Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) have been attracted significant attention due to their high energy efficiency. The electrocatalyst is one of the most important parts. However, state-of-the-art electrocatalysts suffer from several challenges, including 1) low stability under harsh working conditions; 2) low atomic utilization efficiency, especially for noble metals. This thesis, therefore, focuses on the design of highly efficient and stable electrocatalysts from nanoparticles down to single atoms using atomic layer deposition (ALD) and further understand the insight mechanisms.

Firstly, Pt nanoparticles are selectively deposited on the TiO2 modified N-doped carbon nanotubes. The strong metal-support interactions between Pt and TiO2 enhances both the catalytic activity and stability in the oxygen reduction reaction (ORR).

Further, to promote the electronic property of the surface Pt, a surface engineering strategy using Nb single atoms has been developed. The deposition of Nb single atoms is found to promote both the activity and stability of Pt catalysts in the ORR.

Furthermore, to achieve the maximum atomic utilization efficiency, a high-loading Pt single-atom catalyst (SAC) is successfully achieved using N-doped carbon nanosheets as support. The nature of Pt1 atoms under electrochemical hydrogen evolution reaction (HER) is investigated using operando X-ray absorption spectroscopy (XAS).

Finally, the Fe, Co, and Ni SACs are achieved using the pre-located Pt single atoms. The Co and Ni SACs are found to be active in the HER and oxygen evolution reaction, respectively. Moreover, the nature of several types of single atoms under HER is investigated using operando XAS.

In summary, the discoveries in this thesis provide important guidance to achieve electrocatalysts with low cost, high activity, and high stability.

Summary for Lay Audience

With the rapid consumption of fossil fuels and increasing environmental concerns such as air contamination and global warming, the development of eco-friendly and highly efficient energy conversion technologies using clean and sustainable energy sources is highly desired. Polymer electrolyte membrane fuel cells (PEMFCs), a highly efficient energy conversion device that uses alternative chemicals as the fuel source to produce electricity with only heat and non-toxic byproducts have attracted significant attention.

The electrocatalyst that is one of the most important components in PEMFCs suffers from key challenges which limited their practical applications, such as 1) low stability under harsh working conditions; 2) low atomic utilization efficiency, especially for noble metals. This thesis mainly focuses on addressing these challenges on the design of highly efficient electrocatalysts through atomic layer deposition (ALD), as well as the understanding of the reaction mechanism.

A stable Pt electrocatalyst under oxygen reduction reaction (ORR) is first developed. The Pt nanoparticles (NPs) were selectively deposited on the TiO2 after the decoration of N-doped carbon nanotubes by ALD. The strong metal-support interactions between Pt and TiO2 enhances both the catalytic activity and stability.

Further, a surface engineering strategy on Pt NPs using Nb single atoms has been developed through ALD. The deposition of Nb single atoms on Pt NPs is found to promote both the activity and stability of Pt catalysts in the ORR.

Furthermore, to achieve the maximum atomic utilization efficiency, a high-loading Pt single-atom catalyst (SAC) is successfully achieved using N-doped carbon nanosheets as support. The PtSAC exhibits high catalytic performance in electrochemical hydrogen evolution reaction (HER) and the nature of active sites under HER was also investigated.

Finally, Fe, Co, and Ni SACs can be achieved using the pre-located Pt single atoms. The achieved Co and Ni SACs show high catalytic performance in HER and oxygen evolution reaction, respectively. The nature of active sites under HER was also investigated.

In summary, the discoveries in this thesis provide important guidance to achieve electrocatalysts with low cost, high activity, and high stability.

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