Corrosion resistance and electrical conductivity of Ta-modified Titanium bipolar plates for PEMFC

Abstract

Titanium alloy bipolar plates hold significant promise for proton exchange membrane fuel cells (PEMFC) due to their high specific strength, excellent machinability and low density. However, the surface oxide film readily reacts with fluoride ions (F⁻) in service environments, forming porous, layered fluorotitanium compounds that compromise service life and stability. To overcome these limitations, this study utilised magnetron sputtering to deposit a 1 μm-thick tantalum (Ta) coating on TA1 titanium alloy substrates. The microstructure, corrosion resistance and electrical conductivity of the Ta-coated samples were systematically investigated and the mechanisms underlying their enhanced performance were analysed. Results showed that the Ta coating featured a dense, uniform microstructure with strong adhesion to the substrate and no detectable metallurgical defects. A gradient distribution of elements within the diffusion layer further strengthened the coating-substrate interface. Moreover, the formation of a Ta₂O₅ passivation film on the coating surface effectively inhibited fluoride-induced corrosion, reducing the contact resistance from 69.9 to 31.7 mΩ·cm². These findings provide critical theoretical insights and practical guidance for enhancing the corrosion resistance and electrical conductivity of titanium alloy bipolar plates, paving the way for their broader application in PEMFC systems.

Keywords

PEMFC bipolar plate ta modified layer corrosion resistance contact resistance corrosion resistance mechanism

Introduction

Bipolar plates are critical components in proton exchange membrane fuel cells (PEMFC), comprising approximately 70% of the total stack weight and 40% of the overall cost.¹ These plates perform multiple essential functions, including supporting the membrane electrode assembly, facilitating current collection and conduction and preventing direct interaction between oxidants and reducing agents.² To meet the long-term operational requirements of fuel cells, bipolar plates must exhibit several high-performance characteristics, including superior electrical conductivity, excellent hydrophobic properties, high mechanical strength, low mass density and good manufacturability.^3,4 In addition to these properties, bipolar plates must demonstrate exceptional corrosion resistance due to the presence of corrosive ions such as SO₄²⁻, SO₃²⁻, HSO₄⁻, HCO₃⁻ and F⁻ in the PEMFC operating environment.⁵ This capability is vital for withstanding electrochemical degradation in high-temperature and strongly acidic conditions, thereby prolonging service life and enhancing overall fuel cell performance. Consequently, optimising the performance of bipolar plates under these stringent conditions has emerged as a critical focus in the development of PEMFC technology.

Currently, bipolar plates in PEMFCs are predominantly fabricated from materials such as graphite, composites and metals. Among these, graphite bipolar plates, the first material utilised in this application, provide excellent chemical stability and electrical conductivity, particularly in acidic environments.⁶ Despite these advantages, the brittleness, high manufacturing costs and significant gas permeability of graphite limit its applicability in high-power fuel cell stacks, especially in new energy vehicles.⁷ Furthermore, challenges in processing graphite hinder efforts to produce thinner and lighter bipolar plates. Composite bipolar plates have gained considerable attention in recent years for their low weight, corrosion resistance and design flexibility. However, high manufacturing costs, comparatively low mechanical strength and susceptibility to aging during prolonged use significantly restrict their application prospects in PEMFC.⁸ These challenges are particularly pronounced under high-load and long-term operational conditions, where performance degradation is more severe. Metals and their alloys have emerged as promising candidates for bipolar plate materials, owing to their excellent electrical conductivity, low production costs, scalability in manufacturing and capability to be processed into ultra-thin sheets as thin as 0.1 mm. Widely utilised metallic materials include stainless steel, aluminum alloys, nickel-based alloys and titanium alloys.⁹

Titanium and its alloys offer significant advantages for reducing the mass and volume of battery stacks while enhancing energy density, primarily due to their high specific strength, good workability and low density, compared to other metal materials.^10,11 However, titanium alloy bipolar plates are susceptible to corrosion and dissolution in the high-temperature acidic environment on the hydrogen electrode side of PEMFC, which leads to a considerable decline in the activity of electrode electrocatalysts. In particular, fluoride (F⁻) ions in the solution react with the passivation layer on the surface of the titanium alloy, initiating the reaction TiO₂ + 4H⁺ + 6F⁻ → TiF₆²⁻ + 2H₂O.¹² This process results in the formation of porous, layered titanium fluoride compounds, which expose unpassivated regions on the alloy surface. These exposed areas are more vulnerable to corrosive solutions, further accelerating the corrosion process and diminishing the corrosion resistance and stability of titanium alloy bipolar plates. To address this issue, researchers conducted extensive studies, primarily employing micro-alloying and surface coating techniques to enhance the corrosion resistance and electrical conductivity of titanium alloys. For instance, Zhou et al.¹³ demonstrated that increasing the molybdenum (Mo) content significantly improved the corrosion resistance of Ti–Mo alloys, resulting in a passivation current density of less than 10 μA/cm² in a 5% hydrochloric acid (HCl) solution. Zhang et al.¹⁴ developed niobium carbide (NbC)-modified layers on titanium bipolar plates using plasma surface modification technology. Electrochemical tests demonstrated that the current density of the NbC-modified titanium plates remained stable at 0.32 μA/cm², which is one order of magnitude lower than the 6‒7 μA/cm² observed for bare titanium plates. Furthermore, Mou et al.¹⁵ fabricated silicon-containing amorphous carbon films on titanium substrates via plasma-enhanced chemical vapour deposition (PECVD). The incorporation of silicon led to the formation of graphene structures within the amorphous carbon films, significantly enhancing their conductivity and corrosion resistance. While the application of non-metallic or metallic compound-modified coatings on titanium and its alloy substrates can effectively enhance their corrosion resistance and conductivity, the substantial differences in crystal structure, thermal expansion coefficients and thermal conductivity between the coatings and substrates lead to weak adhesion, susceptibility to detachment and inadequate heat transfer.¹⁶ These challenges pose significant barriers to the further advancement of titanium alloy bipolar plates, necessitating extensive research and technological innovations.

To enhance the bonding performance between the coating and substrate and to improve the corrosion resistance and conductivity of titanium alloy bipolar plates in fluoride ion-containing acidic environments, this study employed magnetron sputtering (PVD) technology to deposit a tantalum (Ta) modified layer on the TA1 titanium alloy substrate. Due to the similarities in the physical and chemical properties of Ta and Ti, the objective was to establish a strong metallurgical bond between the coating and the substrate. This work focuses on analysing the microstructure of the Ta-TA1, systematically assessing its corrosion resistance and electrical conductivity under simulated PEMFC operating conditions and elucidating the mechanisms responsible for its enhanced corrosion resistance. The results of this study provide a theoretical foundation and technical reference for improving the corrosion resistance and extending the service life of titanium alloy bipolar plates.

Experimental details

Material preparation

The test substrate comprised commercially pure titanium (Ti ≥ 99.9 wt%). Samples were cut into dimensions of 1 × 10 × 2 mm using a wire-cutting machine. The surfaces of the samples were polished progressively with silicon carbide sandpaper, ranging from 120 to 3000 grit, followed by additional polishing using an automatic polishing machine. After polishing, the samples underwent sequential ultrasonic cleaning in acetone, anhydrous ethanol and deionised water, with each cleaning step lasting 20 min. Following the cleaning process, a hair dryer was utilised to thoroughly dry the samples for subsequent use.

Tantalum (Ta) coatings were deposited using the JCP-350 magnetron sputtering instrument, with a 2-inch tantalum target (Ta ≥ 99.99 wt%) as the target material. During the sputtering process, the distance between the target and the substrate was maintained at 9 cm and the vacuum level in the sputtering chamber was adjusted to 7.0 × 10⁻⁴ Pa. Argon gas was introduced at a flow rate of 5 sccm to stabilise the chamber pressure at approximately 0.5 Pa. The substrate was heated and maintained at 150 °C, while the sputtering parameters included a power of 200 W, a bias voltage of −100 V and a sputtering duration of 5 h. To improve the adhesion of the Ta coating to the titanium substrate, a bias voltage of −300 V was applied to the substrate surface for 20 min prior to sputtering, effectively removing any TiO₂ film layer present on the substrate.

Characterisation of the microstructure

Microstructural and elemental analyses of the sample were performed using a scanning electron microscope (SEM, SU3900, 15 kV) equipped with an energy dispersive spectrometer (EDS). Phase analysis was conducted at room temperature using a Shimadzu XRD 6000 diffractometer (Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation (λ = 0.154060 nm). The X-ray diffraction (XRD) measurements were carried out at a voltage of 40 kV and a current of 40 mA, utilising a scanning rate of 2°/min over a Bragg angle (2θ) range of 10°‒90°. X-ray photoelectron spectroscopy (XPS) was performed using a PHI5000 Versa Probe II to investigate the chemical states of the surface mask layer. The analysis covered a 4 mm² area, employing a 50 W Al Kα X-ray source. The sample was introduced into the analysis chamber under vacuum conditions of less than 2.0 × 10⁻⁷ mbar, with a 400 μm spot size, a 12 kV accelerating voltage and a 6 mA filament current. For wide-spectrum acquisition, a pass energy of 150 eV with a step size of 1 eV was used, while narrow-spectrum scans were conducted with a 50 eV pass energy and a 0.1 eV step size. The spectrometer was calibrated using the C 1 s peak at 284.8 eV as a reference.

Electrochemical measurements

Electrochemical testing was carried out using a Shanghai Chenhua CHI660E electrochemical workstation. The experimental setup employed a standard three-electrode configuration, comprising a saturated calomel electrode (SCE) as the reference electrode, a platinum plate as the counter electrode and the sample as the working electrode. Prior to testing, the sample was carefully cleaned in sequence with acetone, distilled water and deionised water to ensure the removal of any surface contaminants that might affect the electrochemical performance. To simulate the operational conditions of a PEMFC, the anode was maintained at a potential of −0.1 V in a hydrogen (H₂) atmosphere, while the cathode was held at +0.6 V in air. The corrosion testing solution was prepared by dissolving 0.5 mol·L⁻¹ H₂SO₄ and 2 mg·L⁻¹ HF, with the solution heated to 70°C to replicate the typical working temperature of PEMFC systems.¹⁷ In the constant potential polarisation test, the cathode potential was fixed at 0.6 V, while the anode was held at −0.1 V. Current density variations were recorded over a period of 4 h to assess the stability and corrosion resistance under these conditions. Electrochemical impedance spectroscopy (EIS) was conducted using an AC excitation signal with an amplitude of 10 mV, across a frequency range of 10⁻² to 10⁵ Hz, with the scan direction progressing from high to low frequency. The resulting EIS data were analysed and fitted using ZSimpWin software for further interpretation.

Contact resistance measurements

The conductivity of the sample was evaluated using the contact resistance method with the RH-450 proton exchange membrane fuel cell bipolar plate resistance testing system. The contact resistance between the sample and Toray carbon paper (TGP-H-090) was measured under varying pressure conditions utilising the voltammetry method developed by Wang.¹⁸ In this experimental setup, the sample is placed between two layers of conductive carbon paper, creating a sandwich structure. This assembly is then compressed between two copper plates, which are subjected to a controlled compressive force. A constant current of 1 A is applied through these copper plates, and the resulting voltage change is recorded as the compressive force is incrementally increased to 240 N·cm⁻².

The interfacial contact resistance (ICR) between the sample and the carbon paper is calculated using the following formula:

I C R = \frac{R_{2} - R_{1}}{2} \times A_{c o n t a c t}

(1)

where

R_{1}

represents the resistance of the carbon paper under different pressures,

R_{2}

denotes the total resistance of the sample and carbon paper under various pressures,

A_{c o n t a c t}

is the contact area between the bipolar plate and the carbon paper.

Results and discussion

Microstructure and composition

To ascertain the phase composition of the Ta-TA1 coating surface, X-ray diffraction (XRD) analysis was performed, as illustrated in Figure 1. The diffraction peaks observed at 2θ values of 38.47°, 55.55° and 82.46° correspond to the characteristic (110), (200) and (220) crystal planes of body-centered cubic tantalum (Ta). The alignment of these diffraction peaks with the standard PDF card for tantalum (#04-0788) confirms the successful formation of a tantalum coating on the TA1 substrate. Additionally, Figure 1 displays diffraction peaks associated with titanium (Ti) and titanium dioxide (TiO₂). This observation may be attributed to the presence of a thin modified layer, coupled with the strong penetration capability of X-rays. During the reaction process, titanium (Ti) and oxygen (O) readily react to form TiO₂. Although the oxide layer on the Ti substrate was removed by applying a bias voltage prior to the magnetron sputtering process, a residual amount of TiO₂ remained on the substrate surface due to the strong adhesion between TiO₂ and the Ti substrate. This residual TiO₂ was subsequently covered by the tantalum coating during sputtering.

Figure 2 presents the microstructure and elemental distribution of the Ta-TA1 coating's cross-section. The microstructure of the Ta coating is characterised by a uniform and dense morphology, with no observable metallurgical defects such as pores or cracks. This observation indicates a continuous and robust bond at the interface between the coating and the substrate, suggesting that the coating possesses excellent forming quality throughout the preparation process. Based on the cross-sectional morphology and the results from energy dispersive spectroscopy (EDS) line scan analysis, the thickness of the fabricated Ta coating is approximately 1 μm. The coating is primarily composed of two distinct regions: a deposition layer and a diffusion layer. Notably, the alloy elements within the diffusion layer exhibit a gradient distribution, which further corroborates the strong bonding performance between the coating and the substrate. Furthermore, this gradient distribution contributes to reducing stress concentration, enhancing wear resistance and corrosion resistance of the coatings. These advantageous properties render Ta coatings particularly suitable for applications in high-temperature and high-corrosion environments, thus providing a compelling foundation for further research and development.

Electrochemical property

Potentiodynamic polarisation

The potentiodynamic polarisation curves of Ta-TA1 and TA1 in simulated proton exchange membrane fuel cell (PEMFC) cathode and anode environments are presented in Figure 3. As shown, at an anode working potential of −0.1 V, the current density of TA1 is 4.087 μA/cm², while at a cathode working potential of +0.6 V, the current density is 2.725 μA/cm². In contrast, the polarisation curve of Ta-TA1 shifts toward the more positive potential direction and its corrosion current density decreases by approximately an order of magnitude, indicating that the Ta coating significantly enhances the corrosion resistance of TA1 in PEMFC environments.

Notably, Ta-TA1 exhibits an extended passivation range under both anode and cathode conditions. As detailed in Table 1, the passive current densities (Ip) for Ta-TA1 are 1.226 and 1.243 μA/cm² under simulated anode and cathode conditions, respectively, representing reductions of 82.45% and 86.57% compared to TA1. This substantial reduction demonstrates the improved blocking effect of the passivation film, resulting in a lower corrosion rate and enhanced overall corrosion resistance.¹⁹ Moreover, the corrosion potential (E_corr) of Ta-TA1 in the anode environment increases from −0.323 to 0.081 V, while in the cathode environment, it rises from −0.286 to 0.103 V. The positive shift in corrosion potential, coupled with the observed rightward shift in the polarisation curve, underscores the protective efficacy of the Ta coating. Additionally, the corrosion potential of the Ta coating exceeds the anode working potential (−0.1 V), providing effective cathodic protection at this operating point. In summary, the Ta coating significantly enhances the corrosion resistance of the TA1 substrate, as evidenced by reduced current densities, expanded passivation ranges and elevated corrosion potentials, making it highly promising for PEMFC applications.

Table 1.

Corrosion parameters of untreated TA1and Ta-TA1 in simulated PEMFC cathodic/anodic environment.

Sample	Anode			Cathode
Sample	E_corr (V_SCE)	I_p (μA/cm²)	I_−0.1V (μA/cm²)	E_corr (V_SCE)	I_p (μA/cm²)	I _+ 0.6V (μA/cm²)
TA1	−0.323	6.984	4.087	−0.286	9.252	2.725
Ta-TA1	0.081	1.226	−0.235	0.103	1.243	0.335

Constant potential polarisation curve test

The constant potential polarisation curve test effectively reflects the formation process and stability characteristics of the passivation film on bipolar plate materials.²⁰ Figure 4 presents the current density versus time curves for Ta-TA1 and TA1 samples in simulated PEMFC cathode and anode environments over a duration of 14,400 s under constant potential conditions. The graph illustrates that the corrosion current density (in absolute value) of the Ta coating quickly stabilises within a short period, regardless of whether it is in the anode or cathode environment. This rapid stabilisation indicates a fast film formation rate for the Ta coating. Throughout the testing period, the corrosion current density of the Ta coating remained relatively constant, further underscoring the high stability of its passivation film.²¹ In contrast, the corrosion current density of TA1 exhibits a sharp initial decline before gradually stabilising over time. Additionally, the polarisation curve for TA1 displays fluctuations, indicating that the passive film formed at this potential undergoes a dynamic process of dissolution and repair. This behavior suggests that the corrosion mechanisms for TA1 in the PEMFC environment are more complex, reflecting a significantly lower formation and stability of its passivation film compared to that of the Ta coating. Consequently, Ta-TA1 demonstrates superior corrosion resistance in practical applications, establishing a robust foundation for its utilisation in the PEMFC field.

Electrochemical impedance spectroscopy (EIS) testing

Figure 5 presents the electrochemical impedance spectra (EIS) of Ta-TA1 and TA1 under simulated PEMFC conditions. The Nyquist plot clearly shows that the capacitance arc radius of the Ta-coated sample is considerably larger than that of the uncoated TA1. In electrochemical corrosion studies, the radius of the capacitance arc is typically indicative of the charge transfer resistance: a larger arc radius corresponds to a higher charge transfer resistance, which is associated with enhanced corrosion resistance of the material.²² In the Bode impedance modulus plot (log |Z| vs. log f), the value of log |Z| reflects the solution impedance between the sample and the reference electrode in the high-frequency region. As shown in the figure, the slope of the log |Z| vs. log f curves for both the TA1 and Ta coatings is approximately −1, suggesting that the passive films formed on the surfaces of both samples exhibit favourable capacitive resistance characteristics. Moreover, a wider frequency range indicates stronger corrosion resistance. At low frequencies, the impedance modulus |Z| (as f → 0) for the Ta coating is considerably higher than that of TA1, signifying that the Ta coating possesses greater polarisation resistance. Additionally, the Bode phase angle plot shows that both the maximum phase angle and the corresponding frequency range for the Ta coating exceed those of TA1, further supporting the conclusion that the Ta coating demonstrates superior corrosion resistance in simulated PEMFC environments.

Figure 6 illustrates the equivalent circuit models derived from the electrochemical impedance spectra of TA1 and Ta-TA1. For TA1, a two-time constant equivalent circuit model, represented as R_s(Q_pf(R_pf (Q_dlR_ct))), was employed, while a one-time constant equivalent circuit, denoted as R_s(Q_pR_p), was used for Ta-TA1. In Figure 6(a), R_s signifies the solution resistance, R_pf denotes the pore resistance and R_ct represents the charge transfer resistance. The capacitances Q_pf and Q_dl correspond to the double-layer capacitance in contact with the corrosive medium via the pores and the double-layer capacitance of the solution passivation film, respectively. In Figure 6(b), R_s continues to represent the solution resistance, R_p indicates the passivation film resistance and Q_p refers to the capacitance of the passivation film.

The experimental data were fitted using ZSimpWin software and the fitting results are summarised in Table 2. According to Table 2, the pore resistance (R_pf) and charge transfer resistance (R_ct) for TA1 are 2.083 × 10² Ω·cm² and 7.167 × 10³ Ω·cm², respectively. Notably, R_ct is an order of magnitude larger than R_pf, suggesting that the charge transfer process is the rate-limiting step in this electrochemical reaction. Additionally, the passivation film resistance (R_p) of the Ta-TA1 coating is approximately 3.594 × 10⁴ Ω·cm², significantly exceeding the charge transfer resistance (R_ct) of TA1. This finding indicates that the corrosion resistance of the Ta coating is superior to that of the naturally formed oxide film on the surface of TA1.

Table 2.

Electrochemical parameters derived from electrochemical impedance spectra measurement for untreated TA1 and Ta-TA1.

Sample	R_s (Ω·cm²)	Q_pf (Ω⁻¹ ·cm⁻²·s⁻ⁿ)	n _f	R_pf (Ω·cm²)	Q_dl (Ω⁻¹· cm⁻²·s⁻ⁿ)	n _dl	R_ct (Ω·cm²)	R_p (Ω·cm²)
Untreated TA1	14.97	6.712 × 10⁻⁵	0.90	208.3	8.298 × 10⁻⁵	0.75	7.167 × 10³	–
Ta-TA1	20.12	–	0.89	–	6.121 × 10⁻⁵	–	–	3.594 × 10⁴

Analysis of passivation film state of Ta coating

To further elucidate the reasons for the excellent corrosion resistance of Ta-TA1 in the simulated PEMFC environment, we conducted X-ray photoelectron spectroscopy (XPS) analysis on the surface of the Ta-TA1 material. Figure 7 presents the full XPS spectrum of Ta-TA1 before and after constant potential polarisation testing, along with high-resolution XPS spectra for Ta 4f and O 1 s. The Ta 4f spectrum is characterized by four distinct energy level peaks, corresponding to the most stable pentavalent state (Ta₂O₅), sub-oxides (TaO₂, TaO) and metallic tantalum (Ta⁰). During the peak fitting process, we maintained a fixed area ratio of 4:3 for all energy level peaks, with an energy separation of 1.90 eV.^23,24 Specifically, the peak binding energies of Ta 4f_7/2 and Ta 4f_5/2 at 26.7 eV and 28.6 eV, respectively, are attributed to Ta₂O₅.²⁵ The peaks at 25.2 and 27.1 eV correspond to the low oxidation state (TaO₂), while those at 22.5 and 24.4 eV are associated with TaO.²⁶ The low-energy binary states exhibit peak binding energies of 21.9 and 23.8 eV, which are attributed to metallic tantalum (Ta⁰).^27,28 Following the constant potential polarisation testing, we observed a significant reduction in the amounts of Ta sub-oxides and metallic tantalum (see Figure 7(e)), indicating the formation of a protective oxide layer on the surface.

As depicted in Figure 7(c) and (f), the O 1 s spectrum consists of three peaks, with a binding energy of 531.4 eV corresponding to the Ta-O bond in tantalum oxide.²⁹ The peaks at higher binding energies (532.8 and 534.2 eV) are associated with oxygen adsorption.³⁰ A comparison of Figures 7(b) and (e) reveals that, after the constant potential polarisation test, the proportion of surface tantalum in the low-valence state that converted to the more corrosion-resistant Ta₂O₅ increased significantly from 70.18% to 85.43%. We speculate that during the constant potential polarisation process, tantalum undergoes the following reactions:³¹

Ta + H_{2} O \to TaO + 2 H^{+} + 2 e^{-}

(2)

TaO + H_{2} O \to {TaO}_{2} + 2 H^{+} + 2 e^{-}

(3)

2 {TaO}_{2} + H_{2} O \to {Ta}_{2} O_{5} + 2 H^{+} + 2 e^{-}

From this analysis, we conclude that TaO and TaO₂ initially form on the surface of Ta-TA1. These unstable low-valence oxides are further oxidised to Ta₂O₅ during the constant potential polarisation process, which significantly enhances the formation rate and density of the passivation film. Ta₂O₅ exhibits high chemical stability in a 0.5 mol·L⁻¹ H₂SO₄ + 2 mg·L⁻¹ HF solution, conferring superior protective performance in corrosive environments. The dense Ta₂O₅ layer that forms on the surface of Ta-TA1 effectively prevents the reaction TiO₂ + 4H⁺ + 6F⁻ → TiF₆²⁻ + 2H₂O, thereby mitigating the issue of F⁻ ion penetration that can damage the oxide film on the surface of TA1. This mechanism significantly enhances the service stability of Ta-TA1 bipolar plates in fluoride ion environments.

Contact resistance testing

The interface conductivity of bipolar plates is a critical factor influencing the power output, internal heat generation and the current density of PEMFC. Lower contact resistance not only significantly reduces heat generation within PEMFC but also enhances their output power. Figure 8 illustrates the contact resistance of unmodified TA1 and Ta-TA1 in a simulated PEMFC environment. As depicted, at lower pressures, the contact resistance decreases sharply with increasing pressure, attributed to the enhancement of the effective contact area between the sample and carbon paper.³² Notably, at the same pressure level, Ta-TA1 exhibits a considerably lower contact resistance compared to unmodified TA1, underscoring its superior interfacial conductivity.

Under typical operating conditions, the assembly pressure for a PEMFC stack is 140 N·cm⁻². At this pressure, the contact resistance of Ta-TA1 measures 31.7 mΩ·cm², while that of unmodified TA1 is significantly higher at 69.9 mΩ·cm². This disparity indicates that the Ta-modified layer substantially enhances the conductivity of TA1. Additionally, Figure 8 shows that following constant potential polarisation treatment, the contact resistance for both unmodified TA1 and Ta-TA1 exhibits an increasing trend. Specifically, the contact resistance of unmodified TA1 rises to 94.7 mΩ·cm², while that of Ta-TA1 increases to 38.4 mΩ·cm². This represents increases of 35.5% and 21.1%, respectively, compared to their values before the polarisation treatment. These results indicate that unmodified TA1 experiences significant corrosion during the constant potential polarisation process, leading to a marked decrease in its conductivity.

Conclusions

A tantalum (Ta) coating with a thickness of 1 μm was successfully deposited on the surface of the TA1 substrate using magnetron sputtering deposition technology. The resulting Ta coating exhibits a uniform and dense microstructure, characterised by a tight and continuous interface between the coating and the substrate. No significant metallurgical defects were observed. The coating comprises an outer deposition layer and an inner diffusion layer, with elemental distribution exhibiting a gradient within the diffusion layer. This gradient distribution significantly enhances the bonding strength between the coating and the substrate, thereby improving both the mechanical stability and corrosion resistance of the coating.

In a simulated PEMFC environment, the corrosion potential of Ta-TA1 exhibited a significant positive shift compared to TA1, increasing from −0.323 to 0.081 V under anodic conditions and from −0.286 to 0.103 V under cathodic conditions.

Additionally, at an anode working potential of −0.1 V, the Ta coating demonstrated cathodic protection characteristics. The corrosion current densities of Ta-TA1 at anode (−0.1 V) and cathode (+0.6 V) working potentials were measured at 0.189 and 0.190 μA/cm², respectively, which are approximately an order of magnitude lower than those of TA1. Electrochemical impedance spectroscopy (EIS) analysis revealed that the capacitance arc radius of Ta-TA1 is significantly larger than that of TA1, with a polarisation resistance reaching 3.594 × 10⁴ Ω·cm², far exceeding the polarisation resistance of the natural oxide film on TA1. These results indicate that Ta-TA1 possesses excellent corrosion resistance in a PEMFC environment.

In the simulated PEMFC environment, a dense and corrosion-resistant Ta₂O₅ passivation film formed on the surface of Ta-TA1, effectively preventing the reaction TiO₂ + 4H⁺ + 6F⁻ → TiF₆²⁻ + 2H₂O. This passivation mechanism mitigates the damage to the TA1 surface oxide film caused by fluoride ion penetration, thereby significantly enhancing the service stability of Ta-TA1 bipolar plates in fluoride ion-containing environments.

The Ta coating applied to the surface of Ta-TA1 substantially enhances the conductivity of TA1. At a contact pressure of 140 N·cm⁻², the contact resistance of Ta-TA1 is measured at 31.7 mΩ·cm², markedly lower than the 69.9 mΩ·cm² observed for unmodified TA1. Following constant potential polarisation treatment, the contact resistance of unmodified TA1 increased from 69.9 to 94.7 mΩ·cm², representing a 35.5% increase. In contrast, the contact resistance of Ta-TA1 only increased from 31.7 to 38.4 mΩ·cm², a more modest increase of 21.1%.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported financially by the International Science and Technology Cooperation Project of Yunnan Science and Technology Plan (grant no: 202103AF140004); The Basic Research Plan of Yunnan Science and Technology Department (grant no: 202101AU070098); Yunnan Titanium Metal Technology Innovation Center Project (grant no: 202305AK340051); and Key research and development plan of Yunnan Province (grant no: 202203AM140010).

ORCID iDs

Zhentao Yuan

Xiao Wang

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