Current-Producing Chemical Reaction

Reactions in batteries are chemical reactions between an oxidizer and a reducer. In reactions of this type, the reducer being oxidized releases electrons while the oxidizer 

Electrochemical Power Sources Batteries, Fuel Cells, and Supercapacitors, First Edition. Vladimir S. Bagotsky, Alexander M. Skundin, and Yurij M. Volfkovich 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

In the simple case a battery (cell) consists of two electrodes made of different materials immersed in an electrolyte. The electrodes are conducting metal plates or grids covered by reactants active mass), the oxidizer is present on one electrode, the reducer on the other. In silver-zinc cells the electrodes are metal grids, one covered with silver oxide and the other with zinc. An aqueous solution of KOH serves as electrolyte. Schematically, this system can be written as 

When these electrodes are placed into the common electrolyte enabling electrolytic contact between them, an open circuit voltage (OCV) e develops between them (here e = 1.6 V), zinc being the negative electrode. When they are additionally connected by an electronically conducting external circuit, the CXIV causes electrons 

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The different fuel-cell systems differ in the nature of the components selected, and thus in the nature of the current-producing chemical reaction. Each reaction is associated with a particular value of enthalpy and Gibbs free energy -AG) of the reaction and thus also with a particular value of the heat of reaction and of the thermodynamic electromotive force (EMF) e. 

In scouting work, Hasebe [13] described a concept for the application and design rules for a micro structured reactor plant. First the reasons for performing a chemical reaction in a micro device must be clear. Either the target product cannot be produced conventionally, or the production efficiency of the target product can be drastically improved. Currently many chemical reactions are performed in micro reactors (see, e.g., [9]), but it is not clear how many of them fit with the above-mentioned rules. 

The Electrolysis of Molten Sodium Chloride. Molten sodium chloride (the salt melts at 801° C) conducts an electric current, as do other molten salts. During the process of conducting the current a chemical reaction occurs—the salt is decomposed. If two electrodes (carbon rods) are dipped into a crucible containing molten sodium chloride and an electric potential (from a battery or generator) is applied, metallic sodium is produced at the negative electrode—the cathode—and chlorine gas at the positive electrode—the anode. Such electrical decomposition of a substance is called electrolysis. 

An electrochemical cell is a system of electrodes and electrolytes in which either chemical reactions produce electrical energy or electric current produces chemical change. 

Electrochemistry is concerned with the effect of electrical voltages and currents on chemical reactions (ionics) and chemical changes which produce the voltages and currents (electrodics). This is illustrated in Table 9.1 where ionics is governed by Faraday s laws, whereas electrodics is determined by the Nemst equation. 

Petroleum refining, also called petroleum processing, is the recovery and/or generation of usable or salable fractions and products from cmde oil, either by distillation or by chemical reaction of the cmde oil constituents under the effects of heat and pressure. Synthetic cmde oil, produced from tar sand (oil sand) bitumen, and heavier oils are also used as feedstocks in some refineries. Heavy oil conversion (1), as practiced in many refineries, does not fall into the category of synthetic fuels (syncmde) production. In terms of Hquid fuels from coal and other carbonaceous feedstocks, such as oil shale (qv), the concept of a synthetic fuels industry has diminished over the past several years as being uneconomical in light of current petroleum prices. 

Activation Processes. To be useful ia battery appHcations reactions must occur at a reasonable rate. The rate or abiUty of battery electrodes to produce current is determiaed by the kinetic processes of electrode operations, not by thermodynamics, which describes the characteristics of reactions at equihbrium when the forward and reverse reaction rates are equal. Electrochemical reaction kinetics (31—35) foUow the same general considerations as those of bulk chemical reactions. Two differences are a potential drop that exists between the electrode and the solution because of the electrical double layer at the electrode iaterface and the reaction that occurs at iaterfaces that are two-dimensional rather than ia the three-dimensional bulk. 

The most important chemical reaction of chi orohydrin s is dehydrochloriaation to produce epoxides. In the case of propylene oxide. The Dow Chemical Company is the only manufacturer ia the United States that still uses the chlorohydrin technology. In 1990 the U.S. propylene oxide production capacity was hsted as 1.43 x 10 t/yr, shared almost equally by Dow and Arco Chemical Co., which uses a process based on hydroperoxide iatermediates (69,70). More recentiy, Dow Europe SA, aimounced a decision to expand its propylene oxide capacity by 160,000 metric tons per year at the Stade, Germany site. This represents about a 40% iacrease over the current capacity (71). 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

Emission of li t accompanying the passage of an electric current through aqueous solutions and arising from chemical reactions of chemiluminescent species produced during electrolysis. 

Current flow in cells is attended by an overall chemical reaction, more particularly a current-producing (or current-consuming) reaction in which electrons do not appear explicitly. In the example reported above, decomposition of dissolved zinc chloride,... 

In symmetrical galvanic cells, cells consisting of two identical electrodes (e.g., zinc electrodes), current flow does not produce a net chemical reaction in the cell as a whole only a transfer of individual components occurs in the cell (in our example, metallic zinc is transferred from the anode to the cathode). 

It is typical that in Eq. (3.23) for the EMF, all terms containing the chemical potential of electrons in the electrodes cancel in pairs, since they are contained in the expressions for the Galvani potentials, both at the interface with the electrolyte and at the interface with the other electrode. This is due to the fact that the overall current-producing reaction comprises the transfer of electrons across the interface between two metals in addition to the electrode reactions. 

This gives rise to an important conclusion. For nonconsumable electrodes that are not involved in the current-producing reaction, and for which the chemical potential of the electrode material is not contained in the equation for electrode potential, the latter (in contrast to a Galvani potential) depends only on the type of reaction taking place it does not depend on the nature of the electrode itself. 

When the current is anodic, component Red is consumed and the equilibrium in the electrolyte close to the surface is disturbed reaction (13.37) will start to proceed from left to right, producing additional amounts of species Red. In this case the chemical precedes the electrochemical reaction. However, when the current is cathodic, substance Red is produced and the chemical reaction (13.37), now as a subsequent reaction, will occur from right to left. When component Ox rather than component Red is involved in the chemical reaction, this reaction will be the preceding reaction for cathodic currents, but otherwise all the results to be reported below remain valid. 

Each of the particles of Red produced in the chemical reaction will, after some (mean) time t, have been reconverted to A. Hence, when the current is anodic, only those particles of Red will be involved in the electrochemical reaction which within their own lifetime can reach the electrode surface by diffusion. This is possible only for particles produced close to the surface, within a thin layer of electrolyte called the reaction layer. Let this layer have a thickness 5,.. As a result of the electrochemical reaction, the concentration of substance Red in the reaction layer will vary from a value Cg at the outer boundary to the value Cg right next to the electrode within the layer a concentration gradient and a diffusion flux toward the surface are set up. 

Devices for the production of chemical energy differ from the ones described, in that different electrode reactions now occur at the cathode and anode. As the result of overall current-producing and current-consuming reactions, energy-rich products... 

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