Thermoneutral cell voltage

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. 

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. 

A thermoneutral potential, tn( tn = AHh o/zF), is defined in which no heat turnover is observed and tn = 148 V for water electrolysis [16]. If, because of effective catalysis, the total electrolysis cell voltage is close to 1.48 V, then tielectrolysis 1-... 

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 ... 

For water electrolysis, the thermoneutral enthalpy voltage is -1.48 V compared to the electrolysis voltage with a value of —1.23 V. Note that electrochemical cells can operate against a positive AG if an outer source is applied that drives the electrons in the opposite direction (Juettner, 2007). 

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

The thermoneutral potential is 2.72 V this means that during charging at voltage below this value ZEBRA cells will be cooled thermodynamically. 

The actual efficiency of the cell is defined as the actual voltage divided by the thermoneutral potential, that is,... 

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