Pattern Formation in Electrocatalytic Reactions

Spatiotemporal Instabilities

Several electrocatalytical reactions are known to undergo spatiotemporal instabilities under some conditions. We are interested in principle mechanisms leading to pattern formation at the solid/liquid interface and in methods that can be used to control or manipulate pattern formation. Besides a general understanding of how to influence pattern formation, the objective of the latter investigations is to obtain higher yields of electrocatalytic reactions or better performances of electrochemical systems by exploiting their nonlinear properties.

For example, commercial hydrogen is usually produced in a reforming process where hydrocarbons are cracked into H₂ and CO. It is well known that CO acts as a poison in low temperature fuel cells since it occupies free reaction sites on the Pt catalyst. However, CO also induces dynamic instabilities during the hydrogen oxidation, pointing to the possibility that some of the resulting states might possess a catalytic activity towards H₂ oxidation that is superior to the uniform and stationary state.

First simulations reveal that the formation of spatial patterns in the CO coverage can improve the CO tolerance of fuel cells. As prototypical model systems, which at the same time are important from an applied point of view, we use mainly two reactions, namely

(a) the oxidation of hydrogen in the presence of carbon monoxide on Pt electrodes

(b) the electrooxidation of carbon monoxide on Pt electrodes

Both systems (a) and (b) exhibit an S-shaped current-voltage characteristic due to a chemical autocatalytic process and exhibit oscillatory behaviour, although the underlying mechanisms for the latter differ.

Fig. 1: Experimental setup for laterally resolved SEIRAS spectroscopy with imaging optics in the reflected IR beam and array detector. Simultaneously, 4096 IR spectra are recorded which can then be further processed, e.g. in an intensity or frequency map. For further information see ref. [3]

Pattern formation in these systems is studied by means of surface enhanced infrared absorption spectroscopy (SEIRAS) in the so called attenuated total reflection (ATR) configuration.

ATR configuration and optical path of the IR beam allow the simultaneous recording of IR spectra.

Fig. 2: Example of a stationary, scale-free CO domain under galvanostatic conditions. Red color denotes CO coverage at saturation and blue the bare (CO free) Pt surface. The image was obtained from SEIRAS data. Further information can be found in ref. [4]

Fig. 3: Cell design for SEIRAS measurements with the electrolyte flow from one side.

Video: This video shows the spatio-temporal evolution of the CO coverage on a Pt film electrode in the presence of Br- ions in a flow cell as shown in Fig. 3. The electrolyte flows from top to bottom, and dark red corresponds to a high CO coverage, white to the CO free Pt surface.

The spatially resolved investigations are supplemented by classical electrochemical measurements (such as cyclic voltammetry or amperometry), experiments employing feedback control techniques and mathematical modeling. As for the latter, we investigate in particular the dynamics of reaction-diffusion-advection systems, which are adequate to describe the dynamics found with cells as in Fig. 3 or the data shown in the video.

This project is funded by the DFG.

Dynamics of an array of microelectrodes

(in collaboration with Prof. Elena Savinova and Dr. Antoine Bonnefont, Strasbourg, France and Prof. Dr. Rolf Schuster, KIT, Germany)

We also study the dynamics of CO oxidation on an array of Pt micro-electrodes. From a dynamical point of view, the electrode array can be viewed as a network of bistable or oscillatory elements. First results can be found in ref. [7].

References

[1] S. Malkhandi, A. Bonnefont, K. Krischer, Electrochem. Communications 7, 710-716 (2005)

[2] S. Malkhandi, A. Bonnefont, K. Krischer, Surface Science 603, 1646 - 1651 (2009)

[3] R. Morschl, J. Bolten, A. Bonnefont, K. Krischer, J. Phys. Chem. C 112, 9548-9551 (2008)

[4] P. Bauer, A. Bonnefont, K. Krischer, ChemPhysChem 11, 3002 (2010)

[5] S. Malkhandi, P. R. Bauer, A. Bonnefont, K. Krischer, Catalysis Today 202, 144 (2013)

[6] E. Ramirez-Alvarez, R. Rico-Martinez, K. Krischer, Electrochimica Acta, in press (2013)

[7] D. A. Crespo-Yapur, A. Bonnefont, R. Schuster, K. Krischer, E. R. Savinova, ChemPhysChem 114, 1117 - 1121 (2013)