Catalytic Microenvironment Created by Nanoparticle Ligands for CO2 Electrocatalysis

Electrochemical conversion of CO2 into value-added products is one of the most promising technologies for CO2 utilization, which can help realize a circular carbon economy and mitigate the ongoing climate change. Hence, extensive research efforts have been put into developing various nanoparticle electrocatalysts to accomplish selective CO2 conversion. However, given the complex and dynamic reaction environments at the electrochemical interface during CO2 electrocatalysis, conventional approaches to designing nanoparticle catalysts, for example, by tuning their structure and composition, have limitations in order to attain efficient and selective CO2 conversion. To achieve “enzymatic” CO2 electroreduction, approaching unit selectivity with a minimal energy input, a more holistic approach to nanoparticle catalyst design is much needed, taking into account not only the reaction site configurations but also the microenvironment provided or created under operation conditions. In this context, my doctoral research focuses on developing multi-component nanoparticle catalysts that can create a catalytically favorable microenvironment near the nanoparticle surface for selective CO2 electroconversion. More specifically, I have utilized nanoparticle surface ligands, which have normally been considered active-site blocking species, to create a catalytic pocket, assisting the active sites to facilitate the catalytic reaction and enhancing the catalytic performance of the nanoparticle catalysts. This catalytic pocket is created between a nanoparticle surface and an ordered layer of ligands (i.e., nanoparticle/ordered-ligand interlayer, or NOLI), enabling highly selective CO2-to-CO conversion. After briefly discussing the basics of electrochemical CO2 reduction and the importance of microenvironment in CO2 electrocatalysis in Chapter 1, I discuss in Chapter 2 the formation of this unique catalytic microenvironment (the NOLI) near the nanoparticle surface by the collective behavior of nanoparticle surface ligands under electrochemical bias. Also, the catalytic role of the NOLI for CO2 electrocatalysis and resultant improved catalytic performance are discussed. In addition, the interplay at the NOLI between a nanoparticle surface, electrolyte species, organic ligands, and CO2 molecules are described in detail. Furthermore, I discuss modular design of the NOLI-based nanoparticle catalysts, and their translation to high-rate CO2 electrolysis conditions (i.e., gas-diffusion environments). To better understand the collective behavior of the nanoparticle surface ligands under CO2-reducing conditions, which is found necessary for the formation of NOLI, various electrochemical and spectroscopic techniques have been utilized, including sum frequency generation vibrational spectroscopy. In Chapter 3, I discuss nanoparticle assembly induced ligand interactions and their impact on the formation of NOLI. It was found that nanoscopic geometry of nanoparticle such as surface curvature significantly influences the interactions between the organic ligands of neighboring nanoparticles. When the nanoparticles are densely packed on a conducting support, the large surface curvature of the nanoparticle allows for ligand interdigitation, promoting non-covalent ligand interactions. This was found to ensure their collective behavior during CO2 electrocatalysis, creating the NOLI for selective CO2 conversion. In this Chapter, potential ligand layer structure and its catalytically effective coverage are also proposed by combining spectroscopic and electrochemical results. Lastly, in Chapter 4, I conclude this dissertation by summarizing important experimental results and findings in my doctoral research and providing a broad perspective on new opportunities in harnessing nanoparticle ligand interactions or molecular modifiers in general for the development of advanced electrocatalysts

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

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

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