A Microporous and Naturally Nanostructured Thermoelectric Metal-Organic Framework with Ultralow Thermal Conductivity

Microporous metal-organic frameworks (MOFs) offer attributes that make them potentially compelling choices for thermoelectric applications because they combine organic character with long-range order and intrinsically low thermal conductivity. So far, thermoelectricity in this class of materials has required infiltration with external molecules to render the framework electrically conductive. Here, we present thermoelectric studies on an n-type naturally nanostructured microporous MOF, Ni₃(2,3,6,7,10,11-hexaiminotriphenylene)₂, whose pressed pellets exhibit high electrical conductivity and low thermal conductivity. The results here show that by combining the structural rigidity and high crystallinity of inorganic materials, the solution-based synthesis of organic materials, and the unique pore-based tunability and low thermal conductivity, MOFs represent an intriguing new class of thermoelectric materials. Thermoelectric devices convert heat to electricity or vice versa. Their conversion efficiency scales with the thermoelectric figure of merit, ZT, which itself increases with increasing electrical conductivity and Seebeck coefficient, and with decreasing thermal conductivity. Owing to their monodisperse pores in the micro/mesoporous range, complex composition, and tunability, metal-organic frameworks (MOFs) offer promise in the context of thermoelectrics. Indeed, MOFs are excellent thermal insulators and recently have been shown to be good electrical conductors whose electrical conductivity and Seebeck coefficient may be optimized by design and/or post-synthetic tuning. Here, we present the thermoelectric properties of a naturally nanostructured MOF, Ni₃(HITP)₂, which exhibits an ultralow thermal conductivity and a record high ZT in this class of materials at room temperature. These intriguing properties reveal the potential of MOFs for high-performance thermoelectric applications. Thermoelectric devices utilize waste heat to generate electricity or consume electricity to transfer heat. Sun et al. describe high electrical conductivity and ultralow thermal conductivity in the nanoporous material Ni₃(HITP)₂, improve the record of thermoelectric figure of merit in metal-organic frameworks (MOFs), and demonstrate that MOFs are promising candidates for thermoelectrics.