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Unstable Ceria Can Convert Solar Energy Into Hydrogen

Posted on the 04 December 2013 by Dailyfusion @dailyfusion
Unstable Ceria Can Convert Solar Energy Into Hydrogen (Credit: Flickr @ Umair Mohsin

A team of scientists at the Technical University of Denmark (DTU) has successfully increased the reactivity of the ceramic material—ceria—a million fold. One possible application is solar concentrators used to convert solar energy into hydrogen.

Cerium(IV) oxide, also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is used, among other things, in the walls of self-cleaning ovens and catalytic converters. Ceria is known to be a stable ceramic material, but researchers from DTU Energy Conversion have succeeded in increasing its reactivity a million fold. This means that in time it will be possible to produce better fuel cells or so-called solar concentrators that can withstand the heat produced when converting solar energy into fuel.

Ceria comprises numerous small particles or grains which, under normal conditions, are a millionth of a meter—and which only stick together as a result of so-called mass diffusion—i.e the energy between the grains.

The type of ceria used by the researchers in their tests had numerous foreign particles added—in this case the chemical element gadolinium—and was therefore extremely combustion-resistant due to a low mass diffusion value.

“The mass diffusion between all these interfaces keeps the grains together, but under certain conditions the material expands significantly and when we cool it in oxygen, we see a dramatic contraction that causes fissures between the grains,” explains Senior Researcher Vincenzo Esposito from DTU Energy Conversion.

He explains that if ceria is heated to high temperatures, contact between the particles initially contracts and deforms the particles due to mass diffusion and energy reduction between the grain surfaces.

“If you place a red-hot glass rod in cold water, the rod will break because the glass has cracks and invisible flaws through which the stress is dispersed. If, however, you cool down ceria in oxygen, the results are catastrophic—the shock affects each grain interface. All the grain particles disconnect simultaneously—and the object disintegrates into dust”, he explains.

The literature states that this reaction should be avoided as it results in destruction of the material—but no one has ever considered harnessing this reaction for practical purposes—until now. In early September, Vincenzo Esposito and his colleague De Wei Ni showed that they could both alter and harness ceria.

“Disintegration was a common phenomenon but no one paid it any attention. We did. And we can repeat the process with a range of materials under different conditions. We have not only seen the phenomenon and described it. We can also control it,” says DTU Energy Conversion researcher De Wei Ni.

The researchers heated ceria to around 1,000 degrees Celsius, achieving high mass diffusion within the material. They then lowered the temperature very slowly to avoid disintegration. The experiment was a success, paving the way for new ceria applications.

The material possesses the unique ability to absorb and release oxygen—and given that it can also withstand high temperatures—ceria has an important role to play in energy production.

The most obvious application is fuel cells, but the material could also be used in solar concentrators—i.e. huge magnifying glasses that concentrate sunlight into a single hot point on a given material. Water added at the hot point splits into oxygen and hydrogen. The hydrogen is then collected, allowing some of the solar energy to be harvested.

The science is sound, but since the material has to be heated to many hundreds of degrees, it has proved impossible to find a material that could withstand such high temperatures, thus rendering the technology viable. The new, strong, reactive ceria variant can withstand such temperatures and has the potential to make hydrogen harvesting both economically and environmentally viable using solar concentrators.

Vincenzo Esposito, De Wei Ni, Zeming He, Wei Zhang, Aditya Shanker Prasad, Julie A. Glasscock, Christodoulos Chatzichristodoulou, Severine Ramousse, Andreas Kaiser (2013). Enhanced mass diffusion phenomena in highly defective doped ceria Acta Materialia, 61 (16), 6290-6300 DOI: 10.1016/j.actamat.2013.07.012

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