Development of advanced catalysts for electrochemical CO2 reduction reaction to value-added products

Abstract

The carbon dioxide electroreduction reaction (CO2RR), driven by low-carbon electricity, represents a promising approach for achieving sustainable carbon-neutral energy conversion. However, CO2RR involves the sequential or simultaneous occurrence of multiple proton and electron transfer processes, normally accompanied by the competitive hydrogen evolution reaction (HER). These complex processes with unclear mechanisms can lead to diminished selectivity for desired carbonaceous products. Therefore, this research focuses on the development of advanced electrocatalysts, particularly single-atom catalysts (SACs) and copper (Cu)-based catalysts for CO2 electroreduction to C1 products (carbon monoxide, CO and methane, CH4) and C2+ products (ethylene, C2H4). Furthermore, the corresponding structure-function relationships and related reaction mechanisms are systematically investigated.
Firstly, SACs have emerged as attractive materials for CO2RR. Dual-atom catalysts (DACs), an extension of SACs, exhibit more compelling functionalities due to the synergistic effects between adjacent metal atoms. However, the rational design, clear coordination mode, and in-depth understanding of heteronuclear dual-atom synergistic mechanisms remain elusive. Therefore, a heteronuclear Ni-Ag dual-atom catalyst loaded on defective nitrogen-rich porous carbon, denoted as Ni-Ag/PC-N, is synthesized through cascade pyrolysis. The configuration of Ni-Ag dual-atom sites is confirmed as N3-Ni-Ag-N3. Ni-Ag/PC-N demonstrates a remarkable CO Faradaic efficiency (FECO) exceeding 90% over a broad range of applied potentials, i.e., from −0.7 to −1.3 V versus reversible hydrogen electrode (RHE). The peak FECO of 99.2% is observed at −0.8 V vs. RHE. Tafel analysis reveals that the rate-determining step of CO2RR-to-CO is the formation of the *COOH intermediate, and Ni-Ag/PC-N exhibits optimal electrokinetics. In situ Fourier-transform infrared spectroscopy (FTIR) and in situ Raman spectra indicate accelerated production of *COOH intermediates during the CO2RR-to-CO process. Density functional theory (DFT) calculations demonstrate that the coordinated Ni atom lowers the energy barrier of *COOH intermediates formation over the Ni-Ag/PC-N surface, while the adjacent Ag atom mitigates the catalyst poisoning caused by the strong *CO affinity on the Ni atomic site. These findings establish a solid foundation for the practical applications of dual-atom catalyst in CO2RR and potentially other fields, contributing to the development of more efficient and sustainable energy solutions.
Secondly, while CO2RR is extensively researched for generating valuable C1 and C2+ products, the influence of adsorbed hydrogen (*H) on product distribution remains inadequately understood. This work explores the effect by developing bimetallic Cu-based electrocatalysts with varied lanthanum (La) doping ratios. The as-prepared oxide-derived (OD)-La0.10-CuOx catalyst exhibits a FE over 80% for C2+ products at 300  mA cm−2, whereas OD-La0.40-CuOx achieves a 61.4% FECH4 at 400  mA cm−2. Kinetic isotope experiments reveal distinct dependencies of the rate-determining steps on *H transfer for CO2RR in OD-La0.10-CuOx and OD-La0.40-CuOx. In situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and DFT calculations demonstrate that the moderate H2O dissociation capability of OD-La0.10-CuOx lowers the energy barrier for *CHO → *OCCHO conversion, thus increasing the FEC2+. Conversely, OD-La0.40-CuOx, with its strong H2O dissociation capability, favours *CHO → *CH2O, thereby promoting CO2RR-to-CH4. These findings advance the understanding of the role of *H in CO2 electroreduction at industrial current densities and present avenues for tailored CO2RR products via doping engineering.
Thirdly, under industrial current density (> 300 mA cm−2), the insufficient *CO coverage on the catalyst surface induces the competitive HER and sluggish kinetics of C−C coupling, which hinders CO2RR-to-C2+ products. Herein, this work reports europium hydroxide modified oxide-derived CuO nanosheets (Eu(OH)3-Cu NSs) that could effectively optimize the local *CO coverage and C-C coupling, achieves efficient CO2RR-to-C2+ products. The Eu(OH)3-Cu electrocatalyst demonstrates significantly enhanced selectivity for C2+ products, achieving an optimal FE of 81.4% with partial current density of 326 mA cm−2, in contrast to bare CuO NSs. Additionally, compared to CuO component with fast cathodic corrosion, Eu(OH)3 component can be well maintained at current density of 400 mA cm−2 within the flow cell system in hybrid Eu(OH)3-Cu. In situ electrochemical impedance spectroscopy and infrared spectroscopy reveal that the hybrid Eu(OH)3-Cu demonstrates lower onset potential and enrichment of asymmetric *OCCHO intermediates. This hydroxide-metal interface engineering marks a convenient and immensely promising paradigm to enhance the selectivity and stability for CO2RR-to-C2+ products.
To summarize, this thesis provides new insights into high-performance CO2RR electrocatalysts design, including synergistic heteronuclear Ni-Ag dual-atom catalysts, lanthanum-modified CuOx with controllable adsorbed hydrogen, and europium hydroxide modified Cu with optimal *CO affinity. These advancements contribute to improving the activity, selectivity and stability of CO2RR.

Date of Award	15 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Mengxia Xu (Supervisor), Kam Loon Fow (Supervisor), Hainam Do (Supervisor) & Jonathan D. Hirst (Supervisor)