Hydrogen: Materials for More Than Storage
Hydrogen is often discussed as a way to store renewable energy. But hydrogen-based materials can do more than just store hydrogen. A new review article with contribution from Torben R. Jensen from Aarhus University gives an overview of how these materials can support hydrogen transport, compression, and energy storage, such as new types of solid-state batteries.
Hydrogen can play an important role in a future energy system based on renewable electricity. It can be produced from water using green electricity and later used as fuel, chemical feedstock or energy carrier. But before hydrogen can become widely used, several practical challenges need to be solved.
How can hydrogen be stored safely? How can it be transported over long distances? How can it be compressed efficiently? And can hydrogen-based materials create a fundament for the development of new energy technologies?
A new review article in Nature Reviews Clean Technology brings together current research on hydrogen-based materials for energy storage and conversion. The review covers several material classes, including nanoporous materials, metal hydrides, complex hydrides, liquid organic hydrogen carriers and ammonia.
The article shows that hydrogen-based materials are not only relevant for solid state hydrogen storage but also hydrogen compression and heat storage. Some new materials are also promising battery materials.
Key Findings
Hydrogen materials can do more than store hydrogen. They can also help move hydrogen, compress it, improve battery concepts and store heat.
Some porous materials act as sponges and can store large amounts of hydrogen by adsorption to the large inner surfaces.
Organic liquids can also bind hydrogen and facilitate easy storage and transport.
Metal hydrides can be used to new types of hydrogen compressors without a piston with much longer lifetime as compared to conventional compressors.
The next step is practical use. Some hydrogen-based materials have already founded new technologies, which have been demonstrated on a larger scale and are ready to be explored commercially.
Different Materials for Different Tasks related to Hydrogen
Hydrogen, H, is the lightest element. This gives hydrogen the highest energy content per mass among all known substances. However, it is a gas at ambient conditions and very difficult to store compactly. Today, hydrogen can be stored as compressed gas or be condensed to a liquid by cooling to very low temperatures. Both methods are already used, but they come with challenges. Compression requires energy and expensive high-pressure tanks. Liquid hydrogen requires very low temperatures and good thermal insulation to limit hydrogen losses over time.
This review describes several material-based alternatives:
Porous materials, such as metal organic frameworks and activated carbons, work like ‘sponges’. They have small nano-pores and a very large inner surface area, which allow adsorption of large amounts of hydrogen. The advantage is that the adsorption process is very fast and reversible. The challenge is that it usually works best at low temperatures, because the bond between hydrogen and the surface is weak.
Metal and complex hydrides store hydrogen in another way. Here, hydrogen becomes part of a solid material by formation of a chemical bond between hydrogen and another element. Some hydrides can store a large amount of hydrogen in a small volume and at relatively low pressure. This is useful for stationary storage, where weight is less important than safety, size and cost.
Liquid carriers offer a third route. Liquid organic hydrogen carriers and ammonia can hold hydrogen in liquid form. This makes them interesting for long-distance transport and large-scale storage. They can be easier to handle than very cold liquid hydrogen. However, catalysts are needed to release the hydrogen and to load it back into the carrier.
From Storage to Compression, Batteries and Heat
Another point touched upon in the review is that material research has a wider role than hydrogen storage alone.
Hydrides are also important in different types of batteries. Nickel metal hydride batteries are already commercial and have been used in hybrid electric vehicles. But were out-competed by the advent of lithium-ion batteries. However, new research has developed new metal hydrides which allow us to replace the aqueous solutions in the battery with ionic liquids to provide higher energy densities. Moreover, novel classes of complex hydrides have extremely fast cationic conductivity and have been demonstrated to be functional solid-state electrolytes for batteries, which has shed new light on future magnesium and calcium batteries.
Metal hydrides can be used to compress hydrogen. They absorb hydrogen at a lower temperature. When they are heated, they release hydrogen again at a higher pressure. In this way, they can act as compressors without using any moving parts, such as pistons. These new types of compressors are expected to have much longer lifetime. This review describes prototype systems that have reached high pressures. This is relevant for hydrogen refueling stations, which store hydrogen at 1000 bar to fill the cars with hydrogen at pressures up to 700 bar.
Another promising direction is thermal energy storage. Thermal energy (heat) is available directly from the sun but also as waste heat from many industrial processes. Traditionally, heat has often been stored in molten salts, but hydrogen release and uptake in some hydrides are shown to store much more heat. Metal hydrides can store and release heat through reversible reactions with hydrogen. Some hydrides have very high heat storage densities and can work over a wide temperature range.
Why Does It Matter?
Renewable energy production does not match our energy consumption. Wind and solar power can produce large amounts of electricity, but energy systems also need ways to store energy for later use, move it across distances and connect it to industry, transport and heating.
This review reveals that new hydrogen-based materials with unexpected properties may inspire new technologies.
This makes research within hydrogen-based materials important for the green transition. There is a desperate need for new materials with useful properties within heat storage, battery materials etc. at lower costs and with smaller environmental impact to further develop the green transition.
Interested?
You can explore the full review article and learn about the details in Nature Reviews Clean Technology.
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