Graphene can be used to collect hydrogen from moist ambient air to power fuel cells

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.

You can 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.

The Manchester group demonstrated that their one-atom-thick membranes can be used to extract hydrogen from a humid atmosphere. They hypothesise 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.

“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.”

Dr Sheng Hu, a postdoctoral researcher and the first author in this work, added: “It looks extremely simple and equally promising. Because graphene can be produced these days in square meter sheets, we hope that it will find its way to commercial fuel cells sooner rather than later”.

Proton transport through 2D crystals. a, Examples of I‐V characteristics for monolayers of hBN, graphite and MoS2. The upper inset shows experimental schematics. Middle inset: Electron micrograph of a typical graphene membrane before depositing Nafion. Scale bar: 1 µm. In a scanning electron microscope, 2D crystals give rise to a homogenous dark background and can only be seen if contamination, defects or cracks are present (Supplementary Fig. 2). Small (pA) currents observed for MoS2 membrane devices (lower inset) are due to parasitic parallel conductance. b, Histograms for 2D crystals exhibiting detectable proton conductivity. Each bar represents a different sample with a 2 µm diameter membrane. Left and right insets: charge density (in electrons per Å2) integrated along the direction perpendicular to graphene and monolayer hBN, respectively. The white areas are minima at the hexagon centers; the maxima correspond to positions of C, B and N atoms. 

Nature – Proton transport through one-atom-thick crystals

Arxiv – Proton transport through one-atom-thick crystals (19 pages)

Abstract –

Graphene is impermeable to all gases and liquids and even such a small atom as hydrogen is not expected to penetrate through graphene’s dense electronic cloud within billions of years. Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10^‐3 Ohm per cm2 above 250°C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen‐based technologies.

Graphene is increasingly explored as a possible platform for developing novel separation technologies. This interest has arisen because it is a maximally thin membrane that, once perforated with atomic accuracy, may allow ultrafast and highly selective sieving of gases, liquids, dissolved ions and other species of interest. However, a perfect graphene monolayer is impermeable to all atoms and molecules under ambient conditions: even hydrogen, the smallest of atoms, is expected to take billions of years to penetrate graphene’s dense electronic cloud. Only accelerated atoms possess the kinetic energy required to do this. The same behaviour might reasonably be expected in the case of other atomically thin crystals. 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.

Conclusions

Monolayers of graphene and hBN represent a new class of proton conductors. In addition to the fundamental interest in how subatomic particles transfer through atomically thin electron clouds, the membranes can find use in various hydrogen technologies. For example, 2D crystals can be considered as proton membranes for fuel cells. They are highly conductive to protons and chemically and thermally stable and, at the same time, impermeable to H2, water or methanol. This could be exploited to solve the problem of fuel crossover and poisoning in existing fuel cells. The demonstrated current‐controlled source of hydrogen is also appealing at least for its simplicity and, as large‐area graphene and hBN films are becoming commercially available, the scheme may be used to harvest hydrogen from gas mixtures or air.  

SOURCES – Nature, University of Manchester, Arxiv

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