Catalyst drives reaction in both directions

A NEW type of oxide catalyst developed by researchers at Oak Ridge National Laboratory, US, can drive certain chemical reactions in both directions with high efficiency.

While a catalyst that can drive a reaction in two directions may sound counterintuitive, the researchers say it would be extremely useful in energy storage devices such as fuel cells and rechargeable batteries where chemical energy is converted in electrical energy. For example the oxygen reduction reaction effectively extracts electrons from oxygen molecules, which discharges the battery, while an oxygen evolution reaction proceeds in the opposite direction to recharge a battery. A single catalyst which can drive this reaction in both ways, for both discharge and recharge, would be very beneficial.

The catalyst is made from lanthanum nickelate, LaNiO₃, a transition metal oxide system. What is particularly different about the new catalyst is that is makes use of a phenomenon called strain, which effectively changes the electronic structure of the catalyst film.

Research leader Ho Nyung Lee and the team grew a thin film of lanthanum nickelate on a substrate material with different lattice spacing. The mismatch of the lattice pattern introduces the strain, which increases the oxide material’s reactivity. Researcher Daniel Lutterman explains that for a reaction to happen, some bonds need to be made while others need to be broken. A catalyst drives the reaction by interacting with the molecules involved. By introducing strain into the catalyst, the energy required for the interaction to happen is lowered.

“On the surface of nickelate, you have one nickel atom at the centre of a square of four oxygen atoms,” said Valentino Cooper, who helped to research the mechanism. “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.”

In experiments, the catalyst outperformed platinum, which is highly effective at driving both the oxygen evolution and reduction experiments. Lanthanum nickelate without strain can only drive the reaction in one direction.

“In general a catalyst lowers the activation barrier for a reaction to occur,” Lutterman said. '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.”

Journal of the American Chemical Society DOI: 10.1021/jacs.5b11713