(Invited) Moving Electrons and Holes through Porous MOFs and MOF-Supported Catalysts

Abstract

Porous, thin-film, water stable metal-organic frameworks (MOFs) are potentially highly attractive as electrocatalysts as they can present reactant-accessible catalysts at high areal concentration on electrode surfaces. Indeed, such films, if constructed from molecular catalysts, can present the equivalent of several hundred monolayers or more of catalyst -- amounts that can translate into decreases of hundreds of millivolts in kinetic overpotential relative to overpotentials for otherwise equivalent solution-phase electrocatalysts present at millimolar concentrations. Similar conclusions can be reached for catalysts consisting of MOF-node-supported, metal-oxygen or metal-chalcogen clusters. Required for efficacious electrocatalysis, however, is reasonable electrical conductivity, where the basis for conductivity may be site-to-site redox hopping or may instead operationally resemble band-like electronic conductivity. For electrode-supported MOF films having thicknesses of around a micron, site-to-site charge hopping rates on the order of 105 sec-1 should be sufficient to sustain catalytic currents of tens of mA/cm2. Alternatively, MOF electronic conductivities on the order of 10-2 Siemens/cm (roughly seven and a half orders of magnitude less than the conductivity of copper) should be sufficient to sustain catalytic currents of tens of mA/cm2 with only marginal iR penalties. Thus, in comparison to many applications, the degree of conductivity required for MOF thin-film mediated electrocatalysis is modest. This presentation will describe some approaches to rendering highly-porous MOF films sufficiently electrically conductive to support electrocatalysis. Included will be examples of highly anisotropic electrical conductivity, underscoring the role that MOF topology can play in defining both hopping-like and band-like conductivity. Also included may be examples of intentional chemical modulation of MOF conductivity. Based on these and other examples, some potentially transferrable principles for designing electrocatalysis-competent, conductive MOFs will be proffered.