Theoretical Electrocatalysis for Renewable Fuels and Chemicals.

다국어 초록 (Multilingual Abstract)

Energy storage is a key concern to the grid-scale use of intermittent renewable sources like solar and wind. Electrolysis of such compounds as CO2, N2, and H2O into higher chemical potential products represents a possible route towards this goal, yet the conversion process is often severely limited due to insufficient catalysis of the associated chemical reactions. In this work, the electrocatalytic conversion of these three molecules is explored using density functional theory (DFT) methods with the goals of both explaining existing trends in experiment and determining criteria for the design of new systems with improved efficiency. CO2 electroreduction into ethylene and ethanol is highly attractive, since higher hydrocarbons are essential to much of our current fuel and chemical economy. In the first section, the formation of C-C bonds in CO2 electroreduction is discussed. The primarily focus of this section is copper, a catalyst known to convert CO2 into C2 products, and scaling relations for the coupling of *CO to its first hydrogenated derivative, *CHO, can rationalize why it is uniquely suited to do so. Insights into the mechanism of CO dimerization in alkaline conditions on Cu 100, as postulated previously from experiment, are also reported. Water-splitting into hydrogen and oxygen is another possible electrochemical energy storage method, and the oxidation of water into gaseous oxygen is typically coupled to other electroreductions discussed herein as well. In the second section, DFT-predicted oxygen evolution activities on perovskite oxides are correlated with the electronic structure of the catalytic surface. In addition, predicted OER and HER overpotentials on photoabsorbing perovskites suggest that materials with optical properties suitable for water splitting likely do not possess the surface chemistry for efficient catalysis, thus motivating the need for co-catalytic systems in photoelectrochemical water splitting. The last section concerns trends in the theoretical overpotentials for the electroreduction of nitrogen gas to ammonia. Nitrogen electroreduction is severely limited in overpotential by the reductive adsorption of N2 to form *N2H on most materials, and may be limited by reductive desorption of NH and NH2 on more reactive materials. By scaling these two reaction energies on a 2-D volcano, we show that no single transition-metal catalyst is likely to produce ammonia efficiently. This scaling relation does provide a strategy, however, for making new catalysts that might be less limited by these steps, since the selective stabilization of *N2H or selective destabilization of *NH2 should then result in less negative overpotential requirements. In summary, this dissertation uses DFT to describe and rationalize trends in electrocatalysis for three key reactions relevant to the conversion of electricity into chemical fuels. It is our hope that the principles outlined herein may guide the design of new catalytic systems that may ultimately realize the goal of efficient storage of renewable energy.