Fuel cells voltage losses

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. 

For the high current density range, mass transport of reactants and products becomes a limiting factor. So the oxidant and fuel concentration is significantly reduced at high current densities, and they cause fuel cell voltage losses. The mass transport mechanisms within the fuel cell include convection, diffusion, and/or convectimi. Simplification of (17.1) for simple concentration polarization is shown in (17.4). Thus,7ii , can be also defined as current density, where the... 

In the case of membrane electrode dehydration, membrane electrical conductivity decreases and ohmic resistance increases, and so the internal fuel cell voltage loss will be even more serious, leading... 

As we have mentioned earlier, one of the fuel cell voltage losses is the mass transfer loss or concentration loss caused by lower reactant gas concentration distribution at the reaction sites. Mass transport establishes reactant gas concentration distributions in gas supply channels and in the electrodes of a fuel cell, and hence in the distribution of local current densities. The gas supply rates to the anode-membrane and cathode-membrane interface must be sufficient enough to meet the gas consumption rate given by the electrochemical reaction rates. Any insufficient supply of gas to reaction sites may cause sluggishness in the reactions and cause mass transfer loss and reduction in fuel cell output voltage. 

Ohmic losses, in fuel cell voltages, 12 207 Ohmic polarization, batteries, 3 425—426 Ohnesorge number, 23 183, 190 Oil absorption, by silica, 22 371 Oil additives... 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

FIGURE 12.2 Schematic view of various overpotential losses ideal and apparent fuel cell voltage-current characteristics. 

For reformate with a high CO concentration (>10 ppm), a PEM fuel cell with a Pt/Ru alloy anode still suffers from a substantial cell voltage loss, especially in the high-CD region, because the maximum CO oxidation current occurs from 0.39 to 0.6 At its onset potential (<0.1 V), the CO oxidation current density of a Pt/Ru anode is capable of oxidizing only a few ppm CO. The ignition potential, defined as the potential at which the CD increases by approximately two orders of magnitude... 

The performance of a fuel cell is characterized by its output voltage and current density, which is defined as the current per unit area of the cell. The fuel cell voltage drops at higher currents due to increasing catalytic activation losses, ionic and electronic resistances in the cell, and mass transport limitations. The cell efficiency is therefore proportional to the ratio of measured voltage to the ideal cell voltage (1.23 V and 1.21 V for hydrogen and methanol at 25 °C, respectively). 

The fuel cell voltage as a function of current density can be seen in Figure 2. The value of 1,2 V represents a theoretically loss-free voltage. We can see that the actual cell voltage (incl. the open circuit voltage) is always lower than this value. The curve shown in Figure 2 is important with respect to efficiency as the rule states the directly proportional relationship between the efficiency and the fuel cell voltage. 

The shape of the fuel cell voltage-current density characteristic shown in Figure 2 is the result of four basic losses which are in detail described below. 

The kinetics of the HOR on Pt catalysts in a PEMFC are so fast that the cell voltage losses at the anode are negligible [22]. This seems not to be the case in an alkaline membrane fuel cell [3]. 

Figure 1.3 Schematic for the calculation of voltage loss in a fuel cell (for discussion see text). ACL and CCL are the abbreviations for the anode and cathode catalyst layers, respectively. Yellow shaded areas indicate the local polarization voltage r]. For simplicity, the proton conductivity of catalyst layers is taken to be equal to the proton conductivity of the bulk membrane (otherwise the curve loses smoothness at the membrane interfaces). Note that the half-cell voltage loss is given by the value of the overpotential at the catalyst layer/membrane interface.

The above described voltage losses can be surrunarized by a conceptually simple equation, describing the fuel cell voltage, Eceii, as a function of current density, i ... 

Another way to consider the impact of membrane conductivity on fuel cell performance is shown in Fig. 17.5. Figure 17.5a shows the conductivity of a few different EW membranes as a functiOTi of temperature with the atmosphere inside the conductivity cell held at a fixed dew point of 80°C [17]. When the conductivity cell is at 80°C, the %RH is 100%. As the temperature of the cell increases, the %RH at a fixed dew point decreases, causing a decrease in the membrane conductivity. This is similar to the situation in some PEMFC applications where the cell temperature may rise while the humidity level of the incoming gases remains constant. The graph in Fig. 17.5b uses the same data. Here the conductivity is used to calculate the resistance of a 25 pm membrane, and using Ohms law, that resistance is used to calculate the voltage loss (ohmic loss) one would see in a fuel cell at a 0.6 A/cm current density [17]. This represents the fuel cell performance loss due to the loss of membrane conductivity (certainly not the only performance loss under these conditions ). 

UTC Fuel Cells reports that the efficiency of its latest power plants at the beginning of life is 40 percent LHV. The infant life loss reduces the efficiency quickly to 38 percent, but then there is a small decrease in efficiency over the next 40,000 hours (expected cell life) resulting in an average efficiency over life of 37 percent (3). Assuming that the loss in efficiency is due solely to cell voltage loss, the maximum degradation rate can be determined as ... 

The actual fuel cell potential is decreased from its full potential, the Nemst potential, because of irreversible losses. Multiple phenomena contribute to irreversible losses in an actual fuel cell. Eor the hydrogen oxidation reaction, the func-tionahty of fuel cell voltage, E, is typically given by [42-44]... 

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