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Electrochemical and Computational Study of Oxygen Reduction Reaction on Non-Precious Transition Metal/Nitrogen Doped Carbon Materials

Liu, Kexi (2018) Electrochemical and Computational Study of Oxygen Reduction Reaction on Non-Precious Transition Metal/Nitrogen Doped Carbon Materials. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Abstract

An important challenge in low temperature fuel cell development is to find high performance non-precious catalysts replacing expensive platinum cathode catalysts for oxygen reduction reaction (ORR). The recently developed transition metal/nitrogen doped carbon-based materials (TM-N-C) exhibited catalytic activity close to that of platinum, but there are still many questions under debate. In this thesis, electrochemical and computational methods were employed to reveal the ORR mechanism on TM-N-C catalysts.
In the experimental aspect, a cost-effective two-step process, electrospinning and pyrolysis, was developed to produce the TM-N-C catalysts. The electrochemical techniques were applied to the catalytic property characterization of the synthesized catalysts, showing that the Fe-N-C catalyst possesses higher activity than the Co-N-C catalyst and could promote 4e- pathway, while O2 electroreduction was found to proceed mainly with 2e- pathway yielding hydrogen peroxide on the Co-N-C catalyst.
In the computational aspect, the first-principles density functional theory (DFT) was employed to calculate the adsorption energies and activation energies, elucidating the ORR mechanism on TM-N4 type active sites. Based upon the calculation results, the linear correlation between the O2 adsorption energy and the non-bonding d orbitals center in the transition metal macrocyclic complexes was established. The free energy diagram extracted from the calculation results uncovered that the ORR could happen through 4e- associative pathway on the FeN4 site, whereas might end with a 2e- pathway on the CoN4 site due to high energy barrier for O-O bond splitting, supporting the experimental observations. The superior activity of TM-N-C catalysts in alkaline than in acid was well-explained by the activity loss of the coexisted metal-free active sites induced by pyridinic nitrogen protonation in acid. Moreover, the microkinetic analysis was demonstrated to interpret DFT calculated energy parameters as polarization curves. With this tool, the study of the local carbon structures in the Fe-N-C catalysts directly revealed that the introduction of micropores could enhance their catalytic activity through facilitating the formation of FeN4-C8 active sites with high specific activity.


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Details

Item Type: University of Pittsburgh ETD
Status: Unpublished
Creators/Authors:
CreatorsEmailPitt UsernameORCID
Liu, Kexikel91@pitt.edukel910000-0003-3707-1163
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairWang, Guofengguw8@pitt.eduguw80000-0001-8249-4101
Committee MemberMao, Scottsxm2@pitt.edusxm20000-0003-0202-4391
Committee MemberTo, Albertalbertto@pitt.edualbertto0000-0003-2893-8378
Committee MemberNettleship, Iannettles@pitt.edunettles
Committee MemberKeith, Johnjakeith@pitt.edujakeith0000-0002-6583-6322
Date: 25 January 2018
Date Type: Publication
Defense Date: 3 November 2017
Approval Date: 25 January 2018
Submission Date: 3 November 2017
Access Restriction: 2 year -- Restrict access to University of Pittsburgh for a period of 2 years.
Number of Pages: 126
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Mechanical Engineering and Materials Science
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: polymer electrolyte fuel cell, oxygen reduction reaction, non-precious metal catalysts, electrospinning, voltammetry, density functional theory, binding energy, activation energy, nudged elastic band method, catalytic mechanism, microkinetic model
Date Deposited: 25 Jan 2018 21:52
Last Modified: 25 Jan 2018 21:52
URI: http://d-scholarship.pitt.edu/id/eprint/33325

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  • Electrochemical and Computational Study of Oxygen Reduction Reaction on Non-Precious Transition Metal/Nitrogen Doped Carbon Materials. (deposited 25 Jan 2018 21:52) [Currently Displayed]

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