One of the significant issues associated with water splitting is the facile reverse reaction; that is, the catalyst for the hydrogen evolution reaction (HER) is also active in undesired water-formation reactions, such as the oxygen reduction reaction (ORR), or the thermal reaction of H₂ and O₂. Therefore, inhibition of these back reactions is of paramount importance for photocatalytic water splitting.

In this contribution, we report the structural and electrochemical properties of a Mo-coated Pt HER catalyst that is highly active, oxygen-insensitive, and stable in acidic media for overall water splitting. In operando X-ray absorption spectroscopy (XAS) at relevant potential shifts was used to characterize the working electrode. To our best knowledge, the Mo-based modifier developed in this study is the only acid-tolerant material to date that can selectively prevent the water-forming back reaction.

—Garcia-Esparza et al.

The researchers suggested that the molybdenum layer likely hinders oxygen gas permeation, impeding contact with the active platinum. Photocatalytic overall water splitting proceeded using MoO_x/Pt/SrTiO₃ with inhibited water formation from H₂ and O₂—the prevailing back reaction on the bare Pt/SrTiO₃ photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membrane-less electrolysis and photocatalysis.

A key part of the development centered 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 experiments demonstrated that their molybdenum coating strategy has applications in electrocatalysis and photocatalysis devices, added Angel Garcia-Esparza, lead author and currently a postdoctoral researcher from the Ecole Normale Supérieure de Lyon.

Garcia-Esparza helped develop the new catalyst as a graduate student at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia under the direction of Kazuhiro Takanabe, an associate professor of chemical science at KAUST. Takanabe’s research group explored the stability, performance and function of many 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

Another major challenge was finding a way to measure the properties of their molybdenum-coated catalyst, because these molybdenum compounds are not stable when exposed to air. Taking the catalyst out of water perturbs the identity of the material, explained Garcia-Esparza. “Therefore, it was necessary to study the electrocatalyst under working conditions—which is difficult.”

The researchers tested a bare platinum catalyst, with and without a molybdenum coating, during water electrolysis at SSRL, using in operando X-ray absorption spectroscopy with a custom-made electrochemical cell.

In addition, the research team explored photocatalysis applications. They built a photocatalytic water-splitting system using either a standard catalyst of platinum on strontium titanium oxide (Pt/SrTiO₃) or the same catalyst coated with molybdenum. Both systems were tested at KAUST with the lights on and off— that is, with and without an energy source driving the water-splitting reaction.

When the light was on, the standard Pt/SrTiO₃ catalyst increased hydrogen production for only six hours because the system lost efficiency due to the back reaction. When the lights were then turned off, the amount of hydrogen decreased with time—verifying that significant amounts of the gases were recombining to form water.

Graph of the photocatalytic water splitting performance of a 0.3 wt% Pt/SrTiO3 catalyst with and without Mo coating under UV-light irradiation. The Mo-coated catalyst generated increasing amounts of hydrogen gas for 24 hours with the light on, and inhibited water reformation when the light was off. Whereas, the uncoated catalyst increased hydrogen production for only six hours with the light on and the level decreased when the light was off due to water formation. Oxygen production followed a similar pattern but at half the amount of hydrogen, since water has two hydrogen atoms for each oxygen atom. (Angel Garcia-Esparza/KAUST). Click to enlarge.

In contrast, the molybdenum-coated catalyst continuously split water to generate increasing amounts of hydrogen gas for 24 hours, producing about twice as much hydrogen gas as the standard catalyst in one day. In addition, the amount of hydrogen remained stable in the dark, confirming that the coating inhibited water formation.

The results are promising, but more work still needs to be done 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.

—Dimosthenis Sokaras

Resources

• Angel T. Garcia-Esparza, Tatsuya Shinagawa, Samy Ould-Chikh, Muhammad Qureshi, Xuyuan Peng, Nini Wei, Dalaver H. Anjum, Alain Clo, Tsu-Chien Weng, Dennis Nordlund, Dimosthenis Sokaras, Jun Kubota, Kazunari Domen, and Kazuhiro Takanabe (2017) “An Oxygen-Insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-Based Layer for Overall Water Splitting” Angewandte Chemie doi: 10.1002/anie.201701861