Thermoelectric Properties of 2D Ni3(HITP)2 and 3D Cu3(BTC)2 MOFs: First-Principles Studies
Authors: He Yuping , Talin A. Alec , Allendorf Mark D. 
Published: February 2020

Metal-Organic Frameworks are nanoporous structures comprised of metal ions coordinated to electron-donating “linker” molecules. Although their low density and open structure give them low thermal conductivity, most MOFs are electrical insulators. Recently, however, we and others showed that electrically conducting frameworks can be synthesized by several routes, leading to both two- and three-dimensional materials with conductivities as high as 160 S/cm, suggesting potential for thermoelectric device applications. New 2D materials based on MOFs that resemble graphene in their topology are among those with the highest electrical conductivity. These materials, sometimes referred to as Metal-Organic Graphene Analogues (MOGs), display intriguing electronic, thermal, and optical properties, some of which depend on whether a monolayer or multilayer is used. For example, both metallic and semiconducting behavior are predicted, as well as quantum spin Hall and Z2 metallic states in monolayers. Only semiconducting behavior has been experimentally observed so far, however.

In this presentation we describe experimental and theoretical studies of the fundamental charge transport processes controlling the electronic and thermal properties relevant to thermoelectric devices. In particular, we consider the thermoelectric properties of crystalline MOGs with the structure X3(HITP)2 in which X = Ni, Pd or Pt, and HITP = 2,3,6,7,10,11-hexaiminotriphenylene, calculated using ab initio simulations. The dependence of thermoelectric transport properties on the atomic structure are modeled by comparing the calculated band structure, band alignment, and electronic density of states of the three MOGs. We find that the thermoelectric transport properties depend strongly on both the interaction between the linkers and the metal ions and the d-orbital splitting of the metal ions induced by the linker crystal field. Moreover, a significant deviation from the Wiedemann-Franz law arises is predicted for p-type doping of these materials, with a nonlinear behavior of the Lorentz number as a function of hole concentration. The results predict that thermoelectric transport properties of X3(HITP)2 systems can be enhanced by replacing Ni(II) ions in the structure with heavy metal ions, thereby increasing the interaction between the metal ion and the ligands.

We will also discuss a strategy for converting an insulating MOF to an electrically conducting one by inserting redox active molecules in the pores, which call Guest@MOF. The MOF Cu3(btc)2 (btc = benzenetricarboxylate), also known as HKUST-1, becomes electrically conducting when the molecule TCNQ (7,7,8,8-tetracyanoquinodimethane) is infiltrated into the ~1 nm pores of the MOF. TCNQ is a strong π acid that coordinates to unoccupied Cu(II) ions in the framework, generating new charge transfer bands in the UV-visible absorption spectrum. Thermoelectric measurements show that the majority charge carriers are holes, and a Seebeck coefficient of ~400 μV/K is obtained, more than double the value for Bi2Te3.

Together, the results of this work provide fundamental guidance to optimize existing conducting MOFs and to design and discover new families of MOF-like materials for thermoelectric applications.