Voltage loss

Time constant for open circuit voltage (when motor control will use delayed transfer to alternate sources on voltage loss). This value must include the effect of any capacitors applied on the load side of the motor controller. 

To date, as indicated in Fig. 17.3, the effort to reduce cell voltage has been focused on the membrane and the electrodes. In current commercial operations at 4 kA m-2 with a 2 mm electrode gap, the voltage loss of the membrane, anode and cathode has been reduced to approximately 350 mV, 50 mV and 100 mV, respectively, and thus a total of approximately 500 mV for these cell components, or less than one-third the voltage loss of these components in the early years of the commercial membrane process. 

Two distinct classes of cell design exist the monopolar and the bipolar. Most commercial stacks have the bipolar design, which means that the single cells are connected in series both electrically and geometrically. The bipolar cell design has the advantages of compactness and shorter current paths with lower voltage losses. 

Still, the CO tolerance is too low for practical purposes. Ideally, 1000 ppm CO or more should be tolerated without a voltage loss exceeding 20 mV. Moreover, the stability of binary and ternary catalysts under fuel cell operating conditions is an issue. 

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

H2 is the average partial pressure of H2 in the system. At 190°C (374°F), the presence of 10% CO2 in H2 should cause a voltage loss of about 2 mV. Thus, diluents in low concentrations are not expected to have a major effect on electrode performance however, relative to the total anode polarization (i.e., 3 mV/100 mA/cm ), the effects are large. It has been reported (16) that with pure H2, the cell voltage at 215 mA/cm remains nearly constant at H2 utilizations up to 90%, and then it decreases sharply at H2 utilizations above this value. 

As discussed previously, both temperature and CO concentration have a major influence on the oxidation of H2 on Pt in CO containing fuel gases. Benjamin et al. (35) derived Equation (5-12) for the voltage loss resulting from CO poisoning as a function of temperature... 

Electrolyte Structure Ohmic losses contribute about 65 mV loss at the beginning of life and may increase to as much as 145 mV by 40,000 hours (15). The majority of the voltage loss is in the electrolyte and the cathode components. The electrolyte offers the highest potential for reduction because 70% of the total cell ohmic loss occurs there. FCE investigated increasing the porosity of the electrolyte 5% to reduce the matrix resistance by 15%, and change the melt to Li/Na from Li/K to reduce the matrix resistivity by 40%. Work is continuing on the interaction of the electrolyte with the cathode components. At the present time, an electrolyte loss of 25% of the initial inventory can be projected with a low surface area cathode current collector and with the proper selection of material. 

Sulfur compounds H2S, COS, CS2, C4H4S Voltage losses Reaction with electrolyte via SO2... 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

The voltage losses in SOFCs are governed by ohmic losses in the cell components. The contribution to ohmic polarization (iR) in a tubular cell" is 45% from cathode, 18% from the anode, 12% from the electrolyte, and 25% from the interconnect, when these components have thickness (mm) of 2.2, 0.1, 0.04 and 0.085, respectively, and specific resistivities (ohm cm) at 1000°C of 0.013, 3 X 10, 10, and 1, respectively. The cathode iR dominates the total ohmic loss despite the higher specific resistivities of the electrolyte and cell interconnection because of the short conduction path through these components and the long current path in the plane of the cathode. 

If reversibility is assumed at the outlet of each stack, no voltage losses are deducted from the Nernst potential. Therefore, each shaded area represents the maximum power, which each cell could generate. 

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. 

Thin membranes have the advantage of low area specific conductivities and more favorable back diffusion of water in comparison with thicker membranes. In the former case, this means that membranes with lower conductivity values could be tolerated. Analysis of voltage loss versus membrane thickness and specific conductivity has revealed that, if a membrane voltage loss of 25 mV at a current density 1 A cm can be tolerated, then existing materials with conductivity values similar to Nation (0.1 S cm i) could be prepared as 20-30 pm thick membranes. However, thinner membranes also typically exhibit lower mechanical strength than their thicker counterparts and can therefore fail earlier. Therefore, future materials might be suitable with just half the specific conductivity if they can be prepared into membranes of half the thickness and still possess sufficient mechanical strength. ... 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

Under ideal operation of PEFCs, the membrane would retain a uniformly saturated level of hydration, providing the highest proton conductivity, (7p The PEM would therefore perform like a linear ohmic resistance, with irreversible voltage losses ... 

In general. Equations (6.59) and (6.61) highlight the importance of adjusting thickness and effective properties of transport and reaction in CLs in such a way that 6cl 3 Lcl- If we replace dcL by rjg, using Equation (6.59), we obtain an explicit expression for rcL as a function of the catalyst layer voltage loss ... 

Obviously, the CCL not only determines the rate of currenf conversion and the major portion of irreversible voltage losses in a PEFC, but also plays a key role for the water balance of the whole cell. Indeed, due to a benign porous structure with a large portion of pores in the nanometer range, the CCL emerges as favorite water exchanger for PEFCs. Once liquid wafer arrives in gas diffusion layers or flow fields, PEFCs are unable to handle if. 

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