(766g) Mapping Free Energies to Design Molecular Electrocatalysts on a Computer

Density functional theory based computational methods have traditionally been used to rationalize the thermodynamics and kinetics of a particular catalyst or its local derivatives with the luxury of extensive experimental benchmarking. In my PhD work at Stanford University, advised by Prof. Christopher E. D. Chidsey and co-advised by Prof. Robert M. Waymouth, I explored turning this workflow around, that is, using state-of-the-art density functional theory to guide experimental electrocatalyst design. For the specific example of the two-electron reduction of CO₂ to CO and formate, and of protons to H₂, I demonstrated how the use of two thermodynamic descriptors, viz. the free energies of two key intermediates relative to the lowest energy product, streamlines the search for promising catalyst candidates from a large library of transition metals and organic ligands. The predictions were validated by subsequent experimental synthesis and electrochemical studies of an iron complex that was found to be active towards CO₂ and proton reduction at the expected reduction potential. The experimental studies, however, indicated that the in-silico method was underestimating the free energy of CO₂ binding to the electrochemically reduced iron complex, thereby calling for further refinements and validations of the density functionals to adequately model this key step that leads to CO production.

This new catalyst design paradigm was the culmination of several experimental and computational endeavors to understand the electrocatalytic reactivity of transition metal complexes towards CO₂ reduction at low driving forces, some of which included the a) unraveling of a new pattern of CO₂ activation by singly reduced Ru-complexes, b) engineering single step two-electron reductions in first row transition metal complexes to minimize overpotentials associated with electron transfer, c) highlighting a key deactivation step when using transition metal hydrides as alcohol-oxidation catalysts in fuel cells.

Selected Publications

1. Ramakrishnan, Chidsey “Initiation of Electrochemical Reduction of CO₂by a Singly Reduced Ruthenium(II) Bipyridine Complex”, Chem. 2017, 56(14), 8326-8333.
2. Waldie, Ramakrishnan, Kim, Maclaren, Chidsey, Waymouth “Multi-Electron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand” Am. Chem. Soc. 2017, 139(12), 4540-4550.
3. Ramakrishnan, Chakraborty, Brennessel, Jones, Chidsey “Rapid Oxidative Hydrogen Evolution from a Family of Square–Planar Nickel Hydride Complexes” Sci. 2016, 7, 117-127.
4. Ramakrishnan, Waldie, Warnke, de Crisci, Batista, Waymouth, Chidsey “Experimental and Theoretical Study of CO₂ Insertion into Ruthenium Hydride Complexes” Chem. 2016, 55(4), 1623-1632.
5. McLoughlin, Waldie, Ramakrishnan, Waymouth “Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent”, Am. Chem. Soc. 2018, 140(41), 13233-13241.
6. Ramakrishnan, Moretti, Chidsey “Mapping Free Energy Regimes in Electrocatalytic Reductions with Transition Metal Complexes”, submitted.