Energy innovation: Tapping the power of the sun

Energy innovation: Tapping the power of the sun

May 2, 2016

May 2, 2016

One of the secrets to solving the world’s energy demands is as plain as daybreak to the Arizona State University research community.

The most abundant source of renewable energy is the sun----but unfortunately, it is only available for about half the day, weather permitting. Currently, capturing, converting and storing that energy to satiate our technological energy demands can’t compete economically with the use of traditional fossil fuels like oil, coal and natural gas.

That’s why researchers at the Biodesign Institute at Arizona State University are pioneering new ways to develop more efficient, cheap and robust methods of utilizing the sun’s power.

If scientists can engineer better systems to use sunlight and water to directly form fuels more effectively, the promise of green energy could become reality.

“In the bigger picture, an important part of our research is looking to use solar energy to meet global human energy demands,” said Gary F. Moore, an assistant professor in the School of Molecular Sciences and researcher in the Biodesign Institute’s Center for Applied Structural Discovery. “While there are a number of existing ways of doing that, photovoltaics represent one strategy for converting sunlight to electricity…but just making electricity is not enough.”

When electricity is produced using solar panels, it starts its journey as a flow of electrons zipping along wires, eventually making their way to a light bulb or household appliance. In the light bulb the electrical power is spent right away, and as soon as the sun sets, or a storm develops, those panels stop producing electricity.

“The sun, like most renewables, is not always available,” said Moore. “In other words, the wind ceases to blow, the sun sets, and so when these sources are not available, you can’t produce that renewable electricity. So the things we’ve been working on provide a mechanism for storing solar energy when it’s not available, say at night. If we’re successful, you could use the sun all night long.”

One way to make solar fuels is to convert sunlight into hydrogen, but rather than initially storing electrons on metals as they are in batteries, they could be directly stored in fuels as chemical bonds. Fuels have relatively high energy densities compared to batteries because the electrons can be stored much closer in chemical bonds. This, in large part, is why most modern transportation systems depend on their use.

"Societies rely on continuous energy supplies,” said Moore.  “However, current methods of solar energy storage are characterized by relatively low energy densities. For example, the energy density of batteries is over 100 times less than the energy densities of liquid fuels, and hydrogen provides an even greater density.”

By combining solar energy capture, conversion and storage, an energy source can be turned into chemical fuels that can be used on demand.

Moore and his team recently developed a photocathode material, that when hit with solar energy, triggers a chemical reaction on the surface of the photocathode, converting water into hydrogen.

They used a modular approach to create the custom photocathode. During this process, a polymer coating is applied like an ultrathin coat of paint (just six nanometers thick, or 1,000 times smaller than the diameter of a human hair). The polymer acts like a thin protective layer for the semiconductor while also pulling double duty—serving as a sticky glue for assembling the hydrogen producing catalysts in a subsequent step.

This novel assembly method allows for swapping out components (semiconductors, polymers and catalysts to speed up the fuel-forming chemical reactions) as new materials and discoveries emerge. In this way the system can be customized to meet specific consumer and industrial needs.

This winning combination of simplicity and versatility makes it an appealing research area.

“So you would take a semiconductor and now instead of just producing electricity upon illumination, you produce bubbles of hydrogen gas forming at the surface,” said Moore.

Recent results from their research were published in the journals ACS Applied Materials & Interfaces and Industrial & Engineering Chemistry Research. Moore’s team included graduate students in the School of Molecular Sciences, Anna M. Beiler and Diana Khusnutdinova as well as an undergraduate student, Samuel I. Jacob.

Together, they prepared and analyzed the custom materials needed for the project. For them, it’s inspiring to take part in a project that could change our planet’s future energy options.

“I joined the chemistry graduate program so I could work in solar energy,” said Beiler. “But I was a biochemistry undergraduate major, so the path wasn’t immediately clear to me. A lot of chemists go into the semiconductor work. So, the whole field of artificial photosynthesis and using biology as inspiration for solar fuel production is really cool.”

“It involves a seamless integration of many traditional disciplines, not only molecular synthesis, but also work involving solid state materials and the use of new characterization methods for understanding the photochemistry at these hybrid interfaces. It is a very unique opportunity,” said Khusnutdinova, who came from Russia to study at ASU with Moore.

And, in part, they look to nature as inspiration. Plants use photosynthesis, where just adding water and sunlight produces the energy – and the oxygen for life.

“In some sense the chemistry we have been developing is inspired by nature, but it also leverages human ingenuity, engineered materials and imagination,” said Moore.

Their recent findings demonstrate that the polymer coating attachment is not limited to a specific crystal face orientation of a semiconductor. This feature indicates that their technology can potentially be applied across a range of material surfaces. They also show that the selected polymer can be customized to control the rate of fuel production.

But redesigning photosynthesis to power large scale industrial chemistry will require many more steps. Making such systems more durable and scalable will be an ongoing research challenge.

After each experiment, the researchers take advantage of the use of world-class ASU facilities within the LeRoy Eyring Center for Solid State Science and the Magnetic Resonance Research Center to better understand the structure of their constructs and ultimately their functionality.

Like a gem hunter, searching for just the right brilliance and optimal surface structure of a crystal, Moore and his team seek an organization of elements that could provide a shimmering new way to tap the power of the sun.

Although there are many remaining research challenges ahead of them, the future looks bright.

 

Written by: Ally Carr