The efficiency of fuel cells and metal–air batteries is significantly limited by the activation of oxygen reduction and evolution reactions. Despite the well-recognized role of oxygen reaction kinetics on the viability of energy technologies, the governing mechanisms remain elusive and until now have been addressable only by macroscopic studies. This lack of nanoscale understanding precludes optimization of material architecture.

Here, we report direct measurements of oxygen reduction/evolution reactions and oxygen vacancy diffusion on oxygen-ion conductive solid surfaces with sub-10 nm resolution. In electrochemical strain microscopy, the biased scanning probe microscopy tip acts as a moving, electrocatalytically active probe exploring local electrochemical activity. The probe concentrates an electric field in a nanometer-scale volume of material, and bias-induced, picometer-level surface displacements provide information on local electrochemical processes. Systematic mapping of oxygen activity on bare and platinum-functionalized yttria-stabilized zirconia surfaces is demonstrated.

This approach allows direct visualization of the oxygen reduction/evolution reaction activation process at the triple-phase boundary, and can be extended to a broad spectrum of oxygen-conductive and electrocatalytic materials.

—Kumar et al.

The ESM technique is based on a mathematical model, a partial differential equation, that describes the movement of oxygen in various materials. By way of this mathematical description, the measurement data from Electrochemical Strain Microscopy can be visualized on the computer screen.

In the paper, Ciucci and his colleagues applied the technique to a fuel cell. A hydrogen fuel cell consists of two electrodes facing one another and separated by an ion conductor. Electric energy is gained via transfer of ions between the two electrodes. The oxygen in the air reacts with the hydrogen brought in from outside. In this process of oxygen reduction, a catalyst—frequently expensive platinum—plays an essential role as reaction accelerator. In all this, says Ciucci, the oxygen reduction process is the limiting factor in connection with the longevity and efficiency of fuel cells.

To optimize ion transfer between the electrodes, a number of fundamental questions have to be answered. How and where exactly does oxygen reduction occur and how does platinum function as a catalyst? Up to now we have had to do without a suitable instrument for investigating the reaction dynamics involved.

What we have found out from this, is that the catalyst layer of 50-nanometer platinum particles does not allow an equal degree of ion transfer at all points.

—Francesco Ciucci

Resources

• A. Kumar, F. Ciucci, A.N. Morozovska, S.V. Kalinin and S. Jesse (2011) Measuring oxygen reduction/evolution reactions on the nanoscale. Nature Chemistry 3, 707-713 doi: 10.1038/nchem.1112