Practical cell voltage

The theoretical OCV of cells consisting of a hydrogen electrode (in alkaline solution) and a chlorine electrode is 2.17 V. Practical cell voltages of most modem... 

An often very helpful strategy therefore is to modify existing materials. Compared to new compounds, such attempts would primarily not address thermodynamic quantities, such as open cell voltage. Instead, kinetic parameters can be optimized, and quantities such as practical cell voltage, electrode capacity, and power density can be drastically improved. 

This reaction should theoretically lead to a cell voltage of 1.23 V, practically, cell voltages of IV at zero current (open circuit voltage) and 0.5 V during operation are achieved. These low-voltage values per cell lead to the requirement of series connection of various single cells to form a so-called cell stack. 

Because all electrochemical reactions involve anodic and cathodic reactions, polarization will have components for both reactions. As will be explained later, the electrode potentials have two terms for each electrode surface overpotential ija or ijc and concentration overpotential Apart from these overpotentials, electrical energy will also be expended due to the electrical resistance of the cell components such as electrolyte, diaphragm, busbar, etc. Thus the practical cell voltage (, when a net current is flowing through the cell, is the sum... 

In general, current densities of between 1000 and 1500 amps/m are practical, cell voltage of2.5 to 3.0 are achievable and current efficiencies of 95 per cent are attainable. The build-up of impurities in the electrolyte bath is a significant problan in maintaining this performance, and an accumulation of iron for instance can cause current efficiency to decline to 50 per cent. Similarly, copper can cause a major decline in current efficiency. 

Current densities up to 200 amps/m are practical, cell voltages are in the range of 2.6 to 3.0 and operating temperatures around 35 to 40°C. 

Metal Anode Equivalent (Ah/g) Equivalent (Ah/cm ) Theoretical Specific Energy (kWh/kg) Theoretical Ceil Voltage Practical Cell Voltage... 

The investigation of the stability of P -alumina in ZEBRA cells, which always contain some iron, showed an increase of resistance under certain extreme conditions of temperature (370 °C) and of voltage. This is related to the interaction of the P alumina with iron and it was shown that iron enters / -alumina in the presence of an electric field when current is passing, if the cell is deliberately overheated. However, it was found that only the P -phase but not the P"-phase was modified by the incursion of iron. The resistance of the iron-doped regions was high. It was shown that the addition of NaF inhibits access of the iron to the / " -alumina ceramic. By doping practical cells these difficulties have now been overcome and lifetime experiments show that the stability of / "-alumina electrolytes are excellent in ZEBRA cells. 

In practice, the decomposition potential for this overall reaction is found to be about 1.5 V this somewhat high value probably results from polarization and contact resistances. It could be seen that the electrochemical decomposition of alumina to deposit aluminum, using an inert anode, would require a theoretical cell voltage of 2.21 V as opposed to that of 1.18 V when carbon is used as the anode. Thus the participation of carbon in the cell reaction brings down the theoretically required cell voltage by almost 50%. 

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). 

In practical batteries and fuel cells, the influence of the current rate on the cell voltage is controlled... 

Bromide ions serve as a catalyst, although in practice there is some loss due to the formation of bromate in side reactions. The process has also been run with a feed of oxygen through a porous cathode, which eliminates the liberation of hydrogen and decreases the overall cell voltage. 

However, under working conditions, with a current densityj, the cell voltage E j) is lower than eq - see Equation 1.7 - so that the practical energy efficiency, for a DEFC working at 0.5 V and 100 mAcm with complete oxidation to CO2, would be - see Equation 1.9... 

The cell voltages for a fuel cell, as calculated from thermodynamic data, lie a little above one volt. In practice, they are considerably lower and fall still lower as the cell load increases. Figure 2 shows the relationship between cell voltage and current density, and the influences that reduce the cell voltage. 

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