Electrochemical Reduction of Carbon Dioxide

Overcoming the Limitations of Photosynthesis

Frank Marken, David Fermin

• Publisher: Royal Society of Chemistry
• Year: 2018
• Edition: 1
• eBook ISBN:
  9781788014526
• Topic: Sciences physiques
• Subtopic: Science environnementale

One of the crucial challenges in the energy sector is the efficient capture and utilisation of CO2 generated from fossil fuels. Carbon capture and storage technologies can provide viable alternatives for energy intensive processes, although implementation of large-scale demonstrators remains challenging. Therefore, innovative technologies are needed that are capable of processing CO2 emission from a wide range of sources, ideally without additional fossil energy demand (e.g. solar driven or overcoming the limits of photosynthesis). This book covers the most recent developments in the field of electrochemical reduction of CO2, from first-principle mechanistic studies to technological perspectives. An introduction to basic concepts in electrochemistry and electrocatalysis is included to provide a background for newcomers to this field. This book provides a comprehensive overview for researchers and industrial chemists working in environmental science, electrochemistry and chemical engineering.

Chapter 1

Introduction to the Eletrochemical and Photo-electrochemical Reduction of CO₂

David J. Fermin*^a and Frank Marken*^b

^a School of Chemistry, University of Bristol, Bristol BS8 1TS, UK;

^b Department of Chemistry, University of Bath, Bath BA2 7AY, UK

* Email: [email protected]; [email protected]

1.1 Introduction to the (Photo-)Electrochemical Reduction of CO₂

The scale of the challenge in carbon dioxide conversion is enormous, with 35 gigaton of anthropogenic CO₂ generated by fossil fuel combustion/consumption every year. The average percentage of CO₂ in the atmosphere had risen to 403.38 ppm by September 2017 according to the data released by the National Oceanic and Atmospheric Administration of the US Department of Commerce,¹ with serious effects expected on global warming, climate patterns, sea levels, biodiversity, food production, human displacement, and so forth. There is a global consensus that immediate action must be taken to halt the relentless increase in atmospheric carbon dioxide levels, with the Global Apollo programme being a prime example of international initiatives being undertaken.² In addition to the ever growing number of installations of renewable energy sources in different areas of the planet, there are great opportunities in “recycling”^3,4 carbon dioxide with biochars,⁵ by absorption,⁶ or by electrolysis powered by photovoltaic, wind, tidal, hydro-electric, salinity gradient energy, or the so-called “blue energy” systems.⁷ Although CO₂ sequestration has also been proposed,⁸ mimicking natural photosynthesis represents the most attractive but also scientifically challenging avenue.

In the context of artificial photosynthesis, links can be established between catalytic,⁹ electrochemical, and photo-electrochemical conversion of atmospheric CO₂, as schematically illustrated in Figure 1.1. Taking inspiration from nature, integrating functional units at the nanoscale capable of (i) capture solar light, (ii) CO₂ accumulation, and (iii) selective reduction to products can lead to new technologies that can be widely deployed for local mitigation of carbon emission. There are also integration approaches for “semi-artificial photosynthesis”, for example employing PS1 and PS2 apparatus extracted from cells and immobilised onto electrode surfaces. Although synergistic effects can be envisaged from such “nano-integrated” systems, such level of structural complexity currently rarely achieves high efficiency or stability. The electro-reduction of CO₂ is a tough problem so why combine this with the added complexity of an integrated solar cell? Why not just use conventional solar electricity without integration? So far, there are no technological developments that enable answering this question unambiguously. However, there is a large community of scientists, including those contributing to this book, that identifies a direct and selective path of CO₂ to a valuable carbon structure as one of the grand challenges in the path towards a low carbon economy.

Figure 1.1 Schematic depiction of carbon dioxide accumulation, energy harvesting, and catalytic reduction of CO₂ into products at three levels of nano-integrated artificial photosynthesis.

This book primarily focuses on electrochemical conversion of CO₂, establishing correlation between the nature of the catalysts and the electrode potential. Figure 1.1 sketches how the potential bias (input energy) can be provided by solar energy either via photovoltaic solar cells,¹⁰ or by direct photo-generation of carriers at the electrocatalytic site.¹¹ These two approaches, illustrated by Figures 1.1A and B, respectively, represent two different levels of nano-integration. The third level (Figure 1.1C) involves the integration of CO₂ capture moieties directly to the catalysts active site in a combined unit. Depending on the catalyst properties, a variety of products can be generated directly (in situ), or by a separate process exploiting, for example, the formose reaction^12,13 to build up carbohydrates from formaldehyde. A large number of approaches has been presented in the literature for generating higher added value compounds based on various electrochemical reactors,¹⁴ homogeneous catalysts^15,16 or inorganic photosynthetic sites.¹⁷ Independently of the approach used for providing the driving force for CO₂ reduction, investigating the properties of the catalytic sites by (spectro)-electrochemical techniques provides extremely valuable mechanistic information. Some of the most recent developments in spectro-electrochemical tools are reviewed in Chapters 8–10.

Numerous reviews^18–20 and authoritative books^21–23 have been published on the topic of CO₂ reduction and electrocatalysis at electrode surfaces. The high symmetry of the carbon dioxide molecule has often been cited as a key contributor to the activation barrier, with a price in energy often paid by applying a high overpotential (excess potential with respect to the thermodynamic reduction potential). More recent studies, based on the so-called “scaling relations” formalism, point to the fact that the binding energy of intermediate species in multi-electron transfer reactions correlate in a linear fashion, resulting in large overpotentials for a variety of metals.^24–26 Azofra and Sun elaborate on this point in Chapter 6. At the fundamental level, a number of strategies reviewed in this book focus on breaking the challenge posed by this “scaling relation”.

From the thermodynamic point of view, the reduction of CO₂ to give useful hydrocarbons should occur under mild conditions, as summarised by eqn (1.1)–(1.5), and the corresponding Pourbaix diagrams²⁷ (see Figure 1.2). Thermodynamic data for organic media linking to aqueous media have also been reported.²⁸ Reduction to elemental carbon, and indeed nano-carbon products, is only accessible in molten salts and at high temperature.²⁹ The reversible potential for the CO₂/CO redox system (eqn (1.1)) has been elegantly confirmed by experiment (at pH 7) with enzyme-laden electrodes.³⁰CO+H₂O ⇄ CO₂+2 H⁺(aq)+2 e⁻

(1.1)

(1.2)

(1.3)

(1.4)

(1.5)

Figure 1.2 Pourbaix d...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Chapter 1 Introduction to the Eletrochemical and Photo-electrochemical Reduction of CO2
• Chapter 2 Bio-inspired and Bio-electrochemical Approaches in CO2 Reduction Catalysis
• Chapter 3 Copper Catalysts for the Electrochemical Reduction of Carbon Dioxide
• Chapter 4 Single-crystal Surfaces as Model Electrocatalysts for CO2 Reduction
• Chapter 5 Homogeneous M(bpy)(CO)3X and Aromatic N-heterocycle Catalysts for CO2 Reduction
• Chapter 6 DFT Modelling Tools in CO2 Conversion: Reaction Mechanism Screening and Analysis
• Chapter 7 Electrocarboxylation in Ionic Liquids
• Chapter 8 IR Spectro-electrochemistry and Group-6 α-diimine Catalysts of CO2 Reduction
• Chapter 9 Probing CO2 Reduction Intermediates Employing in situ Spectroscopy and Spectrometry
• Chapter 10 Surface-selective and Time-resolved Spectro-electrochemical Studies of CO2 Reduction Mechanisms
• Subject Index