Co-Catalyst Modified Nafion Membrane for Proton Exchange Membrane Fuel Cell Application

Proton Exchange Membrane Fuel Cell (PEMFC) has drawn burgeoning attention in the past decades because of its low working temperature, compact construction and higher current density. Even though these advantages make PEMFC an ideal application for transportation, the requirement of precious metal as catalyst, such as platinum (Pt), limits the cost effectiveness of PEMFC. This study aims to reduce the Pt loading in the catalyst layer of PEMFC membrane electrode assembly (MEA), while increasing the durability and conductivity of the membrane, by utilizing a recently developed co-catalyst consisting of Pt nanoparticles supported on novel nanotube molybdenum carbide (Mo₂C) (also referred to as co-catalyst).

Thanks to their similar electrical and geometric structure to Pt, transition metal carbides, such as Mo₂C, can serve as a good support for Pt. Research has shown that Pt supported on Mo₂C thin film has promising Hydrogen Evolution Reaction (HER) activity. However, the HER activity of Pt deposited on thin film Mo₂C is not as high as bulk Pt [i], partially due to the limited surface area of thin film Mo₂C.  To address the challenge of constrained surface area, this project focuses on using transition metal carbide nano particles as a support, subsequently modifying their surface by island growth of Pt via Atomic Layer Deposition (ALD). Electrocatalytic performance of both bare Mo₂C and Pt ALD modified Mo₂C has been compared with that of commercially available 20% Pt/C. The comparison has demonstrated better HER activity of the co-catalyst, which only has fraction of Pt loading of the commercial 20% Pt/C. High Resolution Transmission Electron Microscopy (HRTEM) images showed rather uniform distribution of Pt nanoparticles that are well adhered to nanotube Mo₂C support. ALD modification has improved the reaction kinetics of Mo₂C towards Oxygen Reduction Reaction (ORR), but currently co-catalyst ORR activity is not yet as high as 20% Pt/C.

MEA performance testing using the co-catalyst on both anode and cathode electrodes has resulted in lower current density than the standard MEA, partially due to the low ORR activity of the co-catalyst. To further our understanding of the co-catalyst performance, we are currently exploring direct co-catalyst nano particles deposition onto the Nafion membrane using electrospray, based on studies that have shown higher electro active area (20-25% improvement) achieved by this technique [ii]. The electrospray technique can distribute the ionomer phase within the mesoporous structure of the catalyst layer, consequently closer contact between the ionomer and catalyst particle and increased conductivity and durability.

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i  BIBLIOGRAPHY Daniel V. Esposito, S. T. (2012). A New Class of Electrocatalysts for Hydrogen Production from Water Electrolysis: Metal Monolayers Supported on Low-Cost Transition Metal Carbides. Journal of the American Chemical Society , 134, 3025-3033.

[ii] T. Fuller, C. H. (2009). Proton Exchange Membrane Fuel Cells 9 (Vol. 25). Pennington, New Jersey, USA: Electrochemical Society.