Researchers at Tyndall National Institute, Cork, are collaborating with scientists from the US and Northern Ireland in a Science Foundation Ireland-supported project which aims to unlock the energy potential in water.
The researchers are bringing skills and knowledge acquired in advanced microelectronics to bear on this issue and are exploring how semiconductor materials and sunlight can be used to isolate energy-laden hydrogen in water by replicating processes found in nature.
Hydrogen is highly inflammable and, after being burned, water is its emission. As such it could make an ideal environmentally-friendly alternative to hydrocarbon fuels if a cheap and energy-efficient means were found to extract it from water.
The €1 million Research into Emerging Nanostructured Electrodes for the Splitting of Water (Renew) project, is led by Prof Martyn Pemble and Dr Paul Hurley at Tyndall, Prof Paul McIntyre at Stanford University and Prof Andrew Mills at Queen's University Belfast.
The researchers’ first task is to create what is effectively an artificial leaf using layers of semiconducting materials such as silicon. This would duplicate the photosynthesis process which occurs naturally in plants to split the molecules of water into hydrogen and oxygen under natural conditions without any additional energy.
“The main focus for the project is a tiny, stacked arrangement of materials that is used for some transistors in the electronic industry,” explains Prof Pemble.
“Previous work has shown that these structures can act as basic artificial leaves for splitting water and the aim now is to make them more efficient. Our primary focus is on creating an efficient structure that will work when sunlight falls on it without us having to give it an extra kick.”
The Renew project had its genesis in work being carried out at Stanford University. “People have been looking at various ways of splitting water for a long time,” says Pemble.
“The issue with using semiconductors is that they usually require ultra-violet light to do it. We thought that it would be better to if they would work with visible light and other more available parts of the light spectrum such as near infra-red.
Corrosive effects
“Our colleagues at Stanford had been doing some very interesting experiments with silicon and they had built a device which showed very promising water splitting qualities. When we got together, we realised that other forms of semiconductor could do a better job.”
The Stanford team showed that if you put the right metal on the surface of a silicon stack and provide light, then you can get it to oxidise water to give oxygen. Then, on another electrode connected to it – perhaps a platinum wire – the electrons produced can be used to reduce water, and this produces hydrogen.
“So it only requires the sunlight to fall on this stack of layers where the water oxidation takes place. Andrew Mills from Queen’s University Belfast is an acknowledged expert on photocatalysis and has shown that the rest of the process is driven by the electrochemistry.”
Pemble points out that silicon is not particularly efficient when it comes to absorbing visible light and that his team at Tyndall is looking at other materials including cadmium sulphide which are more efficient in this respect.
Key to the process will be creating an impenetrable top layer that can withstand water’s corrosive effects, by a process known as atomic layer deposition.
“There are some problems with the devices and one of the chief ones is preventing the photoanode from dissolving in the solution,” Pemble explains.
“We need to be able to place a protective layer on top of it, but that layer must be capable of allowing charged particles to pass through it. We are using atomic layer deposition to achieve this. It is a process used in the production of microprocessors and it allows you to grow materials one atomic layer at a time. We are using titanium dioxide and it seems to be doing a very good job of protecting the device.”
Ultimate goal
That protective layer is particularly important because it is the photoanode which is saturated in light and kick-starts the reaction. It generates the oxygen and the electrons which are created by it move along the circuit and produce the hydrogen at another electrode. The hydrogen-producing electrode doesn’t have to be exposed to the light, so the nature of its protective coating is not so critical.
Reflecting on the Renew partnership, Pemble says: “We have been thinking about doing this for a long time – it is quite obvious that these layered structures can have other applications outside of electronics – and now we have got the opportunity to bring it forward.
“The ultimate goal is to combine our expertise to get to a point where you just drop the electrodes into water and when the sun comes out they would start to bubble away, generating an unlimited, free and completely clean source of hydrogen, as well as oxygen.”
The Renew project is expected to run for the next three years and is jointly funded by the National Science Foundation in the US, Science Foundation Ireland and the Department for Employment and Learning for Northern Ireland under the US-Ireland Research and Development Partnership Programme.
“We are about six months into the project at the moment and we have high hopes that by the end of the three years we will have come up with some real world solutions to this issue,” says Pemple.
“It’s a great project to be involved in. If I say to someone that my research can help electronic devices work faster and better they shrug their shoulders, but when I say that I am working on a device to provide a clean source of free energy for the future you can see how excited it makes people.
“But this really is a team effort – we need all three partners to do what we’re doing. This is an incredibly hot topic in terms of research and that’s part of what makes it really very exciting. We are up against competition from all over the world.
“We are using our electronics knowledge to demonstrate that devices like these can be made to work and can be manufactured cheaply with enormous benefits for us all if we succeed.”
