Electrochemical reduction of carbon dioxide

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

Dual-Facet Mechanism in Copper Nanocubes for Electrochemical CO2 Reduction into Ethylene

Raffaella Buonsanti, Jianfeng Huang, Giulia Mangione

The product selectivity in the electrochemical reduction of carbon dioxide depends on the structure of the copper electrode. Cube-shaped copper catalysts, enclosed by {100} terraces, {110} edges, and {111} corners, exhibit a size-dependent enhanced only selectivity toward C-2 products and ethylene in particular. However, the underlying chemical reasons for such a behavior are not fully understood. This computational work toward ethylene formation investigates the carbon dioxide electroreduction mechanism over copper nanocubes. The analysis of the different pathways illustrates that the thermodynamic picture is limited in describing the formation of this product. Based on the activation barriers associated with the limiting C-2 formation step, we identify a dual-facet mechanism occurring at the interface between the {100} terraces and {110} edges. These results highlight that the reactivity of shape-controlled nanocatalysts goes beyond the facet-selectivity observed in single crystals owing to the possible synergies arising at the intersection between the enclosing crystalline planes.

2019

Copper Nanocrystal Morphology Determines the Viability of Molecular Surface Functionalization in Tuning Electrocatalytic Behavior in CO2 Reduction

Raffaella Buonsanti, Pranit Srinivas Iyengar, Valery Okatenko, James Richard Pankhurst

Molecular surface functionalization of metallic catalysts is emerging as an ever-developing approach to tuning their catalytic performance. Here, we report the synthesis of hybrid catalysts comprising copper nanocrystals (CuNCs) and an imidazolium ligand for the electrochemical CO2 reduction reaction (CO2RR). We show that this organic modifier steers the selectivity of cubic CuNCs toward liquid products. A comparison between cubic and spherical CuNCs reveals the impact of surface reconstruction on the viability of surface functionalization schemes. Indeed, the intrinsic instability of spherical CuNCs leads to ejection of the functionalized surface atoms. Finally, we also demonstrate that the more stable hybrid nanocrystal catalysts, which include cubic CuNCs, can be transferred into gas-flow CO2RR cells for testing under more industrially relevant conditions.

AMER CHEMICAL SOC2021

Cu/Metal Oxide Hybrid Nanocrystals as Electrocatalysts for the CO2 Reduction Reaction

Seyedehbehnaz Varandili

The electrochemical CO2 reduction reaction (CO2RR) is envisioned to play a significant role in achieving carbon neutrality while contributing to storing renewable energies. Cu-based materials are among the most promising electrocatalysts. However, 16 different products are obtained from a Cu foil, therefore strategies to make more selective catalysts are needed. This thesis explores Cu/metal oxide interfaces as a mean to direct selectivity towards hydrocarbon products in CO2RR. The aim is to deviate from the scaling relationships which exist on single metal surfaces. Cu/metal oxides hybrid nanocrystals (HNCs) with tunable morphologies and spatial configuration are synthesized via colloidal chemistry. The achieved level of control and tunability is demonstrated to be crucial in understanding the impact of these parameters on the reaction outcome. First, we start by designing a synthesis procedure for Cu/CeO2-x heterodimers (HDs) and then we study their catalytic behavior toward CO2RR. We show that the Cu/CeO2-x interface promotes CO2RR, particularly methane with around 54% selectivity, which is ~5 times higher than those obtained with a physical mixture of isolated Cu and CeO2-x nanocrystals, thus highlighting the important role played by the intimate bonding at the interface. A miscellanea of characterization techniques indicates the higher extent of ceria reduction in the HDs during CO2RR. DFT elucidates the presence of unique catalytic motifs that enable bidentate adsorption of critical intermediates at both Cu and the CeO2-xO-vacancy site, creating an opportunity for reaction engineering beyond scaling relationship limitations. Having assessed the importance of the Cu/ceria interface, we investigate the impact of increasing the Cu/CeO2 interfacial area on the CO2RR outcome. We find that increasing the number of ceria domain around copper inhibits CO2RR. This result highlights that a balance between interfacial area and accessibility to the active catalytic sites must be sought after to exploit synergies arising at the interface between different domains. Finally, we explore Cu/ZnO HNCs as CO2RR pre-catalysts to elucidate the structural and compositional parameters which direct the reaction pathway towards ethanol. Using different sizes of ZnO seeds for the synthesis of Cu/ZnO HNCs, we are able to achieve two different configurations including ZnO decorated Cu core (Cu@ZnO) and Cu decorated ZnO core (ZnO@Cu) HNCs. Upon activation via cathodic voltage, we find that Cu@ZnO HNCs favor methane production while ZnO@Cu exhibits a noticeable selectivity toward ethanol. Operando XAS, XPS, and electron microscopy reveal that both HNCs transform into Cu@CuZn core@shell NCs during the activation step. However, the Zn content in the surface alloy (i.e. degree of alloying) and the presence of surface cationic Zn species are found to be crucial in determining the selectivity of these catalysts, which possibly elucidates some of the contrasting results reported on Cu-Zn across the literature. Overall, this thesis showcases the importance of well-defined and tunable HNCs as platforms to study structure/properties relations and to advance the current knowledge in CO2RR. Furthermore, the library of nanomaterials accessible through the developed synthesis routes might also be utilized in the future to investigate other catalytic reactions and applications where the hybrid interfaces might be relevant.

EPFL2021

Colloidal Nanocrystals as Heterogeneous Catalysts for Electrochemical CO2 Conversions

Raffaella Buonsanti, Jianfeng Huang

In the past decade, atomically engineered nanomaterials with different sizes and exposed facets have been proven to be excellent model systems to advance catalytic studies. Colloidal chemistry is one of the most powerful wet-chemistry techniques to tailor-make nanomaterials, thus making colloidal nanocrystals an ideal playground to investigate structural and compositional reaction sensitivities but also to study degradation pathways. In this Perspective, we focus on colloidally synthesized nanocrystals as heterogeneous catalysts for the electrochemical CO2 reduction reaction. We discuss very recent studies from us and from others, which encourage the scientific community to explore the tunability offered by colloidal chemistry even further. For example, synergistic interactions of the metallic nanocrystal catalyst with domains of different chemical nature could be exploited with the aim of revealing new catalytic motifs that promote the activity, selectivity, and stability of electrocatalysts for CO2 conversion.

2019

Investigation of Ethylene and Propylene Production from CO2 Reduction over Copper Nanocubes in an MEA-Type Electrolyzer

Raffaella Buonsanti, Valery Okatenko

Previous work carried out in fully liquid environ-ments and at low current densities has demonstrated that highly uniform faceted copper nanocrystals display different selectivity profiles in CO2 reduction compared to polycrystalline copper. As part of ongoing upscaling efforts, it is a matter of interest to investigate whether the high selectivity toward ethylene of copper nanocubes, which show a preferential (100) orientation, is maintained in gas-fed electrolyzers, thus enabling the energy-efficient production of this valuable commodity chemical at industrially relevant current densities. In this work, we assessed the electrochemical CO2 reduction reaction performance of highly uniform copper nanocubes loaded onto gas diffusion electrodes (GDEs) in a zero-gap device. The copper nanocube-loaded GDEs maintained high Faradaic efficiencies toward ethylene at elevated total current densities, resulting in higher overall partial current densities toward this product compared to benchmark electrodes. Interestingly, CO2 reduction to propylene, albeit with low partial current densities, was also observed. However, double-layer capacitance measurements revealed that the performance observed at high current densities is significantly influenced by electrode flooding. The findings of this study can inform future efforts geared toward optimizing the electrodes with this promising class of catalysts.

AMER CHEMICAL SOC2022

Rational design of ionic compounds for electrocatalytic reduction of carbon dioxide

Dmitry Vasilyev

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.

EPFL2019