CO2 Conversion and Utilization E-Book
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CO2 Conversion and Utilization
Comprehensive overview of current development of various catalysts in CO2 conversion and utilization through photocatalytic and electrochemical methods
CO2 Conversion and Utilization systematically summarizes the development of CO2 photo- and electro-conversion and utilization, especially the reaction mechanism, engineering and technology of testing, and preparation methods and physicochemical properties of various catalytic materials. The rational design and preparation of catalysts, development of characterization technologies, and in-depth understanding of catalytic mechanisms are systematically discussed.
In particular, the various parameters influencing the photocatalytic and electrochemical CO2 reduction are emphasized. The underlying challenges and perspectives for the future development of efficient catalysts for CO2 reduction to specific chemicals and fuels are discussed at the end of the text.
Written by a highly qualified author with significant experience in the field, CO2 Conversion and Utilization includes information on:
- Measurement systems and parameters for CO2 photo/electro-conversion, CO2 photo/electro-conversion mechanism, and Cu-based and Cu-free metal materials for electrocatalytic CO2 reduction
- Organic-inorganic, metal organic framework, and covalent organic framework hybrid materials for CO2 photo/electro-conversion
- Single/dual-atom catalysts, homogeneous catalysts, and high-entropy alloys for CO2 photo/electro-conversion
- Semiconductor composite and carbon-based materials for photocatalytic CO2 reduction, novel routes for CO2 utilization via metal-CO2 batteries, and CO2 conversion into long-chain compounds
Providing comprehensive coverage of the subject, CO2 Conversion and Utilization is of high interest for scientific researchers as well as engineers and technicians in industry, including but not limited to photochemists, electrochemists, environmental chemists, catalytic chemists, chemists in industry, and inorganic chemists.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Measurement Systems and Parameters for CO
2
Photo/Electro‐Conversion
1.1 Introduction
1.2 The Measurement Systems for CO
2
Photo/Electro‐Conversion
1.3 The Parameters for CO
2
Photo‐Conversion
References
2 CO
2
Photo/Electro‐Conversion Mechanism
2.1 Introduction
2.2 CO
2
Photo‐Conversion Mechanism
2.3 CO
2
Electro‐Conversion Mechanism
2.4 Summary and Perspectives
References
3 Cu‐Based Metal Materials for Electrocatalytic CO
2
Reduction
3.1 Introduction
3.2 Cu‐Based Metal Materials for Electrocatalytic CO
2
Reduction
3.3 Conclusion and Outlook
Acknowledgment
References
4 Cu‐Free Metal Materials for Electrocatalytic CO
2
Conversion
4.1 Introduction
4.2 CO‐Producing Metals
4.3 HCOOH‐Producing Metals
References
5 Organic–Inorganic Hybrid Materials for CO
2
Photo/Electro‐Conversion
5.1 Hybrid Materials for Photocatalytic CO
2
Reduction Reaction (CO
2
RR)
5.2 Hybrid Materials for Electrochemical CO
2
RR
5.3 Hybrid Materials for Photoelectrochemical (PEC) CO
2
RR
5.4 Challenge and Opportunity
References
6 Metal–Organic Framework Materials for CO
2
Photo‐/Electro‐Conversion
6.1 Introduction
6.2 Photocatalysis
6.3 Electrocatalysis
6.4 Photoelectrocatalysis
6.5 Conclusion and Outlook
Acknowledgment
References
7 Covalent Organic Frameworks for CO
2
Photo/Electro‐Conversion
7.1 Introduction
7.2 COFs for Photocatalytic CO
2
Reduction
7.3 COFs for Electrocatalytic CO
2
Reduction
7.4 Challenges and Perspectives
References
8 Single/Dual‐Atom Catalysts for CO
2
Photo/Electro‐Conversion
8.1 Introduction
8.2 Synthetic Methods of Single/Dual‐Atom Catalysts
8.3 CO
2
Photo‐Conversion
8.4 CO
2
Electro‐Conversion
8.5 Summary and Perspective
References
9 Homogeneous Catalytic CO
2
Photo/Electro‐Conversion
9.1 Introduction
9.2 Homogeneous Catalytic CO
2
Electro‐Conversion
9.3 Homogeneous Photocatalytic CO
2
Reduction
9.4 Summary and Perspective
Acknowledgments
References
10 High‐Entropy Alloys for CO
2
Photo/Electro‐Conversion
10.1 Introduction
10.2 Reaction Pathways and Evaluation Parameters of Electrochemical CO
2
RR
10.3 Characteristics and Synthesis of HEAs
10.4 High‐Entropy Alloys for CO
2
RR
10.5 Summary and Outlook
References
11 Semiconductor Composite Materials for Photocatalytic CO
2
Reduction
11.1 Introduction
11.2 TiO
2
‐Based Composite Photocatalysts
11.3 Metal Oxides/Hydroxides‐Based Composite Photocatalysts
11.4 Metal Chalcogenides/Nitrides‐Based Composite Photocatalysts
11.5 C
3
N
4
‐Based Composite Photocatalysts
11.6 MOFs‐Derived Composite Photocatalysts
11.7 Nonmetal‐Based Composite Photocatalysts
11.8 Conclusions and Perspectives
References
12 Carbon‐Based Materials for CO
2
Photo/Electro‐Conversion
12.1 Advances of Carbon‐Based Materials
12.2 Background of CO
2
Conversion
12.3 EC CO
2
Conversion
12.4 PC CO
2
Reduction
12.5 Carbon‐Based Materials in PEC CO
2
Reduction
12.6 Challenge and Opportunity
References
13 Metal–CO
2
Batteries: Novel Routes for CO
2
Utilization
13.1 Introduction
13.2 The Mechanism for Metal–CO
2
Electrochemistry
13.3 The Electrocatalysts for Metal–CO
2
Batteries
13.4 The Electrolytes for Metal–CO
2
Batteries
13.5 Conclusion and Outlook
References
14 CO
2
Conversion into Long‐Chain Compounds
14.1 Introduction
14.2 Photobiochemical Synthesis (PBS)
14.3 Microbial Electrosynthesis (MES)
14.4 Decoupling Biotic and Abiotic Processes
14.5 Conclusions and Perspectives
References
15 Conclusions and Perspectives
15.1 New CO
2
RR Catalyst
15.2 New CO
2
RR Mechanism
15.3 Industrial CO
2
RR Perspectives
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Industrial use and indicative market prices of some C
1
and C
2+
Chapter 13
Table 13.1 The possible reactions in metal–CO
2
batteries.
Table 13.2 A summary of the strategies for the anode, cathode electrocatalys...
Chapter 14
Table 14.1 Selective summarization of multicarbon products from PBS using CO
Table 14.2 Selective summarization of multicarbon products from MES using CO
List of Illustrations
Chapter 1
Figure 1.1 Schematic for H cell for CO
2
RR.
Figure 1.2 Schematic for cathode flow cell for CO
2
RR.
Figure 1.3 Schematic for MEA for CO
2
RR.
Figure 1.4 Diagram of photocatalytic CO
2
reduction reaction.
Figure 1.5 Schematic diagram of photocatalytic CO
2
reduction system includin...
Figure 1.6 Schematic illustration of three kinds of photocorrosion: (a) oxid...
Chapter 2
Figure 2.1 Global CO
2
emission, 1990–2021 and projected CO
2
emission, 2021–2...
Figure 2.2 Schematic representation of three‐step (light absorption, charge ...
Figure 2.3 Schematic illustration of the energy band structures of some Cu‐b...
Figure 2.4 The possible coordination modes of adsorbed CO
2
δ–
on a...
Figure 2.5 Three possible pathways involving formaldehyde, carbene, and glyo...
Figure 2.6 ESR spectra obtained at 77 K from the photocatalytic reaction sys...
Figure 2.7 Three possible pathways (glyoxal, oxalic acid, and methyl radical...
Figure 2.8 Products from electrochemical CO
2
reduction over polycrystalline ...
Figure 2.9 Standard equilibrium potential for electrochemical CO
2
reaction t...
Figure 2.10 Possible mechanistic pathways proposed for CO
2
electroreduction ...
Figure 2.11 Possible mechanistic pathways proposed for CO
2
electroreduction ...
Figure 2.12 Possible mechanistic pathways proposed for CO
2
electroreduction ...
Figure 2.13 Possible mechanistic pathways proposed for CO
2
electroreduction ...
Chapter 3
Figure 3.1 (a) FEs of various CO
2
reduction products of high‐power reduction...
Figure 3.2 (a) Rate of CO
2
RR to >2e
−
products on different catalysts. ...
Figure 3.3 (a) Partial current density toward C
2+
products on Cu
500
Ag
100
...
Figure 3.4 (a) Illustration of prepared CuPd nanoalloys with different mixin...
Figure 3.5 (a) Illustrations of the reaction process for Cu and Cu–Sn bimeta...
Figure 3.6 (a) The reaction pathways of producing HCOOH through different in...
Figure 3.7 (a) High resolution‐transmission electron microscope (
HR‐TE
...
Figure 3.8 (a) Schematic of a de‐alloyed nanoporous Cu–Al catalyst. (b) FE f...
Figure 3.9 (a) Proposed mechanism for the electroreduction of CO
2
to C
2
H
5
OH ...
Figure 3.10 (A) High‐angle annular dark‐field scanning TEM image of AuAgCu N...
Chapter 4
Figure 4.1 (a) Density of adsorption sites on closed‐shell cuboctahedral Au ...
Figure 4.2 (a) Schematic illustration of hierarchical micro/nanostructured s...
Figure 4.3 (a) The FE
CO
of Zn foil and h–Zn at various constant potentials r...
Figure 4.4 (a) The FE
COOH
of SnO
2
NPs, SnO
2
NTs‐350, and SnO
2
NTs at a serie...
Figure 4.5 (a) Faradaic efficiencies and production rates of formate for FTO...
Chapter 5
Figure 5.1 (a) Scheme of [Ru‐dcbpy]/N–Ta
2
O
5
hybrid photocatalysts for photoc...
Figure 5.2 (a) Z‐scheme CO
2
RR using the hybrid of C
3
N
4
and a binuclear
RuRu′
...
Figure 5.3 (a) Schematic reaction mechanism for the photoreduction of CO
2
by...
Figure 5.4 (a) A schematic presentation of the NiPc molecules anchored on th...
Figure 5.5 (a) Molecular structures of RucP
2+
′ and surface‐bound RucP
2+
...
Figure 5.6 (a) Schematic diagram of PEC conversion of CO
2
to HCOO
−
ove...
Chapter 6
Figure 6.1 Illustration of the roles of MOFs in CO
2
photoreduction: (a) phot...
Figure 6.2 (a) Structure of Ru, Ir, and Cu‐based photosensitizers. (b) Struc...
Figure 6.3 (a) Scheme of photocatalytic CO
2
RR over NH
2
‐Uio‐66(Zr) under visi...
Figure 6.4 (a) Illustration of molecular compartments with precise TiO
2
posi...
Figure 6.5 (a) Synthesis method of Cu SAs/UiO‐66‐NH
2
catalyst. C: gray, O: r...
Figure 6.6 (a) Schematic illustration of the synthesis process of Zr‐BTB@Hem...
Figure 6.7 (a) Schematic illustration of Cu
3
(BTC)
2
MOF structures on GDE for...
Figure 6.8 (a) Structural details of HKUST‐1 and CuAdeAce. Faradaic efficien...
Figure 6.9 (a) Schematic illustration of the synthesis of Cu‐MOF‐CF. (b) Sch...
Figure 6.10 (a) Crystal structures of SU‐101 and modulation of active sites ...
Figure 6.11 (a) TEM image of HKUST‐1 with Cu dimer distortion. (b) Crystal s...
Figure 6.12 (a) Synthesis procedure of ZIF‐8 on Ti/TiO2 nanorod.(b) Sche...
Chapter 7
Figure 7.1 (a) Synthesis of COF and Re‐COF; (b) side view; (c) unit cell of ...
Figure 7.2 The construction of COF‐367‐Co featuring different spin states of...
Figure 7.3 (a) The prepartion of uniformly dispersed POM clusters in COF by ...
Figure 7.4 (a) The chemical structure of TpBpy. (b) Schematic diagram photoc...
Figure 7.5 The synthetic procedure of Bpy‐sp
2
c‐COF and Re‐Bpy‐sp
2
c‐COF.
Figure 7.6 (a) Retrosynthetic interfacial design scheme of heterogeneous cat...
Figure 7.7 Schematic illustration of the synthesis of TTF‐Por(Co)‐COF with v...
Figure 7.8 (a) Schematic illustration for the synthesis of CuPcF
8
‐CoPc‐COF a...
Chapter 8
Figure 8.1 (a) Synthesis process of Fe SAS/Tr‐COFs.(b) Synthesis and str...
Figure 8.2 (a) Synthesis of the Mn
1
Co
1
/CN catalyst.(b) Schematic illustr...
Figure 8.3 (a) Illustration of the synthesis route for Fe
1
NSC. (b, c) TEM i...
Figure 8.4 (a) Scheme illustration for the integrated nanostructural NiSn‐AP...
Figure 8.5 (a) Schematic energy‐level diagram showing the electron transfer ...
Figure 8.6 (a) Photocatalytic activities of samples with double single‐atom ...
Figure 8.7 (a) Positively charged single‐atom metal electrocatalyst accelera...
Chapter 9
Figure 9.1 Structure of Fe tetraphenyl porphyrins and their derivatives.
Scheme 9.1 Oxidative quenching and reductive quenching of PS*.
Figure 9.2 Structure of [Ru(bpy)
3
]
2+
and Ir(ppy)
3
.
Figure 9.3 Structure of BIH.
Figure 9.4 Structure of
fac
‐Re(bpy)(CO)
3
Cl, Ru‐Re1 and Ir–Re supramolecular ...
Figure 9.5 Structure of typical Ru‐based complexes as photocatalysts for CO
2
Figure 9.6 Structure of [Fe(qpy) (OH
2
)
2
]
2+
, [Co(qpy)(OH
2
)
2
]
2+
and [F...
Figure 9.7 Structure of Ni complexes bearing S
2
N
2
‐type tetradentate ligand w...
Figure 9.8 Structure of [ZnCo(OH)L
1
]
3+
and [Co
2
(OH)L
1
]
3+
reported by...
Figure 9.9 Structure of [Fe(dqtpy)(MeCN)]
2+
reported by Lau and Robert [1...
Figure 9.10 Structure of [Fe‐
p
‐TMA]
5+
and Phen2.
Chapter 10
Figure 10.1 Schematic diagram of common CO
2
reduction products of different ...
Figure 10.2 Schematic definition of components for HEAs.
Figure 10.3 Schematic definition of configurational entropy for HEAs.
Figure 10.4 The reaction paths of electrochemical CO
2
RR toward different pro...
Figure 10.5 Evaluation parameters of electrochemical CO
2
RR.
Figure 10.6 Feature overview diagram of HEAs.
Figure 10.7 Overview of the synthesis method of HEAs.
Figure 10.8 (a) An overview of nanodroplet‐mediated conductive deposition us...
Figure 10.9 (a) CO
2
RR reaction diagram of AuAgPtPdCu NPs.(b) Simulate th...
Chapter 11
Figure 11.1 The mechanism for photocatalytic CO
2
reduction.
Figure 11.2 Reactor used for photocatalytic reduction of CO
2
. Left: solid–ga...
Figure 11.3 Photocatalytic activities of CO
2
reduction over TiO
2
, TCx, and C...
Figure 11.4 Schematic diagram of photocatalysts type‐II heterojunction.
Figure 11.5 (a) Schematic llustration of synthesis procedure for transition ...
Figure 11.6 The mechanism for photocatalytic CO
2
reduction.
Figure 11.7 Calculated crystal structures of CN (a), CN‐K
2
(b), and CNNa
2
(c...
Figure 11.8 Fabrication of numerous porous materials from MOFs and MOF‐based...
Figure 11.9 (a) UPS‐determined work functions of GO and Cu/GO hybrids; (b) B...
Figure 11.10 Reaction pathway of photocatalytic CO
2
reduction on SiC@MoS
2
....
Chapter 12
Figure 12.1 The categories of carbon‐based materials as electrocatalysts.
Figure 12.2 (a) HRTEM of a cross‐section of a Fe‐filled CNT. (b) A close loo...
Figure 12.3 (a) Model atomic structures of nanoporous carbon‐based materials...
Figure 12.4 (a) N content distribution at various doping temperatures. (b) F...
Figure 12.5 (a) The FE toward CO for NPCA900, NCA900, PCA900, and CA900 at d...
Figure 12.6 (a) HRTEM images for Bi
2
O
3
‐NDGQDs. (b) FE of formate for Bi
2
O
3
‐N...
Figure 12.7 (a) HRTEM images of CuO
x
@C. (b) Potential‐dependent FE toward et...
Figure 12.8 (a) HRTEM images of HNCM/CNT hybrid membrane. (b) Core–shell str...
Figure 12.9 N
2
sorption isotherms and pore size distributions of (a) NSHCF90...
Figure 12.10 Proposed structure of polymerized C
3
N
4
(blue: carbon; gray: nit...
Figure 12.11 (a) One‐pot synthesis of crystalline CN with tailored grain bou...
Figure 12.12 (a) Molecular structure of a macrocyclic cobalt catalyst. (b) T...
Figure 12.13 (A) The schematic of preparation processes. Evolutions of CH
4
(...
Figure 12.14 (a) The possible electron transfer route of CoTPP/g‐CN in PC CO
Chapter 13
Figure 13.1 (a) The development of metal–CO
2
batteries. (b) Schematic illust...
Figure 13.2 (a) Schematic illustration of the synthesis procedure of N‐CNTs@...
Figure 13.3 (a) Schematic illustration of a Li–CO
2
battery with Ru@Super P a...
Figure 13.4 (a) Schematic illustration of all‐solid‐state Li–CO
2
batteries. ...
Chapter 14
Figure 14.1 Schematic of hybrid biotic–abiotic systems powered by renewable ...
Figure 14.2 Schematic illustration of PBS where photogenerated electrons tra...
Figure 14.3 Schematic illustration of PBS with an external bias, where the p...
Figure 14.4 (a) High‐angle annular dark field () STEM image of a single
M. t
...
Figure 14.5 Schematic illustration of MES where electrons derived from elect...
Figure 14.6 Extracellular electron transfer mode. Direct electron transfer v...
Figure 14.7 The Calvin–Benson–Bassham cycleIn stage 1, the enzyme RuBisC...
Figure 14.8 The Wood–Ljungdahl pathway.One CO
2
molecule is converted to ...
Figure 14.9 The tricarboxylic acid cycle.Eight steps are involved: (1) a...
Figure 14.10 (a) Scanning electron microscopy image of the cathode surface w...
Figure 14.11 Schematic representation of a decoupled electrobiochemical syst...
Figure 14.12 Sketch of the modules used in technical photosynthesis of 1‐but...
Figure 14.13 Schematic illustration of sugar synthesis from a decoupled elec...
Figure 14.14 A decoupled electrochemical–biological system for the productio...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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CO2 Conversion and Utilization
Photocatalytic and Electrochemical Methods and Applications
Edited byZhicheng Zhang
Editor
Prof. Zhicheng ZhangDepartment of ChemistryTianjin UniversitySchool of ScienceNo. 92, Weijin RoadNankai District300072 TianjinChina
Cover Image: © AndriyOnufriyenko/Moment/Getty Images
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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Print ISBN: 978‐3‐527‐35202‐9ePDF ISBN: 978‐3‐527‐84178‐3ePub ISBN: 978‐3‐527‐84179‐0oBook ISBN: 978‐3‐527‐84180‐6
Preface
Rapid industrial development and the combustion of fossil fuels have produced significant volume of CO2. The continuous gathering of CO2 in the atmosphere has crucial negative effects on the climate and environment, such as climate warming, glaciers thaw, and ocean acidification, which has a great influence on the sustainable development of society and threatens survival of mankind. The conversion of CO2 to value‐added chemicals and fuels has been regarded as a promising solution to reduce the emission of CO2. To overcome the thermodynamic and kinetic barriers involved in CO2 activation and conversion, the efficient catalyst as well as the suitable form of energy, such as photocatalytical and electrochemical methods, are necessary.
This book aims to provide a comprehensive overview of current development of various catalysts in CO2 conversion and utilization through photocatalytical and electrochemical methods. The rational design and preparation of catalysts, the development of characterization technologies, and in‐depth understanding of catalytic mechanism, are systematically discussed. Particularly, the various parameters influencing the photocatalytical and electrochemical CO2 reduction are emphasized. Finally, the underlying challenges and perspectives for the future development of efficient catalysts for CO2 reduction to specific chemicals and fuels are discussed. We hope that this book can inspire more people to pay attention to the development of advanced catalysts for the CO2 conversion and utilization through photocatalytical and electrochemical methods.
Zhicheng Zhang
Tianjin University08 January 2022
1Measurement Systems and Parameters for CO2 Photo/Electro‐Conversion
Li Li, Zhenwei Zhao, Xinyi Wang, and Zhicheng Zhang
Tianjin University, School of Science, Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Sciences, No. 92, Weijin road, Nankai district, Tianjin, 300072, China
1.1 Introduction
Fossil fuels are the main fuels in today's society, whose combustion produces a large amount of CO2, bringing a series of environmental pollution problems. The capture and conversion of carbon dioxide have become the focus of current research. It is important to convert CO2 into chemicals and fuels with higher added values, which can alleviate environmental pollution and energy problems. CO2 photo/electric conversion can use renewable clean energy to convert CO2, becoming the most potential conversion method. Here, we summarized the measurement systems and parameters for CO2 photo/electro‐conversion, which mainly include photocatalysis, electrocatalysis, and photo‐electro‐catalysis systems.
1.2 The Measurement Systems for CO2 Photo/Electro‐Conversion
In the chapter, we mainly introduce three photo/electro conversion systems, namely photocatalytic CO2 reduction, electrocatalytic CO2 reduction, and photo‐electro‐catalytic CO2 reduction. The main purpose of this chapter is to explain how to test catalysts and evaluate their advantages and disadvantages, so as to provide readers with a more comprehensive and systematic summary.
1.2.1 The Measurement Systems of Photocatalytic CO2 Reduction
Generally, photocatalytic CO2 reduction reaction systems are mainly divided into two categories: under liquid‐phase reaction system and in gas‐phase reaction system.
1.2.1.1 CO2 Reduction System Under Liquid‐Phase Reaction System
(1) Definition of liquid‐phase reaction system: The liquid‐phase reaction system refers to the reduction reaction occurring in the saturated solution of CO
2
, and the photocatalyst can be uniformly dispersed in the solution.
(2) Characteristics of liquid‐phase reaction system: In the liquid‐phase reaction system, because the solid catalyst dispersed in the solution is always in the agitated state, its charge transfer efficiency and heat transfer efficiency are higher. However, in the liquid‐phase reaction system, the limited solubility and diffusion coefficient of CO
2
in H
2
O will limit the mass transfer efficiency of the photocatalytic CO
2
reduction reaction. Under the reaction conditions of 25 °C and 101.325 kPa, the solubility of CO
2
in H
2
O is less than 0.033 mol L
−1
, which weakens the diffusion of CO
2
molecules from the gas phase to the photocatalyst surface. Compared with neutral and acidic conditions, the solubility of CO
2
under alkaline conditions is higher. The solubility of CO
2
can be improved by increasing the pH value of the solution, or organic solvents such as
acetonitrile
(
ACN
) and
ethyl acetate
(
EAA
) can be added to H
2
O to promote the dissolution of CO
2
.
(3) General reaction device of liquid‐phase reaction system: In the CO
2
reduction reaction experiment in the liquid‐phase system, 300 W xenon lamp light source (which can also be replaced according to the specific experimental test needs) is generally used to simulate solar radiation, and filter or light intensity meter can be used to adjust the appropriate light wavelength and light intensity
[1]
. The whole reaction system is generally carried out at room temperature (special reactions can also be adjusted as required). The specific operations are as follows: weigh a certain amount of catalyst and ultrasonically disperse it in a specific solvent (select according to different materials). Pour its dispersion into a closed device, and inject 99.9% CO
2
into it. After 30 minutes, to ensure that the rest of the interfering gas is discharged, close the reaction device. Put the reaction system under the xenon lamp and turn on the condensing device or other thermostatic devices to ensure that the system is at a certain temperature. The timing starts when the light source is turned on, and the gas samples in the system are collected at certain intervals as required to facilitate the subsequent determination of products.
1.2.1.2 CO2 Reduction System in Gas‐Phase Reaction System
(1) Definition of gas‐phase reaction system: The gas‐phase reaction system is a reduction system in which the photocatalyst is fixed on the substrate support and the mixture of CO
2
and water vapor directly reacts with the photocatalyst.
(2) Characteristics of gas‐phase reaction system: Compared with the liquid‐phase reaction, the gas‐phase reaction is not affected by sacrificing agents, photosensitizers, solvents, and other factors, and is a relatively simple reaction system. The diffusion coefficient of CO
2
in the gas phase is about 0.1 cm
2
s
−1
, which is about 4 orders of magnitude higher than that in the liquid phase. Therefore, in the gas‐phase reaction, the mass transfer efficiency between CO
2
and photocatalyst is higher. Another advantage of gas‐phase photocatalytic CO
2
reduction reaction is that it can effectively inhibit hydrogen evolution reaction. As the reduction of H
2
O to H
2
is more advantageous in thermodynamics and kinetics, the photocatalytic CO
2
reduction reaction in the liquid‐phase reaction may induce hydrogen evolution and reduce the conversion rate of CO
2
. Photocatalytic CO
2
reduction in gas‐phase reaction can effectively solve this problem.
(3) General reaction device of gas‐phase reaction system: The system for testing the photocatalytic CO
2
reduction performance using the gas‐phase reaction system is generally composed of CO
2
cylinder, reactor, light source, detector, computer, and other parts
[2]
. Generally, the reaction is carried out in a closed air circulation system. In the experiment, a 300 W xenon lamp light source (which can also be replaced according to the specific experimental test needs) is generally used to simulate the solar radiation, and a light intensity meter can be used to adjust the appropriate light intensity. The whole reaction system is generally carried out at room temperature (special reactions can also be adjusted as required). Weigh a certain amount of the prepared photocatalyst sample and dissolve it in a specific organic solvent, then evenly coat it on filter paper or flat glass (or according to specific materials) of about a certain specification, put it into a closed system, and add 50 μl water, which is used as an electron source to annihilate holes. High‐purity CO
2
(99.99%) is continuously introduced for 30 minutes to replace the air in the system so that the system is filled with saturated CO
2
. The product sampling method is the same as the above method for reduction in liquid‐phase photocatalytic CO
2
, but no condensing device is required
[3]
.
1.2.1.3 Detection of CO2 Reduction Products
For a finished photocatalytic CO2 reduction reaction, gas chromatograph (GC) can be used to detect gas‐phase products (such as H2, CO, and CH4) [4]. The peak area of the corresponding product can be obtained after the sample extracted in the experiment is detected by GC. The yield of the product can be converted by comparing it with the curve calibrated by the standard gas [5], and the liquid‐phase product can be detected by nuclear magnetic resonance.
1.2.2 The Measurement Systems of Electrocatalytic CO2 Reduction
At present, the system for testing CO2 reduction reaction mainly includes H‐cell, flow cell, and membrane‐electrode assemblies (MEA).
1.2.2.1 Electrocatalytic CO2 Reduction Reaction Test in H‐Cell
H‐cell is the most common measurement system of electrocatalytic CO2 reduction, which is a three‐electrode system, as shown in Figure 1.1[6], including a working electrode (catalyst), a counter electrode (Pt sheets or carbon rod), and a reference electrode (Ag/AgCl electrode in saturated KCl or saturated calomel electrode), as well as a proton‐exchange membrane or cation‐exchange membrane (Nafion 117) in the middle of the cathode and anode, which prevents the products produced by the working electrode from migrating to the surface of the anode and being oxidized.
Figure 1.1 Schematic for H cell for CO2RR.
Source: Li et al. [6], with permission from John Wiley & Sons.
Specifically, the first is the preparation of the working electrode. The method of preparing working electrodes differs depending on the material. Generally, there are two kinds of catalysts: one is materials directly grown in situ on the support, such as carbon paper, carbon cloth, foam copper, and copper foil. These catalysts can be well bonded with the substrate, which is conducive to the transmission of electrons, so the general current and stability will be better. The other is ex situ growth materials, which need to be prepared into ink and dropped onto carbon paper or glassy carbon electrode. Typically, the catalyst ink is prepared by ultrasonicating 5 mg of catalyst with 500 μl of isopropyl alcohol (or ethanol) and 10 or 20 μL of 5 wt % Nafion solutions for at least 30 minutes. And then, the catalyst ink is deposited on carbon paper or glassy carbon electrode to prepare the working electrodes with a load of 1 mg cm−2. (Of course, different catalysts have different maximum loads. It can be flexibly selected according to the characteristics of materials.) It is worth noting that when the conductivity of the catalyst is poor, a certain amount of carbon black can be added during the preparation of catalyst ink to improve the conductivity of the catalyst.
The second step is about the assembly of the H‐cell. Both chambers of the H‐cell are equipped with electrolytes, such as 0.1 M KHCO3 aqueous solution. It is worth noting that the electrolyte in cathode chamber needs to be purged with CO2 for at least 30 minutes before electrochemical measurement.
Finally, electrocatalytic CO2 reduction reaction is conducted on an electrochemical workstation. The catalyst is first stabilized using cyclic voltammetry (CV). Next, linear scanning voltammetry (LSV) is used to evaluate the electrocatalytic activity of the prepared samples in CO2 or Ar‐saturated 0.1 M KHCO3 solution at a scanning rate of 5 mV s−1. The selectivity of CO2RR was tested using current–time (I–t) mode. The gas products are analyzed by a GC equipped with various applied potentials. Usually, the concentration of H2 is analyzed by a thermal conductivity detector (TCD), and the concentration of CO is analyzed by a flame ionization detector (FID). The liquid products are collected at the conclusion of each electrocatalysis and analyzed by the 1H NMR (400 or 600 MHz). For the NMR, typically, 500 μL of catholyte is mixed with 100 μL of D2O and 100 μl of dimethylsulfoxide (DMSO) as the internal standard.
Figure 1.2 Schematic for cathode flow cell for CO2RR.
Source: Wang et al. [7] / with permission of Springer Nature.
1.2.2.2 Electrocatalytic CO2 Reduction Reaction Test in Flow Cell
CO2 has a low solubility in aqueous solutions, which greatly limits its mass transfer rate in electrolytes. Therefore, it has promoted the research and development of CO2 reduction systems. In the flow cell, CO2 can be transported directly to the surface of the catalytic electrode, significantly increasing the mass transfer rate and reaction rate.
Flow cell is an improvement on H‐cell, which is also a three‐chamber cell, as shown in Figure 1.2[7]. The Nafion 117 membrane [8] or Fumasep FAB‐PK‐130 [7] is used to separate the anode and cathode. The electrolyte can be 0.5 M KHCO3 solution, 1 M KOH solution, or other suitable electrolytes for both anode and cathode. The working electrode is usually selected from hydrophobic carbon paper, the counter electrode is a piece of Ni foam, and the reference electrode is a solid‐state Ag/AgCl electrode. The flow rate of CO2 is controlled at 20 sccm by gas mass‐flow controller. The electrochemical data are measured using an electrochemical workstation. The gas‐phase products are monitored in real time by gas chromatography. Similar to H‐cell, the liquid‐phase product is collected after one hour of electrolysis and examined by 400 or 600 MHz NMR. For specific details, please refer to Section 1.2.2.1.
1.2.2.3 Electrocatalytic CO2 Reduction Reaction Test in MEA
To further reduce ohmic losses, MEA are designed to reduce the gap between electrodes. As shown in Figure 1.3, the cathode and anode electrodes are pressed together and sandwiched with an ion‐exchange membrane in the middle, forming a zero‐gap electrolytic cell [9]. The continuously humidified CO2 gas stream is supplied directly to the cathode, and the reduction of carbon dioxide occurs at the boundary between the membrane and the cathode electrode. The cathode gas product is discharged through a simplified cold trap to collect the permeable liquid prior to the GC test. The main advantage of this device over microfluidic flow cells is that the CO2 concentration can be increased relatively easily and significantly by pressurization, resulting in higher current densities and reaction rates.
Figure 1.3 Schematic for MEA for CO2RR.
Source: Wang et al. [7] / with permission of Springer Nature.
1.2.3 The Measurement Systems of Photo‐Electro‐Catalytic CO2 Reduction
1.2.3.1 Basic Device for Photocatalytic CO2 Reduction Experiment
Photocatalytic reduction of CO2 is usually carried out in a closed square quartz pool of a certain volume. The experiment uses a xenon lamp to simulate sunlight, and the irradiation intensity and wavelength can be selected according to the experimental requirements. The reaction device is typically composed of a three‐electrode system [10], i.e. a working electrode, a counter electrode, and a reference electrode, as shown in Figure 1.4[11].
Figure 1.4 Diagram of photocatalytic CO2 reduction reaction.
Source: Li et al. [11] / with permission of John Wiley & Sons.
The most used is the prepared catalyst as a working electrode, that is, a photocathode, the counter electrode generally adopts BiVO4 photoanode, the reference electrode generally adopts saturated calomel electrode, and the electrolyte is generally 0.1 M KHCO3, which can be determined according to the special needs of the experiment [12]. Before proceeding with the reaction, CO2 gas should be introduced into the system for at least 30 minutes to ensure that the CO2 gas in the solution is saturated. Subsequently, experiments are carried out under different biases in the three‐electrode system according to the experimental requirements, and the illumination time is generally two hours or set according to the experiment.
1.2.3.2 Other Devices for Photocatalytic CO2 Reduction
In addition to the photocatalytic CO2 reduction reaction under normal circumstances, the following devices exist. Due to photocatalysis, photocathodes and photoanodes are generally composed of p‐type semiconductors and n‐type semiconductors [13]. According to the configuration of the photoelectrode, the photocatalytic device can be divided into the following three types: (i) photocathodic drive battery (composed of three parts: photocathode, counter electrode, and external bias; see Figure 1.5a); (ii) photoanode‐driven battery (composed of photoanode, counter electrode, and external bias; see Figure 1.5b); and (iii) photocatalytic cells jointly driven by the above two (see Figure 1.5c). Figure 1.5d is an electrode or photoelectrode‐coupled photovoltaic (PV) cell for photoconversion, unlike the above three.
1.2.3.3 Detection of CO2 Reduction Reaction Products
For the completed photocatalytic reaction, GC can be used to detect gas‐phase products (such as H2, CO, and CH4), the sample taken in the experiment can be detected by GC, the peak area of the corresponding product can be obtained, and the yield of the product can be converted by comparing with the curve of standard gas calibration, and the liquid‐phase product can be detected by nuclear magnetic resonance [14].
1.3 The Parameters for CO2 Photo‐Conversion
The main purpose of this chapter is to explain how to evaluate the advantages and disadvantages of catalysts so that experimenters can screen better catalysts.
1.3.1 The Parameters of Photocatalytic CO2 Reduction
It is important to evaluate the photocatalytic activity, selectivity, and stability of CO2RR catalyst with reliable performance parameters to explain the photocatalytic performance, working mechanism, and catalyst design of CO2RR. At present, there is no uniform evaluation parameter for photocatalyst performance in the world, and standardized tests and evaluation methods need to be developed urgently. The most commonly used performance parameters are defined as follows.
Figure 1.5 Schematic diagram of photocatalytic CO2 reduction system including (a) photocathode‐driven system, (b) photoanode‐driven system, (c) photoanode and photocathode‐codriven system, and (d) hybrid photosystem combining the electrode or photoelectrode with a PV cell.
Source: Tang and Xiao [13] / with permission of American Chemical Society.
1.3.1.1 Evaluation Parameters of Photocatalytic CO2 Reduction Activity
Reaction conditions have a direct impact on the photocatalytic activity, including the range and intensity of incident light, reaction temperature, reaction pressure, amount of catalyst and cocatalyst added, pH value of solution, reactor design, and structure [15, 16]. Therefore, it is a difficult task to compare the activity of photocatalysts reported under different conditions.
Generally speaking, yield is the most commonly used parameter to evaluate the activity of photocatalysts, representing the amount of target product produced by the catalyst per unit time and per unit mass. In the photocatalytic CO2 reduction reaction, the unit of catalytic reaction yield is generally μmol h−1 or μmol g−1 h−1. In addition, the commonly used parameters to characterize the photocatalytic activity of the catalyst include apparent quantum yield (AQY), turnover frequency (TOF), and Solar‐to‐fuel energy conversion efficiency (STF).
Apparent Quantum Yield (AQY) AQY refers to the ratio of the number of electrons transferred and the number of incident photons in a reaction system at a specific monochromatic wavelength [13]. It reflects the ability to capture different energy photons during redox reactions on a certain photocatalyst, representing the light‐utilization efficiency of the photocatalyst at a specific wavelength. AQY can be calculated by Eq. (1.1).
Turnover Frequency (TOF) As shown in Eq. (1.2), TOF generally refers to the number of reactions on a unit active site in a unit time at a given temperature, pressure, and reactant ratio and a certain degree of reaction. TOF reflects the frequency of reaction at the active site of the catalyst and the intrinsic activity of the catalyst. It is considered as the most appropriate parameter to compare the activities of different catalysts.
However, it is difficult to determine the exact number of active sites on the photocatalyst, which hinders the application of TOF in the field of photocatalysis. Some researchers prefer to use the number of surface atoms or the number of cocatalysts to calculate TOF, that is, apparent TOF. Considering that the number of active centers may not be proportional to the surface atoms, the apparent TOF should be much lower than the actual TOF [17].
Solar‐to‐Fuel Energy Conversion Efficiency (STF) In addition, STF is used to evaluate the utilization ratio of photocatalysts to sunlight. For a selected product of A, the STF can be calculated by Eq. (1.3)[18].
where ΔG0(A) is the standard Gibbs free energy change in the process of CO2 reduction to A, Pin is the input solar energy intensity and equals 100 mW cm−2 when Air Mass 1.5 Global (AM 1.5G) irradiation is adopted, and t is the consumed time to produce n(A) moles of A.
Photocurrent Density Photocurrent density is often used as a performance parameter in evaluating the photoelectric performance of photocatalysts. After the energy of light radiation is absorbed by the semiconductor, the valence band electrons jump to the conduction band. Under the action of a strong electric field, the conduction band electrons will move directionally to form a current, that is, photogenerated current. As shown in Eq. (1.4), photocurrent density refers to the ratio of photocurrent generated by a photoelectric electrode to the area of light irradiation under the irradiation of solar light. It is usually determined by the optical absorption, the electron–hole pair separation efficiency, and the charge injection efficiency [19]. Therefore, the greater photocurrent response indicates that the catalyst has better charge separation and better catalytic activity.
1.3.1.2 Evaluation Parameters of Photocatalytic CO2 Reduction Selectivity
Photocatalytic CO2 reduction is a reaction involving multiple electrons, which forms a variety of intermediates, making the product of CO2 reduction diversified. Therefore, catalyst selectivity is also an important parameter to measure the quality of catalyst. The product selectivity of photocatalytic CO2 reduction reaction can be defined as the number of electrons required to reduce the target CO2 product compared with the number of electrons required for all reduction reactions. In general, Faraday efficiency (FE) is the most common parameter used to evaluate the selectivity of photocatalysts, quantifying the ratio of the electrons that contribute to yielding product. Hence, for a selected product of A, the corresponding FE can be calculated by Eq. (1.5). An increase in the FE of a specified product suggests an improved selectivity.
where α is the number of electrons needed to reduce CO2 to yield one molecule of A, n(A) is the molar quantity of A, F is the Faraday constant, and Q is the total charge through the circuit in the process of generating n(A) moles of A.
In the photoelectric test, in order to evaluate the catalytic selectivity of the catalyst for a specific product, the partial photocurrent density (jA) is also used. The jA is calculated by multiplying the overall jph with the corresponding FE(A)
1.3.1.3 Evaluation Parameters of Photocatalytic CO2 Reduction Stability
Besides the photocatalytic redox reactions, the photogenerated electrons (holes) with higher (lower) quasi‐fermi energy than the thermodynamic reduction (oxidation) potential of the semiconductor can also drive the degradation or decomposition of semiconductor itself in aqueous solution under illumination, known as photocorrosion of semiconductors, which will greatly reduce the efficiency of catalytic reaction [20, 21] (Figure 1.6).
Therefore, in addition to considering the catalytic activity and selectivity, the stability of the catalytic reaction is also an important indicator to evaluate the catalyst. Long‐term experiments or repeated experiments should be conducted under light to evaluate the long‐term stability of photocatalyst.
1.3.2 The Parameters of Electrocatalytic CO2 Reduction
In CO2RR, generally, the performance of a catalyst is evaluated from the following aspects: selectivity (Faraday efficiency, here is marked as FE), activity (current density), energy efficiency (overpotential), and stability.
Figure 1.6 Schematic illustration of three kinds of photocorrosion: (a) oxidative photocorrosion, (b) reductive photocorrosion, (c) dual photocorrosion, and (d) stable. Er(SC) and Eo(SC) are the thermodynamic oxidation/reduction potentials of semiconductors, respectively.
Source: Li et al. [21] / with permission of American Chemical Society.
The FE means selectivity of the product. The value of FE is calculated by referring to the ratio of the transferred electric energy to the total transferred electric energy for the production of specific products. It can be calculated according to Eq. (1.7):
where e is the number of transferred electrons for each product, F the Faraday constant, Q charge, I applied current, t reaction time, and n total product.
The current density can reflect activity of catalysis. In addition, it reflects the reduction dynamics and hints at the possibility of large‐scale applications. It refers to the amount of current per unit surface area (electrochemical surface area) passing through the catalyst in unit time. It can be calculated according to Eq. (1.8):
where ji and jtotal stand for the current density of the target product and the total current density of the reaction, respectively.
The overpotential reacts to the energy efficiency of the catalyst. The lower the overpotential, the higher the energy efficiency. The overpotential is defined as the difference between the actual potential of the reaction and the thermodynamic equilibrium potential. Overpotential is closely related to current density. When the current density increases, the overpotential also increases.
The stability of the catalyst refers to the time that the catalyst has stable activity under the conditions of use. It can usually be measured according to the I–t curve. Operating under continuous CO2 reduction conditions, the current density versus time behavior was evaluated by means of chronoamperometry.
1.3.3 The Parameters of Photo‐Electro‐Catalytic CO2 Reduction
In PEC CO2RR systems, photocathodes play a pivotal role, from sunlight irradiating the electrodes to product generation. The key parameters describing photocathode performance include starting potential (Von), total photocurrent density (jph), FE for different products, partial photocurrent density (jA), STF, and durability of continuous reaction under light irradiation.
1.3.3.1 Overpotential
The starting potential represents the minimum external bias required to initiate photocatalytic CO2 reduction. The smaller the starting potential, the more likely the photocatalytic reaction is initiated, and the higher the photocathode energy conversion efficiency. Due to the competitive hydrogen evolution reaction, the starting potential does not necessarily mean the beginning of CO2 reduction, and the current density near the starting potential shows a rather slow increase that is difficult to identify precisely. Therefore, for convenience, the starting potential is usually defined as the external bias required for the total photocurrent density to reach 0.1 mA cm−2[11]. In addition, by comparing the difference between the starting potential of photoelectric CO2 reduction and the standard redox potential of CO2, the effective photoelectric voltage generated by the photocathode under light irradiation can be reflected, and the effective photovoltage can play a role in reducing energy consumption and improving energy utilization.
1.3.3.2 Total Photocurrent Density (jph) and Partial Photocurrent Density (jA)
The photocurrent density is usually normalized to the exposed surface area of the photocathode in the electrolyte or the mass of the photocathode catalyst, which can reflect the reaction rate of photocatalysis and measure the catalytic activity of the photocathode. It increases rapidly after exceeding the starting potential and reaches a saturation value at a certain negative potential. The saturation photocurrent density is theoretically determined by the light absorption capacity and charge‐separation efficiency of the semiconductor material, and in general, the greater saturated photocurrent brought by light indicates that the material has better photogenerated carrier generation and separation capabilities. Due to insufficient charge separation/transfer and surface catalytic reactions, the actual photocurrent density may be smaller than the theoretical value [18].
Partial photocurrent density (jA) can be used to evaluate the catalytic activity of the photocathode on a specific product. The higher the partial photocurrent density (jA) of the target product, the higher the selectivity of the photocathode, and the better the catalyst performance. For part of the photocurrent density of target product A, the formula is calculated as follows:
1.3.3.3 Faraday Efficiency (FE)
In general, FE is the most common parameter used to evaluate the selectivity of photocatalyst, quantifying the ratio of the electrons that contribute to yielding product. Hence, for a selected product of A, the corresponding FE can be calculated by Eq. (1.10). An increase in the FE of a specified product suggests an improved selectivity.
where α is the number of electrons needed to reduce CO2 to yield one molecule of A, n(A) is the molar quantity of A, F is the Faraday constant, and Q is the total charge through the circuit in the process of generating n(A) mole amount of A.
1.3.3.4 Solar Energy Conversion Efficiency
According to different reaction devices, STF, applied bias photon‐to‐current efficiency (ABPE), and half‐cell STF are common parameters to evaluate the performance of the photocathodes in photoelectrochemical CO2 reaction [11].
The STF defines the overall solar conversion efficiency of the zero‐/self‐biased two‐electrode PEC CRR cell, and it can be calculated by Eq. (1.11).
where ΔG0(A) is the standard Gibbs free energy change in the process of CO2 reduction to A, Pin is the input solar energy intensity and equals 100 mW cm−2 when AM 1.5G irradiation is adopted, and t is the consumed time to produce n(A) moles of A.
For some two‐electrode PEC CRR cells, an external bias device (Vbias) is applied between the working electrode and the counter electrode to realize the photoelectric catalytic CO2 reduction. ABPE can be used to describe the solar energy conversion efficiency of the photocathode, and it can be calculated by Eq. (1.12).
where jA is the partial photocurrent density (jA) to yield A, E0(A) is the standard redox potential of A generated by CO2 reduction, and Pin is the input solar energy intensity and equals 100 mW cm−2 when AM 1.5G irradiation is adopted. When the anodic reaction is water oxidation reaction, the theoretical voltage of A generated in the two‐electrode PEC CRR cell is (1.23 V − E0(A)).
The calculation of the solar energy conversion efficiency in half‐cell STF is similar to ABPE, except that the external bias is replaced by the applied potential of the photocathode.
1.3.3.5 Apparent Quantum Yield (AQY)
AQY refers to the ratio of the number of electrons transferred and the number of incident photons in a reaction system at a specific monochromatic wavelength [13]. It reflects the ability to capture different energy photons during redox reaction on a certain catalyst, representing the light utilization efficiency of the photocatalyst at a specific wavelength. AQY can be calculated by Eq. (1.13).
1.3.3.6 Electrochemical Active Area (ECSA)
Electrochemical active area (ECSA) refers to the effective area involved in electrochemical reactions, which is determined by the structure and morphology of the catalyst. Photocatalysts with porous or hollow structures usually have a higher electrochemical activity specific surface area and expose richer active sites. In general, the ECSA is proportional to the catalytic performance of the photocathode catalyst. The ECSA value can be calculated from Eq. (1.14)[22].
where Cs and S are constants, representing the specific capacitance and specific surface area of the corresponding surface smoothed sample under the same conditions, respectively. CDL represents the capacitance value of the electric double layer, and the current density and sweep speed are linearly fitted by measuring the CV curve at different sweep speeds, and the resulting slope is the electrochemical electric double layer capacitance.
1.3.3.7 Electrochemical Impedance (EIS)
The Nyquist spectra from electrochemical impedance spectroscopy (EIS) can be used to evaluate the charge transfer rate between the electrode material and the electrolyte. The radius of the semicircle in the high‐frequency regions corresponds to the resistance of the electrode, and the size of the radius of the semicircle in the middle and low‐frequency region is used to explain the charge transfer rate at the interface between the electrode and the electrolyte, and the smaller the radius, the larger the charge transfer rate [22].
1.3.3.8 Tafel Slope (Tafel)
Tafel slope is a common indicator for evaluating photocatalytic reaction kinetics, which can be used to evaluate reaction kinetic rates and predict catalytic mechanisms. The linear part of the Tafel plot can be fitted to the Tafel equation [23]:
where ŋ represents the overpotential, b is the Tafel slope, and j is the current density.
The smaller the Tafel slope, the faster the current density increases, the smaller the overpotential change, and the higher the photocatalytic reaction kinetic rate.
1.3.3.9 Photocatalytic Stability
Besides the photocatalytic redox reactions, the photogenerated electrons (holes) with higher (lower) quasi‐fermi energy than the thermodynamic reduction (oxidation) potential of the semiconductor can also drive the degradation or decomposition of semiconductor itself in aqueous solution under illumination, known as photocorrosion of semiconductors, which will greatly reduce the efficiency of catalytic reaction [20, 21].
Therefore, in addition to considering the catalytic activity and selectivity, the stability of the catalytic reaction is also an important indicator to evaluate the catalyst.
Stability tests such as chronocurrent or chronopotentiometry can be used to assess the stability of a catalyst [24]. The chronocurrent method measures the change in the current of the working electrode with time while controlling the voltage of the working electrode. The chronopotentiometric method measures the change in the voltage of the working electrode over time while controlling the current of the working electrode. However, both the chronocurrent method and the time‐potentiometric method have limitations in measuring the stability of the electrocatalyst and can only be measured at the same current or voltage. In contrast, the multistep chronocurrent method can simultaneously measure the stability of the electrocatalyst at different potentials, that is, control the voltage, from the potential step without an electrochemical reaction to the potential where the electrochemical reaction occurs while measuring the change in the current flowing through the electrode with time.
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2CO2 Photo/Electro‐Conversion Mechanism
Yalin Guo1, Shenghong Zhong2, and Jianfeng Huang1
1Chongqing University, State Key Laboratory of Coal Mine Disaster Dynamics and Control, School of Chemistry and Chemical Engineering, No.55 University Town South Road, Chongqing, 401331, China
2Fuzhou University, College of Materials Science and Engineering, No. 2 Xue Yuan Road, Fuzhou, 350116, China
2.1 Introduction
Rapid industrial development and unrestrained fossil fuel use have led to excessive CO2 emissions into the atmosphere causing severe climate change and environmental issues such as global warming, rising sea levels, and ocean acidification [1]. Taking the year 2021 as an example, CO2 emission has amounted up to 33 gigatons (Gt) as a result of human activity, and it was predicted to increase steadily during the next 30 years (Figure 2.1) [2]. The unprecedented level of CO2 in the atmosphere must be drastically reduced, as per the Paris Agreement adopted in 2015 [3]. While the primary avenue is and will remain to curb emissions at the source, CO2 capture and storage (CCS) and CO2 capture and utilization (CCU) technologies have attracted increasing attention from the perspective of CO2 removal [4]. CCS basically stores CO2 via absorption into the ground or the deep sea, but this method is usually expensive, requires high energy consumption, and is prone to gas seepage. CCU contains CO2 direct and indirect utilization [5]. Typical examples of direct CO2 utilization include the use in food and soft drinks, fire extinguishers, propellants, and concrete building materials [5]. A recent development in the field of indirect utilization concerns carbon recycling approaches such as biochemical [6, 7], thermochemical [8, 9], photochemical [10–12], and electrochemical conversions [13–15], exploiting CO2 as a valuable carbon resource for producing value‐added chemicals and fuels. Photo/electro‐conversion of CO2 is highly promising because of its mild operating conditions, ability to produce tunable products, and utilization of renewable energy resources (e.g. solar, wind, and hydro energy). They represent a sustainable CO2 conversion approach enabling simultaneous carbon fixation and renewable energy storage.
Nevertheless, it should be noted that CO2
