Researchers Cook Up Energy Storage Stew With Hydrogen And Graphene

November 9th, 2015 by Tina Casey

The mobile energy storage units — also known as electric vehicles — have been targeted by Lawrence Livermore National Laboratory for a makeover, and those of you who are sceptical about the role that hydrogen will play in powering the car of the future may be in for a surprise. According to the research team, hydrogen could be used to improve lithium-ion batteries, the current gold standard for electric vehicle batteries.

Hydrogen In A Li-ion Battery, With Graphene

The new energy storage research from Livermore is focused on improving the electrodes of Li-ion batteries. For those of you new to the topic, the basic battery has two electrodes, one positive and one negative. Lithium ions that are hanging on the negative electrode scoot over to the positive electron when the battery is discharging, and then they reverse course when the battery is being charged.

That means the material of which the electrode is made has a lot of work to do. Think of a dog walker with too many leashes, not enough leashes, tangled leashes, or leashes that break at inopportune moments, and you get the picture. Here’s how the folks at Livermore describe the challenge:

Several key characteristics of lithium ion battery performance — capacity, voltage and energy density — are ultimately determined by the binding between lithium ions and the electrode material. Subtle changes in the structure, chemistry and shape of an electrode can significantly affect how strongly lithium ions bond to it.

This is where graphene, our favorite nanomaterial of the new millennium, comes in. Researchers (and this guy, of course) have been tinkering around with graphene as an electrode material for the next generation of high performance, long range EV batteries.

However, there’s a catch. You can DIY your own natural graphene by applying a piece of sticky tape to a chunk of graphite, but recreating the perfect chickenwire structure of the atom-thin material (yes, only one atom thin) at scale for commercial use is notoriously difficult.

The graphene you see in the marketplace today is full of nano-level defects, and more to the point, the typical graphene manufacturing process is based on a chemical synthesis in which hydrogen plays a key role. That could change, but in the meantime there is a lot of residual hydrogen lurking in manufactured graphene, and energy storage sleuths are awfully curious about what impact it has on battery performance.

The Energy Storage Surprise

If you were guessing that hydrogen always degrades performance in electrodes based on graphene, you were wrong (so were we). The team focused their efforts on 3-D graphene nanofoam electrodes, which researchers favor over other forms of graphene for study because the foam is free of binders, so there is no interference from random additives. Another plus is that graphene in a nanofoam state is highly defective. When the team deliberately hit this “defect-rich” graphene with hydrogen, they found that performance improved:

Hydrogen interacts with the defects in the graphene and opens small gaps to facilitate easier lithium penetration, which improves the transport. Additional reversible capacity is provided by enhanced lithium binding near edges, where hydrogen is most likely to bind.

As for the strength of the effect, the team concludes that the hydrogen enhancement has a “surprisingly huge effect on performance.”

What Took So Long For This Energy Storage Breakthrough

Last week we previewed the Energy on the Edge episode of the new Breakthrough series on National Geographic Channel, which showcases some dramatic examples of the collaborative nature of scientific progress underpinning the spectacular leap forward.

Livermore’s new energy storage breakthrough echoes that theme, as described in the full study published at Nature.com under the title Universal roles of hydrogen in electrochemical performance of graphene: high rate capacity and atomistic origins.

As explained by the research team, the hydrogen impact on energy storage performance has been previously studied on “soft” carbon materials produced through pyrolysis, without a conclusive explanation:

The exact mechanisms underpinning this empirically observed behaviour remain a subject of ongoing debate and it is unclear whether this phenomenon also occurs for graphene. A main challenge has been to control the hydrogen content and location in graphene materials, a subject that is also of great interest to hydrogen storage applications.

The team also notes that other experiments on single sheets of graphene indicate that hydrogen would have a negative effect on performance. In order to avoid blind alleys and figure out a new path forward, the team had to cull through the work of previous researchers.

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Photo (cropped): Improved lithium-ion battery via Lawrence Livermore National Laboratory.









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