Limiting Current Investigations for PEM Water Electrolysis in the Presence of Metal Cations
Authors: Friedrichs-Schucht Maurice , Hasché Frédéric , Oezaslan Mehtap 
Published: December 2024

Pretreatment and purification of water is necessary to use it as a feed for the oxygen evolution reaction (OER) at the anode of PEM water electrolysis (PEMWE). Fresh water typically contains several multivalent cations such as Ca2+ and K+. In addition, iron cations may accumulate in PEMWE over a long period of time due to corrosion of system components and fittings of the PEMWE. The presence of these multivalent cations in the feed water causes higher ohmic resistance of the cell and higher iR-free voltages, resulting in a performance loss.[1] Furthermore, the cations can be transported by the water crossover stream from anode to cathode and by diffusion transport.[2] In other words, even traces of metal cations drastically influence the efficiency and durability of PEMWE stacks. Despite this knowledge, contamination with multivalent cations is one to the main reason for the breakdown of PEMWE stacks.[3]

Very interestingly, simulations of cationic contamination for PEM fuel cells (PEMFCs)[4] have shown that the distribution of metal cations in the through-plane direction depends on the proton current through the membrane. The cation concentration profile indicates an accumulation of metal cations close to the cathode side and leads to an increase of the proton transport resistance. In addition, the simulations indicate that in case of a complete substitution of protons by metal cations, the proton transport resistance approaches infinity. Finally, the proton current cannot increase further and a limiting current is reached.[4] The transfer/adoption of these simulations from PEMFC to PEMWE allows to clarify the behavior of the limiting current in dependence of the nature of metal cations such as Ca2+ and K+ and their concentrations, which is still poorly understood to date.

In this work, we investigated if the concept of limiting protonic current from simulations for the PEMFC can be (partially) transferred to PEMWE on a laboratory scale. For this purpose, we evaluated the effect of trace metal cations in the anode feed water on the performance of a PEMWE single cell and quantified the cation concentration in the membrane using spectroscopic techniques such as micro X-ray fluorescence (µ-XRF) and energy-dispersive X-ray (EDX). Electrochemical measurements were carried out in an in-house PEMWE test bench equipped with a single cell of 5 cm2 geometric electrode area and potentiostat with booster. Commercially available catalyst coated membranes with a loading of 0.3 mg cm-2
geo of Pt/C and 1 mg cm-2
geo of IrOx were used. The measurements were carried out at atmospheric pressure and a cell temperature of 80°C. First, a conditioning procedure was performed using highly purified feed water (>20 MΩ cm at room temperature) until a stable cell performance had been achieved. Cation contamination experiments were conducted by introducing various concentrations of metal sulfates (1-100 µmol L-1 cation concentration) into the anode feed water. Performance evaluation was carried out by measuring the polarization curves and electrochemical impedance spectroscopy (EIS) to determine the Tafel slope, overvoltage, and high frequency resistance (HFR). µ-XRF and EDX spectroscopy techniques were used to detect the cation concentration profile along the membrane.

We observed a significant rise of the cell voltage by adding cations to the anode water feed within few hours. More precisely, a limiting current density of 1 A cm-2
geo after 16 hours of exposure to 100 µmol L-1 K+-ions in the anode feed water was determined using the Koutecký-Levich equation. This result is in line with our first simulations. The switch back to purified feed water allows us to monitor the dynamics of the performance recovery process obtained from EIS data. Additional measurements of the polarization curves and HFR were used to distinguish between reversible and irreversible degradation processes due to the cation contamination. Very interestingly, a partial reversibility was observed after the K+-contamination, as the limiting current increased from 1 to 2 A cm-2
geo after 16 hours recovery with purified water feed.

Altogether, this study evaluated the transferability of limiting protonic current concept from simulations in PEMFC to PEMWE to uncover the impact of trace metal cations in the anode feed water on the performance of PEMWE.

[1] C. Immerz, M. Singer, F. Hasché, B. Bensmann, M. Suermann, R. Hanke-Rauschenbach, M. Oezaslan, Meet. Abstr.
2020, MA2020-02, 2456.

[2] M. Friedrichs-Schucht, F. Hasché, M. Oezaslan, ECS Trans.
2023, 111, 3.

[3] N. Danilovic, K. E. Ayers, C. Capuano, J. N. Renner, L. Wiles, M. Pertoso, ECS Trans.
2016, 75, 395.

[4] B. L. Kienitz, H. Baskaran, T. A. Zawodzinski, Electrochim. Acta
2009, 54, 1671.