Spectroscopic Investigations of Renewable Fuel-Forming Reactions Catalyzed by Electrified Interfaces

Abstract

Electrocatalysis can promote reactions that are critical for the sustainable production of fuels and high-value commodity chemicals. For example, the electrochemical reduction of CO2 on various metal electrodes and their alloys has been demonstrated to provide access to a diverse range of industrially relevant products, including carbon monoxide,formate, ethylene, acetate, and ethanol. To enable these and other reduction reactions on a large scale, an abundant supply of electrons and protons is required. The water oxidation reaction has the potential to acts as such a source. The electrochemical reduction of CO2 is generally associated with poor product selectivity and high overpotentials, which are necessary to drive the reaction. Driving the water oxidation reaction requires high overpotentials, too. To address these challenges, it is essential to reveal the interfacial properties that determine the catalytic activity and selectivity of the electrode/electrolyte contact and to probe the reaction mechanisms and their dependence on experimental conditions. In this thesis, we utilize spectroscopic and electroanalytical techniques to reveal the intricate relationships between interfacial properties and catalytic activity and selectivity, and reaction mechanisms and experimental conditions. In the first part of this thesis, we focus on how electrolyte and electrode characteristics influence the reduction of CO2 on copper electrodes. In Chapter 3 of this thesis, using a series of quaternary alkyl ammonium cations (methyl4N+, ethyl4N+, propyl4N+, and butyl4N+), we systematically tuned the properties of the liquid reaction environment to probe how it affects the electrocatalytic reduction selectivity of CO to hydrocarbons on Cu electrodes. Employing differential electrochemical mass spectrometry (DEMS), we observed that ethylene is produced in the presence of methyl4N+ and ethyl4N+ cations, while this product is absent in propyl4N+- and butyl4N+-containing electrolytes. With surface-enhanced infrared absorption spectroscopy (SEIRAS), we show that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. Strikingly, SEIRAS reveals that the hydrophobicity of the two larger cations disrupts the intermolecular interaction between surface-adsorbed CO and interfacial water. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. In Chapter 4 of this thesis, using two types of rough Cu thin-film electrodes, we sought to understand how their distinct atomic-level surface morphologies determine the catalytic activities of the two types of electrodes. DEMS shows that copper films that are electrochemically deposited on Si-supported Au films (CuAu-Si) exhibit an onset potential for ethylene that is 200±65 mV more cathodic than the one of copper films (Cu-Si) that are electrolessly deposited onto Si crystals. Cyclic voltammetry (CV) reveals that the (111) surface facet prevails on CuAu-Si, while the (100) facet is predominant on Cu-Si. SEIRAS reveals the existence of disparate surface morphologies, which manifest themselves in the from of different potential-dependent behaviors of the line shape of the CO stretching band of atop-bound CO.We rationalize the observation with a Boltzmann model that takes into account the difference in CO adsorption energy on terrace and defect sites. This study establishes SEIRAS of surface-adsorbed CO as an important tool for the in situ investigation of the atomic-level surface morphology of rough metal electrodes. In the second part of this thesis, we explore the potential-dependence of the mechanism of the water oxidation reaction on cobalt-oxide based electrocatalysts. To a significant extent, the mechanisms of the water oxidation reaction on heterogeneous catalysts remain obscure. A key elementary step of the water oxidation reaction is the formation of the O-O bond. The intramolecular oxygen coupling mechanism (IMOC) and the water nucleophilic attack mechanism (WNA) have been proposed as possible pathways of O-O bond formation. However, it is still unclear to what extent the accessibility of each pathway is controlled by the applied potential. In Chapter 5 of this thesis, employing water-in-salt electrolytes, we systematically altered the water activity, which enabled us to quantify the impact of the water activity on the rate of the reaction. We discovered that the water oxidation mechanism is sensitive to the applied electrode potential: At relatively low driving force, the reaction proceeds through the IMOC pathway, whereas the WNA mechanism prevails at high driving force.Density functional theory (DFT) calculations provide an explanation for our experimental observations: Prior to water nucleophilic attack, theWNA pathway requires one additional oxidation, which is associated with a high thermodynamic overpotential. Further, using SEIRAS, we detected a superoxo species, a key reaction intermediate on heterogeneouscobalt-oxide based catalyst. This work demonstrates that the IMOC and WNA mechanisms prevail in different potential regimes, an important consideration when determining electrolyzer operation conditions. In summary, the work presented in this thesis provides fundamental insights into the operating principles of electrocatalytic interfaces. These insights are of practical significance and are expected to benefit the design of electrolyzers with high efficiency.