Research Overview: Graphene for Radically New Energy Technologies

Graphene for energy use is emerging as an exciting topic of research, with breakthrough discoveries hitting the headlines every week.

Supercapacitors, the new paradigm for portable energy, are expected to replace traditional batteries in personal devices such as cellphones, laptop computers, tablets, and even electric vehicles. Supercapacitors can recharge and discharge in just seconds, but can hold enough energy to power a handheld device for several weeks. With supercapacitors, even electric cars could be charged at "electric stations" within a few minutes, with enough “gas” to drive around the whole day. Graphene supercapacitors have existed in research labs for only two years, but the progress has been tremendous.

The recently developed process to make graphene supercapacitors, which is economical and industrially scalable, starts from the widely available graphene oxide. Although the aforementioned development is awesome, a most recent one is a serious competitor in the world of graphene supercapacitors. The recent work radically departs from all known supercapacitors, by using technology's favorite material – silicon – as the active energy-storing material.

Instead of storing energy in chemical reactions the way batteries do, “supercaps” store electricity by assembling ions on the surface of a porous material. The problem with the concept of using silicon as the porous material for supercaps is that silicon tends to react with chemicals which form the electrolyte, a chief component of any modern energy-storing device.

The novel device, developed at Vanderbilt University, takes care of the silicon reactivity problem by coating the silicon with a layer of graphene. The graphene layer, only several nanometers thin, passivates the porous silicon surface, preventing any reaction with the electrolyte. The graphene smoothly follows the structure of the pores in silicon. The silicon-graphene composite acts as the electrode of the supercap, resulting in large improvements in energy density compared to bare silicon. Taking into account the ubiquity of silicon, this new approach provides an exciting platform for grid-scale as well as integrated (portable) energy storage.

Only a few weeks ago, engineers at the University of South Carolina published their research on graphene oxide membranes for separating hydrogen from carbon dioxide. The selectivity is based on molecular size, the team reported in the journal Science. Hydrogen and helium pass relatively easily through the membrane, but carbon dioxide, oxygen, nitrogen, carbon monoxide and methane permeate much more slowly. The graphene oxide membrane is less than 2 nanometers thick.

The ultrathin membrane acts like a nano-sieve, letting the smaller hydrogen molecules pass, while blocking the passage of carbon dioxide. The difference in size between a hydrogen and a CO2 molecule is only 40 femtometers. The invention has potential applications in a range of technologies.

With widespread concerns of carbon dioxide as a greenhouse gas, the separation of CO2 from other gases is a valuable addition to the palette of greenhouse gas reduction methods. Furthermore, separating hydrogen from other gases could be of use for hydrogen fuel cells. A different kind of graphene nano-sieve has also been used to clean water from pollutants.

Graphene can not only make an ultrathin filtering membrane, but in combination with other materials can make existing membranes more effective. In last month's research, scientists at Rice University have shown that the addition of graphene to a type of plastic commonly used in packaging beverages can make carbonated drinks have a longer shelf life. Graphene nanoribbons, added to thermoplastic polyurethane, made it 1000 times harder for gas molecules to escape the packaging. The findings could also be used to make better fuel tanks for vehicles that run on natural gas. -- by Marko Spasenovic, Graphenea