pH-Controlled Dealloying Route to Hierarchical Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation by Hydrolysis of Zn in Neutral Water

Abstract

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 H2 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 Zn20Al80 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 H2 and Zn­(OH)2 at ambient temperature and pressure and continuously produce H2 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 H2 applications, during which H2 is produced when needed, without prior storage of this gas compressed in cylinders as it is currently the case.

Phase-transition tailored nanoporous zinc metal electrodes for rechargeable alkaline zinc-nickel oxide hydroxide and zinc-air batteries

“…Optical microscopy confirms the propagation with the color changes, showing that the initially ivory sample turned black first at the edge, where the black color appears similar to previously reported nanostructures of Zn (Fig. 1d ) 25 , 29 .…”

Small-angle X-ray scattering of nanoporous materials

“…H 2 storage and CO 2 capture), 30,35,44,45,50 biotechnology, 51 radiation resistance, 52 plasmonics, 16,[53][54][55][56][57] (photo)catalysis, 17,18,35,38,55,[58][59][60][61][62][63][64][65][66] sensing, 67,68 actuation, 8,[68][69][70][71] and energy storage and conversion. 6,9,11,12,19,21,43,47,60,[72][73][74][75] In all of these applications, the characteristic size, specific interfacial area, and curvature are critical and necessitate proper quantification.…”

Air-Stable Nanoporous Aluminum/Lithium Borohydride Fuel Pellets for Onboard Hydrogen Generation by Hydrolysis with Pure Water

“…An alternative solution to high-pressure H 2 tanks is solid-state H 2 storage in lightweight materials including chemical hydrides such as magnesium hydride (MgH 2 ), aluminum hydride (AlH 3 ), lithium borohydride (LiBH 4 ), and sodium borohydride (NaBH 4 ), which can release H 2 upon heating. However, these chemical hydrides suffer from poor H 2 adsorption/release kinetics under practical temperatures and pressures, as well as a lack of practical regeneration methods. − Another promising alternative to high-pressure H 2 tanks is H 2 generation by metal hydrolysis, during which a reactive metal such as Al, Mg, or Zn spontaneously reacts with water to generate H 2 , heat, and the corresponding metal hydroxide as the only solid byproduct. − The typical hydrolysis reactions for Al, Mg, and Zn are given in eqs − with Δ G calculated at 298 K: …”

“…This typically results in a nanoporous structure of the remaining metal component. In recent years, many researchers have found innovative ways of creating nanoporous nonprecious metallic structures while preventing oxidation from air or aqueous solvents on the surface, resulting in nanoporous Al, , nanoporous Mg, , and nanoporous Zn. , Furthermore, recent studies have shown that H 2 in high quantities can be produced by hydrolysis of these nonprecious nanoporous metals in pure water, that is, water without addition of any cocatalyst to drive the reaction. ,, The high surface area from having nanostructured thin ligaments in these nonprecious nanoporous is critical in overcoming low reaction yields associated with the metal hydroxide passivating layer that naturally forms on the metal surface during the hydrolysis reaction, which prevents further reaction between water and the metal underneath the passivating layer; such a high surface area resulting from these thin nanoscale ligaments will allow for a majority of the metal to react with water. , Thus, it might be possible to overcome the infrastructural and safety issues associated with high-pressure H 2 tanks using nonprecious nanoporous metals such as nanoporous Al, Mg, or Zn to produce H 2 onboard by hydrolysis, provided these nanoporous metals can be safely supplied to end-users through common ground-based distribution channels. Indeed, while nanostructured metals such as Al, Mg, and Zn have high chemical reactivitywhich is desirable for H 2 generation by hydrolysisthey are also pyrophoric and can spontaneously ignite if exposed to air.…”

“…In recent years, many researchers have found innovative ways of creating nanoporous nonprecious metallic structures while preventing oxidation from air or aqueous solvents on the surface, resulting in nanoporous Al, 19,30 nanoporous Mg, 20,31 and nanoporous Zn. 22,32 Furthermore, recent studies have shown that H 2 in high quantities can be produced by hydrolysis of these nonprecious nanoporous metals in pure water, that is, water without addition of any cocatalyst to drive the reaction. 19,20,22 The high surface area from having nanostructured thin ligaments in these nonprecious nanoporous is critical in overcoming low reaction yields associated with the metal hydroxide passivating layer that naturally forms on the metal surface during the hydrolysis reaction, which prevents further reaction between water and the metal underneath the passivating layer; such a high surface area resulting from these thin nanoscale ligaments will allow for a majority of the metal to react with water.…”