Hollow glass microspheres are available as commercial products in large quantities at cost of less than US$2 per kilogram, which would be expected to decrease with economies of scale.
The raw materials are relatively inexpensive, the production processes are well established, and the durability of the spheres is such that they can withstand a large number of filling/outgassing cycles. The materials can easily be recycled to produce new spheres to replace those broken during service.
As a result, hollow glass microspheres appear to be the cheapest of all current materials for the solid-state storage of hydrogen.
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