Atomic structure, electrode-electrolyte

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

Regarding the electrode/electrolyte interface, it is important to distinguish between two types of electrochemical systems thermodynamically closed (and in equilibrium) and open systems. While the former can be understood by knowing the equilibrium atomic structure of the interface and the electrochemical potentials of all components, open systems require more information, since the electrochemical potentials within the interface are not necessarily constant. Variations could be caused by electrocatalytic reactions locally changing the concentration of the various species. In this chapter, we will focus on the former situation, i.e., interfaces in equilibrium with a bulk electrode and a multicomponent bulk electrolyte, which are both influenced by temperature and pressures/activities, and constrained by a finite voltage between electrode and electrolyte. 

It should be apparent from the discussion above that STM possesses tremendous potential for the elucidation of processes at the electrode-electrolyte interface. Particularly promising are the prospects for in situ studies of electrode surfaces. Vibrational, electronic, and structural information is obtainable on an atomic scale for electrodes of importance to basic electrochemical studies. Although relatively few electrochemical applications have been demonstrated to date, the availability of commercial instrumentation (c.f.,95-97) ought to increase the accessibility of STM to electrochemists and widespread use of the technique is expected in the near future. 

J. W. Halley, Studies of the Interdependence of Electronic and Atomic Dynamics and Structure at the Electrode-Electrolyte Interface, Electrochim. Acta. 41 2229 (1996). 

On die electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic structure of tine electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. 

A unique method for studying the phase composition and the atomic structure of crystalline materials. -> Electrodes and -> solid electrolytes are usually crystalline materials with a regular atomic structure that predicts their electrochemical behavior. For instance, the ionic transport in solid electrolytes or - insertion electrodes is possible only owing to the special atomic arrangement in these materials. The method is based on the X-ray (neutron or electron) reflection from the atomic planes. The reflection angle 9 depends on the X-ray (neutron or electron) wave length A and the distance d between the atomic planes (Braggs Law) ... 

As a fundamental basis for all STM studies, electrode-electrolyte interfaces must be prepared reproducibly, and methods must be established to observe these interfaces accurately. Well-defined single crystalline surfaces must be exposed to solution to understand surface structure-reactivity relationships on the atomic scale. Efforts have succeeded to produce extremely well-defined, atomically flat surfaces of various electrodes made of noble metals, base metals, and semiconductors without either oxidation or contamination in solution. 

In this article, electrochemical and tunneling behaviors of p-GaAs/electrolyte interfaces were described and electrochemical AFM was employed to study the atomic structure of and the Cu electrodepostion on the GaAs(lOO) electrodes in various electrolyte solutions. Sub-//m pattern formation by scanning the AFM tip was also attempted. 

IR absorption, emission, and reflection spectra for molecular species either in solid, liquid, or gas phases arise mostly from various changes in energy due to transitions of molecules from one vibrational or rotational energy state to another. The frequency or wavelength of this energy transition is characteristic of the specific chemical bond vibration and/or rotation in the molecule which are determined by the molecular structure, the masses of the atoms, and the associated vibrational energy coupling. Attenuated total reflectance (ATR) and reflection-mode of IR in conjunction with electrochemical methods allow samples to be examined directly in the solid or liquid state without further preparation and are widely used in the characterization of electrode-electrolyte interface properties. Most of ILs are IR-active molecules. Since ILs are stable and chemically inert, the IR characterization can be easily performed on the IL-based system directly. 

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