Eco-friendly Synthesis of Nanoporous Magnesium by Air-Free Electrolytic Dealloying with Recovery of Sacrificial Elements for Energy Conversion and Storage Applications

Abstract

The synthesis of nanoporous Mg with minimal surface oxide coverage has been hindered by its high chemical reactivity. Herein, we demonstrate the fabrication of three-dimensional bicontinuous nanoporous Mg with ligament and pore sizes in the range of 20–30 nm using air-free electrolytic dealloying with recovery of the sacrificial material. The starting material consists of a magnesium–lithium parent alloy with lithium as the sacrificial component. During selective electrolytic leaching in an anhydrous lithium-conducting organic electrolyte solvent, the sacrificial lithium is stripped from the magnesium–lithium parent alloy used as the working electrode and plated on a pure lithium foil used as the counter electrode, enabling sacrificial element recovery, making the process eco-friendly. The morphology of the fabricated nanoporous Mg was thoroughly investigated using electron microscopy techniques, inductively coupled plasma (ICP) spectroscopy, X-ray photoelectron spectroscopy (XPS), small-angle X-ray scattering (SAXS), and X-ray diffraction. The synthesized nanoporous Mg is attractive for energy conversion and storage applications. Here, we show that it can produce hydrogen on-demand by hydrolysis with pure water and that it can also be used as a high-capacity lithium-ion battery anode.

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

“…For these experiments, the pellets were broken into 4–5 bulk pieces by tapping with a spatula, and hydrolysis took place on a single compacted piece of ∼20 mg. We found that 6.01 mL of H 2 was produced per gram of NP-Al/LiBH 4 in the 5 th hour of submersion for the 80:20 wt % composition pellet and 4.72 mL of H 2 produced per gram NP-Al/LiBH 4 for the 90:10 wt % composition. Because a single piece is tested, the expected H 2 generation curve as a function of time is expected to be different from our previous experiments using nanoporous metals crushed into fine, free-flowing powders for H 2 gas generation because of the differing surface areas available between the free-flowing powder form and the compact pellet form. − Thus, a low amount of H 2 is released and observed initially from these pellets; however, a longer and steady H 2 generation rate can be achieved as shown in Supporting Information Figure S1, where 4.81 mL of H 2 per gram is detected after 21 h of submersion for the NP-Al/LiBH 4 80:20 wt % pellet. The prolonged, near-steady H 2 generation is a result of the compressed pellet form factor that reduces the total surface area available for reaction with water and represents the speed at which water can penetrate deeper into the pellet.…”

“…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.…”

“…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, 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.…”

Recent Advancements in the Fabrication of Functional Nanoporous Materials and Their Biomedical Applications

“…In recent years, electrochemical dealloying processes have been modified to produce nanoporous materials from air- and water-sensitive metals, which holds great potential towards producing hydrogen on-demand by hydrolysis and energy storage applications. Fu et al synthesized 3D bicontinuous nanoporous Mg (pore sizes ~20–30 nm) from Mg-Li parent alloy using air-free electrochemical dealloying [ 71 ]. In this process, the sacrificial component of the parent alloy (Li) was recovered by using a pure Li foil as a counter electrode, thus, making the process eco-friendly.…”