A Bulk vs. Nanoscale Hydrogen Storage Paradox Revealed by Material-System Co- Design

Metal hydrides are serious contenders for materials-based hydrogen storage to overcome constraints associated with compressed or liquefied H2. Their ultimate performance is usually evaluated using intrinsic material properties without considering a systems design perspective. An illustrative case with startling implications is (LiNH2+2LiH). Using models that simulate the storage system and associated fuel cell of a light-duty vehicle (LDV), we compared performance of the bulk hydrides with a nanoscaled version in porous carbon, (LiNH2+2LiH)@(6-nm PC). Using experimental material properties, the simulations show that (LiNH2+2LiH)@(6-nm PC) counterintuitively has higher usable gravimetric and volumetric capacities than the bulk counterpart on a system basis despite having lower capacities on a materials-only basis. Nanoscaling increases the thermal conductivity and lowers the desorption enthalpy, which consequently increases heat management efficiency. In a simulated drive cycle for fuel cell- powered LDV, the fuel cell is inoperable using bulk (LiNH2+2LiH) as the storage material but completes the drive cycle using the nanoscale material. These results challenge the notion that nanoscaling incurs mass and volume penalties. Instead, the synergistic nanoporous host-hydride interaction can favorably modulate chemical and heat transfer properties. Moreover, a co-design approach considering application- specific tradeoffs is essential to accurately assess a material’s potential for real-world hydrogen storage.