Improving the Efficiency of PEM Electrolyzers through Membrane-Specific Pressure Optimization

Abstract

Hydrogen produced in a polymer electrolyte membrane (PEM) electrolyzer must be stored under high pressure. It is discussed whether the gas should be compressed in subsequent gas compressors or by the electrolyzer. While gas compressor stages can be reduced in the case of electrochemical compression, safety problems arise for thin membranes due to the undesired permeation of hydrogen across the membrane to the oxygen side, forming an explosive gas. In this study, a PEM system is modeled to evaluate the membrane-specific total system efficiency. The optimum efficiency is given depending on the external heat requirement, permeation, cell pressure, current density, and membrane thickness. It shows that the heat requirement and hydrogen permeation dominate the maximum efficiency below 1.6 V, while, above, the cell polarization is decisive. In addition, a pressure-optimized cell operation is introduced by which the optimum cathode pressure is set as a function of current density and membrane thickness. This approach indicates that thin membranes do not provide increased safety issues compared to thick membranes. However, operating an N212-based system instead of an N117-based one can generate twice the amount of hydrogen at the same system efficiency while only one compressor stage must be added.

Faraday’s Efficiency Modeling of a Proton Exchange Membrane Electrolyzer Based on Experimental Data

“…High Faraday's efficiency can be explained by the fact that recently, PEM electrolyzers have been designed with technical improvements compared to the Faraday's efficiency obtained for alkaline electrolyzers in the 1990s. By increasing the hydrogen output pressure, the Faraday's efficiency is lower due to higher crossover currents through the membrane, corroborating the results obtained in previous works [29][30][31]. The effects of higher pressures are particularly noticeable at low current densities; whereas from 25 A (i.e., 0.5 A.cm 2 ), Faraday's efficiency is similar for the three tests.…”

“…The generated hydrogen is stored in three metal hydride tanks (not shown in Figure 4). The investigated PEM Finally, in [17,[29][30][31], compared to results reported in [10,15,16] for alkaline electrolyzers, the authors have shown that the temperature slightly affects the Faraday's efficiency of PEM electrolyzers. However, membrane thickness and gas pressure have a strong influence on the Faraday's efficiency.…”

“…It can be explained by the fact that the gas pressure of the HYSOLAR electrolyzer is smaller than that of the PHOEBUS electrolyzer. The consequence of gas pressure on Faraday's efficiency has been introduced in previous works [29][30][31] where an increase in gas pressure decreases Faraday's efficiency. It should be remarked that even if the Faraday's efficiency is often sketched at constant temperature, it does not represent the real operation of the electrolyzer.…”

“…On the other hand, the thinner the membrane thickness, the higher the crossover currents. As a result, membrane thickness and operating pressures are challenging issues to improve the efficiency of PEM electrolyzers [29][30][31].…”

“…The increase in pressure has been made possible due to the use of thinner membranes in PEM electrolyzers (i.e., high mechanical resistance) and new materials (i.e., zinc) at the anode and cathode [2][3][4]. Given that higher pressures lead up to the decrease of Faraday's efficiency and energy efficiency as well, the hydrogen pressure and membrane thickness are challenging issues and must be optimized to enhance the energy efficiency of the electrolyzer [31]. To optimize the Faraday's efficiency, some authors [32,33] have proposed to control power electronic converters connected to a PEM electrolyzer for specific operating conditions (i.e., stack voltage and current).…”