Influence of Seawater Cation Contamination on the Performance and Lifetime of PEM Electrolysis Cells
Authors: Gottschalk Torben , Bensmann Boris , Hanke-Rauschenbach Richard 
Published: November 2022

Polymer electrolyte membrane water electrolysis (PEMWE) requires highly purified water (resistivity ≥ 1 MOhm·cm). If the electrolyzer is fed with water of poor quality, both the lifetime and the performance are reduced. [1] Mainly cationic species cause several degradation effects in the membrane and the catalyst layer of the cell. Within the membrane, protons are exchanged by the cations, leading to a reduced ionic conductivity. Further, according to Fenton's reaction, some transition metal cations catalyze the formation of radicals attacking the membrane structure. [2] Additionally, the catalyst layer is occupied with cations, which causes a reduced electrochemical surface area. [3] For offshore applications of PEMWE, the feed water is generated by the purification of seawater, which contains a high amount of impurities such as calcium and magnesium. [4, 5] This purified seawater nevertheless contains some impurities [4] and as soon as the purification process is erroneous, the PEMWE is contaminated with the cations. For that reason, the scope of this contribution is to find a threshold value of a magnesium and calcium contamination in the water supply of a PEM water electrolysis cell, which affects the cell performance and lifetime to a tolerable extent.

To do so, electrolysis cells equipped with commercially available catalyst coated membranes (CCM) are contaminated with magnesium and calcium solutions of different concentrations and operated for up to 100 h. The actual performance of a cell is measured by polarization curve and electrochemical impedance spectroscopy every 25 h. In order to get deeper insights into the degradation phenomena, several in- and ex-situ characterization techniques are used. The investigation contains ion chromatography (IC), inductive coupled plasma mass spectroscopy (ICP-MS), mass spectrometry (MS) and thermo-gravimetric analysis (TGA).

The polarization curves show the cell's overall performance over time but give only a little information about the occurring phenomena. The high frequency resistance of the measured impedance spectra can determine the change in the ionic resistance of the membrane. This ionic resistance increases when the cations occupy the active sides of the membrane. Otherwise, a possible membrane thinning caused by the radical attack decreases the ionic resistance. To distinguish between these antithetical effects, the fluoride concentration in the water supply is measured with an anion IC. The resulting fluoride emission rate correlates with membrane thinning and quantifies this effect. Another method to detect membrane thinning is investigating the process gases with MS. The occurring gas crossover, especially at the anode with its H2 in O2 content, indicates a membrane thickness change. The occupation of the catalyst particles by cations is measurable in-situ by tracking the polarization current at a set potential over time and the impedance spectra reveal changes in the charge-transfer and resistance characteristics as well. Other degradation effects like losing catalyst particles because of weak ionomer structure or catalyst dissolution are investigated with the ICP-MS, which measures the metal concentration in the process water. At the end of the test procedure, the CCMs are analyzed in the TGA to investigate the thermal stability after the operation and the residual weight fraction of the catalyst particles, which is also an indicator for catalyst loss. With these in- and ex-situ characterization techniques, the degradation effects in a PEM electrolysis cell caused by different cation contaminations are investigated separately and their specific contribution is revealed. With this knowledge of different cation concentrations, a relation is derived to find a tolerable concentration of cations in PEM electrolysis operation.

References

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