Electrochemical reduction of carbon dioxide

The electrochemical reduction of carbon dioxide, also known as electrolysis of carbon dioxide, is the conversion of carbon dioxide () to more reduced chemical species using electrical energy. It is one possible step in the broad scheme of carbon capture and utilization, nevertheless it is deemed to be one of the most promising approaches. Electrochemical reduction of carbon dioxide represents a possible means of producing chemicals or fuels, converting carbon dioxide () to organic feedstocks such as formic acid (HCOOH), carbon monoxide (CO), methane (CH4), ethylene (C2H4) and ethanol (C2H5OH). Among the more selective metallic catalysts in this field are tin for formic acid, silver for carbon monoxide and copper for methane, ethylene or ethanol. Methanol, propanol and 1-butanol have also been produced via CO2 electrochemical reduction, albeit in small quantities. The main challenges are the relatively high cost of electricity (vs petroleum) and the fact that most CO2 is diluted wit

Exploring Synthetic Strategies for Nanocrystal/Reticular Framework Hybrids and Their Potential as CO2 Reduction Electrocatalysts

Yannick Thomas Guntern

The electrochemical CO2 reduction reaction (CO2RR) into valuable chemicals has the potential to realize a carbon-neutral energy cycle. Developing catalysts that can achieve high selectivity towards one of the possible reduction products is one of the biggest challenges in this field. Catalysts possessing one single type of active sites cannot fulfil this need, yet to establish design rules for bifunctional catalysts is not trivial. To have material platforms which allow to study the sensitivity of the catalytic behaviour to structural and compositional features of the catalyst is of the uttermost importance to inform catalyst design. This thesis aims at combining well-defined colloidal nanocrystals (NCs) and porous reticular frameworks as hybrid materials to explore their synergistic interactions as CO2RR catalysts. These classes of materials were chosen because they possess complementary chemical properties which are appealing towards eventually construct an ideal catalyst with high performance. However, one of the challenges is to obtain these hybrids with tunable structural features in an unrestricted compositional range. The overall objective is to design and explore synthetic strategies for the formation of new multifunctional composites consisting of colloidal NCs and metal-organic/covalent organic frameworks (MOFs/COFs). First, a synthetic approach for the formation of NC/MOF hybrid thin films is proposed, which enables their application as a novel catalytic platform for CO2RR. We demonstrate that a combination of colloidal chemistry, atomic-layer deposition and solvothermal synthesis allows to form an intimate contact between Ag NCs and an Al-PMOF matrix. Our tuneable Ag@Al-PMOF hybrid catalysts show promoted CO2RR which is attributed to electronic changes in the Ag NC surface and minor mass-transport effects. Moreover, the hybrids improve the morphological stability of the Ag NCs under CO2RR conditions. We show that the synthetic approach can be further extended to form hybrids with other metallic NCs, thereby representing a new tool for the preparation of composite catalysts for CO2RR. One of the limitations of thin film composites is that synthesis conditions might need to be adapted for different substrates. To prepare hybrid materials in the form of dispensible colloidal solutions is highly appealing and better suited for mechanistic studies. The latter are important because understanding the chemistry driving the formation of such composites is eventually the key for achieving compositional and structural tunability. In the second part, we introduce a new strategy to encapsulate various colloidal NCs in the imine-linked COF LZU1. The synthetic approach allows us to tune the shell thickness and crystallization of the COF in the presence of the NCs. After understanding the nucleation and growth mechanisms of the hybrids, we extend the developed synthesis to obtain multi-layered structures with controlled spatial distribution of various NCs with different compositions. Altogether, we demonstrate that our approach can be applied to prepare hybrids with new functionalities that derive from the encapsulated NCs. Overall, this thesis proposes two different approaches which combine colloidal and reticular chemistry to synthesize tunable material platforms which might aid the design of CO2RR catalysts as well as contributing to other applications which require multifunctional porous materials.

EPFL2021

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

A rapidly growing population and industrialization have caused the exploitation of natural resources in keeping with fossil fuels shortage. Due to the combustion of fossil fuels, the concentration of CO2 in the earth's atmosphere, which is a potent greenhouse gas, has been increasing dramatically and contributes to the current climate change. Meanwhile, CO2 can be considered as an abundant and inexpensive carbon feedstock, especially if carbon sequestration is required for future fossil fuel-based power stations. Electrochemical reduction of CO2 to form useful chemicals or fuels is a potentially efficient method of CO2 utilization and recycling. The advantage of the electrochemical CO2 reduction is that natural renewable sources, like solar power, wind power, hydropower or nuclear power can supply electric power for CO2 conversion. Various products, such as carbon monoxide, oxalic acid, formic acid, alcohols and hydrocarbons have been reduced from CO2 First in chapter 1, the general remarks about electrochemical reduction of CO2, especially on metal electrodes are introduced. It also includes the introduction of photoelectochemical CO2 reduction and literature review study about electrocatalytic CO2 reduction with organic molecules as mediators and catalysts. In chapter 2, our interest in Mo-based electrocatalysts led us to study the activity of MoO2 for CO2 reduction. MoO2 microparticles indeed act as an active catalyst for the electrochemical reduction of CO2 in organic solvents such as acetonitrile (MeCN) and dimethylformamide (DMF). In the context of this study, it was found that the CO2 reduction activity is much higher at -20 oC than at RT, and it can be promoted by a small amount of water. The selectivity of CO2 reduction depends on the temperature, overpotential and water concentration. Chapter 3 describes the electrochemical reduction of CO2, improved and modified by using imidazolium ionic liquids (ILs) as an electrolyte in organic solvent with MoO2/Pb electrode. The important role of ILs in changing the pathway of the reduction was observed. Particularly, the use of imidazolium ILs instead of tetraalkylammonium salt as an electrolyte altered the product from oxalate to CO. Besides, the overpotential of the CO2 reduction was shifted to the less negative potentials compared to previous work, and the current density as well increased. At a low water concentration, the catalytic activity was promoted and the total Faradaic efficiency (FE) was up to 100% with more than 60% of CO formation. Furthermore, imidazolium ILs together with our MoO2/Pb electrode lowered the overpotential of CO2 reduction by about 40 mV. In addition to the high selectivity for CO formation, the imidazolium ILs can perform a role as co-catalyst for lowering the CO2 reduction potential. Finally, chapter 4 shows the possibility of photoelectrochemical CO2 reduction to extend our research with various catalysts. In this work, we could derive a photoelectrochemical method to deposit Sn/SnOx catalyst on the surface protected cuprous oxide (Cu2O) photocathodes. The deposition of Sn/SnOx catalyst was optimized on the photocathode and it exhibited reliable CO2 reduction activity in a mild aqueous condition. Sn/SnOx-Cu2O photocathode showed the onset potential of the photocurrent at +0.1 V vs. RHE, which is very promising result. At a constant potential electrolysis of -0.1 V vs. RHE, a FE of 50% for formate formation was obtained and the total FE was around 86%.

EPFL2015

Shape-controlled Copper Nanocrystals as Electrocatalysts for CO2RR

Pranit Srinivas Iyengar

The electrochemical CO2 reduction reaction (CO2RR) has the potential to mitigate the rising CO2 levels while storing renewable energy in chemical bonds. Copper is the only single metal electrocatalyst producing high energy dense hydrocarbons, albeit into 16 products on polycrystalline Cu. Therefore, more selective catalysts need to be developed. Inspired by single crystal studies, well defined metal nanocrystals (NCs) can leverage controllable surface facets and high surface/volume ratio to improve activity and selectivity. This thesis aims to use colloidal Cu NCs with tunable shape and size to firstly demonstrate the importance of these features to direct CO2RR selectivity, and then leverage the facet dependence of Cu NCs to expand on the knowledge regarding mechanistic pathways to high value CO2RR products.Firstly, the known selectivity of Cu{111} to CH4 is investigated at the nano-scale by synthesizing octahedral Cu NCs (Cuoh) with different sizes. The major Cu{111} facet directs CO2RR selectivity to CH4, and the size-dependent performance reveals that the ratio between Cu{111} and {110} edges determines the promotion of CO2RR over the competing hydrogen evolution reaction (HER). The smallest synthesized (75 nm) Cuoh show an overall CO2RR selectivity of 77% with a CH4 faradaic efficiency (FE) of 55%. A contemporary topic of debate is whether the high value product C2H5OH forms via *CHx-*CO coupling or *CO-*CO coupling. As a platform to address this dichotomy, we use CH4 favoring (*CHx populated) Cuoh and C2H4 favoring (*CO-*CO populated) Cucub under elevated *CO coverage induced by Ag NCs. We show that at the expense of CH4, Cuoh selectively promote C2H5OH, while Cucub make both C2H5OH and C2H4. We propose that C2H5OH formation proceeds via *CHx-*CO coupling, and the preferred sites for this coupling are the {111} faces on Cuoh and {110} edges on Cucub. Hence, we posit the counter-intuitive phenomenon of a CH4 selective catalyst becoming C2H5OH selective under CO saturation. Additionally, we suggest that catalysts having larger defect-to-terrace ratios are expected to be C2H5OH selective under CO saturation.We then proceed to tune the Cucub edge/face ratio by size control, as smaller Cucub are expected to possess increased selectivity towards C2H5OH by virtue of their higher proportion of edge sites. Indeed, smaller Cucub show larger C2H5OH/C2H4 ratio under CO saturation conditions further strengthening our hypothesis that Cucub edge sites are C2H5OH selective via the *CHx-*CO coupling pathway.This thesis showcases the strengths of colloidal NCs to reveal structure-sensitivity relations for which the achieved size control (i.e. importance of facet ratio) proved crucial. Furthermore, the possibility to use shape-controlled NCs as a platform to investigate reaction pathways is proposed. The mechanistic insights provided here could aid catalyst design targeting high selectivity for valuable CO2RR products.

EPFL2021

Exploring Synthetic Strategies for Nanocrystal/Reticular Framework Hybrids and Their Potential as CO2 Reduction Electrocatalysts

Yannick Thomas Guntern

EPFL2021

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

EPFL2015

Shape-controlled Copper Nanocrystals as Electrocatalysts for CO2RR

Pranit Srinivas Iyengar

EPFL2021