Hydrogen is considered to be one of the most promising clean fuels for use in portable generators, houses and cars. Hydrogen can also be a sustainable fuel with no carbon footprint when it is produced from water using renewable energy resources.
However, water-splitting systems need an extremely effective catalyst that will help speed up the chemical reaction that splits water into oxygen and hydrogen, while preventing the gases from recombining back into water. A global research team including scientists at the Department of Energy’s SLAC National Accelerator Laboratory has recently developed a new catalyst with a molybdenum coating that has the potential to prevent this problematic back reaction and functions well in realistic operating conditions.
An essential part of the development focused on understanding how the molybdenum coating worked using experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility. The results were reported by the scientists in the April 13 issue of Angewandte Chemie.
When you split water into hydrogen and oxygen, the gaseous products of the reaction are easily recombined back to water and it’s crucial to avoid this. We discovered that a molybdenum-coated catalyst is capable of selectively producing hydrogen from water while inhibiting the back reactions of water formation.
Angel Garcia-Esparza, lead author and currently a postdoctoral researcher from the Ecole Normale Supérieure de Lyon.
The experiments illustrated that their molybdenum coating strategy has applications in photocatalysis and electrocatalysis devices, added Garcia-Esparza. These devices help drive forward a reaction using light or electricity.
Searching for Stability
As a graduate student at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, Garcia-Esparza helped develop the new catalyst under the direction of Kazuhiro Takanabe, an associate professor of chemical science at KAUST. Takanabe’s research group explored the performance, stability and function of several different elements before selecting molybdenum as the coating for a standard platinum-based catalyst.
Finding a coating that worked well in the acid electrolyte used for water splitting was a major challenge for my collaborators, because many materials quickly degrade in the acidic conditions.
co-author Dimosthenis Sokaras, a staff scientist at SLAC.
Of the coatings they tested, “Molybdenum was the best-performing material in acidic media, where the conditions for hydrogen evolution are favorable and facile,” Garcia-Esparza explained.
Testing the Performance
Another significant challenge was discovering a way to measure the properties of their molybdenum-coated catalyst, as these molybdenum compounds fail to be stable when exposed to air.
Taking the catalyst out of water perturbs the identity of the material. Therefore, it was necessary to study the electrocatalyst under working conditions, which is difficult.
So Garcia-Esparza spent a summer carrying out electrochemistry experiments at SSRL in order to characterize the new catalyst under operational conditions.
The idea was to work together to see how the molybdenum-coated catalyst performed and determine its electronic structure when it was operating. We wanted to understand why the back reaction doesn’t happen.
A bare platinum catalyst was tested without and with a molybdenum coating, during water electrolysis at SSRL, using in operando X-ray absorption spectroscopy with a specifically made electrochemical cell.
At SSRL, we were essentially able to do electrochemistry while analyzing the sample with synchrotron radiation. The experiments performed at SLAC were the final piece of the puzzle to determine the local structure and state of the electrocatalyst under the operational conditions of hydrogen production.
Our findings support that the molybdenum layer acts as a membrane to block the oxygen and hydrogen gases from reaching near the platinum surface, which prevents water formation.
The research team also explored photocatalysis applications. They constructed a photocatalytic water-splitting system using either the same catalyst coated with molybdenum or a standard catalyst of platinum on strontium titanium oxide (Pt/SrTiO3). Both systems were tested at KAUST with the lights on and off — that is, they were tested without and with an energy source driving the water-splitting reaction.
The standard Pt/SrTiO3 catalyst increased hydrogen production for only six hours when the light was on because the system lost efficiency due to the back reaction. The amount of hydrogen decreased with time when the lights were then turned off, thus verifying that significant amounts of the gases were recombining to produce water.
In contrast, the molybdenum-coated catalyst constantly split water in order to produce increasing amounts of hydrogen gas for 24 hours, generating about twice as much hydrogen gas as the standard catalyst in one day. Additionally, the amount of hydrogen continued to be stable in the dark, establishing the fact that the coating inhibited water formation
These results prove to be promising, but more work should still be carried out before the catalyst can be used in a practical device.
I think we’re far from actually talking about a commercial device, but it is certainly a huge improvement to have this new catalyst material that prevents the back reaction. Now we need to find a way to make the coating more stable so it produces hydrogen for even longer.
The research team included scientists from SSRL, King Abdullah University of Science and Technology, Fukuoka University, University of Tokyo, and the Center for High Pressure Science and Technology Advanced Research in Shanghai, China. King Abdullah University of Science and Technology supported the work.