pH-Controlled Dealloying Route to Hierarchical
Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation
by Hydrolysis of Zn in Neutral Water
Dealloyed
nanoporous metals made of very-reactive elements have rarely been
reported. Instead, reactive materials are used as sacrificial components
in dealloying. The high chemical reactivity of nonprecious nanostructured
metals makes them suitable for a broad range of applications such
as splitting water into H<sub>2</sub> gas and metal hydroxide. On
the other hand, the same high chemical reactivity hinders the synthesis
of nanostructured metals. Here we use a pH-controlled dealloying strategy
to fabricate bulk nanoporous Zn with bulk dimensions in the centimeter
range via the selective removal of Al from metastable face-centered
cubic bulk Zn<sub>20</sub>Al<sub>80</sub> at. % parent alloys. The
corresponding bulk nanoporous Zn exhibits a hierarchical ligament/pore
architecture characterized by primary ligaments and pores with an
average feature size in the submicrometer range. These primary structures
are made of ultrafine secondary ligaments and pores with a characteristic
feature size in the range of 10–20 nm. Our bulk nanoporous
Zn can split water into H<sub>2</sub> and Zn(OH)<sub>2</sub> at ambient
temperature and pressure and continuously produce H<sub>2</sub> at
a constant rate of 0.08 mL/min per gram of Zn over 8 h. We anticipate
that in this hierarchical bulk architecture, the macropores facilitate
the flow of water in the bulk of the material, while the mesopores
and ultrafine ligaments provide a high surface area for the reaction
of water with Zn. The bulk nanoporous Zn/water system can be used
for on-board or on-demand H<sub>2</sub> applications, during which
H<sub>2</sub> is produced when needed, without prior storage of this
gas compressed in cylinders as it is currently the case