Graphene Has Implications For Solar Power and Organic Lithium Ion Batteries
The long list of Graphene applications continues to grow. It turns out this one atom thick material with excellent conductivity bonds quite well with organic molecules in a lithium ion battery, complementing other compound layers for use in solar power as well.
An organic molecule called hexaazatrinaphthalene (HATN) had been tested before Graphene as a possible source of material for organic cathodes (positive electrode) in a lithium ion battery. Unfortunately, the battery’s liquid electrolyte solution quickly began to dissolve the material ruining it’s bonded molecular structure.
Researchers needed to find something to shield this solid HATN molecule from the onslaught of liquid electrolyes. Furthermore, if they could raise the conductivity of the material simultaneously – that would be ideal.
Scientists are clever, so they figured out that Hydrogen bonds in the HATN molecule bond together well with oxygen atoms in Graphene Oxide, so they decided to try it out. “Graphene oxide has excellent conductivity, and surface oxygen atoms may form hydrogen-bonding interactions with HATN,” Zhang said.
It turns out this molecular combination forms an elaborate nano-rod infrastructure – protecting the easily dissolved HATN core with an outside layer of conducive, resilient Graphene oxide.
“Graphene Oxide solved the dissolution issue of HATN in electrolyte and gave the cathode very good cycling stability,” Zhang says.
A lithium ion battery using this material as its cathode retained 80 per cent of its capacity after 2000 charge/discharge cycles. This is a considerable leap in endurance for a typical lithium battery has a lifespan of only 300-500 cycles.
Furthermore, when they bond graphene oxide with a derivate of HATN called hexaazatrinaphthalene tricarboxylic acid (HATNTA) performance was further increased.
A battery fitted with the HATNTA cathode conserves 86 per cent of its capacity after 2,000 charge/discharge cycles. The team hypothesized that the improved performance arises from HATNTAs polar carboxylic acid groups, which become even more strongly attached to HATNTA due to the addition of graphene oxide.
The discovery was made by Yuzen Zhang and colleagues from A*STAR institute of bioengineering and nanotechnology.
Electrons move through one carbon atom thick graphene at 1/30th the speed of light which is many leagues ahead of any other material currently available to solar panel manufacturers. By retrofitting Graphene to the surface of a solar panel engineers could increase the speed of power generation while simultaneously reducing width and increasing flexibility.
One obstacle was the ultra-short lifetime of excited electrons in graphene (referring to the time the electrons stay mobile). In graphene, this time is only one picosecond or one-millionth of one-millionth of a second
The amount of electrons that can contribute to the current in a solar panel depends on the average time that stay mobile after being activated by sunlight,
“In graphene, an electron stays free for only one picosecond. This is too short for accumulating a large number of mobile electrons.”
However, similar to the Lithium ion battery cathode scientists discovered yet another trick of chemistry. By combining Graphene with Molybdenum Diselenide and Tungsten Disulfide they were able to extend the duration electrons stay mobile in graphene by several hundred times. Zhao and his team created a “tri-layer” material of the MoSe2, WS2, and Graphene layers stacked on top of one another.
The outer most layer facing the sun is made of MoSe2. When sunlight strikes MoSe2 electrons are liberated from the material and begin flowing in to the intermediary layer between MoSe2 and Graphene. This WS2 layer is designed to essentially “suck” the electrons out of MoSe2, and quickly transfer them to the final Graphene layer.
Once in graphene, electrons stay mobile and begin to build up electric current.
In order to test the new material researchers shot an ultra-short laser pulse of 0.1 picoseconds in duration at the MoSe2, liberating some elecrons. In order track the electrons moving through Graphene they used another ultra-short laser pulse to gather data.
What they discovered is that electrons pass through WS2s central layer in about 0.5 picoseconds and maintain their momentum in the final Graphene layer for about 400 picoseconds.
This represents a 400-fold improvement over a single layer of graphene, which researchers also measured in their study.
Their experiments also confirmed that “seats” left in MoSe2 stay unoccupied for the same amount of time. This means that scientists applying their method can control this time, depending on their application, by choosing different “hallway” layers, Zhao said. The work—which researchers plan to continue—paves the way for the use of graphene in novel photovoltaics, potentially adding solar-energy generation to the growing list of applications for graphene, researchers said.
Aside from investigating alternatives to graphene oxide, the team also is working on HATN-based porous polymers for use as organic cathode materials, which should enhance the flow of ions during battery charge and discharge.