Development of Molecular Electrocatalysts for Energy Storage

Abstract

Molecular electrocatalysts can play an important role in energy storage and utilization reactions needed for intermittent renewable energy sources. This manuscript describes three general themes that our laboratories have found useful in the development of molecular electrocatalysts for reduction of CO2 to CO and for H2 oxidation and production. The first theme involves a conceptual partitioning of catalysts into first, second, and outer coordination spheres. This is illustrated with the design of electrocatalysts for CO2 reduction to CO using first and second coordination spheres and for H2 production catalysts using all three coordination spheres. The second theme focuses on the development of thermodynamic models that can be used to design catalysts to avoid high- and low-energy intermediates. In this research, new approaches to the measurement of thermodynamic hydride donor and acceptor abilities of transition-metal complexes were developed. Combining this information with other thermodynamic information such as pKa values and redox potentials led to more complete thermodynamic descriptions of transition-metal hydride, dihydride, and related species. Relationships extracted from this information were then used to develop models that are powerful tools for predicting and understanding the relative free energies of intermediates in catalytic reactions. The third theme is control of proton movement during electrochemical fuel generation and utilization reactions. This research involves the incorporation of pendant amines in the second coordination sphere that can facilitate H-H bond heterolysis and heteroformation, intra- and intermolecular proton-transfer steps, and coupling of proton- and electron-transfer steps. Studies also indicate an important role for the outer coordination sphere in the delivery of protons to the second coordination sphere. Understanding these proton-transfer reactions and their associated energy barriers is key to the design of faster and more efficient molecular electrocatalysts for energy storage.