Author: Cai, Lejuan
Title: Surface engineering for efficient electrocatalytic water splitting and nitrogen reduction
Degree: Ph.D.
Year: 2020
Subject: Hong Kong Polytechnic University -- Dissertations
Water -- Electrolysis
Department: Department of Applied Physics
Pages: vi, 172 pages : color illustrations
Language: English
Abstract: Energy shortage and environment pollution caused by the combustion of fossil fuels force us to seek renewable and clean energy for sustainable development. Hydrogen energy, including hydrogen and hydrogen-containing compounds is regarded as the most promising energy carrier due to its high energy efficiency and zero-emission property. Currently, the main pathway to produce hydrogen energy relies on the catalytic steam reforming and coal gasification, which still require the consumption of hydrocarbon fuels. Hydrogen energy can also be obtained through water splitting driven by renewable energy, such as light, electricity and thermal energy. Among various technologies, hydrogen energy from water electrolysis has attracted tensive attention in last decade because water electrolysis is an important method of energy conversion, which can store the renewable energy in the form of hydrogen. However, current water electrolysis is still limited to small-scale applications, large-scale hydrogen production is hindered by the great electricity consumption induced by large overpotential and poor efficiency. Thus, in this thesis, surface engineering strategy is applied to improve the electrocatalytic activity of active materials for enhanced water splitting performance and N₂ reduction activity. For water splitting part, (1) we investigate the effect of surface functional group on hydrogen evolution reaction (HER) by studying the improved HER kinetics induced by surface hydroxyl group modification. Experimental results indicate different active materials grown on three-dimensional (3D) graphene show enhanced HER performance after crafting massive hydroxyl groups on 3D graphene. The positive role of surface hydroxyl groups on the HER performance is then explained by theoretical investigation, and increased water affinity is regarded as the main cause. Surface hydroxyl group can not only attract abundant H₂O clusters near the cathode surface to constantly supply H₂O molecular for hydrogen evolution, but also balance the interfacial pH environment to reduce the overpotential for HER process. (2) we investigate the effect of heteroatom incorporation on water splitting performance by studying the selective water splitting behaviour of Ni₂P induced by different content of Fe dopant. According to the experimental results, enhanced water splitting performance of Ni2P is successfully achieved when Fe dopant incorporates into Ni₂P matrix. More importantly, the activity of hydrogen evolution and oxygen evolution of Fe-doped Ni₂P can be controlled by adjusting the content of Fe dopant. Higher and lower content of Fe doping in Ni₂P matrix contribute to excellent OER performance and remarkable HER activity, respectively. Finally, the selective water splitting behavior induced by active Fe- and Ni-sites engineering is explained by the key intermediate adsorption theory. Based on above analysis, effective surface engineering strategies including functional group modification and heteroatom doping are proposed and successfully realize improved water splitting performance.
For nitrogen reduction part, (1) we optimize the d-electrons configuration of active transition metal (TM) center via sulfidation process to realize enhanced N₂ activation. Pt, the typical catalytic center with poor N₂ affinity, is selected to act as a model to investigate the significance of sulfidation process. Our DFT calculations predict sulfurized Pt (PtS) possesses reduced number of d electron, which can benefit the σ donation from N₂ molecular, realizing efficient N₂ activation. Besides, PtS shows suppressed HER performance, which may contribute to enhanced Faradaic efficiency of NRR. From the experimental results, PtS shows a reduced overpotential for NRR than Pt, and the NRR Faradaic efficiency of PtS is about 5 times as high as that of metallic Pt, showing consistent results with the theoretical analysis. (2) we propose a universal principle to construct PtS₂ supported single atom centers (SACs) for NRR process. Stability of designed catalysts, the selectivity of HER/NRR and barrier of potential limiting step are considered to screen the most favorable electrocatalyst for NRR process. According to results, SACs with different d electron configurations exhibit different N* affinity, ultimately lead to differ in the energy barrier of the potential limiting step. The barrier of potential limiting step shows liner correlation with the N* binding strength, which is also liner correlated with the integral of the density of unoccupied d orbital states of SACs. In this part, sulfidation method and SACs construction are applied to optimize the d electron configuration of exposed transition metal site. After these engineering strategies, the active transition metal center can realize the combination of unoccupied d orbital and abundant d electron, which is beneficial for N₂ activation and NH₃ production.
Rights: All rights reserved
Access: open access

Files in This Item:
File Description SizeFormat 
991022378658203411.pdfFor All Users7.71 MBAdobe PDFView/Open

Copyright Undertaking

As a bona fide Library user, I declare that:

  1. I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
  2. I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
  3. I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.

By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.

Show full item record

Please use this identifier to cite or link to this item: