Thermoneutral voltage

When energy exactly equal to the enthalpy AH = AG + TAS (=285.83 kj/mol at 1 bar and 25°C) for water splitting is supplied, no heat is absorbed or evolved by the system [10]. The voltage corresponding to this condition, the thermoneutral voltage Vm is given by... 

Thermoneutral potential — (Thermoneutral voltage) The voltage E given by... 

The figure shows a plot of cell voltage (lower line) and thermoneutral voltage (upper line) for the water electrolysis reaction assuming water vapor as reactant. 

CLs), resulting in a drastic drop in cell performance [17], Figure 3.13 also shows the difference between the theoretical cell potential (1.23 V) and the thermoneutral voltage (1.4 V), which represents the energy loss under reversible conditions (the reversible loss) [18], Very often, polarization curves are converted to power density versus current density plots by multiplying the cell voltage by the current density at each point of the curve. 

Reversible Nernst voltage at unit activity of reactants and products, V Thermoneutral voltage V... 

Here Uc,0 = — AG/nF is the cell voltage at equilibrium, Uc is the actual cell voltage under current operation, and is the thermoneutral voltage, Eq. (27), which permits isothermal operation. 

Heat generated per unit area is fcj from ohmic losses through the membrane and Vtn — V — ico) i from electrochemical losses at the cathode catalyst layer, where Vtn is the thermoneutral voltage of the oxygen reduction reaction with product vapor. 

Table 11.1 summarizes the values for the reaction enthalpy AHr and the free reaction enthalpy AGr with the respective voltages for the thermoneutral voltage Vth and the reversible voltage Vj v for different temperatures [7]. 

One important technical evaluation criterion for electrolytic processes is the efficiency, i.e. the cost-benefit ratio for an industrial electrolysis system. When determining the efficiency, it is expedient to utilize the heating value (3.54 kWh Nm ) or the thermoneutral voltage Vth = 1.48 V because in commercial electrolysis systems for alkaline and PEM electrolysis, water is added in its liquid state. As such, the efficiency referring to the heating value of hydrogen specifies how efficiently the electrolyzer or the entire electrolysis system with all auxiliary components can be operated. 

If an electrolyzer works at 100% efficiency (i.e., at 1.48 V under the standard condition), the heat generated from the electrolyzer will be equal to the heat needed for the electrolysis to proceed, and therefore, a thermoneutral situation is achieved. In other words, the electrolyzer neither releases nor absorbs heat to or from the environment. The voltage, 1.48 V under this condition, is called the thermoneutral voltage. Since AH increases with temperature, then the thermoneutral voltage increases slightly with temperature (please refer to Table 2.1 for water in vapor phase). Also, since the difference between AH and AG increases with temperature, the heat needed for the electrolysis to proceed increases with the temperature. 

This is known as the thermoneutral voltage/potential . Hence (theoretically) there is no longer any need for an external heat source and/or coohng system the electricity source itself is sufficient. 

What is the value of the thermoneutral voltage in the case of a SOWE electrolyzer What is the advantage to working at this particular point ... 

Is it advantageous to work at the thermoneutral voltage for a PEMWE electrolyzer ... 

Below 100°C, the behavior of an alkahne water electrolyzer (AWE) will, for the vast majority, be exothermic. Indeed, the irreversible losses relating to the reaction (activation, species transport and charge transport) limit endothermic operation to very low current densities. From the point of view of the current density, the thermoneutral voltage will be surpassed very early on. 

Operation at the thermoneutral voltage is not particularly advantageous for a PEMWE electrolyzer. Indeed, in the same vein as an AWE electrolyzer, because of the irreversible losses caused by the reaction (activation, species transport and... 

If the two half-cells are arranged oppositely, the reactions are reversed and the process refers to water electrolysis. The electrode potential at equilibrium is then negative and ARG gg is positive. Now TARS gg is positive (48.7kJ mol ) and is taken up from the environment or the electrolysis cell cools down. This effect can be compensated if the cell operates at a higher cell voltage to permit isothermal operation, and is denoted as the thermoneutral voltage ... 

A portion of the cell voltage in excess of the thermoneutral voltage (equation 2.56) ... 

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