Year

2021

Degree Name

Doctor of Philosophy

Department

Intelligent Polymer Research Institute

Abstract

Electrochemical CO2 reduction (CO2RR) is an environmentally friendly approach to transform greenhouse CO2 to value-added chemical feedstocks and fuels. One of the promising CO2RR products is formate which is widely applied in chemical, food and energy related industrials. The ideal CO2RR to formate electrolysers should possess features such as high formate conversion Faradaic efficiencies (FEformate) at low overpotentials, high current densities, and outstanding stability to meet industrial requirements. In this thesis, highly selective formate producing catalysts were designed and prepared. The effects of CO2RR catalysts’ structures, the electrolyte alkalinity, the cell configuration, and the full-cell assembly in combination with an oxygen evolution anode toward CO2RR performance were systematically studied.

To study catalyst structural effects on formate selectivity, a novel hierarchical structure of 3 dimensional (3D) mesoporous Pd on highly ordered TiO2 nanotubes were prepared via the electrodeposition method. The product selectivity was found to depend on the TiO2 nanotube length, resulting from the influence of mass transports of CO2, protons and products in the tubes. This work demonstrates the importance of designing efficient hierarchical structures to optimise reactant/product mass transport and electrochemical kinetics.

The electrochemical flow cell was employed to overcome the low current density and mass transfer challenge encountered in H-cell using SnS nanosheet-based catalysts. Alkaline electrolyte (1.0 M KOH) successfully suppressed the hydrogen evolution across all potentials particularly at the less negative potentials, and CO evolution at more negative potentials. This in turn widened the electrochemical potential window for formate conversion. A comparative study to SnOx counterpart indicated sulfur also acts to suppress hydrogen evolution, although electrolyte alkalinity resulting in a greater suppression. Moreover, to achieve a long-term current stability, it is necessary to buffer the carbonate/bicarbonate formed from chemical reactions between CO2 and KOH.

High performance oxygen evolution reaction (OER) catalyst is required to be coupled with CO2RR cathode for the full-cell electrolyser assembly. The ultrathin amorphous iron oxyhydroxide nanosheets were synthesized via cyclic voltammetry (CV) potential modulations on thermally treated iron foils. The size and thickness of nanosheets were controlled by tuning CV cycles, potential range, duration, and electrolytes. By loading of Ni species onto the nanosheets, the OER activity was significantly enhanced, indicating iron oxyhydroxide nanosheets can act as excellent 2D supports to achieve synergies effect of bimetallic catalysis.

A single full-cell CO2 electrolyser under electrochemical flow configuration was developed by employed CO2RR active Bi nanoparticles (NPs)-based cathode and earth-abundant NiFe layered double hydroxide (LDH) anode. The rate determining step of CO2RR to formate is the formation of *OCHO via one electron transfer. The combination of highly active NiFe LDH anode, highly efficient Bi NPs cathode, and highly conductive KOH electrolyte operated in flow cell configuration, all contribute to high-performance non-precious metal catalyst-based electrolyser.

This thesis successfully developed several formate producing CO2RR catalysts, and systematically studied effects of mass transports, electrolyte alkalinity, cell configuration, and anode activity for CO2RR to formate. Such studies in catalyst development and understanding the factors influencing CO2RR performance would assist in developing commercial-relevant large-scale electrolysers.

FoR codes (2008)

0912 MATERIALS ENGINEERING

Download

Share

COinS
 

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.