An Efficient Method to Improve Corrosion Resistance of Ti Bipolar Plate for PEMFC via Electropulsing Treatment

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 1/10 A1109 An Efficient Method to Improve Corrosion Resistance of Ti Bipolar Plate for PEMFC via Electropulsing Treatment Bo Chen (1), Peiyong Yang (2), Daijun Yang (3), Wei Lu (2), Junying Min* (1) (1) School of Mechanical Engineering, Tongji University, 201804 Shanghai/China; (2) State Power Investment Corporation Hydrogen Energy Co., Ltd., 102600 Beijing/China; (3) Clean Energy Automotive Engineering Center & School of Automotive Studies, Tongji University, 201804 Shanghai/China; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract Bipolar plates (BPPs) are key components of proton exchange membrane fuel cells (PEMFCs), where their corrosion resistance and surface conductivity play a vital role in durability and performance of fuel cell stacks. In this study, we propose a novel method to enhance the corrosion resistance of coated Ti BPPs through rapid electropulsing treatment taking less than 2 seconds, which can be easily integrated into the manufacturing process of Ti BPPs. The electropulsing treatment facilitates the formation of a protective oxide layer on the Ti BPP surface, effectively reducing the self-corrosion current density from 0.26 μA/cm² to 0.13 μA/cm² and the corrosion current density at 0.6 V (vs. Ag/AgCl) from 0.33 μA/cm² to 0.09 μA/cm², without sacrificing the interfacial contact resistance (ICR). The finding provides a novel and efficient approach for improving the corrosion resistance of Ti BPPs, advancing their application in PEMFCs. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 2/10 1. Introduction Proton exchange membrane fuel cells (PEMFCs) play a crucial role in hydrogen-toelectricity conversion, offering high energy efficiency, zero carbon emissions, rapid start-up, and low operating temperatures. These advantages make PEMFCs well-suited for applications in electric vehicles and portable power supplies [1]. Bipolar plates (BPPs), as key components of PEMFCs, provide structural support, facilitate the distribution and separation of reactant gases, and enable efficient electrical conduction . However, during operation, BPPs are exposed to an acidic environment containing aggressive anions such as SO₄²⁻ and F⁻, requiring excellent corrosion resistance and high electrical conductivity [2]. Titanium exhibits several advantages, including low density, high specific strength, and superior corrosion resistance [2], making it a promising candidate for next-generation BPPs. Toyota's Mirai series pioneered the application of ultra-thin titanium BPPs, incorporating titanium dioxide and carbon nanocomposite coatings to enhance power density and extend PEMFC lifespan. The exceptional corrosion resistance of titanium stems from its dense surface oxide film, which naturally forms to a thickness of <10 nm under ambient conditions [3]. Various methods have explored to modify the properties of the oxide film to further enhance the corrosion resistance of titanium and its alloys, including thermal oxidation, micro-arc oxidation, anodic oxidation, laser etching etc. Among these methods, thermal oxidation offers several distinct advantages, such as simplicity, cost-effectiveness, uniform oxide layer formation, and high process flexibility [4]. Thermal oxidation has been widely employed for surface modification of titanium and its alloys across various fields, including aviation [5], marine [6], and biomedicine [7][8] etc. Wang et al. [6] fabricated a dense and adhesive oxide film in situ on the surface of titanium alloys through thermal oxidation treatment in air. They investigated the electrochemical corrosion behavior of the oxide film in artificial seawater, using the matrix alloys as control samples. The results showed that, compared to the matrix alloys, the thermally oxidized samples exhibited a 3-6 times reduction in annual corrosion rate, with pitting corrosion nearly eliminated. Xu et al. [8] conducted thermal oxidation treatment on novel titanium-based biomedical medium-entropy alloys at temperatures ranging from 400 to 600 °C for 2 hours in air. They found that the oxide layer enhanced both the wear resistance and corrosion resistance of the alloys. Furthermore, existing studies suggest that factors such as oxidation temperature [9], duration [10], and cooling rate [11] significantly influence the properties of oxide film, thereby affecting corrosion resistance [12]. Therefore, in situ fabrication of oxide films on titanium and its alloys through thermal oxidation is an effective method for improving their corrosion resistance. However, in the field of PEMFCs, studies directly leveraging the oxide film to improve the corrosion resistance of titanium BPPs remain limited. Our previous research [13] demonstrated that the oxide layer introduced by heat-assisted forming enhances the corrosion resistance of coated Ti-BPPs without reducing surface conductivity. Additionally, Joule heating enables rapid metal heating, thereby improving heating efficiency. Increasing evidence indicates that electric current has a potential effect on the microstructural evolution of materials [14][15], as well as the growth of oxides [16][17]. In this work, an efficient method utilizing electropulsing treatment to rapidly generate an oxide layer on the titanium surface was proposed. The corrosion resistance and surface conductivity of titanium BPPs with and without electropulsing treatment were compared. This method enhances the corrosion resistance of titanium BPPs within two seconds and can be easily integrated into the manufacturing process. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 3/10 2. Experiments details Sample preparation In this study, 0.1 mm thick commercially pure titanium sheets are employed, and their chemical composition is provided in Table 1. Table 1 Chemical composition of the 0.1 mm thick commercially pure titanium sheet (wt. %) Element Ti Mg Fe Si Ni Others Content/% 99.8 0.07 0.04 0.02 0.02 Bal. Uniaxial tensile testing was used to simulate the two-step forming process of titanium sheets. First, a 15% pre-straining was applied to the specimen, followed by a 10 % secondary straining. The detailed method for strain calculation can be found in reference [18]. Electropulsing treatment was conducted using the experimental platform shown in Fig. 1a. A pair of electrode clamps is employed to fasten the grip section of the tensile specimens. A pulsed current is subsequently generated by an electropulsing generator, passing along the length of the specimens. The voltage, treatment duration, pulsed current frequency, and pulse duty cycle were set to 140 V, 2 seconds, 4 kHz, and 0.5, respectively. The temperature history of the specimen recorded by an Optris PI400 infrared thermal imager, as shown in Fig. 1b. It was found that the electropulsing treatment rapidly heated the specimen to approximately 750 °C within 2 seconds. The absence of data below 125 °C is due to the measurement range limitation of the infrared thermal imager. The feasibility of the electropulsing treatment for titanium bipolar plates on the experimental platform has been verified in our previous study [18], demonstrating that the proposed electropulsing treatment can be seamlessly integrated into the manufacturing process. Fig. 1 (a) Experimental platform for electropulsing treatment; (b) Temperature history during electropulsing treatment To prepare samples for electrochemical measurements and interfacial contact resistance (ICR) tests, a layer of amorphous carbon was deposited onto both surfaces of electropulsing-treated and untreated samples. The surface morphologies of the coated samples are shown in Fig. 2c and d. The coating process was performed using a composite multi-functional coating machine. Initially, the samples were subjected to ultrasonic cleaning in alcohol for 10 minutes to remove surface contaminants. After drying, ion etching was applied for 20 minutes at an etching power of 2 kW to further eliminate surface impurities. Next, a Ti interlayer was deposited using the arc ion plating method, under a background EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 4/10 vacuum of 0.005 Pa and at a temperature of 200 °C, with a sputtering power of 18 kW for 2 minutes. Amorphous carbon coating was then deposited with a sputtering power of 20 kW and a sputtering time of 25 minutes. Once the coating process was complete, the samples were cooled within the coating machine, followed by a 5-minute ultrasonic cleaning in alcohol and air-drying. Electrochemical tests Electrochemical measurements were conducted using a three-electrode workstation, with a saturated Ag/AgCl electrode as the reference, a platinum mesh as the counter electrode, and specimens with an exposure area of 1.25 cm2 as the working electrode. To simulate the harsh environment in the PEMFC, the corrosion solution was maintained at 80 °C using a circulating water bath system. Throughout the electrochemical tests, oxygen was continuously supplied via a bubbling system to simulate the cathodic environment. The test solution consisted of pH = 3H₂SO₄ with an HF concentration of 2 ppm. Before conducting potentiodynamic and potentiostatic tests, the specimen was allowed to stabilize at the open circuit potential (OCP) for 1 hour. Potentiodynamic polarization was initiated at -0.6 V and progressed to 1.3 V (vs. Ag/AgCl) at a scanning rate of 1 mV/s. Potentiostatic polarization was maintained at 0.6 V (vs. Ag/AgCl) for 1 hour to simulate cathodic operating conditions. Electrochemical impedance spectroscopy (EIS) analysis was performed with a frequency range spanning from 100 kHz to 0.01 Hz, applying a sinusoidal voltage of 10 mV, and 12 measurement points were assigned per frequency decade. Interfacial contact resistance (ICR) measurement An automatic bipolar plate contact resistance testing machine was employed for the ICR measurement of the samples. TGP-H-060 carbon paper was used as the gas diffusion layer (GDL) for the ICR testing. For further details on the ICR measurement procedure, refer to the literature [19]. 3. Results and discussion Surface morphologies To elucidate the surface morphology of the samples with and without electropulsing treatment and coating, the samples were examined using an SS-X3000 micro-nano performance laser microscope, as shown in Fig. 2. It is observed that the surface of the formed sample exhibits numerous cracks and retains the original silver-white color (Fig. 2a). After electropulsing treatment, the surface appears purple and green (Fig. 2b). This distinct color change is associated with the growth of an oxide layer during electropulsing treatment, indicating an increase in the thickness of the titanium oxide layer [20]. After coating, the surfaces of all samples exhibit the gray-black appearance typical of the amorphous carbon coating. Surface cracks remain visible due to the coating’s thickness of approximately 400 nm [13], which is insufficient to fully cover the pre-existing cracks. The surface roughness of different samples is shown in Table 2. The surface roughness (Ra) of untreated and electropulsing-treated samples before coating is 0.426 ± 0.086 μm and 0.530 ± 0.104 μm, respectively. After coating, the surface roughness is 0.497 ± 0.042 μm and 0.446 ± 0.080 μm, respectively. These results suggest that neither the electropulsing treatment nor the coating process significantly affects the surface roughness of the titanium bipolar plates. Furthermore, the influence of surface roughness on the results of the subsequent electrochemical and ICR tests has been excluded. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 5/10 Fig. 2 Surface morphologies of different samples: (a) untreated before coating and (b) electropulsing-treated before coating; (c) untreated after coating and (b) electropulsingtreated after coating Table 2 Surface roughness of different samples Samples Ra (μm) Rz (μm) Untreated before coating 0.426 ± 0.086 2.569 ± 0.344 Electropulsing-treated before coating 0.530 ± 0.104 2.966 ± 0.518 Untreated after coating 0.497 ± 0.042 2.895 ± 0.277 Electropulsing-treated after coating 0.446 ± 0.080 2.610 ± 0.344 Electrochemical analysis To evaluate the effects of electropulsing treatment on the corrosion resistance and surface conductivity of titanium bipolar plates, electrochemical and ICR tests were conducted on the coated samples. Fig. 3a presents the potentiodynamic polarization curves of untreated and electropulsing-treated samples. The self-corrosion potential (Ecorr) of the electropulsingtreated sample is more positive than that of untreated sample, as shown in Fig. 3a, indicating a lower tendency for corrosion. The self-corrosion current densities (Icorr) of both samples were determined by extrapolating the linear segment of the polarization curves to the selfcorrosion potential, yielding values of 0.26 μA/cm² for the untreated sample and 0.13 μA/cm² for the electropulsing sample. A lower corrosion current density corresponds to a slower corrosion rate, demonstrating that the electropulsing-treated sample exhibits superior corrosion resistance. In the potential range of approximately 0.4-1.3 V (vs. Ag/AgCl), the Icorr of both samples increases rapidly, suggesting a significant deterioration in corrosion resistance at higher potentials. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 6/10 Fig. 3b shows the potentiostatic polarization curves at the operating potential of PEMFC (+0.6 VAg/AgCl). The corrosion current densities of untreated and electropulsing-treated samples are 0.33 μA/cm² and 0.09 μA/cm², respectively. The results of potentiostactic polarization further confirm that the electropulsing-treated sample has superior corrosion resistance compared to the untreated sample. This enhancement is attributed to the formation of a protective oxide layer (Fig. 2b) during the electropulsing treatment. Fig. 3 The results of electrochemical testing in pH = 3H2SO4 with 2 ppm HF concentration and O2 bubbling at 80 ℃: (a) potentiodynamic polarization curves; (b) potentiostatic polarization curves (+ 0.6 VAg/AgCl) Nyquist plots and Bode plats for untreated and electropulsing-treated samples before and after potentiostatic polarization are shown in Fig. 4a and b, respectively. As observed in the Bode plots, the electropulsing-treated sample exhibits a more negative phase angle in the low-frequency region. The peak shape of the phase angle variation with frequency suggests the presence of two closely spaced time constants, corresponding to two distinct corrosion processes. The high-frequency corrosion process is associated with the resistance of the amorphous carbon coating, while the low-frequency process corresponds to the resistance of the oxide layer. Notably, in the untreated sample, no time constant related to the oxide layer resistance is observed. For both samples, the phase angle variation in the 104-105 Hz range remains consistent, which is attributed to the phase shift caused by the internal resistance of the reference electrode. Based on these observations, two equivalent circuit models (Fig. 4b) were employed to fit the electrochemical impedance spectroscopy data for the untreated and electropulsing-treated samples. The untreated sample was fitted using circuit (i), where R1 and CPE1 represent the resistance and capacitance of the amorphous carbon coating, respectively, and Rs denotes the solution resistance. The electropulsingtreated sample was modelled using circuit (ii), where R1 and CPE1 still correspond to the resistance and capacitance of the amorphous carbon coating, while R2 and CPE2 represent the resistance and capacitance of the oxide layer, respectively. Due to the surface inhomogeneity, a constant phase element (CPE) was used instead of the ideal capacitance. The fitting results are summarized in Table 3. After corrosion, the slight decrease in R1 indicates a reduction in the corrosion resistance of the coating, which can be attributed to the infiltration of the corrosive medium, leading to the formation of additional defects. In contrast, the increase in R2 after corrosion suggests that the oxide layer undergoes further passivation during the corrosion process, thereby enhancing its protective properties. The overall resistance values (R0 = R1 or R1 + R2) represent the corrosion resistance of the samples. It is evident that the electropulsing-treated samples exhibit significantly higher corrosion resistance than the untreated ones, both before and after corrosion. Specifically, the R0 value of the electropulsing-treated samples before and after corrosion are 9.03 × 105 Ωcm2 and 1.44 × 105 Ωcm2, respectively, which are approximately 3.8 and 7.4 times higher EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 7/10 than those of the untreated samples. Moreover, while the corrosion resistance of the untreated samples deteriorates after exposure to the corrosive environment, the electropulsing-treated samples exhibit improved resistance. This enhancement is attributed to the further passivation of the oxide layer formed during the electropulsing treatment. Fig. 4 The results of electrochemical impedance measurement in pH = 3H2SO4 with 2 ppm HF concentration and O2 bubbling at 80 ℃: (a) Nyquist plots at respective EOCP; (b) Bode plots Table 3 Equivalent-circuit parameters derived from EIS data Samples Rs (Ωcm2) Q1 (Ω-1cm-2sn) n1 R1 (Ωcm2) Q2 (Ω-1cm-2sn) n2 R2 (Ωcm2) χ2 Before corrosion Untreated 1496 2.56 × 10-5 0.90 2.39 × 105 - - - 2.64 × 10-3 Electropulsing-treated 1494 2.41 × 10-5 0.90 2.34 × 105 2.44 × 10-5 0.76 6.69 × 105 2.16 × 10-3 After corrosion Untreated 1417 3.02 × 10-5 0.89 1.96 × 105 - - - 2.79 × 10-3 Electropulsing-treated 1453 2.62 × 10-5 0.90 2.02 × 105 1.81 × 10-5 0.79 1.24 × 106 2.20 × 10-3 Interfacial contact resistance (ICR) The BPPs not only require excellent corrosion resistance but also need to maintain good surface conductivity to ensure the optimal performance of PEMFCs. The ICR, which is a key indicator of surface conductivity, plays a crucial role in controlling the power output of the fuel cell stacks, as it directly affects the efficiency of electron transport between the BPPs and GDLs [1]. The ICR values for both untreated and electropulsing-treated samples, as a function of compaction pressure, are presented in Fig. 5a. As compaction pressure increases, the ICR values for both samples decrease, which is attributed to the increased contact area under higher pressure. The corrosion current densities and ICRs under 1.4 MPa of both untreated and electropulsing-treated sample are shown in Fig. 5b. Notably, the corrosion resistance of the electropulsing-treated sample is significantly improved compared to the untreated sample, due to the oxide layer introduced by the electropulsing treatment (corrosion current density decreased from 0.33 to 0.09 μA/cm²). Meanwhile, the electropulsing-treated sample exhibits comparable surface conductivity to the untreated sample (ICR under 1.4 MPa of 6.5 mΩcm2 vs. 6.2 mΩcm2). EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 8/10 Fig. 5 The ICR as a function of compaction pressure from 0.4 to 2.0 MPa for untreated and electropulsing treated samples 4. Conclusions In this study, a novel method for enhancing the corrosion resistance of coated Ti BPPs through rapid electropulsing treatment was proposed. The corrosion resistance and surface conductivity of titanium BPPs, both with and without electropulsing treatment, were compared, and the following conclusions can be drawn. • Electropulsing treatment facilitates the formation of a protective oxide layer on the titanium surface within 2 seconds (or even shorter), which can be easily integrated into the multi-step forming process of Ti BPPs. • Electropulsing treatment effectively reduces the self-corrosion current density of TiBPPs from 0.26 μA/cm² to 0.13 μA/cm², and the corrosion current density at 0.6 V (vs. Ag/AgCl) from 0.33 μA/cm² to 0.09 μA/cm², without sacrificing the ICR. (ICR under 1.4 MPa of untreated vs. electropulsing-treated: 6.2 mΩcm2 vs. 6.5 mΩcm2) • The oxide layer formed during electropulsing treatment undergoes further passivation in a simulated PEMFC environment, which is the key mechanism for enhancing corrosion resistance of coated Ti-BPPs. This method provides valuable insights into improving the durability of Ti-BPPs in fuel cell stacks. Acknowledgements This study is supported by the National Key Research and Development Program of China (No. 2022YFE0207500). References [1] Leng, Y., Ming, P., Yang, D., Zhang, C.: Stainless steel bipolar plates for proton exchange membrane fuel cells: Materials, flow channel design and forming processes. Journal of Power Sources 451 (2020), 227783. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17476229 A1109 Page 9/10 [2] Yasin, M. C., Johar, M., Gupta, A., Shahgaldi, S.: A comprehensive review of the material innovations and corrosion mitigation strategies for PEMWE bipolar plates. International Journal of Hydrogen Energy 88 (2024), 726-747. [3] Najafizadeh, M., Yazdi, S., Bozorg, M., Ghasempour-Mouziraji, M., Hosseinzadeh, M., Zarrabian, M., Cavaliere, P.: Classification and applications of titanium and its alloys: A review. Journal of Alloys and Compounds Communications (2024), 100019. [4] Lin, N., Xie, R., Zou, J., Qin, J., Wang, Y., Yuan, S., Li, D., Zhao, L., Zhang, L., Wang, Z., Ma, Y.: Surface damage mitigation of titanium and its alloys via thermal oxidation: A brief review. Reviews on Advanced Materials Science 58(1) (2019), 132-146. [5] Ahmed, F. S., El-Zomor, M. A., Ghazala, M. S. A., Elshaer, R. N.: Effect of oxide layers formed by thermal oxidation on mechanical properties and NaCl-induced hot corrosion behavior of TC21 Ti-alloy. Scientific Reports. 12(1) (2022), 19265. [6] Wang, W., Cui, W., Xiao, Z., Qin, G.: The improved corrosion and wear properties of Ti-Zr based alloys with oxide coating in simulated seawater environment. Surface and Coatings Technology 439 (2022), 128415. [7] Hussain, S. N., Abdullah, A. S., Hassan, M. A., Daud, R., Arafat, A., Ahmad, M. F.: A Review on Wear and Corrosion Behavior of Thermal Oxidation on Titanium-based Alloy for Biomedical Application. Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 58(2) (2019), 153-160. [8] Xu, X., Li, Z., Lai, W., Wang, B., Xu, Q., Zhang, Z., You, D., Li, W., Wang, X.: Improved wear and corrosion resistance of biomedical TiZrNbTaMo medium-entropy alloy by thermal oxidation treatment. Tribology International 189 (2023), 108897. [9] Aniołek, K.: The influence of thermal oxidation parameters on the growth of oxide layers on titanium. Vacuum 144 (2017), 94-100. [10] Kumar, S., Narayanan, T. S., Raman, S. G. S., Seshadri, S. K.: Thermal oxidation of CP-Ti: Evaluation of characteristics and corrosion resistance as a function of treatment time. Materials Science and Engineering: C 29(6) (2009), 1942-1949. [11] Jamesh, M., Narayanan, T. S., Chu, P. K.: Thermal oxidation of titanium: Evaluation of corrosion resistance as a function of cooling rate. Materials Chemistry and Physics 138(2-3) (2013), 565-572. [12] Jamesh, M., Kumar, S., Sankara Narayanan, T. S. N.: Effect of thermal oxidation on corrosion resistance of commercially pure titanium in acid medium. Journal of materials engineering and performance 21 (2012), 900-906. [13] Chen, B., Ge, B., Zhang, X., Yang, D., Yang, P., Lu, W., Min, J., Ming, P., Zhang, C.: Role of oxide layer on corrosion resistance and surface conductivity of titanium bipolar plates for proton exchange membrane fuel cell. Journal of Power Sources 624 (2024), 235637. [14] Wang, R., Xu, Z., Jiang, Y., Tang, G., Wan, J., Chou, K. J. C. Z., Li, Q.: The coupling of thermal and athermal effect in high-density multiple pulse continuous treatment of AZ31. Materials & Design 215 (2022), 110495. [15] Yang, Y., Qin, R., Dong, Y., Wang, J., Ye, C.: Crystal plasticity modeling of electropulsing induced plasticity in metals. International Journal of Plasticity 171 (2023), 103828. [16] Piqueras, J., Hidalgo, P.: Growth of metal oxide nanostructures by thermal oxidation of metals under influence of external electric fields and electric current flow. physica status solidi (a) 218(24) (2021), 2100323. [17] Wu, H., Lu, Q., Li, Y., Wang, J., Li, Y., Jiang, R., Hu, W.: Rapid joule-heating synthesis for manufacturing high-entropy oxides as efficient electrocatalysts. Nano Letters 22(16) (2022), 6492-6500. [18] Min, J., Zhang, X., Ma, X., Chen, B.: Forming of ultra-thin titanium sheets with intermediate electropulsing treatment. CIRP Annals 73(1) (2024), 241-244.