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MIT team discovers new family of materials with best performance yet for oxygen evolution reaction; implications for fuel cells and Li-air batteries
19 September 2013
|A diagram of the molecular structure of double perovskite shows how atoms of barium (green) and a lanthanide (purple) are arranged within a crystalline structure of cobalt (pink) and oxygen (red). Grimaud et al. Click to enlarge.|
MIT researchers have found a new family of highly active catalyst materials that provides the best performance yet in the oxygen evolution reaction (OER) in electrochemical water-splitting—a key requirement for energy storage and delivery systems such as advanced fuel cells and lithium-air batteries.
The materials, double perovskites (Ln0.5Ba0.5)CoO3−δ (Ln=Pr, Sm, Gd and Ho), are a variant of a mineral that exists in abundance in the Earth’s crust. Their remarkable ability to promote oxygen evolution in a water-splitting reaction is detailed in a paper appearing in the journal Nature Communications. The work was conducted by Dr. Yang Shao-Horn, the Gail E. Kendall Professor of Mechanical Engineering and Materials Science and Engineering; postdoc Alexis Grimaud; and six others.
The performance of this family of materials, Shao-Horn says, is a step forward from the previous record-holder for a catalyst that promotes electrochemical water-splitting—a material that Shao-Horn and her team reported in a paper in Science two years ago. (Earlier post.) In addition, while the earlier material quickly changes structure during water-splitting, the new material is stable.
The discovery of new cost-effective and highly active catalysts for electrochemical energy conversion and storage is of prime importance to address climate change challenges and develop storage options for renewable energy production. Among the electrochemical processes, the oxygen evolution reaction (OER) on water oxidation is efficiency-limiting for direct solar and electrolytic water splitting (H2O → H2 + ½ O2), rechargeable metal-air batteries (MxO2 → Mx + O2) and regenerative fuel cells. Transition metal oxides such as ABO3 perovskites composed of rare and alkaline earth (A) and 3d transition metal cations (B) are of particular interest as they have intrinsic activities comparable to the gold standards of OER catalysts such as IrO2 and RuO2.
Here we report that the intrinsic OER activities of (Ln0.5Ba0.5)CoO3−δ are among the highest reported to date, with the most active (Pr0.5Ba0.5)CoO3−δ in the series exhibiting activities greater than BSCF, the most active cubic perovskite. Unlike BSCF, these double perovskites are stable under OER conditions based on measurements from cyclic voltammetry (CV), galvanostatic testing and transmission electron microscopy (TEM) imaging. The physical origin of the high activity and stability of these Co-based double perovskites is compared with Co-based pseudocubic perovskites and discussed in term of eg filling of Co ions and the O p-band centre relative to the Fermi level as predicted from density functional theory (DFT).—Grimaud et al.
Electrochemical water-splitting is well understood in principle, but to make it economically viable researchers must find catalysts that are inexpensive, easily manufactured and efficient enough to carry out the conversion without losing too much of the original power. The new finding could be a significant step in that direction, the MIT researchers say.
The specific compounds used in this research were made by combining a lanthanide (praseodymium, samarium, gadolinium or holmium) with barium, cobalt and oxygen. These compounds form a crystal structure with one distinct site for barium and another for the lanthanide. “There’s lots of flexibility in the chemistry and structure,” Shao-Horn says, allowing for a wide variety of potential materials.
While Shao-Horn and her team had previously reported the effect of transition-metal ions such as cobalt or iron on a water-splitting reaction, this work demonstrates how changing the specific lanthanide element has a strong effect on how rapidly oxygen is produced from the catalyst, Shao-Horn says.
|Video shows the strong activity of the new catalyst material (dark circle at center) in promoting the oxygen evolution reaction when submerged in water, as revealed by the bubbles of oxygen forming on its surface. Source: MIT.|
In their tests so far, the catalyst using praseodymium had the greatest activity level of any material tested to date, the researchers report. And, unlike most of the other materials tested in this reaction, tests indicate that this one is stable under repeated use.
In addition, because this represents a new family of compounds for water-splitting, the MIT researchers predict that further research could lead to more-active catalysts.
The pseudocubic perovskites and double perovskites...not only have the highest OER activities in alkaline solution but also exhibit the highest activities for surface oxygen exchange kinetics upon oxygen reduction at elevated temperatures, which highlights the importance of the oxide electronic structure on oxygen electrocatalysis. Future spectroscopic experiments of these oxides are needed to verify the computed O p-band centre trend in this study, and seek activity and stability descriptors that can be measured experimentally. Our study highlights the importance of controlling a transition metal oxide having the O p-band close to the Fermi level as a promising strategy to create highly active oxide catalysts for OER to enable the development of efficient, rechargeable metal-air batteries, regenerative fuel cells and other rechargeable air-based energy storage devices.—Grimaud et al.
“We figured out what physical parameters could control the activity and stability” of the compounds, Grimaud says, providing guidance for future research. These compounds could find use in fuel cells, advanced rechargeable metal-air batteries, and direct-solar splitting of water, they say.
The first perovskite was discovered in 1839, Shao-Horn says, but her group “combed back through the studies” and found that surprisingly little is known about how to tune these oxides for water-splitting. Their work “highlights a new path to potentially connect basic oxide physics with the activity and stability of these perovskites,” she says.
“We know little about the surfaces of these oxides and how they may change under water-splitting conditions,” Grimaud adds, “and what are the active sites.” Future work will be needed to connect oxide bulk properties with oxide surface chemistry and catalytic activities under operating conditions.
Jean-Marie Tarascon, a professor at the University of Picardie in France and director of its laboratory on reactivity and chemistry of solids (who was not involved in this work), says, “This work is significant as it offers an alternative to costly noble-metal catalysts” for the OER. That’s important, he says, because “there is a huge demand for an OER catalyst for direct solar and electrolytic water-splitting. Personally, I think that the true impact of this work is on the fundamental level. The fact that these double perovskites are surface-stable is a great advance.”
The team also included MIT graduate students Kevin May and Wesley Hong; affiliates Christopher Carlton and Yueh-Lin Lee; postdoc Marcel Risch; and Jigang Zhou of Canadian Light Source Inc. in Saskatoon, Saskatchewan. The work was supported by the US Department of Energy’s Hydrogen Initiative Program and the Office of Naval Research.
Alexis Grimaud, Kevin J. May, Christopher E. Carlton, Yueh-Lin Lee, Marcel Risch, Wesley T. Hong, Jigang Zhou & Yang Shao-Horn (2013) Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nature Communications 4, Article number: 2439 doi: 10.1038/ncomms3439
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