Degeneration of Limit Cycle Oscillation in BZ Electrode Reaction System Controlled Externally

A dynamical model of electrode BZ reaction system was establisheed on the basis of three variables Oregonator model and kinetics of electrode process. Under weak periodical constraint approximation, dynamical stability of quasi steady state on the slow manifold of the system is analyzed by means of linearized stability analysis of three variable system. Meanwhile, the corresponding regime favorable for the appearance of limit cycle oscillation is calculated. Computer simulation shows that limit cycle oscillatory regime has degenerated because of the external periodical potential constraint in the electrode phase. In this regime the system behaves as a temporary self-organization. Whereas, outside this regime a kind of response oscillation appear, with same period as the constraint.     

铂电极BZ反应体系的系统动力学分析

在一定的条件下,将铂电极BZ化学反应的六变量高维动力学系约化为三变量体系,同时对该体系进行了全面的系统动力学分析.研究结果表明,通过改变耦合体系的外控参数条件,在将体相保持在均一稳定定态的参数范围内,电极反应相可能进入振荡区,而呈现出电极反应相与体相的动力学行为不一致性.进一步计算出体相处于非振荡状态时,电极反应相产生电化学振荡的外控参数区域.     

外控恒电流对BZ电化学反应体系动力学行为的影响

在使用探头对BZ反应体系进行观测时,原来纯粹的BZ反应系变成了由两个子系通过扩散过程耦合成的一类特殊电化学体系,所用的探头能否如实反映该体系的真实振荡行为值得探讨。本文基于FKN机理及Oregonator模型,分别对Pt电极BZ反应体系建立了动力学模型,讨论了两子系出现极限环振荡的动力学行为不一致性及外控电极电流的影响。     

铂电极B-Z反应系双电层稀疏区电势跃变对电化学振荡的影响

根据连续步骤间非平衡耗散的特征, 定量表示了电化学步骤与扩散步骤的相对滞后性对电极反应体系耗散的贡献, 建立了稀疏区电势跃变ψ的非平衡态热力学计算公式, 给出了其在Boltzman分布近似下的表现形式. 同时, 建立了一个普适的Pt电极B-Z反应动力学模型, 讨论了ψ对电极B-Z反应系状态演化动力学行为的影响, 发现ψ的引入将强化该电极反应体系动力学机制的非线性程度, 导致电化学振荡区的蜕变. 给出了相应的计算机模拟图谱.     

Current oscillations during the electrochemical oxidation of sulfide in the presence of an external resistor

The electrochemical oxidation of sulfide on a polycrystalline platinum electrode was studied under potentiostatic condition when an external resistor is in series with the working electrode. Only two os- cillatory regions can be obtained in the absence of the external resistance, but four oscillatory regions, including two new current oscillations, were found in this system by controlling the external resistance. It is demonstrated that three oscillatory regimes, which arise on the positive branch of current-potential curve, can be classified as HN-NDR (Hidden N-shaped Negative Differential Resistance) oscillators. For the first oscillatory region, various transient complex phenomena, which result from the change of the electrode/electrolyte interface by accumulation of adsorbed element sulfur on the electrode, have been observed. The dynamic behavior of NDR (Negative Differential Resistance) oscillations, appearing along with negative branch of polarization curve, can transform from oscillations into bistability with a sufficient large external resistance in series. Two oscillatory regions in high-potential region classified as HN-NDR type oscillations are separated by a saddle-loop bifurcation. They displayed a sequence of bursting oscillations and irregular oscillations, respectively. The electrochemical oxidation of sulfide provides a model system for studying complex dynamics and possible application in sulfur removal.     

甲酸Pt电极电化学氧化体系中时间自组织导致的可利用功耗失

在电极化学反应步骤为控制步骤的条件下,对甲酸阳极氧化体系中出现的电化学振荡的阈值及跨越阈值后状态演化的极限环路径进行了系统动力学计算.同时,根据非平衡电极过程的耗散-涨落理论,进一步对跨越振荡阈值前后的临界区与电化学震荡区的极化曲线及功耗谱进行了随机热力学分析计算.研究结果表明,跨越临界区后单位电化学反应功耗谱出现突降...     

Role of ohmic losses in appearance of dynamic instabilities in model electrochemical system with cylindrical electrode under potentiostatic conditions

The impedance spectroscopy technique was used for theoretical studies of the conditions of appearance of Hopf instability and saddle-node instability in a model electrochemical system with a preceding homogeneous chemical reaction in the Nernst diffusion layer and electrocatalytic reaction on the surface of a cylindrical electrode under potentiostatic conditions. It is shown that the value of the ohmic resistance parameter in the system can affect the number of bifurcation points and bifurcation frequency. An increase in the ohmic resistance parameter results in the narrowing of the region of Hopf bifurcation giving rise to spontaneous periodical current oscillations and expansion of the region of saddle-node instability leading to bistability. There are threshold values of the ohmic resistance parameters critical for appearance and disappearance of the dynamic instabilities under consideration.     

Effects of hydrogen on current oscillations during electro-oxidation of X70 carbon steel in phosphoric acid

It is the first time that the oscillatory electrodissolution of metals is used to study hydrogen-promoted corrosion, and the primary results prove that it is an effective method for investigating the effect of hydrogen on both the formation and dissolution of a passive film. Effects of hydrogen on the electrochemical oscillations of X70 carbon steel are investigated in 5.0 M H₃PO₄ solution. During the oscillatory electrodissolution of X70 steel electrode, the chemical environment near the surface of the electrode is changed artificially by the oxidation of hydrogen diffused from X70 electrode to surface. With increasing hydrogen pre-charging current density, both the induction time and the ratio of active time to passive time of the current oscillations increase, and the Flade potential also shifts positively. Oxidation of hydrogen decreases the pH value at the interface between the electrode and solution, which retards the formation of a passive film and subsequently promotes its dissolution. This investigation provides further understanding of the effect of hydrogen on the formation and dissolution processes of passive films. It is the first time to observe the periodical changes in the potential at the electrode/electrolyte interface during current oscillations by using scanning reference electrode technology. The scanning reference electrode technology is also used to verify the effect of hydrogen on the current oscillations.     

Ionic strength and pH as control parameters for spontaneous surface oscillations

A system far from equilibrium, where the surfactant transfer from a small drop located in the aqueous bulk to the air-water interface results in spontaneous nonlinear oscillations of surface tension, is theoretically and experimentally considered. The oscillations in this system are the result of periodically arising and terminating Marangoni instability. The surfactant under consideration is octanoic acid, the dissociated form of which is much less surface-active than the protonated form. Numerical simulations show how the system behavior can be controlled by changes in pH and ionic strength of the aqueous phase. The results of numerical simulations are in good agreement with experimental data.     

Externally applied electric fields on immiscible lipid monolayers: repulsion between condensed domains precludes domain migration

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on different lipid monolayers exhibiting surface immiscibility. In the monolayers studied, lipids in the condensed state segregate in discontinuous round-shaped domains, with the lipid in the liquid-expanded state forming the continuous phase. The use of fluorescent probes with selective phase partitioning allows analyzing by epifluorescence microscopy the migrations of the domains under the influence of inhomogeneous electric fields applied to the surface. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the domains until a steady state is reached, indicating the presence of a force opposed to the externally applied electric force. The experimental results were modeled by considering that the opposing force is generated by the dipole-dipole repulsion between the domains.