Investigating the tunability of surface and microenvironment dynamics of Cu nanocatalysts for CO2 electroreduction

The CO2 carbon building block is essential to sustain life on Earth. However, its excessive emissions driven by anthropological activity have irreversibly affected the environment. Therefore, closing the carbon cycle loop through CO2 recycling using renewably sourced electricity not only addresses the growing threat of climate change but is also a powerful way to synthesize the chemicals necessary for the development of present and future generations. Specifically, the combination of CO2, protons, and electrons into value-added products enables the upcycling of CO2 while storing energy into chemical bonds. Given CO2 worldwide availability on Earth as well as in outer space (e.g., Mars), CO2 will always be a relevant feedstock in the future development of chemicals electrosynthesis. In this thesis, I present the prospects of catalyst materials design targeted for improving the utilization of CO2 through electrocatalysis.I introduce in Chapter 1 the current challenges we face for the CO2 electroreduction reaction to have a sizeable impact. There, I specifically discuss within the field of heterogeneous electrocatalysis the various strengths and drawbacks of utilizing nanomaterials to optimize CO2 electroconversion. Nanomaterials are the preferred platform to achieve catalyst fine-tuning that is essential to the CO2 electroconversion to higher-order products. However, in spite of the specific structural design accessible through their synthesis, nanomaterials’ high surface energy makes their structure and resulting properties especially prone to transformation when subject to external activation. The applied bias and reaction environment necessary to electrocatalysis induce in fact great change to such materials. In Chapter 2, I show that although often associated with the degradation of the catalyst surface and thus activity, the structural dynamics in nanocatalysts simultaneously introduces the possibility to design an incredible variety of catalysts constructed in operando. Furthermore, I present in Chapter 3 how the activity of nanocatalysts is not only driven by their surface properties, but also by the reaction environment formed near their surface during the reaction. The unique physicochemical landscape created at this interface can be exploited to tune the progress of complex reactions such as the electrochemical CO2 reduction reaction (CO2RR). Understanding the driving forces behind the formation of such an interface is therefore crucial in guiding the outcome of CO2RR in a controlled manner. I discuss the tools that can be employed to obtain such insights in Chapter 4, including powerful characterization techniques and the fine tuning catalyst structural properties. I emphasize there the importance of in situ and operando characterization techniques to accurately probe the dynamics of nanocatalysts. In addition, I highlight the synthetic advantages intrinsic to utilizing nanomaterials which can help isolate the driving parameters behind their structural evolution during electrolysis. While CO2 electroreduction is a close parallel to photosynthesis, there is a long way to go before it can replicate its selectivity and produce molecules as complex. In Chapter 5, I introduce a first attempt to bridge the synthetic gap between CO2 and sugars in an abiological catalytic process. I conclude this thesis in the last and 6th chapter with an overview of the breadth of advances done on the CO2 valuation through heterogeneous catalysis approaches

The Interactive Dynamics of Nanocatalyst Structure and Microenvironment during Electrochemical CO2 Conversion.

In the pursuit of a decarbonized society, electrocatalytic CO2 conversion has drawn tremendous research interest in recent years as a promising route to recycling CO2 into more valuable chemicals. To achieve high catalytic activity and selectivity, nanocatalysts of diverse structures and compositions have been designed. However, the dynamic structural transformation of the nanocatalysts taking place under operating conditions makes it difficult to study active site configurations present during the CO2 reduction reaction (CO2RR). In addition, although recognized as consequential to the catalytic performance, the reaction microenvironment generated near the nanocatalyst surface during CO2RR and its impact are still an understudied research area. In this Perspective, we discuss current understandings and difficulties associated with investigating such dynamic aspects of both the surface reaction site and its surrounding reaction environment as a whole. We further highlight the interactive influence of the structural transformation and the microenvironment on the catalytic performance of nanocatalysts. We also present future research directions to control the structural evolution of nanocatalysts and tailor their reaction microenvironment to achieve an ideal catalyst for improved electrochemical CO2RR

The presence and role of the intermediary CO reservoir in heterogeneous electroreduction of CO2

Despite the importance of the microenvironment in heterogeneous electrocatalysis, its role remains unclear due to a lack of suitable characterization techniques. Multi-step reactions like the electroconversion of CO2 to multicarbons (C2+) are especially relevant considering the potential creation of a unique microenvironment as part of the reaction pathway. To elucidate the significance of the microenvironment during CO2 reduction, we develop on-stream substitution of reactant isotope (OSRI), a new method which relies on the subsequent introduction of CO2 isotopes. Combining electrolytic experiments with a numerical model, this method reveals the presence of a reservoir of CO molecules concentrated near the catalyst surface that influences C2+ formation. Application of OSRI on a Cu nanoparticle (NP) ensemble and an electropolished Cu foil demonstrates that a CO monolayer covering the surface does not provide the amount of CO intermediates necessary to facilitate C-C coupling. Specifically, the C2+ turnover increases only after reaching a density of ~100 CO molecules per surface Cu atom. The Cu NP ensemble satisfies this criterion at an overpotential 100 mV lower than the foil, making it a better candidate for efficient C2+ formation. Furthermore, given the same reservoir size, the ensemble’s intrinsically higher C-C coupling ability is highlighted by the 4-fold higher C2+ turnover it achieves at a more positive potential. The OSRI method provides an improved understanding of how the presence of CO intermediates in the microenvironment impacts C2+ formation during the electroreduction of CO2 on Cu surfaces

The presence and role of the intermediary CO reservoir in heterogeneous electroreduction of CO2.

SignificanceThe electroconversion of CO2 to value-added products is a promising path to sustainable fuels and chemicals. However, the microenvironment that is created during CO2 electroreduction near the surface of heterogeneous Cu electrocatalysts remains unknown. Its understanding can lead to the development of ways to improve activity and selectivity toward multicarbon products. This work introduces a method called on-stream substitution of reactant isotope that provides quantitative information of the CO intermediate species present on Cu surfaces during electrolysis. An intermediary CO reservoir that contains more CO molecules than typically expected in a surface adsorbed configuration was identified. Its size was shown to be a factor closely associated with the formation of multicarbon products

Photoelectrochemical CO2 Reduction toward Multicarbon Products with Silicon Nanowire Photocathodes Interfaced with Copper Nanoparticles.

The development of photoelectrochemical systems for converting CO2 into chemical feedstocks offers an attractive strategy for clean energy storage by directly utilizing solar energy, but selectivity and stability for these systems have thus been limited. Here, we interface silicon nanowire (SiNW) photocathodes with a copper nanoparticle (CuNP) ensemble to drive efficient photoelectrochemical CO2 conversion to multicarbon products. This integrated system enables CO2-to-C2H4 conversion with faradaic efficiency approaching 25% and partial current densities above 2.5 mA/cm2 at -0.50 V vs RHE, while the nanowire photocathodes deliver 350 mV of photovoltage under 1 sun illumination. Under 50 h of continual bias and illumination, CuNP/SiNW can sustain stable photoelectrochemical CO2 reduction. These results demonstrate the nanowire/catalyst system as a powerful modular platform to achieve stable photoelectrochemical CO2 reduction and the feasibility to facilitate complex reactions toward multicarbons using generated photocarriers

Nanoparticle Assembly Induced Ligand Interactions for Enhanced Electrocatalytic CO2 Conversion.

The microenvironment in which the catalysts are situated is as important as the active sites in determining the overall catalytic performance. Recently, it has been found that nanoparticle (NP) surface ligands can actively participate in creating a favorable catalytic microenvironment, as part of the nanoparticle/ordered-ligand interlayer (NOLI), for selective CO2 conversion. However, much of the ligand-ligand interactions presumed essential to the formation of such a catalytic interlayer remains to be understood. Here, by varying the initial size of NPs and utilizing spectroscopic and electrochemical techniques, we show that the assembly of NPs leads to the necessary ligand interactions for the NOLI formation. The large surface curvature of small NPs promotes strong noncovalent interactions between ligands of adjacent NPs through ligand interdigitation. This ensures their collective behavior in electrochemical conditions and gives rise to the structurally ordered ligand layer of the NOLI. Thus, the use of smaller NPs was shown to result in a greater catalytically effective NOLI area associated with desolvated cations and electrostatic stabilization of intermediates, leading to the enhancement of intrinsic CO2-to-CO turnover. Our findings highlight the potential use of tailored microenvironments for NP catalysis by controlling its surface ligand interactions

Mitochondria Alkylation and Cellular Trafficking Mapped with a Lipophilic BODIPY–Acrolein Fluorogenic Probe

Protein and DNA alkylation by endogenously produced electrophiles is associated with the pathogenesis of neurodegenerative diseases, to epigenetic alterations and to cell signaling and redox regulation. With the goal of visualizing, in real-time, the spatiotemporal response of the cell milieu to electrophiles, we have designed a fluorogenic BODIPY–acrolein probe, <b>AcroB</b>, that undergoes a >350-fold fluorescence intensity enhancement concomitant with protein adduct formation. <b>AcroB</b> enables a direct quantification of single post-translational modifications occurring on cellular proteins via recording fluorescence bursts in live-cell imaging studies. In combination with super-resolution imaging, protein alkylation events may be registered and individually counted, yielding a map of protein–electrophile reactions within the cell lipid milieu. Alkylation is predominantly observed within mitochondria, a source, yet not a sink, of <b>AcroB</b> adducts, illustrating that a mitochondrial constitutive excretion mechanism ensures rapid disposal of compromised proteins. Sorting within the Golgi apparatus and trafficking along microtubules and through the cell endo- and exocytic pathways can be next visualized via live-cell imaging. Our results offer a direct visualization of cellular response to a noncanonical acrolein warhead. We envision <b>AcroB</b> will enable new approaches for diagnosis of pathologies associated with defective cellular trafficking. <b>AcroB</b> may help elucidate key aspects of mitochondria electrophile adduct excretion and cell endocytic and exocytic pathways. Conceptually, <b>AcroB</b> provides a new paradigm on fluorescence microscopy studies where chemical perturbation is achieved and simultaneously reported by the probe

Abiotic sugar synthesis from CO2 electrolysis

CO2 valorization is aimed at converting waste CO2 to value-added products. While steady progress has been achieved through diverse catalytic strategies, including CO2 electrosynthesis, CO2 thermocatalysis, and biological CO2 fixation, each of these approaches have distinct limitations. Inorganic catalysts only enable synthesis beyond C2 and C3 products with poor selectivity and with a high energy requirement. Meanwhile, although biological organisms can selectively produce complex products from CO2, their slow autotrophic metabolism limits their industrial feasibility. Here, we present an abiotic approach leveraging electrochemical and thermochemical catalysis to complete the conversion of CO2 to life-sustaining carbohydrate sugars akin to photosynthesis. CO2 was electrochemically converted to glycolaldehyde and formaldehyde using copper nanoparticles and boron-doped diamond cathodes, respectively. CO2-derived glycolaldehyde then served as the key autocatalyst for the formose reaction, where glycolaldehyde and formaldehyde combined in the presence of an alkaline earth metal catalyst to form a variety of C4 - C8 sugars, including glucose. In turn, these sugars were used as a feedstock for fast-growing and genetically modifiable Escherichia coli. Altogether, we have assembled a platform that pushes the boundaries of product complexity achievable from CO2 conversion while demonstrating CO2 integration into life-sustaining sugars

Scaling Laws of Exciton Recombination Kinetics in Low Dimensional Halide Perovskite Nanostructures.

Carrier recombination is a crucial process governing the optical properties of a semiconductor. Although various theoretical approaches have been utilized to describe carrier behaviors, a quantitative understanding of the impact of defects and interfaces in low dimensional semiconductor systems is still elusive. Here, we develop a model system consisting of chemically tunable, highly luminescent halide perovskite nanocrystals to illustrate the role of carrier diffusion and material dimensionality on the carrier recombination kinetics and luminescence efficiency. Our advanced synthetic methods provide a well-controlled colloidal system consisting of nanocrystals with different aspect ratios, halide compositions, and surface conditions. Using this system, we reveal the scaling laws of photoluminescence quantum yield and radiative lifetime with respect to the aspect ratio of nanocrystals. The scaling laws derived herein are not only a phenomenological observation but proved a powerful tool disentangling the carrier dynamics of microscopic systems in a quantitative and interpretable manner. The investigation of our model system and theoretical formulation bring to light the dimensionality, as a hidden constraint on carrier dynamics, and identify the diffusion length as an important parameter that distinguishes nanoscale and macroscale carrier behaviors. The conceptual distinction in carrier dynamics in different dimensionality regimes informs new design rules for optical devices where complex microstructures are involved