Concentration cell voltage, effects

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

Fundamentally, the eel is simply a living battery. The tips of its head and tail represent the poles of the eel s battery . As much as 80 per cent of its body is an electric organ, made up of many thousands of small platelets, which are alternately super-abundant in potassium or sodium ions, in a similar manner to the potentials formed across axon membranes in nerve cells (see p. 339). In effect, the voltage comprises thousands of concentration cells, each cell contributing a potential of about 160 mV. It is probable that the overall eel potential is augmented with junction potentials between the mini-cells. 

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 reactant gases are consumed in an operating cell, the cell voltage decreases in response to the polarization (i.e., activation, concentration) and to the changing gas composition (see discussion in Section 2). These effects are related to the partial pressures of the reactant gases. 

The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below that required to form nickel sulfide. 

Based on the present understanding of the effect of sulfur on MCFCs, and with the available cell components, it is projected that long-term operation (40,000 hr) of MCFCs may require fuel gases with sulfur " levels of the order 0.01 ppm or less, unless the system is purged of sulfur at periodic intervals or sulfur is scrubbed from the cell burner loop (76). Sulfur tolerance would be approximately 0.5 ppm (see Table 6-3) in the latter case. Considerable effort has been devoted to develop low-cost techniques for sulfur removal, and research and development are continuing (80,81). The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below which nickel sulfide forms. 

The fuel gas composition also has a major effect on the cell voltage of SOFCs. The performance data (33) obtained from a 15 cell stack (1.7 cm active electrode area per cell) of the tubular configuration (see Figure 8-1) at 1000°C illustrate the effect of fuel gas composition. With air as the oxidant and fuels of composition 97% H2/3% H2O, 97% CO/3% H2O, and 1.5% H2/3% CO/75.5% CO2I2OV0 H2O, the current densities achieved at 80% voltage efficiency were -220, -170, and -100 mA/cm, respectively. The reasonably close agreement in the current densities obtained with fuels of composition 97% H2/3% H2O and 97% CO/3% H2O indicates that CO is a useful fuel for SOFCs. However, with fuel gases that have only a low concentration of H2 and... 

Specifically, Figure 16 shows that the current density in a cell with dry cathode gas feed drops nearly instantaneously once the cell voltage is relaxed from 0.6 to 0.7 V due to the fact that the electrochemical double-layer effect has a negligibly small time constant. Further, there exists undershoot in the current density as the oxygen concentration inside the cathode catalyst layer still remains low due to the larger consumption rate under 0.6 V. As the... 

The net cell reaction is an equilibrium reaction. The cell voltage directly relates to the spontaneity of a reaction. The effect of concentration on cell voltages can easily be explained by Le ChMelier s principle. Let us consider the following reaction ... 

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

In a second prototype, the reaction temperature was reduced to 250 °C, which reduced the carbon monoxide concentration from 1.2 to < 1%. Later, the first fuel processor prototype was linked to a meso-scale high-temperature fuel cell developed at Case Western University by Holladay et al. [117], which was tolerant to carbon monoxide concentrations up to 10%. Hence no CO clean-up was necessary to run the fuel processor. A 23 mW power output was demonstrated according to Holladay et al. [118], This value was lower than expected, which was attributed to several factors. First, the hydrogen supply was lower in the reformate. Second, the presence of carbon monoxide (2%) lowered the cell voltage. Third, the presence of carbon dioxide (25%) generated a magnified dilution effect at the gas diffusion layer material of the fuel cell, which was considerably less porous than conventional materials. 

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