Solar fuels: materials breakthrough could open new chapter

An unprecedented number of solar fuels have been identified thanks to the combined efforts of researchers from Caltech and Berkeley Lab. Could the dream of creating clean fuel from just water, sunlight and CO2 soon become a commercial option?

A potential holy grail of clean energy, solar fuel has been a focus for researchers around the world for the last 40 years. Advances are now being made in creating various fuels from a combination of solar energy, water and CO2.

Solar fuels are chemical fuels produced using the sun. As Imperial College London’s Solar Fuels Network explains, we can “use sunlight to drive chemical reactions that make fuels, storing solar energy in the form of chemical bonds. Most simply, we can use sunlight to split water to make hydrogen – a clean, renewable fuel. We can also use sunlight to turn CO2 (a greenhouse gas) into useful fuels”.

Unlike photovoltaic panels which produce electricity, solar fuels provide a clean alternative to coal, oil and other fossil fuels. “We have a big need for fuels because they are the best way to store and transport energy, which is why gasoline-powered vehicles have a longer driving range than battery-powered vehicles,” says lead researcher for the Joint Center for Artificial Photosynthesis (JCAP) John Gregoire. “Solar fuels technology will provide us clean fuels just as solar cells provide clean electricity.”

It is this ability which makes solar fuels so appealing to scientists and researchers, in particular those at JCAP. Set up in 2010, JCAP consists of researchers from California Institute of Technology (Caltech) and Lawrence Berkeley National Laboratory (Berkeley Lab) working together to find an effective solar fuel capable of rivalling petrol, amongst others.

What are solar fuels?

Solar fuels rely on the principle of artificial photosynthesis. The Solar Fuels Institute (SOFI) explains that, “Natural photosynthesis begins with the capture and absorption of sunlight. That light energy is then transported and used to split charges. The charges are further transferred to catalytic sites to perform the fuel forming reactions.”

This is the starting point of solar fuels, sunlight is captured using artificial light-harvesting systems and it is this energy which powers the reactions that create solar fuels. All reactions that result in a fuel rely on electrons, provided by water. “Electrons can be derived from the conversion of water molecules into their constituent oxygen gas and positive hydrogen ions (oxidation). This process, however, is very difficult, and requires special substances— catalysts— to facilitate that reaction,” SOFI explains.

"JCAP has tested 174 vanadates to establish whether they are effective catalysts for artificial photosynthesis."

It is these catalysts, called photoanodes, which the team at JCAP has been working to identify and develop. Sunlight provides the energy to power the redox chemistry, the process that derives electrons from the conversion of water into oxygen and hydrogen. By splitting the water, it is therefore possible to create hydrogen, a popular alternative to current gas sources and according to Gregoire, “the fuel that has been most extensively demonstrated so far”.

Hydrogen is just one of many sought-after potential solar fuels, and important inroads have been made into the solar-powered reduction of CO2 to create fuels. CO2 can be collected using carbon capture and used as part of the photoanodes. This allows researchers to change CO2 into CO, as part of a chemical reaction powered by sunlight. CO is an important component of synthesis gas (syngas), which in turn can be made into fuels, such as synthetic petrol and methane.

JCAP has tested 174 vanadates to establish whether they are effective catalysts for artificial photosynthesis. Of these, twelve have proved successful, enabling the creation of a variety of solar fuels.

The high through-put method

By developing a standardised experimental protocol for the measurement of the activity of catalysts and other materials used in water splitting and CO2 reduction, JCAP has been able to almost double the number of known materials with the potential for use as catalysts. This is largely due to the implementation of its high through-put method. “We demonstrated that we can accelerate discovery of solar fuel materials through integration of theory and experiment and believe that our approach will also enable discovery of materials for lots of other technologies,” says Gregoire. This combination of theory and experiment has not been used before.

Berkley Lab is able to predict which new materials will work much faster than ever before. Following this, JCAP is able to test these materials using their combinatorial sputter deposition system to conduct high through-put material synthesis. This technique combines multiple elements through a process akin to atomic spray painting, and creates thin sheets of new materials to be tested using optical spectroscopy. This allows the team to categorise which materials are promising due to their light absorption properties; just as in nature some plants are capable of more efficient photosynthesis due to quick light absorption.

When JCAP has identified the desirable materials, it can then investigate their ability to convert that solar energy into the chemical reactions that generate fuel. JCAP has invented an instrument to do just that, called the high throughput photoelectrochemical reactor. It is this that sets the research apart, as it is able to conduct these experiments between 100 and 1,000 times faster than standard methods.

How far away are we from a prototype?

JCAP’s research allows the organisation to develop a vast materials database recording solar fuels, including the twelve of 174 vanadates proven successful, along with the 16 already known. Most of these have been photoanodes capable of creating hydrogen, bringing the method the closest to commercial realisation. “We have demonstrated making hydrogen at high-efficiency so from a technology readiness point of view, that fuel is showing promise,” says Gregoire.

The success of solar-produced hydrogen is allowing JCAP to shift its focus. “In the past five years, JCAP has made steady progress towards realising solar hydrogen generation systems that are both efficient and robust and is now turning its focus towards carbon dioxide reduction to produce energy-dense fuels,” JCAP says.

"The benefits of using liquid hydrocarbon fuels show more promise for displacing fossil fuels, which is why we have the strong research focus on generating those fuels using sunlight."

“The benefits of using liquid hydrocarbon fuels show more promise for displacing fossil fuels, which is why we have the strong research focus on generating those fuels using sunlight,” Gregoire explains of the research focus.

JCAPs work brings solar fuels much closer to commercial prototypes. “For solar fuels generators, several groups around the world have made prototype devices, and now a primary focus is finding the right combinations of materials for making devices that are more efficient and less expensive,” Gregoire adds. “Our discovery of a bunch of new candidate materials gives us lots of opportunities for finding the best combinations of materials, but we still have lots of fundamental and applied research ahead of us for making a commercially viable solar fuels technology.”