New efficiency record for solar hydrogen production is 14 percent

September 15, 2015
The tandem cell is covered with a catalyst for hydrogen formation. Credit: M. May

An international team has succeeded in considerably increasing the efficiency for direct solar water splitting with a tandem solar cell whose surfaces have been selectively modified. The new record value is 14 percent and thus tops the previous record of 12.4 percent, broken now for the first time in 17 years. Researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and California Institute of Technology participated in the collaboration. The results are published in Nature Communications.

Solar energy is abundantly available globally, but unfortunately not constantly and not everywhere. One especially interesting solution for storing this energy is . This is what every leaf can do, namely converting sunlight to chemical energy. That can take place with artificial systems based on semiconductors as well. These use the electrical power that sunlight creates in individual semiconductor components to split water into oxygen and hydrogen. Hydrogen possesses very high energy density, can be employed in many ways and could replace fossil fuels. In addition, no carbon dioxide harmful to the climate is released from hydrogen during combustion, instead only water. Until now, manufacturing of at the industrial level has failed due to the costs, however. This is because the efficiency of artificial photosynthesis, i.e. the energy content of the hydrogen compared to that of sunlight, has simply been too low to produce hydrogen from the sun economically.

Record value now exceeded

Scientific facilities worldwide have therefore been researching for many years how to break the existing record for artificial photosynthesis of 12.4 %, which has been held for 17 years by NREL in the USA.

Core component: Tandem Solar Cell

Now a team from TU Ilmenau, Helmholtz-Zentrum Berlin (HZB), the California Institute of Technology as well as the Fraunhofer ISE has considerably exceeded this record value. Lead author Matthias May, active at TU Ilmenau and the HZB Institute for Solar Fuels, processed and surveyed about one hundred samples in his excellent doctoral dissertation to achieve this. The fundamental components are tandem solar cells of what are known as III-V semiconductors. Using a now patented photo-electrochemical process, May could modify certain surfaces of these semiconductor systems in such a way that they functioned better in water splitting.

Stability improved

"We have electronically and chemically passivated in situ the aluminium-indium-phosphide layers in particular and thereby efficiently coupled to the catalyst layer for hydrogen generation. In this way, we were able to control the composition of the surface at sub-nanometre scales", explains May. There was enormous improvement in long-term stability as well. At the beginning, the samples only survived a few seconds before their power output collapsed. Following about a year of optimising, they remain stable for over 40 hours. Further steps toward a long-term stability goal of 1000 hours are already underway.

This small device converts 14 % of the incoming solar energy into chemical energy (in the form of hydrogen). Credit: M. May

Next goals visible

"Forecasts indicate that the generation of hydrogen from sunlight using high-efficiency semiconductors could be economically competitive to fossil sources at efficiency levels of 15 % or more. This corresponds to a hydrogen price of about four US dollars per kilogramme", says Prof. Thomas Hannappel, from the photovoltaics group at TU Ilmenau, who was academic advisor for the work. Prof. Hans-Joachim Lewerenz from the Joint Center for Artificial Photosynthesis at the California Institute of Technology, who worked closely with May, said:"We are nearly there. If we are successful now in reducing the charge carrier losses at the interfaces somewhat more, we might be able to chemically store more than even 17 % of the incident in the form of using this semiconductor system".

Explore further: Nanowire-based design incorporates two semiconductors to enhance absorption of light

More information: May, M. M. et al. Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure. Nat. Commun. 6:8286. DOI: 10.1038/ncomms9286 (2015)

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Eikka
2.3 / 5 (6) Sep 15, 2015
There's a catch if the hydrogen is to be used off-site from the splitters - in vehicles etc.

The water has to be replaced as it splits into gasses, and the replacement water has to be pure of salts and solubles or otherwise any impurities will get concentrated in the cell.

It's basically like boiling a kettle and just adding water. Even clean tap water will leave it covered in a microscopic layer of minerals - water scale - that sticks to the catalyst and builds up, and stops it from working. A kettle can be washed off with caustic chemicals. A PV cell? Probably not. Anything strong enough to dissolve the scale will probably dissolve the catalyst as well.

The requirement for pure water for long term operation completely negates the gains of the system because it takes such a massive amount of energy to purify water. It only makes sense if the hydrogen is burned on-site and the water recycled in a closed loop.

gkam
1.7 / 5 (6) Sep 15, 2015
No, Eikka, in stationary usesthere will be no exhaust it will condense and go back to the cell for re-use. In this case, water is the working fluid.

For autos, it may be that it is re-fillled, but these are stationary sources of water, which can adequately be filtered before use. I think we will capture that pure water in the aurtos and re-use it.
antialias_physorg
3.2 / 5 (9) Sep 15, 2015
The water has to be replaced as it splits into gasses.

You might have heard of these newfangled inventions called 'water pipes'? They're all the rage the world over. You should try them (I know you don't like technology but those are really the bee's knees)

, and the replacement water has to be pure of salts and solubles or otherwise any impurities will get concentrated in the cell

Filters are also not exactly monster tech. This is for large scale installations where a central filtration unit is just a negligible addition to the system.

I know you're arguing against renewables on a "just because" basis - but, man, your arguments are getting beyond ridiculous.
docile
Sep 15, 2015
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docile
Sep 15, 2015
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Eikka
2.5 / 5 (2) Sep 16, 2015
You might have heard of these newfangled inventions called 'water pipes'?


I didn't say -that- was the problem.

Filters are also not exactly monster tech. This is for large scale installations where a central filtration unit is just a negligible addition to the system.


Filtering or conditioning doesn't remove dissolved salts and other minerals. Reverse osmosis or distillation has to be used to remove the solutes, and these require significant amounts of energy.
but, man, your arguments are getting beyond ridiculous.


Of course they seem ridiculous when you deliberately misunderstand them in the first place.

No, Eikka, in stationary usesthere will be no exhaust it will condense and go back to the cell for re-use.


That's what I just said.

For automobiles, you need to split a about 2.5 liters of water to get enough hydrogen to equal one liter of gasoline, so the water consumption will actually be quite enormous.
Eikka
1 / 5 (1) Sep 16, 2015
I think we will capture that pure water in the aurtos and re-use it.


Beause you get 2.5 gallons for every 1 gallon of gasoline equivalent, an ordinary small economy car would produce about a cup per mile.

If you're going to go 200 miles between refueling, you need to collect about 12 gallons of water to return to the station, which then has to ship it back to the solar field to replace the water they've lost. It's a rather complicated system that involves transporting a lot of heavy stuff by road from everywhere around the country, and it -still- requires you to purify that water because it's picked up all sorts of crap along the way from pipes and containers because pure water is actually corrosive and dissolves stuff.

I don't see hydrogen as a viable automobile fuel because it's simply too inconvenient.

For stationary applications where the water can be efficiently recycled, it'll work.
Eikka
3 / 5 (2) Sep 16, 2015
There's another, more economically practical method of producing hydrogen:

https://en.wikipe...trolysis

It involves converting steam instead of liquid water in an electrolytic cell. The heat input directly reduces the amount of electric input needed to split the molecule, which means the energy used in boiling the water into steam is not lost but captured.

And because the water is boiled, any source of water can be used as long as it doesn't contain other harmful volatiles. Seawater, rainwater, toilet water, anything goes.

Wherever there's solar panels, there's also solar heat, so water could be pre-heated by using it to cool down ordinary solar panels - which improves efficiency - then put into mirrored throughs to boil it into steam with sunlight, and then the electricity from the solar panels used to split it into hydrogen.

MR166
not rated yet Sep 17, 2015
"Further steps toward a long-term stability goal of 1000 hours are already underway."

Well they have to do about 200 times better than that for the device to be commercially feasible.

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