(Invited) Thin Film Thermoelectric Metal-Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity

Metal-organic frameworks (MOFs) are extended, crystalline compounds consisting of metal ions interconnected by organic ligands such as benzene carboxylates, forming a crystalline, nanoporous structure. The low atomic density and long, bridge-like bonding characteristic implies that MOFs should exhibit low thermal conductivity, which is an attractive feature for thermoelectric energy conversion. However, most MOFs are electrical insulators due to the non-conjugated character of the organic ligands and poor overlap between their pi orbitals and the valence orbitals of the metal ions. Recently, we discovered that infiltrating the pores of the copper-containing MOF Cu₃(BTC)₂ with redox-active guest molecules TCNQ (7,7,8,8-tetracyanoquinododimethane) increases the electrical conductivity of thin film devices by as much as seven orders of magnitude [1]. Density functional theory indicates that electrical conductivity results from a donor-bridge-acceptor geometry, in which TCNQ binds to two Cu(II) dimer units within the MOF pore. In this paper we report the first thermoelectric properties characterization of a MOF thin film which yield a positive Seebeck coefficient of ~400 uV/K, in qualitative agreement with DFT calculations which also indicate that holes should be the dominant charge carriers in TCNQ@Cu₃(BTC)₂. Finally, we present results of molecular dynamics simulations which indicate that the phonon thermal conductivity of Cu₃(BTC)₂ MOF is indeed low, thus further suggesting that conducting MOFs are promising materials for thermoelectric energy conversion [2].

[1] A. A. Talin, A. Centrone, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Léonard, M. D. Allendorf, Science 343, 66 (2014)

[2] K. J. Erickson , F. Léonard , V. Stavila , M. E. Foster, C. D. Spataru , R. E. Jones , B. M. Foley, P. E. Hopkins, M. D. Allendorf, and A. A. Talin, Adv. Mater. DOI: 10.1002/adma.201501078 (2015).