Oxidation of Porous Transport Layers in Anion Exchange Membrane Water Electrolysis with Alkaline and Pure Water Feed

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244125 A0812 Page 1/6 A0812 Oxidation of Porous Transport Layers in Anion Exchange Membrane Water Electrolysis with Alkaline and Pure Water Feed Luis Hagner (1,2), Severin Vierrath (1,2), Susanne Koch (1,2) (1) Electrochemical Energy Systems, IMTEK - Department of Microsystems Engineering, Universität Freiburg/Germany; (2) Hahn-Schickard, Freiburg/Germany; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract In anion-exchange membrane water electrolysis (AEMWE), porous transport layers (PTLs) can also be applied as porous transport electrodes (PTEs), i.e. with no additional catalyst layer between membrane and PTL. Especially in alkaline conditions, PTEs made from stainless steel have shown high activity and durability, rivaling that of systems with dedicated catalyst layers, such as NiFe-hydroxide powder based catalyst layers.[1] The aim of this study is to investigate electrode activation over time, relating surface oxidation and Fe transport to cell voltage increase. Different fiber electrodes are investigated, measuring polarization curves up to 4 Acm-2 and degradation at 1 Acm-2 in 1.0 M KOH at 60 °C. All stainless steel cells show similar degradation profiles, with degradation rates from 200 – 400 µVh-1 in the last 50 h of testing. After disassembly, metal-containing particles are observed in the cathode-membrane interface. SEM-EDX confirms Fe as the highest mass fraction of these particles (e). Figure 1: Single cell measurements of stainless steel fiber electrodes (PTEs). Electrodes were tested at 60 °C in 1.0 M KOH up to currents of 4 Acm-2 in polarization curves (a) and for 100 h in durability testing at 1 Acm-2. The oxidation of these electrodes depending on operating potential leads to dissolution of Fe and other metals contained in the steel (c), that get deposited in the cathode/membrane interface (d), (e). EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244125 A0812 Page 2/6 Introduction Anion-exchange membrane water electrolysis (AEMWE) is a promising technology for hydrogen production, leveraging the benefits of low-cost materials for catalysts and components and further efficient variable load system operation. The alkaline media, often implemented by using KOH as a supporting electrolyte, enables the use of transition-metal based catalysts (e.g. based on Ni, Fe, Co), reducing raw materials cost significantly by orders of magnitude compared to Ir/Ru-based anode catalysts used in proton-exchange membrane water electrolysis (PEMWE). NiFe-oxyhydroxides (NiFe-OOH) have shown to be highly active materials that can be synthesized as powders or grown directly on substrates. However, electrodes prepared by these methods using a porous-transport-layer (PTL) with dedicated catalyst layer (powder-based or self-supported), have limited mass activity, owed to a low electrical conductivity of these types of catalysts, and because of high-water uptake binders affecting mass-transport and probably particle separation[2]. Also, Ni-based oxyhydroxides suffer from a “charge-trapping” effect during potential cycling, due to different electrical conductivity of the involved Ni-species.[3] In contrast, stainless steel fiber electrodes provide a simple system architecture, combining PTL and catalyst layer in a porous-transport electrode (PTE). The defined interface between PTE and membrane further simplifies the study of these systems. Using a supporting electrolyte of 1 M KOH enables a high volume activity of the PTE, whereas in pure water operation an additional Ionomer coating ensures OHtransport. Stainless steel electrodes have appealing qualities for system architecture, cost and activity of AEMWE systems. Multiple studies conclude that the activation of Fe-containing electrodes changes surfacecomposition of the material, involving leaching of dominantly Fe and other metals such as Cr and Mo[4]. In general, the stability of Fe-containing electrodes is debated.[5] A stable operation at high current densities has been reported, but the authors still detected low amounts of Fe species deposited on the cathode[6]. Also, the effect of cathode (de)activation of the used Pt/C electrodes is not precisely known. In alkaline electrolysis (AWE), deposition of Fe on the cathode was found to increase cathode activity mainly because the deposited Fe increased the surface area of the cathode.[7] From chlorakali technology, so-called “poison-resistant” PtRu cathodes have been developed.[8] The enlargement of surface area and deposition modification made electrode performance less susceptible to deactivation by deposited Fe. Effects on the membrane material and performance related to transport of Fe species through the membrane have not been shown on an AEMWE cell level yet. However, it could be possible that migration of Fe-species from anode to cathode damages the membrane in a Fenton-type of reaction.[9] 1. Scientific Approach Different types of stainless steel electrodes were employed for AEMWE operation using a supporting electrolyte of KOH in different concentration ranges from 1.0 M to 10 mM. Cell voltage over time was monitored at 1 Acm-2 using EIS. Fiber diameters were correlated to electrode capacitance and activity. The corrosion behavior of the PTE led to Fe and Cr leaching, and deposition of these elements in the cathode/membrane interface. For visualization and elemental analysis, Xray Fluorescence mapping (XRF) and SEM-EDX were used. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244125 A0812 Page 3/6 2. Experiments/Calculations/Simulations Materials Stainless steel felts (SS-62/410, SS-71/260, SS-72/300, Bekipor 300 µm) were purchased from Bekaert. Main properties of the steel fibers used are listed in Table 1. Freudenberg H24C5 carbon paper with microporous layer (MPL) was used as cathode GDL with Pt/C (50 wt%, Elyst 5050) catalyst. Anion-exchange membranes (PiperIon 40) and Ionomer (PiperIon A, 5% in EtOH) were supplied by Versogen and FuelCellStore. Potassium hydroxide (KOH) pellets (85 %) was purchased from Carl Roth GmbH + Co. KG. Cathode gas-diffusion electrodes Pt/C cathode gas diffusion electrodes were spray coated using an ultrasonic spray coater (SNR 300, Sonocell). For the spraycoating ink, Pt/C catalyst (150 mg) was mixed with water (2 g), Isopropanol (7 g) and PiperIon A (0.8 g, 5% in Ethanol), Ionomer:Carbon ratio 0.2. The ink was sprayed on Freudenberg H24C5 gas diffusion layer, with a metal loading of 0.55 mg/cm2. Electrochemical measurements Prior to testing, PiperIon A 40 µm membrane and cathode GDE were immersed in 1 M KOH for at least 1 h. Cells were assembled using PTFE gaskets with 5 cm2 active area of varying thickness to ensure ca. 40 % cathode compression. The anode thickness of the stainless steel was matched to have no anode compression. Electrolyte was heated to 60 °C and circulated with 50 mLmin-1. A BioLogic VSP-300 potentiostat with two 10 A/5 V boosters was used to monitor cell voltage. Polarization curves were measured holding each current for 40 s plus 30 s of GEIS measurement (500 kHz to 100 Hz, amplitude 5%, not exceeding 50 mAcm-2). For a break-in of the cell, an additional polarization curve as described above was measured following EIS at 500 and 1000 mAcm-2. The electrode capacitance was determined by CV from 0.4 V to 0.5 V using scanrates from 200, 150, 100, 75, 50 and 25 mVs-1. After this, the final polarization curve used for evaluation was taken, followed by a current hold of 1 Acm-2 of varying time. The characterization of polarization curve, capacitance determination and GEIS were repeated after the current hold. Finally, the cell was checked for electrical shorts at 1.2 V for 5 min. Table 1: Properties of stainless steel fiber PTEs. PTE Type Porosity Thickness Layer type fiber diameter / µm SS-62/410 62 % 410 µm dual 3 SS-78/300 78 % 300 µm dual 5 SS-71/260 71 % 260 µm single 5 Bekipor 300 µm n.A. 300 µm single 10 3. Results The results of single cell testing are shown in Figure 2. All steel PTEs show high activity in the ranges of 2.0 to 2.1 V at a current of 4 Acm-2 (Fig.2a). Polarization curves corrected by ohmic drop (dotted lines) show that the most active are SS-62/410 and SS-B-300. The cells show an activation phase of ca. 30 h until reaching a linear degradation rate of 200 to 400 µVh-1 (Fig. 2b). Since the PTEs are made of the same 316L stainless steel (composition shown in Fig.2c), the improved kinetic activity can be related to the increased surface area of electrodes. Of all samples, SS-62/410 has the smallest fiber diameter in contact with the membrane and supporting electrolyte, leading to a high ECSA (Fig.3a). To confirm this, the electrode capacitance was measured using cyclic voltammetry (CV) at various scanrates (Fig.2d), extracting the net current densities of charge and discharge (Fig.1e). SS62/410 EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244125 A0812 Page 4/6 shows the highest capacitance of 4.0±0.1 mFcm-2 measured in a three-electrode cell setup (Fig.2f), as well as in the single cell test. As the best performing material in polarization curves and current holds, further cell experiments were conducted using this SS62/410 type of PTE. Other reasons for better performance could be improved mass transport and bubble management due to the dual layer configuration of the PTE. A coarse woven mesh at the backside creates additional volume between flowfield lands and PTE, creating more room for gas transport. Figure 2: AEMWE single cell tests with various stainless steel anode PTEs. Polarization curves shown (a), durability tests at 1 Acm-2 in (b). Stainless steel composition as determined by XRF (c), and capacitance measurements by CV d) – (f). Tests were conducted at 60 °C with a 0.5 mgcm-2 Pt/C on carbon paper cathode. After cell testing, disassembled cells were imaged using SEM. As a reference, a cell was assembled as normal, without electrochemical testing, and heated for 1h to 60 °C. This reference cell showed strong protrusion of the steel fibers on the anode side (Fig.3c), especially in the compressed region, where the flowfield land exerted the most force. On the opposite side, the cathode facing a homogeneous flat microporous layer of the GDL did not suffer from permanent deformation (Fig.3d). After 100 h of testing and disassembly, blackbrown particles were visible on both the membrane and cathode side in the cathode/membrane interface. SEM-EDX showed Fe species being the highest mass fraction of these particles, thus confirming that Fe species deposit as a result of anode PTE corrosion (Fig.3e/f). EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244125 A0812 Page 5/6 Figure 3: SEM images of SS-62/410 fiber felt and membrane surfaces after testing. The fine mesh fibers of SS-62/410 shown in a) with a magnification of the highlighted region in (b) protrude the membrane, leading to grooves visible in dry state (c). The cathode-facing side of the membrane shows islands of deposited Fe and detached Pt/C particles from the cathode (e), as determined by EDX (f). References [1] A. W. Tricker, J. K. Lee, J. R. Shin, N. Danilovic, A. Z. Weber, X. Peng, Journal of Power Sources 2023, 567, 232967. [2] a) S. S. Jeon, P. W. Kang, M. Klingenhof, H. Lee, F. Dionigi, P. Strasser, ACS catalysis 2023, 13, 1186; b) M. E. Kreider, H. Yu, L. Osmieri, M. R. Parimuha, K. S. Reeves, D. H. Marin, R. T. Hannagan, E. K. Volk, T. F. Jaramillo, J. L. Young et al., ACS catalysis 2024, 14, 10806. [3] a) D. A. Corrigan, S. L. Knight, J. Electrochem. Soc. 1989, 136, 613; b) M. B. Stevens, L. J. Enman, A. S. Batchellor, M. R. Cosby, A. E. Vise, C. D. M. Trang, S. W. Boettcher, Chem. Mater. 2017, 29, 120. [4] a) Suhas Nuggehalli Sampathkumar, T.B. Ferriday, Hugh Middleton, Jan Van herle 2023; b) S. Han, Jaeyune Ryu, Jeyong Yoon, Chemical Engineering Journal 2024; c) N. Todoroki, T. 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Garcia Barros, A. de Leeuw den Bouter, J. van der Schaaf, M. Theodorus de Groot, iScience 2024, 27, 108695. [8] D. S. Cameron, R. L. Phillips, P. M. Willis in Modern Chlor-Alkali Technology (Eds.: N. M. Prout, J. S. Moorhouse), Springer Netherlands, Dordrecht, 1989, pp. 95–107. [9] A. F. Staerz, M. van Leeuwen, T. Priamushko, T. Saatkamp, B. Endrődi, N. Plankensteiner, M. Jobbagy, S. Pahlavan, M. J. W. Blom, C. Janáky et al., Angewandte Chemie (International ed. in English) 2024, 63, e202306503. Keywords: EFCF2025, H2, LowTemp. Fuel Cells & Electrolysers, AEM, Degradation, Stainless steel electrodes, Electrode oxidation, Electrode corrosion Remark: This work is licensed under Creative Commons Attribution 4.0 International