Towards catalysts for solar hydrogen production

Thin films of molybdenum and sulfur belong to a class of materials that can be considered for use as photocatalysts. Inexpensive catalysts such as these are needed to produce hydrogen as a fuel using solar energy. However, they are still not very efficient as catalysts. A new instrument at the Helmholtz-Berlin Zentrum’s BESSY II now shows how a light pulse alters the surface properties of the thin film and activates the material as a catalyst.

MoS2 thin films of superposed alternating layers of molybdenum and sulfur atoms form a two-dimensional semiconducting surface. However, even a surprisingly low-intensity blue light pulse is enough to alter the properties of the surface and make it metallic. This has now been demonstrated by a team at BESSY II.

Read more on the HZB website

Image: A new instrument at BESSY II can be used to study molybdenum-sulfide thin films that are of interest as catalysts for solar hydrogen production. A light pulse triggers a phase transition from the semiconducting to the metallic phase and thus enhances the catalytic activity.

Credit: © Martin Künsting /HZB

Enhancing solar energy production

Research investigates ways to convert titanium dioxide into a new photoactive material in the visible light range.

The search for clean and renewable energy sources has intensified in recent years due to the increase in atmospheric concentration of greenhouse gases and the consequent increase in the average temperature of the planet. One such alternative source is the conversion of sunlight into electricity through photovoltaic panels. The efficiency in this conversion depends on the intrinsic properties of the materials used in the manufacturing of the panels, and it increases year by year with the discovery of new and better materials. As such, solar energy is expected to become one of the main sources of electric energy by the middle of this century, according to the International Energy Agency (IEA).

Titanium dioxide (TiO2) is an abundant, nontoxic, biologically inert and chemically stable material, known primarily as a white pigment used in paints, cosmetics and even toothpastes. TiO2 is also often used in sunscreens since it is especially capable of absorbing radiation in the ultraviolet region. However, this same property severely limits the use of TiO2 for solar energy conversion, since the ultraviolet emission comprises only 5 to 8% of the total energy of the solar light.

Can this TiO2 property be extended to the visible light region to increase the conversion of sunlight into electricity? To answer this question, Maria Pilar de Lara-Castells et al. [1] conducted an innovative research in which they discuss how a special treatment can change the optical properties of TiO2.

>Read more on the Brazilian Synchrotron Light Laboratory website

Diamond’s 8000th publication: The future of solar cells

A collaboration between researchers in the UK and China recently led to the publication of the 8000th research article describing cutting edge science carried out at Diamond Light Source. Professor David Lidzey from the University of Sheffield and his collaborator Professor Tao Wang from Wuhan University of Technology published their findings in Nano Energy with implications for the future of solar cells.
Fullerene molecules known as “Bucky balls” have been used as charge acceptors in solar cells for a long time. Researchers used Diamond Light Source to investigate new acceptor molecules that would be cheaper to manufacture. They discovered that depending on the molecule and the way that it was blended with polymers, they were able to see a significant efficiency increase over traditional compositions. The added efficiency came from the fact that the new compositions could absorb light over a broader wavelength range. This means that if used in solar cells, they will be able to use more of the sun’s light than is possible using current materials.
The added efficiency comes from the molecules themselves as well as the way they are blended and cast. Using the GWAXS technique at Diamond, the researchers found that flat acceptor molecules were able to stack very efficiently and that the production method allowed them to self-organise on nanometre length scales allowing aggregates to form that extend the wavelengths that can be absorbed.

>Read more on the Diamond Light Source website

Image: A representation of a “bucky ball” or fullerene molecule, commonly used as charge acceptors in solar panels.

Solar–to-hydrogen conversion

Polymeric carbon nitrides exhibit a catalytic effect in sunlight that can be used for the production of hydrogen from solar energy.

However, the efficiency of these metal-free catalysts is extremely low. A team at the Tianjin University in China, in collaboration with a group at the Helmholtz-Zentrum Berlin, has increased the catalytic efficiency of these polymeric carbon nitrides by a factor eleven through a simple process resulting in a larger surface area. The paper was published in the journal Energy & Environmental Science.

One of the major challenges of the energy transition is to supply energy even when the sun is not shining. Hydrogen production by splitting water with the help of sunlight could offer a solution. Hydrogen is a good energy storage medium and can be used in many ways. However, water does not simply split by itself. Catalysts are needed, for instance Platinum, which is rare and expensive. Research teams the world over are looking for more economical alternatives. Now a team headed by Dr. Tristan Petit from the HZB, together with colleagues led by Prof. Bin Zhang from Tianjin University, Tianjin, China, has made important progress using a well-known class of metal-free photocatalysts.

>Read more on Bessy II at HZB website

Image: PCN nanolayers under sunlight can split water.
Credit: Nannan Meng /Tianjin University

Scientists confirm speculation on the chemistry of a high-performance battery

X-ray experiments at Berkeley Lab reveal what’s at work in an unconventional electrode.

Scientists have discovered a novel chemical state of the element manganese. This chemical state, first proposed about 90 years ago, enables a high-performance, low-cost sodium-ion battery that could quickly and efficiently store and distribute energy produced by solar panels and wind turbines across the electrical grid.

This direct proof of a previously unconfirmed charge state in a manganese-containing battery component could inspire new avenues of exploration for battery innovations.

X-ray experiments at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) were key in the discovery. The study results were published Feb. 28 in the journal Nature Communications.

Scientists at Berkeley Lab and New York University participated in the study, which was led by researchers at Natron Energy, formerly Alveo Energy, a Santa Clara, California-based battery technology company.

The battery that Natron Energy supplied for the study features an unconventional design for an anode, which is one of its two electrodes. Compared with the relatively mature designs of anodes used in lithium-ion batteries, anodes for sodium-ion batteries remain an active focus of R&D.

>Read more on the Advanced Light Source website

Photo: An array of solar panels and windmills.
Credit: PxHere

Fuel from the sun: insight into electrode performance

Soft x-ray studies of hematite electrodes—potentially key components in producing fuel from sunlight—revealed the material’s electronic band positions under realistic operating conditions.

In photosynthesis, plants use sunlight to split water into oxygen and hydrogen. The oxygen is released into the atmosphere, and the hydrogen is used to produce molecules—such as carbohydrates and sugars—that store energy in chemical bonds. Such compounds constitute the original feedstocks for subsequent forms of fuel consumed by society.

Photoelectrochemical (PEC) water splitting is a form of “artificial” photosynthesis that uses semiconductor material, rather than organic plant material, to facilitate water splitting. Electrodes made of semiconductor material are immersed in an electrolyte, with sunlight driving the water-splitting process. The performance of such PEC devices is largely determined at the interface between the photoanode (the electrode at which light gets absorbed) and the electrolyte.

>Read more on the ALS webpage

Photo: Roy Kaltschmidt