Closing the Hydrogen and Oxygen Mass Balance for PEM Water Electrolysis

In this study we show the possibility to close the hydrogen and oxygen mass balance for proton exchange membrane (PEM) water electrolysis cells at lab scale at 80 °C. By a combined analysis of the cathode and anode exhaust with respect to the absolute hydrogen and oxygen gas fluxes detected, we can quantify the true Faradaic efficiency of the system with respect to hydrogen production. While both exhausts, anode and cathode, contain an apparent hydrogen flux, there is a certain missing hydrogen share not leaving the cell, corresponding to roughly 2% at ambient pressure operation. This missing hydrogen flux shows a 2:1 correlation with the missing oxygen flux determined from the oxygen mass balance, proving that the inefficiency stems from a cell internal process. We observe a linear correlation of missing hydrogen flux with current density for different operating pressure cases, hinting toward an oxygen crossover based hydrogen loss at the cathode. The effect of the true Faradaic efficiency on the overall production efficiency of the single cells at atmospheric pressure as well as differential and balanced elevated pressure of 10 bar a is shown, revealing a very limited operation window for efficient hydrogen production.