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Free keywords:
Electro-Oxidation, Small Organic Molecules, Kinetic Instabilities, Spatiotemporal Pattern Formation, Electrocatalysis
Abstract:
This work addresses recent research on phenomena of spatiotemporal self-organisation in electrochemical systems. It is written from the viewpoint of electrocatalysis exemplified by the electrocatalytic oxidation of formic acid (HCOOH) and methanol (CH3OH).
First, the electro-oxidation of carbon monoxide was studied, since CO oxidation is an important elementary step in the HCOOH and CH3OH oxidation. A rough Ru (0001) surface has higher catalytic activity for CO oxidation compared with the flat Ru (0001) electrode due to active defects of the surface.
In formic acid oxidation on a Pt ring electrode, reversible transitions between standing waves and travelling pulses were observed when the potential or the concentration of formic acid near the ring electrode were used as control parameters. We also found the anti-phase resp. in-phase edge oscillations on a thin Pt ribbon electrode, when the reference electrode was close to (resp. far away from) the ribbon working electrode. The edge of the ribbon tended to be in a more passive state (high potential) compared with the centre. A surprising experimental observation is phase resetting and controlled pulse reversal induced by external perturbation via a trigger electrode at one location of the ring. Usual phase resetting was obtained for small amplitude perturbations, while stronger perturbations resulted in reversal of the direction of the travelling pulse (i.e., from clockwise to anti-clockwise and vice versa).
All spatiotemporal experimental observations are in very good agreement with theoretical simulations carried out with a reaction-migration model and can be rationalized in terms of short-range positive and long-range negative migration coupling.
The experimental proof of a hidden negative differential resistance oscillator in methanol oxidation suggested that a higher thermodynamic efficiency of a direct methanol fuel cell could be possible by applying autonomous and/or externally driven potential oscillations with appropriate frequency.
