Year

2021

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The increasing energy demand and deteriorating environmental problems stimulate the exploration of sustainable energy technologies. Hydrogen (H2) is expected to attain global carbon neutrality owing to its high energy density and zero-emission. Electrochemical water splitting is considered as an effective approach to produce high-purity H2 cleanly compared with steam reforming reaction and coal gasification. Among various water electrolysis technologies, anion exchange membrane (AEM) water electrolysis cells have attracted significant attention because of their low cost, long-term stability, non-corrosive electrolyte, and compact cell design. However, the sluggish alkaline hydrogen evolution reaction (HER) kinetics is a big obstacle, which requires high noble metal loading. Meanwhile, AEM fuel cells also exhibit many advantages over other fuel cell technologies while the sluggish hydrogen oxidation reaction (HOR) is a major challenge. Therefore, the development of efficient electrocatalysts for HER/HOR in AEM water electrolyzers/fuel cells is highly desirable to better utilize hydrogen energy and attain carbon neutrality. Although platinum-based metals (PGMs) are the most employed electrocatalysts in practical water electrolysis and fuel cells, the high cost and limited element resource would restrict their long-term application. Therefore, it is a great necessity to develop alternative electrocatalysts for PGMs. The catalyst design principle is highly dependent on the understandings of the fundamental HER/HOR processes. Currently, the pH-dependent reaction kinetics of HER/HOR for PGMs is debatable and the universal HER/HOR activity descriptor remains to be established. Therefore, an in-depth understanding of HER/HOR processes by careful catalyst design avenue is required and more efficient HER/HOR electrocatalysts need to be developed.

To this end, in this thesis project, the concept of carbon support functionalization via transition metal single atoms was proposed and the as-prepared carbon support was demonstrated to be effective for the alkaline hydrogen electrocatalysis. Following this concept, we further investigated the effects of transition metals on alkaline hydrogen electrocatalysis and found that the metal-N-C moieties are dedicated to promoting the Volmer step, which is closely associated with electronegativity and d orbital occupancy of the metals. Carbon-based materials possess many wonderful features to be catalyst supports than metal oxides or hydroxides; however, the lack of water dissociation ability cannot ensure fast overall alkaline HER kinetics. For the first time, we proposed a catalyst support functionalization strategy via transition metal single atoms and constructed Ni-N-C/Pt catalyst with highly-improved HER kinetics in contrast to N-C/Pt. The functional catalyst support (Ni-N-C) has added capability in H-OH cleaving step that was induced by Ni single atoms, which endows Ni-N-C/Pt with much higher alkaline HER activity (1.49 A mg-1) than that of N-C/Pt (0.31 A mg-1) and 20% Pt/C (0.74 A mg-1).

FoR codes (2008)

0302 INORGANIC CHEMISTRY, 0306 PHYSICAL CHEMISTRY (INCL. STRUCTURAL)

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.