Temperature Dependence of Water Crossover in Proton Exchange Membrane Water Electrolysis

Abstract

The importance of hydrogen production by proton exchange membrane water electrolysis (PEMWE) in the energy sector makes it worthwhile to understand the system behaviour in failure cases and possible recovery procedures. In particular, insufficient feed water supply to the anode can be caused by inhomogeneous distribution in the stack, pump failure or erroneous operation strategy. For the PEMWE, a distinct turnover point has been reported if feed water supply is reduced in isothermal conditions.[1] Further reduction of the water supply results in higher ohmic resistance due to gradual membrane dehydration and increased mass transport losses.[2] Here, we will use the term “water starvation” to describe this event. The drainage of water from the anode can be explained by oxygen evolution reaction (OER), humidification of oxygen gas and water crossover to the cathode. The last one was measured for well hydrated membranes at atmospheric pressure [3] or below typical operating temperatures [4].In the first part of this work, we present the water crossover in a PEMWE on single cell level to understand their dependence on the operating parameters such as temperature and pressure. Media were supplied and conditioned to a 5 cm2 single cell by using an in-house test bench and a potentiostate with booster. The electrochemical measurements were performed with a commercially available catalyst coated perfluorosulfonic acid membranes (loading of 0.3 mg/cm2 geo Pt/C on cathode and 1 mg/ cm2 geo Ir-Oxide on anode). The anode was kept at atmospheric pressure, whereas the cathode pressure was varied from atmospheric to 400 kPaabs at various current densities and cell temperatures of 40 – 80 °C. To determine the water crossover under several operating conditions, the water vapour was condensed to liquid phase in a heat exchanger and measured by a gravimetric method.Furthermore, the measured water crossover is used to force a water starvation in the second part of this work. After cell assembly, a conditioning procedure was performed until steady cell performance was achieved. Cell performance was evaluated using polarization curve, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and water crossover. The last two are of particular interest, because they provide information about membrane humidification. The Tafel slope and mass transport overvoltage were evaluated from the polarization curves and high frequency resistance (HFR). Water starvation experiments were then carried out at a constant cell voltage by reducing the feed water supply, resulting in a dramatic decay of the current density. The analysis of Tafel slope and EIS data reveals an increase of ohmic and mass transport resistances. Furthermore, the HFR measurements were correlated with water crossover as a function of membrane hydration. We observed that the specific water crossover per current density decreases as a consequence of membrane dehydration. After this step, feed water supply was increased to an unrestricting level for membrane rehydration and performance recovery. For this recovery procedure, different operation modes were employed by varying the current densities. At constant current density, membrane hydration is influenced by the ionic current from the cathode to the anode via proton drag. EIS and water crossover measurements allow us to determine the influence of operation on the dynamics of the membrane rehydration process. Together with the polarization curves, cyclic voltammetry and HFR data, reversible and irreversible degradation processes caused by the water starvation can be identified.Altogether, we present a deeper understanding of water crossover for a wide operating parameter range and provide a guideline for recovery of the performance for PEMWE.[1] Christoph Immerz et al 2020 Meet. Abstr. MA2020-02 2456[2] C. Immerz, B. Bensmann, P. Trinke, M. Suermann, R. Hanke-Rauschenbach, J. Electrochem. Soc. 2018, 165, F1292-F1299.[3] K. Onda, T. Murakami, T. Hikosaka, M. Kobayashi, R. Notu, K. Ito, J. Electrochem. Soc. 2002, 149, A1069.[4] P. Medina, M. Santarelli, International Journal of Hydrogen Energy 2010, 35, 5173.