Cell Current Density

Fig. 4-4 Cell current densities at a defect with an increase in potential AU = 0.5 V, X = 200 /xS cm l —J(d) from Eq. (4-12) without taking into account polarization ( = 0) ...

Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than lO m ... 

There are some differences in the observed dependence of sulfur poisoning behavior on cell current or voltage. For cell testing carried out under the galvanostatic condition, Singhal et al. [59] reported that the relative power output drop caused by exposure to 10 ppm H2S increased from 10.3 to 15.6% when the cell current density increased from 160 to 250 mA/cm2 at 1000°C. Similarly, Waldbillig et al. [65] also reported that when a hydrogen fuel with 1 ppm H2S was used, the relative drop in cell power output was 6.5, 9.8, and 11.8% for a constant cell current density of 250, 500, and 990 mA/cm2, respectively at 750°C. Xia and Birss [74] indicated that the relative cell power output drop caused by 10 ppm H2S increased from 19 to 56% when the current density increased from 130 to 400 mA/cm2 at 800°C. 

However, now there is still the uncertainty as to the relative increase in anode/ electrolyte interfacial resistance under different current densities. Primdahl and Mogensen [39] found that the relative increase in anode interfacial resistance due to sulfur poisoning is independent of temperature and cell current density (up to 100 mA/cm2) when the anode was subject to 35 ppm H2S at 1000°C. Whether this is also the case when the cell temperature is lower (i.e., at 750°C), the H2S concentration is lower (i.e., 1 ppm), and the current density is higher (i.e., up to 1 A/cm2) is not clear at the current stage. 

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. 

Side-reactions As soon as the cell current density surpasses the limiting current density of one reaction, the electrode potential rises until additionally another reaction takes place (in Fig. 1... 

If there are no detrimental organic side reactions, a cell current density in excess of the limiting current density - and as result a loss of current efficiency - may be acceptable for laboratory scale experiments. For example, a hydrogen evolution parallel to an electroorganic cathodic reduction can even be advantageous as it improves the mass transfer by moving gas bubbles and thus enhances the organic cathodic reduction. 

PEM resistance in operational PEFC as a function of the fuel cell current density, comparing experimental data (dots) and calculated results from a performance model based on the hydraulic permeation model for various applied gas pressure differences between anode and cathode compartments. (Reprinted from S. Renganathan et al. Journal of Power Sources 160 (2006) 386-397. Copyright 2006, with permission from Elsevier.)... 

O2 consumption rate becomes smaller under 0.7 V, the O2 concentration at the reaction surface recovers, thus leading to an increase in the cell current density. The current rise time corresponds well with the characteristic time scale of gas phase transport as analyzed above. The rise in the cell current, however, experiences an overshoot because the polymer membrane still maintains a higher water content corresponding to 0.6 V. It then takes about 15 s for the membrane to adjust its water content at the steady state corresponding to 0.7 V. This numerical example clearly illustrates the profound impact of water management on transient dynamics of low humidity PEFC engines where the polymer membrane relies on reaction water for hydration or dehydration. 

Fig. 7.185. In a self-driving cell, the plot of cell overpotential vs. log cell current density should be a straight line if the charge transfers at both electrodes are both rate controlling and valid under the high-field approximation. An apparent /0 for the cell as a whole can be deduced.

The model equations are solved at every time step based on the given instantaneous boundary conditions. The solution starts by solving all nodal electrochemical reaction rates (current densities) given the specified cell voltage (or total current). If a quasi-steady activation loss is assumed (i.e., no double-layer dynamics to be solved), an iterative approach is used to determine the cell current densities at each node - node current at each time step is iterated so as to ensure a uniform cell voltage (e.g., to within 4 microvolt). The balance equation to be iterated at each node is Voeii = EN(I) - J2... 

In contrast to classical chemical reactors, a fuel cell provides the possibility to control the reaction rate directly from outside by setting the cell current, because the local cell current density and the local reaction rate are related by a constant factor. This operation of a fuel cell at constant cell current is more important than the potentiostatic operation from a technical point of view, as fuel cells typically are characterized by current-voltage plots. Because the integral Eq. (15) has to be included in the analysis, the investigation of the galvanostatic operation is more difficult and requires numerical methods. In the following, numerical bifurcation... 

The relationship between the cell voltage and the cell current density can be simply written as the following equation if the fuel cell polarization is larger than 60 mV ... 

Recent reports [22, 23] have demonstrated better CO tolerance with higher loadings (1-2 mg/cm ) PtRu catalysts in PEFC anodes, particularly at cell current densities lower than 200 mA/cm. In contrast, a thin-fihn anode catalyst of very low PtRu loading, prepared as a composite of carbon-supported PtRu (0.15 mg/cm ) and recast ionomer [14], did not exhibit lower losses when 5-20 ppm CO was introduced into the hydrogen feed stream [21]. The same PtRu catalyst was successful, however, in... 

The steady-state water profile across the ionomeric membrane for given cell current density, external humidification conditions, and differential pressurization, is the resultant of these electroosmotic, diffusive, and hydraulic fluxes. 

Fig. 52. Variation of methanol permeation rate in a polymer electrolyte fuel cell at elevated temperature with cell current density for different methanol feed concentrations. The results show that, for methanol concentrations under 1 m, methanol is effectively consumed at the anode, thus minimizing the permeation rate [117], (Reprinted by permission of the Electrochemical Society).

Fig. 23 Air cathode catalyst mass utilization (A mg-1 Pt) for different types of catalyst layers as developed chronologically for hydrogen/air PEFC. Squares PTFE-bonded Pt black at 4 mg Pt/cm2 circles ionomer-impregnated, PA- type electrodes (0.45 mg Pt/cm2) triangles thin-film Pt/C//ionomer composite (0.13 mg Pt/cm2). The relative advantage of thin-film catalyst layers is seen to increase with cell current density, as expected from the lower transport limitations involved (see Sect. 8.3.7.2.3) [10,11].

This expression shows that fuel utilization levels as high as 90% can be achieved even when the membrane is quite leaky to fuel, that is, when /crossover /cell=o is quite large. For example, with /crossover /cen=o as high as 50% of the cell current density targeted, operation at /ceii//iim,an = 0-85 enables, according to Eq. (62), rj i as high as 93% with the leaky membrane. 

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