Graphene Cousin “Flips” For Energy Storage Breakthrough

November 4th, 2014 by Tina Casey

It looks like graphene from above, but flip it on its side and this interesting material reveals a nanoscale sandwich of atoms in three distinct layers. That material would be molybdenum disulfide, and a research team from Rice University has just announced a new energy storage breakthrough based around the sponge-like characteristics of the three-layered edge.

We’re going to drop the h-word up front, as in hydrogen for fuel cells, because the Rice team also foresees an application in hydrogen production. But first let’s check out the energy storage angle.

Energy Storage Breakthrough For Molybdenum Disulfide

Molybdenum disulfide (MoS2) is a semiconductor consisting of a layer of sulfur atoms trapped between two layers of the brittle, silvery transition metal molybdenum.

The flat side shares the now familiar chickenwire structure of atom-thin graphene, but its layered, relatively thick structure provides it with a more robust edge for both energy storage and catalytic reactions.

The new Rice energy storage breakthrough involved exploiting the properties of the edge, as explained by lead researcher and Rice chemist James Tour explains:

So much of chemistry occurs at the edges of materials. A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.

The Rice team isn’t the first to take advantage of MoS2, a widely used industrial material that has been emerging in clean tech applications including next-generation batteries as well as solar energy harvesters and hydrogen production.

The key challenge is making that leap over the “Valley of Death” from labwork to cost-effective, commercial application, and to do that the Rice team has developed a relatively quick, easy way to manufacture thin films of MoS2 while taking full advantage of the edge properties.

First, the Rice team grew a film of molybdenum oxide onto a layer of molybdenum using a room-temperature process anodization, which is commonly used in metals manufacturing.

Then, conversion to MoS2 was achieved by exposing the film to sulfur vapor at 572 degrees Fahrenheit for one hour.

Yep, that’s it.

Supercapacitor Breakthrough Yes, Battery Breakthrough Maybe

One application tested out by the team is supercapacitors, a type of energy storage device that charges and discharges quickly. Supercapacitors also have a longer lifecycle than comparable rechargeable batteries.

Using the new film, the team created supercapacitors that retained 90 percent of their capacity after 10,000 charging cycles and 83 percent after 20,000 cycles.

They haven’t figure out the battery angle quite yet but Tour foresees that the same anodization process could be applied to battery materials, so we’re giving that a solid maybe for now.

Hydrogen Fuel Cell Breakthrough, Yes

We’ve been all over the issue of hydrogen fuel cells for electric vehicles, and the Rice findings add more fuel to the fire by widening the pathway toward more efficient, sustainable hydrogen production.

Aside from technology and efficiency issues compared to battery electric vehicles the sourcing of hydrogen from fossil natural gas is a big sticking point.

However, alternative sources are emerging, including renewable biogas and the solar-driven production of hydrogen from water.

The Rice breakthrough fits into the latter category. You can get hold of the study online but here’s a snippet from the abstract:

The edge-oriented MoS2 film delivers excellent hydrogen evolution reaction (HER) activity with enhanced kinetics and long-term cycling stability.

Typically, the catalysts used to separate hydrogen from water require platinum, so switching over to a relatively cheap, easily manufactured material would help remove one big obstacle to widespread hydrogen use.

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