An unexpected effect found in graphene could be manipulated to design devices capable of directly harvesting solar energy to produce hydrogen fuel - a promising green alternative.
Researchers from the University of Manchester demonstrated an increase in the rate at which graphene membranes conduct protons when simply illuminated by sunlight. This 'photo-proton' effect as it has been dubbed, could be used in devices to artificially mimic photosynthesis. It might also find uses in light-induced water splitting, photo-catalysis and for making new types of highly efficient photodetectors.
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Worldwide, scientists are investigating how to directly use solar energy to produce renewable fuels by trying to replicate photosynthesis in plants. Their man-made 'leaves' will need membranes with very sophisticated properties, including mixed proton-electron conductivity, permeability to gases, mechanical robustness and optical transparency. A mixture of proton and electron-conducting polymers are currently used to make such structures, but they require some important compromises which could be avoided if graphene were used instead.
Graphene – a sheet of carbon just a single atom thick – has an abundance of unique physical and mechanical properties; it is an excellent conductor of electrons and can absorb light of all wavelengths. It is also permeable to thermal protons – the nuclei of hydrogen atoms – researchers have recently found, meaning it might be used as a proton-conducting membrane in various technological applications.
To investigate how light affects the behaviour of protons permeating through the carbon sheet, researchers fabricated pristine graphene membranes decorated on one side with platinum nanoparticles. The team – led by Dr Marcelo Lozada-Hidalgo and Professor Sir Andre Geim – were surprised discover that the proton conductivity of the membrane was enhanced 10 times when illuminated with sunlight.
“By far the most interesting application is producing hydrogen in an artificial photosynthetic system based on these membranes," said Lozada-Hidalgo.
“This is essentially a new experimental system in which protons, electrons and photons are all packed together in an atomically thin volume," said Geim, who won the 2010 Nobel Prize in Physics for his groundbreaking work with graphene. "I am sure that there is a lot of new physics to be unearthed, and new applications will follow.”
Using a combination of mass spectrometry and electrical measurements, the researchers say they measured a photoresponsivity of around 104 A/W, which means around 5,000 hydrogen molecules are being formed in response to every solar photon – or light particle – incident on the membrane. This number is massive compared with existing photovoltaic devices where many thousands of photons are needed to produce just a single hydrogen molecule.
“We knew that graphene absorbs light of all frequencies and that it is also permeable to protons, but there was no reason for us to expect that the photons absorbed by the material could enhance the permeation rate of protons through it,” says Lozada-Hidalgo. “The result is even more surprising when we realized that the membrane was many orders of magnitude more sensitive to light than devices that are specifically designed to be light-sensitive. Examples of such devices include commercial photodiodes or those made from novel 2D materials.”
Photodectectors typically harvest light to just produce electricity, but graphene membranes also produce hydrogen as a by-product. The speed at which they respond to light in the microsecond range is faster than most commercial photodiodes.
Additionally, graphene membranes are being explored for potential uses in filtration - separating organic solvent from water and removing water from a gas mixture - and in coatings on food and pharmaceutical packaging where they could stop the transfer of water and oxygen; thus keeping food and perishable goods fresher for longer. They may also find applications in removing of harmful carbon dioxide released into the atmosphere by power stations.
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