Graphene Shakes Things up for Fuel Cells
The promise for nanomaterials in fuel cells has been a bit lackluster, until the latest discoveries of graphene’s potential
There are two main issues that have hampered the prospects of fuel cells for the powering of automobiles: the costs of isolating hydrogen and building an infrastructure that would deliver that hydrogen to the automobiles.
In what may be one of the most significant contributions of nanomaterials to the potential of the so-called hydrogen economy, researchers at the University of Manchester, led by Nobel Laureate Andre Geim, have shown that the one-atom-thick materials graphene and hexagonal boron nitride (hBN), once thought to be impermeable, allow protons to pass through them.
Ultimately, the impact of this discovery is that it may become significantly easier to isolate hydrogen and you won’t need to build an infrastructure for delivering hydrogen to the automobiles—the fuel cells in combination with some external energy source will be able to produce their own hydrogen.
Geim and his colleagues at Manchester discovered, in research that was published in the journal Nature, monolayers of graphene and hBN are highly permeable to thermal protons under ambient conditions. So hydrogen atoms stripped of their electrons could pass right through the one-atom-thick materials.
This ability of these materials to allow protons to pass through them came as quite a surprise to the researchers. The discovery stands in contradiction to what was thought to be the absolute impermeability of these two materials. So impermeable had graphene been considered that it was estimated it would take the lifetime of the universe (some 14 billion years) for a hydrogen atom to pass through it.
However, with this new understanding, the Manchester researchers claim that a graphene-based membrane could be used for proton-conducting membranes (also known as proton exchange membranes), which are critical to the functioning of fuel cells by conducting protons through it while being impermeable to gases. It is this particular design of fuel cells—so-called polymer exchange membrane fuel cell (PEMFC)—that the US Department of Energy has targeted as the most likely candidate for use in automobiles.
Up until now, the proton-conducting membranes of these fuel cells had been made from polymers that suffered from fuel crossover, which limited their efficiency and durability. The Manchester research suggest that graphene and hBN could be used to create a thinner membrane that would be more efficient while reducing fuel crossover and cell poisoning.
This application would be significant, but it is merely replacing one material with a 2D material to do essentially the same thing that is being done now. What may be the truly remarkable implication of this research is that the graphene membrane could be used to extract hydrogen out of humid air. The researchers believe that the membrane could be used in hydrogen production by extracting hydrogen from humid air to create a kind of mobile hydrogen generator.
“When you know how it should work, it is a very simple setup,” said Marcelo Lozada-Hidalgo, a PhD student and corresponding author of this paper, in a press release. “You put a hydrogen-containing gas on one side, apply a small electric current, and collect pure hydrogen on the other side. This hydrogen can then be burned in a fuel cell.”
Lozada-Hidalgo added: “We worked with small membranes, and the achieved flow of hydrogen is of course tiny so far. But this is the initial stage of discovery, and the paper is to make experts aware of the existing prospects. To build up and test hydrogen harvesters will require much further effort."
According to Lozada-Hidalgo’s explanation, it would seem the hydrogen-containing gas would be the humid air in the atmosphere. However, it’s not clear where the electrical charge that would split the hydrogen and oxygen would come from. Observers have noted that the amount of energy needed to separate hydrogen from water would exceed the energy you would get from combining them again in the fuel cell. So, presumably, the electrical current would need to come from another source, possibly a photovoltaic cell or a battery.
Of course, nanomaterials have been pursued as a way to mimic photosynthesis for splitting water molecules into oxygen and hydrogen for some time from branched nanowire arrays to man-made viruses and companies like HyperSolar having even announced commercial projects. In these efforts, the sun is used as the source of energy that splits the water molecules.
If the potential of this discovery can be exploited as suggested by the researchers, in hindsight we should be pleased that Nobel Laureate Geim decided to forego the career path of pursuing the commercialization of graphene and instead remained focus on fundamental research. He may have just landed upon the “killer app” for graphene.
Graphene and Quantum Dots Vie to Dislodge Platinum as Catalysts in Fuel Cells
Researchers at Rice University have discovered that graphene-based quantum dots are able to serve as better catalysts to the oxygen reduction reactions in fuel cells better than the more costly platinum variety.
Quantum dots are nanocrystals that possess quantum mechanical properties that makes them useful for a wide range of applications, including LCD displays where they already provide richer colors and improved energy efficiency in products available today.
The Rice team used their recently developed process of producing graphene-based quantum dots from coal and discovered that the new material actually outperforms platinum-based catalysts for fuel cell reactions.
“You don’t need to apply as high a voltage as platinum to get the oxygen reduction reaction to occur,” said James Tour, a Rice chemistry professor. “We also get about 70 percent higher current than what platinum would offer.”
The other clear benefit beyond performance is cost. Platinum is one of the most costly and precious metals. Whether the replacement of the catalyst for fuel cells will be the factor that reduces the cost of fuel cells from a cost of $55 per kilowatt (kW) today down to the $30 per kW the US Department of Energy estimated is needed to make fuel cells more price-competitive remains to be seen.
However, Rice in interviews conceded that the real obstacle for adopting hydrogen-based systems is the problem of creating an infrastructure for supplying the hydrogen.
Once again, if the work out of Manchester can live up to its promise of providing an easier way of isolating hydrogen and supplying it to vehicles, it could tip the balance for fuel cells.