Microscopic Insights into Cation-Coupled Electron Hopping Transport in a Metal–Organic Framework

Abstract

Electron transport through metal−organic frameworks by a hopping mechanism between discrete redox active sites is coupled to diffusionmigration of charge-balancing counter cations. Experimentally determined apparent diffusion coefficients, D e app , that characterize this form of charge transport thus contain contributions from both processes. While this is well established for MOFs, microscopic descriptions of this process are largely lacking. Herein, we systematically lay out different scenarios for cation-coupled electron transfer processes that are at the heart of charge diffusion through MOFs. Through systematic variations of solvents and electrolyte cations, it is shown that the D e app for charge migration through a PIZOF-type MOF, Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide (NDI) linkers, spans over 2 orders of magnitude. More importantly, however, the microscopic mechanisms for cation-coupled electron propagation are contingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers, either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, we show that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation−linker interactions can open pathways for concerted cation-coupled electron transfer processes that can outcompete limitations from reduced cation flux.

Solvent-controlled ion-coupled charge transport in microporous metal chalcogenides

“…47 Given that the Fe and Zn systems are isostructural, the substantial difference in conductivity likely arises from the materials possessing distinct transport mechanisms. For the Zn system, redox hopping between clusters necessitates ion movement to maintain charge neutrality, 29,[52][53][54][55][56] whereas charge carrier delocalization in the Fe system maintains charge neutrality through motion of both electrons and holes.…”

“…For example, conductivity studies of the MOF Zr(3-hydroxy-2-[7-(4carboxy-2-hydroxyphenyl)-1,3,6,8-tetraoxo-3,6,7,8-tetrahydro-1H-benzo[lmn] [3,8]phenanthrolin-2-yl]benzoate) revealed that diffusion coefficients derived from chronoamperometry decreased with higher dielectric solvents. 29 However, the scope of the study was limited to few solvents and focused on just this hopping-type material. However, anecdotal evidence abounds for the solvent-and electrolyte-dependence of charge transport in large-pore MOFs and COFs.…”

“…84 This analysis, in combination with solvent-dependent activation barriers, provides some of the rst evidence of redox-type hopping in microporous materials, although this mechanism is commonly invoked for MOFs, such as Zr(3-hydroxy-2-[7-(4-carboxy-2-hydroxyphenyl)-1,3,6,8tetraoxo-3,6,7,8-tetrahydro-1H-benzo[lmn] [3,8]phenanthrolin-2yl]benzoate). 29 Just as PCET provides lower-energy redox pathways in biological systems, 14 the observation of concerted charge transport in TMA 2 ZnGe 4 S 10 suggests high energy intermediates may be avoided in inorganic systems through ICCT. Mixed conductors feature widely in electrochemical Scheme 3 Generalized impedance response for an ideal mixed conductor (black solid trace), an ion-coupled conductor (dotted red trace; TMA 2 ZnGe 4 S 10 ), and ion-limited conductor (dotted blue trace; TMA 2 FeGe 4 S 10 ).…”

“…However, anecdotal evidence abounds for the solvent- and electrolyte-dependence of charge transport in large-pore MOFs and COFs. 29,53,57,78–80 We contend that this solvent dependence likely results from the pervasiveness of hopping-type charge transport in MOFs. Furthermore, we expect these effects to be magnified by nanoconfined ions.…”

“…27,28 Ioncoupled charge transfer (ICCT) has been recently demonstrated to inuence the electron mobility of MOFs as well. 29,30 We propose that ICCT must be considered in materials where external ions, such as electrolytes, exist within Ångstroms of itinerant charges. Conceived classically, the inuence of each charge should scale as 1/r3 r 3 0 , where r denotes the average intercharge distance, 3 r the operative dielectric constant, and 3 0 the permittivity of vacuum.…”