The conduction of electrons and ions is central to the operation of modern technologies. In particular, it is the redistribution of charge within a closed system that enables the production of energy, the storage of information, and the transduction of physicochemical phenomena. Progress on these fronts dictates that underlying component materials also progress towards superior performance and greater specificity in supplying a set of properties tailored to one of an ever growing and nuanced list of applications. Necessarily, new conductive materials must be developed continuously in order to keep up with societal demands for a more electrified and computerized planet.
Historically, functional materials reside with a relatively small set of classes: metals, ceramics, glasses, polymers, zeolites, and carbons. While there have been fantastic advances over the last century among every one of these classes, polymers and zeolites stand out as uniquely modern technologies. Synthetic polymers trace back to 1830, with the development of vulcanized rubbers. The advent of synthetic polymers revolutionized the level of control and tunability possible in materials science by leveraging the near infinite structural diversity possible in organic chemistry. It wasn’t until one hundred and twenty years later that synthetic zeolites were commercialized. Zeolites are ceramics that distinguish themselves by their large crystalline void spaces capable of hosting gases and small molecules. Microporous zeolites represented a new competitor to porous carbons, with key advantages of long-range crystalline order and a unique surface chemistry particularly amenable to molecular sieving, gas separations, and the catalytic transformation of commodity chemicals. However, like porous carbons, structural and chemical diversity is limited for zeolites. Today, there are only about 230 known zeolite structures, and the discovery of new zeolite structure types is a notoriously difficult endeavor.
More recently a new set of porous crystals has been discovered, metal-organic frameworks. These hybrid materials are particularly well suited to the task of controlling structure and surface chemistry at the atomic level. With both an inorganic component providing the rich structural diversity unique to transition metal coordination chemistry and a prominent organic linker component providing near infinite tunability akin to that of organic polymers, the development of metal-organic frameworks has ignited a renaissance in crystal engineering and porous materials chemistry. Over the last twenty years alone, the structures of many thousands of new metal-organic frameworks have been determined, dwarfing zeolites, carbons, and other porous materials. Further, metal-organic frameworks are steadily proving to be vastly superior in performance across many applications, especially for gas storage and separations.
Many obvious implementations of metal-organic frameworks as replacements for zeolites and porous carbons have been widely developed. All the while, the understanding and control of charge transport and electronic communication rested just beyond the horizon. Unlike zeolites and other porous crystals that are exclusively excellent electronic insulators, metal-organic frameworks do not share the same chemical limitations. The possible co-existence of intrinsic porosity, unprecedented structural diversity, intuitive tunability, and long-range electronic communication could fully leverage the potential of this class of materials. That is, the ability to manipulate a bulk electronic structure with precision and dramatic effect may result in new and unpredictable technologies.
Yet, it is not without reason that electronically conductive metal-organic frameworks are rare. Electronic communication is strongly dependent upon the relative distances between interacting valence electrons and vanishes steeply with increasing distance. Likewise, ion mobility in crystalline materials also requires extremely short hopping distances, typically less than half a nanometer. Within a class of materials that routinely boast void fractions in excess of 60%, it is not surprising that finding a high density of valence states or ion hopping sites has proven difficult. As such, it is of fundamental interest to explore how these structures can be coaxed into displaying the bulk transport of charge, and the physical means by which it occurs. By first learning the rules of synthesizing conductive metal-organic frameworks and transporting charge therein, it may then be possible to rationally tailor porous conductors for a targeted property or application.
To this end, Chapter 1 discusses the nature of ion transport in metal-organic frameworks, its relation to traditional ion conductors, and reviews the prospects of ion conducting frameworks in a number of proposed applications. Surprisingly, though there are many proof-of-concept reports of using metal-organic frameworks in electrochemical devices, the investigation of ion transport is routinely ignored. This is despite it likely being the performance limiting factor in many cases. In fact, many implementations of metal-organic frameworks as component materials in supercapacitor electrodes are predicated on intrinsic porosity translating to fast ion transport. As will be shown, this is decidedly false for many canonical framework materials.
Nonetheless, with considerable optimization and targeted selection of pore structure and surface chemistry, it is possible to obtain ionic conductivities high enough to be useful in electrochemical systems. In fact, with judicious selection of host lattice and host-guest interactions it is possible to conduct ions that have been particularly intransigent to bulk transport in traditional materials like ceramics and polymers. Chapter 2 highlights the synthesis of several metal-organic framework-based ion conductors capable of conducting magnesium, demonstrating an alternative strategy for multivalent ion conduction. Discussed in detail are the critical factors that dictate ion insertion and bulk transport in metal-organic frameworks and multiple strategies for maximizing ionic conductivity.
With these tools in hand, Chapter 3 tackles mixed electron-ion conductivity in a similarly structured metal-organic framework. In some respects, the interrogation of ion transport in mixed conductors is made easier by the possibility of probing ion transport over a wide range of concentrations, electrochemically measuring the transport of a single ionic species, and directly observing host-guest interactions by changes in electrochemical potential. While many frameworks have been reported as candidate electrodes, Chapter 3 represents the first to thoroughly discuss the role of host-guest interactions, and a surprising sensitivity to changes in the included electrolyte.
Finally, Chapter 4 focuses on the modulation of electronic conductivity with host-guest interactions in a non-ionic metal-organic framework. A new technique for interrogating conductivity in porous solids is presented that is based on in-situ gas dosing and the simultaneous determination of conductivity and a gas adsorption isotherm. Unlike previous methods based on vapor exposure at ambient pressure, the measurements reported are referenced to the pure metal-organic framework, which possesses a consistent and well-defined chemical composition. Thus, equilibrium conductivity-composition profiles are directly extracted from the measurement. This is in contrast to the complex and relatively slow exchange reactions that occur by vapor exposure methods, which often lead to results that are difficult to interpret. Thus, a standard method of measuring and reporting the chemresistive response of gaseous and vaporous adsorbates is proposed and demonstrated.
