Sink electrochemical reaction

The charged reactant for the sink electrochemical reaction is supplied by the solid electrochemical cell of a PEVD system. The solid phase (E) is an exclusive ionic conductor for (A +) or (A ), and serves as the solid electrolyte. (C) and (W) are solid electronic conducting phases, and contact (E) from both sides as counter and working electrodes, respectively. They coimect with the external electric circuit, which consists of a dc source and other possible measurement devices. Because the conductivity changes in nature from ionic to electronic at the electrode/electrolyte interfaces, the solid electrochemical cell in a PEVD system effectively separates the transport paths of ionic and electronic charged carriers... 

As schematically shown in Figure 7a, initial PEVD reaction and product nucleation occurs at the three-phase boundary of solid electrolyte (E), working electrode (W) and the sink vapor phase (S) which contains vapor phase reactant (B). Only here are all reactants available for the half-cell electrochemical reaction at the sink side of a PEVD system. Although the ionic and electronic species can sometimes surface diffuse at elevated temperature to other sites to react with (B) in the vapor phase, the supply of the reactants continuously along the diffusion route is less feasible and the nuclei are too small to be stabilized under normal PEVD conditions. Only along the three phase boundary line are all the reactants available for further growth to stabilize the nuclei. Consequently, initial deposition in a PEVD process is restricted to certain areas on a substrate where all reactants for the sink electrochemical reaction are available. 

Among electrode processes with at least one charge transfer step, several different types of reaction can be found. The simplest interfacial electrochemical reactions are the exchange of electrons across the electrochemical interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface. The electrode in this case is merely the source or sink of electrons, uptaking electrons from the reduced species and releasing them to the oxidized redox species in solution. Examples of simple electron transfer reactions are... 

Fick s law of diffusion, J = —D9C/9x, applies when there is a source and a sink, with a vector current between then proportional to the concentration gradient. An equilibrium electrochemical reaction on open circuit is not a sink for reactants nor a source of product. It is a zero-flow blockage, albeit with balanced exchange currents. The flow of ions in the cell is a balance the drive V is balanced by an opposing concentration difference. 

As with CVD and EVD techniques, PEVD process design starts by considering the PEVD product (D). Either an anodic or cathodic halfcell electrochemical reaction at the sink side of the system can lead to product (D) formation and subsequent deposition. 

In order to carry out the electrochemical reaction for product (D) deposition in a PEVD process, the neutral reactant (B) and other possible reactants for doping are transported to the deposit (D) surface by diffusion in the sink vapor phase (S). Furthermore, if there are reaction products other than (D), they must be driven away from the reaction sites though the sink vapor phase. 

Usually, the working electrode (W) is a porous metallic electrode in PEVD. Thus, reactant (B) in the vapor phase can reach the surface of the solid electrolyte for initial electrochemical reaction at a three-phase boundary of solid electrolyte (E), working electrode (W) and sink vapor phase (S) as shown in Eigure 3 (location II). All reactants for the sink side electrochemical reaction (1) or (2) are only available there. Subsequent reaction and deposition of the product (D) requires both electrons and ions to travel through product (D) to the surface to react with vapor phase reactant(s) electrochemically at location III in Eigure 3. 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

Because the atmosphere at both the source and sink sides was the same, the chemical potential of sodium was equal and fixed by the Na COj phase in equilibrium with the same atmosphere at all three Pt electrodes, when no electric field was present. At this point, the equilibrium electrochemical reaction... 

The kinetics of electrochemical reactions are often modified by the nature of the electrode material, and by the presence of atomic and molecular species either adsorbed on the surface or in the bulk solution [14]. Electrocatalysis is primarily concerned with the study of this phenomenon and, particularly, with the factors that govern enhancements in the rates of redox processes. Implicit in this general statement is the ability of the species responsible for these effects, or electrocatalyst, or the electrode itself, to carry out the reaction numerous times before undergoing possible deactivation. Electrocatalytic processes in which the electrode simply serves as a source or sink of electrons to generate solution phase species that... 

Details of the core mass, momentum, energy, and species conservation and transport features of FLUENT are documented in detail in the FLUENT user manual [2]. Details of the electrochemical reactions, loss mechanisms, electric field computation, and electrode porous media constitutive relations are documented by Prinkey (ref. 1). This reference also documents the treatment of species and energy sources and sinks arising from the electrochemistry at the electrode-electrolyte interfaces. 

An electronic conductor that acts as a source or sink of electrons that are involved in electrochemical reactions. 

Since by definition 7t-electrons are extensively conjugated in tltese classes of polymers, it would be much easier for one to oxidize or reduce the polymers rather than their monomeric counterparts. The polymer would act as either an electron source during oxidation or an electron sink during reduction. As a result, the polymers would be electroactive and participate in the electrochemical reactions. For this reason, electrochemistry has been playing a central role in characterizing the 7t-conjugated polymers. 

The LE serves not only as a medium for selective transport of ionic charge but also, in many cases, as the source and sink for the reactants and products involved in the electrochemical reaction. The electrodes... 

The steady-state flux of hydrated protons in the agglomerate is due to diffusion and migration in the internal electric field. It is dictated by the Nernst-Planck equation, with a sink term, ip, due to electrochemical reactions at the dispersed Pt water interfaces. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

The physics of multiple phases through a porous medium is further complicated here with phase change and the sources and sinks associated with the electrochemical reaction. The equations used to describe transport in the gas diffusion layers are given below. 

Fuel cell electrodes are porous gas diffusion electrodes, which are usually described in modeling using homogenization [144]. This means that the pore electrolyte and the electrode material share the same geometrical domain and the electric potential in the electronic and ionic conductors are present in the same geometrical domain. Also the concentration variables for the species in the gas phase, the species dissolved in the electrolyte, and the constituents of the electrolyte may be present in the same geometrical domain defined by the gas diffusion electrodes. The electrochemical reactions that occur at the interface between the pore electrolyte and the electrode are introduced as sources or sinks in the material and current balances. To calculate the chemical composition in the electrolyte and in the gas phase in every point in space in a geometrical domain, material balances for each of the species in the solution as well as a conservation of mass for the whole have to be defined. The conservation of mass for the whole solution may eliminate one of the species material balances, which for a dilute solution usually is the solvent s material balance. The constitutive relations in the electrolyte may be the... 

Note that an electrochemical reaction consumes gaseous species that disappear from the gas phase and form species dissolved in the liquid electrolyte. For example, hydrogen oxidation forms hydrogen ions, where the net reaction is then a consumption of hydrogen in the anode gas in the gas pores. This means that the electrochemical reactions are mass sinks or sources in the mass balances for the gaseous mixture in the pores. In the case of hydrogen oxidation, the... 

The temperature distribution in the fuel cell is described by the equations for the conservation of energy, which results in the heat transfer equations with heat production from the electrochemical reactions and other sources and sinks (ohmic and protonic heating, irreversible reaction heat, and reaction entropy, and heat transfer between the phases) [145]. The heat transfer equations are defined for porous and free media, and in solids. The flux may take into account heat transfer by conduction, advection, and radiation. Alternatively, and depending on the size of the cell and experimental setup, heat transfer can be described using two different temperature fields in two coupled thermal balances, where sources and sinks in the respective thermal balance represent transfer of heat between the two phases. This is done in the presence of large temperamre difference between the gas temperature (fluid-(gas)-phase temperature) and the cell operating temperature (sohd-phase temperature). 

All relevant electrochemical reactions in PEFCs exhibit peculiar sensitivities to the surface structure of the catalyst (Boudart, 1969). The abundances of the different surface sites, for example, edge sites, comer sites, or sites on crystalline facets, are related to the size of nanopartieles (Kinoshita, 1990). Support-particle interactions may alter the electronic stmeture of catalyst surface atoms at the rims with the support (Mukerjee, 2003). Moreover, the support may serve as a source or sink of reactants via the so-called spillover effect (Eikerling et al., 2003 Liu et al., 1999 Wang et al., 2010 Zhdanov and Kasemo, 2000). 

In Equation 6.72, the first term represents the unsteady accumulation term S is the volumetric source or sink term given by the three-phase electrochemical reaction region in the active layer of the electrodes. The volumetric reaction term is neglected from the mass species transport equation for the electrode if the active layer is assumed as the electrode-membrane interface with surface reaction and this is taken into account in the assigned boundary condition at the interface. 

The CE (also known as auxiliary electrode), is an electrode that is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g., platinum, gold, graphite, and glassy carbon) and it does not participate in the electrochemical reaction (Thomas and Henze, 2001). Because the current is flowing between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be higher than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical process imder investigation. 

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