Where chemistry meets electricity

The combination of chemistry and electricity is best known in the form of electrochemistry, in which chemical reactions take place in a solution in contact with electrodes that together constitute an electrical circuit. Electrochemistry involves the transfer of electrons between an electrode and the electrolyte or species in solution. It has been in use for the storage of electrical energy (in a galvanic cell or battery), the generation of electrical energy (in fuel cells), the analysis of species in solution (in pH glass electrodes or in ion-selective electrodes), or the synthesis of species from solution (in electrolysis cells). 

However, there are many other options to combine electricity with chemistry. One that has been studied intensively for a variety of different applications is plasma chemistry (see Fridman, 2008 for a recent overview). A plasma is a partially ionized gas, in which a certain percentage of the electrons is free instead of bound to an atom or molecule. Because the charge neutrality of a plasma requires that plasma currents close on themselves in electric circuits, a plasma reactor shows resemblance to an electrochemical cell, although due to the much lower ionization degree and conductivity, a plasma discharge will typically be operated in the range of hundreds of volts, compared to a few volts in the case of an aqueous electrochemical cell. 

The above combinations of electricity with chemistry deal with the generation of charged species in either a gas of a liquid medium. This requires ionization, which occurs by electron transfer and transport of charged species in a closed electrical circuit. The charged species themselves, or the radicals generated by them (e.g., radicals generated by electron impact in the gas phase, in a plasma) can be used as activated species taking part in chemical reactions. 

Finally, one may use charging or polarization of surfaces, induced by external electric fields, to control the adsorption and desorption of molecules and the state of these adsorbed molecules, in order to control their chemical reactivity. This is an upcoming field that has not yet been explored to its fullest potential. It involves aspects of nanotechnology and nanoscience, like the fabrication of structures of several nanometers and stimuli generated by scanning tunneling microscopic probes. The outcome of the research in this field is generally of a fundamental nature. The topic of electronic control of reactions at surfaces will be discussed in the last section of this chapter. 

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In the second chapter, Anil Agiral and Han J.G.E. Gardeniers take us to a fascinating world wherein "chemistry and electricity meet in narrow alleys." They claim that microreactor systems with integrated electrodes provide excellent platforms to investigate and exploit electrical principles as a means to control, activate, or modify chemical reactions, or even preparative separations. Their example of microplasmas shows that the chemistry can take place at moderate temperatures where the reacting species still have a high reactivity. Several electrical concepts are presented and novel principles to control adsorption and desorption, as well as the activity and orientation of adsorbed molecules are described. The relevance of these principles for the development of new reactor concepts and new chemistry is discussed. 

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