The Effect of Compression on Water Uptake and Predicted Performance of a High Pressure PEM Electrolysis Cell Model

Abstract

In the electrolysis of water it is convenient and advantageous to directly compress hydrogen at the cathode. This reduces system requirements and energy consumption associated with subsequent mechanical compression [1–3]. The simplest and most energy efficient system configuration is the so-called asymmetric setup, where only the cathode is operated at elevated pressures [2]. Under these conditions a large pressure difference forms across the polymer electrolyte membrane which mechanically compresses the polymer backbone and the anode electrode. While titanium felt has to be used as a gas diffusion layer at the anode, to withstand the mechanical compression, a conventional perfluorinated sulfonic acid (PFSA) membrane can be used as the electrolyte. Although PFSA membranes like Nafion exhibit high thermo-mechanical stability, water uptake and transport properties are significantly affected by mechanical compression depending on the temperature, relative humidity and the amount of liquid water present [4]. Consequently, it is of great importance to accurately account for the physiochemical relationship between mechanical compression and equilibrium water uptake, when attempting to predict the electrochemical behavior of an asymmetric pressurized PEM electrolysis cell In this work, a non-equilibrium formulation of the water uptake model by Kusoglu et al. [4] has been implemented in a two-dimensional, two-phase, multi-component and non-isothermal PEM electrolysis model. The non-equilibrium formulation of the water uptake model was chosen in order to account for interfacial transport kinetics between each fluid phase and the PFSA membrane. Besides modeling water uptake, the devised membrane model accounts for water transport through diffusion and electro-osmotic drag in the polymer phase, and hydraulic permeation in the liquid phase. Charge transport and Butler-Volmer electrochemistry are likewise included. In order to investigate the effect of accounting for membrane compression, a parametric study is carried out with and without the compression corrected water uptake model by Kusoglu et al. [4]. The obtained simulation results confirm and underline that the predicted water uptake and cell voltage are highly dependent on the applied water uptake model.