energies Article Improving the Efficiency of PEM Electrolyzers through Membrane-Specific Pressure Optimization Fabian Scheepers 1,* , Markus Stähler 1, Andrea Stähler 1, Edward Rauls 1 , Martin Müller 1, Marcelo Carmo 1 and Werner Lehnert 1,2 1 Forschungszentrum Juelich GmbH, Institute of Energy and Climate Research, IEK-14, Electrochemical Process Engineering, 52425 Juelich, Germany;
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[email protected]; Tel.: +49-2461-61-2177 Received: 20 December 2019; Accepted: 23 January 2020; Published: 1 February 2020 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.