(301g) Transition Metal Carbide Electrodes Modified By High-Powered Impulse Magnetron Sputtering

Transition metal carbides (TMCs) play a critical role in the functionality of many engineered systems related to energy storage, as well as applications requiring corrosion resistance, high temperature and high strength. It is well known that the physics regulating the material properties of TMCs is controlled by its structure and chemistry at both the atomic and mesoscales. Indeed, unconventional carbide phases published in the last few years have shown the ability to access enhanced electronic conductivity^1-2, and catalytic activity^2-3. Transition metal carbides are typically synthesized under thermal equilibrium conditions using furnaces often operating in excess of 1500 ºC. By contrast, the non-equilibrium⁴ nature of low- temperature plasmas such as in high-powered impulse magnetron sputtering (HiPIMS) makes this technique an excellent candidate to synthesize novel non-equilibrium TMCs with unique material properties.

Our study of non-equilibrium TMC thin films generated by HiPIMS is characterized using microscopy, electrochemical corrosion, and electrochemical impedance spectroscopy in an effort to build empirical electrochemical-material relationships.

Materials and Methods

Molybdenum carbide (Mo₂C) micro particles synthesized by co-reduction carburization were immobilized on carbon paper thus mimicking a catalyst support dispersed on a gas diffusion layer (GDL). Thin films of deposited or implanted Mo, Ta, and Zr were grown using a range of kick pulses up to +700 V. Pt catalyst loading on the particles was 20 wt% and at 57 μg/cm²). Diffractometry and spectroscopic techniques were used to determine material composition. Equilibrium corrosion curves (potential scan rate 0.167 mV/s) were acquired in high and low pH media. Chronopotentiometric titration (constant current, varying pH) was performed to determine passivation and dissolution currents similar to those produced in a Pourbaix diagram⁵. Electrochemical impedance spectroscopy (EIS) was used to determine resistance to an oxygen exchange reaction.

Results and Discussion

The threshold for improved performance was determined to be as low as 5 nm of Ta deposited/implanted using a kick pulse of 500-700 V. Mo₂C receiving this treatment displayed an increased corrosion potential of 100-110 mV compared to untreated Mo₂C, See Figure 1. Bulk crystallographic phase of the Mo₂C was unchanged, all alloys formed were in the ~ 5 nm closest to the surface leaving overall electrical conductivity unchanged.

Significance

With the treatment described above, a fuel cell utilizing Mo₂C catalyst supports would be able to run at approximately 20% higher average power without negative repercussions on device performance or lifetime.

Figure 1. Equilibrium corrosion curves for Mo₂C/C supports with and without the HiPIMS Ta treatment

References

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3. Schmuecker, S.M.; Clouser, D.; Kraus, T.J.; Leonard, B.M. Dalton Trans. 46, 39 (2017).
4. Wang, Z.; Zhang, Y.; Neyts, E.C.; Cao, X.; Zhang, X.; Jang, B.W L.; Liu, C.-j. ACS Catal. 8, 3 (2018).
5. Weidman, M.C.; Esposito, D.V.; Hsu, Y.C.; Chen, J.G. Power Sources. 202 (2012).