When Strontium Is Away, Iridium Comes Out to Play

The Science

As an alternative to producing gasoline from crude oil, scientists are looking to synthesize fuels from water and sunlight. The challenge of splitting water, besides being an uphill process that requires significant amounts of energy, is that steps in the reaction are kinetically slow. Researchers have developed a new, stable catalyst that splits water molecules through a solar-powered process. The catalyst performs the reaction 100 times faster than previously possible, while maintaining activity under acidic conditions.

The Impact

In addition to performing water splitting rapidly and being remarkably robust, the new catalyst is a major step towards decreasing the amount of rare and costly iridium in solar devices. The reduction in cost should increase the viability of sustainable hydrogen gas generation, which would circumvent current methods of production that rely on petroleum as a feedstock. Hydrogen gas is vital to many renewable energy-related applications, including transportation fuels, a major component of commodity chemical synthesis, and a potential energy source for home heating and vehicle transportation.

Summary

Water splitting, which generates hydrogen and oxygen gas, plays a key role in renewable energy technologies by producing molecules that can be used as fuels or as feedstocks to prepare high-value chemicals. However, the slow kinetics of one step involved in the process of water splitting, known as the oxygen evolution reaction, limits the performance and commercialization of the process. Additionally, proton exchange membrane electrolyzers, which are critical components of devices that carry out water splitting, require acid-stable catalysts. Scientists at Stanford University and SLAC National Accelerator Laboratory developed an iridium oxide/strontium iridium oxide (IrO_x/SrIrO₃) catalyst that is formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO₃. The group demonstrated that the catalyst remains active for 30 hours of continuous cycling in the presence of an acidic electrolyte, indicating remarkable stability. Theoretical calculations using density functional theory suggest highly active IrO_x surface layers that form during strontium leaching are responsible for the impressive activity and robustness. Further, the calculations suggest IrO₃ or anatase-IrO₂ overlayers are particularly active for oxygen evolution reactions and favorable to form from the SrIrO₃ precursor. The IrO_x/SrIrO₃ catalyst outperforms known iridium and ruthenium oxide (IrO_x and RuO_x) systems, the only other oxygen evolution reaction catalysts that have reasonable stability in the presence of acidic electrolytes. The acid-stability and high intrinsic activity of the catalyst show promise for its integration into renewable energy technologies.

Funding

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract 
DE-SC0008685 and through the SUNCAT Center for Interface Science and Catalysis. Partial support was provided by the Center
 on Nanostructuring for Efficient Energy Conversion at Stanford University, an Energy Frontier Research Center funded by the DOE, Office of Science, under award DE-SC0001060. L.C.S. received fellowship support from the DARE (Diversifying Academia, Recruiting Excellence) Doctoral Fellowship supported by the Vice Provost for Graduate Education at Stanford University. Part of this work was performed at the Stanford Nano Shared Facilities. K.N., Y.H., and H.Y.H. acknowledge support from the DOE, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 (thin film synthesis), and from the Global Climate and Energy Project at Stanford University (modifications under ionic tuning). C.F.D. and C.S.K. acknowledge support from the National Science Foundation Graduate Research Fellowship program under grant DGE-114747. C.S.K. also acknowledges support from the Morgridge Family Stanford Graduate Fellowship.

Publication

L.C. Seitz, C.F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H.Y. Hwang, J.K. Nørskov, and T.F. Jaramillo, “A highly active and stable IrO_x/SrIrO₃ catalyst for the oxygen evolution reaction.” Science 353, 1011-1014 (2016). [DOI: 10.1126/science.aaf5050]