N8- polynitrogen stabilized on carbon-based supports as metal-free electrocatalyst for oxygen reduction reaction in fuel cells
Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical and Materials Engineering
Edward L. Dreyzin
Kamalesh K. Sirkar
The sluggish oxygen reduction reaction (ORR) kinetics at the cathode is one of the key factors limiting the performance of polymer electrolyte membrane fuel cell (PEMFC). Platinum-based materials are the most widely studied catalysts for this ORR reaction while their large-scale practical application in fuel cells is hindered due to their scarcity and low stability. Therefore, highly active, low cost and robust non-Pt catalysts are being developed to overcome the drawbacks. Recently, a novel polynitrogen N8- (PN) stabilized on multiwall carbon nanotube (MWNT) was synthesized under ambient condition for the first time by our group and demonstrated high ORR activities. It is promising for replacing platinum-based catalysts. However, the substrate effect was not covered in our previous work. Moreover, the PN synthesis mechanism and its catalytic properties for ORR and ORR mechanisms are still not fully understood.
The main objectives of this research are to investigate the catalytic properties of PN on different carbon-based substrates, to identify the active sites and mechanisms of ORR, and eventually to provide guidelines for optimizing the synthesis of PN-series catalysts as well as increasing the efficiency of ORR.
Polynitrogen N8- (PN) deposited on multiwalled carbon nanotubes (PN-MWNT) are synthesized by cyclic voltammetry (CV) with UV irradiation and further used for oxygen reduction reaction (ORR). Compared to the sample synthesized without UV, a larger amount of N8- is synthesized and is found to distribute more uniformly on MWNT with 254nm UV irradiation (PN-MWNT-254nm); this indicates the production of more azide radicals as the precursors for synthesis of N8- by photoexcitation of azide ions is a rate-limiting step for PN synthesis. The PN-MWCNT-254nm sample shows higher ORR current density than that from a commercial Pt catalyst. Kinetic studies indicate a four-electron pathway on N8- while a two-electron one on N3- . In situ Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) analysis reveals that the side-on and end-on 02 adsorption occurs at N8- and N3-, respectively, confirming the electron transfer process. Calculation results from natural bonding orbital (NBO) analysis are used to identify the possible active sites for oxygen chemisorption and further clarify the ORR mechanism.
PN deposited on graphene (G), nitrogen-doped graphene (NG) and boron-doped graphene (BG) are synthesized experimentally. The formation of PN on G, NG and BG is confirmed by ATR-FTIR and temperature-programmed desorption (TPD). Moreover, a larger amount of N8- is obtained on NG and BG substrates than that over pure G. Electrochemical tests show that PN-NG and PN-BG possess superior activity toward the ORR and favored a four-electron pathway.
This work provides facile strategies to efficiently synthesize PN under ambient condition and deep understanding of its intrinsic oxygen reduction activity.
Yao, Zhenhua, "N8- polynitrogen stabilized on carbon-based supports as metal-free electrocatalyst for oxygen reduction reaction in fuel cells" (2019). Dissertations. 1653.
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