Theoretical Design of Molecular Electrocatalysts with Flexible Pendant Amines for Hydrogen Production and Oxidation

Abstract

The design of hydrogen oxidation and production electrocatalysts is important for the development of alternative renewable energy sources. The overall objective is to maximize the turnover frequency and minimize the overpotential. We use computational methods to examine a variety of nickel-based molecular electrocatalysts with pendant amines. Our studies focus on the proton-coupled electron transfer (PCET) process involving electron transfer between the complex and the electrode and intramolecular proton transfer between the nickel center and the nitrogen of the pendant amine. The concerted PCET mechanism, which tends to require a lower overpotential, is favored by a smaller equilibrium Ni-N distance and a more flexible pendant amine ligand, thereby decreasing the energetic penalty for the nitrogen to approach the nickel center for proton transfer. Our calculations provide predictions about designing catalysts that incorporate these properties. These design principles will be useful for developing the next generation of hydrogen catalysts.

Proton-Coupled Electron Transfer: Moving Together and Charging Forward

“…On the basis of this prediction, several Ni catalysts with more flexible pendant amines were examined. 88 However, often the catalysts with more flexible amines (i.e., with a lower frequency associated with the Ni---N motion) exhibited larger equilibrium Ni---N distances, thereby counteracting the advantage, although other factors such as inner-sphere reorganization energy could favor the flexible amine ligands. Thus, the EPT mechanism requires a balance between a well-positioned pendant amine with a short equilibrium Ni---N distance and a flexible pendant amine that facilitates a further decrease in this distance.…”

Hydrogen Tunneling in Enzymes and Biomimetic Models

“…Furthermore, the inner-sphere reorganization energy is smaller for the Ni(PN) catalyst than for the Ni(P 2 N 2 ) 2 catalysts with equivalent substituents, so the rate constant k EPT ( R ) is expected to be greater for the Ni(PN) catalyst. 62 As a result, it is possible that the overall rate constant given in Eq. (22) will be large enough for the Ni(PN) catalyst with methyl substituents to favor the concerted PCET mechanism.…”

“…61–62 Unfortunately, these two characteristics often compete against each other because greater flexibility of the pendant amine is typically associated with a larger equilibrium Ni–N distance. Reducing the bulkiness of the substituents on the other ligands, however, may result in a smaller equilibrium Ni–N distance for such flexible amines.…”

“…These catalysts have been studied experimentally 274,284 and theoretically. 62 Reprinted with permission from Ref. 62.…”

“…54–60 The PCET reaction involving proton transfer between the pendant amine and the nickel center and electron transfer between the nickel complex and the electrode has been studied with the nonadiabatic EPT formalism. 61–62 An important objective of such theoretical studies is to help guide the design of more effective molecular catalysts for hydrogen production in an effort to develop environmentally friendly and renewable alternative energy sources.…”