In research out of the University of Manchester in the UK led by Nobel Laureate Andre Geim, it has been shown that the one-atom-thick materials graphene and hexagonal boron nitride (hBN), once thought to be impermeable, allow protons to pass through them. The result, the Manchester researchers believe, will be more efficient fuel cells and the simplification of the heretofore difficult process of separating hydrogen gas for use as fuel in fuel cells.
This latest development alters the understanding of one of the key properties of graphene: that it is impermeable to all gases and liquids. Even an atom as small as hydrogen would need billions of years for it to pass through the dense electronic cloud of graphene. In fact, it is this impermeability that has made it attractive for use in gas separation membranes.
But as Geim and his colleagues discovered, in research that was published in the journal Nature, monolayers of graphene and boron nitride 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.
The surprising discovery that protons could breach these materials means that that they could be used in proton-conducting membranes (also known as proton exchange membranes), which are central to the functioning of fuel cells. Fuel cells operate through chemical reactions involving hydrogen fuel and oxygen, with the result being electrical energy. The membranes used in the fuel cells are impermeable to oxygen and hydrogen but allow for the passage of protons.
It is these proton exchange membrane fuel cells that are thought to be the most viable fuel cell design for replacing the internal combustion engine in vehicles. However, the polymer-based membranes that have been used to date suffer from fuel crossover that limits their efficiency and durability.
The implication of this latest research is that graphene and hBN could be used to create a thinner membrane that would be more efficient while reducing fuel crossover and cell poisoning. The end result is that it could give the fuel cell the technological push that it has needed to make hydrogen a viable alternative to fossil fuels.
Another, even more remarkable prospect highlighted by this discovery is that these one-atom-thick materials could be used to extract hydrogen from a humid atmosphere. This could be a huge bend in the road that points us towards the so-called hydrogen economy.
One of the inconvenient truths about fuel cells for powering automobiles is that it is extremely costly and energy intensive to isolate hydrogen gas. The main push in nanomaterials for hydrogen gas separation has been artificial photosynthesis in which sunlight rather than electricity is used to split the hydrogen from a water molecule. In fact, another two-dimensional material, molybdenum sulfide (MoS2), has been used as a somewhat effective catalyst for producing hydrogen gas in a solar water-splitting process.
But what Geim and his colleagues are suggesting with this latest research stands this paradigm on its head. It is conceivable, based on this research, that hydrogen production could be combined with the fuel cell itself to make what would amount to a mobile electric generator fueled simply by hydrogen present in air.
“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."
While some have been frustrated that Geim has focused his attention on fundamental research rather than becoming more active in the commercialization of graphene, he may have just cracked open graphene’s greatest application possibility to date.