Laser structured PTL: Porosity vs. electrical contact in in proton exchange membrane water electrolysis (PEM-WE) cells

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 1/7 B1401 Laser structured PTL: Porosity vs. electrical contact in in proton exchange membrane water electrolysis (PEM-WE) cells Sebastian Daniel Hirt* (1), Philipp Maack (2), Pascal Sous (1), Vimanshu Chanda (1), Cemal Esen (2), Hesham Solh (1), Andreas Ostendorf (2), Natalia Levin (1) (1) The hydrogen and fuel cell center, Duisburg/Germany; (2) Applied Laser Technology, Ruhruniversität Bochum, Bochum/Germany; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract The global transition to a decarbonized economy is projected to require a hydrogen production capacity of approximately 5000 TWh by 2030. PEM-WE is well-suited for this task due to its flexibility in responding to load fluctuations (arising from renewable energies) and its ability to produce hydrogen at high pressures. The PEM-WE cell consists of a catalyst-coated membrane where electrochemical water splitting occurs through the oxygen and hydrogen evolution reactions. For efficient PEM-WE operation, it is crucial to ensure good electrical contact between the catalyst layer and the membrane, while also facilitating homogeneous transport of water to the membrane and removal of the generated oxygen at the anode side. Inhomogeneous flow or gas bubble accumulation over the catalyst particles increases internal resistance, leading to higher cell voltage and reduced process efficiency. The porous transport layer (PTL), typically made of titanium, plays a key role in media transport within the cell and the electrical connection of bipolar plate and catalyst layer.[1] This work investigates the fabrication of defined porosities in titanium foils through laser drilling. Based on systematic electrochemical results with different custom-made foils, we identify key parameters for PTLs, showing that the mass transport resistance significantly decreases with a 2D porosity above 1% (Figure below). The correlation of porosity with the electrical connection of the catalyst was also investigated. Figure 1: left: Picture and SEM image of a structured titanium foil. right: Polarization curve of cells with PTL with different porosities. References: [1] T. L. Doan et al., Int. J. Energy Res., 2021, 45, 14207–14220. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 2/7 Introduction The global transition towards sustainable energy systems has brought green hydrogen to the forefront as a critical enabler of decarbonization. Among the various hydrogen production technologies, proton exchange membrane water electrolysis (PEM-WE) stands out due to its ability to generate high-purity hydrogen at high current densities, as well as its excellent compatibility with fluctuating renewable energy sources such as solar and wind.[1] A central component influencing PEM-WE efficiency is the porous transport layer (PTL), which is positioned between the catalyst layer (CL) and the flow field. It serves multiple roles: delivering water to reaction sites, removing generated gases, conducting heat and electrons, and supporting the membrane mechanically.[2] The structure of the PTL—particularly its porosity, pore size distribution, tortuosity, and thickness—directly impacts ohmic losses and mass transport overpotentials.[3,4] Recent research has shown that while higher porosity can facilitate gas removal, it may also compromise mechanical integrity and increase ohmic resistance.[5] Conversely, denser structures improve electrical conductivity but hinder water and gas flow. The use of microporous layers (MPLs) on the CL-facing surface of the PTL has emerged as a solution to mitigate this trade-off. MPLs improve the contact between the CL and PTL, enhance catalyst utilization, and reduce interfacial resistances.[3,6,7] An emerging strategy to further tailor PTL performance is surface modification by laser structuring. Femtosecond laser texturing of titanium PTLs has demonstrated significant improvements in electrochemical performance. This is attributed to the increased interfacial area and enhanced wettability, which facilitate improved water permeation and oxygen removal.[1,8] For example, Suermann et al. observed that laser-treated PTLs reduced both ohmic and mass transport losses by at least 6 and 2 mΩ cm² respectively at 4 A cm⁻², compared to untreated samples.[1] In addition to surface structure, PTL thickness plays a crucial role in balancing performance and durability. Studies by Weber et al. have shown that reducing PTL thickness can lower mass transport overpotentials. However, when the PTL is too thin, it may result in insufficient compression, increased water transport resistance under the flow field landings, and membrane deformation.[9] The optimal PTL thickness is typically around half the width of the flow field land.[9,10] 1. Scientific Approach Porous titanium foils were fabricated by laser drilling in a liquid environment, yielding precisely defined hole networks with minimal thermal damage. The resulting PTL microstructures were characterized by confocal laser scanning microscope (CLSM) to determine hole diameter, spacing, and taper geometry. Their impact on PEM-WE performance was then assessed via polarization curves and EIS: polarization data quantified overall cell voltage and ohmic resistance, while impedance spectra—through equivalentcircuit fitting and DRT analysis—revealed charge-transfer kinetics and mass-transport limitations as a function of two-dimensional porosity. By correlating microstructural features with electrochemical responses, this integrated approach delivers clear guidelines for designing PTLs that balance conductivity, catalytic accessibility, and reactant/product transport for high-efficiency water electrolysis. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 3/7 2. Experiments Laser structuring of Ti foils:[11] The 25 μm thick grade 1 titanium foil (Ankuro TiGr1 ASTM B265) used throughout the experiments was cut into 55 mm² pieces and mounted on an adhesive carrier inside a liquidflow chamber. The carrier was then secured onto a machined precision surface inside the processing chamber. Subsequently, the chamber was sealed with a lid, ensuring precise parallel alignment and a uniform 2.50 mm water film between the foil surface and the integrated laser window. During the experiments, two pumps were constantly circulating deionized water at a maximum flow rate of 10.5 l min–1 in a closed-loop configuration. A secondary loop with nanoparticle filters were needed to clean the liquid. A peristaltic pump raised the absolute system pressure to 180 kPa to minimize gas contamination and bubble formation. For percussion drilling of the titanium foils inside the flow-chamber a Light Conversion Carbide CB3-40W laser system with the first harmonic at 1032 ± 10 nm was used. The optical emission was set to 193 fs long laser pulses in all experiments while the repetition rates were ranging from single pulses up to 2 MHz, with additional inter- (63 MHz) and intraburst (2.5 GHz) pulse trains. When using pulse trains, which effectively increase the repetition rate for up to ten subsequent pulses, the overall processing efficiency was enhanced. Within the optical scanning assembly, a water-cooled Hamamatsu LCOS SLM (Liquid Crystal on Semiconductor Spatial Light Modulator), a Scanlab Excelli Scan 14 galvanometer, and a telecentric f-theta objective with a focal length of 70 mm were used. While the scanner-system is utilized for fine positioning of the laser beam during processing, the SLM was used to generate up to nine parallel multi-beams. Fine adjustment of the focal plane was handled by a motorized mechanical z-axis, while larger structures requiring repositioning of the flow-chamber in x-y-direction were managed with a motorized Linos x.act XY 100 cross slide. High-resolution magnetic encoders from RLS ensured precise stage positioning for accurate intermeshing on the edges of several dozen micromachined sub-sections, resulting in seamless manufacturing of 25 mm x 25 mm areas while simultaneously achieving very high hole densities above 900 mm –2. The structure and geometry of the structured samples was analyzed by CLSM with a µsurf (NanoFocus AG) and a ZEISS Axio Imager A2 microscope. Electrochemical characterization: The PEM-WE test bench enables controlled operation of electrolysis cells under industrially relevant conditions. Key parameters include a temperature range of 40–80 °C, current densities up to 8 A cm–2, and atmospheric pressure. The system supplies process water in a closed loop with flow rates of 2–10 mL min–1 cm–², and features an external heater circuit with constant flow. The test cell (BalticFuelCells) has an active area of 5 cm². Electrical power is provided by a programmable power supply (up to 40 A). Electrochemical measurements, including electrochemical impedance spectroscopy (EIS), were conducted using a Gamry Reference 3000 potentiostat in combination with a power booster. Measurement and control systems monitor voltage, current, temperature, pressure, and water conductivity. Data acquisition is performed via a LabVIEW interface. Safety features include pressure relief valves, leak detectors, overtemperature protection, and automated shutdown protocols. Before testing, the cell was conditioned by heating to 60 °C under a continuous water flow of 50 mL min–1. Subsequently, 20 cyclic voltammograms were performed between 0.05 V and 1.5 V at a scan rate of 100 mV s–1. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 4/7 Polarization curves were recorded from 0.002 A cm–2 to 5 A cm–² with a scan rate of 0.01 A s–1. A cutoff criterion of 2.3 V was applied to avoid damage to the membrane. Electrochemical impedance spectroscopy (EIS) covered a frequency range from 100 kHz to 0.1 Hz with 10 points per decade. The DC current was set to 1 A cm–², and the AC perturbation amplitude was 0.5 A. Figure 1:Schematic overview on the test plan for the electrochemical measurements. 3. Results and discussion Four laser-structured titanium foils were prepared and investigated as PTL for PEM water electrolysis. The samples differed in three key geometric parameters: hole spacing (140 – 280 µm), hole diameter (18.16 – 23.22 µm), and resulting porosity (0.38 – 2.56%) (Table 1). The hole diameters were determined via optical microscopy, while the hole spacing was defined during the laser structuring process. The two-dimensional porosity was calculated based on the measured hole diameters and spacing, assuming a uniform hole distribution and neglecting material depth. This simplification is justified by the absence of a complex 3D architecture in the foil structure. Table 1: Overview on the samples and their key parameters. Sample Hole to hole distance µm hole diameter µm porosity % 1 140 23.22 2.56 2 210 19.13 0.76 3 280 22.04 0.56 4 280 18.16 0.38 The laser drilling process was carried out in a water environment to improve thermal management during ablation. Immersing the samples in water enhances heat dissipation and is expected to reduce thermal damage such as local melting or burr formation at the hole edges.[11] This approach is intended to enable the fabrication of uniform and welldefined microstructures, with minimal defects. The use of controlled laser parameters and submerged processing aims to ensure consistent hole geometry across the entire PTL surface, which is crucial for reproducible structure–property correlations in electrochemical testing. For analyzing the hole geometry and the surface structure CLSM images were taken (Figure 2): EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 5/7 Figure 2: CLSM images of the laser structured PTL with porosities of a) 2.56%, b) 0.76%, c) 0.56% and d) 0.38%. CLSM measurements confirmed that the hole spacing corresponded well to the predefined values set during laser structuring. The laser-drilled holes displayed an elliptical shape, deviating from ideal circular symmetry. The holes also exhibited a conical geometry, with the diameter decreasing along the drilling axis. This tapering is attributed to the timedependent laser exposure: as the beam penetrates deeper into the material, the ablation front narrows, leading to a gradual reduction in hole diameter. Therefore, hole shape and size are strongly influenced by laser exposure time. Additionally, CLSM revealed the presence of rim zones around the holes, typically 10 – 210 µm wide. These areas likely result from localized heat accumulation or redeposition effects during the drilling process. The polarization curves of the four laser-structured PTLs reveal a pronounced dependence of cell voltage on two-dimensional porosity (Figure 3a). At 1 A cm⁻², the two highest-porosity foils (Samples 1 and 2 with 2.56% and 0.76% porosity, respectively) both reach 1.76 V, while the 0.56% sample (Sample 3) requires 1.78 V and the lowest-porosity foil (0.38%, Sample 4) 1.80 V. Increasing the current density to 2 A cm⁻² amplifies these differences: the 2.56% porosity layer rises to 2.01 V, the 0.76% to 2.02 V, and the 0.56% remains at 2.17 V, whereas the 0.38% sample reached the cut of voltage before at1.73 A cm–2. In the activation regime (< 0.2 A cm⁻²), all curves converge near the thermodynamic voltage (~ 1.45 V), but higher porosity clearly lowers charge-transfer losses. In the ohmic region (0.2 – 2 A cm⁻²), the shallower slopes of the 2.56% and 0.76% samples reflect reduced membrane and contact resistances relative to the denser foils. Finally, the sharp voltage upturn observed above 1.00 A cm⁻² for the 0.38% sample and above 1.30 A cm⁻² signals the onset of mass-transport limitation, while the most porous PTL maintains a more EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 6/7 moderate increase, underscoring the critical role of optimized pore networks in enhancing both charge-transfer kinetics and reactant/product transport in PEM electrolysis. Figure 3: Electrochemical characterization of laser-structured PTLs with varying twodimen-sional porosities. a) Polarization curves recorded at 60 °C and 1 bar, showing cell voltage as a function of current density for samples with porosities of 2.56%, 0.76%, 0.56% and 0.38%. b) Nyquist plots (symbols) and equivalent-circuit fits (solid lines) using the model shown in d). c) Distribution-of-relaxation-times (DRT) spectra. In the Nyquist plot (Figure. 3b) the spectra of the EIS measurements are shown. The data were fitted with the equivalent circuit shown in Figure 3d. The small inductive element L₀ (≈10⁻¹⁰ – 10⁻⁹ H) is attributed to the test‐cell cabling and electrical connections and does not influence the intrinsic electrochemical response. The ohmic resistance (Rohm) strongly correlates with the porosity of the PTL samples. The cell with a PTL porosity of 2.56% Rohm was 197 mΩ cm² and rised to 242 mΩ cm² with a porosity of 0.38%. The R₁-attributed to the activation resistance of the oxygen evolution reaction (OER)- increases as two‐dimensional porosity decreases, rising from 42.8 mΩ cm² at 2.56% porosity to 45.5 mΩ cm² at 0.76%, 64.7 mΩ cm² at 0.56%, and reaching 154 mΩ cm² at 0.38% porosity. This trend reflects progressively impeded electron transport to the catalyst sites in less porous foils. The distribution‐of‐relaxation‐times (DRT) analysis of the impedance data (Figure 3c) reveals three well‐separated processes whose relative intensities and time constants evolve strongly with PTL porosity. At the shortest times (<10⁻4 s) a small, porosity‐independent feature reflects pure ionic conduction and contact effects. A major mid-frequency peak around 10⁻2 s was assigned to the electrochemical charge-transfer step at the catalystelectrolyte interface (OER).[12] This peak grows in amplitude and shifts slightly towards longer relaxation times as porosity decreases, mirroring the increase in R₁ and constant EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476292 B1401 Page 7/7 phase element of the anode reaction (CPE₁) heterogeneity. Most notably, a pronounced low‐frequency peak between 10 and 10¹ s, which corresponds to gas–liquid mass‐transport in the porous network,[13] is minimal for the 2.56% sample but dominates the DRT spectrum of the 0.38% foil (reaching > 12 mΩ cm²). This increase in diffusion-related relaxation confirms that low-porosity PTLs cause significant transport limitations under operating conditions. Interestingly, the high-porosity layers display narrower, more distinct DRT peaks. In contrast, the low-porosity samples exhibit broader, overlapping features, suggesting increased structural and transport heterogeneity. Together, these observations emphasize that optimizing PTL porosity is essential for reducing charge-transfer resistance and mitigating mass-transport bottlenecks in PEM electrolysis. In conclusion, we evaluated four laser-drilled PTL samples with porosities ranging from 0.38% to 2.56%. Submerged drilling produced uniform, elliptical, conically tapered micro-holes that were confirmed by CLSM. Electrochemical testing revealed that increasing two-dimensional porosity markedly improves the electrical connectivity of active catalyst sites and lowers both ohmic and mass-transport resistances. Consequently, the tailored PTL architecture exerts a strong, quantifiable impact on PEM-WE performance, providing a clear pathway for designing more efficient and durable electrolyzer electrodes. References [1] Y. Tao, M. Wu, M. Hu, X. Xu, M. I. Abdullah, J. Shao, H. Wang, SusMat, 2024, 4, e230. [2] T. L. Doan, H. E. Lee, S. S. H. Shah, M. Kim, C.-H. Kim, H.-S. Cho, T. Kim, Int. J. Energy, Res., 2021, 45, 14207-14220. [3] B. Hasa, U.R. Aryal, S. Higashi, N. E. Tolouei, J.T. Lang, B. Erb, A. Smeltz, I. V. Zenyuk, G.Zhu, Appl. Catal., B, 2025, 361, 124616. [4] G. Stelmacovich, S. Pylypenko, ChemElektroChem, 2024, 11, e202400377. [5] R.-T. Liu, Z.-L. Xu, F.-M. Li, F.-Y. Chen, J.-Y. Yu, Y. Yan, Y. Chen, B. Y. Xia. [6] M. F. Ernst, V. Meier, M. Kornherr, H. A. Gasteiger, J. Electrochem. Soc., 2024, 171, 074511. [7] C. Qui, Z. Xu, F.-Y. Chen, H. Wang, ACS. Catal., 2024, 14, 921-954. [8] M. Suermann, T. Gimpel, L. V. Bühre., W. Schade, B. Bensmann, R. HankeRauschenbach, J. Mater. Chem. A., 2020, 8, 9, 4898-4910. [9] C. C. Weber, T. Schuler, R. De Bruycker, L. Gubler, F. N. Büchi, S. De Angelis, JOPS Adv., 2022, 15, 100095. [10] Z. Kang, S. M. Alia, J. L. Young, G. Bender, Electrochim. Acta, 2020, 354, 136641. [11] P. L. Maack, C. Esen, A. Ostendorf, JLMN, 19, 3, 192-203. [12] A-L. Chan, H. Yu, K. S. Reeves, S. M. Alia, J. Power Sources, 2025, 628, 235850. [13] Y. Li, Y. Jiang, J. Dang, X. Deng, B. Liu, J. Ma, F. Yang, M. Ouyang, X. Shen, Chem. Eng. J., 2023, 451, 138327. Acknowledgements This work was financially supported by the Federal Ministry of Economic Affairs and Climate Action on the basis of a decision of the German Bundestag (IGF: 21971N). Keywords: EFCF2025, H2, LowTemp. Fuel Cells & Electrolysers, PEM-WE, porous transport layer, laser structuring Remark: This work is licensed under Creative Commons Attribution 4.0 International