Artificial photosynthesis: Converting Solar Energy to Hydrogen: A New – Stable Photocathode with Potential


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       HZB Institute for Solar Fuels

Many of us are familiar with electrolytic splitting of water from their school days: If you hold two electrodes into an aqueous electrolyte and apply a sufficient voltage, gas bubbles of hydrogen and oxygen are formed. If this voltage is generated by sunlight in a solar cell, then you could store solar energy by generating hydrogen gas. This is because hydrogen is a versatile medium of storing and using “chemical energy”. Research teams all over the world are therefore working hard to develop compact, robust and cost-effective systems that can accomplish this challenge. But it is not that simple, because an efficient hydrogen generation preferably proceeds in an acidic electrolyte corroding very fast solar cells. Electrodes that so far have been used are made of very expensive elements such as platinum or platinum-iridium alloys.

New photocathode with several advantages


Under the “Light2Hydrogen” BMBF Cluster project and an on-going “Solar H2” DFG Priority program, a team from the HZB Institute for Solar Fuels has now developed a novel photoelectrode that solves these problems: It consists of chalcopyrite (a material used in device grade thin film solar cells) that has been coated with a thin, transparent, conductive oxide film of titanium dioxide (TiO2). The special characteristics are: The TiO2 film is polycrystalline and contains a small amount of platinum in the form of nanoparticles. This new composite presents some special talents. Firstly, it produces under sun light illumination a photovoltage of almost 0.5 V and very high photocurrent densities of up to 38 mA/cm2; secondly, it acts as a catalyst to accelerate the formation of hydrogen, and finally, it is chemically protected against corrosion as well. Since TiO2 is transparent, almost all sun light reaches the photoactive chalcopyrite, leading to the observed high photocurrent density and photovoltage comparable with those of a conventional device-grade thin-film solar cell.

HZB recipe and technology
The recipe for this novel and elegant coating was developed by Anahita Azarpira in the course of her doctoral studies in a team headed by Assoc. Prof. Thomas Schedel-Niedrig. She uses a chemical vapor coating technique (sprayed ion-layer gas reaction/Spray-ILGAR) that was developed and patented at the HZB Institute for Heterogeneous Material Systems (EE-IH). In this process, the titanium dioxide and platinum precursors are dissolved in ethanol and converted to a fog using an ultrasonic bath. The produced aerosol is directed over the heated substrate using a stream of nitrogen gas resulting into a polycrystalline thin film grown on the chalcopyrite substrate over time with embedded nanoparticles of platinum.

More than 80% of light converted
Azarpira and her colleagues varied the amount of platinum in the precursor solution in order to optimize the properties of the novel composite photoelectrode device. The properties were optimal with a volumetric proportion of about 5% platinum (H2PtCl6) in the precursor solution.” Artificial Photosynth ext

More than 80% of the incident visible sunlight was photoelectrically converted by this composite system into electric current available for the hydrogen generation”, says Schedel-Niedrig. That means little light is lost and the quantum efficiency is virtually very high. In addition, it has been reported in the very recently published article that the composite shows high long-term stability over 25 hrs and reveals large photoelectrocatalytic activity of about 690 hydrogen molecules produced per second and per active center at the surface under illumination.

Feasibility demonstrated
However, there is still a lot to do. Currently, the majority of the required voltage between the composite photocathode and a platinum counter electrode of around 1.8 V is still coming from a battery. Hence the solar-to-hydrogen efficiency has to be clearly improved. “But anyway, we demonstrate the feasibility of such future-oriented chemical robust photoelectrocatalytic systems that have the potential to convert solar energy to hydrogen, i.e to chemical energy for storage. As a consequence we have successfully developed and tested a demonstrator device for solar hydrogen production with a company in Schwerin under the Light2Hydrogen project, according to Schedel-Niedrig.

Source: Helmholtz-Zentrum Berlin für Materialien und Energie

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Artificial Photo-Synthesis ‘Solar-Fuels’ – One Step Closer?


Artificial Photosynth extCaltech scientists, inspired by a chemical process found in leaves, have developed an electrically conductive film that could help pave the way for devices capable of harnessing sunlight to split water into hydrogen fuel.

When applied to semiconducting materials such as silicon, the film prevents rust buildup and facilitates an important chemical process in the solar-driven production of fuels such as methane or hydrogen.

“We have developed a new type of protective coating that enables a key process in the solar-driven production of fuels to be performed with record efficiency, stability, and effectiveness, and in a system that is intrinsically safe and does not produce explosive mixtures of hydrogen and oxygen,” says Nate Lewis, the George L. Argyros Professor and professor of chemistry at Caltech and a coauthor of a new study, published the week of March 9 in the online issue of the Proceedings of the National Academy of Sciences (PNAS), that describes the film.

The development could help lead to safe, efficient artificial photosynthetic systems—also called solar-fuel generators or “artificial leaves”—that replicate the natural process of photosynthesis that plants use to convert sunlight, water, and carbon dioxide into oxygen and fuel in the form of carbohydrates, or sugars.

The artificial leaf that Lewis’ team is developing in part at Caltech’s Joint Center for Artificial Photosynthesis (JCAP) consists of three main components: two electrodes—a photoanode and a —and a membrane. The photoanode uses sunlight to oxidize water molecules to generate oxygen gas, protons, and electrons, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane, which is typically made of plastic, keeps the two gases separate in order to eliminate any possibility of an explosion, and lets the gas be collected under pressure to safely push it into a pipeline.

George L. Argyros Professor and Professor of Chemistry Nate Lewis and postdoc Ke Sun, who together have helped develop a protective film that is rust-resistant, highly transparent, and highly catalytic. This new thin-film could help pave the …more

Scientists have tried building the electrodes out of common semiconductors such as silicon or gallium arsenide—which absorb light and are also used in solar panels—but a major problem is that these materials develop an oxide layer (that is, rust) when exposed to water.

Lewis and other scientists have experimented with creating protective coatings for the electrodes, but all previous attempts have failed for various reasons. “You want the coating to be many things: chemically compatible with the semiconductor it’s trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels,” says Lewis, who is also JCAP’s scientific director. “Creating a protective layer that displayed any one of these attributes would be a significant leap forward, but what we’ve now discovered is a material that can do all of these things at once.”

The team has shown that its nickel oxide film is compatible with many different kinds of semiconductor materials, including silicon, indium phosphide, and cadmium telluride. When applied to photoanodes, the nickel oxide film far exceeded the performance of other similar films—including one that Lewis’s group created just last year. That film was more complicated—it consisted of two layers versus one and used as its main ingredient titanium dioxide (TiO2, also known as titania), a naturally occurring compound that is also used to make sunscreens, toothpastes, and white paint.

“After watching the photoanodes run at record performance without any noticeable degradation for 24 hours, and then 100 hours, and then 500 hours, I knew we had done what scientists had failed to do before,” says Ke Sun, a postdoc in Lewis’s lab and the first author of the new study.

Lewis’s team developed a technique for creating the nickel oxide film that involves smashing atoms of argon into a pellet of nickel atoms at high speeds, in an oxygen-rich environment. “The nickel fragments that sputter off of the pellet react with the oxygen atoms to produce an oxidized form of nickel that gets deposited onto the semiconductor,” Lewis says.

Crucially, the team’s nickel works well in conjunction with the membrane that separates the photoanode from the photocathode and staggers the production of hydrogen and oxygen gases.

“Without a membrane, the photoanode and photocathode are close enough to each other to conduct electricity, and if you also have bubbles of highly reactive hydrogen and oxygen gases being produced in the same place at the same time, that is a recipe for disaster,” Lewis says. “With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once.”

Lewis cautions that scientists are still a long way off from developing a commercial product that can convert sunlight into fuel. Other components of the system, such as the photocathode, will also need to be perfected.

“Our team is also working on a photocathode,” Lewis says. “What we have to do is combine both of these elements together and show that the entire system works. That will not be easy, but we now have one of the missing key pieces that has eluded the field for the past half-century.”

Explore further: New method stabilizes common semiconductors for solar fuels generation