Developing High-Performance 2D Heterostructured Electrocatalysts and Photocatalysts for Hydrogen Production and Utilizationsts and Photocatalysts for Hydrogen Production and Utilization

Author

Xiaohan MaFollow

Date of Award

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Committee Chair/Advisor

Jianhua Tong

Committee Member

Ming Tang

Committee Member

Ming Yang

Committee Member

Luiz Jacobsohn

Abstract

H₂ is a pivotal chemical in modern society, not only as a clean energy carrier but also as a versatile chemical reactant. However, traditional hydrogen production and utilization heavily rely on thermocatalysis, which is highly energy-intensive and can result in heavy carbon emission and severe environmental problems. Photocatalysis and electrocatalysis are greener alternatives to thermocatalysis that can capitalize on the renewable sunlight and electricity and thus dramatically reduce energy requirements. However, heterogeneous electro/photocatalysts are still far from application to hydrogen economy due to the lack of design principles that can lead to sufficient efficiency. To address this challenge, the dissertation primarily focuses on developing high-performance electrocatalysts and photocatalysts by understanding the impact of surface defects and interactions between different phases on catalytic performance. With the obtained understanding, electro/photocatalysts with high efficiency in H₂ production and utilization (herein, transfer hydrogenation) can be facilely fabricated. To better achieve an in-depth understanding of fabricating electro/photocatalysts used for the hydrogen economy, my thesis work starts with the research on H₂ evolution reaction (HER) via electrocatalysis, and then moves to the HER using the more challenging photocatalytic approach and finally proceeds to the most challenging part, photocatalytic transfer hydrogenation.

For electrocatalytic HER, MoS₂ nanosheets are in situ grown on carbon fiber paper for the fabrication of the proton exchange membrane (PEM) cell electrode. Impressively, this integrated electrode with an ultralow MoS₂ loading of 0.14 mg/cm² can achieve small cell voltages of 1.96 and 2.25 V under 1 and 2 A/cm², respectively, in a practical PEM cell, which is superior to most cells using noble-metal-free HER electrocatalysts even with extremely high catalyst loadings of 3~6 mg/cm² under the similar cell operation conditions. The ultrahigh activity of the as-synthesized electrode is attributed to the intimate contact between MoS₂ and CFP, vertical alignment of MoS₂ nanosheets on CFP, the coexistence of 1T and 2H multiphase MoS₂ and the existence of various defects on MoS₂.

For photocatalytic HER, an Au nanocage/MoS₂ system is investigated to understand the effect of localized surface plasmon resonance (LSPR) on photocatalysis. The match between the LSPR wavelength of Au nanocages and the optical absorption edge of MoS₂ is found to be critical to the activity of the composite. When the match is achieved, a 40-fold activity increase over the bare MoS₂ is observed, while the other unmatched counterparts show much less activity enhancement (~15-fold). The near field enhancement (NFE) is proposed to govern the LSPR process with the energy of surface plasmon transferred from Au to MoS₂ to promote electron excitation in MoS₂, the efficiency of which maximized when the LSPR wavelength of Au matches the MoS₂ absorption edge.

In the photocatalytic transfer hydrogenation case, phenylacetylene (PA) semi-hydrogenation is selected as a model reaction to understand how vacancies in 2D semiconductors may be utilized to manipulate photocatalytic efficiency. 2D g-C₃N₄ nanosheets loaded with Ni single-atoms (SAs) are used as the catalyst for this reaction. By controlling both the Ni loading and the density of surface vacancies on g-C₃N₄, it is found that the numbers of vacancies and Ni SAs had a synergistic impact on the activity of the catalyst. Therefore, a fine tuning of both factors should be important to achieve an optimal hydrogenation activity.

Overall, all research examples highlight the important role played by surface defects and metal-semiconductor interactions, and the findings from the research can be potentially used to guide the design of high-performance photocatalysts for hydrogen evolution and hydrogenation reactions.

Recommended Citation

Ma, Xiaohan, "Developing High-Performance 2D Heterostructured Electrocatalysts and Photocatalysts for Hydrogen Production and Utilizationsts and Photocatalysts for Hydrogen Production and Utilization" (2023). All Dissertations. 3489.
https://tigerprints.clemson.edu/all_dissertations/3489

Author ORCID Identifier

0000-0003-3536-0981