Rational design of ionic compounds for electrocatalytic reduction of carbon dioxide

Electrochemical reduction of carbon dioxide is one of the plausible approaches towards renewable energy carriers. When coupled with electricity, which can be provided by sustainable technologies, it becomes a method of high importance with potential value for future energy challenges. However, the electrochemical version of the CO2 reduction reaction (CO2RR) proceeds through highly energetic intermediates, therefore efficient catalytic and co-catalytic systems are needed. Recently, considerable attention was dedicated to application of ionic liquids (ILs) as promising promoters for the CO2RR. While most efforts in this domain have been concentrated on spectroscopic and electrochemical investigation of existing IL-based systems and on the development of more active electrodes, there are only a limited number of studies discussing the structure/activity relationships of ILs in the CO2RR. Our aim was to fill this gap and to find new classes of ILs that are able to promote CO2RR. Another aim of our project was to delineate the basic trends in the structures of the active co-catalysts and provide descriptors for the promoting activity. In this work three new classes of ILs are evaluated for the CO2RR. Within each IL series the structures of the cationic core were varied, the dependencies of the stabilities and activities of the ILs on the structure are discussed. Additionally, a fundamentally new type of ionic systems based on deep eutectic solvents was applied to the CO2RR and found to be a highly active, cheap and non-toxic alternative for the conventional ILs. Based on the obtained results, the charge and its accessibility are suggested to be the main descriptors for the activity of the ionic catalysts in non-aqueous environments.

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

This thesis deals with the electrocatalytic properties of two-dimensional metal organic coordination networks (2D-MOCNs) self-assembled on electrode surfaces and catalytic activity of 2D-MOCNs is demonstrated for the first time. The low-coordinated metal centers within these supramolecular structures resemble the catalytically active sites of metallo-enzymes. The activity of the metal centers is determined by the type of the metal and the coordination environment; both parameters can be carefully adjusted in 2D-MOCNs. In order to study and correlate electrocatalytic properties with the atomic structure of 2D-MOCNs, a scanning tunnelling microscope operating in ultra-high vacuum (UHV) was combined with an electrochemical (EC) setup. Sample preparation and scanning tunneling microscopy (STM) were conducted under UHV conditions. By STM the composition and structure of the networks were controlled and characterized prior to EC experiments. A transfer system between UHV and EC instrumentation was constructed to keep the sample at all times in a clean and controlled environment. It is demonstrated that the mechanism of the oxygen reduction reaction (ORR) can be influenced by the type of the metal center in the network. The networks formed by benzene-tricarboxylic acid (TMA) with either Fe or Mn atoms are structurally identical, but their electrochemical signal differs significantly. Both networks catalyze the reduction of O2 to H2O, but on different pathways. These results emphasize the key role of the unsaturated metal centers in the electrocatalytic reduction. The electrocatalytic response in the ORR can also altered by the distinct coordination environment; this is shown for Fe atoms nitrogen-coordinated by either tetracyanoquinodimethane (TCNQ), bis-pyridyl-bipyrimidine (PBP) or phythalocyanine (Pc). Depending on the specific coordination environment formed by the different ligands the the final product (H2O2 or H2O) and the mechanism of the ORR is changed. In the last part of this thesis the two metal binding sites of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) are used to selectively incorporate different metal centers in a fixed organic environment and create homo- and hetero-bimetallic networks. The coupling of the two metals centers promoted by the organic environment modifies the electrocatalytic response. The correct combination of two metal centers in the right positioning makes an efficient catalyst. In the ORR the peak and onset potential is varied depending on the metal combination. In the case of the oxygen evolution reaction a non-linearly increased catalytic activity by the cooperative bimetallic effect is obtained. The results presented in this thesis demonstrate the high potential of 2D-MOCNs for heterogeneous catalytic chemical conversions. Engineering of the network structure and the distinct coordination environment of the metal centers offers the possibility to tune their catalytic activity. This opens up a new route for the design of a new class of nanocatalyst materials.

EPFL2015

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

Principal Descriptors of Ionic Liquid Co-catalysts for the Electrochemical Reduction of CO2

Paul Joseph Dyson, Mohammad Khaja Nazeeruddin, Erfan Shirzadi, Serhii Shyshkanov, Dmitry Vasilyev

Electrochemical reduction of carbon dioxide (CO2RR) is promoted by ionic liquid (IL) co-catalysts, and several mechanisms have been proposed to explain their role. Due to the complexity of the CO2RR and the limited number of active IL co-catalysts, a consensus on the precise role of ILs has not been reached, and it is not possible to improve their activity in a rational way. Herein, we describe guanidinium (Gua) ILs that act as co-catalysts for the CO2RR when employed in non-aqueous electrolytes. The peripheral substituents of the Gua cation were systematically modified allowing the IL co-catalytic properties to be fine-tuned, and on the basis of the observed substitution effects, charge delocalization and availability were shown to be the critical descriptors determining co-catalytic activity. These descriptors can be used to rationalize activity trends for other classes of IL co-catalysts.

AMER CHEMICAL SOC2020

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

EPFL2015

Electrochemical Reduction of CO2 Catalyzed by Metal Oxides

Yeonji Oh

EPFL2015