Overpotential Analysis in High Pressure Water Electrolysis

To store or transport hydrogen (and optionally oxygen) in high pressure tanks or pipelines pressures up to 1000 bar are needed. Pressurized polymer electrolyte electrolyzers are typically operated at around 30 bar to reduce downstream mechanical compression and gas drying stages. Interestingly, often the cell voltage does not show the expected thermodynamic voltage increase 1,2. Consequently beneficial influences are compensating the compression work, at the expense of higher faradaic losses due to increased gas crossover. In this work we analyze cell voltages at gas pressures up to 100 bar as function of temperature up to 70 °C and current densities up to 4 A/cm 2 , considering the three main overpotentials (ohmic, kinetic, mass transport) using a zero-order analysis based on the Tafel kinetics model. As expected, the ohmic overpotential is mainly a function of temperature and almost independent from pressure. Therefore the overpotential gains with increasing pressure, observed for current densities above about 0.8 A/cm 2 , stem from the kinetics and/or the mass transport. Both overpotentials decrease with increasing pressure, but over a wide current density region kinetics contribute about two thirds to the voltage gain. The improved kinetics can be attributed to an increased apparent exchange current density, whereas the increased mass transport might be related to smaller gas volumes or velocities resulting in an improved two-phase flow in the porous structures. Literature: 1 S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy, Int J of Hydrogen Energy, 31 (2006) 171-175. 2 P. Millet, R. Ngameni, S.A. Grigoriev, N. Mbemba, F. Brisset, A. Ranjbari, C. Etiévant, PEM water electrolyzers: From electrocatalysis to stack development, Int J of Hydrogen Energy, 35 (2010) 5043-5052.