Half cell element

Eig. 19. CME monopolar electrolyzer a, membrane b, cathode element c, half-cathode element d, current distributor e. Teflon tube f, CI2 + depleted brine manifold g, conductor rod h, CI2 + depleted brine outlet nozzle i, base frame j, recycled NaOH manifold k, recycled NaOH inlet nozzle 1, gasket (the gasket-to-element ratio is quite small) m, tie rod n, anode element o, H2 + NaOH manifold p, end plate, q, under cell bus bar (simplifies piping... 

Halbelement, n. Elec.) half-cell, half-element, halben, v.t. halve. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

It was mentioned earlier that the equilibrium cell voltage A%, is equal to the difference between the equilibrium potentials of its half-cells e.g., for the Daniell element,... 

The elemental reaction used to describe a redox reaction is the half reaction, usually written as a reduction, as in the following case for the reduction of oxygen atoms in O2 (oxidation state 0) to H2O (oxidation state —2). The half-cell potential, E°, is given in volts after the reaction ... 

In each case, the set up consist of two half cells, each containing an electrode dipping into a solution of an appropriate electrolyte, separated by a salt bridge or similar device. Atoms of elements having a greater tendency to lose electrons (Ni, Zn) are oxidized at the anode, giving up electrons which travel through the external circuit to the cathode, where they combine with the cation (Cu2+, H+) which is most readily reduced. An illustrative list of cells with different types of electrode is shown in Table 6.9, while an illustrative list of various types of half-cell is shown in Table 6.10. 

There have principally been two main pathways by which cells have been described. One description begins from the very basic elements relating to the fact that when two suitable half-cells are combined, an electrochemical cell results. The combination is built by bringing the solutions in the half-cells into communication, so that ions can pass between them. If these two solutions are similar, no liquid junction is present, and one has a cell with an absence of transference (Figure 6.6). If the solutions are dissimilar, the transport of ions across the junction will bring about irreversible changes in the two cells, and one has a cell with presence of transference. 

In Table 7-1 the relative tendencies of certain elements to react were listed qualitatively. We can give a quantitative measure of relative tendency to react, called standard reduction potential, as shown in Table 14-2. In this table, the standard half-cell potential for each half-reaction, as a reduction, is tabulated in order with the highest potential first. If we turn these half-reactions around, we change the signs of the potentials and we get oxidation potentials. We thus have half-reactions including both elementary metals and elementary nonmetals in the same table, as well as many half-reactions that do... 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

Ref. Year Precursor materials Half cell RRDE PEMFC ESR IR Mossbr NMR Raman SIMS UV XAS XPS XRD SEM TEM Elemental Thermal... 

In the same procedure, electrolyser elements capable of ODC operation as well as elements in the hydrogen mode are tested in parallel. As a supplementary effect it was seen that the performance of the element (which in the anode half-cell was optimised for the special demands of the thermohydraulics of finite-gap ODC operation) demonstrated excellent operational results. The standardised power consumption of 4kAm-2 remains below 2050 kWh tonne-1 NaOH even after several months of operation. 

The standard electrode potential of an element is defined as its electrical potential when it is in contact with a molar solution of its ions. For redox systems, the standard redox potential is that developed by a solution containing molar concentrations of both ionic forms. Any half-cell will be able to oxidize (i.e. accept electrons from) any other half-cell which has a lower electrode potential (Table 4.1). 

Figure 54. Measured (a) and simulated (b) effect of electrode misalignment, (a) Total-cell and balf-cell impedances of a symmetric LSC/rare-earth-doped ceria/LSC cell with nominally identical porous LSC x= 0.4) electrodes, measured at 750 °C in air based on tbe cell geometry shown. (b) Finite-element calculation of tbe total-cell and half-cell impedances of a symmetric cell with identical R—C electrodes, assuming a misalignment of the two working electrodes (d) equal to the thickness of the electrolyte (L). ...

Figure 55. Simulated half-cell impedances of the cell shown in Figure 53, calculated using finite-element analysis. (a) Half-cell responses assuming an electrode misalignment dlL equal to 1, as defined in Figure 54c. (b) Half-cell responses assuming perfect electrode alignment [dlL = 0).

When faced with a cell drawing or a line diagram, first write reduction reactions for each half-cell. To do this, look in the cell for cm element in two oxidation states. For the cell... 

We described the left half-cell in terms of a redox reaction involving Pb because Pb is the element that appears in two oxidation states. We would not write a reaction such as F2(g) + 2e 2F, because F2(g) is not shown in the line diagram of the cell. 

But what is the reaction in the left half-cell The only element we find in two oxidation states is hydrogen. We see that H2(g) bubbles into the cell, and we also realize that every aqueous solution contains H h. Therefore, hydrogen is present in two oxidation states, and the halfreaction can be written as... 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-cell reactions which control the rate of corrosion. 

Europium, Eu, is one of the lanthanide elements.) Use the data in Table 18.1 to calculate the standard reduction potential for the Eu3+/Eu2+ half-cell. 

Of course, these reactions may be very much more complicated. Since the pH is specified, H + is not included as a reactant, and a reactant may be a sum of species if the reactant has pKs in the pH region of interest. These biochemical reactions do balance atoms of elements other than hydrogen, but they do not balance electric charges. When the half-reactions occur in half-cells connected by a KC1 salt bridge, the difference in electric potential between the metallic electrodes... 

A battery requires two half cells, each of which must involve two oxidation states of an element. Thus, in the Daniell cell, one of the half cells consists of copper metal (oxidation number = 0) in contact with... 

In these formulae / and Tin indicate the standard electrode potentials which are established, when all the half cell components arc in their standard state and have activities equalling unity. The relation between the standard oxidation and standard reduction potentials of the same element is exactly the same as those formed with other potentials, i. e. for example s) = — n. ... 

Further, it follows from the table that the reduced form of any metal or ion in the standard state (a = 1) is capable reducing an oxidized form of any metal or ion in the standard state with a less positive standard oxidation potential. Thus e. g. with the element Pt Sn++, Sn+-n + is e° = — 0.15 V and with the half cell Pt Hga++, Hg++ is e° = — 0.910 V. The reduced form of the first half... 

It has already been mentioned that it is important to remember in which direction the reaction proceeds in half cells, when calculating the EMF of a cell from the potentials of the half cells, forming that cell, and to draw a correct distinction between the quantities e° and 7t°. Thus e. g. the standard oxidation potential of the element Pt Sn++, Sn++++ has a negative value e° = — 0.15 V. This means that at the electrode in combination with the... 

When measuring an unknown electrode potential the half cell under examination M M+ is combined with a reference electrode (e. g. a calomel one) in the manner illustrated in Fig. 13. In this figure A is the calomel electrode, B the element to be measured and C the salt bridge, which forms the electric connection between A and B and eliminates at the same time the liquid junction... 

Reactive electrodes refer mostly to metals from the alkaline (e.g., lithium, sodium) and the alkaline earth (e.g., calcium, magnesium) groups. These metals may react spontaneously with most of the nonaqueous polar solvents, salt anions containing elements in a high oxidation state (e.g., C104 , AsF6 , PF6 , SO CF ) and atmospheric components (02, C02, H20, N2). Note that ah the polar solvents have groups that may contain C—O, C—S, C—N, C—Cl, C—F, S—O, S—Cl, etc. These bonds can be attacked by active metals to form ionic species, and thus the electrode-solution reactions may produce reduction products that are more stable thermodynamically than the mother solution components. Consequently, active metals in nonaqueous systems are always covered by surface films [46], When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47], In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/MZ+ half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/Mz+ couple [48],... 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

Weston normal element (cell) — Electrochemical -> standard cell showing a particularly stable and reproducible cell voltage. In the international Weston normal element a cadmium amalgam (cadmium content in the solid phase approx. 15 wt %, in the liquid phase approx. 5wt%, total average 12 to 12.5 wt%, the electrode potential depends only on the temperature, not on the mass ratio of liquid and solid phases) and a mercury electrode (half-cell) are combined according to... 

This system is known as the standard hydrogen electrode (SHE). The electric potential established by this equilibrium is assigned the value of 0 V. Therefore, when the SHE is compared with a half-cell comprised of a dissolved chemical species in equilibrium with its solid, elemental form or with an inert electrode (i.e., an electrode made of a material like platinum that facilitates electron exchange but does not react with the medium), the electric potential can be measured with a voltmeter and a table of potentials can thus be generated. 

In this PEVD system, the source (O) will be a vapor phase, which contains elemental solid-state transported reactant (A), and an anode half-cell reaction... 

The Ag/AgCl electrode is used both as an internal reference element in potentiometric ISEs, and as an external reference electrode half-cell of constant potential, required to complete a potentiometric cell (see Figure 4-1). In both cases, the Ag/AgCl electrode must be in equilibrium with a solution of constant chloride ion activity. 

A half-cell contains the oxidized and reduced forms of an element, or other more complex species, in contact with one another. A common kind of half-cell consists of a piece of metal (the electrode) immersed in a solution of its ions. Consider two such half-cells in separate beakers (Figure 21-6). The electrodes are connected by a wire. A voltmeter can be inserted into the circuit to measure the potential difference between the two electrodes, or an ammeter can be inserted to measure the current flow. The electric current is the result of the spontaneous redox reaction that occurs. We measure the potential of the cell. 

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