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August 25th, 2011

Imaging the electrochemical reaction in solid oxide fuel cells

Published on August 25th, 2011 | By: Eileen De Guire

Electrochemical strain microscopy images ion mobility. The overlay shows electrochemical activity of platinum nanoparticles on a yttria-stabilized zirconia surface, showing ionic activity along the triple phase boundaries. Credit: ORNL

My favorite part of the TV hospital drama, House, is the beginning when the failure occurs. The show opens with the patient-of-the-week doing normal stuff and the camera cuts to the deconstruction going on unbeknownst inside the unlucky patient’s body. The camera zooms around, darting through veins, leaping across synapses, undulating in the ebb and flow within until—zonk!—something goes terribly wrong. The crackerjack team of doctors sure could use a probe like the one the show’s producers have—one that shows from within how things flow, interact and fail. Instead they have the brilliant but irascible Dr. House.

There are times when materials scientists, too, would benefit from getting an inside view. It can be especially helpful to be able to characterize a material dynamically and capture the material’s response as it is happening. For example, the key reaction in a solid oxide fuel cell is the oxygen reduction reaction that occurs at the triple phase boundary. The triple phase boundary (pdf) is where the solid electrolyte, catalyst and gas are in contact. It’s where the action is in a fuel cell with the oxygen reduction reaction at the cathode and the hydrogen oxidation reaction at the anode, converting chemical energy into electricity.

The TPB is a very difficult region to characterize. If the electrochemical reactions could be observed or imaged, it should be possible to understand the fundamental mechanisms controlling the material’s performance and to design improved materials.

“If we can find a way to understand the operation of the fuel cell on the basic elementary level and determine what will make it work in the most optimum fashion, it would create an entirely new window of opportunity for the development of better materials and devices,” says Amit Kumar in an ORNL press release.

Kumar is lead author of a new paper out of Oak Ridge National Laboratory that describes a new technique—electrochemical strain microscopy—that allows scientists to directly measure oxygen reduction/evolution reactions and oxygen vacancy diffusion on ion conducting solid surfaces, like yttria-stabilized zirconia.

ESM measures electrochemical reactivity and ionic current in solids on a scale of ten nanometers or less. By applying a periodic bias to a scanning probe microscopy tip in contact with the surface, ionic movement is induced, the surface deforms and the deformations are mapped. As explained in an article (pdf) by Asylum Research, a partner with ORNL on developing the technique, “The intrinsic link between concentration of ionic species and/or oxidation states of the host cation and molar volume of the material results in electrochemical strain and surface displacement.”

Regarding the importance of the capability afforded by ESM, coauthor and ACerS member Sergei Kalinin says, “When you want to understand how a fuel cell works, you are not interested in where single atoms are, you’re interested in how they move in nanometer scale volumes. The mobile ions in these solids behave almost like a liquid. They don’t stay in place. The faster these mobile ions move, the better the material is for a fuel cell application. Electrochemical strain microscopy is able to image this ion mobility.”

The technique can be used to characterize ionic conductivity for other applications as well, such as lithium batteries, metal-air batteries and semiconductors.

Kalinin, co-theme leader for Functional Imaging on the Nanoscale at ORNL, says in an email to us, “The ORR/OER directly underpin the operation of fuel cells and metal–air batteries, and hence their probing on the level of a single electrocatalytic nanoparticle or structural defect is of direct interest for energy storage and conversion. Furthermore, these processes can severely affect (through oxygen nonstoichiometry) the functionality of the materials of interest for condensed matter physics community. After all, materials used in fuel cells—manganites, cobaltites, etc.—are the same as those studied for colossal magnetoresistance or nanoscale phase separation.”

The paper is “Measuring oxygen reduction/evolution reactions on the nanoscale,” A. Kumar, et. al., Nature Chemistry 3, 707-713 (2011) doi:10.1038/nchem.1112
Published online 14 August 2011

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