Univ. of Manchester team finds monolayer graphene permeable to protons; implications for PEM fuel cell and other hydrogen technologies

28 November 2014

Researchers at the University of Manchester in the UK have found that monolayers of graphene—which, as a perfect monolayer is impermeable to all gases and liquids—and its sister material boron nitride (BN) are highly permeable to protons, especially at elevated temperatures and if the films are covered with catalytic nanoparticles such as platinum. The finding could have a significant impact on proton exchange membrane fuel cell technologies and other hydrogen-based technologies.

The discovery is reported in the journal Nature by an international team led by Professor Sir Andre Geim, who, with Professor Sir Kostya Novoselov succeeded in producing, isolating, identifying and characterizing graphene in 2004 at the University of Manchester, an achievement for which the pair won the Nobel Prize in Physics in 2010. (Graphene had been studied theoretically as far back as 1947; professors Geim and Novoselov were the first to fabricate and to study the material.)

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Graphene is a single layer of carbon packed in a hexagonal (i.e., like a honeycomb) lattice, with a carbon-carbon distance of 0.142 nm. It was the first truly two-dimensional crystalline material and is representative of a whole class of 2D materials including monolayer Boron-Nitride (BN) and Molybdenum-disulfide (MoS₂), both of which were produced after 2004.

A perfect graphene monolayer—i.e., a one-atom thick graphene sheet—is impermeable to all atoms and molecules under ambient conditions, including the smallest of atoms, hydrogen. Only accelerated atoms possess the kinetic energy required to penetrate graphene’s dense electronic cloud.

(Graphene is also being increasingly explored as a possible platform for developing novel separation technologies because, once the impermeable monolayer is perforated with atomic accuracy, it may allow ultrafast and highly selective sieving of gases, liquids, dissolved ions and other species of interest.)

The Manchester team was experimenting to see if protons are also repelled by graphene; their expectation was that protons would indeed be blocked, as existing theory predicted as little proton permeation as for hydrogen.

Instead, the researchers found that protons pass through the ultra-thin crystals surprisingly easily.

Here we report transport and mass spectroscopy measurements which establish that monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons under ambient conditions, whereas no proton transport is detected for thicker crystals such as monolayer molybdenum disulphide, bilayer graphene or multilayer hBN.

Protons present an intermediate case between electrons (which can tunnel easily through atomically thin barriers) and atoms, yet our measured transport rates are unexpectedly high and raise fundamental questions about the details of the transport process. We see the highest room-temperature proton conductivity with monolayer hBN, for which we measure a resistivity to proton flow of about 10 Ω cm² and a low activation energy of about 0.3 electronvolts. At higher temperatures, hBN is outperformed by graphene, the resistivity of which is estimated to fall below 10−3 Ω cm² above 250 degrees Celsius. Proton transport can be further enhanced by decorating the graphene and hBN membranes with catalytic metal nanoparticles. The high, selective proton conductivity and stability make one-atom-thick crystals promising candidates for use in many hydrogen-based technologies.

The discovery makes monolayers of graphene, and its sister material boron nitride, attractive for possible uses as proton-conducting membranes in fuel cells. Fuel cells use oxygen and hydrogen as a fuel and convert the input chemical energy directly into electricity. Without membranes that allow an exclusive flow of protons but prevent other species to pass through, this technology would not exist.

Despite being well-established, fuel-cell technology requires further improvements to make it more widely used. One of the challenges is a fuel crossover through the existing proton membranes, which reduces their efficiency and durability.

The University of Manchester research suggests that the use of graphene or monolayer boron nitride can allow the existing membranes to become thinner and more efficient, with less fuel crossover and poisoning. This can boost competitiveness of fuel cells.

Hydrogen harvesting. The Manchester group also demonstrated that their one-atom-thick membranes can be used to extract hydrogen from a humid atmosphere. They hypothesize that such harvesting can be combined together with fuel cells to create a mobile electric generator that is fueled simply by hydrogen present in air.

When you know how it should work, it is a very simple setup. You put a hydrogen-containing gas on one side, apply small electric current and collect pure hydrogen on the other side. This hydrogen can then be burned in a fuel cell. 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.

The work is an international collaboration involving groups from China and the Netherlands who supported theoretical aspects of this research.

Resources

• S. Hu, M. Lozada-Hidalgo, F. C. Wang, A. Mishchenko, F. Schedin, R. R. Nair, E. W. Hill, D. W. Boukhvalov, M. I. Katsnelson, R. A. W. Dryfe, I. V. Grigorieva, H. A. Wu A. K. Geim (2014) “Proton transport through one-atom-thick crystals,” Nature doi: 10.1038/nature14015

• K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov (2004) “Electric Field Effect in Atomically Thin Carbon Films” Science 666-669 doi: 10.1126/science.1102896