Development of non-noble (Co-N/MWCNT) and polybenzimidazole-modified (Pt-PBİ/MWCNT) electrocatalysts for high-temperature PEM fuel cell applications

Date

2020

Author

Eren, Enis Oğuzhan

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Due to the extremely high cost of platinum group metals, the development of highly efficient, non-noble electrocatalysts, especially for the oxygen reduction reaction (ORR) is essential for the commercialization. In recent studies, state-of-the-art Metal-N-C (Metal = Fe or Co) catalysts are considered as promising alternatives to expensive Pt/C catalysts because of their excellent electrochemical and physical properties. They can somehow provide decent mass activity and electrochemical performance with considerably low material and synthesis cost. In this thesis, the main course is the development of a non-noble, high-performance Co-N/MWCNT electrocatalyst for high-temperature proton exchange membrane fuel cell (HT-PEMFCs) application, and the second course is the development of polybenzimidazole (PBI) modified carbon-nanotubes as a support material for platinum-based electrocatalysts to increase catalytic durability at elevated temperatures. Upon completion of the studies, synthesis of the Co-N/MWCNT catalyst with a high-temperature pyrolysis method was successfully demonstrated by XPS, ToF- SIMS, XRD, Raman, and HR-TEM analysis. Results showed that the uniform dispersion of Co nanoparticles and the formation of active pyridinic Co-N4 sites v were appropriately achieved. It is believed that the formation of stable Pyridinic-N sites could promote more electron pathways through the ORR, and increases the catalyst’s stability. Rotating disc electrode (RDE) analysis demonstrated that the kinetics of the Co-N/MWCNT catalyst is sufficient enough to operate as a cathode for a typical PEM fuel cell operation. From the HT-PEMFC test results, at the 150°C and 160°C, the peak current/power density of the cobalt-based MEA was somehow higher than that of the commercial Pt/C. The only drawback is the stability issue at higher temperatures (170°C or more) in an acidic medium. Results partially confirmed that the Co-N/MWCNT catalyst could be the promising alternative to replace noble Pt/C catalysts at the cathode side. In the sub-study, polybenzimidazole wrapped carbon-nanotubes and microwave- assisted reduction of Pt nanoparticles were successfully characterized by TGA, XPS, XRD, and TEM analysis. Modifying carbon-nanotubes through the polybenzimidazole film ensured fine (about 2 – 5 nm) and uniform dispersion of Pt nanoparticles. Cyclic voltammetry (CV) analysis concluded that the Pt- PBI/MWCNT catalyst had a 43 m2·g-1 of electrochemically active surface area (ECSA) to catalyze hydrogen oxidation, which is parallel to the literature but slightly lower than that of commercial Pt/C and Pt/MWCNT catalysts due to hydrophobic behavior of PBI molecules in an aqueous electrolyte. On the other hand, it has shown superior durability compared with the commercial Pt/C and Pt/MWCNT. After the 1000th CV, it has retained almost 80% of its initial ECSA, which makes the Pt-PBI/MWCNT is much more durable than the Pt/C and Pt/MWCNT. However, HT-PEMFC tests indicated that the peak current/power density values of the PBI-modified catalyst were lower than that of commercial- grade Pt/C, suggesting that some balance between durability and performance has to be taken into consideration.

Subject Keywords

Fuel cells., Co-N/MWCNT, Pt-PBI/MWCNT, PEM Fuel Cell, High- Temperature, Polybenzimidazole.

URI

http://etd.lib.metu.edu.tr/upload/12625464/index.pdf
https://hdl.handle.net/11511/45676

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Graduate School of Natural and Applied Sciences, Thesis