電気化学的二酸化炭素還元のための銅触媒と合成システムの合理的な設計

概要

九州大学学術情報リポジトリ
Kyushu University Institutional Repository

Rational Design of Cu-based Catalysts and
Synthetic Systems for Carbon Dioxide
Electroreduction
孫, 明旭

https://hdl.handle.net/2324/7157291
出版情報：Kyushu University, 2023, 博士（理学）, 課程博士
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権利関係：Public access to the fulltext file is restricted for unavoidable reason (3)

（様式３）

氏

名

：Mingxu Sun

論 文 名 ：Rational Design of Cu-based Catalysts and Synthetic Systems for Carbon
Dioxide Electroreduction (電気化学的二酸化炭素還元のための銅触媒と合成システムの合理的な
設計)

区

分

：

甲
論

文

内

容

の

要

旨

With the glowing threat of global climate change, reducing CO2 emissions and achieving sustainable
resource use have become urgent priorities. Electrocatalytic CO2 reduction reaction (CO2RR) therefore has
significant environmental and economic implications by converting CO2 into organic compounds, fuels, and
chemical feedstocks. However, the current limitations of CO2RR technologies include poor selectivity
towards specific products, low reaction rates, insufficient carbon utilization, and a lack of sufficient stability.
This study aims to explore catalysts and operating systems for CO2RRs to convert CO2 into high-value
chemicals.
Chapter 1 presents a comprehensive systematic review of CO2RR research. Chapter 2 focuses on the
elucidation of the reaction mechanism and design/optimization of catalysts to develop highly selective
electrocatalysts for various specific products. Chapter 3 emphasizes the design and application of operating
systems to enhance reaction rates and carbon utilization, with the aim of achieving efficient and stable
CO2RRs. Chapter 4 focuses on the exploration and prospecting of the CO2RR pathway, with the aim of
understanding the reaction process at the molecular level and establishing a comprehensive CO2RR network.
Chapter 1: General Introduction
The CO2RR is an emerging technology with the potential to convert CO2 into valuable products. The
successful development and optimization of CO2RR necessitate a systematic study encompassing the
fundamental principles, performance evaluation indicators, and essential experimental components. In this
introduction, I systematically introduce the basic principle of CO2RR, the indicators used to evaluate its
performance, and the important components required for conducting experiments in CO2RR.
Chapter 2: Understanding the Roles of Hydroxide in CO2 Electroreduction on a Cu Electrode for
Achieving Variable Selectivity
Hydroxide-derived copper (OH/Cu) electrodes exhibit excellent
performance for the electrocatalytic CO2RR. However, the role of hydroxide
(OH) in CO2RR remains controversial and the origin of the selectivity
enhancement emerging on OH/Cu has not been fully understood.
In this study, the author synthesized three electrodes characterized by
significantly different OH amounts: small (Cu foil), moderate (M-OH/Cu), and
large amounts of OH (L-OH/Cu), in which the M-OH/Cu and L-OH/Cu were
prepared by electrooxidation method followed by reduction with different degree.
The OH amount on the Cu surface was characterized by electroadsorption
technique, which is the first time used for OH amount evaluation. The
electroadsorption suggested that the amount of OH and the OH-to-Cu0 ratio on

Fig.1.
Faradaic
efficiency (FE) and
the
ratio
of
C2+ -to-CH4
in
CO2RR.

L-OH/Cu were 4.7 times and 2.7 times, respectively, larger than those on M-OH/Cu, whereas obvious
adsorption was not observed on Cu foil. Fig. 1 presents Faradaic efficiencies (FE) for CO2RR on prepared
Cu electrodes. Cu foil showed a high CH4 selectivity with an average CH4 FE of 67% and M-OH/Cu
provided a FE of multicarbon products (C2+ FE) of 71%. L-OH/Cu having large OH amount exhibited a
lower C2+ FE (54%) than M-OH/Cu (71%) but the ratio of C2+-to-CH4 on L-OH/Cu is higher than 355 (Fig.
1). Density functional theory calculation conducted by Prof. A. Staykov represented that the OH coverage
modifies the work function of Cu surfaces and the reaction energetics for the formation of *CHO and
*COCHO, which are deeply related to the variable selectivity observed on the Cu electrodes.
Chapter 3: Designing Enhanced Gas Diffusion Electrode with Ultrathin Super Hydrophobic
Macropore Structure for Acidic CO2 Electroreduction
The development of flow reactors assembled with gas diffusion layer (GDL) electrodes has accelerated
the industrial application of CO2RR technology. Most of CO2RR to date has focused on alkaline conditions
due to the significant selectivity towards carbon products observed under alkaline conditions. However, a
major drawback of alkaline electrolytes is their consumption of input CO2 through reactions with hydroxide
ions (OH−), leading to reduced carbon utilization efficiency. CO2RR in acidic electrolytes would weaken
carbonate formation, however, its CO2RR rate is generally limited by the slow CO2 diffusion compared with
alkaline electrolytes.
In this study, the author established a model to
investigate the factors affecting CO2 diffusion based on a
commercially available GDE structure and found that the CO2
diffusion efficiency can be improved by optimizing the
thickness, pore size, and hydrophobicity of the GDE.
Therefore, a new Cu-GDL with an ultra-thin structure,
macroporous pores, and superhydrophobicity was newly
designed.

The

preparation

method

is

based

on

the

Fig.2. Surface characterization
Cu-GDL. a-c SEM, d CA and TEM.

of

improvement of the chapter 2. Characterization techniques such as scanning electron microscopy (SEM),
transmission electron microscopy (TEM), and contact angle (CA) measurements revealed the unique
structure and superhydrophobic characteristics of the Cu-GDL (Fig. 3). By using Cu-GDL, the author
achieved a Faradaic efficiency (C2+ FE) of 87% while achieving a high partial current density (C2+ j) of up to
−1.6 A cm−2 for multicarbon (C2+) product. Furthermore, to assess the potential of CO2RR technology in
practical applications, we systematically evaluated the influence of CO2 concentration on CO2RR
performance. The results demonstrated that diluted CO2 concentrations had minimal impact on CO2RR
selectivity. Even with a diluted 25% CO2 concentration, we achieved a C2+ j of −0.34 A cm−2, meeting
industrial standards.
Chapter 4: Exploration and Prospect of CO2RR Pathway Based on In Situ Raman Spectroscopy
Although great progress has been made, Cu catalysts remain the only viable option for CO2RR to form
multicarbon (C2+) compounds. Catalyst development is largely dependent on the understanding of the
CO2RR pathway, in particular the key step in the construction of carbon skeletons, namely the C–C coupling.
Knowledge of the details of the CO2RR process at the molecular level is therefore critical to success. In this
exploration and perspective, we have conducted a preliminary analysis of the CO2RR pathway based on our
observations using in situ Raman spectroscopy, laying the foundation for future exploration of the CO2RR
pathway.

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