Nemst potentials

A similar thermodynamic procedure is applicable to the electron transfer reaction, Eq. (32.4). By using the eqnilibrium condition, which expresses the equality of the sum of the electrochemical potentials of reactants and products, an expression for the equivalent Nemst potential dilference can be derived ... 

Since the separation between the tip and the surface is such that their respective double layers do not overlap, the nanostmcturing process can be described simply through the diffusion of the ions toward the surface. Thus, the concentration profiles of the diffusing ions dehne effective Nemst potential prohles that can be employed to predict the regions where the oversaturation conditions will contribute to metal nucleation and growth. 

FIGURE 36.5 Model calculation employed to study the dependence of the cluster diameter on the distance between the STM and the substrate surface for localized electrochemical nucle-ation and growth. The lower part of the figure shows the profile of the Nemst potential for the Co /Co potential as a function of the distance from the tip center. The fines indicate a constant Co concentration. (From Schindler et al., 2000, with permission from the American Institute of Physics.)... 

The question arises of the extent to which the build-up of an electrode potential may significantly alter the original concentration of the solution in which the electrode is placed. Let us take the example of a silver electrode. Once the electrode has been immersed in an Ag+ solution, part of the Ag+ ions will be discharged by precipitation of the corresponding amount of Ag and to an extent such that the Nemst potential has been reached. In fact, a double layer at the electrode/solution interface has been formed whose structure cannot be as precisely described as has appeared from the model proposed by... 

The concentration overpotential rjc is the component directly responsible for the steep increase in potential observed as the current approaches the limiting current, since the Nemst potential difference (Eq. 6) becomes very large as the concentration of the reacting ion at the electrode approaches... 

The ideal performance of a fuel cell is defined by its Nemst potential represented as cell voltage. The overall cell reactions corresponding to the individual electrode reactions listed in Table 2-1 are given in Table 2-2, along with the corresponding form of the Nemst equation. The Nemst... 

The standard Nemst potential (E°) is the ideal cell voltage at standard conditions. It does not include losses that are found in an operating fuel cell. Thus, it can be thought of as the open circuit voltage. 

Since each system achieves the same total fuel utilization (90%) across the same total area, each stack has the same average current density. Irreversible voltage loss is mainly a function of current density and stack temperature. Since these parameters are equivalent in each stack, it is assumed that the Nemst potential of each stack would be reduced by the same amount. 

For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nemst potential. The Nemst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. 

Two ASPEN (Advanced System for Process Engineering, public version) simulations compare the performance of conventional and networked fuel cell systems having identical recycle schemes and steam bottoming cycles. Each simulated system was composed of three MCFC stacks operating at the same temperature and pressure. The Nemst potential of each MCFC in both systems was reduced by 0.3 volts due to activation, concentration and ohmic voltage... 

Butler in 1924 developed the idea that the Nemst equilibrium potential for an electrochemical process is the potential at which the forward and back reactions proceed at the same rate [37]. Following this, Bowden and Rideal [38] introduced the term jo as the value of the forward and back current density at the reversible Nemst potential and wrote the Tafel equation in the form of Equation 1.6. 

The maximum open-circuit photo voltage, attainable in a dye-sensitized solar cell, is the difference between the Fermi level of the solid under illumination and the Nemst potential of the redox mediator. However, for these devices this limitation has not been achieved and Voc is in general much smaller. It appears that Voc is kinetically limited and the diode Equation 17.12 can be applied for an n-type semiconductor in a regenerative cell.23... 

In Chapter 6 we have seen that metal M will be deposited on the cathode from the solution of M"+ ions if the electrode potential E is more negative than the Nemst potential of the electrode M/M"+. However, it is known that in many cases metal M can be deposited on a foreign substrate S from a solution of M"+ ions at potentials more positive than the Nemst potential of M/M"+. This electrodeposition of metals is termed underpotential deposition (UPD). Thus, in terms of the actual electrode potential E during deposition and the Nemst equilibrium potential (M/M"+) and their difference AE = E — (M/M"+), we distinguish two types of electrodeposition ... 

In general it may be said that for the normal ion exchange membranes there is a concentration area for which the Nemst equation holds true. At high electrolyte concentrations deviations occur due to the participation of the co-ions in the transport. At very low concentrations deviations are found too. As causes are here mentioned the influence of unstirred surface layers and hydrolysis of the charged groups. With membranes of a high permeability to the solvent, the Nemst potential is not reached, because the transport of the solvent reduces the membrane potential. The decrease is some per cent at most. 

Fig. 6.23 Dependence of the Nemst potential on the density of the proton-binding sites and on adsorption. Upper curves within the bracketed sets correspond to the absence of adsorption. The lower curves show the effect of 100 mM charged adsorbate (adapted from Sandifer 1998)...

The conversion of the reaction at the specific temperature, pressure and initial gas compositions is governed by its thermodynamic equilibrium. According to Equation (2.16), the maximum available work of the fuel cell can be determined by the Nemst potential, which represents the electrical potential of the reaction. 

Furthermore, an average Nemst potential can be defined taking the integral overpotential into account, which is inversely proportional to the total required cell area. 

The corrected cell potential, Ecot, is obtained by subtracting the ohmic (jjohm), concentration (jjcone), and activation (//act) losses (i.e. overpotentials) from the ideal Nemst potential, E ... 

Pressure can affect the operation of a fuel cell in several ways, such as in its thermodynamic performance, transport of reactants, kinetics of reactants, etc. Here, only the more prominent pressure benefit to the fuel cell resulting from the Nemst potential will be considered. The Nemst potential is given by ... 

In this case, the operating temperature of the fuel cell (used in the Nemst potential, and overpotential calculations) is assumed to be the outlet cathode gas temperature. Initial fuel cell parameters used in the various simulations are summarized in Table 8.5. 

It is assumed that there is no temperature gradient in the discrete anode/ electrolyte/cathode unit, and so the Nemst potential is a function of the average temperature 1] in the ith anode/electrolyte/cathode unit and the oxygen partial pressure l h at both the electrodes. Accordingly, the Nemst potential is given as... 

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