Interfacial electron transfer, molecular electrochemical processes

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

Our interest in SERS stemmed from our research activities concerned with establishing connections between the molecular structure of electrode interfaces and electrochemical reactivity. A current objective of our group is to employ SERS as a molecular probe of adsorbate-surface interactions to systems of relevance to electrochemical processes, and to examine the interfacial molecular changes brought about by electrochemical reactions. The combination of SERS and conventional electrochemical techniques can in principle yield a detailed picture of interfacial processes since the latter provides a sensitive monitor of the electron transfer and electronic redistributions associated with the surface molecular changes probed by the former. Although few such applications of SERS have been reported so far the approaches appear to have considerable promise. 

In this section, selected studies are presented in which self-assembled monolayers have been used to address topics such as transition-state structures and sequential electron transfer. These studies were selected because they address fundamental mechanistic processes. SAMs have also been used to investigate such basic electrochemical phenomena as the potential profile near an electrode [134, 135], interfacial capacitance [136], the influence of redox [134] or polarizable [137] moieties on double-layer structure and the behavior of ultramicroelectrodes approaching molecular size [138]. These important topics are beyond the scope of this chapter, and the interested reader is directed to the literature for more information. 

Mechanistic views and theoretical formalism of molecular STM processes are addressed in some detail in Chapter 8. Views of single molecule electron transport are rooted in theories of interfacial electrochemical electron transfer but offers new theoretical features and even phenomena. At the same time puzzles remain, resolution of which requires substantial computational efforts in the form of molecular dynamics and quantum chemical computations. Efforts along such lines are only just beginning. We provide here a few observations of immediate relevance to the data and images shown above. 

All these areas are covered in a broad literature, overviewed, for example in refs. 24 and 25. We do not here address all these elements of molecular charge transfer theory. Instead we discuss the two central factors in the interfacial (bio)electrochemical electron transfer process, first the nuclear reorganization (free) energy and then the electronic tunneling factor. 

An additional distinction between weak and strong electronic interactions between the molecular redox level and the electrodes is important. In the former limit the overall steady-state STM process can be viewed as two consecutive, environmentally relaxed interfacial singleelectron transfer steps, each analogous to electrochemical electron transfer. Section 2, giving steady-state tunneling current forms such as... 

The typical IL system could be considered as a solvent-free system, in which it can simplify the EIS analysis significantly which spurs its wide use in the characterization of the IL-electrode interface. However, due to low mobility of ions in an IL and multiple molecular interactions present in an IL, more time is needed to reach to a steady state of IL-electrode interface structure and arrangement, when a potential is applied. Furthermore, the electron-transfer process in ILs is different from that in traditional solvents containing electrolytes. Thus, the interfacial structures of IL are more complex than other systems. Even the electrode geometry could affect the EIS results of IL systems. It is noted that the bulk ILs could not be simply described by a resistor (R ) as in classic electrochemical systems. And the electrode double layer in IL electrolyte couldn t be simply depicted as a capacitor. So the Randle equivalent circuit is not sufficient to describe an IL system. Significant efforts have been made to illustrate the properties of diffusion layer and the bulk ILs with equivalent circuits. However, currently there is no general equivalent circuit model to describe the interface of an IL system. 

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