Hydrogen shows great potential as a clean energy carrier to power vehicles, help stabilize power grids with renewable energy, and replace natural gas. However, one major hurdle is developing affordable ways to store hydrogen efficiently and safely. Overcoming storage challenges is key to unlocking the many benefits of a hydrogen economy.

Physical Adsorption Storage
Physical adsorption onto high-surface-area solids holds promise for mobile Hydrogen Storage . Adsorbing hydrogen onto porous materials like metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) exploits weak van der Waals forces. These frameworks self-assemble from light organic molecules linked by metal clusters, yielding micro-porous solids with immense internal surface areas exceeding 5,000 m2/g. At 77 K, MOFs can adsorb over 7.5% of their mass as hydrogen. However, additional work is needed to improve uptake at higher, more practical temperatures. Researchers continue exploring new framework designs and chemistries to optimize binding interactions and hydrogen capacity.

One challenge is that van der Waals forces impart only modest heats of adsorption, in the range of 5-10 kJ/mol H2. As temperature increases, hydrogen is less strongly bound and more difficult to retain. Porous polymers displaying open metal sites show enhanced adsorption through stronger interactions but struggle with limited surface areas. Overall, ongoing research aims to develop robust sorbents combining high surface area with localized strong binding able to meet US Department of Energy system targets.

Chemical Hydrogen Storage
Storing hydrogen chemically in solid-state materials provides another potential route. One approach involves complex metal hydrides that reversibly absorb and release hydrogen via proton addition/removal reactions. Sodium aluminum hydrides (NaAlH4) and borohydrides (NaBH4) showed initial promise but suffered from slow kinetics and thermal decomposition above 100°C, precluding use in fuel cells.

Lithium hydrides could potentially offer higher capacities than traditional metal hydrides. For example, lithium nitride (Li3N) reacts exothermically with hydrogen according to the reversible reaction Li3N + 6H2 ↔ LiNH2 + 2LiH + heat. This system stores 11.5 wt% hydrogen, exceeding the DOE system target. However, challenges preventing practical utilization include sluggish hydrogen sorption kinetics and inadequate thermodynamic reversibility near ambient conditions. Additional engineering efforts focus on improving sorbent properties through compositional modifications like alloying or nanoconfinement.

Another approach involves hydrogenating unsaturated light hydrocarbons. For instance, cycloalkanes like cyclohexane vapor can reversibly absorb up to 6 wt% H2 through catalytically-enabled hydrogenation/dehydrogenation. Researchers design catalyst systems promoting fast reaction rates near ambient temperatures and pressures. However, purity control poses difficulties, as byproducts from incomplete conversion degrade performance. Overall, significant chemistry advances are still needed to develop sturdy, reversible solid-state hydrogen carriers meeting cost and performance goals.

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