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As the automotive industry in particular has become drawn to the concept of integrating hydrogen energy as a fuel source, the challenges associated with successfully storing this material have greatly limited its practicality for this purpose.
To combat this dilemma, a group of Researchers from the Lawrence Berkeley National Laboratory in California have recently discovered a new mechanism by which graphene-wrapped magnesium oxide nanoparticles (MgO-NP) successfully reduces the surface reactivity and oxidation of MgO-NP to allow for the optimal storage of hydrogen energy.
As the most abundant and simplest element present within the universe, hydrogen energy is a nontoxic fuel source whose only emission is water vapor. This zero emission property is highly valuable, especially when considering the harmful emissions that are produced following the use of the more popular energy fuel sources such as gasoline, coal and nuclear energy.
While these sources must often be obtained through various extraction processes, hydrogen is virtually everywhere, thereby allowing the generation of hydrogen to be much easier and in higher demand1. Despite the vast number of benefits associated with hydrogen energy, its storage is exceptionally difficult due to the extremely small size of hydrogen molecules that make this energy source more susceptible to leaks in storage.
Two current ways in which hydrogen can be stored involve its adsorption onto the surface of or within solid materials, which involves the attachment of the hydrogen molecules in the form of H2 or atoms (H) onto the surface of the material2. When heated, adsorbed hydrogen will be released from the surface, thereby allowing for its use as an energy source. Some materials that have been successful in this type of hydrogen storage include various metal hydrides, amides and other organic molecules, however, these materials are limited in their storage capacity.
Recent research has pointed to magnesium oxide (MgO) as a practical material to be used for the storage of hydrogen energy, as its slow kinetics allow for a gradual uptake and release of this useful molecule. When MgO is reduced to the nanosize, its ability to adsorb hydrogen is increased as a result of the greater surface area of the material, however, this increased surface area will also make this material more susceptible to deleterious surface reactions such as oxidation, essentially effecting the hydrogen storage capacity.
In an effort to address this challenge, previous work has shown that embedding Mg nanoparticles into graphene or graphene oxide materials can reduce these types of reactions while simultaneously allowing for the permeation of hydrogen molecules for energy production.
The work performed by the Lawrence Berkeley Researchers points to a bulk-like crystalline MgO layer that is present within the graphene-encapsulated/Mg nanocomposite, which plays an important role in allowing for the dissociation of hydrogen molecules out of the material while maintaining the mechanical and chemical stability of this combined material. To investigate the ability of this oxide layer to potentially affect the overall uptake and release of hydrogen molecules, the Researchers employed various techniques. These included X-ray photoelectron spectroscopy, which revealed a small fraction of the oxidative state present within the Mg-NP and X-ray diffraction for the bulk characterization of the materials, which determined that no magnesium oxide was formed in the graphene-wrapped MgO material.
By investigating the properties of this material by these methods, the Authors concluded that the outermost atomic layer of magnesium that comes into direct contact with the graphene oxide layer becomes oxidized in this state. This oxidation allows for a honeycomb MgO structure to arise, which has been shown to be beneficial for the dissociation of hydrogen molecules when used as an adsorption material in hydrogen storage systems.
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References:
- “What is Hydrogen Energy?” – Conserve Energy Future
- “Hydrogen Storage – Basics” – Energy.gov
- “Atomically Thin Interfacial Suboxide Key to Hydrogen Storage Performance Enhancenements of Magnesium Nanoparticles Encapsulated in Reduced Graphene Oxide” L. Wan, Y. Liu, et al. Nano Letters. DOI: 10.1021/acs.nanolett.7b02280.
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