Research on Transition Metal-Based Electrocatalysts Modulated by Cerium Element for Electrochemical Water Splitting

Abstract

With the gradual consumption of fossil fuels and rigorously increasing pressures from environment pollutions, it is important and necessary to develop and harvest clean renewable energy in large scales to fulfill the global sustainable development in economy and society. Hydrogen generated from electrochemical water splitting can not only replace fossil fuels as clean and efficient energy carrier, but also realize the large-scale conversion and storage of sustainable energy-produced electricity. Electrocatalytic water splitting consists two half reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and extra additional potential is required because of the limitation in slow cathodic and anodic kinetics. Non-noble metal-based electrocatalysts (such as manganese, iron, cobalt, and nickel) are of huge advantages in low cost and rich abundance as compared with noble metals, but their activity and stability are still not satisfactory for practical application. Rational design the structures and composites of electrocatalysts using various strategies can significantly decrease the overpotentials for efficient water splitting. Methods including controlling the morphology, employing conductive substrates as supports, regulating the ratios of various components, engineering defects, doping foreign elements, as well as surface coating and decoration are applied to enhance the catalytic activity and stability. Recently, more attentions are paid on regulating the electrocatalytic activity of electrocatalysts by incorporating with rare earth elements. Because of the stable chemical property and unique electronic structure of rare earth elements, their collaboration with transition metal-based electrocatalysts is developed as a novel method to modify the structure and property of electrocatalysts. In this thesis, cerium as a typical rare earth element, is selected and introduced into transition metal-based electrocatalysts to improve their HER and OER property. The role of cerium-based species and their influences on the structures and properties of electrocatalysts are also investigated. Detailed researches and results are listed below.

(1) Doping Ce into transition metal phosphides to improve their HER property. Theoretical calculations demonstrated the decreased hydrogen adsorption free energy during HER process for CoP and modified charge state of Co after doping Ce element. Series characterizations proved the successful doping of Ce into CoP. Electrochemical hydrogen evolution test results showed the significant decrease in overpotentials in both acidic and alkaline electrolytes for Ce-doped CoP, and the increased surface catalytically active sites, faster turnover frequency and good stability for HER. Furthermore, similar improvement in HER activity is also observed when doping Ce into nickel phosphide and iron phosphide, suggesting this method as a general approach to modulate the catalytic property of transition metal phosphides.

(2) Effect of spatial configuration of CeO2 on the OER property of NiO. Spatial configurations of CeO2 in NiO can affect its structure and oxygen evolution property. Therefore, two kinds of different structures are synthesized, namely, CeO2-embedded NiO structure and CeO2-surface-loaded NiO structure, and then characterized by various techniques to learn their differences in crystal structures, surface chemical states, and coordination numbers of Ce. Results in OER performance showed the better activity of CeO2-embedded NiO structure, as compared with CeO2-surface-loaded NiO structure and pure NiO. Further analysis showed their difference in oxygen adsorption capacity and its effect on OER property. This work gave an insight to the promotion in OER property of electrocatalysts caused by CeO2, which would also work as a guideline for designing high-performance OER electrocatalysts.

(3) Highly active OER electrocatalyst of cerium phosphate decorated NiCo2O4. Decorating CePO4 layer on the surface of NiCo2O4 nanowire arrays can be realized by a facile wet chemical deposition method, and the coating structure is characterized and confirmed by different techniques. Excellent activity and good stability of CePO4 coated NiCo2O4 in alkaline solution witnessed the huge improvement of OER property by decorating CePO4. The accelerated conversion of intermediates promoted by phosphate group and the oxygen adsorption ability of cerium element in CePO4 contributed to the faster generation of oxygen. This result provide a novel method and approach to prepare and modify efficient electrocatalysts for OER.

In summary, introducing this typical rare earth element of cerium and its compounds into various transition metal-based electrocatalysts can modulate their structures, furthermore improve the activity for HER and OER, paving a novel method and pathway to realize highly efficient and low-cost electrochemical water splitting.

Date of Award	11 Jul 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Johnny Chung Yin HO (Supervisor) & Yongquan Qu (External Supervisor)