The numerical analysis conducted in this study proposes a guideline to maximize the high-temperature effect, which is expected to reduce the electrolysis voltage of the polymer electrolyte membrane water electrolyzer. High-temperature operation is intuitively thought to reduce activation overvoltages. However, a further consideration predicts that high temperature, especially a temperature higher than the saturated temperature regulated in the operation pressure, decreases the liquid saturation and causes shortage of water, leading to a large increase in overvoltages. This high temperature problem is analyzed using the developed theoretical model, which considers gas/liquid behavior. The analysis suggests that, if the gas saturation in the anode catalyst layer is kept at or below 0.3 by increasing the pressure, liquid water in the catalyst layer is sufficient to OER catalytic ability regulated by exchange current density, demonstrating that the high-temperature effect works. According to this guideline, increasing the temperature with pressurization can monotonically reduce the anode activation overvoltage. For instance, raising the temperature from 100 to 120°C and raising the pressure from 0.13 to 0.22 MPa can prevent the gas saturation from increasing beyond 0.3 and allows the lower electrolysis voltage to vary from 1.57 to 1.51 V.

This study challenges to decrease water electrolysis voltage by thermodynamic coupling between boiling and water electrolysis. Boiling, once a system to cause boiling is given, spontaneously advances and causes entropy generation. When boiling is superimposed on an electrode where electrochemical reaction of water electrolysis progress, the entropy generation by the boiling possibly accelerates the reaction of water electrolysis, leading to reduction of electrolysis voltage. To confirm this new concept, electrolysis voltage for a unit cell of PEMWE is measured in the region from 80°C to 120°C under a condition that cell pressure and electrolysis current are kept constant. The measurement results showed that the electrolysis voltage abruptly decrease as cell temperature crosses boiling point and then turn to increase at a few degree higher than the point. These features in the measurement were reproduced in the theoretical analysis based on a mathematical model considering the thermodynamic coupling.

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