Initially, a systematical hazard and operability (HAZOP) study was performed to evaluate the risks for pressurized polymer electrolyte water electrolysis (PEWE) operation in the order of 100 bar with a safety factor of three. Based on the results of the HAZOP a test bench and an electrolysis cell were designed and commissioned. With the high pressure test bench, different material combinations were electrochemically investigated as a function of temperature (30 to 70 °C), pressure (up to 100 bar) and current density (up to 4 A/cm2) by recording current-voltage characteristics and high frequency resistance (HFR) measurements. Furthermore a zero order model according to Tafel was applied to distinguish between the three main overpotentials: kinetic, ohmic and mass transport. As expected, all overpotentials increase with increasing current density, the mass transport overpotential occurs above around 0.2 A/cm2. With respect to the pressure, isothermal compression behavior is expected from thermodynamics. While this holds true for differential pressure operation, for balanced pressure operation and at relevant current densities above around 1 A/cm2 the cell voltage is increasingly independent of the balanced pressure. Consequently beneficial processes have to take place which could be mainly related with improved oxygen evolution kinetics. Concerning the compression losses, losses on the cell voltage level and losses due to gas crossover were considered. Even if the gas crossover losses are almost doubled for the balanced pressure compared to the differential pressure operation, it can be shown that balanced pressure operation ia energetically equal or even more beneficial for a wide pressure range, especially at higher current densities, due to the beneficial processes.
