The best of two worlds: Solar hydrogen production breakthrough

Jul 29, 2013
When light hits the system, an electrical potential builds up. The metal oxide layer acts as a photo anode and is the site of oxygen formation. It is connected to the solar cell by way of a conducting bridge made of graphite (black). Since only the metal oxide layer is in contact with the electrolyte, the silicon solar cell remains safe from corrosion. A platinum spiral serves as the cathode where hydrogen is formed. Credit: Image: TU Delft

Using a simple solar cell and a photo anode made of a metal oxide, HZB and TU Delft scientists have successfully stored nearly five percent of solar energy chemically in the form of hydrogen. This is a major feat as the design of the solar cell is much simpler than that of the high-efficiency triple-junction cells based on amorphous silicon or expensive III-V semiconductors that are traditionally used for this purpose.

The photo anode, which is made from the metal oxide bismuth vanadate (BiVO4) to which a small amount of tungsten atoms was added, was sprayed onto a piece of conducting glass and coated with an inexpensive cobalt phosphate catalyst. "Basically, we combined the best of both worlds," explains Prof. Dr. Roel van de Krol, head of the HZB Institute for Solar Fuels: "We start with a chemically stable, low cost metal oxide, add a really good but simple silicon-based thin film solar cell, and – voilà – we've just created a cost-effective, highly stable, and highly efficient solar fuel device."

Thus the experts were able to develop a rather elegant and simple system for using sunlight to split water into hydrogen and oxygen. This process, called , allows to be stored in the form of hydrogen. The hydrogen can then be used as a fuel either directly or in the form of methane, or it can generate electricity in a fuel cell. One rough estimate shows the potential inherent in this technology: At a solar performance in Germany of roughly 600 Watts per square meter, 100 square meters of this type of system is theoretically capable of storing 3 of energy in the form of hydrogen in just one single hour of sunshine. This energy could then be available at night or on cloudy days.

Metal oxide as photo anode prevents corrosion of the solar cell

Van de Krol and his team essentially started with a relatively simple silicon-based thin film cell to which a metal oxide layer was added. This layer is the only part of the cell that is in contact with the water, and acts as a photo anode for oxygen formation. At the same time, it helps to prevent corrosion of the sensitive silicon cell. The researchers systematically examined and optimized processes such as light absorption, separation of charges, and splitting of water molecules. Theoretically, a solar-to-chemical efficiency of up to nine percent is possible when you use a photo anode made from bismuth vanadate, says van de Krol. Already, they were able to solve one problem: Using an inexpensive cobalt phosphate catalyst, they managed to substantially accelerate the process of oxygen formation at the photo .

A new record: More than 80 percent of the incident photons contribute to the current!

The biggest challenge, however, was the efficient separation of electrical charges within the bismuth vanadate film. Metal oxides may be stable and cheap, but the charge carriers have a tendency to quickly recombine. This means they are no longer available for the water splitting reaction. Now, Van de Krol and his team have figured out that it helps to add wolfram atoms to the bismuth vanadate film. "What's important is that we distribute these wolfram atoms in a very specific way so that they can set up an internal electric field, which helps to prevent recombination," explains van de Krol. For this to work, the scientists took a bismuth vanadium wolfram solution and sprayed it onto a heated glass substrate. This caused the solution to evaporate. By repeatedly spraying different wolfram concentrations onto the glass, a highly efficient photo-active film some 300 nanometers thick was created. "We don't really understand quite yet why bismuth vanadate works so much better than other metal oxides. We found that more than 80 percent of the incident photons contribute to the current, an unexpectedly high value that sets a new record for metal oxides" says van de Krol. The next challenge is scaling these kinds of systems to several square meters so they can yield relevant amounts of hydrogen.

Explore further: Building the ideal rest stop for protons

More information: The research is published today in Nature Communications at DOI: 10.1038/ncomms3195

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User comments : 12

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MR166
2.4 / 5 (10) Jul 29, 2013
This appears to be a useful and cost effective way to produce useable fuel. Let's hope that it is durable and can be produced in quantity.
antialias_physorg
3.7 / 5 (3) Jul 29, 2013
Have to replace the Platinum coil with another material - otherwise this will get expensive in mass production (or just go with another energy storage solution like batteries).

Still: Sounds rather promising. And I'm sure this can and will be further tweaked.
hemitite
4 / 5 (4) Jul 29, 2013
Sounds good, although I wish they had stuck to using the elemental name tungsten rather than its seldom used moniker wolfram that most readers are not likely to be unfamiliar with.
hemitite
1 / 5 (1) Jul 29, 2013
Oh cripe, it must be Monday! My last post should have read, "...that most readers not likely to be familiar with."
PeterParker
2.3 / 5 (3) Jul 29, 2013
Almost as good as the photoelectric to chemical conversion efficiency of photosynthetic plants.

antialias_physorg
3.7 / 5 (6) Jul 29, 2013
Almost as good as the photoelectric to chemical conversion efficiency of photosynthetic plants.

Slightly better, as plants have only about 2.5-3% conversion efficiency.

This research is economically nonsensical.

It's a one stop shop solution. What's nonsesical about that? Distributed systems are hard to set up (or move if you're need a mobile system)
A modular system like this has the advantage that it scales directly with the amount of units you set up. For a distributed producing/storage system you may not be able to scale as easily as they may come in different 'chunk' sizes.
although I wish they had stuck to using the elemental name tungsten rather than its seldom used moniker wolfram

Wolfram is the name more commonly used in the Netherlands.
wwqq
3.7 / 5 (6) Jul 29, 2013
Almost as good as the photoelectric to chemical conversion efficiency of photosynthetic plants.


That's because plants are rubbish at capturing sunlight; with many being less than 1% efficient.

Plants must shade competitors, reducing efficiency; they must produce offspring and they must contain the machinery for doing so. They must capture CO2 from the low concentrations that exist in air. They must capture nutrients and water in an optimal way. In short, they're not designed for efficiency; trade-offs that increase efficiency reduce fitness.
Requiem
1.7 / 5 (6) Jul 30, 2013
Almost as good as the photoelectric to chemical conversion efficiency of photosynthetic plants.


That's because plants are rubbish at capturing sunlight; with many being less than 1% efficient.

Plants must shade competitors, reducing efficiency; they must produce offspring and they must contain the machinery for doing so. They must capture CO2 from the low concentrations that exist in air. They must capture nutrients and water in an optimal way. In short, they're not designed for efficiency; trade-offs that increase efficiency reduce fitness.


Please post more often.
antialias_physorg
3.7 / 5 (3) Jul 30, 2013
They must capture nutrients and water in an optimal way.

Agree with what you say - would like to qualify this part, though. Plants do not need to be optimal. They are products of evolution - which means: they must be just good enough to survive (i.e. be just a tad bit better than any kind of competition). Being vastly better is not selected for (this goes for all traits - be it speed or intelligence or photosynthesis efficiency).

Additionally this "tad bit better" only goes for the whole package. Survival for plants is not all about who is the most efficient. Lots of other factors come into play (e.g. many trees use aggressive chemicals to keep competitors in the vicinity down. Others grow quickly in height before starting to put out larger leaves - which mean they will be out of the shade of other plants and will, in turn, shade/kill/stunt them).

Strategy is as important (or more so) than photosynthetic efficiency in the plant world.
Tennesse
1 / 5 (2) Jul 30, 2013
The hydrogen technology has no market. Nobody produces the hydrogen with solid state solar cells, despite they already have 20% efficiency and they're easy to install and maintain - so why someone should produce the hydrogen with solar cells with ten-times lower efficiency filled with electrolyte and connected with water and gas pipes prone to freezing and green weed growing?

It would be an economical suicide.
manifespo
1 / 5 (1) Aug 01, 2013
The researchers might try to use an external electrostatic field to help separate the excitons into holes and electrons.
manifespo
1 / 5 (1) Aug 01, 2013
Plants keep us alive. i would hardly say they are rubbish, since their photosynthetic oxygen and carbon sequestration stabilize our planet's biogeochemical cycles! nature has had 3.7 billion years of research & development to produce trees and plants and other photosynthetic organisms like algae- biomimicry is one way out of our self-made eco-mess!