(377c) Surface Modification of Transition Metal Carbide Nanoparticles Via Atomic Layer Deposition For Energy Applications

This work centers around surface modification of transition metal carbide (TMC) via Atomic Layer Deposition (ALD) for energy applications such as serving as catalysts for  electrolyzers or proton exchange fuel cells. Transition Metal Carbides (TMCs) (W, Mo, V, Ti, Ta) are known to have similar electronic and geometric structure as Pt-group metals [1]. Among the TMCs, Mo₂C has shown good stability in acidic environment and sufficient hydrogen evolution reaction (HER) activity [2]. Therefore, Mo₂C could be a potential alternative to the conventional Pt. However, Mo₂C has not shown the same electrocatalytic activity as Platinum (Pt). Data in our group has further demonstrated that Mo₂C has order of magnitude higher Tafel slopes than Pt for HER and oxygen reduction reaction (ORR), which corresponds to a slower kinetics. For this reason the past decade or has seen burgeoning interest of modifying Mo₂C surface using Pt. For example, Pt supported on carbon nanotube has shown higher electrocatalytic activity than conventional Pt supported on Powder carbon [3]. Pt supported on Mo₂C is expected to show higher catalytic activity, thanks to its similar electronic structure to Pt and synergistic chemistry between Mo₂C and Pt. In this study ALD technique has been adopted to deposit Pt nanoparticles onto Mo₂C  nanotubes, due to its simplicity, reproducibility, and molecular or even atomic level control on the deposited particle size and distribution. 

Despite of the progress made in recent years on minimizing Pt loading, the challenge of low surface area of TMC support still persists. To address the challenges, the objectives of the present study are to modify the surface of TMCs with Pt and further investigate the synergetic effect between the nanotube TMC support and nanoparticle Pt deposited onto TMC nanotube via ALD. The hypothesis is that the nanosize of the support and deposited metal will allow maximized active areas of both Pt atoms and synergetic interaction areas between Pt nanoparticles and the TMC nanotube supports. 

To screen the Pt-TMC co-catalyst activity, HER activity of both unmodified and modified TMCs were determined by cyclic voltammetry in N₂ saturated 0.5M H₂SO₄ solution and compared with the commercially available 20% Pt/C. Based on the tafel plots, ALD modified Mo₂C support with Pt nanoparticles showed higher HER activity than the commercial 20% Pt/C. Oxygen Reduction Reaction (ORR) activity of both the catalysts were determined by cyclic voltammetry into O₂ saturated 0.5M H₂SO₄ solution. Bare Mo₂C shows very limited stability towards oxidation, but after the surface modification oxidation stability has been increased, which is evidenced by cyclic voltammograms of ORR. The morphology of the ALD modified metal carbide was examined via High Resolution Transmission Electron Microscopy (HRTEM), which proved the island-growth of Pt on TMC with narrow size distribution. 

Bibliography

1. Levy, R.B. and Boudart, M., Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis. Science, 1973. 181(4099): p. 547-549.

2. Weidman, M.C., Esposito, D.V., Hsu, Y.-C., and Chen, J.G., Comparison of Electrochemical Stability of Transition Metal Carbides (Wc, W2c, Mo2c) over a Wide Ph Range. Journal of Power Sources, 2012. 202: p. 11-17.

3. Matsumoto, T., Nagashima, Y., Yamazaki, T., and Nakamura, J., Fuel Cell Anode Composed of Mo[Sub 2]C Catalyst and Carbon Nanotube Electrodes. Electrochemical and Solid-State Letters, 2006. 9(3): p. A160.