The Anode Half-Cell Reaction

The different intermediate states are called S0-S4, where So corresponds to water, and S4 to O2. Between each state, an electron is removed, and four protons are also removed from So-. S 4. 

For an electrode surface, it is not meaningful just to remove electrons since for the electrode surface, the number of electrons is not constant, but only the chemical potential for the electrons is constant. Because the electrode is an electronic conductor, the electrons can flow to and from the surface without any barrier or delay. In order 

The reactions mechanisms the catalyst will follow may depend on the electrode material. The difference is that in reaction mechanism 2, the last reaction step doesn t include an electron and proton. It is clear that reaction mechanism 2 can only be relevant if the barrier for recombining the two oxygen atoms is small. Later, it will be argued why including mechanism 2 in the analysis doesn t change the conclusion. 

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This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

The anodic half-cell reaction occurring at the photoanode/ electrolyte interface may be written ... 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

Following the procedure described previously (using ideal gas as secondary reference state for the SHE) it is easy to show that for the anode half-cell reaction (Eq. 3), the standard cell potential with respect to SHE is given by... 

Alkaline batteries A more efficient alkaline dry cell, shown in Figure 20.9, is replacing the standard zinc-carbon dry cell in many applications. In the alkaline cell, the zinc is in a powdered form, which provides more surface area for reaction. The zinc is mixed in a paste with potassium hydroxide, a strong base, and the paste is contained in a steel case. The cathode mixture is manganese(IV) oxide, also mixed with potassium hydroxide. The anode half-cell reaction is as follows. 

Fig. 3 Electrochemical framework for intergranular corrosion described by an Evans diagram depicting the anodic half-cell reaction kinetics for the grain boundary zone and the grain matrix. In this case, enhanced active dissolution occurs in both the grain boundary region and in the matrix. At a fixed potential, given by Egpp, the anodic dissolution rate is accelerated along the grain boundary compared to the matrix.

The relations established in the following are developed on the example of Equation (17.1) for the anodic half-cell reaction or Equation (17.3) for the overall cell reaction. 

Entropy change in the anodic half-cell reaction. -27... 

The anode catalyzes the oxidation of the fuel using 02- ions delivered through the electrolyte-producing electrons that flow through the external circuit to the cathode. If H2 is used as the fuel, the anode half-cell reaction is as follows ... 

Figure 9.3 The lead storage battery. The key to obtaining electrical energy from a redox chemical reaction is to physically separate the two half-cell reactions so that electrons are transferred from the anode through an external circuit to the cathode. In the process, electrical work is accomplished.

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

We suggest that these results arise from adsorption effects which are the cathodic complements to the anodic phenomena outlined above. The electrolysis half-cell reaction at the photocathode ... 

A review of photo-assisted electrolysis studies performed with p-type semiconductor photocathode/dark Pt anode systems suggests that a complementary phenomena arising from the presence of OH ions produced during the reduction half-cell reaction,... 

Although the anode half-cell always appears on the left in the shorthand notation, its location in a cell drawing is arbitrary. This means that you can t infer which electrode is the anode and which is the cathode from the location of the electrodes in a cell drawing. You must identify the electrodes based on whether each electrode half-reaction is an oxidation or a reduction. 

The anode is the main focus of this chapter (see Figure 3.2.1). The total half-cell reaction involves four charge transfer steps. 

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... 

Therefore for a given reaction to take place, the cell potential must be positive. The cell potential is taken as the difference between the two half-cell reactions, the one at the cathode minus the one at the anode. The half-cell potential exists because of the difference in the neutral state compared to the oxidized state, such as Fe/Fe + or, at the cathode, the difference between the neutral state and the reduced state, as in These reduction-oxidation (redox) potentials are measured relative to a standard half-cell potential. The chart shown in Table 2 lists potentials relative to the which is set as zero. 

A carbon (graphite) rod in the center of the dry cell serves as the cathode, but the reduction half-cell reaction takes place in the paste. An electrode made of a material that does not participate in the redox reaction is called an inactive electrode. The carbon rod in this type of dry cell is an inactive cathode. (Contrast this with the zinc case, which is an active anode because the zinc is oxidized.) The reduction half-cell reaction for this dry cell follows. 

The following equation represents the oxidation half-cell reaction at the anode where lead is oxidized from the zero oxidation state to the +2 oxidation state. 

Each electrode reaction, anode and cathode, or half-cell reaction has an associated energy level or electrical potential (volts) associated with it. Values of the standard equilibrium electrode reduction potentials E° at unit activity and 25°C may be obtained from the literature (de Bethune and Swendeman Loud, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, 1964). The overall electrochemical cell equilibrium potential either can be obtained from AG values or is equal to the cathode half-cell potential minus the anode half-cell potential, as shown above. 

To receive electrons from the oxidation half-reaction To relieve the buildup of positive charge in the anode half-cell... 

The lead battery is used primarily in cars where they deliver current to the start engine. The reason for this widely use through many years is that lead batteries work well with good performance at typical outdoor temperatures. The anode in the lead battery is the lead electrode while the cathode typically consists of a lead electrode covered with lead oxide. Both electrodes are placed in an electrolytic solution of sulphuric acid. The following half cell reaction takes place at the anode ... 

For the use in watches and pocket calculators small galvanic cells are used. There are numerous different types of dry cell batteries. In an acid dry cell battery the inner shell is made of zinc functioning as anode and a carbon rod in the centre of the cell, being in contact with solid Mn02 and solid NH4CI functioning as cathode. The following half cell reaction takes place at the anode ... 

A more modem version of the dry cell battery is the alkaline version where the solid NH4CI is replaced by KOH and NaOH. Hereby the following half cell reaction takes place at the anode ... 

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