Nanoporous polymers have emerged as a promising class of materials for hydrogen storage through physisorption, offering advantages such as lightweight structures, tunable pore sizes, and synthetic versatility. Among these, porous aromatic frameworks (PAFs) and covalent organic frameworks (COFs) stand out due to their high surface areas, structural stability, and customizable architectures. Unlike metal-organic frameworks (MOFs) and zeolites, nanoporous polymers rely entirely on weak van der Waals interactions for hydrogen uptake, avoiding the complexities of chemical hydrides or covalent bonding mechanisms.
The primary advantage of nanoporous polymers lies in their tunability. By adjusting the building blocks and synthesis conditions, researchers can precisely control pore size distribution, surface area, and overall framework geometry. For instance, PAFs, constructed from rigid aromatic units, exhibit exceptional thermal and chemical stability while maintaining high porosity. COFs, on the other hand, are crystalline and benefit from predictable pore geometries due to their ordered structures. Both materials can achieve surface areas exceeding 1000 m²/g, with some PAFs reaching upwards of 5000 m²/g, rivaling the best-performing MOFs.
Hydrogen storage performance in these materials is governed by the interplay between surface area, pore volume, and adsorption enthalpy. At cryogenic temperatures (77 K), nanoporous polymers can achieve gravimetric capacities of 2-5 wt%, comparable to many MOFs and superior to conventional zeolites. However, at ambient temperatures, their capacities drop significantly due to the weak nature of physisorption. To enhance room-temperature performance, strategies such as introducing narrow micropores (below 1 nm) or incorporating polar functional groups have been explored. These modifications increase the adsorption enthalpy from 4-6 kJ/mol to 6-10 kJ/mol, though still below the 15-25 kJ/mol range considered ideal for ambient storage.
Synthesis of nanoporous polymers typically involves crosslinking or templating approaches. Crosslinking methods, such as Friedel-Crafts alkylation or Sonogashira coupling, create robust networks with permanent porosity. Templating techniques, using sacrificial materials like silica nanoparticles, allow for precise pore size control but require additional steps for template removal. Compared to MOFs, which often rely on metal coordination, or zeolites, which demand high-temperature crystallization, polymer synthesis is generally more flexible and scalable. However, achieving high crystallinity in COFs remains challenging, often requiring long reaction times or specific catalysts.
When compared to MOFs and zeolites, nanoporous polymers exhibit distinct trade-offs. MOFs often outperform in terms of surface area and hydrogen uptake at low temperatures, but their stability under moisture or mechanical stress can be problematic. Zeolites, while highly stable, suffer from lower surface areas and limited tunability. Nanoporous polymers bridge these gaps by offering stability akin to zeolites and tunability approaching MOFs, albeit with slightly lower absolute capacities.
A critical consideration is the scalability of these materials. While laboratory-scale synthesis of PAFs and COFs is well-established, transitioning to industrial production poses challenges. Crosslinking reactions may require hazardous solvents, and templating methods can be cost-prohibitive at large scales. In contrast, zeolites benefit from mature manufacturing processes, and some MOFs are nearing commercial production.
Future research directions for nanoporous polymers include optimizing synthesis protocols for reduced costs, enhancing adsorption enthalpies through pore engineering, and integrating these materials into composite systems for practical applications. Their lightweight nature makes them particularly attractive for mobile storage, where system weight is a critical factor.
In summary, nanoporous polymers represent a versatile and stable alternative for hydrogen physisorption, with tunable properties that can be tailored for specific storage needs. While they currently lag behind MOFs in absolute performance, their synthetic flexibility and robustness position them as strong contenders in the evolving landscape of hydrogen storage materials. Advances in pore design and scalable synthesis will be key to unlocking their full potential.