Investigation of highly efficient Bismuth-based electrocatalysts for CO2 electroreduction to formate
Creators: Liu, Shaoqing
Publisher: name: University of Alberta Library
Year: 2022

<p>Renewable energy-powered electrochemical CO2 reduction (ECR) is considered as a promising approach to producing high value-added chemicals to mitigate the ever-increasing accumulation of CO2 in the atmosphere. It could offer a sustainable route to achieving both carbon neutrality in the energy cycle and the storage of renewable energy. Among all the possible products of CO2 reduction, formate has attracted great attention because it is being widely explored as a hydrogen storage material and chemical fuel for fuel cells. It is therefore highly desirable to pursue efficient electrocatalysts for ECR toward formate production. However, the high reaction energy barrier and complicated multi proton-coupled electron transfer steps result in the slow kinetics and broad distributions of the products over most electrocatalysts. Meanwhile, the highly competitive side reaction, hydrogen evolution reaction (HER), always occurs accompanying with ECR. Therefore, the key issue for practical formate production via ECR lies in searching for efficient catalysts that can actively and selectively transform CO2. In this thesis, I aim to develop efficient, cost-effective, and earth-abundant bismuth-based catalysts for simultaneously achieving high activity and selectivity towards ECR, and also to explore and reveal the reaction mechanisms that lead to the enhancement effects of the developed catalysts.<br>
One of the key issues for most of the existing Bi-based catalysts toward ECR is that they do not simultaneously possess high activity (current density &gt; 45 mA cm–2) and high selectivity [faradaic efficiency (FE)] &gt; 90 % in H-cell. Hence, I developed a catalyst of S-doped Bi2O3 nanosheets (NSs) coupled with carbon nanotubes (S-Bi2O3-CNT) and investigated its ECR performance. Compared with the undoped control catalyst (Bi2O3-CNT), the S-Bi2O3-CNT not only exhibits an outstanding catalytic activity for CO2 to formate conversion through ECR, but also can maintain high faradaic efficiency (FE ˃ 90 %) of formate at high current density (48.6 mA cm–2), which outperforms most of the previously reported catalysts. Theoretical calculations reveal that the S doping causes the electronic delocalization of Bi sites, which can benefit the CO2 adsorption and stabilize *OCHO intermediates while suppressing HER through hindering *H adsorption. All these synergistically favor formate production over H2 evolution.<br>
The limited solubility and diffusion of CO2 in an aqueous solution greatly limit the current catalysts to maintain the high formate selectivity and activity over a wide potential window. I developed a catalyst of Bi2O3 NSs grown on carbon nanofiber (Bi2O3@C/HB) with inherent hydrophobicity and applied it for ECR toward formate production. In H-cell system, Bi2O3@C/HB achieved high formate Faradaic efficiency (FEformate) of ˃93 % over an extremely wide potential window of 1000 mV and a peak formate current density of 102.1 mA cm–2. In addition, the developed Bi2O3@C/HB demonstrated a good antiflooding capability when operating in a flow-cell system and can deliver a current density of 300 mA cm–2. Molecular dynamics simulations indicate that the hydrophobic can create a favorable solid-liquid-gas triple-phase boundary which boosts the ECR activity through forming a highly concentrated CO2 layer and meanwhile inhibiting HER by reducing proton contacts.<br>
Thermodynamic analysis suggests the counter anodic oxygen evolution reaction (OER) consumes ~ 90 % of the overall energy input in a full ECR electrolyzer because of its sluggish kinetics. Its product, O2, is not of significant economic value. Herein, I present a pair-electrosynthesis tactic for low energy generation of formate via coupling selectively anodic electrochemical ethylene glycol oxidation (EGO) and cathodic ECR. I used Ni modified Co phosphide grown on nickel foam (NiCoP/NF) and single Cu atom doped Bi (BiCu) as efficient anodic and cathodic electrocatalysts, respectively. The prepared NiCoP/NF exhibited high performance for anodic EGO to formate conversion with &gt;90 % selectivity and a current density of 500 mA cm–2. Meanwhile, the cathodic BiCu catalysts can simultaneously achieve high activity, selectivity, and long stability for formate production from ECR. Consequently, the new coupled ECR-EGO system can operate at a cell voltage of ~ 760 mV lower as compared to the conventional ECR-OER system.</p>