Porous silicon has emerged as a promising candidate for solid-state hydrogen storage due to its high surface area, tunable pore structure, and favorable chemical properties. The material’s ability to adsorb hydrogen through physisorption and chemisorption mechanisms makes it a subject of significant research interest. This article examines the adsorption mechanisms, capacity limits, and key factors influencing hydrogen storage in porous silicon, focusing on material properties rather than energy harvesting applications.
The primary mechanism for hydrogen storage in porous silicon is physisorption, where hydrogen molecules adhere to the material’s surface through weak van der Waals forces. The high surface area of porous silicon, often exceeding 500 m²/g, enhances this process by providing numerous adsorption sites. Pore size distribution plays a critical role, with micropores (less than 2 nm) being particularly effective due to overlapping potential fields from opposite pore walls, which strengthen the binding energy of hydrogen molecules. Mesopores (2–50 nm) contribute to faster diffusion kinetics, while macropores (greater than 50 nm) facilitate bulk transport but offer limited adsorption capacity.
Chemisorption also occurs in porous silicon, particularly when surface modifications or dopants are introduced. Hydrogen molecules can dissociate and form covalent bonds with silicon atoms, especially at defect sites or in the presence of catalysts such as palladium or platinum nanoparticles. This mechanism typically requires higher temperatures for hydrogen release compared to physisorption but can contribute to overall storage capacity. The balance between physisorption and chemisorption depends on factors such as temperature, pressure, and material preparation.
Experimental studies have demonstrated hydrogen storage capacities in porous silicon ranging from 1 to 3 wt% at cryogenic temperatures (77 K) and moderate pressures (up to 50 bar). At room temperature, capacities generally decrease to below 1 wt% due to the weaker binding energies associated with physisorption. However, chemically modified porous silicon, such as hydride-terminated surfaces or metal-doped variants, has shown improved room-temperature capacities, reaching up to 1.5 wt% in some cases. These values remain below the U.S. Department of Energy’s targets for onboard hydrogen storage but are competitive with other porous materials like activated carbon or metal-organic frameworks under similar conditions.
The crystalline structure and surface chemistry of porous silicon significantly influence its hydrogen storage performance. For instance, (100)-oriented silicon wafers etched electrochemically in hydrofluoric acid produce a more uniform pore distribution compared to (111)-oriented substrates, leading to higher reproducibility in storage measurements. Surface passivation with hydrogen or oxygen also affects adsorption: hydrogen-terminated surfaces exhibit stronger interactions with hydrogen molecules, while oxidized surfaces may introduce additional binding sites through hydroxyl groups.
Temperature and pressure are critical operational parameters. At lower temperatures, physisorption dominates, but the trade-off is the energy required for cooling. Higher pressures increase hydrogen uptake but introduce engineering challenges related to system design and safety. The enthalpy of adsorption for hydrogen in porous silicon typically falls between 4 and 8 kJ/mol, which is lower than that of metal hydrides but sufficient for reversible storage under moderate conditions.
Degradation and cycling stability are practical considerations. Repeated adsorption-desorption cycles can lead to pore collapse or surface oxidation, reducing storage capacity over time. Studies indicate that porous silicon retains approximately 80% of its initial capacity after 100 cycles when stored under inert conditions. Doping with stabilizing elements or coating with protective layers, such as carbon or silicon nitride, has been shown to improve cyclability.
Comparative analysis with other storage media highlights the advantages and limitations of porous silicon. While metal hydrides offer higher gravimetric capacities, they often suffer from slow kinetics and high dehydrogenation temperatures. Porous silicon, in contrast, exhibits faster kinetics and lower desorption temperatures but struggles with absolute capacity. Composite materials, such as porous silicon infused with nanoscale metal hydrides, represent a potential pathway to synergize these properties, though such systems are beyond the scope of this discussion.
Theoretical modeling and computational studies have provided insights into optimizing porous silicon for hydrogen storage. Density functional theory (DFT) calculations suggest that silicon nanowires with controlled diameters and surface terminations could achieve binding energies closer to the ideal range of 15–25 kJ/mol for room-temperature storage. Molecular dynamics simulations further indicate that hierarchical pore structures, combining micro- and mesopores, could enhance both capacity and kinetics.
In summary, porous silicon presents a viable but not yet optimal solution for solid-state hydrogen storage. Its performance is governed by pore structure, surface chemistry, and operational conditions, with current capacities limited by the weak interactions in physisorption-based systems. Future research directions may focus on advanced surface modifications, hybrid materials, and nanostructuring to push the boundaries of its storage potential while maintaining the inherent advantages of silicon-based systems. The material’s compatibility with existing semiconductor fabrication techniques also offers a unique pathway for integrating storage and electronic functionalities in future devices.