In-situ observation of the electrochemical processes at the solid-liquid interface identifying the active sites

Abstract

Due to the explosive population growth and energy consumption, carbon and carbon dioxide emission rates are expected to increase significantly. In addition, since the world’s primary energy resources are mostly fossil fuels, the main reason for global pollution, it is challenging to address the rising global concerns, such as climate changes and limited energy resources. Therefore, developing sustainable pathways to produce carbon-neutral fuels, like hydrogen, is the most critical challenge for human beings. In the sustainable hydrogen production landscape, (photo)electrochemical water-splitting is one of the promising candidates for driving the carbon-zero energy cycles because of using the most abundant resource on the earth

water. However, due to the four electron-proton coupled reactions, the sluggish kinetics of the oxygen evolution reaction requires a huge overpotential, resulting in a relatively low energy conversion efficiency. The molecular catalysts could be one of the key strategies for the efficient water-splitting reaction by supporting the electron-proton transfers through the active sites. In addition, we can successfully improve the reaction performance based on the many advantages, such as clear structures and active sites, easy tenability, and single metal atoms economy. However, the homogeneity of molecular catalysts under operating conditions is still debated, as it is not revealed in detail whether the true active catalyst on the electrode is the robust molecular complex itself or the metal oxide compounds produced by their decomposition. This controversy is because there are few in-situ studies on the solid-liquid interface at the atomic scale. Moreover, it leads to ambiguous conclusions about what exactly happens at the active sites during the electrochemical reaction. Therefore, fully understanding the mechanisms of proton and electron transfer at the solid-liquid interface is essential for developing enhanced (photo)electrochemical devices. Thus, we extensively investigate the mechanism of chemical reactions ((photo)electrochemical induced electron transfer, water-splitting, and $CO_2$ electroreduction reaction) to provide molecular information at the solid-liquid interface using various molecular catalysts and metal electrodes. In this dissertation, Chapter 1 explains the importance of studying the solid-liquid interface and modern surface science analytical tools. This chapter will also introduce the research backgrounds of molecular catalysis. Chapter 2 introduces the theoretical knowledge and working principle of scanning tunneling microscopy (STM), mainly employed for observing the solid-liquid interface. Furthermore, the process of modifying a beetle-type STM into an electrochemical STM is described in detail. We demonstrated that our modified electrochemical STM shows highly stable operating performance during the electrochemical reaction condition. In Chapter 3, the formation of well-defined molecular structures of free-base porphyrin, copper, cobalt, and nickel porphyrins is observed in a dodecane, non-polar solution. In particular, the structural changes are in-situ monitored at the molecular level under ultraviolet (UV) light irradiation. It is found that the formation of the molecular catalyst film is different depending on the presence or absence of the center metal and that the center metal has a significant impact on the rate of electron transfer through the adjacent supramolecules. In Chapter 4, we observe the formation of the molecular arrangements of porphyrins in one of the other non-polar solutions, octanoic acid. We observed that porphyrin molecules build different molecular packing structures despite a similar non-polar solution. In the early stages of molecular film construction, the parallel molecular arrangements (~ $0.32 molecule/nm^2$) are mainly seen, but the hexagonal molecular arrangements (~ $0.55 molecule/nm^2$) become dominant over time due to their thermodynamically stable structures. This molecular restructuring is accelerated by an external energy factor called UV light. In Chapter 5, we monitor the in-situ structural changes of the manganese porphyrin molecular catalyst during the oxygen evolution reaction. After the first redox reaction, the catalytic activity is dramatically increased by showing the newly emerged active species. The irreversible morphological changes presumably result from the decomposition of manganese porphyrin during the irreversible oxidation-reduction reaction, followed by the restructuring of manganese oxide ($MnO_x$) as an active species. It is confirmed by in-situ electrochemical STM imaging and ex-situ X-ray photoelectron spectroscopy. Still, the remaining question is the chemical nature of the active species. In order to confirm the active species is the manganese oxides or not, in Chapter 6, we decided to carry out an in-depth study by chemical treatment. The active species are selectively removed with an acidic solution, and suppressed by adsorption of phosphonic acid. Given that an acidic solution is one of the well-known oxide removals, and the phosphonic acid is one of the oxide favorable adsorption materials, we strongly estimate that metal oxide mainly constitutes the real active catalyst. In Chapter 7, the electrocatalytic performance is investigated according to the surface structure of the metal electrodes. We present STM results of the surface structural sensitivity of Au single crystals itself for electrocatalytic $CO_2$ reduction as a simple model study. Au(hkl) is the most active electrocatalyst for $CO_2$ conversion into CO, showing structural dependency on coordinated sites, such as the terrace of Au(111) and the steps of Au(332). Through in-situ electrochemical STM, we have confirmed that these are the actual active sites for $CO_2$ reduction. In-situ atomic-scale observations of catalytic processes at the solid-liquid interface covered in this dissertation play an important role in developing the next-generation water-splitting devices by providing the fundamental information, focusing on understanding the chemical and structural properties of the electrocatalysts.