Using rust and water to store solar energy as hydrogen

November 11, 2012, Ecole Polytechnique Federale de Lausanne

How can solar energy be stored so that it can be available any time, day or night, when the sun shining or not? EPFL scientists are developing a technology that can transform light energy into a clean fuel that has a neutral carbon footprint: hydrogen. The basic ingredients of the recipe are water and metal oxides, such as iron oxide, better known as rust.

Kevin Sivula and his colleagues purposefully limited themselves to inexpensive materials and easily scalable production processes in order to enable an economically viable method for production. The device, still in the experimental stages, is described in an article published in the journal .

The idea of converting into hydrogen is not a new one; researchers have been working on it for more than four decades. During the 1990s, EPFL joined the fray, with the research of Michaël Grätzel. With a colleague form University of Geneva, he invented the photoelectrochemical (PEC) tandem solar cell, a technique for directly from water. Their prototypes shared the same basic principle: a dye-sensitized solar cell – also invented by Michael Grätzel – combined with an oxide-based semiconductor.

The device is completely self-contained. The electrons produced are used to break up and reform the pieces into oxygen and hydrogen. In the same liquid, two distinct layers in the device have the job of generating electrons when stimulated by light; an oxide semiconductor, which performs the oxygen evolution reaction, and a dye-sensitized cell, which liberates the hydrogen.

The most expensive part? The glass plate

The team's latest prototype focused on resolving the main outstanding problem with PEC technology: its cost. "A U.S. team managed to attain an impressive efficiency of 12.4%," says Sivula. "The system is very interesting from a theoretical perspective, but with their method it would cost 10,000 dollars to produce a 10 square centimeter surface."

So the scientists set themselves a limitation from the start – to use only affordable materials and techniques. It wasn't an easy task, but they managed. "The most expensive material in our device is the ," explains Sivula. The efficiency is still low – between 1.4% and 3.6%, depending on the prototype used. But the technology has great potential. "With our less expensive concept based on , we hope to be able to attain efficiencies of 10% in a few years, for less than $80 per square meter. At that price, we'll be competitive with traditional methods of ."

The semiconductor, which performs the oxygen evolution reaction, is just iron oxide. "It's a stable and abundant material. There's no way it will rust any further! But it's one of the worst semiconductors available," Sivula admits.

Silicon-enhanced nano-rust

That's why the iron oxide used by the team is a bit more developed than what you'd find on an old nail. Nanostructured, enhanced with silicon oxide, covered with a nanometer-thin layer of aluminum oxide and cobalt oxide – these treatments optimize the electrochemical properties of the material, but are nonetheless simple to apply. "We needed to develop easy preparation methods, like ones in which you could just dip or paint the material."

The second part of the device is composed of a dye and titanium dioxide – the basic ingredients of a dye-sensitized solar cell. This second layer lets the electrons transferred by the iron oxide gain enough energy to extract hydrogen from water.

An outstanding potential – up to 16%

The results presented in the Nature Photonics paper represent a breakthrough in performance that has been enabled by recent advances in the study of both the iron oxide and dye-sensitized titanium dioxide, and both of these technologies are rapidly advancing. Sivula predicts that the tandem cell technology will eventually be able to attain an efficiency of 16% with iron oxide, while still remaining low cost, which is, after all, the attractiveness of the approach. By making it possible to store solar energy inexpensively, the system developed at EPFL could considerably increase the potential of solar energy to serve as a viable renewable energy source for the future.

Explore further: Harnessing the energy of the sun: New technique improves artificial photosynthesis

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2 / 5 (6) Nov 11, 2012
What we need is to mobilize the garage scientists. We need people to experiment, SAFELY, with this type of thing and interact with each other though the internet. There should be easy access to professionals when need be to answer tough questions and evaluate prototypes.

Too bad our govt./industry wants to help themselves and not everyone.
2.7 / 5 (3) Nov 11, 2012
I don't see how this is an 'energy storage' system. It's a 'hydrogen generating system.'

Storage, if hydrogen is to be a storage medium, requires another step: pressurization.

Adding that step causes the economic feasibility of hydrogen storage for solar energy to collapse. No matter how cheap the hydrogen-generating materials, pressurization entails significant energy losses.

Since this isn't a storage solution, but only a hydrogen-generating solution, it isn't clear to me why this approach makes better economic sense than simply sticking electrodes in water and cranking out hydrogen from grid current.
1.5 / 5 (2) Nov 11, 2012
Of course, the system which will separate the solar electricity generation and hydrogen electrolysis can be always optimized separately, so they will provide a better yield. Not to say about technical difficulties connected with installation and maintenance of solar cells full of water solution, separation of bubbles, algae and bacteria from it, etc...
1 / 5 (2) Nov 11, 2012
Even if hydrogen were free, it would still be too expensive to use.

Unfortunately oil is cheaper now than it was mid 1975...
1 / 5 (1) Nov 12, 2012
Unfortunately oil is cheaper now than it was mid 1975...

How is "oil cheaper now than it was" shortly after an embargo in any way "unfortunate"?
1 / 5 (2) Nov 12, 2012
ricarguy questioned oddly
How is "oil cheaper now than it was" shortly after an embargo in any way "unfortunate"?
Be careful here I didnt say anything about an embargo & had no reason to as the price didnt drop significantly after mid 1970's.

Clearly if oil were more expensive now then it would significantly favour development of a number of renewable energy sources which are only recently starting to approach overall system costings of fossil fuels in terms of trends etc.

Many of the earlier speculative methods were never developed further (& given the chance for diverse exploration in terms of manufacturing permutations) in the late 1970's & 1980's due to relaxation of the availability of oil & the also unfortunate fact some older oil wells have increasing levels due to strata seepage etc.

Say the price per barrel was 20 (pre-embargo) in 1975, then by 2015, in line with general inflation rates, it should be ~USD 320 per barrel (plus or minus $100).

Strange times indeed !
1 / 5 (2) Nov 12, 2012
lengould100 correctly observed
..then simply attach the hydrogen atoms to some carbon chains....
I wouldnt bother with cheaper methods due to consequential infrastructure changes (not costed) & long term safety issues, H2 is potent greenhouse gas.

Far more efficient, natural & effective to do what lengould100 & nature have done for millenia.

Now with bio engineering and synthetic biology and the already proven methods for using yeasts to produce diesel fuel by fermentation and other methods we might even be able to reduce reliance on coal - which has consistently added radiation to our biosphere of not-insignificant levels, some recent suggestions in USA is an increase in thyroid cancers in last 30 years. Although most of these are curable, other cancers and esp with the young are getting more prevalent and not as easy to cure (yet).

Radiation from fossil fuels & nuclear is moving our biological changes & mutations faster than ever & more unpredictable.

Strange times indeed !

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