MXenes represent a class of two-dimensional transition metal carbides, nitrides, and carbonitrides with unique properties that make them promising candidates for hydrogen storage. Their high surface area, tunable surface chemistry, and excellent electrical conductivity contribute to their effectiveness in physisorption, chemisorption, and hydrogen spillover mechanisms. Compared to metal-organic frameworks (MOFs) and carbon-based materials, MXenes exhibit distinct advantages and challenges in hydrogen storage applications.

Physisorption is a key mechanism for hydrogen storage in MXenes, relying on weak van der Waals interactions between hydrogen molecules and the material’s surface. The layered structure of MXenes provides abundant adsorption sites, while their surface functional groups, such as oxygen, fluorine, or hydroxyl groups, influence hydrogen uptake. Theoretical and experimental studies suggest that Ti3C2Tx, a widely studied MXene, can achieve hydrogen storage capacities of up to 8.8 wt% under cryogenic conditions (77 K) and moderate pressures. This performance is competitive with activated carbon and some MOFs, which typically exhibit capacities in the range of 5-10 wt% under similar conditions. However, the practical application of physisorption is limited by the requirement of low temperatures to achieve significant hydrogen uptake.

Chemisorption involves stronger chemical bonding between hydrogen atoms and the MXene surface, often leading to higher storage capacities at near-ambient temperatures. The presence of transition metals in MXenes, such as titanium or vanadium, facilitates the dissociation of hydrogen molecules into atoms, which then bind to the material’s surface or intercalate between layers. For instance, Ti2C MXene has demonstrated a theoretical hydrogen storage capacity of 5.3 wt% at room temperature through chemisorption. However, the strong bonding can lead to slow kinetics during hydrogen release, requiring elevated temperatures or catalysts to desorb hydrogen efficiently. In contrast, MOFs rely primarily on physisorption, while carbon materials like graphene or nanotubes exhibit limited chemisorption due to the absence of active metal sites.

Hydrogen spillover is another mechanism where MXenes show promise. In this process, hydrogen molecules dissociate on a catalytic metal nanoparticle (e.g., palladium or platinum) supported on the MXene surface, and the resulting hydrogen atoms migrate onto the MXene. The high electrical conductivity of MXenes enhances this effect by facilitating electron transfer during the dissociation and migration steps. Experimental studies have reported that spillover can increase hydrogen uptake in MXenes by up to 50% compared to pure physisorption. MOFs also benefit from spillover effects, but their insulating nature can limit electron transfer, reducing efficiency. Carbon materials, while conductive, often lack the tailored surface chemistry required for optimal spillover.

When comparing MXenes with MOFs and carbon materials, several factors stand out. MOFs excel in physisorption due to their ultrahigh porosity and surface areas exceeding 5000 m²/g, but their stability under repeated cycling and moisture exposure remains a challenge. Carbon materials, such as activated carbon or graphene, offer good cyclability and lower costs but suffer from lower hydrogen capacities at ambient temperatures. MXenes strike a balance with moderate surface areas (typically 200-500 m²/g), tunable chemistry, and the ability to leverage multiple storage mechanisms. However, scalability and cost-effective synthesis of MXenes remain hurdles for large-scale applications.

In summary, MXenes offer a versatile platform for hydrogen storage through physisorption, chemisorption, and spillover effects. Their performance is competitive with MOFs and carbon materials, with distinct advantages in conductivity and surface chemistry. While challenges in synthesis and cycling stability persist, ongoing research into MXene modifications and hybrid systems may further enhance their viability for hydrogen storage applications.