The Molecular Nature of Redox-Conductive Metal-Organic Frameworks

Abstract

ConspectusRedox-conductive metal-organic frameworks (RC-MOFs) are a class of porous materials that exhibit electrical conductivity through a chain of self-exchange reactions between molecularly defined, neighboring redox-active units of differing oxidation states. To maintain electroneutrality, this electron hopping transport is coupled to the translocation of charge balancing counterions. Owing to the molecular nature of the redox active components, RC-MOFs have received increasing attention for potential applications in energy storage, electrocatalysis, reconfigurable electronics, etc. While our understanding of fundamental aspects that govern electron hopping transport in RC-MOFs has improved during the past decade, certain fundamental aspects such as questions that arise from the coupling between electron hopping and diffusion migration of charge balancing counterions are still not fully understood.In this Account, we summarize and discuss our group's efforts to answer some of these fundamental questions while also demonstrating the applicability of RC-MOFs in energy-related applications. First, we introduce general design strategies for RC-MOFs, fundamentals that govern their charge transport properties, and experimental diagnostics that allow for their identification. Selected examples with redox-active organic linkers or metallo-linkers are discussed to demonstrate how the molecular characteristics of the redox-active units inside RC-MOFs are retained. Second, we summarize experimental techniques that can be used to characterize charge transport properties in a RC-MOF. The apparent electron diffusion coefficient, D_e^app, that is frequently determined in the field and obtained in large perturbation, transient experiments will be discussed and related to redox conductivity, σ, that is obtained in a steady state setup. It will be shown that both MOF-intrinsic (topology, pore size, and apertures) and experimental (nature of electrolyte, solvent) factors can have noticeable impact on electrical conductivity through RC-MOFs. Lastly, we summarize our progress in utilizing RC-MOFs as electrochromic materials, materials for harvesting minority carriers from illuminated semiconductors and within electrocatalysis. In the latter case, recent work on multivariate RC-MOFs in which redox active linkers are used to "wire" redox catalysts in the crystal interiors will be presented, offering opportunities to independently optimize charge transport and catalytic function.The ambition of this Account is to inspire the design of new RC-MOF systems, to aid their identification, to provide mechanistic insights into the governing ion-coupled electron hopping transport mode of conductivity, and ultimately to promote their applications in existing and emerging areas. With basically unlimited possibilities of molecular engineering tools, together with research in both fundamental and applied fields, we believe that RC-MOFs will attract even more attention in the future to unlock their full potential.

Figure 1

Schematic representation of diffusional electron hopping transport in an RC-MOF film grown on an electrode surface after a large step potential experiment. Electron injection at the electrode-MOF interface transforms electronically isolated redox-active species O to R, and meanwhile, charge balancing counterions Z⁺ are moving from the electrolyte solution into the MOF film to maintain overall charge neutrality. The color gradient inside the MOF film represents the concentration gradient of R.

Figure 2

Schematic representation of diffusional electron hopping transport within a RC-MOF in transient electrochemical experiments, and a “molecular” CV profile (a). Schematic representation of an RC-MOF with a 50:50 distribution of reduced and oxidized linkers obtained by applying a potential that corresponds to the E₀ of the redox active component for an extended period of time. The 50:50 steady state redox composition of the film at its E₀ is a direct outcome of the Nernst equation. Deviating from the E₀ will alter the redox composition of the film and lead to a drop in redox conductivity. Consequently, a bell-shaped redox conductivity curve as a function of applied potential is observed for a RC-MOF (b).

Figure 3

Crystal structure of Zr(NDI) (a) and UU-100(Co) (d) as determined by microcrystal electron diffraction and the molecular structure of corresponding NDI (b) and cobaloxime linker (e). CVs of thin films grown on FTO substrates (in black), and the corresponding redox conductivity curves, as deduced from electrochemical impedance spectroscopy of Zr(NDI) (c) and UU-100(Co) (f). The “molecular” CV waves, and the bell-shaped conductivity curves are characteristic of RC-MOFs. Reproduced from ref (4). Available under a CC-BY 4.0 license. Copyright 2023 Springer Nature.

Figure 4

Representative CV responses of the same RC-MOF film on FTO surface at different scan rates; “surface” wave at 10 mV s^–1 (a) and “diffusion” wave at 200 mV s^–1 (b). Reproduced with permission from ref (14). Copyright 2023 American Chemical Society.

Figure 5

Schematic illustration of intrinsic MOF-born and extrinsic parameters that are important for experimental D_e^app.

Figure 6

(a) Crystal structure of Zn(NDI) MOF with the projection of the pyrazolate-bridged Zn²⁺ chain (top) and viewing along the c direction (bottom). (b) Cyclic voltammetry (CV) scan (5 mV s^–1) and steady-state redox conductivity measurement of MOF thin-films in Ar-saturated DMF with KPF₆ as the supporting electrolyte (0.1 M). Reproduced from ref (4). Available under a CC-BY 4.0 license. Copyright 2023 Springer Nature.

Figure 7

CVs and corresponding electrochromic properties (bottom panel) of Zn(PMDI) (a), Zn(NDI) (b), and Zn(PDI) (c) MOF films. Reproduced with permission from ref (14). Copyright 2023 American Chemical Society.

Figure 8

(a) Biomimetic redox-conductive PCN-700-based MOF with NDI linkers as electron mediators and [FeFe] hydrogenase as active site models for HER. (b) Schematic representation of a multivariate RC-MOF with two diffusional electron hopping transport channels (green arrows) that communicate through thermodynamically driven electron transfers from the higher to the lower energy channel. (c) Schematic representation of surface grown Zr(NDI) on p-type semiconductor surfaces. Reproduced with permission from refs (5, 48, 50). Copyright 2021, 2024 American Chemical Society and available under a CC-BY 4.0 license; 2020 Springer Nature.