Small-Cell Current Density

Perry, Newman and Cairns (Perry et al., 1998) obtained a numerical solution to the problem of CCL performance and provided the asymptotic analytical solutions for large and small cell current densities jo- Eikerling and Kornyshev (Eikerling and Kornyshev, 1998) derived the explicit analytical solution to the system for the case of small overpotentials in the general case they reported numerical results. 

Finally, at small cell current densities, expansion of the arcsinh function simplifies Equation 1.77 to... 

Note that both Ip and Id are independent of the exchange current density f. Physically, the dependence on f appears in the problem if both Ip and Id are much larger than Icl- This case corresponds to a small current regime discussed in the section Small Cell Current Density. If one of Id and Ip is less than Icl, the ratedetermining process is the transport of the respective reactant, and the reaction penetration depth is independent of the catalyst active surface, which is incorporated in/. 

A small amount of sulfur in the fuel dramatically degrades the performance of Ni-YSZ anodes due to the adsorption of sulfur on Ni surfaces. The extent of sulfur poisoning, as measured by the relative increase in cell resistance, always increases with H2S concentration in the fuel, but decreases with cell operating temperature and cell current density. Sulfur poisoning of Ni-based anode is generally more reversible as the cell temperature increases and as H2S concentration or exposure time is reduced. 

Figure 2.12. Capillary pressure as a function of position in the CCL for different fuel cell current densities as indicated on the graphs. For small jo, Tc z) indicating operation in the optimal wetting state. For jo >parts of the CCL are completely flooded, i.e. r z)>ryi, and the fuel cell performance is critically impaired [25],...

The Joule term in (2.149) is proportional to the square of cell current density. The distribution of local current over the cell surface is usually very nonuniform and we may expect the effects due to temperature variation along the cell surface. Furthermore, in stacks with poor heat management these thermal nonuniformities may be further enhanced. Though temperature variation across the CL in the stack is still small, the absolute temperatures at different points of the cell surface may differ quite strongly. The respective temperature fields and effects can be studied with the models of a higher dimensionality (Chapter 5). These models usually utilize the boundary condition (2.149), where T, q and jo are considered as local values. 

FIGURE 1.18 Schematic of the two large-current regimes of CCL operation for the cases of (a) poor proton transport and (b) poor oxygen transport. The membrane is at the left side, while the GDL is on the right. In either case, the electrochemical conversion runs in a small conversion domain or Ij) at the CCL interface, throngh which poorly transported particles arrive. Note that in both the cases, the thickness of the conversion domain is inversely proportional to the cell current density. 

Thus, at small currents, the overpotential drop increases linearly with the cell current density. 

In this case, the PEM operates like a linear ohmic resistance, with irreversible voltage losses t]pem = jolpEM/ p, where jo is the fuel cell current density. In reality, this behavior is only observed in the limit of small Jo- At normal current densities of fuel cell operation, y o l A cm , the electro-osmotic coupling between proton and water fluxes causes nonuniform water distributions, which lead to nonlinear effects in tipem. These deviations result in a critical current density Jpc, at which the increase in tipem incurs a dramatic decrease of the cell voltage. It is, thus, crucial to develop membrane models that could predict the value of Jpc on the basis of primary experimental data on structure and transport properties. 

To rationalize the effect of oxygen stoichiometry X on the potential loss, consider first the case of a small cell current. In this case, the local polarization curve is given by Equation 5.43. To calculate the polarization curve as a function of mean cell current density rjo (/), it is advisable to divide the terms under the logarithms in Equation 5.43... 

Figure 5.10 shows the overpotentials corresponding to the terms in Equation 5.74 for curve 2. At a typical current of 1 A cm , the ratio of activation resistive CCL transport losses is about 2 1 0.1. The transport loss in the GDL is small. Note that the potential loss due to oxygen transport in the CCL markedly increases with the growth of the cell current density. 

Equations 5.118 and 5.119 determine the small-current impedance spectrum as a function of the dimensionless cell current density. Examples of these spectra are shown in Figure 5.16. 

Note that the peak value of f is 2.5 times larger than the mean cell current density, that is, a small ring around the spot generates much higher proton current on the anode side (Figure 5.39b). The Joule heat in the membrane is proportional to the square of the proton current density hence, inside this ring, a risk of local overheat and of membrane drying increases dramatically. 

The coefficient on the right-hand side of the equation is 61 mV. Now, assuming no external current is drawn, = 0, and a cathode exchange current density of 10 A/cm, the overpotential caused by current losses versus the internal current density is shown in Figure 5.26. For an internal current density of 1 mA/cm, the cell overpotential is 0.28 V. From the figure, we see a steep increase in overpotential at small internal current density. Thus, even if the cell current density to the load is zero, the open-circuit voltage of the cell is 0.92 V for an internal current density of 1 mA/cm. ... 

Small amounts of propionitrile and bis(cyanoethyl) ether are formed as by-products. The hydrogen ions are formed from water at the anode and pass to the cathode through a membrane. The catholyte that is continuously recirculated in the cell consists of a mixture of acrylonitrile, water, and a tetraalkylammonium salt the anolyte is recirculated aqueous sulfuric acid. A quantity of catholyte is continuously removed for recovery of adiponitrile and unreacted acrylonitrile the latter is fed back to the catholyte with fresh acrylonitrile. Oxygen that is produced at the anodes is vented and water is added to the circulating anolyte to replace the water that is lost through electrolysis. The operating temperature of the cell is ca 50—60°C. Current densities are 0.25-1.5 A/cm (see Electrochemical processing). 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

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