Harnessing solar energy

[International Desk, DIU]

Solar energy, despite its abundance, has proven to be a very difficult form of energy to harness effectively. However, an international team of scientists and engineers from Europe now believe that they may be on their way to changing this state of affairs, using the same principles involved in photosynthesis to harness solar energy in the form of hydrogen fuel.

NANOPEC is a project that is aiming to revolutionise the way in which solar energy is harnessed. By addressing the challenges of photon capture and energy conversion, the international team have begun investigations into solar-driven hydrogen production via photoelectrochemical water splitting.

Although scientists have long considered the idea of hydrogen as a fuel, no photocatalyst material as yet has been found that has acceptable performance, stability and cost levels to make it a viable concept. However, with the dawn of the nanotechnology revolution, this is all about to change, as Kevin Sivula explains: “The notion of converting solar energy directly into chemical energy via photoelectrochemical reactions has been known for many years,” he begins, “but it is really only since nanotechnology has come to the forefront of a lot of new research that the field has been ‘rediscovered’.

“The crucial task that nanotechnology can perform is to enable inexpensive materials - semiconductor oxides - to perform at a much greater level than they would naturally within these solar-to-hydrogen reactions.

“There are three things that need to happen in an electrode for photoelectrochemical energy conversion to occur,” explains Sivula. “First of all, you need light to be absorbed by one material. This light absorption event creates a charge, and this charge then needs to be transported through the electrode material to be collected by a wire or transferred to another electrode for a redox reaction.

“After that, you need a material for the catalysis of the reaction occurring, because unlike photovoltaic conversion which just creates electricity, we are creating an excited state in the material that has to then be transferred into a chemical bond.”

The NANOPEC project has applied nanotechnology to all three of these stages, but it is the methods by which the nanotechnology is being applied that Sivula believes is one of the strongest aspects of the project. They have developed methods using simple solution processing techniques that create nanostructures by controlling the kinetics in a solution reaction, enabling them to create nanostructures in bulk form very easily.

“There is a lot of nanotechnology out there that makes for some very impressive demonstrations,” says Sivula, “but which is not particularly practical. When each nanowire needs to be created individually, and then taken off the substrate using an atomic force microscope tip, it is not really a technique that is scalable to industry level. That is why we have focused on using simple bottom-up approaches.”

One of the chemical engineering partners has been working towards scaling up some of the concepts demonstrated and making them work over a larger area, in the hope of preparing a prototype electrode for a photelectrochemical cell. “This is essential work,” says Sivula, “as it also means that we have the opportunity to test stability and how well the electrode works in different conditions such as high light intensity and temperatures. This will bring us closer to fully understanding the feasibility of the approaches and devices we are developing.”

Eight partners from across Europe comprised of a mixture of engineers and scientists have been brought together to tackle the overall goal of creating inexpensive and efficient devices that work on the photoelectrochemical concept. The goal for the end of the project is to have created a demonstrable device of relatively large size (100cm2) that has a solar-to-hydrogen conversion efficiency of 7% and which utilises the inexpensive semiconductor oxide materials enhanced by the nanotechnology.

“As well as this overall technological goal, there are a number of other aspects that we have pursued during the project,” continues Sivula. “One of these is was demonstrate composite nanotechnological techniques; this involves what we call the host-scaffold guest-absorber approach, which basically means using two materials in combination to overcome the limitations of one of the materials by using the other ‘host’ material to facilitate charge transport.

“We have also been investigating the advantages that metal nanostructures can bring. These structures can enhance light absorption in a semiconductor that is in high proximity to them, through what is known as plasmonic resonance. This is something that has shown a lot of promise for photovoltaic energy conversion, but until now has not been investigated in terms of photoelectrochemical energy conversion.”

Another aspect of research covered by the NANOPEC projects has been in the development of new materials, specifically oxynitride semiconductor materials. It is hoped that these new materials will have a lower electronic band gap, meaning that they would absorb more sunlight and therefore have higher potential conversion efficiency.

The NANPEC project has yielded a significant amount of success since its inception, with over 50 papers published over the three years it has been running. Sivula expands on some of the achievements of which he is particularly proud.

“One of the materials we have focused on for many aspects of the project is iron oxide. This is a widely available material; iron is the 6th most common atom in the earth’s crust, and easily oxidises in air and water to give iron oxide. Its red colour tells us that it absorbs sunlight, and is therefore useful to us.

“However, iron oxide does not work well as a semiconductor unless you use nanotechnology to control its morphology and electrical characteristics. We have been able to demonstrate a greatly improved performance of this material using our techniques.”

An even more successful demonstration, the results of which were published in the journal Nature Materials, was that of very high photocurrents in copper oxide. Copper oxide is also very good at absorbing solar energy, but its drawback lies in the fact that it is unstable when in contact with water. Using a technique called atomic layer deposition, a team from NANOPEC deposited a layer of a more stable oxide only a few nanometres thick on top of the copper oxide, affording it greatly enhanced stability. Using this engineered material to create stabilised copper oxide electrodes, they were able to observe record-breaking photocurrents.

Interest in the field has increased dramatically as the project has gained momentum. “At the EMRS (European Materials Research Society) conference last spring in Nice,” tells Sivula, “we held our own symposium; the first symposium of its kind. The room was absolutely packed out – there weren’t even enough chairs to go around! I believe that this level of interest is a great sign, and we hope that our work will really help to further the development of these types of electrodes and this type of solar conversion.

“It is essential for humanity that we figure out how to convert solar energy inexpensively, and also how to store the energy; after all, the sun does not shine all day. If we can store this energy in the form of a fuel, then we can begin to look at using it for tasks such as powering vehicles. I truly believe that hydrogen is a very promising candidate for this role.”

Looking to the future, Sivula expects that the next step for the people involved in the project will be to continue the development of their technologies towards industrialisation. The Swiss Energy Office has displayed interest in funding another three-year project in which the goal would be to create a demonstrable device that could be used to create a start-up company or could be sold directly to industry.

“It is important that we keep demonstrating what we have achieved and that we keep achieving the project milestones,” Sivula explains. “Once we are able to create a device that is working at around 10% efficiency, and we can create that device at a cost of $100 per m2, then we will have reached a level at which we can produce hydrogen at the same cost as we can produce it now via steam methane reforming.

“This would be a huge achievement, as it would signal the point at which we were able to economically compete. We believe this is a totally realistic goal, due to the fact that we have actively pursued the use of inexpensive materials and techniques.”

Project Webpage: http://nanopec.epfl.ch/