Operando Sub-Second Neutron Imaging of Cationic Species Migration in a Proton Exchange Membrane Water Electrolyzer

Abstract

In recent years, it became clear that implementing energy storage on a large scale is a necessary route for the power production sector to maintain a stable and sustainable grid with increasing share of intermittent renewable sources of energy. Proton exchange membrane water electrolyzers show promise especially because of their suitability to be coupled to renewables in remote areas decentralizing the energy grid. There are, however, some challenges, which have to be tackled before market penetration can occur. One of the most pressing issues is the susceptibility of the proton exchange membrane to feed water impurities in form of cations [1], [2]. Titanium porous transport layers, piping and other system components will introduce slight amounts of cations during normal operation (Fe, Na, Cu, Ca, Ti), which over a longer period will become the cause of significant overpotential [3]. It is therefore necessary to understand the contamination mechanism of proton exchange membranes and its impact on the electrochemical performance of the cell. Motivated by the fact that the reversible degradation is one of the key factors which dictate the operability of an electrolyzer, we carried out experiments which bring further insights into the influence and behavior of the cationic species in the proton exchange membrane water electrolyzer under operation and standby conditions. Neutron imaging techniques were employed to observe the model contaminant in form of Gd3+, which is a perfect fit due to its large neutron cross-section. We found that under the effect of the electric field the cations migrate and accumulate near cathode catalyst layer/porous transport layer interface. A sub-second imaging setup enabled us to correlate the position of cations within the membrane with electrochemical data. Subsequently we regenerated the cell by operating it with diluted sulfuric acid (below 1mMol/L) injected into cathode compartment during the neutron imaging. Influence of the accumulation of the cations near the cathode catalyst layer on the electrolyzer performance and optimal operando regeneration procedures will be discussed. [1] S. Sun, Z. Shao, H. Yu, G. Li, and B. Yi, “Investigations on degradation of the long-term proton exchange membrane water electrolysis stack,” J. Power Sources, vol. 267, pp. 515–520, 2014. [2] X. Wang, L. Zhang, G. Li, G. Zhang, Z. G. Shao, and B. Yi, “The influence of Ferric ion contamination on the solid polymer electrolyte water electrolysis performance,” Electrochim. Acta, vol. 158, pp. 253–257, 2015. [3] U. Babic, M. Suermann, F. N. Büchi, L. Gubler, and T. J. Schmidt, “Critical Review—Identifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development,” J. Electrochem. Soc., vol. 164, no. 4, pp. F387–F399, 2017. Figure 1