Operando neutron imaging of PEWE cation mitigation

M. Zlobinski, U. Babic, V. Manzi-Orezzoli, M. Siegwart, L. Gubler, T.J. Schmidt, P. Boillat

Since hydrogen has been introduced as a fuel for mobility and stationary applications, making electrolysers efficient, safe and economical has been a topic of high importance. PEWE (Polymer Electrolyte Water Electrolysers) are especially suit-able for high purity hydrogen generation because Anode (H2O splitting side) and Cathode (H2 evolution side) are separated gas wise. Another important reason why PEWE is so attractive is that it can generate hydrogen efficiently under pressurised conditions what excludes compressors from the system. In or-der to make this technology competitive on the market it is necessary to grasp better understanding of processes taking place during operation, including the degradation processes.

This study shows a first attempt to image contamination of electrolysers by cations across the membrane which could be another small step to making PEWE durable and efficient. The ultimate goal of this research is to determine the cation behav-iour in the MEA under various operating conditions and to tell whether it is possible to regenerate the membrane without cell disassembly or at least slow down the degradation signifi-cantly. In order to image cation movement within PEM it was necessary to find a suitable element which is a good neutron absorber and could represent PEWE usual ionic contaminants such as Fe³⁺ relatively well. To this purpose, we chose to mim-ic the iron contamination using Gadolinium ions, whmim-ich are found in the same oxidation state and provide an exception-ally high cross section for neutrons.

Experimental setup

Measurements were carried out at SINQ Neutra beamline us-ing a tilted detector assembly to improve the resolution. The used setup features a pixel size of 6 µm and an effective reso-lution of approximately 20 µm which is enough to investigate the PEM cross section (thickness ~ 200 µm). The setup used during these experiments was prepared in a way that repro-duces normal operating conditions as well as possible ones, even though due to possible test bench contamination it was necessary to use external storage tanks.

Scheme 1. Simplified experimental setup schematic.

The electrolyser key components were mostly provided by commercial suppliers, including MEAs (Membrane Elec-trode Assemblies) based on N117 membranes delivered by Greenerity. For PTLs (Porous Transport Layers) we used sin-tered titanium T10 (36 % porosity) material provided by Sika.

The cell housing volume was relatively large compared to the active area which minimised the impact of operating condi-tions on the water temperature. Heating regulation of the sys-tem was designed in a way that ensures a stable and desired temperature in the flow fields even if water in the supply tank differs significantly from the temperature set point.

Two feed water tanks were used: one filled with ultrapure wa-ter and another one with a Gadolinium salt in ultrapure wawa-ter solution (concentration 1 mMol). The selection of the supply tank was done using solenoid valves placed before the pump and manually operated valves were used for the return lines.

The tanks were assembled in a way that provided relatively uniform temperatures of both fluids inside. The piping provid-ing the fluid to the cell itself was equipped with a controlled heating system.

Results

Initial measurements consisted of several IV curves and a conditioning period i.e. for MEA hydration, PTL hydration and reaching the desired temperatures of the system. The Elec-trolyser was left operating for 12 hours at a current density of 1 A/cm² to analyse the initial degradation rate of the MEA which is caused by the test bench itself, as no ion exchanger was placed in the loop. Once the cell has been characterised the contaminant was introduced in the loop. The response of the system was immediate.

Figure 1. Cell performance over time depending on feed water content.

Figure 1 shows the impact of the contaminant in the loop on the cell performance. Once the feed water containing the Gadolinium has been introduced, accelerated degradation oc-curred. This shows how much electrolyser cells are sensitive to the feed water purity. Upon switching back to the clean water loop steady regeneration could be observed. The mechanism behind the regeneration is yet to be explored.

Figure 2. Impedance spectra, contamination and regeneration.

The observations from neutron imaging give us hints on the cation content and behaviour.

Figure 3. Gadolinium migration toward Cathode catalyst layer over time under galvanostatic 1 A/cm² operating conditions.

Figure 3 shows accumulation of Gadolinium cations near the cathode catalyst layer while middle areas of membrane stay relatively pure. This information alone suggests that re-generation methods should focus on utilisation of the cath-ode compartment for cations removal from the membrane, for example by introducing a slightly acidic environment in that compartment. To further assess how the cations move as a function of current density, investigations with start/stop cycles were conducted.

Figure 4. Cation movement within the membrane during start stop operation cycles.

The images (Figure 4) show that, as soon as we stop drawing current through the electrolyser, the cations diffuse from the cathode catalyst layer to somewhere else and reappear upon applying current. Such behaviour can partially explain typical short voltage peaks at electrolyser start-ups resulting from cation rearrangement in the MEA. By plotting intensity

pro-files across the membrane (Figure 5), we clearly see a distinct dip of intensity during ON state as well as an intensity gradient across the membrane.

Figure 5. Intensity profiles across the MEA for ON/OFF.

We also note that during OFF state average intensity in the membrane decreases meaning that Gadolinium deposited around the cathode catalyst layer diffused back into the mem-brane.

Conclusions

The presented results using Gd cations to emulate usual con-taminants as Fe provided insight into the processes occurring within a heavily contaminated PEWE MEA. The membrane is able to partially regenerate itself provided that the feed water is pure.

The cations were observed to move towards the cathode catalyst layer under the effect of current and to accumulate there, though they seem to be less detrimental in this location than in the membrane or in the anode catalyst layer. Using this observation, it might be possible to design a regeneration procedure using the cathode side under operating conditions.

Acknowledgement

The Swiss Federal Office for Energy is greatly acknowledged for financial support of our research. Additionally, the authors would like to thank SINQ beamline scientists for making pre-liminary measurements possible.

References

[1] S. Sun, Z. Shao, H. Yu, G. Li, B. Yi, J. Power Sources 267 , 515–520 (2014).

[2] X. Wang, L. Zhang, G. Li, G. Zhang, Z. Shao, B. Yi, Electrochim.

Acta 158 , 253–257 (2015).

SCIENTIFIC ACHIEVEMENTS 2017