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.
