molecules. This improves the intake capacity of hydrogen by AC.

In theory, the uptake of hydrogen by AC can reach 4.0 wt% at 77 K. However, the uptake capacity decreases to less than 1.0 wt% at 100 bar and room temperature. This leads to poor commercial practicality. Chemically modifying AC, for example, through metal doping and potassium hydroxide (KOH) treatment, can enhance the hydrogen storage performance of AC.

A recent study shows that preparing AC materials from a KOH treatment and polypyrrole precursor displayed a large surface area of 3000 to 3500 m2g–1. The capacity of hydrogen storage was around 7.03 wt% at 20 bar and 77 K.
Further, studies have reported that the hydrogen spillover technique is an effective approach for improving the binding energy between the surfaces of carbon materials and hydrogen molecules at room temperature.

Also, studies indicate that doping hydrogen storage materials with transitional metals, such as nickel (Ni), palladium (Pd), and platinum (Pt) can increase the capacity of hydrogen storage and stability due to the spillover phenomenon. MAHYTEC, a solid hydrogen storage company, uses hydrides, a metallic powder that can absorb and store hydrogen in significant quantities at room temperature and low pressure. The company applies a wide range of hydride types including FeTi and LaNi5 in their metallic hydride hybrid systems.

Carbide-derived carbons (CDCs)

Carbide-derived carbons are manufactured through the selective extraction of metal or metalloid atoms from carbide precursors. Several studies have used a range of binary and ternary carbides. The pore sizes of the resulting materials are very well-defined.

The common method of synthesis of CDCs is halogenation, primarily chlorination. However, reactions with organic salts, acid etching, thermal decomposition, and hydrothermal treatment are viable alternatives. The pore structure of CDCs can be affected by both the process conditions and the precursor.

Carbide-derived carbons manufactured through the chlorination of ZrC and Ti3SiC2 precursors, for instance, have been discovered to have narrower pore size distributions and smaller pores when processed at lower temperatures. Studies have reported that annealing CDCs in hydrogen increases the hydrogen adsorption capacities of CDCs, as a result of the elimination of residual chlorine from the pores.

Future directions

Importantly, the capacity of hydrogen storage of nanoporous carbon materials largely depends on their synthesis methods, adsorbed (doping) species, oxygen-containing functionalities, impurity contents, and shapes.

The natural abundance and diverse structures of nanoporous carbon materials are highly advantageous. Improved material designs and productions will be needed for large-scale hydrogen storage purposes.

Future research can focus on enhancing the stable qualities of nanoporous carbon materials. Also, it can focus on producing cost-efficient nanoporous carbon materials, which are essential for commercial applications. Last but not the least, the research can also focus on other nanoporous carbon materials like carbon cryogels, aerogels, and xerogels, carbon molecular sieves, and templated carbons.