Mobile electrolyte

The majority of alkaline fuel cells are of this type. The main advantage of having the mobile electrolyte is that it permits the electrolyte to be removed and replaced from time to time. This is necessary because, as well as the desired fuel cell reactions of equations 5.1 and 5.2, the carbon dioxide in the air will react with the potassium hydroxide electrolyte 

The potassium hydroxide is thus gradually changed to potassium carbonate. The effect of this is that the concentration of OH ions reduces as they are replaced with carbonate COs ions, which greatly affects the performance of the cell. This major difficulty is discussed further in Section 5.6, but one way of at least reducing it is to remove the CO2 from the air as much as possible of, and this is done using a CO2 scrubber in the cathode air supply system. However, it is impossible to remove all the carbon dioxide, so the electrolyte will inevitably deteriorate and require replacing at some point. This mobile system allows that to be done reasonably easily, and of course potassium hydroxide solution is of very low cost. 

The disadvantages of the mobile electrolyte centre around the extra equipment needed. A pump is needed, and the fluid to be pumped is corrosive. The extra pipework means more possibilities for leaks, and the surface tension of KOH solution makes for a fluid that is prone to find its way through the smallest of gaps. Also, it becomes harder to design a system that will work in any orientation. 

To summarise then, the main advantages of the mobile electrolyte-type alkaline fuel cell are as follows  

This mobile electrolyte system was used by Bacon in his historic alkaline fuel cells of the 1950s and in the Apollo mission fuel cells. It is almost universally used in terrestrial systems, but the Shuttle Orbiter vehicles use a static electrolyte, as described in the next section. 

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In the opposite case to that considered above, Cs >ic2 and the difference in concentration Cs of the mobile electrolyte inside and outside the gel may be comparable in magnitude to the concentration C2/ of counter-anions. Hence the ion osmotic pressure is greatly reduced. Calculation of Cs — Cs for this case (see Appendix B) gives for the osmotic pressure due to the mobile ions... 

Since we are interested in the excess mobile electrolyte concentration, we introduce the variable tj defined by... 

It is interesting to compare these results with the electrophoretic measurements made under identical electrolyte concentrations. Figure 8 shows that the variation of electrophoretic mobility with sodium chloride concentration is different for the bare and the PVA-covered particles. For the bare particles, the mobility remains constant up to a certain salt concentration, then increases to a maximum and decreases sharply, finally approaching zero. The maximum in electrophoretic mobility-electrolyte concentration curve with bare particles has been explained earlier (21) by postulating the adsorption of chloride ions on hydrophobic polystyrene particles. In contrast, for the PVA-covered particles, the mobility decreases with increasing electrolyte concentration until it approaches zero at high salt concentration. 

In many cell designs, the electrolyte is circulated (mobile electrolyte) so that heat can be removed and water eliminated by evaporation (6). Since KOH has the highest conductance among the alkaline hydroxides, it is the preferred electrolyte. Approximately of the water formed at the anode migrates across the electrolyte and exits in the cathode. 

Figure 1 A distributed resistor network models approximately how the apphed potential is distributed across a DSSC under steady-state conditions. For various values of the interparticle resistance, fiT,o2, and the interfacial charge transfer resistance, Rc the voltage is calculated for each node of the Ti02 network, labeled Vj through V . This is purely an electrical model that does not take mobile electrolytes into account and, therefore, potentials at the nodes are electrical potentials, whereas in a DSSC, all internal potentials are electrochemical in nature.

The distributed resistor model neglects the effect of mobile electrolyte ions. Much of our following discussion of the electrolyte s influence neglects, for simplicity, the distributed resistance. In a real dye cell, both effects operate simultaneously. Both tend toward the same result An applied potential will be more or less confined near the substrate electrode, depending on the relative rates of charge transport and interfacial charge transfer and on the concentration of electrolyte. 

This analysis is valid for all solar cells that consist of interpenetrating chemical phases—of which there are an increasing number [51]. For those without mobile ions, the distributed resistor model alone leads to the conclusion that dark currents cannot be quantitatively compared to photocurrents for those with mobile electrolyte, the effect is quantitatively reinforced by the field-induced motion of the electrolyte ions. 

For this reason (and others), it is not trivial to make a solid-state version of the dye cell. Initial attempts did not include a mobile electrolyte and thus had no way of neutralizing the Coulomb attraction between the photogenerated charge pairs [42,53]. The best results were achieved by Tennakone et al. [13] in a cell with solid Cul as the hole conductor—in which the ionic mobility of the Cul may have helped neutralize the Coulomb attraction. Later attempts included mobile electrolyte ions, which improved performance [9,54]. 

This has implications for the design of high-surface-area solar cells in general If the bulk of the device is essentially field-free at equilibrium, then mobile electrolyte and nanoporosity are required to eliminate the photoinduced electric fields that would otherwise inhibit charge-carrier separation. On the other hand, if the particle size is substantially larger than in the conventional dye cell or if there is no mobile electrolyte, then an interfacial or bulk built-in electric field... 

The photoinduced difference between the quasi-Fermi level for electrons in the Ti02 and the quasi-Fermi level for holes in solution, EFn = EFn, - EFp.solution, sets an upper limit to the photovoltage, Voc, because it is this potential difference and the fact that electrons and holes are confined to separate chemical phases that drives electrons toward the substrate electrode and holes toward the counterelectrode. Although VEFn is mainly comprised of V x in DSSCs, there is, nevertheless, a possible role for q at interfaces where the field cannot be entirely screened by mobile electrolyte. 

Organic semiconductor photovoltaic cells share many characteristics with both DSSCs and conventional cells. Charge generation occurs almost exclusively by interfacial exciton dissociation, as in DSSCs, but, in contrast, OPV cells usually contain no mobile electrolyte and thus rely on Vcharge separation. OPV cells may have planar interfaces, like conventional PV cells, or highly structured interfaces, like DSSCs. They provide a conceptual and experimental bridge between DSSCs and conventional solar cells. 

The Poisson equation is valid under conditions of zero ionic strength. If dissolved, mobile electrolytes are present in the solvent, the Poisson-Boltzmann (PB) equation applies instead... 

Now we assume that the distribution of mobile electrolyte ions is always at thermodynamic equilibrium within the membranes as well as in the surrounding solution, then the potential far inside the membranes remains constant during interaction. In the present system the potentials far inside membranes 1 and 2, respectively, remain constant at iAdon i Adon 2-... 

Detailed examination of the electron density in the supercage of Ca- and La-exchanged faujasites showed a nonzero electron density without any really significant peaks. Baur (7) correlated the absence of definite peaks with a variety of physical data, suggesting that the water molecules and cations act as a mobile electrolyte solution. 

CEC Distribution between a solid stationary phase and mobile electrolyte solution Neutral compounds Weak acids and bases Ions... 

Electrostatics. Mobile electrolyte ions reorganize to effect charge screening and compensate charging of nanoparticles. This may improve conduction but at possible cost to the cell voltage. 

Figure 9. Ionic-strength dependence of solute permeation in alkaline solutions. Large circles with + or - signs stand for permeating solute molecules, small circles with - signs for membrane-fixed charges, and + and - signs for small mobile electrolyte ions.

Figure 7.30(b) shows an electrode with solid contact gel. The gel is sticky and serves both as contact electrolyte and for electrode fixation. For this electrode, EEA = EA. The electrolyte conductivity is rather low because the solution is a gel with low ionic mobility. Electrolyte series resistance may be the dominating factor of electrode/skin impedance at high frequencies. This may introduce problems for use (e.g., for impedance plethysmography around 50 kHz). With this electrode type the skin is not wetted. With such constructions it is... 

The solution properties of polyelectrolytes in general are markedly different from those of polyelectrolyte solutions with added salts. These differences are very strikingly revealed in their viscometric behaviors. Viscosity, as pointed out in the previous section, is related to the size of polymer molecules and therefore is affected by molecular expansion. When a small amount of a simple salt, such as sodium chloride, is added to a dilute polyelectrolyte solution, the ionic strength of the solution outside of the polymer coil is increased relative to the strength of the solution inside of the coil. Consequently, some of the mobile electrolyte diffuses into the polyion coil and the thickness of the ionic atmosphere around the polymer chain is reduced. This effect produces a significant contraction of the polyion coil and is reflected in decreased values of the viscosity. 

However, the requirement of a certain quantity of CO2 at the cathode to form the carbonate ion is a drawback in the plant management. Furthermore, the high contact resistances and the cathode resistance increase overpotentials and decrease efficiency, and concentration polarization limits the electric power densities. Finally, the main challenge for MCFC developers derives from the very corrosive and mobile electrolyte, which requires the use of nickel and high-grade stainless steel for the cell hardware. Higher temperatures promote material problems, and also cathode... 

The basic structure of the mobile electrolyte fuel cell is shown below in Figure 5.3. The KOH solution is pumped around the fuel cell. Hydrogen is supplied to the anode. 

Figure 5.3 Diagram of an alkaline fuel cell with mobile electrolyte. The electrolyte also serves as the fuel cell coolant. Most terrestrial systems are of this type. 

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