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This voltage, 1.481 volts, is required for splitting liquid water. It is the voltage at which an electrolysis cell operating at 25°C can operate without producing excess heat. (Practical cells operate above this voltage and produce excess heat.) It also is the voltage that corresponds to the HHV of hydrogen and therefore represents a more reasonable value to use when calculating cell and stack voltage efficiency. The formula for calculating the voltage efficiency of a cell or cell stack thus becomes the following. Voltage efficiency = Thermal neutral voltage (E) Cell operating voltage (V) A similar calculation can be performed for water vapor using the LHV. The thermoneutral voltage for splitting water vapor at 25°C is 1.253 volts. Steam Electrolysis and High-Temperature Cells The above discussion applies primarily to electrolysis cells operating at temperatures that are less than the boiling point of water; these include the PEM and alkaline electrolysis cells. There is a class of high-temperature steam electrolysis cells under development, however, that operates in the 800° to 1,000°C temperature range, where the thermodynamics are significantly different. As the temperature climbs, the LHV of hydrogen increases and the Gibbs free energy decreases. At 1,000°C, for example, the LHV of hydrogen is 249.2 J/mole and the Gibbs free energy for the reaction is 179.9 kJ/mole. The water-splitting reaction at 1,000°C can thus be written as follows. H2O(steam) + 179.9 kJ/mole electricity + 69.3 kJ/mole heat → H2 + 1⁄2 O2 The thermodynamic voltage for this reaction—which corresponds to both the open-circuit voltage for the solid oxide fuel cell and the solid oxide electrolyzer cell—is 0.932 volts. The thermoneutral voltage for the electrolysis reaction is 1.291 volts. These high-temperature cells are considerably more efficient in that they have lesser internal resistance losses and improved reaction kinetics as compared to their low-temperature PEM counterparts. It is well within the realm of possibility that a practical high-temperature electrolysis cell could operate below the thermoneutral voltage. In this case, the heat requirement must be made up by an external heat source. A high-temperature electrolyzer operating at 1,000°C and 1.200 volts, for example, would not generate sufficient heat via internal resistance to keep the electrochemi- cal reaction going. As the cell operated, the electrochemical reaction would withdraw heat from the cell components and cool the cell to the point that it ceases operating. Therefore, to maintain temperature, sensible heat must be supplied to the cell components from an outside source. In the high-temperature case, calculating the voltage efficiency of the cell can be straightforward and the thermodynamic voltage can be used. In the case of global system efficiency, however, both the electrical input and the heat input from the external source must be included, otherwise the calculation produces a nonsensical answer and an efficiency that is greater than 100%. Voltageefficiency(cell) =Thermodynamicvoltage(E) Operating voltage (V) 7PDF Image | Hydrogen Production: Fundamentals
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