Electrochemical reaction dynamics: a review

The structure-giving role of Rb+ ions for water-ice nanoislands supported on Cu(111)

We characterize the effect of rubidium ions on water-ice nanoislands in terms of area, fractal dimension, and apparent height by low-temperature scanning tunneling microscopy. Water nanoislands on the pristine Cu(111) surface are compared to those at similar coverage on a Rb+ pre-covered Cu(111) surface to reveal the structure-giving effect of Rb+. The presence of Rb+ induces changes in the island shape, and hence, the water network, without affecting the nanoisland volume. The broad area distribution shifts to larger values while the height decreases from three bilayers to one or two bilayers. The nanoislands on the Rb+ pre-covered surface are also more compact, reflected in a shift in the fractal dimension distribution. We relate the changes to a weakening of the hydrogen-bond network by Rb+.

Regulation of electrocatalytic activity by local microstructure: focusing on catalytic active zone

Traditional regulation methods of active sites have been successfully optimized the performance of electrocatalysts, but seem unable to achieve further breakthrough in the catalytic activity. Unlike the conventional viewpoint of focusing on single active site, the concept of local microstructure active zone is more comprehensive and new suits of methods to regulate reaction zone for electrocatalytic reactions are developed accordingly. The local microstructure active zone refers to the zone with high catalytic activity formed by the interaction between active atoms and neighboring coordination atoms as well as the surrounding environment. Instead of the traditional single active atom site, the active zone is more suitable for the actual electrochemical reaction process. According to this concept, the activity of the electrocatalysts can be coordinated by multiple active atoms. This strategy is beneficial to understand the relationship between material, structure and catalysis, which realizes the scientific design and synthesis of high performance electrocatalysts. This review provides the research progress of this strategy in electrocatalytic reactions, with the emphasis on their important applications in oxygen evolution reaction, urea oxidation reaction and carbon dioxide reduction.

A numerical investigation of the effect of external resistance and applied potential on the distribution of periodicity and chaos in the anodic dissolution of nickel

Low density, elongation, and suppression of the shrimp-like structures in the resistance-potential phase diagrams have been observed in the oscillatory dissolution of nickel.

0D–2D heterostructures as nanocatalysts for self-oscillating reactions: an investigation into chemical kinetics

Ceria-decorated graphene nanocomposites as an efficient catalyst for the oscillatory Belousov–Zhabotinsky reaction.

Potentiostatic current and galvanostatic potential oscillations during electrodeposition of cadmium

Cathodic current and potential oscillations were observed during electrodeposition of cadmium from a cyanide electrolyte on a vertical platinum electrode, in potentiostatic and galvanostatic experiments. Electrochemical impedance spectroscopy experiments revealed a region of negative real impedance in a range of non-zero frequencies, in the second descending branch with a positive slope of the N-shape current-potential curve. This kind of dynamical behaviour is characteristic of the HN-NDR oscillators (oscillators with the N-Shape current-potential curve and hidden negative differential resistance). The oscillations could be mainly attributed to the changes in the real active cathodic area, due to the adsorption of hydrogen molecules and their detachment from the surface. The instabilities of the electrochemical processes were characterized by time series, Fast Fourier Transforms and 2-D phase portraits showing quasi-periodic oscillations.

Spatially distributed current oscillations with electrochemical reactions in microfluidic flow cells

The formation of spatiotemporal patterns is investigated by using a chemical reaction on the surface of a high-aspect-ratio metal electrode positioned in a flow channel. A partial differential equation model is formulated for nickel dissolution in sulfuric acid in a microfluidic flow channel. The model simulations predict oscillatory patterns that are spatially distributed on the electrode surface; the downstream portion of the metal surface exhibits large-amplitude, nonlinear oscillations of dissolution rates, whereas the upstream portion displays small-amplitude, harmonic oscillations with a phase delay. The features of the dynamical response can be interpreted by the dependence of local dynamics on the widely varying surface conditions and the presence of strong coupling. The patterns can be observed for both contiguous and segmented metal surfaces. The existence of spatially distributed current oscillations is confirmed in experiments with Ni electrodissolution in a microfluidic device. The results show the impact of a widely heterogeneous environment on the types of patterns of chemical reaction rates.

Preferred frequency responses to oscillatory inputs in an electrochemical cell model: linear amplitude and phase resonance

We investigate the dynamic mechanisms of generation of amplitude and phase resonance in a phenomenological electrochemical cell model in response to sinusoidal inputs. We describe how the attributes of the impedance and phase profiles change as the participating physicochemical parameters vary within a range corresponding to the existence of stable nodes and foci in the corresponding autonomous system, thus extending previous work that considered systems close to limit cycle regimes. The method we use permits us to understand how changes in these parameters generate amplifications of the cell's response at the resonant frequency band and captures some important nonlinear effects.

Experimental assessment of the sensitiveness of an electrochemical oscillator towards chemical perturbations

In this study we address the problem of the response of a (electro)chemical oscillator towards chemical perturbations of different magnitudes. The chemical perturbation was achieved by addition of distinct amounts of trifluoromethanesulfonate (TFMSA), a rather stable and non-specifically adsorbing anion, and the system under investigation was the methanol electro-oxidation reaction under both stationary and oscillatory regimes. Increasing the anion concentration resulted in a decrease in the reaction rates of methanol oxidation and a general decrease in the parameter window where oscillations occurred. Furthermore, the addition of TFMSA was found to decrease the induction period and the total duration of oscillations. The mechanism underlying these observations was derived mathematically and revealed that inhibition in the methanol oxidation through blockage of active sites was found to further accelerate the intrinsic non-stationarity of the unperturbed system. Altogether, the presented results are among the few concerning the experimental assessment of the sensitiveness of an oscillator towards chemical perturbations. The universal nature of the complex chemical oscillator investigated here might be used for reference when studying the dynamics of other less accessible perturbed networks of (bio)chemical reactions.

Electrochemical Oscillations of Nickel Electrodissolution in an Epoxy-Based Microchip Flow Cell

We investigate the nonlinear dynamics of transpassive electrodissolution of nickel in sulfuric acid in an epoxy-based microchip flow cell. We observed bistability, smooth, relaxation, and period-2 waveform current oscillations with external resistance attached to the electrode in the microfabricated electrochemical cell with 0.05 mm diameter Ni wire under potentiostatic control. Experiments with 1mm × 0.1 mm Ni electrode show spontaneous oscillations without attached external resistance; similar surface area electrode in macrocell does not exhibit spontaneous oscillations. Combined experimental and numerical studies show that spontaneous oscillation with the on-chip fabricated electrochemical cell occurs because of the unusually large ohmic potential drop due to the constrained current in the narrow flow channel. This large IR potential drop is expected to have an important role in destabilizing negative differential resistance electrochemical (e.g., metal dissolution and electrocatalytic) systems in on-chip integrated microfludic flow cells. The proposed experimental setup can be extendend to multi-electrode configurations; the epoxy-based substrate procedure thus holds promise in electroanalytical applications that require collector-generator multi-electrodes wires with various electrode sizes, compositions, and spacings as well as controlled flow conditions.

Vesicle-to-micelle oscillations and spatial patterns

A pH oscillator is coupled to and controls rhythmic interconversion of nanoscopic vesicles and micelles made of fatty acids. When changes in pH are combined with diffusion, self-assembly produces spatially extended patterns of vesicle/micelle "stripes" or concentric "shells".

Scaling relationship for oscillating electrochemical systems: dependence of phase diagram on electrode size and rotation rate

Dynamics of oscillations in electrochemical systems are affected by both chemical and physical properties of the systems. Chemical properties include the type of electrochemical reaction, the electrode material, the composition of the electrolyte, etc., while physical properties include the solution resistance, the cell constant, the electrode size, the rotation rate, the external resistance, etc. Earlier, we proposed the application of cell-geometry-independent phase-diagrams to characterize the oscillatory regions in the electrode potential vs. external resistance parameter plane. In this report, we investigate how this type of phase diagram changes with the surface area (electrode radius) and the rotation rate of an electrode. Based on linear stability analysis of a general, two-variable model for negative-differential resistance (NDR) type electrochemical oscillators we propose a scaling relationship. It predicts that all scaled data points derived from the critical values of parameters (resistance and potential) characterizing the onset of oscillations should fall - independently of the size of the electrode and the rotation rate - on a single plot. The analytical predictions are tested in both numerical simulations and experiments with copper electrodissolution in phosphoric acid.

Frequency of negative differential resistance electrochemical oscillators: theory and experiments

An approximate formula for the frequency of oscillations is theoretically derived for skeleton models for electrochemical systems exhibiting negative differential resistance (NDR) under conditions close to supercritical Hopf bifurcation points. The theoretically predicted omega infinity (k/R)1/2 relationship (where R is the series resistance of the cell and k is the rate constant of the charge transfer process) was confirmed in experiments with copper and nickel electrodissolution. The experimentally observed Arrhenius-type dependence of frequency on temperature can also be explained with the frequency equation. The experimental validity of the frequency equation indicates that 'apparent' rate constants can be extracted from frequency measurements of electrochemical oscillations; such method can aid future modeling of complex responses of electrochemical cells.