Spatio-temporal interfacial potential patterns during the electrocatalyzed oxidation of formic acid on Bi-modified Pt

Synchronized Current Oscillations of Formic Acid Electro-oxidation in a Microchip-based Dual-Electrode Flow Cell

We investigate the oscillatory electro-oxidation of formic acid on platinum in a microchip-based dual-electrode cell with microfluidic flow control. The main dynamical features of current oscillations on single Pt electrode that had been observed in macro-cells are reproduced in the microfabricated electrochemical cell. In dual-electrode configuration nearly in-phase synchronized current oscillations occur when the reference/counter electrodes are placed far away from the microelectrodes. The synchronization disappears with close reference/counter electrode placements. We show that the cause for synchronization is weak albeit important, bidirectional electrical coupling between the electrodes; therefore the unidirectional mass transfer interactions are negligible. The experimental design enables the investigation of the dynamical behavior in micro-electrode arrays with well-defined control of flow of the electrolyte in a manner where the size and spacing of the electrodes can be easily varied.

Normal-form approach to spatiotemporal pattern formation in globally coupled electrochemical systems

We show that the experimental global coupling (GC) of spatially extended electrochemical oscillators is weak close to a supercritical Hopf bifurcation. A center manifold reduction allows then the normal form which comprises the GC and the naturally existing nonlocal (migration) coupling (NLC) to be derived. We show that the interaction between NLC and GC widens the spectrum of coherent structures found in globally coupled oscillatory media and allows for wavelength selection of standing waves, stabilization of phase clusters without breaking phase invariance, and creation of heteroclinic networks connecting families of oscillatory states characterized by different spatial symmetries.

Edge effects in an electrochemical reaction: HCOOH oxidation on a Pt ribbon

The use of a ribbon-shaped Pt electrode gives rise to edge effects of the interfacial potential, as is predicted from the potential theory in the form of the corresponding reaction-migration equation. They are studied in the bistable region of formic acid oxidation. Essentially, the edges tend to be more passive than the bulk of the electrode, which also causes a passivation (activation) transition to originate from the edges (center) of the ribbon. The experimental results are in agreement with simulations of the reaction-migration system.

Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum: A Mathematical Modeling and Simulation

We have modeled temporal potential oscillations during the electrooxidation of formic acid on platinum on the basis of the experimental results obtained by time-resolved surface-enhanced infrared absorption spectroscopy (J. Phys. Chem. B 2005, 109, 23509). The model was constructed within the framework of the so-called dual-path mechanism; a direct path via a reactive intermediate and an indirect path via strongly bonded CO formed by dehydration of formic acid. The model differs from earlier ones in the intermediate in the direct path. The reactive intermediate in this model is formate, and the oxidation of formate to CO2 is rate-determining. The reaction rate of the latter process is represented by a second-order rate equation. Simulations using this model well reproduce the experimentally observed oscillation patterns and the temporal changes in the coverages of the adsorbed formate and CO. Most properties of the voltammetric behavior of formic acid, including the potential dependence of adsorbate coverages and a negative differential resistance, are also reproduced.

Influence of Bi Modification of Pt Anode Catalyst in Direct Formic Acid Fuel Cells

The influence of Bi modification of Pt anode catalyst on the performance of direct formic acid fuel cells was investigated. Compared with the unmodified Pt anode, the Bi modified Pt (PtBi(m)) electrode prepared by under-potential deposition (UPD) caused faster electrocatalytic oxidation of formic acid at the same value of the overpotential, and thus, PtBi(m) resulted in an increase in the power performance of direct formic acid fuel cells. Electrochemical impedance spectra helped to explain the difference of performance between the unmodified Pt and Bi modified Pt electrodes. Solution conductivity and dehydration phenomena occurring in highly concentrated formic acid solutions can also explain the higher power performance of PtBi(m).

A hierarchy of global coupling induced cluster patterns during the oscillatory H2-electrooxidation reaction on a Pt ring-electrode

We report experimental results on spatiotemporal pattern formation during the oscillatory hydrogen electrooxidation reaction on a Pt ring-electrode under negative (desynchronizing) global coupling (GC). Spatially one-dimensional profiles of the interfacial potential drop along the angular direction of the ring electrode are recorded by means of a potential probe. The dynamics is investigated as a function of two control parameters, the applied voltage U and the strength of the global coupling. The latter is adjusted either by varying the distance between the working electrode (WE) and the reference electrode (RE) or by inserting a negative impedance device in series with the WE. In the absence of global coupling, uniform oscillations were destabilized by migration coupling, and electrochemical turbulence developed at large values of U (H. Varela, C. Beta, A. Bonnefont and K. Krischer, Phys. Rev. Lett., 2005, 94, 174104; ). Already low global coupling strengths sufficed to suppress turbulence. Instead, regular two-phase clusters formed. At higher coupling strength, a second type of two-phase cluster was observed as well as two types of irregular cluster patterns, which were connected with an irregular motion of the cluster boundaries and the emergence and disappearance of clusters through splitting and merging of the boundaries, respectively. Upon increasing the coupling strength even further, five-phase clusters were stabilized and at the highest coupling strength applied the cluster patterns transformed into strongly modulated pulses. The two types of two-phase clusters and the five-phase clusters are analyzed employing several signal processing techniques.

Theory of electrochemical pattern formation

The spatial coupling in electrochemical systems is mediated by ion migration under the influence of the electric field. Since field effects spread very rapidly, every point of an electrode can communicate with every other one practically instantaneously through migration coupling. Based on mathematical potential theory we present the derivation of a generally applicable reaction-migration equation, which describes the coupling via an integral over the whole electrode area. The corresponding coupling function depends only on the geometry of the electrode setup and has been computed for commonly used electrode shapes (such as ring, disk, ribbon or rectangle). The pattern formation observed in electrochemical systems in the bistable, excitable and oscillatory regime can be reproduced in computer simulations, and the types of patterns occurring under different geometries can be rationalized. (c) 2002 American Institute of Physics.

Spatiotemporal Mixed-Mode Oscillations on a Ring Electrode

We present spatiotemporal mixed-mode oscillations in the electrocatalytic oxidation of formic acid on a platinum ring electrode modified by bismuth deposition. On the anodic scan, we observed spontaneous reversal of direction of a rotating activation pulse occurring repeatedly at the same location of the ring. In the constant potential method, spatiotemporal mixed-mode oscillations of pulse reversal were shown to constitute a fraction of an ordered Farey sequence.