Carbon Dioxide Electrochemistry

Homogeneous and Heterogeneous Catalysis

Marc Robert, Cyrille Costentin, Kim Daasbjerg, Marc Robert, Cyrille Costentin, Kim Daasbjerg

• Publisher: Royal Society of Chemistry
• Year: 2020
• Edition: 1
• eBook ISBN:
  9781788019408
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Conversion of light and electricity to chemicals is an important component of a sustainable energy system. The exponential growth in renewable energy generation implies that there will be strong market pull for chemical energy storage technology in the near future, and here carbon dioxide utilization must play a central role. The electrochemical conversion of carbon dioxide is key in achieving these goals. Carbon Dioxide Electrochemistry showcases different advances in the field, and bridges the two worlds of homogeneous and heterogeneous catalysis that are often perceived as in competition in research. Chapters cover homogeneous and heterogeneous electrochemical reduction of CO2, nanostructures for CO2 reduction, hybrid systems for CO2 conversion, electrochemical reactors, theoretical approaches to catalytic reduction of CO2, and photoelectrodes for electrochemical conversion. With internationally well-known editors and authors, this book will appeal to graduate students and researchers in energy, catalysis, chemical engineering and chemistry who work on carbon dioxide.

CHAPTER 1

Approaches to Controlling Homogeneous Electrochemical Reduction of Carbon Dioxide

A. JACOB A. BARRETT,*^a B. FELIX M. BRUNNER,^a C. PO LING CHEUNG,^a D. CLIFFORD P. KUBIAK,^a E. GWENDOLYENNE L. LEE,^a F. CHRISTOPHER J. MILLER,^a G. KATE M. WALDIE^b AND H. ALMAGUL ZHANAIDAROVA^a

^a Department of Chemistry and Biochemistry, University of California, San Diego 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093, USA

^b Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States

*Emails: [email protected]; [email protected]

1.1 Introduction

Carbon dioxide is the end product of combustion and respiration, and its reduction back to energy-rich products has challenged chemists for well over a hundred years. The first reports of the direct electrochemical reduction of CO₂ appeared in the early 1900s.^1,2 As the field of homogeneous catalysis grew during the 1960s and 1970s, chemists recognized that molecular electrocatalysis was also feasible. The first reports of homogeneous electrochemical reduction of CO₂ by molecular catalysts appeared in the 1970s and 1980s. These included metal phthalocyanine complexes,^3,4 metal tetraaza-macrocycles,^5,6 late metal phosphine complexes,⁷ and Re (I) bipyridyl carbonyl complexes.⁸ A molecular electrocatalyst both participates in an electron transfer reaction (at an electrode) and increases the rate of a chemical reaction. In organic electrochemistry, mediators are often used to mediate the flow of charge between an electrode and a chemical substrate, thus acting as an outer sphere electron donor or acceptor toward the substrate.⁹ An electrocatalyst both supplies charge and performs inner sphere reduction or oxidation of an otherwise kinetically stable substrate. Both the electron transfer and chemical kinetics must be fast for an electrocatalyst to be efficient. Additionally, the electrocatalyst must display a good thermodynamic match between the redox potential (E⁰) for its electron transfer reaction and the chemical potential difference between the products and reactants (i.e. thermodynamic driving force) for the reaction that is being catalyzed (e.g. reduction of CO₂). A significant advantage of molecular electrocatalysis is that these factors can be optimized by chemical tuning of the metal centers via appropriate ligand design.

Over the past ten years, several comprehensive reviews of the homogeneous electrochemical reduction of CO₂ have appeared.^10–15 In addition to the effects of metal type, d-electron configuration, and ligand type, new dimensions in the performance of molecular catalysts have been identified. Many breakthroughs in our understanding of how molecular catalysts respond to their local environment in the electrochemical cell, e.g. solvent, electrolyte, and applied bias have occurred over the last decade. One example of innovative catalyst design is the use of pendent proton relays, as well as charged and hydrogen bonding groups attached to the catalysts have been reported. This chapter presents much of the new knowledge that has been gained over the past ten years in the homogeneous electrochemical reduction of CO₂, with the aim of identifying areas where new research may produce further breakthroughs in our understanding and control of the reduction of CO₂ for its conversion to high-value chemicals like CO, formate, methanol, and methane.

1.2 Overview of Parameters for Evaluating Electrocatalysts

This section will give a brief introduction to the parameters commonly used to describe molecular electrochemical catalysis. A more in-depth discussion of these parameters can be found in the ‘Catalyst Comparisons’ section of this text.

Catalytic Tafel plots are used to visualize catalyst performance using the overpotential (η) and turnover frequency (TOF). The overpotential is the additional potential applied beyond the thermodynamically determined potential (E⁰) required for the electrochemical reaction to transpire. The reaction conditions, such as solvent and pK_a of all present proton sources (i.e. water, phenol, or acetic acid), must be considered to appropriately determine E⁰ of the redox reaction. Furthermore, the products formed (i.e. CO, MgCO₃, HCO₃⁻) are considered in the determination of E⁰_cat. Thus, the calculation of overpotential is not trivial but enables the comparison of electrocatalysts under different reaction conditions. The TOF is the number of chemical conversions of the substrate per unit of time (typically in seconds). Catalysts that have a high TOF for a selective transformation with minimal overpotential are ideal.

The catalytic potential for the electron transfer event is reported in several ways. The potential value at the peak catalytic current (E⁰_cat) is typically reported versus a standard electrode potential such as saturated calomel electrode (SCE), silver chloride electrode (Ag/AgCl), or normal hydrogen electrode (NHE). Additionally, this potential may be adjusted based on the redox couple of an internal standard like ferrocene/ferrocenium (Fc^+/0). The potential at half the catalytic current (E⁰_cat/2) is normally found at the steepest part of the catalytic wave and may also be reported. The Faradaic efficiency (FE) describes the amount of product produced per number of electrons transferred to facilitate the electrochemical reaction. The following sections will provide many of these parameters for an assortment of electrocatalysts.

1.3 Overview of Metals Utilized in Electrochemical CO₂ Reduction

Molecular catalysts can typically be divided into classes based on ligand architecture. Our research group previously described five main classes of catalysts in a 2009 tutorial review published in Chem. Soc. Rev.: metals ligated by porphyrins, cyclams, bipyridyls, polypyridyls, and phosphines.¹⁰ This section will briefly recount the molecular catalysts used for electrocatalytic CO₂ reduction by Periodic Group in the literature to date.^10–15 Improvements in catalytic activity via changes to the metal identity and/or ligand structure, including tuning of the steric and electronic properties, are discussed. The discussion will also describe new strategies for improving catalytic selectivity (i.e. pendent proton relays, Lewis acid additives, hydrogen bonding, and others).

1.3.1 Group 6

The Kubiak and Cowan groups have examined electrocatalytic CO₂ reduction with bipyridine complexes of Group 6 metals. The metal tetracarbonyl bipyridyl, [M(R-bpy)(CO)₄] (M=Mo, W; R=H, tBu), complexes are active for CO₂ reduction with quantitative Faradaic efficiency (FE) for CO at −2.3 V vs. saturated calomel electrode (SCE) [ca. −2.7 V vs. ferrocenium/ferrocene (Fc^+/0)] in MeCN.¹⁶ The reaction rates are significantly slower than ...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Approaches to Controlling Homogeneous Electrochemical Reduction of Carbon Dioxide
• Chapter 2 Homogeneous Electrochemical Reduction of CO2. From Homogeneous to Supported Systems
• Chapter 3 Heterogeneous Electrochemical CO2 Reduction
• Chapter 4 Nanostructures for CO2 Reduction: From Theoretical Insight to Material Design
• Chapter 5 Theoretical Approach to Homogeneous Catalytic Reduction of CO2: Mechanistic Understanding to Build New Catalysts
• Chapter 6 Bridging Homogeneous and Heterogeneous Systems: Atomically Dispersed Metal Atoms in Carbon Matrices for Electrocatalytic CO2 Reduction
• Chapter 7 Bridging Homogeneous and Heterogeneous Systems—Photoelectrodes for CO2 Electrochemical Conversion
• Chapter 8 Hybrid Biological–Inorganic Systems for CO2 Conversion to Fuels
• Chapter 9 In Situ Spectroscopic Methods to Study Electrochemical CO2 Reduction
• Chapter 10 Electrochemical Reactors
• Subject Index