Niobium Interlayer Coating: Is it a Practical Approach to Tune the Protective Pt Loading in PEM Water Electrolyzers?
Authors: Kalhori Hossein , Johar Mohammadhossein , Moradizadeh Leila , Shahgaldi Samaneh 
Published: December 2024

The surge in energy demand, along with global warming emissions from natural energy sources, has accelerated the transition towards renewable energies at an unprecedented rate. Among the array of potential candidates, green hydrogen stands out as a particularly promising alternative. In this regard, proton exchange membrane water electrolyzers (PEMWE) have gained widespread acceptance for green hydrogen generation due to their high efficiency and low operating temperatures. However, the relatively high cost of PEMWE components like catalysts, porous transport layer (PTL) and bipolar plates remains a challenge. For instance, commercial PTLs, comprising over 20% of the total PEM stack, often incorporate precious metal coatings to prevent passivation. In this study, we aimed to address the cost issue by replacing precious metal with non-precious metals. Niobium is known for its corrosion resistivity within acidic electrolytes [1]. Here, niobium thin films were deposited onto commercial titanium felt PTLs using pulsed laser deposition (PLD), a technique that allows precise control over film thickness and composition. Corrosion tests were conducted using an electrochemical potentiostat, with end-of-life tests employed for comparison. Results showed improvements in the performance of niobium-supported titanium felts compared to uncoated felts, indicating promise for cost optimization in PEM stacks.

To deposit niobium thin films, a pulsed KrF laser with a wavelength of 248 nm was employed. Prior to each deposition, the chamber was evacuated to 5 × 10−5 mbar, followed by the introduction of specific gases such as N2 or He into the chamber during deposition. The choice of background gas is presumed to impact the composition of the niobium based thin films on PTL [2]. The corrosion resistivity of the produced niobium-coated titanium PTLs using two different gases were evaluated using a three electrodes cell. An ex-situ electrochemical setup was designed and 0.5 M H2SO4 electrolyte was used to simulate the electrolyzer media. The temperature was set at 70 °C. Chronopotentiometry was employed to assess the end-of-life characteristics of the samples over a period of at least 10 hours.

The thickness of each sample was controlled by X-ray reflectivity (XRR) accompanied by the cross-section scanning electron microscopy (SEM). Corrosion tests were performed on Nb-based thin films with a thickness of 120 nm. Figure 1(a) demonstrate the chronopotentiometry results of niobium-coated samples using nitrogen and helium in compared to bare PTL. In these tests, a constant current density of 400 mA/cm² was applied for an extended duration, allowing the potential to change within the 0-10 V range. These graphs show that with the same current density, the electrochemical potential was found to be lower in niobium-coated Ti PTLs compared to the uncoated counterpart, indicative of enhanced surface conductivity post-coating. It is observed that the potential of uncoated titanium PTL rises after 3 hours of this test. In contrast, both niobium-coated films demonstrated prolonged test duration, with no cessation even after 10 hours, underscoring the advantage of niobium-coated films over bare titanium PTLs. Intriguingly, niobium films subjected to nitrogen gas exhibited a lower potential than counterparts grown in helium, suggesting the potential presence of nitride compounds replacing oxides within the thin film structure of niobium. Given that the electrical conductivity of NbN is higher than that of the NbO structure [3], it can be inferred that the presence of nitrogen may enhance both the electrical conductivity and corrosion resistance of the PTL. Figure 1 (b)-(d) reveals the SEM images of these samples after three hours of the corrosion tests. These results confirm that the surface of the niobium thin film grown in nitrogen exhibited less peeling compared to other niobium-coated and uncoated titanium PTLs. Hence, it is inferred that niobium thin films grown in a nitrogen-rich environment exhibit noteworthy potential for prolonged stability when contrasted with both the uncoated PTL and the niobium-coated PTL developed under nitrogen-deficient conditions.

Fig. 1 (a) Chronopotentiometry curve obtained for different coated and uncoated titanium PTLs. The SEM images of the (b) Nb:He, (c) Nb: N2, and (d) uncoated titanium PTLs after three hours of corrosion

We would like to acknowledge the support of the Natural Sciences and Engineering Research Council of Canada, Canada Research Chair and PRIMA Québec and Niobay team for their constant support.

References

[1] Lettenmeier, P.; et al, Scientific Reports
2017, 7 (1), 44035.

[2] Roch, T.; et al, Applied Surface Science, 2021, 551, 149333.

[3] Daudt, N. F.; et al, Journal of Materials Engineering and Performance, 2020, 29, 5174.

Figure 1