With lithium-ion (Li-ion) batteries becoming the de-facto energy source for next-generation vehicles, some of us remember that there was a time when fuel cells were thought to be the most viable solution for powering vehicles after the internal combustion engine.
Of course, this is only a perception based on how companies like Tesla have made the Li-ion battery seem to be the best option. However, the US Department of Energy (DoE) has set benchmarks for what storage materials will need to deliver in order to compete for a place in post-fossil fuel vehicles.
Now researchers at Rice University have developed a nanomaterial for fuel cells that consists of layers of graphene separated by nanotube pillars of boron nitride. The material might tick all the boxes established by the DoE for next-generation vehicles.
In research described in the journal Langmuir, the Rice researchers demonstrated in computer models that 3D architecture of the hybrid nanomaterial would be able to store enough hydrogen to become a practical fuel for light-duty vehicles.
According to DoE’s benchmark figures, a medium would need to store at least 5.5 percent of its weight in hydrogen to be economically feasible. The Rice team has modeled a material that can store nearly 12 percent of its weight in hydrogen at room temperature. When the temperature is brought down to -196ºC that percentage bumps up to 15 percent.
To get to these numbers, the researchers juggled a few different combinations of nanomaterials. The models tested pillared structures of boron nitride and pillared boron nitride graphene doped with either oxygen or lithium. The best of these turned out to be oxygen-doped boron nitride graphene.
In the un-doped pillared born nitride graphene, the hydrogen atoms bond to the material because of van der Waals forces, which are forces of attraction or repulsion between molecules that are not based on covalent or ionic bonds. But when oxygen is used to dope the material the atoms bond very strongly with the hybrid material, leading to a better surface for incoming hydrogen.
“Adding oxygen to the substrate gives us good bonding because of the nature of the charges and their interactions,” said Rouzbeh Shahsavari, a materials scientist at Rice, in a press release. “Oxygen and hydrogen are known to have good chemical affinity.”
This hybrid material allows for a large degree of tunability, according to Shahsavari, making it possible to tailor it for particular operating temperatures and pressures.
With this structure, Shasavari and his colleagues are confident that the material can meet the DoE requirement that a fuel cell be able to withstand 1500 charge-discharge cycles.