Solar-powered water splitting is a promising means of generating clean and storable energy, say German university researchers.
In the light of global climate change, there is now an urgent need to develop efficient ways of obtaining and storing power from renewable energy sources.
The photocatalytic splitting of water into hydrogen fuel and oxygen provides a particularly attractive approach in this context, say the researchers based at Germany’s Wurzburg and Ludwig-Maximilians (LMU) Universities.
But there’s a problem. It turns out that efficient implementation of this process, which mimics biological photosynthesis, is technically very challenging, since it involves a combination of processes that can interfere with each other.
Now, LMU physicists led by Dr Jacek Stolarczyk and Professor Jochen Feldmann, in collaboration with chemists at Wuzburg led by Prof Frank Wurthner, have succeeded in demonstrating the complete splitting of water with the help of an all-in-one catalytic system for the first time.
Technical methods for the photocatalytic splitting of water molecules use synthetic components to mimic the complex processes that take place during natural photosynthesis.
In such systems, semiconductor nanoparticles that absorb light quanta (photons) can, in principle, serve as the photo-catalysts.
Absorption of a photon generates a negatively charged particle (an electron) and a positively charged sub-atomic particle species known as a ‘hole’.
The two must be spatially separated so that a water molecule can be reduced to hydrogen by the electron and oxidized by the hole to form oxygen.
“If one only wants to generate hydrogen gas from water, the holes are usually removed rapidly by adding sacrificial chemical reagents,” says Stolarczyk.
“But to achieve complete water splitting, the holes must be retained in the system to drive the slow process of water oxidation.”
The problem lies in enabling the two half-reactions to take place simultaneously on a single particle while ensuring that the oppositely charged species do not recombine.
In addition, many semiconductors can themselves be oxidised and thereby destroyed, by the positively charged holes.
Stolarczyk: “We solved the problem by using nanorods made of the semiconducting material cadmium sulphate, and spatially separated the areas on which the oxidation and reduction reactions occurred on these nanocrystals.
“The researchers decorated the tips of the nanorods with tiny particles of platinum, which act as acceptors for the electrons excited by the light absorption.”
The LMU group had already previously shown that this configuration provides an efficient photocatalyst for the reduction of water to hydrogen.
The oxidation reaction, on the other hand, takes place on the sides of the nanorod. But the challenge is to facilitate the efficient generation of oxygen while minimising damage to the rods.
The study was carried out as part of the interdisciplinary project Solar Technologies Go Hybrid. This was launched in 2012 and its mission of which is to explore innovative concepts for the conversion of solar energy into non-fossil fuels.
Now that a novel catalyst based on semiconductor nanoparticles has been shown to facilitate all the reactions needed for artificial photosynthesis, getting to pre-commercial let alone fully commercial production of hydrogen (and oxygen) appears to be some years off.
SOUTH KOREANS MAKE GREENHOUSE GAS-TO-FUEL CONVERSION BREAKTHROUGH
Meanwhile, in the Far East, a team of scientists are attempting to harness solar in a different way by developing photo catalysts that can convert carbon dioxide into usable energy such as methane or ethane.
The team is led by Professor Su-Il In at the Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science & Technology in South Korea.
As CO2 emissions increase, the Earth’s temperature rises and interest in finding ways of reducing the level of this major greenhouse gas culprit is becoming increasingly urgent.
In addition, the need to develop credible sources of sustainable, large-scale energy is growing. And in the basket of potential future solutions is the push to develop the photo-catalysts deemed necessary to the conversion of CO2 and water (H2O) into hydrocarbon fuels.
According to the Korean team, although many semiconductor materials with large band gaps are often used in photo-catalyst studies, they have so far been limited in their ability to absorb solar energy.
So they have been working on “improving the “structure and surface” of photo-catalysts to enable them to better harness the solar energy required to fuel CO2 and water processing.
Prof In’s research team has developed what they claim to be a high-efficiency photocatalyst that can convert carbon dioxide into methane (CH4) or ethane (C2H6) by placing graphene on reduced titanium dioxide in a stable and efficient way.
The photocatalyst developed by the research team can selectively convert CO2 from a gas to methane or ethane.
Moreover, they have so far achieved conversion rates of 5.2% and 2.7% respectively higher than conventional reduced titanium dioxide photo-catalysts.
The Koreans claim that, for ethane generation, they ir volume, this result shows the world’s highest efficiency under similar experimental conditions.
The catalyst material developed by the research team is expected to be applied to a variety of areas such as high-value-added material production.
In turn, this could be used to help solve global warming problems and energy resource depletion issues by selectively producing higher levels of hydrocarbon materials using sunlight.
Prof In expects that follow-up research will help improve the conversation rates further and lead to commercialisation.
ORIGAMI INSPIRES SOLAR TO WATER INVENTION
Water covers most of the globe, yet many regions still suffer from a lack of clean drinking water.
If scientists could efficiently and sustainably turn seawater into clean water, a looming global water crisis might be averted.
Now, inspired by origami, the Japanese art of paper folding, researchers have devised a solar steam generator that apparently approaches 100% efficiency for the production of clean water.
Solar steam generators produce clean water by converting energy from the sun into heat, which evaporates seawater, leaving salts and other impurities behind.
Then, the steam is collected and condensed into clean water. Existing solar steam generators contain a flat photo-thermal material, which produces heat from absorbed light.
Although these devices are fairly efficient, they still lose energy by heat dissipation from the material into the air.
However, Prof Peng Wang and colleagues at the Kaust Solar Centre, King Abdullah University of Science & Technology in Saudi Arabia wondered if they could improve energy efficiency by designing a three-dimensional photo-thermal material.
They based their structure on the Miura fold of origami, which consists of interlocking parallelograms that form “mountains” and “valleys” within the 3D structure.
They made their solar steam generator by depositing a light-absorbing nanocarbon composite onto a cellulose membrane that was patterned with the Miura fold.
They found that their 3D device could achieve a 50% higher evaporation rate than a flat 2D device.
In addition, the efficiency of the 3D structure approached 100% compared with 71% for the 2D material.
The researchers say that, compared to a flat surface, origami “valleys” capture the sunlight better so that less is lost to reflection. In addition, heat can flow from the valleys toward the cooler “mountains,” evaporating water along the way instead of being lost to the air.
Their work continues. The next stage for the team is to test the device with real water samples and its ability to deal with salt and biological fouling before they work on scaling up the process.
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