Charge Transfer within Metal-Organic Frameworks: The Role of Polar Node in the Electrocatalysis and Charge Storage

Abstract

Metal-organic frameworks (MOFs) assembled from photo and redox active building blocks such as porphyrins and pyrenes, as well as numerous post-synthesis processes that enable incorporation of required chemical functionality within the pores, have signified these crystalline molecular assemblies as emerging class of compositions for energy conversion, and storage systems. In particular, electrocatalytic energy conversion and storage applications require efficient charge transport process within the framework. It is known that the difference in the redox potentials between the linkers and metal-oxo nodes enforces a self-exchange/hopping type charge transport processes. For example, porphinato-iron based frameworks are shown to provide high electrode-surface concentration of electrocatalytic reduction of CO2 to CO. However, in such processes, as counter ion migration accompanies a (electrochemical) charge transfer reaction within the narrow pores of the MOFs, it leads to unique situations including a slower kinetic and selective redox reaction [i.e. a modified self-exchange reaction: L⁺—PF₆̅ + L⁰ → L⁰—PF₆̅ + L⁺]. This paper will discuss how the charge-transfer process is inherently different within the pores and that the metal-oxo node can play a critical role in the CT kinetics.