Investigations of Bismuth- and Cobalt-based Electrocatalysts for Carbon Dioxide Reduction Reactions
Creators: ZHU, MENGNAN
Publisher: name: University of Alberta Library
Year: 2023

<p>The electrochemical reduction of carbon dioxides (CO2RR) is a promising solution for mitigating CO2 emissions while simultaneously generating value-added chemicals. From the perspective of materials engineering, understanding the structure-function relationships is crucial in guiding the rational design of electrocatalysts and advancing CO2RR technologies. Considering its significance attached, this thesis endeavors to investigate the i) BaBiO3 (BBO) perovskite, ii) Bi- and iii) Co-based electrocatalysts with atomically distributed active sites, with particular focus on how the key structural features including potential-driven reconstructions, coordination environment, and local geometry affect the catalytic performances of CO2RR. <br>
The first part of this thesis delves into the investigation of the voltage-driven structural reconstructions in the perovskite BBO, resulting in the generation of Bi metallene (eBBO) that manifests an efficient production of formate/formic acid (FA) with appreciable selectivity and partial current densities. Furthermore, Ba2+ ions are released into the electrolyte during the course of reconstructions owing to the irreversible exsolution of Bi cations and phase transformations. The combined insights from the time-resolved Fourier-transform infrared spectroscopy (FTIR) and in situ Raman analysis reveal that the adsorption of Ba2+ ions at eBBO surface facilitates CO2 adsorption, leading to an enhanced CO2-to-FA conversion. This work is of direct significance in uncovering the structural evolutions during electrolysis and elucidating the cooperative catalysis between A- and B- site cations in perovskites for room-temperature CO2RR.<br>
The second part of this thesis presents a simple and versatile nanosecond pulsed lasers method for the simultaneous confinement of the Bi “single atom” (SA) on graphene-based support for CO2RR. The outcomes of the reactions could be controlled by adjusting the compositions of the graphene support. Electrochemical studies disclose that the Bi ensemble supported by pristine or nitrogenated graphene preferably produces FA or syngas, respectively, as the dominant CO2RR products. X-ray absorption spectroscopy (XAS) and theoretical modeling collectively reveal that the product tunability originates from the distinct electronic characteristics of the doped Bi SAs induced by their different first-shell coordination (either carbon or nitrogen), which modify the hybridizations between the Bi center and <em>OCHO/</em>COOH intermediates, alter the energy barriers of the rate-determining steps and ultimately trigger bifurcated reaction pathways. This study demonstrates a practical and scalable ultrafast laser approach to tailoring the fine-structures and catalytic properties of supported catalysts and offers atomic-level insights into the mechanisms of CO2RR on ligand-modified Bi SAs with potential applications in various fields.<br>
The third part of this thesis presents the geometric tuning of the CoN8C8 motif in cobalt phthalocyanine (CoPc) through varying the radius of carbon nanotubes (CNTs) supports, revealing the underlying volcanic correlations between the magnitude of compressive deformation/curvature within CoN8C8 and its catalytic performance in CO2RR. The optimally curved CoN8C8 boosts the turnover frequency by 2.4-fold at a potential of -0.8 V vs RHE via regulating the energy barrier of the rate-determining step (RDS), as supported by Tafel analysis and computational simulations. Such structural features further secure a near 4-fold CoPc mass preservation, affording a halved degradation rate in CO partial current density in the optimally strained sample in contrast to the least strained one by virtue of the slower CoPc demetallation and more efficient charge transfer.</p>