ORNL team engineers 1st high-performance, two-way oxide catalyst; outperforms platinum; potential for new electrochemistry systems
A research team led by Oak Ridge National Laboratory (ORNL) has created the first high-performance, two-way oxide catalyst and filed a patent application for the invention. The new bi-directional catalyst can outperform platinum in oxygen reduction and oxygen evolution reactions (ORR and OER). The accomplishment is reported in the Journal of the American Chemical Society.
The discovery may guide the development of new material systems for electrochemistry. Energy storage devices, such as fuel cells and rechargeable batteries, convert chemical energy into electricity through a chemical reaction. Catalysts accelerate this process, making it more efficient. In particular, an oxygen reduction catalyst extracts electrons from oxygen molecules, while an oxygen evolution catalyst drives the reaction in the opposite direction. Catalytic reactions that proceed in both directions are required for charging and discharging of regenerative energy storage devices.
|Bifunctional overpotential (η) to attain 30 μA/cm2 for both OER and ORR reactions show compressed LaNiO3 surpassing Pt and IrO2. Credit: ACS, Petrie et al. Click to enlarge.|
Advancements in energy storage are essential for driving the development of more sophisticated mobile technologies as well as continuing the trend toward a greener economy. At the forefront of this push are high energy density devices, such as regenerative fuel cells and metal-air batteries. In these and related electrochemical systems, both the oxygen reduction and oxygen evolution reactions (ORR and OER, respectively) are crucial toward successful operation. Traditionally, conductive catalysts incorporating noble metals (e.g., Pt and IrO ) have been used to facilitate these reactions near room temperature.
To alleviate costs and poor stabilities during OER in alkaline solutions, significant efforts have focused on transition-metal oxides (TMOs) with multivalent Ni, Fe, Co, and Mn. Similar to alloying in noble metals, the majority of research into increasing oxygen activities of TMOs involves cationic doping, which often promotes either the ORR or OER but not bifunctionality. Here, we explore how another factor, i.e., strain, can influence bifunctionality in TMOs.—Petrie et al.
Oxide materials are workhorses of energy generation and storage. Several oxide materials contain transition metals, which can easily exchange electrons. Other work has shown that strain greatly influences low-temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity. However, the catalytic impact of strain on transition-metal oxide thin films, such as perovskites, is not widely understood.
The ORNL researchers epitaxially strained the conducting perovskite LaNiO3 (lanthanum nickelate) to determine the influence of strain on both the oxygen reduction and oxygen evolution reaction.
|“We found a catalyst that is very good at driving both the opposing oxygen evolution reaction and the oxygen reduction reaction.”|
—senior author Ho Nyung Lee
The researchers made a thin film of the transition metal oxide through heteroepitaxy, which grows one material on a substrate with different lattice spacing. The lattice mismatches introduce strain into the system. The strain changed the film’s electronic structure without altering its chemical composition, creating a catalyst better at driving chemical reactions.
In their study, the researchers showed that compressive strain as small as −1.2% can enhance the bifunctional ORR and OER activities of the lanthanum nickelate above that of the best performing noble metal, platinum.
Though nickelates can exceed the performance of platinum in one catalytic direction, they perform poorly in the other, limiting their bifunctionality. Strain engineering in this study has enhanced their performance in both directions, fulfilling the goal of surpassing well-known catalysts such as platinum in bifunctionality.
Although applied to a perovskite, there is no reason such strain cannot increase the activity of other TMO-based catalysts, such as Mn2O3 and NiFeOx. Previously, the possibilities of tensile strain on LNO- based heterostructures have attracted great interest due to physical properties, such as theoretical hints of cuprate-like superconductivity. Here, our new discovery further expands the importance of strain engineering TMOs into the electro-chemical realm.—Petrie et al.
Engineering strain. Reactions required the formation and breaking of bonds, explained ORNL’s Daniel Lutterman.
That’s very much dependent upon the energy of these orbitals and how well they’re able to overlap with the orbitals of the small molecules that are coming to the surface. By affecting those energies through strain, we’re affecting that bond-making and bond-breaking process.
In general a catalyst lowers the activation barrier for a reaction to occur. If you lower it even further through strain, you’re making a better catalyst. It’s still the same material because it’s a lanthanum nickelate, but because those bonds are elongated, it’s an enhanced lanthanum nickelate.—Daniel Lutterman
Lead author Jonathan Petrie led the epitaxial synthesis of strained oxide materials and catalytic testing, and Tricia Meyer assisted thin film deposition using a technique that employs a high-power excimer laser to vaporize material and deposit it as high-quality thin films under precisely controlled conditions.
John Freeland of the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory, contributed soft X-ray absorption spectroscopy, a technique for understanding orbital structure changes, and related data analysis. Valentino Cooper of ORNL’s Materials Science and Technology Division performed theory calculations.
Both theory and experiment have long shown the importance of a specific orbital in defining catalytic activity on the surfaces of transition metals. Here, using theory, we can give insights into how orbital splitting correlates with bifunctionality at oxide surfaces—which hadn’t been seen before.—Valentino Cooper
Straining thin films controls orbital splitting—the stretching of clouds of valence electrons.
On the surface of nickelate, you have one nickel atom at the center of a square of four oxygen atoms. If you strain that square and push the oxygen atoms closer, then the nickel–oxygen bond becomes unstable. When an oxygen molecule comes in and wants to react with that surface, much less energy is needed to break the oxygen–oxygen bond in the oxygen molecule. In other words, the transition state for the reaction to proceed is lower in energy.—Valentino Cooper
This new insight into how strain can be used to tune orbital splitting opens the door to developing new strategies for catalyst design and innovation.
ORNL’s Laboratory Directed Research and Development Program and the DOE Office of Science supported the study. Researchers used resources at the Center for Nanophase Materials Sciences, a DOE User Facility at ORNL. The ORNL-led Fluid Interface Reactions, Structures, and Transport center, an Energy Frontier Research Center funded by the DOE Office of Science, helped characterize catalytic performance.
Jonathan R. Petrie, Valentino R. Cooper, John W. Freeland, Tricia L. Meyer, Zhiyong Zhang, Daniel A. Lutterman, and Ho Nyung Lee (2016) “Enhanced Bifunctional Oxygen Catalysis in Strained LaNiO3 Perovskites” Journal of the American Chemical Society 138 (8), 2488-2491 doi: 10.1021/jacs.5b11713